Electrochemiluminescence and Photoinduced Electrochemiluminescence Probing of Energy Conversion Catalysis

Abstract

Electrochemiluminescence (ECL) and its photoinduced variant, photoinduced electrochemiluminescence (PECL), have progressed beyond analytical detection tools to emerge as powerful spatiotemporal probes for interfacial electrocatalysis and photoelectrochemical processes. By transducing local electron-transfer events, reactive intermediates, and charge-carrier dynamics into photon emission, ECL and PECL provide a distinctive electrochemical–optical platform that combines high sensitivity, negligible optical background, and precise spatiotemporal controllability. This review systematically surveys recent methodological advances and representative applications of ECL and PECL in energy conversion catalysis. ECL-based approaches are shown to enable visualization of catalytic heterogeneity, dynamic evolution, and structure–activity relationships across a wide range of catalytic materials. Particular emphasis is placed on mechanistic insights into key electrocatalytic reactions, including the oxygen reduction, oxygen evolution, and hydrogen evolution reactions, in which spatial and temporal resolved ECL signals elucidate reaction pathways, intermediate dynamics, and local microenvironmental variations. In parallel, PECL is discussed as a selective probe for photoinduced charge-carrier utilization, recombination, and interfacial kinetics in photocatalytic and photoelectrochemical systems. Finally, current challenges and future perspectives for advancing ECL- and PECL-based methodologies in energy-related catalysis are outlined.

1. Introduction

Electrochemiluminescence (ECL) refers to the generation of electronically excited states through electrochemical reactions at electrode interfaces, followed by photon emission upon relaxation to the ground state. − Because light emission is directly initiated by interfacial redox processes, ECL provides an optical signal that is intrinsically correlated with electrochemical current. , Over the past decades, this unique electrochemical-optical transduction mechanism has been extensively exploited in analytical chemistry. − ECL-based assays offer outstanding sensitivity, robustness, and reproducibility, enabling broad commercialization in clinical immunoassays and bioanalysis. These advances have established ECL as a mature and reliable detection modality with well-understood photophysical and electrochemical foundations. In semiconductor systems, however, photogenerated charge carriers can also drive interfacial redox reactions. Photoinduced electrochemiluminescence (PECL) approaches further extend this electrochemical–optical framework by coupling light excitation with electrochemical processes, enabling optical probing of photoelectrochemical reactions at semiconductor interfaces.

Beyond its success in analytical applications, the fundamental characteristics of ECL and its photoinduced variant PECL make them particularly attractive for probing redox reactions at interfaces. − Photon emission occurs only when electrochemical reactions proceed, which enables direct correlation between photon output, applied potential, and reaction timing. − In ECL and PECL measurements, the experimentally recorded observables are the photon emission intensity and its temporal evolution under electrochemical control, which reflect the formation and consumption dynamics of excited luminophore species at the electrode interface. These signals can be linked to mechanistic targets such as the formation of reactive intermediates, variations in the local microenvironment near the electrode interface, and the characteristic time of catalytic reactions. Through spatially and temporally resolved measurements combined with electrochemical analysis, these observables enable inference of reaction pathways and the formulation of hypotheses regarding rate-determining steps. In addition, compared with scanning electrochemical probe methods, ECL enables wide-field, noncontact imaging without mechanical scanning. − Nevertheless, ECL-based readouts remain indirect and may be influenced by probe-induced perturbations or coupling with mass-transport processes. Despite these limitations, the unique electrochemical–optical coupling and imaging capability of ECL and related PECL approaches still make them a powerful platform for visualizing interfacial electrochemical processes with spatial and temporal resolution.

Electrochemical and photoelectrochemical energy conversion processes, including electrocatalysis and photocatalysis, are governed by complex interfacial phenomena spanning multiple length and time scales. Macroscopic performance metrics such as activity, efficiency, and durability emerge from microscopic processes. − These processes include charge transfer, mass transport, and structural transformations of catalytic materials under operating conditions. However, conventional electrochemical measurements largely provide ensemble-averaged information, obscuring spatial heterogeneity and transient dynamics, while many spectroscopic and microscopic techniques lack direct coupling to electrochemical control parameters. Addressing these challenges requires measurement strategies that can directly link interfacial reaction events to spatially and temporally resolved observables. In this context, ECL and PECL offer unique opportunities by translating interfacial redox reactions into optically detectable photon emission.

In this review, we present a comprehensive overview of recent progress in ECL measurement and imaging for energy electrochemical systems, with an emphasis on its emerging role as a probe for interfacial activity, heterogeneity, and reaction mechanisms. We first introduce the fundamental mechanisms and instrumental platforms underlying ECL and PECL. We then discuss how ECL is employed to evaluate catalyst activity, stability, and spatial heterogeneity across representative material classes. Subsequently, we highlight mechanistic studies of key energy-relevant electrocatalytic reactions, including oxygen reduction, oxygen evolution, and hydrogen evolution, followed by recent advances in PECL for probing photocatalytic and photoelectrochemical processes. By organizing these developments within a unified electrochemical-optical framework, this review highlights what ECL can uniquely resolve at catalytic interfaces. We further discuss future opportunities and challenges for deploying ECL in energy electrochemistry.

2. Mechanistic Pathways of ECL

In a typical ECL process, an applied potential at the electrode drives redox reactions of luminophores and/or coreactants, leading to the generation of reactive intermediates such as radical ions. , These intermediates subsequently undergo follow-up chemical reactions to form electronically excited states, which relax to the ground state with the emission of photons. , The resulting light emission thus serves as a direct optical readout of interfacial electrochemical processes.

Fundamentally, ECL generation mechanisms can be classified into two major pathways: annihilation and coreactant pathways. , In the annihilation pathway, both oxidized and reduced forms of the reactant species (denoted as A^•+ and B^•–) are generated electrochemically. These electrogenerated oxidant and reductant species subsequently undergo an annihilation reaction to produce an electronically excited state of the luminophore (A*), which emits light upon relaxation to the ground state. Notably, the oxidized and reduced species may originate from the same molecular entity, enabling self-annihilation ECL. A prototypical example is the Ru­(bpy)₃ ²⁺ system, in which electrochemically generated Ru­(bpy)₃ ³⁺ and Ru­(bpy)₃ ⁺ annihilate to form the excited Ru­(bpy)₃ ²⁺* (Figure a). In contrast, the coreactant pathway involves an auxiliary species that participates in the electrochemical process to facilitate excited-state generation. In this mechanism, the coreactant is electrochemically activated to form highly energetic radicals, which react with the luminophore to produce its excited state. Depending on whether the luminophore is oxidized or reduced at the electrode prior to the chemical step, coreactant ECL mechanisms are commonly categorized into oxidative-reduction and reductive-oxidation pathways. For instance, in the widely used Ru­(bpy)₃ ²⁺/tripropylamine (TPrA) system, oxidative activation of TPrA generates strong reducing radicals that react with Ru­(bpy)₃ ³⁺ to form the emissive Ru­(bpy)₃ ²⁺* (Figure b). ,

1.

Schematic illustration of the two fundamental ECL pathways in Ru­(bpy)₃ ²⁺ systems. (a) Annihilation pathway, where oxidized and reduced species are generated sequentially under alternating potentials. (b) Coreactant pathway with TPrA.

The practical applicability of annihilation and coreactant pathways differs substantially depending on the electrochemical and experimental objectives. Annihilation ECL relies on the simultaneous generation of both oxidized and reduced forms of the luminophore, which typically requires wide potential windows and electrochemical stability of both redox states. As a result, annihilation ECL is most commonly implemented in aprotic organic solvents and under well-controlled conditions, where competing side reactions are minimized. Owing to its mechanistic simplicity and absence of auxiliary reagents, annihilation ECL has played an important role in elucidating fundamental ECL mechanisms and excited-state energetics. −

In contrast, coreactant ECL employs an auxiliary species to facilitate excited-state generation, thereby significantly relaxing the electrochemical constraints on the luminophore. By decoupling excited-state formation from the need to access both extreme redox potentials of the luminophore, coreactant pathways enable efficient ECL generation in aqueous media and under mild electrochemical conditions. This feature is particularly advantageous for bioanalysis, , clinical application, , as well as energy-related electrochemical systems, , where reactions are typically conducted in water-based electrolytes. Moreover, the choice of coreactant allows ECL processes to be selectively coupled to specific reaction intermediates, such as reactive oxygen species, providing additional chemical sensitivity and mechanistic insight. −

3. Fundamental Apparatus

3.1. Instrumentation for ECL Detection and Imaging

For intensity-based ECL detection of single entities, a typical experimental setup consists of a photomultiplier tube (PMT), an electrochemical cell, and an electrochemical workstation (Figure a). Commercially available instruments have been developed to enable precise synchronization between the PMT and the electrochemical workstation, allowing simultaneous acquisition of optical and electrical signals. To achieve single-entity sensitivity, ultramicroelectrodes (UMEs) or nanoelectrodes can be employed. Quantitative analysis in PMT-based ECL measurements is typically achieved by establishing a correlation between the emitted ECL intensity and the concentration, activity, or reaction efficiency of the target analyte or electrochemical process under investigation.

2.

Schematic diagram of (a) the ECL measurement setup and (b) the ECL imaging configuration.

The instrumental configuration for ECL imaging, or ECL microscopy (ECLM) (Figure b), is similarly straightforward. A typical ECLM setup comprises an electrochemical cell integrated with an electrochemical workstation and a wide-field optical microscope equipped with a charge-coupled device (CCD) or an electron-multiplying CCD (EMCCD). , High-numerical-aperture objectives are generally preferred to maximize photon collection efficiency. A function or signal generator can be incorporated to synchronize the camera acquisition with the electrochemical control. Upon switching off external illumination, electrochemical reactions are initiated using potential steps or cyclic voltammetry, while the sCMOS or EMCCD captures ECL images in real time, enabling direct visualization of interfacial electrochemical activity. In ECLM, quantitative information is extracted not only from the total emission intensity but also from the spatial and temporal distribution of the ECL signal. Pixel-wise photo counts analysis enables mapping of local electrochemical activity across heterogeneous electrode surfaces, while time-resolved imaging allows extraction of dynamic parameters.

3.2. PECL and All-Optical ECL (AO-ECL) Instrumentation

Beyond conventional electrically driven ECL platforms, PECL and AO-ECL configurations have emerged as powerful extensions that introduce optical excitation.

In a typical PECL setup, a light source such as a laser or light-emitting diode (LED) is coupled to the electrochemical cell to illuminate the electrode surface while electrochemical bias is applied (Figure a). Optical excitation can generate charge carriers in semiconducting electrodes or photoactive materials, modulate interfacial energetics, or selectively activate reaction pathways, thereby enhancing or spatially modulating the ECL response. −

3.

(a) Scheme showing light conversion by PECL. Reproduced with permission from ref . Copyright 2022, Royal Society of Chemistry. (b) Scheme of monolithic AO-ECL at a nanostructured Au-pnn ⁺⁺Si–Pt junction under illumination. (c) Hole transfer mechanism leading to visible photons generation at the solid–liquid interface, P represents reaction products. (d) Normalized spectra of the incident excitation light-emitting diodes (brown/red curves) and the resulting AO-ECL emission of luminol/H₂O₂ at Au-pnn ⁺⁺Si–Pt (blue curve); the largest anti-Stokes shift is represented by a pink dashed arrow. Reproduced with permission from ref . Copyright 2023, American Chemical Society.

Recently, AO-ECL platforms have been developed by Sojic’s group, in which both the electrochemical activation and the subsequent luminescence readout are achieved entirely by optical means, without direct electrical wiring to the working electrode. In a typical AO-ECL setup, the electrochemical interface is illuminated by a focused or patterned light source that induces localized charge separation or redox activity at the electrode–electrolyte interface (Figure b). This light-driven electrochemical process generates emissive excited states, giving rise to ECL emission. From an instrumental perspective, AO-ECL configurations eliminate conventional potentiostats, electrical contacts, and reference electrodes. Instead, they integrate an optical excitation module (e.g., continuous-wave or pulsed laser, LED, or patterned illumination system) with a wide-field or microscopic optical detection unit. The emitted ECL photons are collected either by a standard optical microscope equipped with a CCD/CMOS camera for spatially resolved imaging, or by simple imaging devices such as smartphone cameras for macroscopic detection, depending on the required spatial resolution and signal intensity. The absence of electrical connections enables contact-free operation and flexible spatial programmability, as the electrochemical activity can be defined by the illumination geometry rather than by electrode patterning. As a result, AO-ECL setups are particularly suited for large-area imaging, parallel excitation of multiple sites, and dynamic modulation of reaction regions.

By introducing light as an external and programmable control parameter, ECL can be spatially and temporally modulated beyond purely electrochemical regulation. These emerging optically controlled ECL platforms provide a new instrumental framework for visualizing electrochemical and energy-conversion processes, while offering high adaptability in excitation geometry and detection scale. ,

4. Application of ECL in Energy Conversion and Catalysis

4.1. Catalyst Activity, Stability, and Heterogeneity Revealed by ECL

ECL offers a sensitive and versatile probe for evaluating the activity, stability, and heterogeneity of catalytic materials in electrochemical and photoelectrochemical energy conversion. By transducing localized interfacial electron-transfer events into photon emission, ECL provides intrinsic signal amplification, as well as spatiotemporal resolution that go beyond ensemble-averaged electrochemical measurements. , As a result, ECL can reflect not only overall catalytic activity but also site-specific reactivity, dynamic evolution, and susceptibility to deactivation. , This strategy is broadly applicable to diverse catalyst classes, including noble metals, carbon-based and framework materials, as well as semiconductor photoelectrodes. In the following sections, several representative catalyst systems are selected to illustrate how ECL reveals activity, stability, and heterogeneity.

4.1.1. Noble-Metal Nanocatalysts

Noble-metal catalysts, particularly Pt-based materials, are widely investigated in electrocatalysis owing to their well-defined electronic structures, high intrinsic activity, and robustness toward key interfacial electron-transfer processes. − Such catalysts also serve as well-defined model platforms for fundamental studies of electrocatalysis, enabling detailed probing of interfacial charge transfer, surface coordination, and catalytic heterogeneity. , Because of these attributes, noble-metal nanostructures have become the earliest and most informative model systems for developing ECL-based methodologies, allowing interfacial reaction competence to be transduced into high-contrast optical signals. ,,

Early single-particle ECL microscopy on individual Au and Pt nanostructures revealed pronounced spatial heterogeneity in photon emission, even under uniform electrochemical polarization. For example, ECL imaging at single Au nanowire electrodes showed that emission is localized to specific regions rather than uniformly distributed, directly visualizing heterogeneous interfacial electron-transfer activity on an individual metallic object. Similarly, ECL imaging of isolated Au nanoparticles resolved localized redox-active regions on otherwise nominally identical particles, establishing that interfacial electron-transfer events are intrinsically heterogeneous at the nanoscale.

Spatiotemporal ECL imaging further demonstrated that such heterogeneity is dynamic rather than static. On single two-dimensional Au nanoplates, ECL intensity patterns were shown to evolve over time during electrochemical excitation, with initially bright edge or corner regions progressively dimming upon repeated potential cycling. This behavior was attributed to surface oxidation and restructuring that homogenize local electronic states and suppress ECL activity, providing a direct optical manifestation of activity decay coupled to surface transformation. Importantly, these results revealed that sites exhibiting the strongest ECL signals-often associated with low-coordination atoms-are also the most susceptible to deactivation. The physical picture was sharpened by the introduction of super-resolution ECL microscopy, which pushed spatial resolution well below the optical diffraction limit. Using super-resolution radial fluctuations (SRRF)-ECL analysis, local ECL activity on individual Au nanostructures could be reconstructed with ∼100 nm resolution, enabling facet- and defect-dependent interfacial reactivity to be quantitatively resolved (Figure ). Time-lapse super-resolved maps further revealed that regions with initially enhanced ECL intensity undergo faster decay, directly linking local electronic structure, ECL activity, and susceptibility to oxidation or dissolution within the same particle. Operando mapping showed that electrochemical oxidation and etching can dynamically generate new reactive edges, enhancing local ECL activity and underscoring that surface degradation and activation can coexist during operation. Related time-resolved ECL microscopy studies have similarly visualized dynamic activity redistribution during surface evolution of noble-metal-based nanocatalysts.

4.

(a) Schematic illustration of the ECLM system for single-particle imaging. E and C represent the luminophore and coreactant, respectively, which are Ru­(bpy)₃ ²⁺ and TPrA. (b) Basic principle of SRRF analysis of multiple images. Wide-field and super-resolution ECL images of (c) a single Au nanosphere and (d) separated Au nanospheres and corresponding line profiles taken from the regions between the yellow arrowheads. Scale bar: 500 nm. (e) Wide-field and super-resolution ECL images of one nanorod and spatial distribution ECL intensity obtained using Matlab (scale bar: 200 nm). (f) Wide-field and super-resolution ECL images of one nanoplate and spatial distribution ECL intensity obtained using Matlab (scale bar: 200 nm). Reproduced with permission from ref . Copyright 2021, American Chemical Society.

4.1.2. Single-Atom and Atomically Dispersed Catalysts

Single-atom catalysts (SACs) have emerged as a pivotal class of materials in energy-related electrocatalysis, offering maximal metal utilization, uniform and well-defined active centers, and unique electronic structures that are inaccessible in nanoparticle catalysts. , By isolating metal atoms within specific coordination environments, SACs provide an ideal platform for elucidating intrinsic structure–activity relationships and probing elementary interfacial electron-transfer processes at the atomic limit. However, their extreme dispersion and often low metal loadings pose significant challenges for conventional electrochemical and spectroscopic characterization, which typically average over large populations of sites and obscure subtle heterogeneity. −

ECL offers a particularly suitable approach for interrogating SAC performance because ECL directly converts localized interfacial electron-transfer events into photon emission with single-molecule sensitivity. , In contrast to nanoparticle catalysts that expose multiple facets and coordination motifs, SACs feature atomically dispersed metal centers with more well-defined local coordination environments. As a result, ECL responses can be more readily correlated with the local chemical environment and catalytic behavior of SACs. From a physical chemistry perspective, ECL readouts in SAC systems reflect not merely catalytic turnover, but the energetics and kinetics of intermediate formation, excited-state generation, and the competition between radiative and nonradiative relaxation pathways that collectively determine photon output.

Early representative studies established that ECL intensity can serve as a sensitive descriptor of SAC population and electronic metal–support interactions (EMSI). For example, Liu and Shi employed Pt single atoms anchored on MXene supports to quantitatively correlate ECL intensity with trace Pt atom concentrations, demonstrating that isolated Pt sites efficiently regulate reactive oxygen species (ROS) generation pathways. This work revealed that EMSI plays a decisive role in controlling interfacial electron transfer and ROS-mediated ECL processes, providing insight into how atomic dispersion alters catalytic pathways rather than simply enhancing apparent activity. Similarly, Zhu’s group systematically compared Ni single-atom catalysts with distinct Ni–N₄ and Ni–N₂O₂ coordination environments, showing that subtle changes in local atomic geometry lead to markedly different ORR pathways and ECL responses.

ECL methodologies have been extended from ensemble measurements to spatially and temporally resolved performance evaluation of SACs. We introduced ECLM to investigate SAC performance at the individual particle level. By achieving single-photon sensitivity and temporal resolution down to 100 ms, this imaging platform enabled real-time visualization of heterogeneous electron-transfer behavior across SAC-containing particles. The ECLM approach was particularly sensitive to low concentrations of H₂O₂, allowing quantitative mapping of ROS generation efficiency and dynamic electron-transfer rates. Importantly, the imaging results revealed pronounced heterogeneity and temporal fluctuation in SAC activity, directly linking atomic-site electronic structure to catalytic efficiency and stability. In the case of Fe–N–C SACs, the superior peroxidase-like activity was attributed to efficient oxygen activation and ROS generation at specific Fe–N coordination motifs, as visualized by localized ECL emission (Figure ). It should be noted that the ECL imaging resolves signals from individual catalyst particles or nanoscale domains containing single-atom active sites, rather than directly detecting emission from an individual metal atom. Direct ECL detection at the level of a single atom remains extremely challenging due to the limited photon flux associated with a single catalytic turnover and the spatial resolution limits of current optical imaging techniques.

5.

(a) Schematic illustration of in situ imaging of SACs at single particle level via ECLM. (b) ECL image and 3D ECL intensity diagram of individual SACs. (c) ECL image and 3D ECL intensity diagrams (inset Figure) of Fe-NC SACs at different potentials, scale bar: 5 μm; (d) Emission of one nanoparticle obtained using Matlab, and corresponding photon counts profile along with the horizontal (blue) and vertical (red) lines; (e) Φ_ECL calculated based on ECL emission photon counts at individual Fe-NC SACs versus potential. Reproduced with permission from ref . Copyright 2024, Wiley-VCH.

4.1.3. Semiconductor and Hybrid Catalysts

Semiconductor and hybrid catalysts underpin a broad range of energy-conversion systems, including photoelectrochemical water splitting and photoassisted electrocatalysis, where performance is dictated by band-structure-mediated charge generation, separation, and interfacial utilization. , Compared with metallic catalysts dominated by surface states, semiconductor-based materials exhibit strongly coupled bulk-surface behavior, making their activity and stability highly sensitive to defects, surface states, and junction architectures. PECL provides a particularly informative window into these materials because photon output is contingent on successful interfacial charge utilization, thereby reporting not only carrier generation but also charge-transfer competence at the catalyst/electrolyte interface. ,

PECL at silicon electrodes in water demonstrated that photogenerated carriers can directly drive ECL reactions, enabling optical interrogation of interfacial charge injection and relaxation dynamics at semiconductor/liquid junction (Figure ). This framework has since been expanded toward energy-relevant oxide photoelectrodes. On BiVO₄ photoanodes, PECL emission was shown to sensitively reflect surface reaction efficiency and interfacial kinetics-highlighting that only a fraction of photogenerated carriers is productively utilized at the reactive interface. Analogous behavior was reported on nanostructured hematite (α-Fe₂O₃), where PECL responses captured the interplay between recombination losses and surface reaction competence, linking defect/surface-state chemistry to photoelectrochemical performance.

6.

(a) Scheme showing an n-Si/SiO _x /Ni MIS photoanode during PECL (the side product of the TPrA radicals formed during the ECL process is denoted by p). (b) Scheme showing the Si/SiO _x /Ni electrodes employed in this work with ECL emission (λ_ECL). (c) IPCE spectra recorded on n-Si/SiO _x (green curve) and n-Si/SiO _x /Ni (1.5 nm) (purple curve) at 0.7 V vs SCE. (d) Normalized absorption spectrum of the electrolyte (black curve) and PECL emission spectra recorded on n-Si/SiO _x (green curve) and n-Si/SiO _x /Ni (1.5 nm) (purple curve) at 0.6 V vs SCE under illumination at λ_LED = 810 nm. These measurements were performed in PBS (pH 7.3) with 5 mM [Ru­(bpy)₃]²⁺ and 100 mM TPrA. Reproduced with permission from ref . Copyright 2019, American Chemical Society.

Recent progress has further broadened semiconductor and hybrid material scopes by pushing PECL into new spectral and architectural regimes. Anti-Stokes PECL enabled near-infrared excitation to access ECL readouts at photocathode platforms, providing a materials route to expand optical interrogation beyond visible excitation windows. Complementarily, a wireless semiconductor bipolar electrode architecture was introduced to realize Anti-Stokes PECL in a wiring-free configuration, demonstrating that semiconductor device design can directly regulate PECL excitation/collection and interfacial charge transfer. Hybrid metal–insulator-semiconductor (MIS) photoanodes further enabled ultrastable and site-selective upconversion behavior, underscoring the critical role of protection layers and junction energetics in stabilizing semiconductor interfaces.

4.2. Mechanistic Studies of Electrocatalytic Reactions by ECL

Early applications of ECL in electrocatalysis were often viewed with caution, as ECL was primarily considered a probe-based approach that reports catalytic activity indirectly through auxiliary luminophores rather than the catalytic reaction itself. This perception raised concerns regarding its applicability to mechanistic studies of energy-relevant electrocatalytic processes. In recent years, however, an increasing number of studies have shown that, when ECL is judiciously coupled to reactive intermediates and implemented with precise temporal control and spatial resolution, it can directly interrogate key steps of electrocatalytic reactions. , By enabling time-resolved and spatially resolved monitoring of reaction pathways, intermediate formation, and kinetic evolution, ECL has progressed from a qualitative activity reporter to a powerful mechanistic probe. , In the following sections, several representative and intensively investigated electrocatalytic reactions are selected to illustrate how ECL can be employed to elucidate mechanisms of electrocatalytic reactions.

4.2.1. Oxygen Reduction Reaction (ORR): Reaction Pathways and Reactive Intermediates Revealed by ECL

The ORR is a central process in electrochemical energy conversion systems such as fuel cells and metal-air batteries. , Owing to its multistep nature, ORR can proceed through either a four-electron (4e^–) pathway to water or a two-electron (2e^–) pathway that produces hydrogen peroxide as an intermediate or final product. Discriminating between these pathways and identifying reactive oxygen species (ROS) generated at electrified interfaces are therefore essential for catalyst evaluation and mechanistic understanding. While Bard and co-workers established the fundamental framework of ECL in the early 1970s, the mechanistic understanding of luminol-based ECL emerged later from studies building upon the classical luminol chemiluminescence system and adapting it to electrochemical triggering. Haapakka and Kankare demonstrated quantitative correlations between ECL intensity and peroxide yields measured by rotating ring-disk electrode (RRDE) techniques, validating ECL as an optical reporter of the 2e^– ORR pathway and bridging ECL signals with classical electrochemical metrics. It should be noted that quantitative correlations between ECL intensity and peroxide yield rely on controlled experimental conditions. In particular, the concentrations of luminophore and coreactant, photon collection efficiency, and the effective ECL quantum yield should remain constant, while variations arising from quenching or optical detection efficiency are assumed to be negligible. Under such conditions, ECL signals can be correlated with peroxide production measured by RRDE to provide quantitative insight into the 2e^– ORR pathway. However, when factors such as electrode geometry, local pH, or the accumulation of reactive oxygen species vary significantly, the relationship between ECL intensity and catalytic kinetics becomes system-dependent, and ECL responses should be interpreted primarily in a semiquantitative manner.

Recent developments have extended ORR-ECL studies into time-resolved and spatially resolved regimes. Zhang’s group introduced a time-dependent ECL (Td-ECL) strategy (Figure ), in which the temporal evolution of luminol emission following ORR initiation was analyzed to extract kinetic information on ROS generation. By combining optical transients with kinetic modeling, this approach enabled pathway discrimination between 2e^– and 4e^– ORR and provided insight into intermediate–mediated ORR kinetics using purely optical readout. Ma and co-workers combined ECL microscopy with scanning electrochemical probe techniques to achieve subparticle-level mapping of ORR activity and pathway selectivity, revealing facet-dependent behavior in which edge sites preferentially catalyze the 4e^– pathway while basal planes favor 2e^– ORR on single Au-based nanoplate. Related concepts have also been explored in catalyst design, where metal-free perylene-based polyimide frameworks were shown to enhance ECL emission through oxygen reduction–related pathways that promote the formation of reactive oxygen species involved in the luminol ECL process.

7.

(a) Responses to H₂O₂ at different concentrations using ECL and RRDE methods. Colored areas represent standard errors. (b) Reaction pathways of ORR in the modified Damjanovic model. (c) Brief setup of the Td-ECL method for ORR kinetics analysis and electrochemical/ECL signals at different time during Td-ECL measurement. The enlarged area and line charts show the distributions of dissolved O₂, ROS, and Ap* in the diffusion layer. Reproduced with permission from ref . Copyright 2023, Wiley-VCH.

Transient ECL microscopy (transient ECLM) has recently been introduced to image the establishment of ORR at individual nanocatalysts by converting transient, locally generated H₂O₂ into ECL photons using L-012 within a double-pulse-relaxation waveform. By separating the ORR-driving cathodic pulse from the anodic ECL-probing pulse and employing cycle-to-cycle relaxation with photon accumulation, transient ECLM isolates early Faradaic product formation from capacitive interference and resolves ORR evolution. This enables definition of a characteristic establishment time, which links steady-state kinetics with transient processes at the catalyst-electrolyte interface and provides deeper mechanistic insights into the rate-determining step of ORR.

Collectively, these ORR-ECL studies demonstrate that ECL can interrogate ORR pathways, ROS intermediates, and kinetic heterogeneity across ensemble, single-particle, and single-site levels. By converting intermediate formation into optical signals under operando conditions, ECL provides a mechanistically rich and spatially resolved perspective on ORR that complements and extends traditional electrochemical approaches.

4.2.2. Oxygen Evolution Reaction (OER): Intermediate Formation and Rate-Determining Steps Probed by ECL

The OER is the key anodic process in water splitting and rechargeable metal-air batteries, , and its sluggish kinetics arise from multistep proton-coupled electron transfer and strongly potential-dependent interfacial microenvironments. Because molecular oxygen and short-lived oxygenated species produced during OER can participate in the luminol-based ECL pathway as coreactants, the OER process can be indirectly visualized and monitored through ECL signals that reflect local oxygen generation dynamics. Building on this concept, recent studies have demonstrated that electrocatalytic oxygen reactions can be deliberately coupled to ECL as an optical readout of interfacial reaction activity. , For example, a Co₃O₄/NiCo₂O₄ heterojunction was reported to promote luminol anodic ECL under oxygen evolution reaction conditions, where OER-related and oxygen-involved reactive oxygen species, together with evolved O₂ participate in the luminol ECL pathway and thereby amplify photon output. Beyond this specific catalyst system, this work illustrates a general concept that catalytic regulation of OER kinetics can be quantitatively transduced into ECL photon output.

Recent studies have further established ECL imaging as a powerful optical readout for probing OER beyond conventional electrochemical measurements. ,, In a representative single-entity study, spatiotemporally resolved ECL imaging was correlated with visible absorption spectroscopy to investigate OER on individual cobalt layered hydroxide nanoplates, where short-lived oxygenated intermediates generated during OER acted as coreactants in the luminol ECL pathway, enabling direct visualization of local reaction activity and heterogeneity at the single-particle level. This approach revealed stepwise potential-dependent evolution of active sites and demonstrated that ECL can distinguish competing two- and four-electron OER pathways through their distinct intermediate signatures. Complementarily, ECL has also been employed as a chemically selective probe to disclose key OER intermediates. Using metal–organic frameworks as model catalysts, ECL signals derived from a luminol derivative were quantitatively linked to the formation and accumulation of OOH* species during OER, allowing the rate-determining step and spatially heterogeneous activity to be assessed from ECL onset potentials and intensity evolution. Reaction-pathway-resolved OER imaging was further achieved by constructing single Ag nanowire@ITO Mott–Schottky junctions, where interfacial charge redistribution selectively activated superoxide-mediated OER pathways that could be chemically distinguished by luminol ECL. This study demonstrates that ECL can sensitively report pathway switching governed by interfacial electronic structure rather than overall activity alone.

Zhang’s group further extended the scope of OER-ECL coupling by demonstrating that ECL can serve as a quantitative probe of local proton dynamics during oxygen evolution (Figure ). By developing a hybrid ECL emitter based on resonance energy transfer between robust carbon nitride nanosheets and pH-responsive dyes, they achieved real-time visualization of local pH gradients generated near OER electrocatalysts under operating conditions. In this system, proton accumulation produced during OER modulated the ECL intensity in a chemically selective manner, enabling subsecond-resolved mapping of interfacial acidification adjacent to catalysts such as RuO₂. Importantly, the ECL-derived pH profiles were further translated into quantitative descriptors of proton generation rates and apparent OER kinetics, revealing pronounced local acidification and its direct correlation with catalyst degradation.

8.

Experimental setup of ECL-based local H⁺ generation detection. (a) CN-FITC-coated electrode (yellow) was positioned close to a RuO₂-coated electrode (black). RuO₂ was biased at a constant potential to carry out OER and generate H⁺, while CN-FITC was repetitively biased at −1.2 and 0 V for 0.5 s, respectively. (b) Scheme of H⁺ diffusion from the RuO₂- to CN-FITC-coated electrode surface. (c) ECL pulse of the CN-FITC electrode. Pulse interval: 30 ms; potential: −1.2 V. (d) Theoretical (solid line) and experimental (dots) correlations between the measured ECL intensity and solution pH. (e) ECL profile on CN-FITC-coated electrodes positioned close to RuO₂-coated electrodes. The RuO₂-coated electrodes were biased at 1.1 V (black), 1.3 V (red), and 1.5 V (blue), while the CN-FITC-coated electrodes were repetitively biased at −1.2 and 0 V for 0.5 s each. (f) The pH profiles at different distances from the RuO₂-coated electrode over 180 s. Electrolytes: 0.1 M KCl and 0.005 M K₂S₂O₈. Reproduced with permission from ref . Copyright 2025, American Chemical Society.

By coupling OER-triggered oxygenated species and proton dynamics to light emission, ECL enables optical access to interfacial reaction activity, intermediate chemistry, and local microenvironment evolution with high chemical selectivity and spatiotemporal resolution. ,, Beyond visualizing oxygen generation, ECL-based approaches provide quantitative descriptors of reaction pathways, rate-determining steps, and proton-coupled kinetics that are difficult to resolve by conventional electrochemical techniques alone. These advances collectively establish ECL as a powerful platform for probing OER mechanisms at both ensemble and single-entity levels, opening new opportunities for mechanistic understanding and rational catalyst design.

4.2.3. Hydrogen Evolution Reaction (HER): Nanoscale Reaction Dynamics and Gas Evolution Visualized by ECL

Compared with OER, the hydrogen evolution reaction (HER) introduces an additional layer of complexity for optical interrogation because gas nucleation, growth, coalescence, and detachment dynamically reshape the local electric field, mass transport, and interfacial wettability. ECL offers a distinctive route to visualize these processes, as photon generation can be coupled either to catalytic activity at nanoscale sites or to bubble-induced microenvironment modulation. A seminal example is the “ECL blinking” strategy developed by Ma et al. to monitor HER at the single-nanoparticle level (Figure ), where stochastic H₂ nanobubble formation on individual hollow carbon nitride nanospheres intermittently switched ECL emission ON/OFF, thereby translating bubble generation-growth-collapse events into time-resolved optical trajectories. Importantly, the power-law statistics of the ON/OFF durations revealed heterogeneous and intermittent reactivity among multiple catalytic sites on a single particle, highlighting ECL as an optical reporter of nanoscale reaction dynamics rather than an averaged current response. Complementarily, Qin et al. demonstrated that HER activity on single Pt nanoparticles can be spatially mapped using a bipolar nanoelectrode array, in which HER occurring on individual bipolar nanoelectrodes was converted into a spatially resolved ECL signal with submicrometer resolution. Beyond serving as a readout of catalytic turnover, ECL is also sensitive to the presence of gas cavities themselves: ECL imaging has been used to show that surface-adhered bubbles create distinct interfacial microenvironments (e.g., ionic enrichment in the bubble “corona”) that can markedly alter local reactivity and the onset of oxidation chemistry, underscoring that gas evolution is not merely a transport artifact but an active determinant of interfacial electrochemistry. In HER studies, ECL provides mechanistic insight by translating bubble nucleation–growth–detachment dynamics and catalytic site heterogeneity into spatiotemporally resolved photon trajectories. These observations reveal nanoscale heterogeneity of catalytic activity and the dynamic coupling between bubble evolution and interfacial reaction environments.

9.

(a) Schematic illustration of the ECL microscopy setup. Colocalization analysis of the ECL image (b), FL image (c) and SEM image (d) of a same set of HCNSs. Electrolyte: 100 mM K₂SO₄ aqueous solution containing 100 mM K₂S₂O₈ coreactant. The ECL image is an accumulation of all bright original ECL images (exposure time of EMCCD, 1 s) within the first five cyclic voltammetry scans (scan range from 0 V to −1.5 V; scan rate, 0.1 V/s) to improve the signal-to-noise ratio. The exposure time of FL image is 1 s. Reproduced with permission from ref . Copyright 2020, American Chemical Society.

Collectively, these advances establish ECL as an emerging modality for HER that can simultaneously report single-site catalytic heterogeneity and bubble-regulated reaction microenvironments, enabling mechanistic insights into gas-evolving electrocatalysis that are difficult to extract from conventional ensemble electrochemical measurements alone.

4.3. Mechanistic Probing of Photocatalytic and Photoelectrocatalytic Processes by PECL

With the continued development of PECL, its application to photocatalytic and photoelectrocatalytic systems has entered a phase of rapid expansion. Beyond serving as an auxiliary optical signal, modern PECL has evolved into a mechanistically informative probe that links photoinduced charge generation, separation, and interfacial utilization to photon emission through well-defined chemical pathways. By selectively reporting the fraction of photogenerated carriers that participate in interfacial redox reactions, PECL enables direct interrogation of charge-carrier dynamics that are difficult to disentangle using conventional photocurrent or spectroscopic measurements alone. ,, The integration of PECL with spatiotemporal imaging, light modulation, and device-level architectures has further broadened its scope, allowing photocatalytic activity, carrier recombination, and interfacial reaction heterogeneity to be visualized with high sensitivity. ,− As a result, PECL has become an increasingly powerful platform for probing mechanisms in photocatalytic and photoelectrocatalytic energy-conversion processes, as discussed in the following sections.

4.3.1. PECL-Based Measurements of Photoinduced Charge-Carrier Dynamics

In photocatalytic and photoelectrocatalytic energy systems, performance is dictated by how efficiently photogenerated minority carriers are separated, transported, and finally injected into interfacial redox reactions. Yet conventional photocurrent measurements often conflate bulk transport with surface kinetics and provide limited, ensemble-averaged insight into carrier utilization at reactive interfaces. PECL offers a distinctive measurement modality in which photoexcited carriers are transduced into ECL photon output through a well-defined chemical pathway, thereby selectively reporting the fraction of carriers that actually reach the interface and participate in electron-transfer chemistry. , A landmark in this direction was established by Sojic’s teams in the first aqueous PECL implementation at silicon electrodes, where photogenerated holes enabled the classical [Ru­(bpy)₃]²⁺/TPrA ECL at markedly reduced applied bias, making the PECL onset and intensity evolution sensitive descriptors of minority-carrier injection efficiency rather than overall current response.

This PECL-based “carrier-to-photon” readout was subsequently generalized to metal-oxide photoelectrodes relevant to solar fuels. For instance, BiVO₄ photoanodes exhibited pronounced PECL amplification at low bias, enabling optical quantification of interfacial carrier utilization under illumination, while nanostructured hematite electrodes further highlighted how PECL onset and intensity evolution sensitively reflect recombination losses and surface reaction efficiencies in sluggish oxide systems. Beyond steady-state bias-assisted schemes, PECL also enabled energetics-oriented interrogation through anti-Stokes configurations and ultrastable MIS photoanodes, where photovoltage and PECL efficiency provide quantitative handles on carrier separation and injection under prolonged operation. − Importantly, the same mechanistic principle has recently been extended to AO-ECL, in which light alone addresses both carrier generation and the ECL readout at semiconductor diode platforms, enabling purely optical quantification of charge injection/relaxation dynamics without external wiring.

4.3.2. Spatiotemporal Imaging of Photocatalytic and Photoelectrocatalytic Interfaces by PECL

In photocatalytic and photoelectrocatalytic energy systems, spatial heterogeneity in charge-carrier generation, separation, and interfacial transfer plays a decisive role in governing overall performance. Local variations in crystal facets, defect distributions, band bending, and junction architecture can lead to highly nonuniform carrier utilization, which is often obscured in ensemble-averaged photocurrent measurements. By coupling photoexcitation with ECL readout, PECL imaging provides a direct strategy to visualize where photogenerated carriers effectively drive interfacial redox reactions. , A first class of PECL-based imaging approaches relies on light-guided or light-patterned photoelectrodes, in which illumination itself defines the spatial domain of electrochemical reactivity. Using oxide photoanodes relevant to solar fuel conversion, such as hematite, dynamic light guidance has been shown to confine PECL emission to illuminated regions, enabling mask-free and reconfigurable mapping of photoanodic reaction zones without physical electrode patterning. In these systems, the spatial extent and temporal evolution of PECL emission directly reflect the balance between carrier diffusion, recombination, and surface reaction kinetics, providing mechanistic insight into how illumination geometry and local electronic structure regulate photoelectrochemical activity.

Beyond ensemble photoelectrodes, PECL imaging has been extended to resolve nanoscale heterogeneity in individual photoelectrocatalysts. In a representative advance, PECL microscopy (PECLM) was introduced to image photoinduced electrochemical processes at single nanocatalysts, converting localized carrier-driven redox reactions into spatially resolved photon emission. , By selectively reporting interfacial charge-transfer events rather than bulk photogeneration, PECLM revealed pronounced particle-to-particle variations in carrier utilization efficiency, highlighting intrinsic heterogeneity that is inaccessible to conventional photocurrent or ensemble optical measurements. More recently, PECL imaging concepts have been further generalized through fully optical or all-optical implementations, in which light alone governs both spatial addressing and ECL readout at semiconductor interfaces. , In energy-relevant photoelectrodes, such approaches enable spatiotemporal control of reaction zones without wired microelectrode arrays, offering a flexible platform to interrogate carrier injection and relaxation dynamics across extended surface (Figure ). These all-optical strategies conceptually bridge PECL with light-addressable electrochemistry, further expanding the design space for imaging charge-carrier dynamics in photoelectrocatalytic systems.

10.

(a) Scheme of the PECL microscopy setup. (b) Top-view SEM picture showing the Ni catalyst pattern deposited onto n-Si, with NiNPs of diameters ranging from 5 μm to 500 nm (note that NiNPs with lower diameters are located below but are not visible in this image, as indicated by the right arrow). (c–f) PECL images recorded at different potentials: 0.2 (c), 0.4 (d), 0.6 (e), and 0.8 V (f). (g) PECL intensity profile extracted for the NiNPs indicated by the white square in (f); inset: magnification of the plot for y-coordinates >100 μm (y-axis is indicated in (b)). Reproduced with permission from ref . Copyright 2025, American Chemical Society.

Collectively, these developments establish PECL imaging as a versatile methodology for mapping charge-carrier generation, separation, lateral transport, and interfacial transfer in photocatalytic and photoelectrocatalytic energy systems. By transforming local carrier utilization into spatially and temporally resolved photon maps, PECL-based imaging complements traditional electrochemical techniques and provides mechanistic insight into how nanoscale heterogeneity governs macroscopic energy-conversion performance.

5. Conclusion and Outlook

ECL has evolved from a highly sensitive analytical detection technique into a versatile measurement and imaging framework for energy conversion catalysis. As summarized in this review, recent advances in ECL and PECL have enabled direct visualization and quantification of catalytic activity, heterogeneity, and reaction pathways across a broad range of electrocatalytic and photoelectrochemical systems. By converting interfacial electron-transfer events and reactive intermediate formation into photon emission, ECL provides a unique electrochemical–optical transduction route that complements conventional electrochemical and spectroscopic techniques, particularly in resolving spatial heterogeneity and transient behavior that are obscured in ensemble measurements.

5.1. Future Directions in ECL/PECL Methodology

Looking forward, continued development of ECL and PECL methodologies is expected to further expand their capability for probing catalytic interfaces. One central direction lies in advances in measurement and imaging technologies. Improvements in temporal control, synchronization, and signal acquisition are enabling ECL to move beyond quasi-steady-state observation toward direct probing of fast transient electrochemical processes. These include the early stages of reaction initiation, intermediate accumulation, and kinetic establishment at catalytic interfaces. Access to such short-lived regimes is essential for identifying rate-determining steps and dynamic reaction pathways that ultimately govern macroscopic catalytic performance.

Further enhancement of spatial resolution and imaging precision will also be critical for capturing the intrinsic heterogeneity of catalytic materials under operating conditions. Catalysts rarely function as static entities; instead, their active sites, local coordination environments, and interfacial microstructures may evolve continuously during electrochemical or photoelectrochemical operation. The ability of ECL and PECL to visualize such changes at the level of individual particles or nanoscale domains provides a promising route to correlate structural evolution with catalytic activity and stability. Continued progress in super-resolution imaging, single-photon detection, and light-addressable excitation schemes is expected to further narrow the gap between optical observables and the true spatial scales of catalytic phenomena.

5.2. Toward Operando Realism and Probe-Aware Measurements

Another important direction is the integration of ECL and PECL with increasingly realistic reaction environments. Extending measurements toward higher current densities, complex electrolytes, dynamic potential or illumination waveforms, and coupled mass-transport processes will allow optical approaches to interrogate catalytic reactions under conditions closer to those encountered in practical energy conversion systems.

At the same time, the luminophore–coreactant chemistry used for optical readout may itself influence the interfacial electrochemical environment. For example, luminol-based systems are strongly coupled to ROS chemistry, because luminol emission is promoted by oxygen-derived reactive species such as superoxide and peroxide-related intermediates. As a result, the luminol reaction network can consume, amplify, or redistribute ROS intermediates, thereby altering the local ROS balance near catalytic interfaces. In addition, luminol reactions are strongly pH dependent, and the radical-mediated steps involved in luminol activation may locally perturb proton or hydroxide concentrations, particularly in weakly buffered electrolytes.

Other coreactant systems may introduce different perturbations. Persulfate-based ECL involves the generation of sulfate radical species, which are strong oxidants and may modify interfacial redox pathways or alter local charge distribution within the electrochemical double layer. TPrA, a widely used coreactant in anodic ECL systems, forms reactive radical intermediates that can participate in proton-coupled electron-transfer processes and may influence local proton activity or interfacial ion organization. In gas-evolving reactions such as HER or OER, the presence of probe molecules and their intermediates may also affect bubble nucleation, growth, or detachment by modifying local wettability, pH gradients, or gas supersaturation conditions near catalytic surfaces.

These considerations imply that the reliability of ECL and PECL readouts depends on maintaining experimental conditions under which probe chemistry remains subordinate to the catalytic process of interest. In practice, mechanistically reliable ECL measurements are generally more readily achieved at moderate current densities, where the reaction remains closer to the kinetic regime and probe-driven effects are less likely to be amplified by severe mass-transport limitations. Buffered electrolytes are also preferred to minimize strong local pH gradients arising from either catalytic reactions or probe activation pathways. For luminol-based systems, mildly alkaline conditions are typically required to sustain efficient emission while avoiding excessive pH fluctuations near the electrode interface. By contrast, persulfate- and TPrA-based systems can be compatible with a wider range of conditions depending on the emitter and interfacial reaction chemistry, but they may become increasingly sensitive to radical accumulation or interfacial charge redistribution at high reaction rates. Under conditions involving very high current densities, strong local pH shifts, or vigorous gas evolution, ECL signals may increasingly reflect probe-coupled chemistry rather than intrinsic catalytic processes, and should therefore be interpreted with caution.

For this reason, rigorous experimental controls remain essential when deploying ECL or PECL under operando-like conditions. Systematic variation of luminophore and coreactant concentrations, comparison with independent electrochemical measurements such as RRDE, and control experiments performed in the absence of the probe species can help verify that observed photon signals originate from intrinsic catalytic activity rather than probe-induced artifacts. Continued progress in probe design, calibration strategies, and standardized validation protocols will be crucial for enabling ECL and PECL measurements under increasingly realistic catalytic environments.

5.3. Expanding ECL/PECL to Emerging Energy Conversion Reactions

Beyond the extensively studied ORR, OER, and HER systems, an important future direction is to extend ECL and PECL methodologies to more complex energy conversion reactions, such as CO₂ reduction (CO₂RR), nitrogen reduction (NRR), and organic electrosynthesis. These reactions are typically governed by competing pathways and short-lived intermediates, making mechanistic analysis particularly challenging using conventional electrochemical measurements alone. In this context, spatially and temporally resolved optical reporting could provide valuable insight into intermediate formation, pathway competition, and local catalytic heterogeneity under operating conditions.

For CO₂RR, ECL probes that respond to key reduction products or oxygenated intermediates may enable spatially resolved mapping of pathway selectivity and local activity during CO₂ conversion. In the case of NRR, coupling ECL detection with ammonia formation or nitrogen-containing products could potentially provide an alternative strategy for visualizing catalytic sites involved in nitrogen activation. Realizing these possibilities will require probe chemistries with improved intermediate or product specificity, as well as careful management of electrolyte complexity and probe-induced perturbations. Continued advances in reaction-selective probe design and validation with complementary analytical techniques will be essential for extending ECL-based approaches to these emerging catalytic systems.

In summary, ECL and PECL are evolving from simple activity reporters into increasingly informative probes of reaction dynamics in energy conversion catalysis. Continued advances in temporal and spatial resolution, together with probe-aware experimental design and integration into realistic operating environments, will further strengthen their capability to reveal how catalytic materials function, evolve, and ultimately fail under working conditions. Such developments are expected not only to deepen mechanistic understanding of electrocatalytic and photoelectrochemical processes, but also to provide new experimental foundations for the rational design and optimization of next-generation energy conversion catalysts.

Glossary

Vocabulary

Electrochemiluminescence (ECL)

ECL is light emission generated by electrochemical reactions that produce excited states of luminophores at electrode interfaces. The emitted photons arise from the relaxation of these electrochemically generated excited species to the ground state.

Photoinduced electrochemiluminescence (PECL)

PECL refers to ECL processes in which photoexcitation generates charge carriers in a semiconductor or catalyst that subsequently participate in electrochemical reactions leading to light emission. It couples photophysical excitation with electrochemical pathways to probe photoelectrochemical processes.

All-optical electrochemiluminescence (AO-ECL)

AO-ECL refers to ECL processes in which both excitation and readout are controlled optically without applying an external electrical bias. Light irradiation generates charge carriers in a photoactive electrode or semiconductor that drive redox reactions leading to ECL emission.

Operando imaging

Operando imaging refers to visualization techniques that monitor physical or chemical processes in real time while the system is actively operating under working conditions. In electrochemical studies, it enables direct observation of interfacial reactions during ongoing electrochemical reactions.

Spatiotemporal resolution

Spatiotemporal resolution describes the combined ability of an imaging technique to resolve both spatial details and dynamic changes over time. High spatiotemporal resolution allows processes occurring at small length scales and short time scales to be captured simultaneously.

The authors declare no competing financial interest.