The Role of Local pH in Electrocatalysis: Measurement, Impact, and Control Strategies

Abstract

The local pH at the electrode–electrolyte interface exerts a profound, yet often underappreciated, influence on the kinetics, selectivity, and stability of electrocatalytic reactions. This review critically examines the origins, dynamics, and consequences of pH gradients in processes such as water electrolysis, CO₂ reduction, nitrate reduction, and alcohol oxidation. We survey state-of-the-art experimental and computational approaches for probing local pH, including scanning electrochemical microscopy, operando spectroscopy, density functional theory, and multiscale modeling. These complementary methods reveal how near-surface (local) pH evolves with current density, electrode morphology, and electrolyte composition, thereby reshaping catalytic pathways and shifting reaction mechanisms. Particular attention is devoted to the influence of local pH on catalyst degradation and support corrosion, including phase dissolution, carbon oxidation, and membrane failure. We also discuss mitigation strategies such as buffer optimization, electrode architecture design, and gas diffusion layer engineering that enable control over local reaction environments. By integrating mechanistic insight with advanced diagnostics, this review highlights that controlling local pH is essential for improving both performance and durability in electrochemical systems. The concepts presented herein provide a framework for designing next-generation catalysts and reactors capable of operating under extreme or fluctuating (local) pH conditions.

Introduction

Electrocatalytic reactions are strongly influenced by the local pH at the electrode–electrolyte interface, which can deviate markedly from the bulk electrolyte pH due to ion-transport limitations, reaction kinetics, surface charge effects, and surface-adsorbate-solution interactions. In this review, local pH denotes the proton activity in the electrolyte region adjacent to the electrode spanning the electrochemical double layer (EDL) (interfacial pH) and the electroneutral reaction/diffusion layer (near-surface pH). Under faradaic operation, this region develops non-equilibrium local pH gradients, whose extent depends on transport limitations and device architecture. Given the non-ideal and system-dependent nature of interfacial acid–base behavior, Table defines local pH in terms of both interfacial location and operating regime (i.e., under high current density) and, in parallel, offers a concise synopsis of the experimental and modeling approaches used to measure or estimate local pH, which are discussed in detail in the following sections.

1. Distinction between Interfacial pH in the Electrochemical Double Layer and Non-Equilibrium Local pH Gradients under Faradaic Conditions, with Associated Length Scales, Governing Physics, and Representative Experimental and Theoretical Methods.

pH spatial regime	definition	length scale	dominant physics	experimental access	modeling approaches
Interfacial pH (compact/Stern layer)	Proton activity within the inner electrochemical double layer adjacent to the electrode surface	<1 nm	Electrostatics, specific adsorption, solvent structure, surface acid–base equilibria	Not directly accessible by conventional pH probes; inferred from surface-sensitive spectroscopy and electrochemical observables	Continuum double-layer models, grand-canonical DFT, constant-potential methods, explicit-solvent MD/AIMD
Interfacial pH (diffuse layer)	Proton activity within the diffuse part of the electrochemical double layer under near-equilibrium conditions	∼1–10 nm (ionic-strength dependent)	Electrostatic ion partitioning, double-layer structure	Indirect inference via potential-dependent kinetics and interfacial spectroscopy	Poisson–Boltzmann-type theories and variants, MD/AIMD simulations
Near-surface pH (reaction layer)	Proton activity just outside the double layer, where homogeneous equilibria and transport begin to dominate	∼10–100 μm (boundary-layer dependent)	Reaction–diffusion, buffering equilibria, ionic migration	SECM pH microelectrodes, fluorescence or confocal probes, operando IR or Raman spectroscopy of buffer species	Nernst–Planck transport models with homogeneous reactions (electroneutral or Poisson-coupled)
Local pH (non-equilibrium)	Spatially and temporally varying pH field under Faradaic current	∼10 μm to >100 μm (including porous electrodes and GDEs)	Coupled electrochemical kinetics, mass-transport, buffer capacity, convection	SECM mapping, fluorescence imaging, NMR or MRI of carbonate systems, fast OCP-decay methods	Multiphysics finite-element simulations, microkinetic–transport coupling

The local pH is particularly important in processes such as water electrolysis, CO₂ reduction (CO₂RR), N₂ reduction (N₂RR), and alcohol oxidation, where the availability of protons (H⁺) and hydroxide ions (OH^–) directly influences surface coverages, reaction pathways, catalyst stability, wettability, and product selectivity. ,

Local pH gradients arise from the production or consumption of H⁺ and OH^– at electrode surfaces during electrocatalytic reactions. For example, in the hydrogen evolution reaction (HER, eq ) under acidic conditions, rapid reaction rates can locally deplete H⁺ and cause interfacial basification. Conversely, in the oxygen evolution reaction (OER, eq ) under alkaline conditions, OH^– consumption can induce localized acidification, thereby affecting both reaction kinetics and material stability. Similar effects arise in CO₂ electrolysis (e.g., to produce ethylene in acid, eq , or ethanol in base, eq ), where cathodic reduction can lead to local neutralization or alkalization, influencing CO₂ solubility, (bi)­carbonate equilibria, and selectivity toward C₁ versus C₂₊ products. Such local (near-surface) pH shifts can also accelerate degradation pathways, including metal leaching, carbon corrosion, and membrane failure.

$$2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2$$ 1

$$4\mathrm{O}{\mathrm{H}}^{-}\to {\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}$$ 2

$$2\mathrm{C}{\mathrm{O}}_2+12{\mathrm{H}}^{+}+12{\mathrm{e}}^{-}\to {\mathrm{C}}_2{\mathrm{H}}_4+4{\mathrm{H}}_2\mathrm{O}$$ 3

$$2\mathrm{C}{\mathrm{O}}_2+9{\mathrm{H}}_2\mathrm{O}+12{\mathrm{e}}^{-}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+12\mathrm{O}{\mathrm{H}}^{-}$$ 4

Understanding and controlling local pH is therefore essential for optimizing both the performance and durability of electrocatalytic systems. Recent research has focused on developing techniques to measure, model, and regulate local pH using approaches that span operando spectroscopy, scanning electrochemical microscopy (SECM), density functional theory (DFT), ab initio and classical molecular dynamics, multiscale simulations, and buffering. Complementary strategies involving advanced electrode architectures, tailor-made electrolytes, and gas diffusion layers have also emerged as effective means to control local reaction environments.

From a theoretical standpoint, the relationship between local and bulk pH can be quantified using Boltzmann statistics. The local concentration of an ion i at an interfacial potential Ψ _s (or electrochemical double-layer potential), C _i,local, is related to its bulk concentration, C _i,bulk, by

$$C_{i,\mathrm{local}}=C_{i,\mathrm{bulk}}\exp \left(\frac{-z_{i}e{\mathrm{\Psi }}_s}{k_{\mathrm{B}}T}\right)$$ 5

where z _i is the ion valence, e is the elementary charge (1.602 × 10^–19 C), k _B is the Boltzmann constant (1.381 × 10^–23 J K^–1), and T is the absolute temperature (K). For protons (z _H⁺ = +1), this expression yields

$${\mathrm{pH}}_{\mathrm{local}}={\mathrm{pH}}_{\mathrm{bulk}}+\left(\frac{e{\mathrm{\Psi }}_s}{k_{\mathrm{B}}T\ln 10}\right)$$ 6

At 298.15 K, this relation can be written in a convenient numerical form as

$$\mathrm{\Delta }\mathrm{pH}\approx \frac{e\mathrm{\Delta }\varphi }{k_{\mathrm{B}}T\ln 10}\approx \frac{\mathrm{\Delta }\varphi }{(2.303k_{\mathrm{B}}T)/e}\approx \frac{\mathrm{\Delta }\varphi }{0.059\mathrm{V}}$$ 7

which corresponds to approximately 16.9 pH units per volt. Here, Δϕ denotes the local electrostatic potential drop experienced by H⁺, for example across the diffuse layer. Therefore, even in the absence of Faradaic H⁺/OH^– production or consumption, a purely capacitive change of |Δϕ| = 25–100 mV corresponds to an interfacial pH modulation of ΔpH ≈ 0.4–1.7 units. In practice, Δϕ is generally considerably smaller than the total applied potential, particularly in concentrated electrolytes where electrostatic screening is strong and a substantial fraction of the potential drop occurs across the compact layer. Accordingly, purely capacitive pH shifts are often modest compared with Faradaic, mass-transport-driven pH excursions.

These relationships highlight the direct influence of interfacial potential on proton concentration and, by extension, underpin the interpretation of pH-dependent interfacial phenomena including proton-coupled electron transfer (PCET), surface catalysis, and ion transport. While the interfacial potential–pH relationship provides a useful first-order framework, local pH effects at electrochemical interfaces are often not captured by simply applying a bulk Nernst-type expression. The interfacial region features strong electric fields, specific adsorption, and altered solvation/ion pairing, such that acid–base equilibria can deviate substantially from bulk expectations. In particular, the apparent pK _a of electrode-bound (or interfacial) species can shift under polarization, and experimentally observed dependencies may therefore be less straightforward than idealized treatments suggest. These non-idealities, documented in classical studies by White et al. , and highlighted in recent work by Mayer et al., underscore the need for direct experimental/operando determination of pH-related interfacial phenomena, especially under conditions where the structure and composition of the double layer evolve with potential and current density. Experimentally resolving local pH with high spatial and temporal resolution remains challenging, often requiring indirect probes or nanoscale measurements, while its accurate modeling, together with correlative near-surface electrolyte effects, is conceptually complex and computationally demanding.

In this review, we examine the role of local pH across a range of electrocatalytic reactions, emphasizing its measurement, mechanistic impact, and control strategies. Although this review focuses on electrocatalysis, analogous interfacial proton gradients are also central to aqueous energy-storage electrodes, whose charge compensation is mediated by proton-coupled redox chemistry. In particular, proton-insertion coupled electron transfer (PICET) materials can also locally consume or release protons during (dis)­charge, creating pronounced pH gradients in the electrolyte-filled region adjacent to (and within) porous electrodes. These gradients can shift equilibrium potentials, alter apparent kinetics, and influence stability, making “local pH” a shared design variable across electrocatalysis and aqueous energy-storage technologies. By consolidating theoretical concepts, diagnostic techniques, and mitigation approaches, we aim to provide a framework for the design of next-generation electrocatalysts and reactors capable of operating efficiently and stably under extreme or fluctuating pH conditions.

Experimental and Theoretical Approaches for Local pH Measurement

In Situ and Operando Techniques

Accurately determining local pH in electrochemical systems is essential for understanding and optimizing processes that involve PCET steps, including HER, OER, CO₂RR, and nitrate or nitrite reduction. A variety of direct and indirect approaches are available, each with characteristic spatial and temporal resolution, chemical selectivity, and operational stability. This section outlines these methods and illustrates their capabilities and limitations with representative examples from recent literature.

Direct electrochemical probes infer pH from pH-sensitive redox or open-circuit potentials (OCP) measured in close proximity to the electrode surface. Potentiometric microelectrodes, most commonly IrO _x -modified, are widely used and can exhibit near-Nernstian or even super-Nernstian pH responses. Nevertheless, limitations arise from both material behavior and measurement physics. For IrO _x specifically, dissolution in acidic electrolytes and comparatively slow response can hinder measurements in transient or highly dynamic environments. More fundamentally, potentiometry is the established standard for bulk pH but becomes increasingly difficult to interpret near polarized electrochemical interfaces. As probes are miniaturized to achieve spatial resolution, drift, noise, and long-term instability typically increase, and the probe itself can perturb the local microenvironment. In addition, steep potential gradients, strong electric fields, and non-equilibrium ion distributions near an operating electrode mean that the measured potential reflects not only proton activity but also local electrostatic and transport contributions. Consequently, potentiometry remains robust for bulk electrolyte pH, yet can be progressively less informative for resolving near-surface pH under operando conditions where reaction–transport gradients dominate. These challenges have motivated alternative sensing materials and complementary operando approaches for local pH mapping under harsh electrochemical conditions.

SECM enables high-resolution mapping of local pH using potentiometric or voltammetric pH-sensitive tips. Voltammetric probes typically offer faster response and improved robustness under fluctuating reaction conditions. For example, Monteiro et al. functionalized Au ultramicroelectrodes with a self-assembled monolayer containing the 4-hydroxylaminothiophenol/4-nitrosothiophenol (4-HATP/4-NSTP) redox couple (Figure a). The resulting probe exhibited a reversible redox response and a Nernstian mid-peak potential shift of 57 mV per pH unit, enabling accurate pH mapping under both Ar and CO₂ atmospheres (Figure c). The probe also maintained high temporal resolution during CO₂RR, while avoiding stability issues common in polymer-based sensors.

1.

SECM pH sensor synthesis and calibration. (a) Voltammogram showing conversion of 4-NTP into the pH-sensitive redox couple 4-HATP/4-NSTP on Au ultramicroelectrodes (Au-UMEs). (b) pH sensor voltammetry in 0.1 M Li₂SO₄ solutions adjusted to different pH, taken at 200 mV s^–1. (c) Calibration curves of the modified probe under different gas atmospheres. (d,e) Diffusion-layer dynamics showing pH recovery following interruption of HER (d) or CO₂RR (e). Potentials are reported versus Ag/AgCl. (f) Schematic of homogeneous reactions occurring alongside electrode processes. Figure adapted from ref with permission from American Chemical Society.

SECM also provides insight into the temporal evolution of near-surface pH. Monteiro et al. followed pH relaxation after switching off HER or CO₂RR (Figure d,e). In Ar, local pH rapidly returned to the bulk value due to fast H⁺/OH^– diffusion (Figure d), while in the presence of CO₂, buffering by carbonate species delayed relaxation (Figure e), with recovery times dependent on the applied potential and thus the extent of local alkalization. These results highlight the interplay between homogeneous buffering reactions and ionic transport in shaping near-surface pH dynamics during and after electrocatalysis (Figure f).

Bare Au micro- and nanoelectrodes have also been used as SECM probes. Li et al. demonstrated their ability to detect local acidification during OER in both acidic and alkaline electrolytes by exploiting the super-Nernstian shift of the Au/Au₂O₃ redox couple as a pH indicator. In contrast to Pt, Au minimizes interference from oxygen reduction, which is particularly important under O₂-evolving conditions.

Optical fluorescence methods, particularly fluorescence confocal laser scanning microscopy (CLSM), provide spatially resolved pH maps with micrometer-scale resolution. Rudd et al. used fluorescein, a dye with pH-dependent emission intensity near neutral pH, to image pH gradients around microelectrode arrays. Although CLSM offers excellent spatial resolution, its utility is constrained by the narrow pH-responsive range of fluorescein (pH ≈ 6–8) and by the need for optically transparent or semi-transparent electrodes.

Surface-sensitive vibrational spectroscopy, such as attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and surface-enhanced Raman spectroscopy (SERS), enables indirect pH determination by monitoring vibrational signatures of buffer species (e.g., phosphate, carbonate). Hicks et al. used SEIRAS to quantify phosphate speciation near Cu electrodes during acidic CO₂RR, correlating the intensities of H₂PO₄ ^– and HPO₄ ^2– bands with local pH under varying current densities and cation concentrations. ATR-SEIRAS has also been used to track the HCO₃ ^–/CO₃ ^2– interconversion during alkaline CO₂RR on Cu, with isotopic substitution (¹³CO₂) enhancing spectral resolution. Using a complementary approach, Lu employed in situ SERS to monitor phosphate-based Raman signatures across a wide pH range in reactions including NO₃RR and OER. These studies illustrate the broad applicability of vibrational spectroscopy for buffered interfacial environments.

Nuclear magnetic resonance (NMR) offers a chemically specific, operando means to quantify carbonate equilibria. Schatz et al. used ¹³C NMR to follow the HCO₃ ^–/CO₃ ^2– equilibrium during CO₂RR, enabling estimation of local pH without relying on electrode-dependent properties. Although powerful, NMR typically requires isotopically enriched species and specialized instrumentation.

Open-circuit potential (OCP) decay transients have recently emerged as a rapid and non-invasive method to quantify interfacial pH under high current density conditions. Sauvé et al. used the reversible H₂/H⁺ couple on Pt gas-diffusion electrodes (GDEs) to determine surface pH immediately after electrolysis. The method leverages the fast (∼20 ms) capacitive discharge following galvanostatic operation, during which the electrode potential reflects interfacial pH.

As shown in Figure a, in strong electrolytes such as 1 M H₂SO₄ and NaOH, the OCP returns to the bulk reversible hydrogen electrode (RHE) potential within 20 ms. In contrast, in 0.1 M sodium phosphate buffer (NaP_i), the potential remains approximately 0.30 V negative, corresponding to more than five pH units of basification. Increasing the cathodic current density from −10 to −100 mA cm^–2 in 1 M NaP_i (Figure b) proportionally increases both the duration of the transient and the magnitude of ΔpH, confirming that the proton consumption rates govern the extent of the pH shift. Raising the phosphate concentration from 0.1 to 1.0 M at −30 mA cm^–2 (Figure c) reduces the pH swing from about five to less than two units, illustrating the central role of buffering capacity.

2.

Determination of interfacial pH swings in various buffered and unbuffered electrolytes. (a–c) OCP decay transients recorded immediately after galvanostatic polarization; (a) Measurements at 100 mA cm^–2 in stirred solution (700 rpm) of 1 M H₂SO₄, 1 M NaOH, and 0.1 M NaP _i . (b) Effect of current density (−10 to −100 mA cm^–2) in 1.0 M NaP _i . (c) Influence of buffer strength (0.1 to 1.0 M NaP _i ) at −30 mA cm^–2. (d) Calculated ΔpH from post-electrolysis OCP decays. Top traces (↓) correspond to ΔpH at t = 0, bottom traces (↑) to ΔpH at t = 20 ms. Dotted lines serve as visual guides. Figure adapted from ref with permission from Elsevier.

Figure d compares ΔpH measured at 0 ms (the surface pH at the moment electrolysis is stopped) and at 20 ms, when relaxation has begun through diffusion and buffering. This comparison highlights how current density and buffer strength jointly determine both the magnitude of the interfacial pH perturbation and its early-stage recovery. In this study, Sauvé et al. validated the OCP-decay approach and showed that even electrodes in strongly buffered or acidic media can experience interfacial pH swings of 12–16 units under modest currents. These extreme shifts underscore the importance of mass-transport limitations in practical electrocatalysis. The method is particularly well suited to Pt GDEs and conditions relevant to commercial electrolyzers. However, it is important to note that in the system studied by Sauvé et al. the presence of Pt in combination with evolved hydrogen and protons in solution effectively establishes a local RHE. This situation is largely specific to Pt (and a limited class of related materials) and should therefore not be generalized when interpreting local pH or interfacial potential effects in other electrocatalytic systems.

Fluorescence pH sensor foils, as shown by Obata et al. provide a complementary route to spatially resolved pH visualization. Their study enabled the first direct observation of local pH changes during water splitting in near-neutral electrolytes within a complete two-electrode cell. In unbuffered 0.5 M K₂SO₄ at 1 mA cm^–2, the pH shifted beyond the detection range of the sensor. In contrast, in 0.1 M KPi, pH changes remained confined within 1 mm of the electrodes and stabilized after approximately three minutes. Natural convection driven by buoyancy effects, validated through coupled electrochemical and computational fluid dynamics (CFD) simulations, replenished buffer species and mitigated pH gradients even at low current densities (< 2 mA cm^–2). These findings highlight the important interplay between electrolyte composition, transport phenomena, and pH regulation in electrochemical cells.

Recent work is rapidly extending operando local-pH diagnostics into regimes where gradients are fastest and most heterogeneous, including gas-diffusion electrodes and membrane-based devices. For example, electrochemiluminescence reporters positioned near OER catalysts have enabled qualitative, real-time tracking of local pH gradients with sub-second time resolution, providing direct evidence for proton accumulation and degradation pathways under high-rate operation.

In parallel, in situ measurements combining a pH ultramicroelectrode with scanning electrochemical microscopy have been implemented in PEM electrolyzer configurations to monitor local pH in device-relevant environments and quantify how catalyst-layer modifications regulate microenvironment acidity. Complementary optical approaches, including fluorescence mapping and Raman detection using vibrational buffer reporters, continue to provide spatially resolved views of evolving pH fields during CO₂RR (including in acidic media) and related reactions, emphasizing that local pH must be measured directly before being invoked to rationalize mechanism or selectivity trends.

While the case studies above illustrate how different probes capture local pH (often: near-surface pH) under distinct operating regimes, selecting an appropriate method in practice requires a side-by-side comparison of their key figures of merit. In particular, spatial and temporal resolution, the accessible current-density window, and pH sensitivity jointly determine whether a technique can resolve fast, localized pH excursions versus slower, averaged gradients. Table compiles representative performance metrics across the most widely used approaches.

2. Quantitative Performance Metrics of Representative Techniques for Local pH Measurement in Electrochemical Systems .

technique	spatial resolution	temporal resolution	current density range	sensitivity (E/pH)
Potentiometric sensors	xy: 1–10 μm (microelectrodes)	ms to s	Low to moderate (<100 mA cm–2)	High (sub-millivolt)
	z: 10–1000 μm (point probe at user-defined distance from surface)
Voltammetric probes	xy: 1–10 μm (microelectrodes)	ms to s	Low to moderate (<100 mA cm–2)	High (sub-millivolt)
	z: 1–100 μm (point probe at user-defined distance from surface)
SECM	xy: ∼tip radius (0.1–25 μm; <1 μm with nano-SECM)	ms to s	Low to moderate (<100 mA cm–2)	Moderate (probe-dependent)
	z: 10–100 nm (controlled tip-surface gap)
Fluorescence microscopy	1–5 μm	ms to s	Low (typically <10 mA cm–2)	Moderate (fluorophore-dependent)
	xy: 0.3–1 μm (diffraction-limited; setup-dependent)
	z: 1–2 μm optical section (confocal CLSM)
ATR-SEIRAS	1–2 μm	s to min	Moderate (typically <100 mA cm–2)	Moderate (depends on vibrational bands)
	xy: 10–100 of μm (1–2 μm IR microscopy/synchrotron)
	z: near-surface enhanced region <10 nm
SERS	xy: 1–2 μm	ms to s	Low-Moderate (typically <100 mA cm–2)	High (single-molecule sensitivity)
	z: 2–10 nm (near-field hotspot region)
OCP-decay analysis	N/A	ms	High (>100 mA cm–2)	High (sub-millivolt)

^aApproximate spatial and temporal resolution, applicable current density ranges, and pH sensitivity are summarized. Values are indicative only and depend on probe design, experimental configuration, and operating conditions.

Beyond resolution and sensitivity, local-pH measurements are constrained by how the probe couples to transport and geometry. Techniques that perform robustly on planar electrodes can become ambiguous in porous or membrane-coupled systems, where the relevant pH field is spatially heterogeneous and strongly time-dependent. To clarify these practical boundaries, Table maps the suitability of common approaches to high current densities, GDE/MEA, and 3D electrode architectures.

3. Applicability of Local pH Measurement Techniques to High-Current-Density Operation and Device-Relevant Electrode Architectures .

technique	high current densities (>100 mA cm–2)	GDE/MEA configurations	3D porous electrodes
OCP-decay analysis	Well suited for high current densities; enables rapid, time-resolved assessment of interfacial pH	Readily applicable to GDEs/MEAs; non-invasive and compatible with realistic device operation	Applicable to porous electrodes; enables real-time pH monitoring
SECM	Generally effective up to ∼100 mA cm–2; potential challenges above this range	Can be applied to GDEs in selected configurations but limited by tip-electrode distance control and gas evolution	Challenging for complex 3D architectures; emerging multi-tip and scanning strategies may improve accessibility
Fluorescence imaging	Typically limited to low-to-moderate current densities (up to 100 mA cm–2)	Applicable to thin or optically accessible MEAs; constrained by optical transparency and probe stability	Strongly limited by light scattering, probe penetration, and heterogeneous pore geometry
ATR-SEIRAS	Generally limited to low-to-moderate current densities	Primarily tested on planar or thin-film electrodes; difficult to implement in GDEs due to IR penetration	Not well suited for thick, porous electrodes due to scattering and limited light penetration
SERS	Suitable at low-to-moderate current densities; signal stability can degrade at high densities	Primarily limited to planar model electrodes due to optical access and signal reproducibility	Limited by light penetration and scattering in 3D porous systems

^aSuitability and key practical limitations are summarized for high current densities (>100 mA cm^–2), gas-diffusion electrodes (GDEs) and membrane-electrode assemblies (MEAs), and three-dimensional porous electrodes.

The constraints highlighted in Table are not fundamental in all cases, but often reflect practical barriers in probe integration, optical access, and signal stability under gas evolution and strong transport gradients. Rapid methodological development is therefore aimed at extending local-pH diagnostics into the high-current, device-relevant regime. Table summarizes representative strategies being pursued to broaden the operational window of each technique for GDE/MEA configurations and complex porous electrodes.

4. Emerging Strategies for Extending Local pH Measurement Techniques to High-Current-Density and Device-Relevant Electrochemical Architectures.

technique	strategies for high current densities	strategies for GDE/MEA configurations	strategies for 3D porous electrodes
OCP-decay analysis	High temporal resolution for accurate pH shifts under high currents	Integration into high-throughput and automated electrolysis platforms	Direct implementation in large-area GDEs with real-time interfacial pH tracking
SECM	Multi-tip and array-based SECM for faster acquisition and improved spatial coverage	Integration with microfluidic cells to enable controlled flow and parallel measurements	Microelectrode arrays and tailored probe geometries to improve access to recessed regions
Fluorescence imaging	Use of near-infrared dyes and ratiometric or dual-dye pH/pOH imaging for improved robustness	Fiber-optic or wave-guide-based illumination and detection integrated into MEAs	Transparent or semi-transparent electrodes and higher-sensitivity fluorophores to mitigate scattering
ATR-SEIRAS	Thin-film or IR-transparent substrates to maintain signal under elevated current densities	Microstructured or patterned electrodes to enhance evanescent-field coupling	Nanoparticle-enhanced or structured substrates to improve signal collection from confined pores
SERS	Nanostructured plasmonic substrates to preserve enhancement at high currents; lateral SERS (L-SERS) to decouple optics from reaction zones	Advanced optical geometries with reduced scattering and improved collection efficiency	Fiber optic or embedded plasmonic probes to access buried catalytic regions
Voltammetric probes	Optimization of probe chemistry and conditioning for stability under high current densities	Integration of microelectrode probes into GDE/MEA test cells for continuous pH monitoring	Flexible, miniaturized probes or probe arrays to navigate complex porous architectures

In summary, available methodologies for probing local pH each have characteristic strengths and limitations. Table summarizes the main constraints and practical implications of the most widely used techniques.

5. Limitations and Practical Implications of Local-pH Measurement Techniques at Electrode Interfaces.

technique	main limitation(s)	practical implications
Potentiometric sensors	Signal drift over time; frequent calibration	Suitable for point measurements, but require regular maintenance and recalibration
Voltammetric probes	Accuracy depends on consistent electrode conditioning	Sensitive to surface fouling or roughness; may require frequent cleaning
Optical methods (fluorescent/absorbance)	Signal convolution, photobleaching, spectral overlap	High spatial resolution, but require careful probe selection and calibration
Vibrational spectroscopy (IR/Raman/SEIRAS/SERS)	Area-averaged signals; requires transparent substrates or vibrationally active species	Excellent for mechanistic studies on compatible electrodes; less suited for heterogeneous surfaces
NMR (including MRI or microcoils)	Poor spatial resolution; often requires isotopic labelling	Highly precise for bulk speciation; challenging for interfacial gradients
OCP-decay analysis	Assumes rapid proton equilibration; slow re-equilibration can underestimate pH swings	Fast and non-invasive for interfacial environments; requires validation of kinetic assumptions

Computational Approaches

Atomistic Modeling

There is still no consensus among theoreticians on the most suitable computational framework to describe local pH and the associated solvent and electrolyte effects. As in most computational studies, a balance must be struck between accuracy and computational cost, and the selected approach often varies across research groups and depends on the objectives of the study and the anticipated influence of local pH and ionic species.

A common starting point are static DFT calculations with an implicit solvent. Some studies augment this with a small number of explicit solvent or electrolyte molecules, an approach commonly referred to as micro-solvation. − Other efforts construct fully explicit interfacial models, for example static water layers with embedded electrolyte species, to represent local electric fields and specific ionic environments. These strategies differ in how they balance computational expense with the ability to capture directional solvent–adsorbate interactions and specific ion effects. Building on these static descriptions, ab initio molecular dynamics (AIMD) has been employed to treat thermal fluctuations and dynamic solvent structures more faithfully. −

Implicit solvent models offer computational efficiency but cannot accurately represent directional interactions such as hydrogen bonding between solvent and adsorbates. , Moreover, the dielectric properties and ionic distributions near the interface differ markedly from those in the bulk: cation concentrations may reach values 60–80 times higher than in the bulk solution, leading to substantial variations in double-layer capacitance and local electric fields. , These deviations challenge the straightforward use of bulk parameters to describe interfacial environments. Nevertheless, implicit solvation facilitates the implementation of grand-canonical DFT (GC-DFT) calculations at constant-potential, providing an attractive alternative to conventional constant-charge formalisms. −

The micro-solvation approach, either alone or combined with implicit solvation, provides a cost-effective route to introduce specific hydrogen bonds and local electrolyte motifs. It has been shown to reproduce experimental onset potentials with reasonable accuracy and can accommodate selected electrolyte species. , However, determining the optimal number and configuration of explicit molecules remains nontrivial and is often approached iteratively, and long-range interactions are still underrepresented. To account for cation effects, several studies have employed static water layers containing explicit electrolyte species and described their influence in terms of local electric fields. Other authors attribute these effects to direct electrostatic interactions between cations and adsorbates, , and cation-mediated reaction pathways have also been proposed, assigning cations an active catalytic role. Complementing these static pictures, AIMD has been used to describe adsorbate–solvent interactions with higher fidelity, , to model elementary reaction steps, , and to probe the structure and dynamics of the electrochemical double layer.

A major challenge remains in the disentanglement of pH, electrolyte, and double-layer effects, which are often interdependent. − Establishing causal relationships between local pH and specific cation or anion effects is particularly difficult. , Considerable attention has been devoted to non-Nernstian behavior in electrocatalysis, for example the pH dependence of features in the hydrogen region of Pt electrodes during cyclic voltammetry. These studies indicate the importance of cation and hydroxide co-adsorption and competition with reaction intermediates for surface sites. − More recently, the distinct influence of Na⁺, K⁺, and Mg²⁺ ions on the adsorption energies of C₁ intermediates in CO₂RR has been illustrated. The slopes and offsets of linear adsorption-energy scaling relations among C₁ species vary with both metal and cation identity, reflecting specific cation–adsorbate interactions. Such correlations enable quantitative predictions through multivariate regression models that incorporate system-dependent descriptors. Establishing these relationships is essential for the integration of local pH and electrolyte effects into high-throughput catalyst screening and materials design. Several studies have highlighted the importance of active sites with tailored coordination and local environment to achieve targeted selectivity, , while others have advocated for design descriptors that extend beyond simple adsorption energies. −

An additional limitation arises from the restricted accuracy of standard exchange-correlation functionals, particularly for molecules and charged species. For static DFT calculations, semiempirical corrections and thermodynamic cycles have been employed to mitigate these errors or avoid the explicit calculation of charged species. − In contrast, for AIMD these errors persist throughout the trajectories and may only be resolved through the use of improved functionals and/or (pseudo)­potentials.

Finally, not all double-layer models require DFT input. Continuum models that incorporate local pH, ionic effects, and mass-transport considerations can yield valuable insights even without first-principles data. Moreover, static DFT and molecular dynamics simulations, whether ab initio or based on machine-learned interatomic potentials, can be integrated within multiscale frameworks that connect thermodynamic, kinetic, and transport phenomena across multiple length and time scales, thereby providing a more comprehensive description of the double layer and interfacial electrocatalytic phenomena. − The following subsection discusses this type of simulations in detail.

Multiphysics Simulations

Describing the behavior of chemical and catalytic systems at the atomic scale using ab initio calculations can benefit substantially from complementary approaches that map atomistic predictions onto measurable observables for model validation and parametrization. Metrics such as open-circuit-potential decay, overpotential transients, product selectivity, current density, and local pH shifts depend sensitively on micro- to macroscale properties, including geometry, characteristic length and time scales, intrinsic reaction rates, fluid-flow regime, and the distribution of electrostatic potential in both electrolyte and solid phases.

A practical workflow involves two main steps. First, microkinetic modeling connects the atomistic level to the microscale by translating DFT-derived energetics into intrinsic rate constants using transition-state theory, including surface coverage and activity effects. Second, multiphysics simulations couple these reaction kinetics with mass, charge, momentum, and heat transport to predict device-level behavior. This second step is particularly powerful for extracting observables in complex settings, such as electrochemical reactions in porous electrodes with intricate geometries, where mass-transport, interfacial kinetics, acid–base equilibria, double-layer and capacitive effects, and electric fields jointly govern interfacial properties, including local pH at electrode–electrolyte interface.

A representative example of such a multiscale framework was presented by Fornaciari et al., who focused on oxidative catalysis and combined DFT calculations, microkinetic modeling, and transport analysis to map OER activity across acidic, neutral, and alkaline conditions. Their results revealed that at low current densities, near-neutral pH can outperform both acidic and alkaline regimes due to a dual-pathway mechanism, whereas at higher current densities (>20 mA cm^–2) the alkaline route dominates. Their framework also showed that a bulk pH of 9 can correspond to a local surface pH of 2.4, emphasizing the strong impact of transport limitations on apparent catalytic behavior.

A similar multiscale framework was developed by Heenen et al. to showcase the power of coupling DFT simulations, microkinetic models, and mass-transport in CO₂RR, specifically to rationalize selectivity toward acetate compared to other C₂ products under varying conditions (bulk pH, potential, surface roughness, etc.). The authors concluded that local pH gradients are driven not only by the applied potential, which controls the flux of OH^– generated at the cathode, but also by catalyst roughness. Increased roughness leads to more pronounced local pH variations because the larger surface area enhances ketene adsorption and reduces its reaction with OH^–, thereby diminishing acetate selectivity.

Other investigations based solely on multiphysics simulations have also elucidated the presence of local pH changes and their impact on different electrochemical processes. For example, Wosiak et al. applied finite-element modeling to water electrolysis under pulsed currents and showed that, even in an unbuffered electrolyte, interfacial pH swings can exceed nine units, significantly increasing overpotentials. Pulsed operation delayed the onset of these shifts but did not eliminate them. As shown in Figure a, during galvanostatic electrolysis at 1 mA cm^–2, the interfacial pH initially drifts slowly from the bulk value owing to the buffering capacity of water. Once a critical charge is passed and the dominant ionic species are depleted, the interfacial pH abruptly shifts by nearly nine units (from acidic to alkaline at the cathode and vice versa at the anode). Bulk pH values closer to neutrality accelerate this transition, whereas more extreme values delay it. The concomitant rise in cell potential reflects the abrupt change in interfacial composition. Comparing continuous and pulsed operation (10 Hz, 10% duty cycle) at the anode (Figure b), pulsing delays the pH jump by nearly threefold with respect to charge passed. The off-periods allow partial relaxation of OH^– build-up, which slows extreme increases in interfacial pH and reduces the voltage penalty until additional charge is delivered. This study underscored the importance of coupling a tertiary current distribution, Butler–Volmer kinetics, and mass transport for an accurate description of interfacial phenomena.

3.

(a) Comparison between near-surface pH at the anode (pH_a) under alkaline conditions and at the cathode (pH_c) under acidic conditions at different bulk pH values (10, 11, 12, 13 and 1, 2, 3, 5, respectively) during water electrolysis at 1 mA cm^–2. (b) Interfacial pH at the anode under continuous and pulsed operation (10 Hz, 10% duty cycle) at the same current density. Figure adapted from ref with permission from Elsevier.

Subsequently, Nagita et al. developed a steady-state one-dimensional (1D) finite-element model for the HER in porous electrodes. By including water self-ionization, they showed that even when the bulk pH is 1, the pore-scale pH can rise above neutrality. Their analysis identified pore size and catalyst-layer thickness as key parameters governing proton and hydroxide fluxes, thereby influencing local potentials and guiding the design of GDEs for both water electrolysis and CO₂RR.

In the context of bioelectrochemical systems, Picioreanu et al. proposed a “multiscale” framework that combines macro-scale mass balances for the bulk liquid with microscale mass, charge and momentum balances for the biofilm on the electrode. By coupling Nernst–Planck transport, convective flow from CFD simulations, and microbial kinetics, they quantified pH gradients within electrogenic biofilms and across proton-exchange membranes. Their results showed that performance is closely linked to control of the anolyte pH through buffering capacity and, fundamentally, to proton transport to the cathode. Geometry and hydrodynamics shape local pH fields, which in turn govern mediator kinetics and current density.

At a more mechanistic level, Carneiro-Neto et al. employed 1D finite-element simulations to compare Volmer–Tafel and Volmer–Heyrovský mechanisms in acidic buffers. They found that, under Volmer–Heyrovský kinetics at pH 5, the near-surface electrolyte becomes locally alkaline despite the acidic bulk, explaining side reactions such as metal-hydroxide deposition during electrodeposition.

More recently, Veroneau et al. developed an algebraic model capable of predicting OER-induced pH gradients as a function of bulk pH, buffer capacity, and diffusion-layer thickness. Validated using an acid-stable PbO _x catalyst, the model indicated that even well-mixed, well-buffered systems can exhibit local pH deviations approaching one unit until currents exceed 1 A cm^–2, highlighting the persistent need for acid-tolerant OER materials. Finally, Wu et al. introduced a 1D finite-element framework for the HER to systematically assess the effects of bulk pH, buffer composition, temperature, and applied potential. They concluded that increasing bulk pH and buffer strength is far more effective than temperature control in limiting interfacial ΔpH to within one unit, providing practical design guidelines for stable operation under dynamic electrolysis conditions.

A closely related body of work in aqueous energy storage highlights how multiphysics frameworks can quantitatively resolve local pH gradients beyond electrocatalysis. Makivić et al. examined reversible proton insertion in TiO₂ in mild aqueous electrolytes and combined experiments with modeling/multiphysics simulations to show that strong pH gradients develop at the electrode interface during proton insertion and disinsertion. Importantly, the magnitude and sign of these gradients depend on the available proton donor/acceptor chemistry (e.g., water vs NH₄ ⁺), and the resulting local pH profiles provide a mechanistic basis for changes in electrode voltage in unbuffered versus buffered media. These results reinforce the broader message of this review: local proton activity is a spatially heterogeneous variable that must be measured and/or modeled explicitly when interpreting electrochemical performance, whether in electrocatalysis or in PCET/PICET-based energy-storage materials.

Collectively, these studies illustrate the evolution from fully resolved multiphysics simulations, which explicitly treat ionic transport, reaction kinetics, and electrostatics, to reduced analytical models that retain essential predictive power. This progression provides researchers with a robust framework for mitigating interfacial pH fluctuations and for optimizing both efficiency and stability in electrochemical energy systems. When parametrized and validated against operando measurements, such kinetic and transport models can be used not only to rationalize observed behavior, but also to predict how electrode architecture, electrolyte composition, and operating conditions influence local pH under realistic current densities. Accordingly, kinetic and transport models play a central role in translating local pH control from a qualitative concept into a predictive design tool when used in conjunction with experimental validation.

However, it is important to note that quantitative discrepancies between model predictions and experiments are most often attributed to differences in the probed length scale (inner double layer versus reaction–diffusion layer), uncertainties in buffer reaction kinetics and effective transport parameters, electrode porosity and tortuosity, and the influence of convection and gas evolution, which are commonly simplified or neglected in continuum descriptions. Nevertheless, across diverse systems and methodologies, both modeling and experiment lead to the conclusion that substantial, non-equilibrium local pH shifts are an intrinsic feature of high-rate electrochemical reactions.

Local pH Effects in Key Electrocatalytic Reactions

Water Electrolysis (HER/OER)

Influence of the Local pH in Water Electrolysis

Local pH at electrode–electrolyte interfaces plays a pivotal role in the catalytic performance, stability, and mechanistic pathways of both OER and HER. During the OER in acidic media, proton generation leads to local acidification, which affects both the formation and stability of the active catalyst phase. In alkaline media, local OH^– depletion slows the rate of the conventional adsorbate evolution mechanism (AEM). Whether the reaction transitions to a lattice-oxygen-mediated (LOM) pathway is governed primarily by M–O covalency and the energetics of oxygen-vacancy formation rather than on OH^– scarcity alone. In addition, the relative importance of LOM versus AEM depends on the interplay between chemical and electrochemical steps: chemical steps do not shift with applied potential, whereas electrochemical PCET steps do, following the computational hydrogen electrode framework. As a result, LOM-type chemical pathways tend to become more competitive at lower potentials and elevated temperatures, while electrochemical AEM steps dominate at higher potentials. When operative, the LOM pathway can accelerate OER kinetics but often compromises stability, because repetitive lattice-oxygen redox promotes irreversible oxygen release, vacancy accumulation, and transition-metal dissolution, progressively eroding the active surface and degrading long-term performance. These dependences have been examined theoretically and verified experimentally.

In contrast, the HER in acidic media consumes hydronium ions (H₃O⁺), while in alkaline media it proceeds via H₂O dissociation. In both cases, reaction turnover can lead to local alkalization near the cathode. These dynamic processes indicate that interfacial pH is not merely a background parameter, but a central determinant of reaction pathways, selectivity, and long-term durability.

Studies on cobalt oxide-based catalysts have correlated changes in Co oxidation state (Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺) and interfacial charge accumulation with variations in local pH (Figure ). Operando Co K-edge X-ray absorption spectroscopy and voltammetric analysis revealed that alkaline electrolytes facilitate low-potential Co redox transformations and surface reconstruction, which correlate with improved catalytic performance (η ≈ 376 mV at 20 A g^–1) and lower Tafel slopes (∼46 mV dec^–1). In neutral and acidic environments, poorer charge accumulation and slower Co oxidation result in higher overpotentials (η ≈ 500 mV in alkaline and 531 mV in acid) and steeper Tafel slopes (>64 mV dec^–1). Overall, local pH controls both the potential window and the durability of the active Co oxidation state. Alkaline environments enable low-potential interfacial charge build-up and reversible CoOOH/CoO _x surface reconstruction, whereas neutral and acidic conditions shift activation to more positive potentials, slow Co–O oxidation, and often destabilize high-valent Co–O species with ligand-hole or oxyl character, thereby increasing overpotentials and dissolution rates.

4.

(a) OER polarization curves collected by steady-state chronoamperometry at different constant applied potentials. Vertical dashed lines indicate the formal redox potentials V° = (V _pa + V _pc)/2, where V _pa and V _pc are the potentials of the anodic and cathodic redox peaks, respectively, for the Co³⁺/Co⁴⁺ couple in different electrolytes. (b) Stacked bar chart summarizing η₂₀, ΔV ₁ = V°(Co^III/IV) – 1.23 V and ΔV ₂ = η₂₀ – V°(Co^III/IV) in various media. (c) Tafel plots derived from the OER polarization curves. (d) ΔE _edge calculated by taking E _edge at the start of the first CV measurement as the initial state, observed during CV in different electrolytes. Figure adapted from ref with permission from Springer Nature.

A complementary perspective was provided by Veroneau et al., who developed a model to quantify local pH gradients during OER by incorporating bulk pH, buffer capacity, diffusion coefficients, and current density. Even under strong buffering conditions and high mass-transport (e.g., rotating disk electrodes at 2500 rpm), their model and experiments on an acid-stable PbO _x catalyst show that at current densities approaching 1 A cm^–2, the local pH at the anode can drop sharply, often by more than five units relative to the bulk. This local acidification arises from the accumulation of protons produced by the OER that cannot be rapidly neutralized or transported away. The emergence of current plateaus in cyclic voltammetry and shifts in RRDE ring currents further support the transition from hydroxide-mediated to water-mediated OER pathways. Such changes in the local environment influence not only reaction energetics but also catalyst degradation, as acid-intolerant surfaces may undergo dissolution or restructuring under these conditions.

Regarding HER, alkaline media traditionally exhibits sluggish kinetics compared to acidic systems, with rates 2 to 3 orders of magnitude lower at pH 13 than at pH 1. Several factors have been proposed to account for this slowdown, ranging from the pH dependence of hydrogen binding energies, to the higher kinetic barrier for water dissociation compared to hydronium reduction, which is associated with the formation and cleavage of H–OH bonds. Ledezma-Yanez et al. further correlated sluggish alkaline HER kinetics with the deviation of the applied potential from the potential of zero charge. This deviation implies a strong interfacial electrical field that increases the rigidity of interfacial water and makes its reorganization more difficult during charge transfer within the double layer. More recently, a combination of AIMD simulations and in situ SEIRAS showed that the limited kinetics of alkaline HER also arise from reduced connectivity of the H-bond network at the alkaline interface, which increases the barrier for hydrogen transfer. Consequently, the already slower HER kinetics in alkaline media are further challenged by local pH variations. For example, even moderate current densities of 30 mA cm^–2 can induce local pH increases of more than seven units at Pt GDEs for some systems. Such local alkalinization represents a significant challenge in designing efficient and durable alkaline HER electrocatalysts.

Strategies to Control and Tune the Local pH

A major advantage of alkaline water splitting over its acidic counterpart is that OER catalysts can be based on earth-abundant transition metals rather than noble metals. However, the sluggish HER kinetics in alkaline media increase the overall overpotential, limiting the practical attractiveness of alkaline electrolysis. In this context, strategies that induce local acidification at the cathode–electrolyte interface can significantly improve HER kinetics.

One such strategy is nanostructuring of the catalyst. Recent work has shown that nanostructured Pt-based catalysts can exhibit anomalously high HER activity in highly alkaline electrolytes (1–2 M KOH), with Tafel slopes of ∼30 mV dec^–1 and apparent activation energies as low as ∼1.1 kJ mol^–1, values that resemble those observed for acidic HER. In situ Raman spectroscopy revealed strong signals corresponding to hydronium ions at the catalyst surface during operation, despite the negligible H₃O⁺ concentration in the bulk solution. These observations suggest that water dissociation and proton adsorption can locally generate a transient acid-like environment, forming H₃O⁺ intermediates within the electric double layer. This effect appears to be specific to nanostructured materials, where abundant surface defects and high local current densities facilitate water splitting and stabilize the hydronium-rich layer. The presence of H₃O⁺ near the surface shifts the rate-determining step from the Volmer (water dissociation) step to the Heyrovsky or Tafel steps, enabling faster proton–electron transfer. Moreover, electrochemical stripping and cyclic voltammetry show that increasing alkalinity decreases the binding strength of *H intermediates on nanostructures, further enhancing turnover. These findings indicate that nanoscale design not only modifies surface energetics but also enables dynamic modulation of local chemical environments that can override bulk thermodynamic constraints.

More recently, Tan et al. proposed an alternative approach to induce acidic local environments in alkaline HER by engineering the catalyst support and local reaction environment. Using Pt nanoparticles supported on MgO, they observed by operando Raman, synchrotron infrared, and X-ray absorption spectroscopy that H₃O⁺ species accumulate at the surface and that Pt nanoparticles carry a net negative charge, thereby generating acid–like environments around the Pt sites. This behavior was attributed to the presence of oxygen vacancies in MgO, which promote H₂O dissociation, as well as to charge transfer from Mg to Pt. With this catalyst, a current density of 10 mA cm^–2 was achieved at an overpotential of 39 mV, comparable to that in acidic media. The effect of the support on Pt nanoparticles, the resulting change in activity, and a schematic of the reaction mechanism are illustrated in Figure .

5.

(a) Schematics of electron transfer between Pt nanoparticles and two supports (oxygen-deficient MgO vs graphite). (b) HER polarization (LSV) curves comparing Pt supported on oxygen-deficient MgO (Pt/MgO) vs carbon-supported Pt (Pt/C) and bare MgO in 1 M KOH (with Pt/C in acid shown for reference), illustrating how support choice modifies the catalytic activity. (c) Schematic representation of the reaction mechanism of hydrogen evolution in Pt/MgO. Figure adapted from ref with permission from Springer Nature.

A related strategy was reported by Zhao et al., who synthesized an amorphous NiMoB catalyst with an oxygen–vacancy–rich structure that also promotes H₂O dissociation. In addition, Mo incorporation weakens the binding strength of hydrogen intermediates. The resulting higher surface concentration of hydronium ions was confirmed in situ by Raman spectroscopy. The catalyst delivered overpotentials of 38 and 48 mV at 10 mA cm^–2 in alkaline media, validating its effectiveness as an alkaline HER catalyst.

Together, these studies indicate that carefully designed catalyst nanostructures and supports can paradoxically induce local acidification during alkaline HER, thereby enhancing activity and reducing the overall overpotential required for alkaline water splitting. They also highlight that local pH control at the nanoscale can be as important as, and in some cases more decisive than, the bulk electrolyte pH in determining practical device performance.

CO₂ Electrolysis

Influence of the Local pH in CO₂ Electrolysis

The local pH near the electrode surface also plays a crucial role in determining the activity and selectivity of the electrochemical CO₂RR. Its influence arises from a complex interplay between reaction kinetics, mass-transport, adsorption equilibria, and electrolyte composition. Recent advances combining operando techniques, simulations, and catalyst microengineering have shown how local pH governs the near-surface chemical environment of electrodes, thereby modifying reaction pathways and product distributions.

During CO₂RR, local pH can deviate markedly from the bulk because net OH^– is generated at high interfacial flux (from CO₂RR and/or water reduction) while proton donors are depleted in the near-surface region. This shift changes (i) CO₂ availability via acid–base speciation and rapid (bi)­carbonate formation, and (ii) the kinetics and coverages of adsorbed intermediates, thereby modulating the competition between CO₂RR and HER. For instance, catalysts such as Au and Ag predominantly yield CO, and an increase in local pH tends to enhance CO₂ activation while suppressing HER due to reduced H⁺ availability. However, increasing local alkalinity does not necessarily translate into a monotonic CO₂ activation, because elevated alkalinity can reduce local dissolved CO₂ via fast carbonate formation. , Consistently, operando studies in bulk acidic media show that bulk pH is a poor descriptor for near-surface proton activity (local pH) under working currents, and must be evaluated together with transport and speciation constraints.

A seminal conceptual framework for electrolyte–pH coupling was introduced by Singh et al. who proposed that alkali cations can buffer near-surface alkalization through (effective) cation hydrolysis under electrochemical conditions. Larger, more weakly hydrated cations form more acidic aquo complexes (lower pK _a values) and buffer interfacial alkalization more effectively. This buffering maintains a higher local CO₂ concentration and thereby influences activity and selectivity. This mechanism was later validated by Ayemoba and Cuesta, who provided the first direct, in situ spectroscopic evidence that alkali metal cations buffer near-surface pH via size-dependent hydrolysis during CO₂RR on Au electrodes. By monitoring ATR-SEIRAS bands of CO₂ and HCO^3– during slow linear-sweep voltammetry, they showed that the local pH at the electrode–electrolyte interface increases following the cation trend: Li⁺ > Na⁺ > K⁺ > Cs⁺. Crucially, they demonstrated that earlier models overestimated cation-induced pKa shifts, particularly for larger cations, due to misestimated interfacial charge densities, and emphasized that electrolyte purity can qualitatively affect observed cation trends by perturbing interfacial chemistry.

Direct quantification approaches further strengthened the link between cations and near-surface pH. Zhang and Co used RRDE techniques to directly quantify local pH during CO₂RR on Au, revealing increases of up to 3 pH units, modulated by the nature of the alkali cation. The observed “local alkalinity” sequence Li⁺ > Na⁺ > K⁺ > Cs⁺ correlates with cation hydration energy and buffering capacity, and thus with how effectively each cation maintains a favorable microenvironment for CO₂RR. Together, these studies consolidate the picture that alkali cations regulate interfacial pH and thereby CO₂RR/HER competition, while also underscoring that cations can influence reactivity through additional mechanisms (double-layer structure, electric fields, and intermediate stabilization).

In this context, Monteiro et al. broadened the picture by examining multivalent cations in acidic media (pH ≈ 3), identifying two mechanistic regimes as a function of overpotential. Explicit-field AIMD simulations show that less acidic, weakly hydrated cations accumulate more strongly at the outer Helmholtz plane and stabilize *CO₂ ^–, providing a promotion pathway distinct from, yet complementary to, simple pH buffering. −

On Cu-based catalysts, which are known for forming C₂₊ products, local pH modulates not only HER suppression but also key steps such as C–C coupling. Schatz et al. employed chemical shift-resolved MRI to map spatial pH gradients and observed steeper gradients in NaHCO₃ than in KHCO₃, correlating with higher acetate selectivity (Figure ). These correlations are consistent with mechanistic analyses that assign acetate formation to solution-phase ketene hydrolysis by OH^–, highlighting that not only the absolute pH but also the spatial distribution of alkalinity can bias product branching.

6.

Product analysis using ex situ ¹H NMR with water suppression. (a) Concentrations of liquid products detected in 0.1, 0.5, and 1 M NaHCO₃ and KHCO₃ after in operando experiments at ca. −1.4 V vs RHE, as well as concentrations of impurities in pristine electrolytes. (b) Faradaic efficiencies of CO₂RR products in the same electrolytes. (c) Schematic of the solution-based acetate formation mechanism proposed by Heenen et al. Figure adapted from ref with permission from American Chemical Society.

Figure a shows the liquid-phase product concentrations, while Figure b reports the corresponding faradaic efficiencies. In all electrolytes, CO₂RR generates formate, acetate, dissolved methane, and acetaldehyde, whereas alcohols (ethanol ≫ methanol) are formed only with K⁺-containing electrolytes. Increasing the bicarbonate concentration increases absolute yields. The authors proposed that cation identity dictates selectivity: Na⁺ amplifies acetate production (up to about 15% FE at 1 M) while suppressing alcohols, consistent with a pH-driven mechanism in which ketene is converted to acetate in solution; K⁺ maintains high formate yield and introduces ethanol (up to about 6% FE) but suppresses acetate. The least buffered 0.1 M electrolytes favor formate formation, consistent with facile 2e^– reduction under strongly alkaline interfacial conditions. This “microenvironment-to-selectivity” link is consistent with the acetate mechanism proposed by Heenen et al. (Figure c), where ketene formed at the surface can undergo solution-phase reaction with OH^–; therefore, sharper local alkalinity gradients and OH^– retention can increase the relative contribution of acetate formation via solution chemistry rather than purely surface-bound pathways. Overall, Figure indicates that electrolyte identity and buffer capacity provide a powerful lever, in addition to catalyst design, for steering CO₂RR toward desired liquid products. This behavior was linked to cation hydrolysis, where Na⁺-induced local buffering is less effective than that of K⁺, leading to sharper pH increases that favor solution-phase reactions such as ketene hydrolysis to acetate.

Interfacial acid–base chemistry can also differ qualitatively from bulk expectations. Martínez-Hincapié et al. showed by in situ FTIR and voltammetry that even at low bulk pH (∼1), adsorbed carbonate and bicarbonate can exist on Pt(111) electrodes, governed by surface acid–base equilibria that differ from bulk behavior. The electrode potential modulated the deprotonation of adsorbed bicarbonate, illustrating how surface charge density influences pK _a shifts and, consequently, interfacial speciation.

Similarly, Welch et al. systematically examined how local pH evolves within GDEs during CO₂RR, revealing strong links between electrode microstructure, local environment, and catalytic activity. Using confocal fluorescence microscopy with a ratiometric pH-sensitive dye (DHPDS), they mapped pH distributions in three dimensions with micrometer resolution under operating conditions.

Their key finding is that local pH rises markedly with current density, from pH ∼6.8 at open circuit to above pH 10 at −28 mA cm^–2 in a mildly buffered 100 mM KHCO₃ electrolyte. Notably, this alkalinization is not spatially uniform. Figure shows how pH changes as a function of current density and position relative to the Cu GDE surface. At zero current, pH is homogeneous; as current density increases, gradients develop both across the electrode and within microporous trenches. At low currents (−1.6 to −3.4 mA cm^–2), pH maps exhibit heterogeneous alkaline “hotspots” that coincide with highly active regions. At higher currents, the surface becomes uniformly alkaline. Gradients are especially pronounced in the microporous structure, where confinement enhances OH^– retention. Narrow trenches exhibit consistently higher pH than wider ones, a trend that correlates with higher selectivity toward multicarbon (C₂₊) products and suppression of HER. These results emphasized that local confined hotspots in GDEs, rather than the spatially averaged pH, can dominate the electrode reactivity, making microstructure a key design parameter for microenvironment control.

7.

Operando mapping of solution pH in three dimensions over a Cu GDE. Maps are recorded at the same lateral position but at different heights above and below the electrode surface, and at different current densities. From top to bottom, rows correspond to 27 μm above, 0 μm (at the surface), 15 μm below, and 30 μm below the electrocatalyst surface. From left to right, columns correspond to 0 mA cm^–2 (no reaction under OCP), −1.6, −3.4, −14.7, and −28.0 mA cm^–2. The pH color scale and scale bar apply to all images. Figure adapted from ref with permission from American Chemical Society.

Recently, elevated-temperature operation has been analyzed as an additional control knob that can reshape local pH gradients and thereby shift apparent selectivity trends. Brandão et al. used confocal Raman spectroscopy to extract near-surface pH profiles near Cu catalysts and found that higher temperature produces steeper pH gradients (larger surface–bulk differences over shorter boundary layers). At −0.6 V vs RHE, the surface–bulk pH difference at 75 °C was 1.8 pH units higher than at 25 °C due to temperature alone, suggesting that “warm” electrolyzers may operate under more alkaline microenvironments than room-temperature tests would imply. They further show that temperature can move the interface across a microenvironment window: moderate heating can align with improved multicarbon formation, while at higher temperatures competing hydrogenation pathways become more competitive and can reduce C–C coupling.

Strategies to Control and Tune the Local pH

In CO₂RR, operating in acidic bulk media can reduce bulk carbonate formation and thereby improve single-pass carbon efficiency, which is a long-standing limitation of neutral and alkaline systems where CO₂ rapidly equilibrates with OH^– to form (bi)­carbonates. Importantly, at high current density, local OH^– generation can still drive near-surface CO₂ neutralization even when the bulk is acidic, so “acidic bulk” does not automatically imply carbonate-free interfacial chemistry. However, acidic conditions inherently favor HER, which strongly competes with CO₂RR and other reduction processes at negative potentials. A central direction in CO₂RR research is therefore the design of interfaces that decouple bulk electrolyte composition from the local catalytic environment, by controlling interfacial pH (proton activity) and ion distribution.

Key strategies include the use of ion-conducting polymer overlayers to modulate proton and hydroxide transport, nanostructured or hydrophobic architectures to confine reaction zones and restrict proton penetration, membrane or interlayer designs that enrich certain cations, and local cation-engineering to tune buffering capacity. Effective interfacial engineering must address both ionic selectivity (e.g., retaining OH^–, limiting H⁺ access, enriching alkali cations) and structural confinement (e.g., preventing OH^– diffusion into the bulk, where carbonate would reform), because either lever alone is usually insufficient at ≥100 mA cm^–2. That is, high-rate OH^– generation and steep transport gradients demand simultaneous control of ion transport/selectivity and electrode architecture (porosity, wettability, catalyst-layer design) to avoid CO₂ depletion, carbonate loss, and stability penalties. The interplay of these chemical and structural elements determines the ability to sustain a favorable micro-pH without compromising stability or scalability. Mechanistic and operando studies consistently indicate that either lever in isolation can be insufficient to stabilize a target micro-pH under industrially relevant currents.

At the device scale, gas-diffusion architectures remain a central platform to impose sharp interface gradients and restrict proton access. By tailoring catalyst-layer thickness, hydrophobic barriers, and ion-transport pathways, such systems suppress HER and enhance CO₂ reduction kinetics, particularly favoring multicarbon products such as ethylene.

For example, the multilayer GDE concept of Dinh et al., illustrates how an abrupt interface design can promote C–C coupling under strongly alkaline operation by sustaining a highly alkaline near-surface environment and high CO availability. Their multilayer GDE comprised a graphite support, a carbon nanoparticle layer, a thin Cu catalyst, and a hydrophobic PTFE membrane. Operating in 10 M KOH, this configuration sustained a highly alkaline local environment that accelerated CO dimerization, as rationalized by DFT calculations, while suppressing HER. These results showed that not only electrolyte composition but also spatial confinement of OH^– via catalyst–layer thickness is critical for maintaining reaction selectivity, underscoring the importance of local pH control in decoupling the reaction interface from limitations imposed by bulk CO₂ solubility and carbonate formation. Although this architecture fairly illustrates how microenvironment control in concentrated alkaline media can promote C–C coupling, carbon-efficiency losses associated with CO₂ neutralization to (bi)­carbonate remain a challenge in strongly basic operation, which has motivated recent approaches to keep the bulk electrolyte acidic while engineering a decoupled interfacial microenvironment.

Within this broader paradigm, it is increasingly clear that the microenvironment is shaped not only by the catalyst layer but also by the porous transport medium. The porous transport layer can therefore act as a mesoscale “microenvironment knob” that governs CO₂ delivery, OH^– removal/retention, and wetting. Dolmanan et al. showed that tuning GDL pore architecture (size) and associated transport constraints can measurably shift local pH and redirect product pathways. They reported that smaller pores lead to higher local pH due to hindered CO₂ diffusion, resulting in formate selectivity shifts even on Ag, a metal typically biased toward CO. Confocal microscopy confirmed that reduced porosity elevates the pH near the catalyst surface, supporting a switch in pathway via pH-controlled stabilization of different intermediates. Building on this mesoscale “microenvironment knob”, a complementary strategy is to impose confinement directly at the nanoscale, where diffusion lengths and ion fluxes can be engineered inside catalyst-adjacent voids.

In this context, structural confinement of OH^– at the nanoscale offers another route to engineer local alkalinity, while maintaining the bulk acidic. Li et al. introduced hollow Ag@C nanostructures capable of confining OH^– generated during CO₂RR. Ag nanoparticles line the inner walls of a thin porous carbon shell (≈40 nm thick, with pores of about 2.6 nm), so that the reaction takes place inside nanocavities that hinder H⁺ ingress and OH^– egress. This design is proposed to create a locally alkaline environment even in strongly acidic bulk electrolyte (0.5 M K₂SO₄ adjusted to pH ∼1.1). HER is effectively suppressed, and FE > 95% for CO are achieved at 300 mA cm^–2, comparable to that achieved in alkaline electrolytes, as shown in Figure . At low CO₂ flow (2 sccm), a single-pass carbon efficiency of 46.2% is obtained, approximately twice that of an alkaline control under similar conditions. Simulations support the hypothesis that diffusion-limited transport within the nanocavities generates a stable high-pH zone conducive to selective CO formation, reproducing restricted proton entry, limited hydroxide escape, and a low carbonate fraction (≤0.2 at 300 mA cm^–2). Control architectures (Ag/C or C@Ag) do not reach similar performance, highlighting the importance of diffusion-mediated confinement in creating a local alkaline environment without reopening the carbonate loss channel.

8.

Faradaic efficiency to CO of hollow structured Ag@C and conventional Ag/C catalysts at different current densities in (a) 1.0 M KOH (pH 13.7), and (b) 0.5 M K₂SO₄ (pH 1.1). Figure adapted from ref with permission from RSC.

Beyond conventional buffer design and electrode structuring, a complementary strategy is to engineer the interfacial ion environment in strongly acidic bulk electrolytes so that proton activity/flux at the catalyst is effectively lowered while CO₂RR intermediates are stabilized. Huang et al. reported that concentrating K⁺ in the vicinity of electrochemically active sites enables CO₂RR on Cu at pH < 1, achieving single-pass CO₂ utilization of 77% (with 50% conversion efficiency toward multicarbon products) at current densities exceeding 1 A cm^–2. In their reactor–catalyst design, the bulk is kept strongly acidic (1 M H₃PO₄, pH ≤ 1), while the interface is conditioned to form a proton-poor, K⁺-rich microenvironment that favors CO₂RR over HER. K⁺ cations accumulated near Cu sites, generate strong local electric fields, stabilize key intermediates, and lower the barriers for CO₂ activation and C–C coupling. To sustain this enrichment during electrolysis, the authors introduced a cation-augmenting layer (CAL), a perfluorosulfonic acid ionomer blended with carbon nanoparticles, which acts as a K⁺ reservoir.

A particularly clear example of a confinement-in-acid strategy was shown by Feng et al., who used confined La-doped Cu spheres with channel architectures to retain OH^– and enrich cations locally despite a strongly acidic bulk (reported pH ≈ 1). Using operando SERS signatures of (bi)­carbonate together with modeling, they inferred locally highly alkaline conditions (maximum local pH reported up to ≈13.7 at −700 mA cm^–2) inside the confined regions and linked this microenvironment to high C₂₊ performance at high rates (C₂₊ FE reported up to ≈86% at partial current densities on the order of 775 mA cm^–2, depending on conditions).

Recent results make clear that improved selectivity in acid does not always need to be framed purely as maximizing local alkalinity. A complementary strategy is to manipulate surface coverage so that HER is kinetically suppressed even when large pH shifts are not the central explanatory variable. For example, Yu et al. reported an acidic CO₂ electrolysis approach that achieves high formic acid selectivity (reported ≈91% FE at 1.2 A cm^–2, pH ≈ 1) and uses in situ ATR-FTIR to associate performance with increased coverage of key intermediates that suppresses the HER. This provides a useful balance point: microenvironment engineering can be expressed through proton-transport control and/or through surface-coverage control, and the dominant lever may differ by product target (e.g., C₂₊ vs formate) and reactor regime.

Furthermore, Zhao et al. engineered a hybrid interfacial layer composed of covalent organic framework (COF) nanoparticles and a perfluorosulfonic acid ionomer. This adlayer created nanochannels that restricted proton flux while maintaining cation enrichment, resulting in a locally alkaline microenvironment that favored C–C coupling on Cu. Multicarbon selectivities above 75% were obtained together with improved energy efficiency in acidic media.

Altogether, these studies establish local pH as a central parameter in regulating CO₂RR performance. It alters the relative thermodynamic and kinetic favorability of CO₂RR versus HER, governs the coverage and stabilization of key adsorbed intermediates (e.g., CO*, *CHO, *COH), and modulates solution equilibria that control CO₂ availability through (bi)­carbonate formation. Accordingly, precise microenvironment tuning through cation identity and accumulation, catalyst-layer architecture, wetting/confinement, and mass-transport engineering, provides a powerful route to steer activity and selectivity, and is increasingly viewed as essential for efficient, product-specific CO₂ electrolysis at technologically relevant rates. At the same time, broader syntheses emphasize that carbonate chemistry remains a persistent constraint linking local pH directly to carbon efficiency, motivating strategies that decouple interfacial conditions from bulk electrolyte composition while maintaining robust operation.

While many interfacial-engineering approaches are indeed structural (static) in nature, maintaining catalytic selectivity and stability at high current density ultimately requires a dynamic function: continuous refilling of local proton or hydroxide sources within the diffusion layer. Under these conditions, classical buffering becomes ineffective because replenishment is limited by transport rather than equilibrium capacity. To address these limitations, we propose the integration of dynamic replenishment systems capable of actively sustaining the local pH environment. Inspired by proton-relay mechanisms in molecular catalysis, such approaches could involve electrodes incorporating materials that undergo reversible protonation and deprotonation cycles, − or the use of advanced ionic-liquid-based electrolytes that retain high buffering capacity even under extreme operating conditions. , In parallel, microstructural modifications of gas diffusion layers, such as increased porosity or tailored ion-conducting pathways, could enhance the transport of protons or hydroxide ions to the electrode surface, thereby supporting continuous local pH regulation. By coupling these advanced dynamic buffering mechanisms with optimized electrode architectures and adequate operation conditions (e.g., high convection and potential pulsation), it may become possible to maintain stable local pH conditions at high current densities, sustaining electrocatalytic performance while mitigating degradation processes commonly associated with static buffering strategies.

Seawater Splitting

Influence of the Local pH in Seawater Splitting

Direct production of hydrogen from seawater electrolyzers, without intermediate desalination or purification, would greatly enhance the viability and scalability of water splitting technologies. However, the complex composition of seawater imposes harsh operating conditions that degrade electrodes and ultimately cause electrolyzer failure. A central challenge arises from strong local pH alterations at both electrodes: pH increases at the cathode and decreases at the anode, with deviations of up to 5–9 pH units relative to the bulk seawater pH ∼8. These large near-electrode pH fluctuations occur even at modest current densities in weakly buffered media and impact both cathodic scaling and anodic selectivity.

Because seawater contains significant concentrations of Ca²⁺ and Mg²⁺, the increase in local pH at the cathode promotes the formation of insulating hydroxide precipitates that block active sites and clog porous structures. At the anode, the OER competes with chloride oxidation. Although the equilibrium potential for OER (1.23 V vs RHE) is lower than that of chloride oxidation (1.36 V vs RHE), chloride oxidation is a two-electron process (2Cl^– → Cl₂ + 2e^–) involving essentially one key intermediate, whereas the OER is a four-electron process involving at least three adsorbed intermediates (*O, *OH, *OOH) that are constrained by scaling relations. As a result, chlorine evolution is often kinetically favored.

The pH range that maximizes the thermodynamic potential difference between OER and chloride oxidation (for a chlorine concentration of 0.5 M at 25 °C), as emphasized by Dionigi et al., lies above pH 7.5. That work introduced an “alkaline design criterion”, stating that catalysts able to sustain the target current density with η_OER ≤ 0.48 V in alkaline media are well suited to achieve nearly 100% OER selectivity over chlorine-based products. Figure illustrates this criterion, showing the maximum allowed OER overpotential as a function of pH and indicating the dominant chloride oxidation products.

9.

Maximum allowed OER overpotential for electrolyzer catalysts to ensure 100% selective water splitting. Values are obtained as the difference between the standard electrode potentials of the three relevant chloride oxidation reactions (chlorine, hypochlorous acid, and hypochlorite formation) and the OER as a function of pH. Figure adapted from ref with permission from Wiley.

A straightforward strategy to suppress chloride-derived products is bulk alkalization of seawater, for example by adding KOH, so that the electrolyte pH is increased and operation occurs at overpotentials below ∼0.48 V. Operating at pH 13 ensures that, even when interfacial pH decreases at the anode under load, the surface remains in a regime where OER is thermodynamically favored over chloride oxidation. However, such strong alkalinization exacerbates hydroxide precipitation of Ca²⁺ and Mg²⁺, which then must be removed, effectively reintroducing a water treatment step and adding the cost of the alkalizing agent.

Strategies to Control and Tune the Local pH

More recently, strategies have emerged to use natural seawater directly by controlling local pH at the electrode surface rather than modifying the bulk electrolyte. One such approach employs a Lewis acid layer deposited on top of a transition metal oxide electrode. In a representative study, a Lewis acid-modified anode was prepared by coating CoO _x with Cr₂O₃. Cr³⁺, as a hard Lewis acid, binds strongly to OH^– and can capture it at the electrode surface. Using IrO _x -modified pH-sensing electrodes, the authors observed that the surface pH at an uncoated anode was lower than the bulk seawater pH, whereas the Cr₂O₃ adlayer drove the local pH up, approaching pH 14 at 1.60 V vs RHE. As shown in Figure a, the resulting OH^– concentration at the coated anode exceeds the level required to repel chloride ions, thereby suppressing chloride oxidation.

10.

(a) Measured OH^– concentration and theoretical concentration of excess OH^– required to prevent Cl^– corrosion on Cr₂O₃-coated and uncoated CoO _x anodes in natural seawater. (b) OH^– concentration of bulk seawater adjacent to Cr₂O₃-coated and uncoated CoO _x cathodes at various H₂ production current densities after 30 min in natural seawater. (c) Schematic of charge repulsion during HER in seawater at a Pt/NiN₃ cathode covered with V₂O₃. (d) Schematic HER mechanism on Pt/WO₂ illustrating the formation of a local acid-like environment through hydrogen supply from H _x WO _y . Panels (a, b) were prepared using data from ref ; panel (c) was adapted from ref with permission of AAAS; and panel (d) was adapted from ref with permission of American Chemical Society.

At the cathode, Cr₂O₃ coatings also increase the local pH, which benefits HER kinetics under seawater conditions. In addition, the strong binding of OH^– to the adlayer reduces the availability of free OH^– to react with Ca²⁺ and Mg²⁺, mitigating hydroxide precipitation near the cathode. This effect is evident in the OH^– concentration difference between coated and uncoated CoO _x cathodes at various current densities (Figure b). Collectively, these features enable seawater electrolysis at industrially relevant current densities.

A similar concept was pursued by Hu et al., who deposited a V₂O₃ overlayer on a dual Pt–NiN₃ cathode catalyst. In this system, the adlayer reduced salt precipitation and mitigated chloride-induced corrosion through charge repulsion (Figure c). At the same time, V₂O₃ promoted better Pt dispersion, increasing the density of active sites. An anion-exchange membrane electrolyzer incorporating Pt–NiN₃@ V₂O₃/NF as the cathode and NiFe LDH/NF as the anode delivered stable operation for 500 h at 500 mA cm^–2.

Although controlling surface pH with a Lewis acid is a promising route for seawater electrolysis, there is an inherent trade-off, since overlayers can partially block the underlying active sites. To address this, Hong et al. developed a catalyst that generates local alkaline microenvironments without excessively covering the surface. They studied OER in neutral water using a CoO-based catalyst with Fe–O and Cr–O ligands, where the ligands act as atomic-level Lewis acid sites. The synergistic action of both ligands increases the OH^– concentration at the catalyst surface, as evidenced by IrO _x -based pH probes: with Cr alone, the surface pH reached ∼12, whereas with both Fe and Cr, it approached ∼14. According to their analysis, Fe increases the oxidation state of Cr³⁺ but prevents overoxidation to Cr⁶⁺, thereby avoiding Cr leaching. The presence of local alkaline microenvironments was confirmed by enhanced Raman and ATR-FTIR spectroscopies. Pairing this catalyst with commercial Pt/C yielded an electrolyzer capable of supplying 100 mA cm^–2 at 1.5 V for 500 h.

In a complementary direction, local pH regulation has also been used to generate acid-like microenvironments for cathodic HER. A notable example is the use of Pt/WO₂ as a cathodic electrocatalyst for hydrogen production from natural seawater. In this system, WO₂ is dynamically converted in situ to H _x WO _y , which can supply H⁺ to Pt active sites during HER (Figure d). This mechanism creates a local acidic microenvironment near the Pt surface that suppresses the rise in local pH, keeping it below 9.5 and thereby limiting hydroxide precipitation. However, chloride-induced corrosion remains an important challenge in this configuration.

Although several challenges remain before efficient, large-scale seawater electrolysis can be realized, local pH regulation has emerged as a promising strategy to achieve long-term stability and corrosion resistance. For cathodic electrocatalysts, both acidic and alkaline microenvironments have shown benefits: a local acidic environment limits hydroxide precipitation, while a local alkaline environment can suppress chloride-induced corrosion and reduce the formation of deposits by tightly binding OH^– near the surface. For the anode, catalysts that form locally alkaline environments while maintaining a high density of active sites are particularly attractive. At the same time, catalyst leaching under alkaline anodic conditions must be carefully controlled. Bulk alkalization alone is often insufficient, since the anode interfacial pH can still decline under load. Strategies that maintain adequate transport and buffering while preserving local alkaline advantages are therefore essential for achieving high OER selectivity and durability in seawater splitting.

Other Electrocatalytic Reactions

Local pH at the electrode–electrolyte interface exerts a decisive influence on activity, selectivity, and stability across a wide range of electrocatalytic reactions, even in electrolytes that are nominally well buffered. These effects arise from the coupling between PCET steps, adsorption equilibria, homogeneous acid–base chemistry, and reaction-driven pH shifts, all of which reshape the near-surface environment under operating conditions.

In formic acid oxidation on Pt, interfacial pH shifts reflect both acid–base equilibria and competitive anion adsorption. In weakly adsorbing electrolytes such as perchlorate or sulfate, the current increases with pH up to pH ∼5 and then plateaus to pH ∼10, tracking the rise in formate (HCOO^–) concentration. In contrast, phosphate- or chloride-containing media exhibit strong anion adsorption that blocks surface sites and can modify the reaction mechanism. These interactions flatten or even invert the pH-activity trend, producing bell-shaped current–pH profiles. This behavior originates from the ability of phosphate species, depending on their protonation state and coverage, to either stabilize key intermediates or suppress their adsorption. Consequently, local pH is shaped not only by proton–transfer dynamics but also by the protonation and adsorption equilibria of the anions themselves.

Local pH similarly governs the mechanistic landscape in nitrite reduction on Pt. In acidic bulk solutions (pH ≈ 1), fast homogeneous decomposition of HNO₂ generates NO, which couples with adsorbed *NO to form N₂O. This manifests as a characteristic wave at 0.3–0.5 V vs RHE. At high cathodic currents, proton consumption rapidly increases the interfacial pH above the pK_a of HNO₂, suppressing NO formation and shifting the mechanism toward the direct 4- or 6-electron reduction of NO₂ ^– to hydroxylamine (NH₂OH) or ammonia (NH₃) at more negative potentials. Once the local pH exceeds ≈10, adsorbed hydrogen becomes strongly inhibiting, explaining the sharp fall-off in activity under strongly alkaline interfacial conditions.

Building on these operando pH maps and Raman reporters, a recent BPM–MEA nitrate electrolysis study makes the “local pH/proton availability” knob very explicit: Huang et al. systematically varied the proton flux delivered to the cathode microenvironment by the bipolar membrane and showed that nitrate-to-ammonia selectivity is not simply a catalyst property, but a microenvironment outcome set by how quickly protons are supplied and consumed near the active sites. By tuning the near-catalyst proton availability via interposer layers, proton-scavenging carbonate electrolytes (CO₃ ^2–), and catalyst-layer configurations, they identified a moderate proton supply regime that strongly biases the network away from nitrite accumulation and toward NH₃, as illustrated in Figure . They report a 576% increase in NH₃ yield relative to a standard MEA configuration and a selectivity ratio of 26 (NH₃ over NO₂ ^–) at 200 mA cm^–2, directly linking macroscopic operating conditions to the interfacial acid–base landscape that governs branching in NO₃RR.

11.

(a) Simplified illustration of the pH gradient within the interposer layer (IPL) and its effect on proton inhibition in a bipolar membrane (BPM) based membrane electrode assembly (MEA) system. (b) Faradaic efficiency to NH₃ in 1 M KCl and 0.5 M K₂CO₃ electrolytes containing 70 × 10^–2 M KNO₃, measured with and without the IPL over a range of current densities. Figure adapted from ref with permission from Wiley.

Ethanol oxidation on Pt provides a complementary example. In phosphate buffers, the reaction favors a C₂ pathway because HPO₄ ^2– blocks the multiatom ensembles required for C–C scission. At bulk pH values above 11, however, solution-phase deprotonation to ethoxide renders the first electron transfer rate-limiting and increases the current by two orders of magnitude (Figure a). Under high-rate electrolysis, hydroxide is consumed rapidly at the anode; if the electrolyte lacks sufficient buffer capacity, the interfacial pH collapses, re-acidifying the surface and re-enabling *CO formation, which increases the overpotential. Gold avoids this limitation because its surface remains metallic until ≥ 0.8 V. On Au, the activity for primary alcohol oxidation scales linearly with the aqueous pK _a of the substrate: once the alkoxide forms in solution, the β-H/2e^– elimination step proceeds rapidly on Au–*OH ensembles that tolerate high current densities (Figure b). These findings highlight that electrolyzers for nitrite valorization or alcohol upgrading must pair appropriate catalysts with electrolytes that can sustain high buffer fluxes or implement forced convection and thin diffusion layers. Only then does the double-layer pH remain within the narrow window that supports the desired mechanistic channel while preventing mass-transport-induced performance decay.

12.

(a) Corrected maximum peak currents and currents at 1.2 V vs RHE from cyclic voltammograms for the oxidation of 0.5 M ethanol in 0.1 M phosphate buffers of pH 2–12. (b) Onset potential versus pK _a for various alcohols. Values in parentheses correspond to pK _a and oxidation overpotential for each substrate. Adapted from ref with permission of American Chemical Society.

Voltammetric pH probes, including syringaldazine-functionalized Au UMEs, have been used to map interfacial pH gradients during the oxygen reduction reaction (ORR). These measurements reveal correlations between local pH, onset potential, and product selectivity. For example, during ORR on Pt, the interfacial pH can rise to ∼12, shifting the ORR onset and increasing the relative production of hydrogen peroxide. Similarly, voltammetric pH sensors applied to nitrate reduction on Cu detect rapid interfacial alkalization arising from proton consumption and OH^– accumulation. These shifts correlate with changes in mechanistic pathways and kinetics, underscoring the reaction-dependent nature of local pH modulation. ,,

Lu et al. further illustrated the dynamic character of local pH during several reactions using in situ Raman spectroscopy, employing phosphate species as vibrational pH reporters. In NO₃RR (Figure a,b), proton consumption increases the local pH from 5.8 to approximately 10.5, evidenced by the transition from H₂PO₄ ^– to HPO₄ ^2– signatures. During nitrobenzene reduction (Figure c,d), the local pH increases from 5.8 to ∼8, corresponding to partial proton depletion and the formation of aniline. In contrast, benzylamine oxidation in PBS at pH 12 generates protons, lowering the interfacial pH to ∼11.5 and producing a shift from PO₄ ^3– to HPO₄ ^2– species (Figure e,f). Across all systems, ΔpH scales with applied potential and current density.

13.

Left panels: in situ Raman spectra of (a) nitrate reduction, (c) nitrobenzene reduction, and (e) benzylamine oxidation. Right panels: corresponding currents and ΔpH for (b) nitrate reduction, (d) nitrobenzene reduction, and (f) benzylamine oxidation at each potential. Figure adapted from ref , with permission from American Chemical Society.

Together, these results suggest that local pH shifts are reaction-specific, strongly current-dependent, and persistent even in buffered systems. Interfacial pH is therefore not a passive consequence of electrolysis but an active, dynamic parameter that shapes mechanistic pathways, adsorbate energetics, and catalytic stability. Tailoring this local environment, through electrolyte design, binder chemistry, electrode structuring, or operating regime, can unlock enhanced activity, selectivity beyond what bulk properties alone would suggest. To consolidate these reaction-dependent trends and their practical consequences, Table summarizes typical local (near-surface) pH shifts, the dominant pH-mediated pathways, and associated mitigation concepts across major electrocatalytic reactions.

6. Cross-Reaction Summary of Local pH Effects in Electrocatalysis .

reaction	medium/device	(typical) electrolyte and bulk pH	local pH trends at relevant j	local pH-mediated pathways	mitigation strategy/mechanism	refs
OER (anode)	Acidic (PEM-like)	0.05–1 M HClO4 or H2SO4 (pH ∼ 0–1)	↓ local pH (acidification, H+ generation).	Performance: Acidification shifts PCET/adsorbate coverages; can move from OH–-assisted to H2O-mediated pathways at high rate.	Increase buffer/transport (thin diffusion layer, forced convection, flow fields).	,,,,
			Can exceed −5 pH units at high j (∼1 A cm–2) even with high mass-transport.	Durability: Redox-transient dissolution (Ir) and acid-sensitive oxide dissolution/leaching under local acidic pockets	Avoid potential excursions (start/stop protocols).
	Alkaline (AWE/AEM)	0.1–1 M KOH/NaOH (pH > 13)	↓ local pH (acidification, OH– depletion).	Performance: Local OH– depletion can increase overpotential and alter apparent kinetics.	Acid-stable catalyst/support.
			Magnitude depends on j and buffer (often 1–several pH units)	Durability: Ni/Fe/Co (oxy)hydroxides show poor stability. Acidification increases metal solubility (dissolution and phase transformation).	Maintain OH– supply (flow/forced convection)
				LOM/oxygen release can accelerate vacancy formation and leaching	Catalyst composition to suppress leaching (Fe/Co control), stabilize oxyhydroxide phases
	Near neutral/buffered (AEMEL/sulfate–phosphate)	Weak buffer or supporting salt: phosphate buffer (KPi/NaPi, pH ∼6–12) or sulfate (K2SO4) with pH adjusted	↓ local pH (acidification, H+ generation).	Performance: Weak buffer can induce steep gradients.	Coatings that retain OH– without excessive site blocking (Lewis-acid, overlayers, ligands)
			Specific catalysts can create local alkaline microenvironments (surface pH ∼12–14) in neutral bulk.	Durability: Gradients promote metal-ion leaching and precipitation of hydroxides/carbonates in catalyst layer: pore blockage, loss of area, delamination
Seawater electrolysis (OER+HER)	Natural seawater (weak buffering)	Natural seawater (pH ∼ 8, Cl– ∼ 0.5 M, Ca2+/Mg2+).	Cathode: ↑ local pH(alkalinization)	Performance: Anode acidification increases chloride-derived products. Cathode alkalinization affects HER kinetics.	Local pH control via coatings/overlayers (e.g., Lewis-acid layers capturing OH– to repel Cl–; layers that reduce free OH– to limit Ca/Mg precipitation).	,−
		Bulk alkalization by diluted KOH.	Anode: ↓ local pH Cl– oxidation competitiveness.	Durability: Ca/Mg hydroxide precipitation clogs pores, chloride chemistry and local pH accelerate corrosion/leaching, trade-off between OER selectivity and scaling	Bulk alkalization can help OER selectivity but intensifies scaling.
			Deviations up to ∼5–9 pH units even at moderate j.
HER (cathode)	Acidic (PEM/strong acid)	0.05–1 M H2SO4/HClO4 (pH ∼ 0–1)	↑ local pH (H+ depletion near surface) at high rate.	Performance: Interfacial basification can shift from H+ reduction to water reduction and change kinetics.	Microenvironment engineering to create acid-like interface regions.	,,−
			Small in strong acid/high j	Durability: Effects on rate/overpotential
	Alkaline (AWE/AEM)	0.1–1 M KOH/NaOH (pH >13)	↑ local pH (OH– accumulation/proton-donor depletion) near cathode	Performance: Higher kinetic barriers for H2O dissociation. Increased barrier for hydrogen transfer.	Catalyst design to improve water dissociation (oxygen-vacancy catalysts, proton-supplying phases)
			Can be several pH units depending on diffusion layer and buffer	Durability: Can accelerate support/ionomer degradation
CO2RR (cathode)	Acidic	K2SO4, H3PO4 (pH ∼ 0–3)	↑ local pH (>3 pH units, pH 14)	Performance: Higher local pH can suppress HER but also neutralizes CO2 near surface via (bi)carbonate (bulk pH is a poor descriptor under working currents).	Decouple bulk (acid) vs interface (alkaline): acidic bulk operation + engineered microenvironment (overlayers/COF + ionomer nanochannels), tune wetting/confinement, cation enrichment, (GDE architecture)	,− ,,− ,,
				Durability: Salt/carbonate accumulation can clog pores
	Alkaline	Concentrated KOH/NaOH (pH >13) + CO2 feed		Performance: Strong HER suppression but CO2 availability limited by fast carbonate chemistry.	Increase buffer capacity/flux; manage cation identity (effective buffering via cation hydrolysis), thin diffusion layers, design pore structure.
		Often flow cell/GDE.
	Near neutral/buffered	0.1–1 M KHCO3 (pH ∼ 6.8) with alkali cations.		Durability: Carbonate/bicarbonate precipitation blocks pores and active sites (transport losses and delamination)
		Often flow cell/GDE.
NO3RR/NO2RR (cathode)	Acid	Strong acid (H2SO4/HClO4/HNO3) (pH ∼0–2)+ KNO3 salt	↑ local pH (>5 pH units)	Performance: Homogeneous HNO2 → NO can lead to N2O pathways. At high anodic currents, local pH rise can shift the dominant pathway (NO, NH2OH/NH3).	Electrode/interface design to control pH window via buffer/transport, catalyst/site design to favor certain intermediates, avoid large alkalization in porous layers, match catalyst to buffer chemistry.	,,,−
				Durability: HER competition and local pH fluctuations can destabilize catalysts/support
	Alkaline	KOH/NaOH (pH ∼ 11–14) + KNO3 salt	↑ local pH (often to very alkaline values)	Performance: HER suppression can raise FE to NH3 (catalyst dependent). Excessively alkaline microenvironments can limit hydrogenation steps (limit *H/proton availability for deep reduction) and shift product distribution.	Engineer interface for controlled proton supply (local proton donors, tailored ionomers) while keeping the HER low
				Durability: Precipitates possible with multivalent ions
Small-molecule oxidation (anode)		Often phosphate buffers (0.1 M) (pH 2–12)	↓ local pH (>2 pH units)	Performance: pH sets reactant speciation (acid/base forms), anion adsorption and reaction trends.	Local environment design (local pH reaction specific and current dependent): binder chemistry, electrode structuring, operating regime.	− ,
			Magnitude is reaction- and buffer-dependentThe magnitude is reaction- and buffer-dependent	Durability: At high-rate, local pH collapse can re-enable poisoning channels (e.g., *CO on Pt) and increase overpotential	Electrolyte selection to control anion adsorption and sustain buffer flux/thin diffusion layer
ORR (cathode)	Aqueous ORR (Pt, pH probes)	Varies by system	↑ interfacial pH (up to ∼12) during ORR on Pt under operating conditions	Performance: Higher relative H2O2 production at higher local pH.	Mitigate via controlling mass-transport and microenvironment to avoid peroxide-favoring pH	,
				Durability: H2O2 production can drive reconstruction/dissolution of catalysts and carbon support

^aFor each reaction class (column 1), the table summarizes: representative electrolytes and bulk pH (column 2); typical direction and literature-reported ranges of local (near-surface) pH shifts at relevant current densities, noting the dependence on device architecture and operating regime (column 3); dominant mechanisms by which local pH modulates activity and selectivity, including competition with parallel reactions (column 4); and common local-pH-linked deactivation pathways and mitigation strategies (column 5). Reported values are context-specific and depend on cell design, transport conditions, potential referencing and iR correction, and the pH-probing method and length scale

Beyond catalyst microenvironment engineering, classical mitigation strategies such as increasing buffer capacity and promoting convection/flow can measurably suppress pH gradients and reduce the associated transport overpotentials. For example, fluorescence imaging in a full two-electrode water-splitting cell showed that increasing phosphate buffer concentration to 0.5 M limited the local pH shift to <0.5 units even at 10 mA cm^–2, whereas weaker buffering produced larger gradients and an abrupt ≈300 mV cell-voltage penalty when the current density increased from 1 to 2 mA cm^–2, once buffer capacity was exceeded.

To connect these interfacial design strategies to experimentally quantified outcomes, Table summarizes representative reports spanning CO₂ electrolysis and related systems, including cases where local pH moderation enables high single-pass carbon utilization or suppresses competing HER under acidic bulk conditions.

7. Representative Quantitative Outcomes of Local pH Mitigation Strategies across Electrocatalytic Systems .

mitigation strategy	quantitative impact	mechanistic link to local pH/gradients	refs
Increase buffer capacity	At 0.5 M KPi, ΔpH stays <0.5 pH units even at 10 mA cm–2; the sharp ∼300 mV voltage increase observed in 0.1 M KPi when going 1 → 2 mA cm–2 is not observed at 0.5 M KPi	Higher buffer capacity sustains proton donor/acceptor supply, reducing concentration overpotential caused by local pH drift
Cell geometry/promote convection. Exploit buoyancy/bubble-driven convection	Simulations show that without buoyancy-driven convection, surface pH continues drifting and becomes numerically unstable after ∼180 s, whereas with buoyancy, local pH stabilizes after convection develops	Convection adds a transport pathway beyond diffusion/migration, thinning boundary layers and limiting pH build-up
Electrolyte engineering + separator design for near-neutral electrolysis. Concentrated phosphate + porous separator	100 mA cm–2 at 1.56 V of total cell voltage with 99.9% H2 purity; performance within ∼50 mV of an alkaline benchmark (1.51 V of total cell voltage in concentrated KOH) at 100 mA cm–2	Dense buffering mitigates local pH shifts; high molality also reduces dissolved gas crossover; separator blocks bubble crossover while keeping iR manageable
Device architecture. Bipolar membrane (BPM) to “recycle” (bi)carbonate back to CO2	Achieves a single-pass CO2 utilization (SPU) of 78% and reports ∼10-fold reduction in downstream CO2 separation energy compared with past systems	BPM-driven local acidification enables in situ conversion of (bi)carbonate to CO2 near the cathode, mitigating alkalinity-driven CO2 loss/crossover
Reverse-bias BPM to avoid carbonate formation + acid-tolerant catalyst	Selective CO2RR in acidic microenvironment: CO FE up to 63% ± 7 (25 mA cm–2); still >30% CO FE at 100 mA cm–2	BPM maintains acidic interface (reducing carbonate formation), but requires catalysts that favor CO2RR over HER; shows viability of “acidic microenvironment” route
Membrane selection	With AEM: anolyte pH drops 14 → 8, triggering +1.2 V increase at 45 mA cm–2; anode CO2/O2 ratio = 3.6. With BPM: anode CO2/O2 ratio = 0.38 and overall lower steady-state voltage	BPM suppresses (bi)carbonate crossover/CO2 pumping and slows anolyte neutralization, reducing voltage penalties linked to OER overpotential + resistance
BPM vs AEM comparison to mitigate carbon crossover

^aReported values correspond to the specific operating conditions used in the cited studies

Effect of Local pH on Electrode Degradation

Local pH measurements and operando spectroscopic probes can serve not only as mechanistic tools, but also as diagnostics for catalyst and device health during electrochemical operation. Catalyst degradation often manifests as changes in activity, selectivity, or stability that are preceded by subtle structural or chemical transformations. Operando techniques such as X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy enable real-time monitoring of oxidation states, coordination environments, and surface chemistry, providing early indications of phase transitions or dissolution processes. For example, changes in metal oxidation state or coordination during OER can signal the formation or loss of active phases and foreshadow long-term deactivation.

In situ Raman spectroscopy can detect surface poisoning or passivation by tracking changes in vibrational modes, such as the appearance of new bands or shifts in peak intensities, which indicate the formation of unwanted surface layers or progressive structural degradation.

Membrane and ionomer degradation is another key factor affecting the long-term stability of electrochemical systems, especially in PEM electrolyzers. Over time, membranes can undergo chemical corrosion, mechanical stress, and ionomer leaching, leading to the loss of ionic conductivity and structural integrity. Electrochemical impedance spectroscopy (EIS) is an effective diagnostic tool for tracking changes in membrane resistance, which can indicate phenomena such as membrane swelling, cracking, or ion-exchange degradation. In addition, membrane performance can be assessed by monitoring changes in water uptake and ion-exchange capacity, which directly correlate with the ability of the membrane to conduct protons and maintain mechanical stability under operating conditions. An increase in resistance or a decrease in ion-exchange capacity over time is therefore indicative of progressive membrane failure.

The local reaction environment, particularly the pH near the electrode surface, plays a crucial role in electrocatalytic performance and stability. Techniques such as SECM and open-circuit potential (OCP) decay measurements can be used to monitor pH gradients during operation. Local acidification or alkalinization can strongly affect catalyst stability, promoting processes such as metal leaching, surface reconstruction, or ionomer degradation. OCP decay measurements provide a rapid means to assess the magnitude of pH fluctuations, as the electrode potential shifts immediately after interrupting the electrochemical reaction, directly reflecting changes in local proton activity. By monitoring these pH transients, researchers can gain early insight into regions and operating conditions where degradation is likely to occur, enabling improved prediction of long-term system performance. Such real-time diagnostics are thus critical for assessing both the durability of electrocatalytic materials and the local reaction environments driving their degradation.

Structural Instability of Catalysts

Local pH fluctuations play a critical, yet often underappreciated, role in electrocatalyst degradation during the OER. Even when the bulk electrolyte composition appears benign, sharp pH gradients develop within the nanometer-scale reaction zone, significantly altering catalyst speciation, dissolution kinetics, and structural integrity. This section highlights key degradation pathways that originate from, or are exacerbated by, local pH changes.

In alkaline media, catalysts such as Ni, Fe, and Co-based (oxy)­hydroxides or oxides exhibit high OER activity but are particularly vulnerable to pH-induced dissolution and phase transformation. Giordano et al. demonstrated that pH influences both PCET steps and the thermodynamic stability of active oxyhydroxide surfaces. Local acidification at the anode, which can arise from proton production during the OER or from mass-transport limitations, increases the solubility of transition metals and destabilizes metastable active phases. This promotes partial dissolution, especially of Fe and Co, followed by surface reconstruction or loss of the catalytically active oxyhydroxide layers. These processes reduce intrinsic activity and compromise long-term stability even under conditions that appear stable from a bulk perspective.

In acidic PEM electrolyzers, local pH at the catalyst interface can deviate from the bulk during dynamic operation, including current ramps or gas bubble evolution. Iridium, the benchmark anode catalyst, is generally considered stable under high potentials and low pH. However, Cherevko et al. , showed that dissolution is initiated by redox transitions of Ir species during potential excursions rather than by steady-state OER. These transitions, which occur at the metal or oxide interfaces, are intensified by local acidification and rapid fluctuations in potential during startup and shutdown.

A pronounced example of pH-dependent degradation is observed in Co₃O₄ under acidic OER. Priamushko et al. showed that Co dissolution is transient yet strongly sensitive to the immediate interfacial environment. Abrupt potential changes perturb local proton activity and trigger rapid leaching of Co species, even under nominally stable operating conditions.

In neutral or near-neutral media, such as those relevant to anion–exchange membrane electrolyzers (AEMELs), local pH gradients form readily due to weak buffering. These gradients promote metal–ion leaching from catalyst surfaces along with precipitation of metal hydroxides or carbonates in the catalyst layer. Both effects lead to pore blockage, loss of active area, and eventual delamination of catalytic films.

Local pH also modulates the formation and stability of reactive oxygen species and peroxides, which influence degradation during the ORR. Hardwick and Galloway showed that species such as LiO₂ and other metal peroxides evolve under ORR conditions depending on interfacial pH and electrolyte composition. These intermediates induce surface reconstruction, dissolution, and morphological changes in both carbon supports and transition metal catalysts, and these processes accelerate performance loss.

Carbonate accumulation represents an additional degradation pathway in near-neutral CO₂RR and OER systems. As shown by Wu et al., local alkalinization facilitates carbonate and bicarbonate formation, which can precipitate at the catalyst–electrolyte interface. This leads to pore clogging, blockage of active sites, and progressive deactivation. Salt buildup also disturbs local pH and alters the double–layer structure, which intensifies degradation by promoting mechanical stress and surface delamination.

Taken together, these examples show that local pH is both a driver and a diagnostic marker of catalyst degradation. Its influence spans changes in surface speciation, enhanced dissolution, restructuring of active phases, precipitation of insulating deposits, and mechanical instability. Effective mitigation strategies should therefore combine local pH buffering, controlled operation protocols, and rational catalyst and support design, including protective layers, corrosion-resistant supports, and materials engineered for resilience under dynamic interfacial pH conditions.

Corrosion of Electrode Supports

Although the bulk stability of catalyst supports such as carbons, metal oxides, and metal-organic frameworks (MOFs) is often well established, their behavior under rapidly shifting interfacial pH conditions remains insufficiently understood. , In this section we examine how localized pH gradients influence the chemical degradation and surface transformation of these supports under electrolysis-relevant conditions.

Carbon-based supports, which are widely used in both acidic and alkaline electrolysis, are highly susceptible to pH-driven degradation. In strongly acidic bulk environments (pH ∼0–1), carbon corrosion through electrochemical oxidation is well documented at OER-relevant potentials above 1.6 V vs RHE. This process generates oxygenated surface groups, reduces hydrophobicity, and eventually leads to structural breakdown through evolution of CO and CO₂. However, even under alkaline or near-neutral bulk conditions, local acidification caused by water oxidation or insufficient buffering can transiently lower the interfacial pH to levels that promote carbon oxidation or functional group hydrogenation. Local acidic pockets can protonate and hydrogenate surface oxygen groups, altering both the electronic structure and the corrosion resistance of the carbon substrate. High current densities intensify these effects because they accelerate reaction rates and amplify the speed and magnitude of pH changes.

In CO₂ reduction systems, carbon-based GDEs commonly experience significant local alkalization. This promotes carbonate and bicarbonate precipitation that blocks pores and restricts gas transport. These deposits are often attributed to ionic strength or salt effects, yet the primary driver is the rapid local pH rise caused by OH^– production and inadequate removal. The resulting carbonate species can also interact chemically with the support, inducing delamination or passivation of catalytic layers.

Metal oxides, which serve as supports or active phases for the OER and HER, are not immune to local pH effects. Although oxides such as TiO₂ and SnO₂ are considered stable over a wide pH range, their surface chemistry is highly sensitive to transient pH variations. Local shifts in proton activity influence surface hydroxylation states, thereby affecting electron transport, interfacial adhesion, and ion conductivity. −

Another degradation pathway involves the hydrogenation and dehydroxylation of surface groups. At low local pH, surface-bound OH^– groups can be protonated, which modifies the surface energy and reactivity. Conversely, highly alkaline microenvironments promote deprotonation and the formation of reactive oxide anions. These species destabilize the lattice or leach cations from the support, a concern especially relevant for multi-cation oxides and MOF-derived materials. , Transient dissolution during redox-induced transitions between oxide and metal phases, which occurs when the potential changes rapidly, represents an additional degradation route for materials such as Ir or Co oxides, even when the bulk materials appear thermodynamically stable.

Although MOFs are not discussed extensively here due to limited electrolysis-specific studies, their hybrid inorganic–organic architectures and high surface area make them particularly susceptible to bond cleavage, linker protonation, or metal–leaching events when exposed to abrupt pH changes. Under dynamic electrochemical operation, these effects may critically limit their long-term durability.

Despite the evident risks associated with local pH fluctuations, this factor remains insufficiently considered when evaluating catalyst–support stability. Most durability studies rely on bulk electrolyte control and therefore overlook the steep gradients that arise under practical operating conditions, especially at high current densities. Standard characterization techniques also struggle to capture transient or spatially confined degradation processes, underscoring the need for in situ and operando diagnostics that probe interfacial chemistry at appropriate time and length scales.

In summary, stability in the bulk electrolyte does not guarantee long-term durability of catalyst supports. Carbon and oxide materials, which often appear robust under static conditions, can experience significant degradation when exposed to local pH extremes during electrolysis. ,, Addressing this challenge requires improved material design and more comprehensive electrochemical testing protocols that incorporate realistic interfacial conditions. As the field advances toward higher current densities and industrial operation, the influence of local pH on support corrosion deserves much closer attention.

Membrane and Ionomer Degradation

Degradation of membranes and ionomers in electrochemical devices, including water electrolyzers, arises from a combination of chemical, mechanical, and electrochemical stressors. A detailed understanding of how local pH gradients and conductivity losses contribute to this degradation is essential for the rational design of durable membrane–electrode assemblies (MEAs).

Severe local pH gradients are a central, yet often underestimated, factor in membrane and ionomer failure. These gradients are intrinsic to systems that combine acidic and alkaline environments, such as bipolar membrane electrolyzers, or that operate under uneven hydration and gas crossover. In hybrid acid–alkali bipolar membrane (BPM) devices, local differences in pH across the membrane can reach values as large as 14, especially under high current densities that promote water dissociation at the BPM junction (Figure ).

14.

Proposed chemical and electrochemical BPM degradation pathways in hybrid water electrolyzers. Figure adapted from ref with permission from American Chemical Society.

Although the anion exchange layer (AEL) is often considered the weakest component because of its vulnerability in alkaline environments, recent studies indicate that the cation–exchange layer (CEL) may degrade even more severely. X-ray computed tomography revealed the formation of microporosity and localized thinning primarily on the CEL side, suggesting that electrochemical corrosion facilitated by proton flux and voltage gradients is a dominant degradation mechanism specific to the CEL (Figure ). Loss of ionic conductivity is both a consequence and a driver of membrane failure. As pores and defects propagate, water and ion transport pathways are disrupted, which increases ohmic resistance. In AEM catalyst layers, the ionic resistance of the ionomer has been observed to increase substantially during durability tests, with values rising from 0.6 to 6.4 Ω cm in some cases. This increase is accompanied by significant reductions in double-layer capacitance and electrochemically active surface area (ECSA), indicating that diminished ion transport at the catalyst–ionomer interface directly weakens electrode kinetics.

15.

XRCT images and 3D reconstructions of (a, b) pristine BPM and (c, d) reacted BPMs after 12 h of operation in a hybrid electrolyzer for water splitting. (e, f) Soaked BPM in 1.0 M KOH at room temperature for 12 h. The dashed box includes the front views of CEL and AELs, a cross-section slice of BPM, as well as the model of the reconstructed pore distribution analysis. Figure adapted from ref with permission from American Chemical Society.

Degradation is strongly influenced by the local chemical environment and by interfacial phenomena. In alkaline media, hydroxide ions attack quaternary ammonium groups through elimination (E2) or nucleophilic substitution (SN2) pathways. These reactions accelerate with increasing pH and temperature, and decomposition of ionic groups often proceeds more rapidly than cleavage of the polymer backbone. FTIR analysis supports these observations, with a clear loss of sulfonic acid signatures in the CEL after operation in water electrolysis, consistent with SO₃H group degradation through direct reaction with hydroxide or through electrochemically driven pathways.

At the catalyst–ionomer interface, degradation can be even more complex. Ha et al. identified potential-dependent degradation mechanisms that vary depending on the ionomer and the catalyst surface. For example, sulfonic acid groups in Nafion undergo SO₃ cleavage at potentials as low as 1.4 V vs RHE on IrO₂. Sustainion undergoes ring oxidation to form alcohols, and Versogen exhibits double deprotonation in alkaline environments. These reactions not only degrade the ionomer but also generate soluble side products, such as alcohols or sulfate species, that can poison active catalytic sites or alter the kinetics of the OER.

Local pH gradients also generate physical stresses. Steep pH differences across membranes, particularly under dynamic load conditions, lead to local osmotic swelling. This swelling produces mechanical stress that promotes delamination and crack formation. In BPMs, water dissociation at the membrane interface generates net water fluxes and swelling that occur predominantly on the CEL side. This asymmetric deformation accelerates pore formation and facilitates further acid–base neutralization, ultimately reducing membrane effectiveness and increasing parasitic voltage losses.

Membrane and ionomer degradation is further complicated by morphological effects. Perfluorinated ionomers such as Nafion and Aquivion possess microphase-separated structures with acidic groups confined to hydrophilic domains. These domains can reorganize in response to changes in solvent composition, pH, and ionomer concentration, thereby altering the apparent acidity and the transport behavior of the ionomer film. Figure a presents a schematic illustrating how local pH in Nafion-based dispersions depends strongly on solvent composition and ionomer concentration, which together influence particle aggregation. In water-rich environments, sulfonic acid side chains extend into the solvent, increasing proton accessibility and lowering local pH. This promotes electrostatic interactions that stabilize the dispersion.

16.

(a) Schematic of 2D slice of potential structure representing individual chains and aggregates of Nafion (Diameter as measured by dynamic light scattering), showing the side-chain orientation differences (pH differences) as a function of aggregation and solvent content. (b) Average diameters and ζ–potentials of carbon–ionomer aggregates at different water fractions and ionomer:carbon (I:C) ratios between 0 and 1.5 made by first adjusting the dispersion pH to 0, 2.5, or 9. Figure adapted from ref with permission from American Chemical Society.

Figure b shows that increasing the water fraction and the ionomer content exposes and dissociates more sulfonate groups, which increases acidity and ionic strength. The resulting compression of the electrical double layer reduces the magnitude of the ζ-potential and weakens electrostatic repulsion, allowing larger ionomer–carbon aggregates to form. When suspensions are compared at matched pH, most solvent-dependent differences disappear, indicating that local proton activity, rather than the identity of the solvent, primarily controls both aggregation behavior and ζ-potential.

In summary, membrane and ionomer degradation in electrochemical cells is strongly influenced by local pH gradients, which activate chemical, electrochemical, and mechanical instability pathways. These effects are compounded by dynamic restructuring of the ionomer morphology and by complex interactions at the catalyst–ionomer interfaces. Effective mitigation requires strategies that incorporate spatially resolved pH control, optimization of interfacial binding and ionomer design, and preservation of ionic conductivity under operational stress. Further progress will depend on combining operando diagnostics with predictive theory to guide the development of durable electrolyzer components.

Challenges and Future Directions

Despite major progress in probing and controlling interfacial proton activity, several key challenges remain before local pH can be fully harnessed as a design parameter in electrocatalysis. A central limitation lies in the absence of real-time, high-resolution mapping techniques that can capture fast and spatially heterogeneous pH fluctuations at operating electrodes. Existing methods, including SECM, operando vibrational spectroscopy, and OCP-decay transients, provide valuable information but fall short in either temporal resolution, spatial precision, or chemical specificity. Moving beyond these limitations will require hybrid methodologies that combine spatially resolved experimental probes with kinetic and transport modeling, enabling quantitative interpretation of operando pH dynamics across relevant length scales. By hybrid methodologies we refer to the combination of spatially resolved experimental probes (e.g., scanning electrochemical microscopy, fluorescence- or vibrational-based pH reporters, and indirect electrochemical probes such as rotating ring disk electrode (RRDE)) with multiscale modeling frameworks coupling continuum reaction–transport descriptions to atomistic representations of the electrochemical double layer, enabling quantitative interpretation of operando pH dynamics across relevant length scales. When parametrized and validated against experiment, such models can move beyond qualitative description and provide predictive guidance on how electrode architecture, electrolyte composition, and operating conditions influence local pH under technologically relevant current densities.

The influence of local pH is often intertwined with other interfacial phenomena and should be measured directly before being invoked as the primary driver of activity or selectivity. Alkali-cation hydrolysis and buffering provide a grounded method to tune interfacial proton activity without modifying the intrinsic acidity of the active site, but separating these effects from double–layer electric fields remains a substantial challenge. , Attributing observed trends to cation identity also requires rigorous electrolyte purification and documentation. This includes verifying each hydroxide or supporting salt lot by ICP-OES and ion chromatography, , reporting Li, Mg, and Cs content to low-ppm levels, preparing hydroxides from high-purity oxides when possible, and conducting salt-swap controls at matched pH, ionic strength, and buffer capacity. Spike–recovery tests at ppb to ppm concentrations can further reveal the sensitivity of activity and selectivity to unintended dopants. These coupled determinants of interfacial proton activity, spanning kinetics, double-layer structure, confinement, buffering/ion exchange, and operating environment, are summarized in Figure .

17.

Schematic overview of the primary factors that shape local proton activity/local pH (near-surface pH) at electrified interfaces, including kinetic proton consumption/production, electric double-layer structure, confinement/transport limitations, buffering and ion-exchange equilibria, and environmental parameters (temperature, ionic strength, and gaseous CO₂).

Several factors beyond pH can produce similar macroscopic signatures, including redistribution of double-layer fields, ,, the presence of hydrophobic or hydrophilic (micro)­environments, , interfacial morphology, and electronic structure. , Robust attribution therefore requires operando pH measurements combined with proper potential referencing, strict control of ionic strength and electrolyte purity, and orthogonal perturbations that vary the microenvironment at fixed pH, or vary pH while maintaining all other parameters constant. These measurements should ideally be complemented by probes of interfacial structure and electronic response, while explicitly accounting for cell geometry and product speciation. These constraints also highlight the need for more consistent reporting to enable fair comparisons across studies and platforms.

Beyond improved diagnostics, community-wide minimum reporting metadata would substantially strengthen cross-study comparability of local-pH effects. We encourage authors to report the full electrolyte composition (all salts, molarities, cation/anion identity, impurity control) and the measured bulk pH together with buffer identity/concentration (and, when relevant, buffer capacity or total alkalinity). The reactor/architecture (H-cell/flow cell/GDE/MEA, electrode area/spacing, catalyst-layer/ionomer details) and transport conditions (stagnant/stirring/RPM/flow, gas feed rate/pressure/humidity, temperature) should be stated explicitly. Likewise, potential reporting (reference, scale, calibration, iR correction method/values) and current normalization (geometric area plus at least one additional basis such as ECSA or mass activity) are essential for fair benchmarking. “Local pH” should be defined, as seen in Table , operationally by specifying where it is probed (distance/position; OHP vs microns) and how it is obtained (technique, calibration, perturbation controls). Finally, consistent selectivity reporting (FE with uncertainty and analytical method) and stability descriptors (steady-state duration, dissolution/metal loss, scaling, membrane/ionomer diagnostics) are needed to connect interfacial pH swings to performance and durability.

Current methods, such as SECM and operando spectroscopy, however valuable, are limited by their temporal and spatial resolution. To overcome these limitations, future work should focus on integrating real-time, high-resolution sensors with advanced data analysis techniques, such as machine learning algorithms. This integration would enable the continuous and precise monitoring of pH fluctuations, fostering a deeper understanding of how local pH influences electrocatalytic behavior. Furthermore, the development of multi-probe sensor arrays that can capture pH variations at multiple locations on the electrode surface would allow for more comprehensive and accurate mapping of the local electrochemical environment, especially under dynamic operating conditions.

In parallel, there is great potential in exploring new measurement principles, such as the use of nanostructured sensors integrated in the device that provide localized pH detection while simultaneously offering real-time feedback mechanisms to adjust local pH. These technologies would be particularly useful for creating tailored reaction environments at the electrode-electrolyte interface, where pH fluctuations are most pronounced. Additionally, microfluidic reactors with embedded pH sensors provide an exciting opportunity for integrating real-time pH regulation with electrocatalytic processes, enabling more precise control over local reaction environments.

Free-electron lasers (FEL) are promising for local-pH related interfacial studies because they combine femtosecond pulse durations with extremely high peak brilliance. This enables pump–probe and, in favorable cases, single-shot, element- and site-specific spectroscopy of transient interfacial species. Importantly, FELs are unlikely to provide a direct pH readout; rather, they can resolve ultrafast proton-coupled processes, interfacial solvation dynamics, and transient changes in local speciation that underpin local pH effects at electrified interfaces, although operando implementations remain in an early stage. The main hurdles involve designing electrochemical cells compatible with FEL irradiation and electrical control, such as thin-layer or ATR geometries equipped with IR or soft-X-ray windows, as well as mitigating beam-induced heating and radiolysis through low-fluence operation, , rapid electrolyte exchange, and controlled timing. The most promising directions include FEL–ATR spectroscopy for tracking OH-stretch envelopes and anion speciation under potential control, soft-X-ray FEL O–K-edge X-ray absorption spectroscopy pump–probe to monitor protonation dynamics within the double layer, and tender FEL ambient pressure X-ray photoelectron spectroscopy for resolving acid–base chemical shifts. Quantitative extraction of local pH will require benchmarking against storage-ring measurements and molecular simulations.

In parallel, catalyst development must target materials capable of withstanding the extreme and fluctuating pH conditions encountered under realistic operation. Many high-performance catalysts degrade when exposed to sharp proton or hydroxide gradients that trigger dissolution, phase transformations, or ionomer corrosion. Rational design of acid/alkali-tolerant catalytic motifs, protective adlayers, and dynamic proton-relay structures will be essential to ensure stability without compromising kinetics. Such motifs can be realized by stabilizing Ru/Ir-free sites through heteroatom coordination and d-band tuning (e.g., Ru–O_bri–W or Ru–O–Ta/Mo/Nb linkages), and by employing corrosion-resistant conductive oxides such as Sb-doped SnO₂ or Ti_0.7Nb_0.3O₂ for the OER. Protective adlayers may take the form of ultrathin conformal Nb₂O₅, Ta₂O₅, TiO₂, or WO _x shells deposited by atomic layer deposition, which are proton-permeable yet resistant to dissolution, or phosphate and pyrophosphate layers that self-replenish during operation. − Dynamic proton-relay structures can be introduced by tethering Brønsted base–acid pairs (e.g., imidazole–sulfonate or sulfonimide–phosphonate couples) or by engineering W–O_bri–Ru motifs that enable bridging-oxygen-assisted deprotonation pathways. Collectively, these strategies aim to buffer local pH excursions while preserving site accessibility and conductivity, offering practical routes to catalysts that combine high activity with long-term durability.

Equally important is the integration of pH-control strategies into scalable electrolyzer architectures. Although nanoconfinement, ionomer engineering, and cation or anion modulation have demonstrated impressive improvements at the laboratory scale, translating these concepts to industrial devices requires precise control of gas–liquid–solid interfaces, membrane transport, flow-field design, and mechanical robustness under large current densities. − Flow-field and two-phase management can intentionally shape boundary-layer thickness, reactant residence time, and bubble removal, thereby tuning local proton activity. Interdigitated and flow-through architectures promote convection across porous transport layers and suppress extreme local pH gradients. Shallow ribs, herringbone microgrooves, or 3D lattices can generate secondary flows that refresh the diffusion layer with minimal pressure penalty. Serpentine channels with tapered manifolds and dedicated venting features, combined with wettability gradients such as hydrophilic PTL for electrolyte wicking and hydrophobic stripes for gas evacuation, help rapid bubble detachment and prevent pH-skewing stagnation pockets. Operational parameters, including increased flow rates, pulsed flow, or periodic back-flushing, can temporally reset interfacial environment. Buffer capacity, ionic strength, cation identity, and recirculation-loop volume determine the steady–state pH profile. Matching transport–layer pore structure to the channel design (flow-through for strong gradients and flow-by when milder control is desired) and optimizing ionomer distribution within the catalyst layer ensure that convective renewal is translated into predictable local-pH control. Incorporating these strategies into reactor design will be essential for maintaining performance at current densities above 1 A cm^–2, where interfacial gradients are most severe.

A further challenge lies in establishing quantitative links between local pH excursions and long-term degradation. Although transient acidification or alkalization is increasingly recognized as a driver of dissolution, corrosion, and support failure, very few studies have correlated pH dynamics with durability over thousands of hours. Progress in this area will require accelerated stress testing, long–term operando diagnostics, and multiscale modeling to connect interfacial chemistry with macroscopic device lifetimes.

From a computational perspective, the main difficulties depend on the theoretical level chosen. Researchers seeking close correspondence with experiments require simulations of larger systems and longer time scales that capture multiple intertwined phenomena. Machine-learning interatomic potentials are a promising pathway toward this goal. A complementary strategy involves constructing simplified models that capture the essential physics of interfacial processes with reasonable computational cost. In either case, the accuracy of DFT for gas-phase species, anions, cations, and adsorbates must be assessed carefully, and corrections are often necessary where thermochemistry is poorly described. When combined with experimental validation, multiphysics kinetic–transport models provide a practical route to predict trends and operating regimes that mitigate extreme local pH excursions, even if absolute values remain sensitive to model assumptions. Multiphysics models that integrate thermodynamic, kinetic, and mass-transport effects in systems prone to local pH variations are also highly desirable, provided they remain both computationally tractable and conceptually transparent.

Addressing these challenges will elevate local pH from a qualitatively appreciated phenomenon to a powerful design principle. Achieving this transition will require coordinated advances across diagnostics, (multiscale) modeling, catalyst design, and device engineering, ultimately enabling the next generation of robust, efficient, and scalable electrochemical systems.

∇.

J.W.R.-A. and A.E. contributed equally to this work and are considered co-first authors.

The authors declare no competing financial interest.