Skip to main content
NCBI home page
Royal Society of Chemistry logoLink to Royal Society of Chemistry
. 2026 Oct 27. Online ahead of print. doi: 10.1039/d6cp01946c

A mesoporous electrode strategy for probing ions at the electrode–electrolyte interface using in situ X-ray absorption spectroscopy

Jan Sebastian Dominic Rodriguez a,b, Alfred Larsson a, Nipon Deka c, Rianne Jorritsma a, Hassan Javed a, Kees Kolmeijer a, Ajsa Cuprija d, Wanli Liu e, Nicole Barber e, Adam M Squires e, Detre Teschner f,g, Aliakbar Ghafari f,g, Cheng-Hao Chuang b, Rik V Mom a,✉
PMCID: PMC13726401  PMID: 42969468

Abstract

Electrolyte ions present at the electrode–electrolyte interface critically influence electrocatalytic reactions, yet direct, quantitative detection of interfacial ions under operating conditions remains challenging due to dominant bulk electrolyte signals. Here, we present an interface-sensitive soft X-ray absorption spectroscopy (XAS) approach that enables quantitative tracking of interfacial ion behavior at electrified platinum (Pt) surfaces. Mesoporous Pt films with high surface-area-to-volume ratios were fabricated via soft-templated electrodeposition onto SiNx windows, strongly enhancing the relative contribution of the interfacial electrolyte within nanoscale pores while effectively attenuating signals from the bulk. Using Na+ as a model cation, Na K-edge XAS measurements reveal pronounced dependencies of interfacial ion concentration on applied potential, electrolyte pH, and bulk concentration. The observed trends highlight the role of surface chemistry in interfacial ion behavior. In addition to concentration changes, potential-dependent modifications of the Na+ hydration shell are identified, indicating re-organization of the solvation of the ions at the interface. These results highlight the capability of our methodology to quantitatively probe ion-specific behavior in the electrical double layer under operando conditions. We foresee that the method can be extended to a wide range of ionic and molecular species relevant to electrocatalysis and energy conversion.

Electrolyte ions at the interface critically influence electrocatalysis, yet their direct, quantitative detection during operating conditions remains challenging due to dominant bulk electrolyte signals.graphic file with name d6cp01946c-ga.webp

Introduction

For electrochemical reactions such as the hydrogen evolution (HER), oxygen evolution (OER), and oxygen reduction (ORR), ions present at the electrode–electrolyte interface have a strong impact on the catalytic activity.1–5 Exploiting this, the type and concentration of ions in the electrolyte, and the electrolyte's pH have all been used to tune electrocatalytic performance. These effects stem from the interactions of water molecules, ions, and adsorbates at the electrode–electrolyte interface. However, the detailed structure of this interface, and how it changes with potential and surface chemistry, is still not fully understood.

In particular, the behavior of cations near the electrode surface has proven challenging to resolve. In the bulk electrolyte, cations are solvated and well-distributed. However, at the charged electrode surface, cations can be attracted or repelled and may be either solvated or partially desolvated, depending on the applied conditions.6–8 This interfacial ion behavior has a significant impact on electrocatalysis, as the interfacial ion structure tunes the catalytic reaction environment by changing the local electric field, modifying the interfacial water structure, or by stabilizing intermediates through ion–adsorbate interactions.1,2,5 Therefore, it is important to quantify how many cations are located near the surface during reaction conditions.

A main experimental challenge to detect interfacial cations is that the electrode–electrolyte interface, where they are located, is only a very small part of the whole electrochemical cell. For most characterization techniques, the signal primarily comes from the bulk electrolyte, making it difficult to isolate signals from the interface. To solve this, several interface-sensitive techniques have been developed, which use specific types of light-matter interactions to induce interface sensitivity.9 For example, surface X-ray diffraction (XRD) exploits total reflection,10–12 sum-frequency generation (SFG) spectroscopy uses interface-specific selection rules,13–15 and plasmon-based methods such as surface-enhanced Raman spectroscopy (SERS) and surface-enhanced infrared spectroscopy (IRAS) enhance signals locally through electronic resonance.4,11,16 While each of these strategies has successfully provided insight into interfacial ion behavior, they do have limitations in the types of ions and electrodes that can be studied. Furthermore, it is often difficult to quantify the interfacial ion concentration from the spectra for a series of potentials and electrolytes, which is crucial to relate interfacial ion behavior to catalytic performance trends.

Here, we present an interface-sensitive X-ray absorption spectroscopy approach that enables such quantitative tracking of interfacial ion behavior. We use mesoporous Pt electrodes prepared by soft templating to increase the amount of electrode–electrolyte interface, thereby strongly favoring interfacial ion signals over the bulk signal in (soft) X-ray absorption spectroscopy (XAS). We exemplify this with the case study of Na+–Pt interactions, showing how both the applied potential and electrolyte pH strongly impact the interfacial Na+ concentration.

Methods

Mesoporous electrode preparation

To obtain electrodes that are suitable for interface-sensitive XAS, mesoporous Pt films were electrodeposited on a 20 nm Au/SiNx chip (Silson, 100 nm 1 × 1 mm2 window) via a soft templating process.17–19 Prior to the deposition, the SiNx chip was cleaned by soaking in a 0.5 M H2SO4, 1 g L−1 KMnO4 solution, followed by soaking in a dilute Piranha solution and rinsing with ultrapure water. The plating solution was prepared by dissolving 0.125 g of Brij 58 surfactant and 1.2 mL of 8 wt% hexachloroplatinic acid (HCPA) in 10.77 mL of ultrapure water. The SiNx chip was submerged into a glass beaker containing the plating solution, except for a small corner used to make electrical contact. The reference and counter electrodes of the deposition cell were an Ag/AgCl electrode (Innovative Instruments LF-1.6-48 leak-free Ag/AgCl) and a Pt wire, respectively. Deposition of Pt was carried out at 0.27 VRHE unless stated otherwise. As the SiNx chip itself is electrically insulating, electrodeposition only occurred on the Au-coated part of the chip. After electrodeposition, the surfactant template was removed by repeated soaking and washing in ethanol and 1 : 1 ethanol–water mixtures, leaving behind a Pt film with mesopores.

Electrode characterization

The electrochemically active surface area (ECSA) of the mesoporous Pt films was determined using the hydrogen underpotential deposition (Hupd) charge in 0.1 M HClO4, employing the known Hupd charge of 210 uC cm−2 as conversion factor.20–22 Assessment of the film thickness and morphology were carried out using scanning electron microscopy (SEM) using a Zeiss EVO microscope. Further structural characterization was performed using a Anton Paar SAXSpoint 2.0 setup was operated using a Cu source beam (1.54 Å, 8 keV) with detector distance of 350 mm, to obtain small angle X-ray scattering (SAXS) 1-D profiles.

In situ X-ray absorption measurements

The in situ XAS measurements were performed at the ISISS beamline endstation at the BESSY II synchrotron facility in Berlin, Germany, which provides monochromatized X-ray photon beams in the soft X-ray regime (80–2000 eV) using a bending magnet (D41) and a plane grating monochromator (PGM). The total fluorescence yield (TFY) signal was recorded using photodiode detectors, with incidence and exit angles of 57° and 55° vs. sample normal, respectively. Since the recorded spectra were measured at the Na K-edge, the analysis chamber was kept under high vacuum conditions.

The spectro-electrochemical flow cell features a Pt wire as the counter electrode and an Ag/AgCl electrode (Innovative Instruments LF-1.6-48 leak-free Ag/AgCl) as the reference electrode. The cell can accommodate 1 × 1 cm2-frame SiNx chips. In this study, we used chips with Au coating and a 1 × 1 mm2 100 nm window (Silson). Electrolyte circulation was achieved by delivering Ar purged electrolyte solutions into the flow cell using a peristaltic pump. Electrochemical control was provided by an SP-200 BioLogic potentiostat, enabling the application of standard electrochemical techniques, including cyclic voltammetry (CV) and chronoamperometry during operando spectroscopic measurements.

Prior and during the measurements, several checks were regularly performed to verify the proper wetting, cleanliness, stability and electrochemical behavior of the system. These are described in detail in SI Section S2. A detailed description of the XAS data treatment is provided in SI Section S3.

Results

Fig. 1 displays the experimental geometry of the interface-sensitive XAS experiments. The basis of the experiment is the SiNx window type flow cell that is available at several beamlines,23–30 in which a SiNx window provides a high-vacuum leaktight separation between the liquid environment in the cell and the vacuum of the spectroscopy chamber. The mesoporous Pt film was coated onto the SiNx window, serving as the working electrode. In this geometry, incident X-rays entering the cell first travel through the mesoporous film before they reach other parts of the cell. If the Pt film is sufficiently thick, the X-ray beam is fully scattered and absorbed within the mesoporous film, so that only the mesoporous film and the electrolyte inside the pores are probed in the XAS measurements. Due to the mesoporous structure of the film, a large fraction of this in-pore electrolyte is in contact with the Pt electrode. As a result, the (fluorescence yield) XAS signal from the absorption edges of the ions in the pores is strongly interface-sensitive. In the following, we will show that this makes it possible to detect subtle changes in interfacial ion concentration and solvation structure.

Fig. 1. Schematic diagram of the interface-sensitive XAS approach to probe the electrode–electrolyte interface. (a) Sample and (b) liquid flow cell geometries, where the working electrode is composed of (c) a mesoporous Pt film deposited on an Au-coated SiNx window with a Ti seed layer. (d) Schematic diagram of the possible adsorbate and electrolyte structures inside the pores.

Fig. 1

Open in a new tab

A key factor in our measurement strategy is that only the electrolyte in the pores should be probed, and not the bulk electrolyte behind the mesoporous film. To this end, the mesoporous film thickness should be chosen such that X-rays are traveling to and from the bulk electrolyte are (almost) fully attenuated. The relative contribution of pore and bulk ion signal can be calculated using:

graphic file with name d6cp01946c-t1.webp 1
graphic file with name d6cp01946c-t2.webp 2

where Ipore and Ibulk are the fluorescence signal intensities from ions inside the pores, and in the bulk, respectively. λinPt and λoutPt are the attenuation lengths of the incident and the emitted X-rays inside the mesoporous Pt catalyst, respectively. Inline graphic and Inline graphic are the attenuation lengths of the incident and the emitted X-rays within the bulk, respectively. Values for the attenuation lengths of both the mesoporous film and bulk electrolyte can be calculated as described in SI Section S1, exploiting the CXRO database.31cionfilm and cionbulk are the concentrations of electrolyte ions within and outside the pores of the film, respectively. Finally, ϑ and ϕ are the X-ray incidence angle and fluorescence collection angle, respectively.

The calculated fluorescence contribution from within the pores increases with film thickness, approaching unity once the majority of the excitation and emitted photons originate from within the porous layer. As shown in Fig. 2a, variations in incident and exit angles slightly shift the thickness required to reach full pore contribution, reflecting changes in the effective X-ray path length through the film. In contrast, Fig. 2b demonstrates a stronger dependence on the attenuation lengths of different electrolyte ions. Species with lower energy edges (e.g. Na), and therefore shorter X-ray attenuation lengths, reach near-complete pore contribution at smaller porous film thicknesses, while ions with longer attenuation lengths (e.g. K) require substantially thicker films before the fluorescence signal becomes dominated by the pore volume.

Fig. 2. Ion signal contribution from inside the pores for varying mesoporous Pt film thickness. (a) Calculated pore signal contribution, Ipore/Itotal, as a function of catalyst film thickness for different incident and exit angles relative to the surface normal. (b) Calculated pore signal contribution for representative commonly used electrolyte ions (Na, S, Cl, and K) at their respective K-edges. X-ray attenuation length around the Na K-edge of (c) bulk NaOH electrolyte solutions and (d) a mesoporous Pt layer with NaOH within the pores at varying concentrations.

Fig. 2

Open in a new tab

For quantitative analysis of the interfacial ion concentration, it is important to ensure that there is a linear relationship between the ion concentration in the pores and the XAS intensity. This condition is met if the ions do not contribute significantly to the overall X-ray attenuation (i.e. there is essentially no self-absorption). As shown in Fig. 2c, in bulk electrolyte (i.e. without the presence of the mesoporous Pt), there is a clear effect on the attenuation length for concentrated electrolytes (∼4.5% change in attenuation length across the Na K-edge), whereas the effect is minimal in dilute solutions (∼0.3% change in attenuation length). Within the mesoporous Pt film, the effect of X-ray absorption by Na+ on the attenuation length is much smaller, because the Pt film dominates the overall X-ray attenuation (see Fig. 2d). Even at a high in-pore ion concentration of 1 M, the X-ray attenuation length hardly changes across the Na K-edge. Therefore, self-absorption effects are expected to play a role only for extremely concentrated electrolytes (e.g. water-in-salt electrolytes or ionic liquids).

As discussed above, the preparation of mesoporous Pt films with controlled thickness and pore size is crucial to facilitate XAS measurements on interfacial ions. Such controlled synthesis is possible using soft-templated Pt electrodeposition. In this methodology, self-assembling surfactant structures such as micelles are used to create nanometric voids during Pt electrodeposition (Fig. 3a). Here, we used a robust and straightforward one-pot synthesis process with Brij-58 micelles as the template.18,32 The amount of Brij-58 that was added in a dilute solution of HCPA is just slightly above its critical micelle concentration (∼0.8 wt.%) to ensure formation of independent micelles. Under these conditions, Pt4+ precursor species preferentially associate with the outer shell of the micelles, where the hydrophilic heads of the Brij-58 chains are located.18 As electroreduction proceeds, Pt nucleates at the electrode surface and grows around the surfactant assemblies, forming a continuous metallic framework. Subsequent removal of the surfactant yields a mesoporous Pt structure with a well-defined pore structure, as inferred from the pronounced peak in the 1-D SAXS profile in Fig. 3b. The peak maximum at ∼0.82 nm−1 implies a dominant pore-to-pore distance of ∼7.7 nm. At lower q values, no further features are observed, indicating that the pore network is disordered. However, at the larger scale, it can be observed that the film grows extremely uniform (Fig. 3c and d). In the context of the interface-sensitive XAS this is highly relevant, as it means that a well-defined film thickness can be chosen.

Fig. 3. Formation of mesoporous Pt films. (a) Concept of synthesis via soft templating process using a non-ionic surfactant (Brij-58). The blue circle represents the hydrophilic head of the surfactants. (b) SAXS profile of a 500 nm-thick Pt film. (d) In-plane and (c) cross-sectional SEM images of mesoporous Pt films.

Fig. 3

Open in a new tab

By tuning the electrodeposition parameters, both the pore structure and thickness of the mesoporous Pt films can be optimized. To identify an appropriate deposition potential, the electroreduction behavior of HCPA was first characterized using linear sweep voltammetry (LSV) (Fig. 4a). A gradual onset of Pt electrodeposition current is observed below 0.5 VRHE. At lower potentials, the Pt reduction current is convoluted with features near 0.08 and 0.27 VRHE, arising from hydrogen adsorption on the freshly deposited Pt.33,34 Below 0 VRHE, hydrogen evolution also contributes to the current. This is problematic, as it skews the relation between the current that has passed (charge density) and the total amount of deposited Pt. To stay outside of hydrogen evolution region, and to reduce mass transport limitations induced by high Pt deposition rates that could lead to inhomogeneous deposition, we estimate the practical potential window for deposition to be from 0.15 VRHE to 0.45 VRHE. Fig. 4b shows the current during electrodeposition at 0.27 VRHE. After an initial sharp drop in current due to local consumption of the HCPA, the current transient stabilized into stable steady-state deposition. To quantify the electrochemically active surface area (ECSA) of the resulting film, cyclic voltammograms were recorded in 0.1 M HClO4 (Fig. 4c), and the charge associated with Hupd was used as a measure of accessible Pt surface sites. The roughness factor was then defined as the ratio of the ECSA to the geometric electrode area, providing a quantitative descriptor for the surface area amplification induced by the mesoporosity of the film.

Fig. 4. Electrodeposition parameters of mesoporous Pt film preparation. (a) Linear scan voltammetry profile in 0.8 wt% HCPA + 1 wt.% Brij-58, recorded at 5 mV s−1. The orange arrow shows the direction of the scan. (b) Current and charge densities during electrodeposition at 0.27 VRHE. (c) Typical cyclic voltammogram in 0.1 M HClO4 to assess the ECSA of the resulting Pt films. (d) Thickness and roughness factor vs. amount of deposited material, expressed in deposition charge density (C cm−2). The thickness values were obtained from cross-sectional SEM images.

Fig. 4

Open in a new tab

By varying the deposition time, and thereby the deposition charge density, both film thickness and roughness factor can be controlled (Fig. 4d). The film thickness, determined independently by scanning electron microscopy (SEM), and the roughness factor both exhibit a linear dependence on the deposited charge. This scaling behavior indicates consistent, uniform growth of the porous network during deposition. This means that variation of the deposition time allows tuning of the film thickness, while maintaining a constant pore morphology.

To obtain more detailed insight into the pore morphology of the mesoporous Pt film, we combined SEM data, deposition charge, and roughness factor to calculate the porosity and average pore size, assuming a cylindrical pore geometry:

graphic file with name d6cp01946c-t5.webp 3

With tmeasured the film thickness measured by SEM and tcalculated film thickness expected for a non-porous Pt film based on the deposition charge (236 nm per (C cm−2)−1).

graphic file with name d6cp01946c-t6.webp 4

With dpore, Vpore and Apore the pore diameter, volume and area, respectively, and RF the roughness factor. For the deposition at 0.27 VRHE, we find a porosity of approximately 31% and pore size of ∼4 nm.

To vary the pore morphology, the deposition potential can be used. When the potential is lowered below 0.27 VRHE, the deposition current increases, which can affect the degree to which the accessible space around the surfactant micelles is fully filled with Pt. Indeed, deposition at slightly lower potential, 0.22 VRHE, and correspondingly a larger Pt reduction current (∼1.7 mA cm−2) leads to a more open pore structure with a film porosity of ∼50% and pore size of ∼9.7 nm. We attribute this increase in porosity and pore size to “messier” growth at higher current density, where the space around the micelles is filled to a lesser extent. Another contributing factor is the increasing level of hydrogen adsorption on the growing Pt film, which has also been shown to affect the deposition process.34

The pore size and porosity that can be tuned through the deposition potential affect the in situ XAS measurements in several ways. From the electrochemical point of view, the higher porosity and pore size produced at lower potential is advantageous, as such an open film structure facilitates better mass transport under electrocatalytic conditions and more bulk-like electrolyte behavior. In addition, the porosity affects the probing depth of the XAS measurements. This can be quantified using eqn (1) and (2), considering the X-ray attenuation length calculated using SI Section S1. For our experimental geometry the attenuation length at the Na K-edge is ∼158 nm for the film deposited at 0.27 VRHE, whereas it is ∼214 nm for the film deposited at 0.22 VRHE. Hence, a thicker film is required to attain full attenuation of the X-rays within the film if it has a more open structure. Finally, the pore size of the film also impacts the degree of interface-sensitivity of the measurements. For example, if we approximate the pores as cylindrical and assume an electrical double layer thickness of 0.5 nm, than 44% of the electrolyte in the film is part of the double layer for the 4 nm pores produced at 0.27 VRHE. For the 9.7 nm pores produced at 0.22 VRHE, this is 20%. In our proof-of-concept here, we chose to optimize for maximum interface sensitivity. Therefore, we proceeded with films deposited at 0.27 VRHE.

To demonstrate how the mesoporous Pt film facilitates soft XAS studies on interfacial ions, we tracked the behavior of Na+ ions at the Pt-electrolyte interface in several electrolytes. The mesoporous Pt films deposited on SiNx chips were mounted in a polyether ether ketone (PEEK) flow cell using a metal holder that also served as electrical contact. The cyclic voltammogram recorded in the flow cell in 0.1 M NaOH (Fig. 5b) shows the sharp Hupd and oxide features typical of clean polycrystalline platinum in alkaline media, confirming the clean, bubble-free assembly of the cell and proper electrical contact. To facilitate alignment of the X-ray beam, Ag paste was applied to the SiNx chip to cover all regions except the thin membrane window (see Fig. 5c). The intensity of the Ag M2,3-edges (∼603.8 and ∼573.0 eV) provides a reliable marker for locating the membrane window, enabling reproducible alignment of the X-ray beam with the active electrode area. Note that the same can be done in the tender X-ray range using the Ag L-edges. Once aligned, O K-edge spectra can be used to verify that the pore structure is free of surfactant template remnants and fully accessible to the electrolyte (see Fig. S2 in the SI and the corresponding discussion).

Fig. 5. (a) Spectro-electrochemical flow cell at the ISISS beamline at BESSY II. (b) Cyclic voltammetry of a mesoporous Pt film mounted inside the spectro-electrochemical flow cell in 100 mM NaOH. (c) Ag paste applied on the SiNx chip exposed in the endstation analysis chamber for the alignment of the window to the incident X-ray beam and to the detector.

Fig. 5

Open in a new tab

Using the Na K-edge as an element-specific probe, Na+ ions were monitored in alkaline media at 5 mM NaOH (pH 11.7) and 100 mM NaOH (pH 13.0), as well as in acidic media in 50 mM NaClO4 + 50 mM HClO4 (pH 1.3). Spectra were recorded for three Pt surface states: the H-covered surface in the Hupd region, the OH/H2O covered surface in the “double layer” region, and the surface oxide. Importantly, all spectra are recorded with the same sample and a fixed alignment, so that the absolute intensities of the spectra can be compared. These intensities, particularly at the post-edge, are thus a direct measure for the variation in Na+ concentration at the Pt-electrolyte interface.

As shown in Fig. 6a and b, the dataset recorded in this manner makes it possible to assess the effects of pH, electrolyte concentration, and surface structure/potential on the interfacial Na+ concentration. To interpret these effects, it is common to use the framework of the potential of zero charge (PZC). The PZC is a material-specific property of the electrode surface, which determines at what potential the surface will be charge neutral. At potentials below the PZC, the electrode surface charges negatively and cations are increasingly attracted. Along the same lines, the electrode surface charges positively above the PZC, repelling cations and attracting anions. Thus, the difference between the applied potential and the PZC provides a first indication of the interfacial ion concentration that can be expected.

Fig. 6. Observation of the interfacial Na+ cation concentration and hydration shell via Na K-edge XAS. Background-subtracted spectra of (a) alkaline (pH 13) vs. acid (pH 1.3) and (b) 100 mM vs. 5 mM NaOH, at different applied potentials covering the oxide, double layer, and Hupd regions. (c) Normalized spectra of 5 mM NaOH in the oxide (1.29 V) and Hupd (0.01 V) regions. Potentials are expressed vs. RHE.

Fig. 6

Open in a new tab

We first use the potential vs. PZC to analyze the strong pH effect that can be observed in Fig. 6a. When comparing spectra recorded at the same potential vs. RHE, it is clear that cations are much more strongly attracted to the Pt surface in pH 13 compared to pH 1.3. The reason for this is that the PZC is a pH-independent property,35 whereas the RHE scale is pH-dependent. As a result, potential vs. PZC is (13–1.3) × 0.059 = 0.69 V lower in pH 13 compared to pH 1.3, making it much more attractive to cations.

Fig. 6b shows the comparison between 5 mM NaOH and 100 mM NaOH, where again a significant difference in interfacial Na+ concentration can be observed. This difference is again in line with expectation, as there is a difference in pH and bulk Na+ concentration, both of which steer the interfacial Na+ concentration to higher values for the 100 mM NaOH case.

Finally, we can also assess the effect of surface structure/potential on the interfacial Na+ concentration. It can be observed that in alkaline electrolytes, the cation concentration rises when increasing the applied potential, against the naïve expectation of the PZC picture. In acidic electrolyte, this is not observed. These observations show that surface chemistry necessitates nuancing of the basic PZC picture, which will be the topic of future studies.

In addition to the interfacial ion concentration quantification, the Na K-edge spectra can also be used to analyze the local hydration environment of the Na+ ions. As shown in Fig. 6c, the Na K-edge shows a potential dependent shoulder feature at approximately 1078–1081 eV. Modeling of Na K-edge spectra of solvated Na+ ions has shown that the intensity increase in this shoulder can be attributed to the distortion or partial opening of the Na+ hydration shell.36 Based on this, it is clear that the oxide-covered surface (1.26 VRHE) induces more distortion in the hydration shell of the interfacial Na+ ions than the hydrogen-covered surface (0.01 VRHE). This may indeed be expected, because the Na+ concentration at the interface is higher for the oxide covered surface, causing more crowding-induced distortion of the hydration shell. In addition, it may be that the O or OH groups on the oxide-covered surface partially replace water as ligands.

Conclusion

In summary, we demonstrated an approach for probing ions at the electrode–electrolyte interface under well-defined electrochemical conditions using in situ XAS. Central to this methodology is the use of mesoporous Pt films with an exceptionally high surface-area-to-volume ratio, which strongly boosts the interfacial ion signals with respect to those from the bulk. The mesoporous Pt films were fabricated by electrodeposition onto SiNx windows via a straightforward and well-controlled soft-templating process, yielding homogeneous mesoporous electrodes suitable for in situ XAS measurements. Using this approach, we showed that it is possible to track the interfacial Na+ concentration and to probe changes in the ion hydration shell. We show how measurements while varying the electrolyte and potential enable studies on the effects of e.g. pH, ion concentration, and surface structure/potential on the formation of the electrical double layer. These measurements demonstrate that in situ XAS can directly capture ion-specific behavior at electrified interfaces, providing insight into interfacial processes that are difficult to access with conventional electrochemical or spectroscopic techniques. We anticipate that this methodology can be extended to other ionic or molecular species that participate in electrocatalytic and energy storage processes, offering new opportunities to elucidate interfacial phenomena in electrochemical energy conversion and catalysis.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

CP-OLF-D6CP01946C-s001
CP-OLF-D6CP01946C-s002

Acknowledgments

J. S. D. R. acknowledges Tamkang University for a PhD stipend. A. L acknowledges the Swedish Research Council for funding under grant number 2024-00351. K. E. K acknowledges the Leiden Institute of Chemistry for a PhD stipend. R. V. M. and H. J. acknowledge the Dutch Organization for Scientific Research (NWO) for funding under grant number ECCM.TT.ECCM.001. C.-H. C. gratefully acknowledges the financial support provided by the National Science and Technology Council (NSTC) of Taiwan, Grant No. 114-2112-M-032-002- and 114-2112-M-032-011-. The Helmholtz Zentrum Berlin is acknowledged for awarding beam time.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6cp01946c.

References

  1. Huang B. Rao R. R. You S. Hpone Myint K. Song Y. Wang Y. Ding W. Giordano L. Zhang Y. Wang T. Muy S. Katayama Y. Grossman J. C. Willard A. P. Xu K. Jiang Y. Shao-Horn Y. Cation- and pH-Dependent Hydrogen Evolution and Oxidation Reaction Kinetics. JACS Au. 2021;1(10):1674–1687. doi: 10.1021/jacsau.1c00281. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Chen X. McCrum I. T. Schwarz K. A. Janik M. J. Koper M. T. M. Co-Adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chem., Int. Ed. 2017;56(47):15025–15029. doi: 10.1002/anie.201709455. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. van der Heijden O. Eggebeen J. J. J. Trzesniowski H. Deka N. Golnak R. Xiao J. van Rijn M. Mom R. V. Koper M. T. M. Li+ Cations Activate NiFeOOH for Oxygen Evolution in Sodium and Potassium Hydroxide. Angew. Chem., Int. Ed. 2024;63(18):e202318692. doi: 10.1002/anie.202318692. [DOI] [PubMed] [Google Scholar]
  4. Kumeda T. Kubo R. Hoshi N. Nakamura M. Activation of Oxygen Reduction Reaction on Well-Defined Pt Electrocatalysts in Alkaline Media Containing Hydrophobic Organic Cations. ACS Appl. Energy Mater. 2019;2(5):3904–3909. doi: 10.1021/acsaem.9b00582. [DOI] [Google Scholar]
  5. Xu P. Wang R. Zhang H. Carnevale V. Borguet E. Suntivich J. Cation Modifies Interfacial Water Structures on Platinum during Alkaline Hydrogen Electrocatalysis. J. Am. Chem. Soc. 2024;146(4):2426–2434. doi: 10.1021/jacs.3c09128. [DOI] [PubMed] [Google Scholar]
  6. Huang J. Li M. Eslamibidgoli M. J. Eikerling M. Groß A. Cation Overcrowding Effect on the Oxygen Evolution Reaction. JACS Au. 2021;1(10):1752–1765. doi: 10.1021/jacsau.1c00315. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Michael J. D. Demeter E. L. Illes S. M. Fan Q. Boes J. R. Kitchin J. R. Alkaline Electrolyte and Fe Impurity Effects on the Performance and Active-Phase Structure of NiOOH Thin Films for OER Catalysis Applications. J. Phys. Chem. C. 2015;119(21):11475–11481. doi: 10.1021/acs.jpcc.5b02458. [DOI] [Google Scholar]
  8. Liu Y. Kawaguchi T. Pierce M. S. Komanicky V. You H. Layering and Ordering in Electrochemical Double Layers. J. Phys. Chem. Lett. 2018;9(6):1265–1271. doi: 10.1021/acs.jpclett.8b00123. [DOI] [PubMed] [Google Scholar]
  9. Kolmeijer K. E. Mom R. V. Operando Methods for the Elucidation of Electrolyte Effects in Electrocatalysis. Curr. Opin. Electrochem. 2025;51:101700. doi: 10.1016/j.coelec.2025.101700. [DOI] [Google Scholar]
  10. Strmcnik D. van der Vliet D. F. Chang K.-C. Komanicky V. Kodama K. You H. Stamenkovic V. R. Marković N. M. Effects of Li+, K+, and Ba2+ Cations on the ORR at Model and High Surface Area Pt and Au Surfaces in Alkaline Solutions. J. Phys. Chem. Lett. 2011;2(21):2733–2736. doi: 10.1021/jz201215u. [DOI] [Google Scholar]
  11. Kumeda T. Tajiri H. Sakata O. Hoshi N. Nakamura M. Effect of Hydrophobic Cations on the Oxygen Reduction Reaction on Single–crystal Platinum Electrodes. Nat. Commun. 2018;9(1):4378. doi: 10.1038/s41467-018-06917-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Tanaka S. Tajiri H. Sakata O. Hoshi N. Nakamura M. Interfacial Structure of Pt(110) Electrode during Hydrogen Evolution Reaction in Alkaline Solutions. J. Phys. Chem. Lett. 2022;13(36):8403–8408. doi: 10.1021/acs.jpclett.2c01575. [DOI] [PubMed] [Google Scholar]
  13. Patrow J. G. Sorenson S. A. Dawlaty J. M. Direct Spectroscopic Measurement of Interfacial Electric Fields near an Electrode under Polarizing or Current-Carrying Conditions. J. Phys. Chem. C. 2017;121(21):11585–11592. doi: 10.1021/acs.jpcc.7b03134. [DOI] [Google Scholar]
  14. Ge A. Rudshteyn B. Videla P. E. Miller C. J. Kubiak C. P. Batista V. S. Lian T. Heterogenized Molecular Catalysts: Vibrational Sum-Frequency Spectroscopic, Electrochemical, and Theoretical Investigations. Acc. Chem. Res. 2019;52(5):1289–1300. doi: 10.1021/acs.accounts.9b00001. [DOI] [PubMed] [Google Scholar]
  15. Rebstock J. A. Zhu Q. Baker L. R. Exploring the Influence of Interfacial Solvation on Electrochemical CO2 Reduction Using Plasmon-Enhanced Vibrational Sum Frequency Generation Spectroscopy. ChemCatChem. 2024;16(14):e202301301. doi: 10.1002/cctc.202301301. [DOI] [Google Scholar]
  16. Lee S. Y. Kim J. Bak G. Lee E. Kim D. Yoo S. Kim J. Yun H. Hwang Y. J. Probing Cation Effects on *CO Intermediates from Electroreduction of CO2 through Operando Raman Spectroscopy. J. Am. Chem. Soc. 2023;145(42):23068–23075. doi: 10.1021/jacs.3c05799. [DOI] [PubMed] [Google Scholar]
  17. Akbar S. Elliott J. M. Squires A. M. Anwar A. Optimum Conditions for Electrochemical Deposition of 3-D Mesoporous Platinum Framework. J. Nanopart. Res. 2020;22(6):170. doi: 10.1007/s11051-020-04912-9. [DOI] [Google Scholar]
  18. Wang H. Wang L. Sato T. Sakamoto Y. Tominaka S. Miyasaka K. Miyamoto N. Nemoto Y. Terasaki O. Yamauchi Y. Synthesis of Mesoporous Pt Films with Tunable Pore Sizes from Aqueous Surfactant Solutions. Chem. Mater. 2012;24(9):1591–1598. doi: 10.1021/cm300054b. [DOI] [Google Scholar]
  19. Akbar S. Boswell J. Waters S. Williams S. Elliott J. M. Squires A. M. Control of Pore and Wire Dimensions in Mesoporous Metal Nanowire Networks through Curvature Modulation in Lipid Templates: Implications for Use as Electrodes. ACS Appl. Nano Mater. 2021;4(6):5717–5725. doi: 10.1021/acsanm.1c00505. [DOI] [Google Scholar]
  20. Dix S. T. Lu S. Linic S. Critical Practices in Rigorously Assessing the Inherent Activity of Nanoparticle Electrocatalysts. ACS Catal. 2020;10(18):10735–10741. doi: 10.1021/acscatal.0c03028. [DOI] [Google Scholar]
  21. Mayrhofer K. J. J. Strmcnik D. Blizanac B. B. Stamenkovic V. Arenz M. Markovic N. M. Measurement of Oxygen Reduction Activities via the Rotating Disc Electrode Method: From Pt Model Surfaces to Carbon-Supported High Surface Area Catalysts. Electrochim. Acta. 2008;53(7):3181–3188. doi: 10.1016/j.electacta.2007.11.057. [DOI] [Google Scholar]
  22. Rudi S. Cui C. Gan L. Strasser P. Comparative Study of the Electrocatalytically Active Surface Areas (ECSAs) of Pt Alloy Nanoparticles Evaluated by Hupd and CO-Stripping Voltammetry. Electrocatalysis. 2014;5(4):408–418. doi: 10.1007/s12678-014-0205-2. [DOI] [Google Scholar]
  23. Pfeifer V. Jones T. E. Velasco Vélez J. J. Arrigo R. Piccinin S. Hävecker M. Knop-Gericke A. Schlögl R. In Situ Observation of Reactive Oxygen Species Forming on Oxygen-Evolving Iridium Surfaces. Chem. Sci. 2017;8(3):2143–2149. doi: 10.1039/C6SC04622C. https://dx.doi.org/10.1039/c6sc04622c [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Knop-Gericke A. Pfeifer V. Velasco-Velez J. J. Jones T. Arrigo R. Hävecker M. Schlögl R. In Situ X-ray Photoelectron Spectroscopy of Electrochemically Active Solid-Gas and Solid-Liquid Interfaces. J. Electron Spec. Relat. Phenom. 2017;221:10–17. doi: 10.1016/j.elspec.2017.03.010. [DOI] [Google Scholar]
  25. Grinter D. C. Venturini F. Ferrer P. van Spronsen M. A. Arrigo R. Quevedo Garzon W. Roy K. Large A. I. Kumar S. Held G. The Versatile Soft X-Ray (VerSoX) Beamline at Diamond Light Source. Synchrotron Radiat. News. 2022;35(3):39–47. doi: 10.1080/08940886.2022.2082181. [DOI] [Google Scholar]
  26. Vantelon D. Trcera N. Roy D. Moreno T. Mailly D. Guilet S. Metchalkov E. Delmotte F. Lassalle B. Lagarde P. Flank A.-M. The LUCIA Beamline at SOLEIL. J. Synchrotron Rad. 2016;23(2):635–640. doi: 10.1107/S1600577516000746. [DOI] [PubMed] [Google Scholar]
  27. Zangrando M. Zacchigna M. Finazzi M. Cocco D. Rochow R. Parmigiani F. Polarized High-Brilliance and High-Resolution Soft x-Ray Source at ELETTRA: The Performance of Beamline BACH. Rev. Sci. Instrum. 2004;75(1):31–36. doi: 10.1063/1.1634355. [DOI] [Google Scholar]
  28. Zhu S. Scardamaglia M. Kundsen J. Sankari R. Tarawneh H. Temperton R. Pickworth L. Cavalca F. Wang C. Tissot H. Weissenrieder J. Hagman B. Gustafson J. Kaya S. Lindgren F. Källquist I. Maibach J. Hahlin M. Boix V. Gallo T. Rehman F. D’Acunto G. Schnadt J. Shavorskiy A. HIPPIE: A New Platform for Ambient-Pressure X-Ray Photoelectron Spectroscopy at the MAX IV Laboratory. J. Synchrotron Rad. 2021;28(2):624–636. doi: 10.1107/S160057752100103X. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Fung H. S. Yuh J. Y. Huang L. J. Tseng T. C. Perng S. Y. Wang D. J. Tsang K. L. Chung S. C. A Soft X-Ray (300–1000 eV) Active Grating Monochromator Beamline at NSRRC. AIP Conf. Proc. 2007;879(1):563–566. doi: 10.1063/1.2436123. [DOI] [Google Scholar]
  30. Yu P. Xia Y. Feng X. Zheng S. Ren G. Zhang N. Liu X. Chen C. Guo X. The Evolution of Local Structure of Mo6S8 during Li+ Electrochemical Storage Studied by in-Situ Tender X-Ray Absorption Spectroscopy. Prog. Nat. Sci.: Mater. Int. 2022;32(6):739–744. doi: 10.1016/j.pnsc.2022.09.006. [DOI] [Google Scholar]
  31. Henke B. L. Gullikson E. M. Davis J. C. X-Ray Interactions: Photoabsorption, Scattering, Transmission, and Reflection at E = 50-30 000 eV, Z = 1-92. At. Data Nucl. Data Tables. 1993;54(2):181–342. doi: 10.1006/adnd.1993.1013. [DOI] [Google Scholar]
  32. Lim H. Kani K. Henzie J. Nagaura T. Nugraha A. S. Iqbal M. Ok Y. S. Hossain Md. S. A. Bando Y. Wu K. C. W. Kim H.-J. Rowan A. E. Na J. Yamauchi Y. A Universal Approach for the Synthesis of Mesoporous Gold, Palladium and Platinum Films for Applications in Electrocatalysis. Nat. Protoc. 2020;15(9):2980–3008. doi: 10.1038/s41596-020-0359-8. [DOI] [PubMed] [Google Scholar]
  33. Yasin H. M. Denuault G. Pletcher D. Studies of the Electrodeposition of Platinum Metal from a Hexachloroplatinic Acid Bath. J. Electroanal. Chem. 2009;633(2):327–332. doi: 10.1016/j.jelechem.2009.06.020. [DOI] [Google Scholar]
  34. Kriek R. J. Mogwase B. M. S. Vorster S. W. Relation of the Electrochemical Interplay between H2PtCl6 and H2O/H3O +/H2+ and the Hydrogen-Evolution Reaction. Electrochem. Sci. Adv. 2022;2(2):e2100041. doi: 10.1002/elsa.202100041. [DOI] [Google Scholar]
  35. Rizo R. Sitta E. Herrero E. Climent V. Feliu J. M. Towards the Understanding of the Interfacial pH Scale at Pt(111) Electrodes. Electrochim. Acta. 2015;162:138–145. doi: 10.1016/j.electacta.2015.01.069. [DOI] [Google Scholar]
  36. Aziz E. F. Zimina A. Freiwald M. Eisebitt S. Eberhardt W. Molecular and Electronic Structure in NaCl Electrolytes of Varying Concentration: Identification of Spectral Fingerprints. J. Chem. Phys. 2006;124(11):114502. doi: 10.1063/1.2176616. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

CP-OLF-D6CP01946C-s001
CP-OLF-D6CP01946C-s001.zip (6.7MB, zip)
CP-OLF-D6CP01946C-s002
CP-OLF-D6CP01946C-s002.pdf (386.9KB, pdf)

Data Availability Statement

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6cp01946c.

Articles from Physical Chemistry Chemical Physics are provided here courtesy of Royal Society of Chemistry

RESOURCES