Electrochemical phase transitions are ubiquitous in energy materials, including rechargeable battery electrodes, electrocatalysts for water electrolysis, and electrochromic windows. Because different phases exhibit vastly different properties, their spatial distribution critically determines electrode performance and device functionality. Consequently, these transitions have long attracted interest in fundamental and applied research.

Most studies on operando imaging of phase transitions during battery cycling focus on individual particles or small particle ensembles, achieving nanometer-scale resolution. While observing phase transitions in single particles in real time is a remarkable achievement, it raises an important question: do single-particle dynamics adequately represent the spatiotemporal evolution across an entire electrode? The prevailing assumption is that macroscopic electrode behavior during cycling simply reflects the collective microscopic dynamics of its constituent particles. As a result, macroscopic emergent phenomena have received little attention. Yet, in other fields—such as catalysis—macroscopic self-organization and complexity play a decisive role, as demonstrated by Gerhard Ertl’s Nobel Prize–winning work. This raises the possibility that similar effects may occur in battery electrodes.

To address this question, we investigated macroscopic spatiotemporal dynamics of electrochemical phase transitions in nickel (oxy)hydroxide electrodes. These materials are electrochromic, changing color from light green nickel hydroxide (Ni(OH)₂) to dark grey nickel oxyhydroxide (NiOOH) upon charging, and reverting upon discharge. This color change enables direct monitoring of the phase transition using simple optical imaging. Although limited to spatial features larger than ~1 μm, optical imaging offers a large field of view encompassing the entire electrode and allows fast video recording, making it well suited for tracking phase propagation in real time.

Using translucent thin-film electrodes, we recorded videos of the phase transition during charging [1]. Strikingly, instead of a uniform color change across the electrode, we observed lateral propagation of a macroscopic phase front across millimeter-length scales, perpendicular to the applied electrochemical driving force. This behavior contradicts the prevailing model of a planar phase front advancing uniformly through the electrode thickness.

The observed lateral phase-front propagation reveals strongly nonuniform charging, with some regions becoming fully charged while others remain largely discharged. This nonuniformity has critical consequences: overcharged regions can initiate premature oxygen evolution, limiting access to the electrode’s full capacity. Oxygen evolution also promotes battery swelling and electrolyte leakage, undermining safety and lifetime. Our findings therefore uncover a critical and previously overlooked aspect of battery electrode behavior—macroscopic nonuniform charging driven by emergent phase-transition dynamics.

The oxygen evolution reaction (OER) plays a crucial role in (photo)electrochemical devices that use renewable energy to produce synthetic fuels. While the mechanism of this reaction is still debated, recent measurements on semiconducting oxides like Fe₂O₃, BiVO₄, TiO_{2 and} WO₃ [1,2] have shown that the dependence of the rate of OER on the surface hole density is a power law, suggesting a multihole mechanism via surface hole accumulation. This is reminiscent of the mechanism for OER promoted by the oxygen evolving complex in Photosystem II and in stark contrast with metallic oxides like IrO₂ [3], where the dependence is exponential. Modeling this reaction requires all-atom simulations to capture the effects of the H-bond network of the solvent on the reactants, long molecular dynamics simulations to sample accurately the solvent degrees of freedom and enhanced sampling to model activated process like O-O bond formation. In this work, focusing on hematite (Fe₂O₃), we have used machine learning interatomic potentials (MLIP) trained on DFT energy and forces to achieve this goal. In contrast to what was previously assumed, we find that the reaction proceeds via direct coupling of oxygen adsorbates, triggered by the oxidation of terminal oxygen sites. This process is considerably faster than competing mechanisms like the nucleophilic attack of a water molecule or hydroxide ion. We tested the accuracy of these MLIP predictions, validating the kinetics of this process with DFT calculations. Microkinetic modeling was then used to predict the overall rate of the catalytic process and to relate the photocurrent so the surface charge accumulation. We find a power-law with a third order dependence, in agreement with experiments, and we assign its origin to the facile formation of the superoxo intermediate, a step whose activation energy is weakly dependent on the surface hole coverage.

The semiconductor–electrolyte interface plays a central role in determining the efficiency of photoelectrochemical water splitting, where light absorption, charge separation, charge transfer, and catalytic reactions are intimately coupled. Although significant progress has been made in developing nanostructured photoelectrodes, understanding how interfacial properties govern these processes remains a major challenge for the realization of efficient and durable solar hydrogen production.

This contribution presents selected studies on nanostructured semiconductor photoelectrodes, with the aim of discussing how interface engineering influences charge-transfer processes and water-splitting performance. Through the combination of controlled nanostructure synthesis with structural, morphological, optical, and photoelectrochemical characterization, we examine the effects of morphology, crystallographic structure, surface chemistry, and catalyst integration on carrier transport, surface recombination, and reaction kinetics.

Particular attention will be devoted to the opportunities offered by surface functionalization and heterostructure design to reduce charge transfer resistance and promote interfacial reactions, as well as to the limitations that still hinder the widespread implementation of these approaches. By highlighting both the strengths and the shortcomings of current interface engineering strategies, we will discuss the challenges associated with improving stability, understanding dynamic interfacial processes, and establishing reliable structure–property relationships.

The presented examples illustrate how correlating nanoscale interfacial characteristics with photoelectrochemical performance can provide valuable insight into the design of more efficient photoelectrodes. At the same time, they underscore the need for complementary in situ and operando characterization together with multiscale modelling to achieve a comprehensive understanding of solid–liquid interfaces and to guide the development of next-generation materials for solar fuel production.

Technology at the nanoscale has emerged as one of the primary challenges in science, as new physical and chemical effects can be manipulated at will. As advancements in materials science push the boundaries of physics and chemistry, it becomes crucial to understand the origins of these unique properties and how they relate to changes at the atomic scale, particularly those linked to structural alterations in materials, often associated with crystal defects or surface terminations. This understanding is especially vital for low-dimensional materials designed for energy and environmental applications, where crystallography and the distribution of atomic species play a critical role in determining their physical properties, thereby enhancing their performance, including efficiency and selectivity in specific reactions.

In this presentation, I will demonstrate how the combination of advanced electron microscopy imaging and electron spectroscopy, utilizing aberration-corrected scanning transmission electron microscopy (STEM), enables us to investigate elemental composition and structure with unprecedented spatial resolution. Multimodal analytical techniques can be now combined to extract information down to the atomic scale.[1] This approach allows us to determine growth mechanisms and correlate structural properties with performance.

Typically, materials intended for physical applications require perfect crystallinity, free from defects, disorder or inhomogeneities. However, I will illustrate how, in the context of chemistry applications such as catalysis, defects can sometimes enhance the reactivity of certain nanomaterials, particularly in two-dimensional systems. This shift in perspective reveals that defects such as inhomogeneous grain boundaries [2], presence of vacancies [3,4] or even amorphization [5] can lead to the formation of free radicals or an increased density of dangling bonds at the surface, which may improve catalytic properties for selective molecules. I will show how in-situ reactions can now be conducted within the transmission electron microscope (TEM), facilitating precise visualization of active sites.[6,7] Finally, I will discuss how artificial intelligence can assist in automating the analysis of defects and vacancies, enabling statistically significant evaluations of our systems and reactions.[8-10]

A detailed understanding of catalyst surface and interfacial phenomena is essential to the rational design of more efficient energy conversion systems. In this talk, I will demonstrate how liquid-phase transmission electron microscopy (TEM) affords valuable insight into the complex organic/inorganic and solid/liquid/gas interfaces that govern catalytic behavior.

I will first review recent advances in confined TEM microcell design, before briefly discussing their application in real-time imaging of copper catalysts during the CO₂ reduction reaction (CO₂RR) [1]. I will then extensively cover how electrochemical liquid-phase TEM, applied to cobalt-based oxygen-evolving oxides, elucidates the effects of electrowetting and the active Co²⁺/Co³⁺ redox transition as the catalyst surface becomes increasingly hydrophilic [2]. Finally, I will present recent findings on iridium oxide-based catalysts, integrating experimental observations with molecular dynamics simulations and density functional theory calculations. 

These advanced electron microscopy diagnostics offer fundamental insights into the 'true' starting state of catalytic systems and their evolution during electrocatalytic processes at a scale that is otherwise inaccessible, providing a powerful complement to other characterisation techniques.

Understanding how electrochemical materials evolve during operation remains a central challenge in energy conversion and storage technologies. While conventional electrochemical measurements provide macroscopic performance metrics, the nanoscale structural processes governing degradation often remain hidden. Electrochemical liquid-phase electron microscopy (EC-LP-EM) offers direct access to dynamic solid–liquid interfaces, yet quantitative interpretation is frequently hindered by radiolysis, complex electrochemical environments, and uncertainties regarding the relationship between nanoscale observations and bulk behavior.

In this contribution, I will present a quantitative workflow that bridges operando nanoscale imaging with complementary electrochemical and analytical techniques to establish causal links between structural evolution and macroscopic electrochemical responses. Using silver electrodeposition and dissolution on platinum as a model system, EC-LP-EM is combined with automated image analysis, radiolysis assessment, conventional electrochemistry, and on-line scanning flow cell inductively coupled plasma mass spectrometry (SFC-ICP-MS). This correlative approach enables the direct quantification of nanoscale structural transformations and their translation into measurable electrochemical observables.

The experiments reveal a surprising phenomenon: freshly electrodeposited silver dissolves spontaneously under open-circuit conditions immediately after deposition. Real-time nanoscale observations show that dissolution initiates preferentially at the electrode–electrolyte interface and coincides with equilibration of the open-circuit potential. Correlative electrochemical analysis demonstrates that this behavior originates from thermodynamic and kinetic equilibration processes involving silver oxidation, oxygen reduction, and platinum surface chemistry rather than from externally applied electrochemical stimuli. Beyond elucidating this specific mechanism, the study establishes a general framework for correlating dynamic nanoscale processes with bulk electrochemical behavior. The results demonstrate that substantial structural transformations can occur even under nominally inactive open-circuit conditions and highlight the dynamic nature of electrochemical interfaces. More broadly, the presented methodology provides a blueprint for investigating degradation phenomena in electrocatalysts, batteries, corrosion science, and other energy materials, thereby advancing the development of predictive structure–performance relationships across electrochemical technologies.[1]

Understanding the role of surface oxidation states in governing electrocatalytic activity is crucial for the rational design of efficient and durable catalysts based on scarce noble metals. In this work, we present a unified view of electrocatalysis across a series of Pt- and Pd-based systems, including intermetallic compounds (PtSn4 and PdSn4), nanostructured catalysts for alcohol electrooxidation, and oxide-promoted materials for the hydrogen evolution and oxidation reactions.

A common feature emerging from these studies is that catalytic performance is governed by the dynamic evolution of the surface oxidation state rather than by the pristine electronic structure. In Pd-based catalysts for ethanol electrooxidation, the formation of PdO surface layers under operating potentials leads to activity losses, whereas their in situ reduction restores metallic active sites and enhances catalytic turnover. Likewise, the introduction of reducing agents or suitably reactive environments enables control over the Pd/PdO equilibrium, directly influencing catalytic activity, selectivity, and stability.[1–3]

In parallel, oxide–metal interfaces play a crucial role in modulating catalytic pathways. In Pd–NiO and Pd–CeO2 systems, strong metal–support interactions alter the oxidation state of palladium and promote the formation of catalytically active Pd–OH species, thereby enhancing the kinectics of the hydrogen evolution and hydrogen oxidation reactions in alkaline media. These findings highlights the beneficial role of partially oxidized or hybrid metal–oxide interfaces in facilitating charge transfer and stabilizing key reaction intermediates.[4–8]

Consistently with this picture, surface oxidation in PtSn4 and PdSn4 intermetallic compounds generates SnOₓ-rich surface layers that actively participate in catalysis, leading to enhanched hydrogen evolution activity and CO tolerance. Across all the systems, the active phase is therefore more appropriately described as a dynamic metal/oxide interface rather than as a static metallic surface.[9–11]

Collectively, these findings establish surface redox chemistry as a unifying descriptor of electrocatalytic activity and provide a general framework for the rational design of advanced electrocatalysts based on controlled surface oxidation states and operando surface transformations.

Polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology for converting renewable electricity into hydrogen, owing to its stable operation under fluctuating power input. However, both the current density achievable at practical full‑cell voltages and the long‑term durability still do not meet societal and industrial expectations. While catalyst development is commonly used to overcome these limitations, catalyst ink composition may also play a significant role, as ink optimization is widely used to improve fuel cell performance. Nevertheless, its effects in PEMWE remain insufficiently understood. In this study, we varied catalyst ink ratios and evaluated their impact on current density and full‑cell voltage characteristics.

Both cathode and anode inks consisted of catalyst, ionomer, water, and alcohol. The cathode catalyst was 20 Pt-wt% Pt/C, and the anode catalyst was amorphous IrO₂; Nafion (20 wt% in water and 1‑propanol) served as the ionomer. We first examined the influence of alcohol species, that is, ethanol versus 1‑propanol. Although the current densities at 1.9 V and 80°C were similar (2.2–2.4 A/cm²), ethanol yielded slightly higher values, likely due to the higher porosity of catalyst layers formed by its faster evaporation during drying.

Next, the catalyst‑to‑ionomer ratio was varied. For the cathode, the carbon‑to‑ionomer ratio in Pt/C inks was adjusted from 1:0.45 to 1:1.5, while for the anode, the IrO₂‑to‑ionomer ratio ranged from 1:0.1 to 1:1.5. At 1.8 V and 80°C, the resulting current densities spanned 1.6–1.85 A/ cm² for Pt/C and 0.8–1.85 A/cm² for IrO₂. Constant‑current operation at 2.0 A/cm² for several hours revealed an optimal cathode ratio of 1:1 for current density stability, whereas the optimal anode ratio was less distinct. The optimal current density was obtained with 1:0.22 (or 0.1) for the anode ratio, whereas the differences were small for the cathode ratio. The broader current density variation observed for the anode likely reflects the stronger rate limitation imposed by the oxygen evolution reaction relative to proton transport through the electrolyte. Current stability appears to correlate with the mechanical robustness of the cathode catalyst layer.

These findings demonstrate that, in addition to catalyst development, the physicochemical environment of the catalyst layer plays a critical role in determining PEMWE performance.

Electrochemical intercalation in layered materials is a key process in energy storage and surface science, enabling the formation of confined phases, local strain, and chemical heterogeneities at the nanoscale [1]. In highly oriented pyrolytic graphite (HOPG), intercalation leads to the formation of blister-like nanostructures that host trapped species and display complex mechanical, structural, and electronic behaviours [2,3]. In this work, individual HOPG blisters are investigated through a multimodal nanoscale approach combining atomic force microscopy (AFM), scanning tunnelling microscopy (STM), Raman spectroscopy, and tip-enhanced Raman spectroscopy (TERS). This combination enables the correlation of topographic, mechanical, atomic-scale, and vibrational information across both the blister body and its highly structured edge regions. AFM and STM reveal the morphology, stiffness, and surface features of the intercalation-induced structures, while Raman and TERS provide spatially resolved spectroscopic fingerprints with enhanced sensitivity to local strain, disorder, and confined molecular species. Particular attention is devoted to blister borders and coalescence regions, where interlayer deformation and chemical confinement are expected to be most pronounced. Local changes in the Raman G-band response and TERS spectra highlight the heterogeneous nature of these nanostructures and suggest the possible encapsulation of gaseous species such as CO, CO₂, and O₂. These findings establish a meaningful analogy between electrochemically generated graphite blisters and gas-filled bubbles in two-dimensional materials. Overall, this study demonstrates that the integration of scanning probe microscopy with advanced Raman techniques is a powerful strategy to unravel the physicochemical complexity of intercalated stratified systems at the nanoscale, with implications for electrochemical interfaces, surface science, and energy-related layered materials.

Transient electrochemical intermediates often exist on timescales far shorter than the temporal resolution accessible to conventional operando spectroscopic techniques. As a result, many key steps of electrocatalytic reaction mechanisms remain experimentally inaccessible. While substantial progress has been made in the development of in situ and operando methods over the past decade, the typical time resolution of structural spectro-electrochemical techniques remains limited to approximately 0.1 s.

In this talk, I will present a general approach for extending operando spectroscopy into the sub-millisecond regime through synchronized electrochemical reaction activation and spectroscopic probing. First, I will demonstrate how a conventional Raman spectroscopy setup can be adapted to perform time-resolved investigations of electrochemically driven processes. The method enables direct observation of bond formation and bond breaking events with a time resolution of 0.2 ms, representing an improvement of nearly two orders of magnitude over conventional operando Raman measurements.

I will then show how the same concept can be translated to X-ray absorption spectroscopy through the development of a transient spectro-electrochemical Hard XAS setup implemented at the Balder beamline at MAX IV. The technique provides access to transient changes in the electronic and structural state of electrocatalysts on sub-millisecond timescales and is demonstrated using the oxygen evolution catalyst NiOOH. Finally, I will present our preliminary efforts to extend this methodology to the soft X-ray regime through the development of the first operando Soft X-ray flow cell capable of sustaining current densities up to 100 mA cm-2, tested at the Elettra Synchrotron. This new platform opens the way to time-resolved operando Soft X-ray spectroscopy under technologically relevant reaction conditions.

Together, these developments represent the first step towards a systematic resolving of short-lived reaction intermediates of electrocatalytic processes.

Nickel-based oxides and hydroxides are among the most promising oxygen evolution reaction (OER) electrocatalysts for alkaline water electrolysis, yet understanding how admixed elements influence both bulk redox transformations and formation of surface-active intermediates remains challenging. NiOOH is widely recognized as the active phase in alkaline OER, while Raman studies have highlighted the importance of oxygen-related intermediates formed under anodic polarization [1,2]. Here, we investigate the influence of lanthanum incorporation on sol–gel-derived Ni-based oxide/hydroxide powders and corresponding thin films. By combining different characterization techniques with in situ Raman and in situ UV–visible spectroelectrochemistry, correlations between composition, redox evolution, and OER activity are followed, Ni- and Ni/La-based materials containing 2, 5, and 10 mol% La were synthesized and thermally treated at 300 °C. Structural characterization reveals NiO nanocrystallites embedded in a partially amorphous hydrated matrix. The parallel investigation of powders and thin films, prepared from the same Ni- and Ni/La-sols, enables direct comparison between electrocatalytic response and electro-optical redox behavior [3].

Electrochemical measurements in 0.1 M KOH show progressive activation of NiO into the Ni(OH)₂/NiOOH redox pair during cycling, with a strongly concentration-dependent influence of La [4]. Whereas 2 mol% La causes only minor changes, 5 mol% reduces activity through structural rearrangement, while 10 mol% enhances OER performance. In situ Raman spectroelectrochemistry directly follows the anodic formation of NiOOH through the emergence of characteristic bands at 482 and 556 cm⁻¹ and reveals low-intensity features in the 800–1150 cm⁻¹ region associated with oxygen-related surface species formed under OER conditions [1,2]. Complementary in situ UV–visible spectroelectrochemistry performed on thin films tracks potential-dependent coloration during Ni²⁺/Ni³⁺ oxidation [2,3]. Measurements show that lanthanum not only modifies the catalyst structure, but also influences the course of the electrochemical activation and accessibility of redox-active nickel centers. Together, both in situ techniques provide insight into how lanthanum modifies Ni-based electrocatalysts and demonstrate the importance of combining vibrational and optical spectroelectrochemistry to distinguish between bulk redox processes and surface catalytic transformations during OER [3].

Semiconductor/cocatalyst interfaces strongly influence the efficiency and selectivity of photoelectrochemical photoanodes. In hematite- and BiVO₄-based systems, transition-metal oxyhydroxide overlayers are often described as hole-collecting cocatalysts, yet their actual role under operating conditions involves more complex redox chemistry governed by bias, illumination, catalyst composition, and substrate identity.

Here, we present operando X-ray absorption spectroscopy as an element-selective platform to track redox dynamics at photoanode/cocatalyst interfaces during PEC operation. Using custom PEC cells integrated with synchrotron XAS detection, oxidation-state changes and local structural rearrangements can be monitored while the electrode is biased, illuminated, and exposed to electrolyte.

Three representative approaches are discussed. First, steady-state XAS and fixed-energy X-ray absorption voltammetry reveal light-driven redox changes in catalytic overlayers, including substrate-specific charge trapping and illumination-dependent oxidation in CoFeOx-modified WO₃/BiVO₄ photoanodes¹. Second, potential-modulated X-ray electrochemical impedance spectroscopy synchronizes XAS detection with periodic voltage perturbations, isolating the catalyst redox response and providing access to potential-driven oxidation and reduction kinetics. Third, light-modulated XAS probes transient photoinduced processes by correlating the catalyst absorption signal with modulated illumination. Applied to hematite photoanodes with Ni-based overlayers, this method reveals frequency-dependent Ni oxidation and reduction pathways, highlighting the active role of the cocatalyst in charge transfer, recombination, and back-transfer processes.

Together, these examples show that operando PEC-XAS can move beyond static characterization toward a dynamic, chemically resolved description of working photoanode interfaces. Combining steady-state, potential-modulated, and light-modulated measurements provides mechanistic insight for designing semiconductor/cocatalyst architectures with improved charge selectivity and enhanced solar-driven oxidation performance.

Figure 1: Operando PEC-XAS approaches for probing redox dynamics at photoanode/cocatalyst interfaces. a) Fixed-energy X-ray absorption voltammetry, FEXRAV, records the element-specific fluorescence response during a linear potential sweep, here shown under dark/light conditions. b) X-ray electrochemical impedance spectroscopy, XEIS, follows the catalyst redox response to a sinusoidal potential modulation, allowing frequency-resolved extraction of potential-driven oxidation/reduction dynamics. c) Intensity-modulated XAS, IM-XAS, probes photoinduced redox processes by correlating the X-ray fluorescence signal with a sinusoidally modulated light excitation. In all cases, the incident X-ray energy is kept fixed at the selected absorption-edge energy while the electrical or optical stimulus is varied.

In the future, biomass, plastic waste, water and carbon dioxide could replace crude oil, coal and natural gas as the major feedstocks for a circular and sustainable production of fuels and chemicals.¹ Techno-economic studies already indicate that it could also become profitable to use renewable power to electrochemically convert carbon dioxide directly or using cascade routes into ethylene, ethanol and propanol - from which a broad range of value added products could be produced.²

Over the past years we have studied films of soft-landed copper (oxide) nanocrystals as cathode catalyst layers for both the direct and cascade CO₂ and CO electroreduction routes, exploring their behavior from the particle-level to the device-level.^3-4 The electrochemical performance of these catalysts has been evaluated in H-cells, flow-by and more recently in membrane electrode assembly cells, yielding valuable insights with respect to the interplay between loading, surface area and operating conditions on performance and product distribution (multicarbon products such as ethylene and ethanol being the major ones).

While the majority of the work has been applied in nature, we also carried out in-situ synchrotron X-ray absorption spectroscopy experiments in diluted bicarbonate (CO₂ reduction) and potassium hydroxide (CO reduction) to better understand the dynamic behavior of the nanoporous catalyst layers under bias. These experiments were combined with porosity and surface area information from electron tomography and Pb underpotential deposition to establish the link between electrochemically active surface area, in-situ electronic structure and faradaic efficiency ratio of the most important products (methane/ethylene and C₂₊/acetate, respectively). For example, the data indicate that the oxide CLs reduce to metallic Cu under alkaline conditions without significant compaction of the film. In addition, the analysis shed light on the extent and significance of catalyst reconstruction in the case of <50 and 200 nm-thick nanoporous CLs, which helped us explain the observation of irreversible and reversible methane formation in membrane electrode assembly cells under CO₂ reduction conditions. Together, this work establishes a baseline understanding and framework for operating ionomer-free porous catalyst layers in these applications.

In recent decades, Cu has attracted considerable attention over other pure metals catalysts for its exceptional performance of the electrocatalytic reduction of CO2 (CO2RR) into valuable hydrocarbons and alcohols [1]. However, the low selectivity and stability of this catalyst remain significant challenges. Consequently, understanding and controlling the dynamics of electronic properties at the solid-liquid interface during CO2RR using operando techniques are essential.

By accessing transition-metal L-edges and light-element K-edges (O, N, C), soft X-ray Absorption Spectroscopy (sXAS) is the ideal tool to track catalytic redox mechanisms while simultaneously capturing local environmental details. At the BACH beamline within the ELETTRA Synchrotron facility (IT), a microfluidic electrochemical cell for operando XAS has been developed [2], enabling a detailed characterisation of oxidation states and electronic structure directly at the catalyst-electrolyte interface. The microfluidic electrochemical cell (ME-cell) features inlet and outlet channels, which allow for the renewal of the electrolyte, and a three-electrode system, comprising an Ag/AgCl leakless as reference electrode (RE), a Pt wire as counter electrode (CE) and a working electrode (WE) made of an Au-coated Si3N4 membrane onto which the catalytic material is deposited.

Our ex situ and in situ sXAS study revealed that Nafion alters the chemical environment of the pristine Cu catalyst, leading to the formation of Cu²⁺ species, likely via partial dissolution induced by the ionomer's acidity, followed by coordination of dissolved copper species with the sulfonic groups of Nafion. These results were corroborated by ex situ Cu K-edge measurements, demonstrating the powerful synergy achieved by combining soft and hard XAS methodologies. Furthermore, to the best of our knowledge, this is the first report highlighting Nafion-induced dissolution effect on Cu-based catalytic materials.

Moreover, Faradaic efficiency (FE) measurements revealed differences in product selectivity when Nafion is added via spin-coating or drop-casting: while both approaches favour HCOOH as the primary product, spin-coating enhances CO formation and facilitates ethylene generation. These findings underscore the dual role of Nafion as both a structural binder and an active modifier of the first-stage catalytic behaviour, demonstrating the importance of in situ XAS for elucidating the catalyst-binder interactions during CO2RR.

Electrocatalytic interfaces and reactions are highly complex and embracing this complexity in atomistic simulations remains incomplete due to the difficulties in modeling for the experimentally relevant reaction conditions in the simulation. These issues call for the development, implementation, and usage of advanced computational methods.

In this contribution, I will show how grand canonical density functional theoretical (GC-DFT) methods enable the simulation of electrochemical systems at constant potential. I will first introduce the theoretical foundations of GC-DFT and present its practical implementations. This is followed by demostrations of GC-DFT to simulate and understand electrochemical thermodynamics, kinetics, and interfaces as function of the electrode potential. I will particularly highlight how and why the electrode potential, surface charge, and surface coverage together shape the electrocatalytic mechanisms, thermodynamics, and kinetics which cannot be simulated without the explicit inclusion of the electrode potential in the simulation protocol. Finally, I will discuss and outline some underlying challenges of GC-DFT and its future extensions. 

Bismuth vanadate (BiVO₄) represents a promising candidate for photoelectrochemical (PEC) water splitting. It has been shown that polarons play an important role in the water-splitting mechanism by introducing charge transition levels (CTLs) in the band gap and modifying the band alignment. Additionally, the 
introduction of oxygen vacancies, which can interact with electron polarons, has been reported to improve the PEC efficiency. However, explicit simulations of water interfaces require large numbers of atoms, and calculations of CTLs involve integration of free energies over long timescales, making ab initio methods 
prohibitive. In this work, we train a machine-learning potential that captures electron polarons and oxygen vacancies in multiple charge states to investigate their stability at an explicit BiVO₄-water interface. By computing finite-temperature free energies, CTLs, and oxygen-vacancy formation energies as a function of distance from the interface, we determine how the aqueous environment modifies charge trapping and defect energetics relative to bulk BiVO₄. While the free-energy differences are temperature independent in bulk BiVO₄ due to the harmonic response of the VO₄ units, we find that the aqueous interface introduces a clear temperature dependence. By further analyzing the local structural distortions associated with electron localization, we provide a microscopic link between interfacial structure and the stability of localized charges. Finally, we observe vacancy migration near the interface, highlighting the strong impact of the aqueous interface on the stability of oxygen vacancies.

The efficiency and durability of photoelectrodes for solar fuel production are largely determined by the dynamic processes that take place at the solid–liquid interface. Resolving these processes calls for correlative characterization across length and time scales, from operating devices down to the nanoscale. We present such a multiscale approach to elucidate performance-limiting mechanisms and to identify pathways toward improved photoelectrode design. 

To probe degradation processes more broadly, we recently introduced operando spectroscopic ellipsometry as a quantitative tool for real-time monitoring of photocorrosion, revealing strong dependencies on crystallinity, illumination, and electrolyte conditions in TiO₂ model systems. Complementary nanoscale analysis using time-resolved Kelvin probe force microscopy further establishes direct correlations between local morphology and charge carrier dynamics, highlighting superior charge separation and transport in crystalline regions. Extending this correlative approach to other photocathodes, we uncover competing redox-driven degradation pathways and implement a protection strategy based on catalyst integration and heterojunction design, enabling stable CO₂ reduction with high selectivity.

Taken together, these results provide a multiscale framework linking interfacial chemistry, microstructure, and charge transport to photoelectrode performance and stability, offering actionable design principles for next-generation materials in sustainable energy technologies.