Cation identity and concentration strongly influence electrocatalytic processes, yet their effects remain insufficiently understood. Taking hydrogen evolution reaction in alkaline media as a model system, variations in cation concentration induce complex, sometimes inverted, activity trends. Increasing cation concentration can either promote or inhibit electrocatalytic activity depending on cation identity, electrode material and solution pH. These Janus-faced effects of cations challenge the current understandings of cation effects in electrocatalysis, which typically emphasize either promotional or inhibitory roles. Here, we propose a mechanistic rationale for the promoter-inhibitor transitions of cation effects and identify cation position in the electric double layer as the key factor governing this behavior. The theoretical framework distinguishes two cation states: cations electrostatically attracted in the diffuse layer, or cations specifically adsorbed at the inner Helmholtz plane. Incorporating the electric field effect on water dissociation beyond the Frumkin corrections, we show that the two cation states modulate the local electric field and thus kinetics in opposite ways. The observed inversions result from their competition, governed by cation size and adsorption strength. The framework and insights will be relevant to other electrocatalytic reactions at strongly negatively charged surfaces, such as CO2 reduction.

The passivation of silicon dangling bonds by hydrogen is a crucial requirement for silicon-based optoelectronic technology, especially for solar cells. Recent experiments on intense light soaking of silicon heterojunction solar cells unveiled interesting dynamical aspects of hydrogen passivation that are linked to Si-H bond breaking and repassivation. These processes take place predominantly in porous regions near the amorphous/crystalline interface, where hydrogen can exist in molecular form. This work addresses the question of whether molecular H_2 directly participates in Si-H depassivation and repassivation. Using density functional theory, we calculate and compare formation energies of point defects, multivacancy cavities and the Si(100) surface to identify relevant passivated and depassivated states. Furthermore, we employ nudged elastic band calculations to determine the activation barriers of the corresponding pathways. We find that sufficient local free volume enables a direct double-H pathway for depassivation through the formation of confined molecular H_2. Despite involving the breaking of two Si-H bonds, the double-H process can be energetically and kinetically competitive with the single-H process and can exhibit a reverse repassivation barrier as low as 0.15eV under p-type conditions. These findings provide a plausible atomistic explanation for passivation recovery during light soaking and illuminated annealing in porous regions near amorphous/crystalline silicon interfaces.

Charge redistribution at the contact of two dissimilar metals, referred to as contact electrification, is ubiquitous in nanotechnologies. However, individual nanoscale heterojunctions remain difficult to access experimentally, particularly in terms of real-space charge redistribution and the corresponding electric field. Here we provide local, quantitative access to the hidden phenomena at individual nanoparticle–substrate heterojunctions by combining off-axis electron holography with a predictive computational twin based on orbital-free density functional theory. Electron holography measurements on a grounded metallic architecture isolate phase shifts that originate from work-function difference. In parallel, the computational twin model, calibrated solely by the work functions of the constituent silver and gold, reproduces the measured phase shifts without fitting to the holography data. This quantitative agreement unambiguously identifies work-function difference as the microscopic origin, revealing “work functions at work”. Moreover, charge redistribution is found not merely at the buried contact region but, unexpectedly, at the external surfaces of nanoparticles, causing electric field perturbations in surrounding environment. This spatially-resolved information—inaccessible to ensemble-average techniques—reveals how a single nanoparticle alters its local electrostatic conditions via contact electrification, with direct implications for catalytic properties. Our framework further provides a new capability to probe local contact potentials and to infer individual nanoparticle work function, enabling studies of metal–support interactions to single-particle resolution.

Joo and East have recently published a Comment on our article (F. Parisi et al., Phys. Chem. Chem. Phys., 2024, 26, 28037, https://doi.org/10.1039/D3CP06047K). The Comment is based on the wrong assumption that we misassigned the infrared spectrum of liquid diethylmethylammonium triflate [DEMA][TfO]. The authors incorrectly claim that our hypothesis was that the two bands are due to the NH stretch mode in two different ion-pair structural types. We clarify here that our original analysis did not invoke two separate, static ion-pair structures, but rather a continuum of dynamically evolving hydrogen-bonding environments that naturally produce a broadened, bimodal band shape. The results presented in our paper are aligned with the ones presented in the Comment. The Comment brings up the concept of Fermi resonance, which indeed gives a plausible explanation of the features seen in the experimental absorption spectra.

The electrical double layer (EDL) at the metal-solution interface is a nanoscale region where quantum mechanical metal electrons meet almost classical electrolyte species. Describing metal electrons with orbital-free density-functional theory (DFT), the recently developed density-potential functional theoretical (DPFT) model constitutes a computationally efficient approach to modeling the EDL. However, the performance of orbital-free DFT is less studied for interfaces than for bulk phases. Herein, we develop a constant-potential Kohn-Sham-Poisson-Boltzmann theory with exact kinetic energy as a benchmark for DPFT models. Solving Kohn-Sham and Poisson-Boltzmann equations self-consistently, we obtain electron density, electrostatic potential, and double-layer capacitance of the EDL, which are then used to assess DPFT models with Thomas-Fermi-von Weizsäcker (TFvW) or Pauli-Gaussian kinetic energy functional. In general, TFvW outperforms the Pauli-Gaussian kinetic energy functional for modeling EDL. In addition, a much smaller gradient coefficient in the TFvW functional than the default value of 5/3 is suggested for modeling the EDL. These findings are instrumental to the future development of orbital-free DFT for electrochemical interfaces.

Atomic-scale steps markedly influence electrochemical activity and stability and exhibit structural instability under electrochemical conditions. Yet the microscopic mechanisms that cause these behaviors remain largely unclear. Herein, we study the microstructure and thermodynamics of the electrical double layer at stepped electrodes, using the semiclassical density-potential functional theory. The theory captures trends observed in experiments regarding the differential capacitance and the potential of zero free charge (PZFC) with step density for stepped Au and Ag . Departing from the case of flat electrodes, the PZFC deviates from the potential of minimum capacitance at stepped electrodes, necessitating local PZFCs to describe heterogeneous surface charging conditions. Furthermore, linking step-induced PZFC shifts to changes of the surface tension, the theory predicts that step bunching is thermodynamically driven at more positive electrode potentials and sensitive to the electrolyte composition.

Electrocatalyst nanoparticles, attached to an electronically conductive support material, are key components that determine the performance and lifetime of electrochemical devices like fuel cells and electrolyzers. Differences in electronic and electrochemical properties between nanoparticles and support induce phenomena subsumed as electro-ionic metal-support interactions. These phenomena are responsible for heterogeneously distributed electron densities and electrical double-layer properties over the surface. The resulting local reaction environment (LRE), qualitatively different from that of single-crystalline extended surfaces, remains poorly understood. In an effort to address this shortcoming, the current work introduces the effective ion concentration as a quantitative descriptor for the LRE around supported nanoparticles. This property is defined as the average ion concentration over the reaction plane. Using gold-supported silver nanoparticles immersed in acidic solutions as a model system, we investigate how the effective proton concentration depends on the size and the packing density of nanoparticles, Fermi levels of nanoparticle and support materials, bulk electrolyte concentrations, and electrode potential. To further rationalize its impact on electrocatalytic activity, we define a complementary LRE descriptor that incorporates the effect of the local electrostatic potential. Based thereon, an activity descriptor is introduced by combining the two reaction-agnostic LRE descriptors with two reaction-specific kinetic parameters, viz., reaction order and transfer coefficient. Results are discussed in view of the suitability of the descriptors to be used in the design and optimization of nanoparticle-based electrocatalysts for electrochemical applications.

Electrocatalysis is greatly influenced by the local reaction environment, which is governed by the structure of the catalyst, the distribution of the electrolyte, and the local electric field. In catalytic systems comprised of complex molecular species like ionomers, the distribution of electrolyte can vary substantially, resulting in divers local reaction environments. In order to gain atom-scale insight into this micro-environment we construct a model system consisting of a platinum surface, varying levels of water, and a Nafion thin film and conduct molecular dynamics simulations. We employ a construction based on Voronoi tesselation to assemble a dense film of ionomer that fully covers the platinum substrate. An energy analysis reveals that water film configurations with thickness of less then 1.3 nm are stable. Simulations with charged platinum surfaces are analysed in view of electrostatic conditions and differential capacitance of the interface configuration. Trends observed in these properties can be interpreted in view of the crowding of hydronium ions or the Nafion film at the platinum surface. The presented workflow can be easily applied to investigate novel ionomers for use in PEMFCs.

The degradation of Pt in the cathode catalyst layer remains a formidable challenge for the longevity of polymer electrolyte fuel cells. In recent years, high surface area carbons have emerged as promising support materials for Pt in order to yield long lifetimes. Existing degradation models for catalyst layers usually assume homogeneity of composition, microstructure and conditions prevailing in catalyst layers that result in uniformly distributed rates of degradation processes. Our work presents an extension of a well-established physical-statistical model that includes dissolution, redeposition, coagulation, and detachment. For the first time, we examine the impact of structural heterogeneity as well as the role of local structural reconfiguration during operation on performance. Pt nanoparticles in proximity to ionomer are exposed to a significantly higher proton concentration and are therefore expected to experience higher rates of dissolution than particles not in proximity to ionomer, suggesting that two populations of nanoparticles should be distinguished. This distinction of two separate populations is implemented into our modified model. Over time, the microstructure surrounding these particles reconfigures, exerting a strong impact on ECSA loss and other degradation characteristics. Our parameter study reveals peculiar influences of kinetic rates and structural changes. This allows guidelines to be provided for the design of catalyst layer microstructures, including the distribution of ionomer therein.

The electric double layer (EDL) that forms at the interface between metals and ionic solutions is at the heart of various energy technologies. Recent experimental data have challenged our traditional understanding of the EDL charging behavior, which is based on mean-field Gouy-Chapman-Stern-type (GCS) models. In this article, we present a classical theory for the EDL, derived from first-principles statistical mechanics, that accounts for electron-ion correlation effects using the method of image charges and systematically extends beyond the mean-field level. Such electron-ion correlations introduce an additional interaction between the metal surface and electrolyte ions, significantly altering the EDL structure. Our theory, valid in the limit of dilute electrolyte solutions and weakly charged metal surfaces, achieves quantitative agreement with experimental capacitance data across a wide range of electrode materials and electrolyte solvents, and thus resolves long-standing questions on the origin of discrepancies to GCS predictions. Thereby, the framework conceptually unifies the processes of double-layer charging and ion adsorption (electrosorption), which are typically considered as distinct phenomena, but are shown to be manifestations of the same fundamental electrostatic principles.

Vanadium redox flow batteries (VRFBs) are a promising technology to capture and store energy from renewable sources, reducing the reliance on fossil fuels for energy generation. However, during the charging process, the parasitic hydrogen evolution reaction at the negative electrode affects the performance and durability of VFRBs. The evolution of hydrogen bubbles causes the loss of effective reaction area and blocks the transport of reactants. We employ the lattice Boltzmann method to investigate the two-phase flow transport in the negative electrode of VRFBs. Systematic parametric analyses reveal that increased gas production leads to uneven gas removal from the electrode, while an optimal flow rate can effectively remove bubbles and reduce external pumping energy. Additionally, increasing the compression ratio hinders gas removal but enhances electrode electrical conductivity. Overall, the present study provides valuable mechanistic insights into bubble generation at the negative electrode of VRFBs and offers a theoretical reference for designing and optimizing VRFBs.

Cation effects on the hydrogen evolution reaction (HER) have been recognized for nearly a century, yet their physical origins, particularly their strong coupling with pH, remain debated. Experiments show pronounced and often inverted trends in HER activity with cation identity and concentration, which vary with pH, electrode material, and applied potential. Despite extensive research, no unified framework has reconciled these observations. In this Perspective, we revisit these trends from an electrostatic point of view. Using Butler–Volmer–Frumkin theory, we show that inverted cation trends between acidic and alkaline electrolytes arise naturally from the opposite roles of Frumkin corrections in proton- versus water-mediated Volmer steps. Multiple inversions observed under alkaline conditions further expose the limitations of single-factor, primarily atomistic, explanations, while a recent electrostatic framework accounting for local potential and electric field effects offers a coherent picture of many trends. These considerations highlight interfacial electrostatics as a critical factor and motivate integrated approaches bridging continuum and atomistic descriptions.

Electrochemical impedance spectroscopy is widely used to probe transport and reaction processes in polymer electrolyte membrane fuel cell catalyst layers. However, the relationship between catalyst ink composition and catalyst-layer impedance remains poorly understood, despite its potential value for catalyst-layer design and degradation analysis. Here, a previously developed impedance model is coupled with a structure-based wetting model that predicts water-retention curves and ionomer-film connectivity from ink composition. The resulting framework is also used to assess possible origins of degradation-induced impedance changes. Using a semi-global fitting strategy, the model reproduces experimental spectra for different ionomer-to-carbon ratios. Increasing the ionomer-to-carbon ratio produces steeper water-retention curves and greater ionomer-film connectivity. The associated decrease in polarization resistance and contraction of the low-frequency impedance arc arise from the coupled effects of water uptake on oxygen transport and ionomer connectivity on proton transport. A single composition change is insufficient to explain the experimentally observed non-monotonic impedance evolution during accelerated stress testing. Instead, the degradation response likely reflects coupled changes in electrochemically active surface area, oxygen-reduction-reaction kinetics, pore structure, wettability, and transport properties. This work establishes a framework linking ink composition, catalyst-layer structure, and impedance, and demonstrates that degradation-induced impedance evolution requires a multi-parameter interpretation.

The structure and properties of the catalyst-ionomer interface at the cathode exert a major impact on the performance of polymer electrolyte membrane fuel cells. The interface is affected by both the chemical structure of the ionomer and the potential-dependent changes to the catalyst/support surface during operation. This work presents molecular dynamics simulations of the catalyst-ionomer interface for an expanded Pt/C-ionomer thin film model. Simulations reveal that the structure of the ionomer film is sensitive to the oxidation state of the carbon support, with the preferential ionomer orientation shifting from backbone-towards-carbon to sidechaintowards-carbon oxide. The equivalent weight of the ionomer is shown to determine ionomer packing at the catalyst surface, which could impact the local oxygen transport resistance. The equivalent weight also influences the local proton concentration (or pH) and the proton conductivity at the catalyst-ionomer interface. Shorter sidechain length also increases conductivity by forming larger water clusters that act as channels for hydronium mobility. Overall, the presented simulations demonstrate how the ionomer composition could be tuned to enhance performance via its impact on kinetic, ohmic, and transport losses in fuel cell voltage.

A classical coulombic correlation functional in one-loop (1L) and local-density-approximation (LDA) is derived for electrolyte solutions, starting from a first-principles many-body partition function. The 1L-LDA functional captures correlations between electrolyte ions and solvent dipoles, such as screening and solvation, which are ignored by conventional mean-field theories. This 1L-LDA functional introduces two parameters that can be tuned to the experimental dielectric permittivity and activity coefficients in the bulk electrolyte solution. The capabilities of the 1L-LDA functional for the description of metal-electrolyte interfaces are demonstrated by embedding the functional into a combined quantum-classical model. Here, the 1L-LDA functional leads to a more pronounced double-peak structure of the interfacial capacitance with higher peaks and shorter peak-to-peak distance, significantly improving the agreement with experimental data and showing that electrolyte correlation effects exert a vital impact on the capacitive response.

The accurate interpretation of measured electrochemical impedance spectroscopy (EIS) data is crucial for understanding degradation mechanisms in solid oxide cells (SOCs) [1]. EIS is a powerful tool for measuring SOCs in operando, but the analyses of EIS data requires physical models to decipher and interpret the underlying cell processes [2]. In this contribution, two connected physical models are presented. The first part focuses on a model of degradation processes in SOC. The key feature of the model that will be presented is the hierarchical approach linking local degradation at the particle level with performance effects on the electrode level. In the second part, a model for the electrochemical impedance spectrum of the anode active layer and the anode support layer will be presented. The underlying white box approach allows the interpretation of cause and effects involved in EIS experiments. In combination, the two models connect materials, component and cell properties, with the operating conditions of a SOC to the outcome of EIS experiments. The whole modeling framework allows fast computation on a normal laptop. In the first part, the degradation model will be described in more detail. The degradation model focusses on the fuel electrode, since the most severe degradation contribution originates in it [3]. The degradation framework, illustrated in Figure 1, comprises three interdependent structural levels. On the particle level , degradation takes place by Ostwald ripening, coagulation or poisoning of particles [4, 5, 6, 7, 8]. The rate of these particle degradation processes is influenced by the local reaction environment. First and foremost, the local electrode potential drives Ostwald ripening. On the electrode level, the ensemble of solid particles within the electrode is modeled. A statistical electrode is assumed and the local potential and current are calculated. The statistical electrode is built up in three steps with essential parameters passed from one to the next. We start with spherical particles describing the electrode. Two particle radius distributions (PRDs) – for metal and ceramic particles – capture the structural evolution. While ceramic particles remain static, the metal particles change with time under the influence of local conditions during cell operation and dependent on electrode properties [4, 5, 6, 7, 8]. In the “percolation theory” step (see Fig. 1), the electrode’s statistical microstructure is calculated from the PRDs. Assuming that the electrode is described by a porous medium, we can use percolation theory to extract the essential microstructural properties, such as the triple phase boundary length, and electronic and ionic conductivity [9, 10, 11, 12, 13]. These parameters provide a bridge between the structural evolution and the macroscopic performance of the electrode. In the performance model, we assume that the electrode consists of two interpenetrating homogeneous phases. Hence the electrode can be described by the phase’s conductivity and the reaction area between them. With porous electrode theory, spatially dependent variables, such as phase potentials and current distribution, are derived [14]. These local variables is fed back to the degradation processes, focusing on potential-driven Ostwald ripening as the dominant mechanism. This feedback loop allows for the iterative, self-consistent calculation of degradation in the fuel electrode. At the device level, the model computes the performance and health indicators like remaining useful life as functions of operational conditions over the lifetime of the cell. Additionally, diagnostic information like polarization curves and EIS spectra can be analyzed [15]. The description of the numerical model for electrochemical impedance spectra follows [15]. It is assumed that the SOC is isothermal and the pressure gradient across the cell is negligible. For the anode active layer, reaction processes are captured by the Butler-Volmer-equation and charge conservation is assumed. The transport of hydrogen through the anode support layer is described using Fick’s law of diffusion. The set of ordinary differential equations (ODEs) in the frequency domain, obtained for this system, can be solved numerically as a boundary value problem. Solving the ODE system for a range of perturbation frequencies leads to the electrochemical impedance spectrum. Our research has shown that the Warburg element alone is insufficient to describe the transport-related impedance contribution [15]. This contribution depends not only on the support layer's parameters but also on the electrochemical activity and capacitance of the adjacent active layer. The transport impedance of the support layer always depends on the adjacent electrode, especially on the electrode’s capacitance [16]. In case of a planar electrode, which was the adjacent electrode in case of Warburg’s experiment, the capacitance is three orders of magnitude smaller than the capacitance of a porous electrode [16]. In an SOC, the support layer is next to a porous electrode, and thus its transport impedance is not sufficiently described by the Warburg impedance [16]. This knowledge provides new access to understand impedance data as well as may help unravel degradation mechanisms in the electrode. From fitting, the impedance model can reveal effective parameters of the electrode like the effective ion conductivity and hydrogen diffusivity. When connecting this impedance model on the right loop in Figure 1 with the degradation model in the left loop, we found that the growth rate of metal particles increases with local potential, leading to faster growth near the dense electrolyte layer. This reduces the active surface area and the reaction rate in the active layer. Hence the anode active layer impedance increases. The model for the electrochemical cell impedance [15] shows that this local electrode degradation is connected to an increase of the transport impedance in the adjacent anode support layer. Furthermore, over time particle growth results in a loss of structural connectivity. When the metal particle network collapses, affected regions of the electrode become inactive and contribute only as ion-conducting domains. Over time, this leads to a shift the most active zones from the region near the electrolyte to region near the porous layer, increasing the ohmic resistance obtained from the impedance spectra. The feedback loop between the particle level and electrode-level properties allows the model to calculate lifetime, performance loss, and impedance spectra.We introduced a white-box modeling approach that captures the impact of local structural changes in the metal phase on the performance and impedance of a SOC fuel electrode, including its support. Two main observations become clear when performing a simulated degradation experiment: local degradation, originating at the dense electrolyte, affects hydrogen transport impedance in the anode support layer and the anode active layer shifts due to connection losses, increasing the ohmic resistance of the SOC. References [1] C. Mänken, et.al., J. Electrochem. Soc. 171 (2024) pp. 064503. [2] D. D. MacDonald, Electrochim. Acta 51 (2006) pp. 1376-1388. [3] M. B. Mogensen, et al., Fuel Cells 21 (5)(2021) pp. 415. [4] R. M. Darling, J. P. Meyers, J. Electrochem. Soc . 150 (2003) pp. A1523. [5] P. Urchaga, et al., Electrochim. Acta 176 (2015) pp. 1500-1510. [6] A. Kregar, T. Katrašnik, Open Physics 17 (1) (2019) pp. 779. [7] I. Lifshitz, V. Slyozov, J. Phys. Chem. Solids 19 (1961) pp. 35. [8] C. Wagner, Z. Elektrochem . 65 (1961) pp. 581. [9] P. Costamagna, et al ., Electrochim. Acta 43 (1998) pp. 375. [10] D. Bouvard, F.F. Lange , Acta. Metal. Mater. 39 (12) (1991) pp. 3083. [11] D. Stauffer, A. Aharony, Introduction to Percolation Theory: Second Edition, 1992, Taylor & Francis. [12] D. Chen,et al., J. Power Sources 191 (2) (2009) pp. 240. [13] S. H. Chan, Z. T. Xia, J. Electrochem. Soc . 148 (4) (2001) pp. A388. [14] M. Eikerling, A. A. Kornyshev, J. Electroanal. Chem . 453 (1998) pp. 89. [15] M. Knappe, A. Kulikovsky, J. Electroanal. Chem. 975 (2024) pp. 118773. [16] A. Kulikovsky, Electrochemistry Communications 84 (2017) pp. 28-31. Figure 1

The performance of water electrolyzers hinges on the availability of effective catalyst materials for the oxygen evolution reaction (OER). The electrocatalysis of the OER is particularly challenging in proton exchange membrane water electrolysis (PEMWE). Only few materials can withstand the corrosive conditions at the PEMWE anode whilst being active towards the OER. Iridium oxide, the state-of-the-art OER catalyst in PEMWE, represents an exception, providing both high activity and stability under oxidizing potentials in acidic electrolyte. The scarcity of iridium, however, makes it necessary to develop next-generation catalysts with improved iridium utilization that enable significantly reduced iridium loadings at PEMWE anodes. This contribution will discuss the atom-level principles underlying the unique properties of iridium oxide in defying the often-observed correlation between OER activity and corrosion of metal-oxide catalysts [1–3]. Commonly considered OER mechanisms require the cleavage of at least one metal–oxygen bond, which therefore must not be too strong for achieving optimal activity—a consequence of the Sabatier principle. On the other hand, a high strength of metal–oxygen bonds is required for (bulk) stability of the oxide lattice, resulting in a common tradeoff between OER activity and stability of metal-oxide catalysts [4]. The exceptional performance of iridium dioxide is explained based on a recently proposed OER mechanism that proceeds without requiring the splitting of metal–oxygen bonds [5]. Instead, the oxygen molecule evolves via a peculiar Ir–OOOO–Ir transition state, leaving all Ir–O bonds intact. This explains why iridium dioxide can present strong Ir–O bonds whilst still being highly active towards the OER, emphasizing the unique role of crystalline IrO 2 as anode catalyst in PEMWE. To achieve improvements in iridium utilization in practice, we recently developed a synthesis strategy leveraging the outstanding properties of crystalline iridium dioxide. Common synthesis routes require high-temperature steps to form the crystalline phase, which concomitantly leads to particle growth and a decreased active surface area. On the contrary, the new synthesis method proceeds under mild temperatures in the presence of a strong oxidizing agent to afford small (about 2 nm) IrO 2 nanoparticles with a high degree of crystallinity [6]. The obtained catalyst demonstrated high mass-specific OER activity with very good stability in ex situ glass-cell experiments. The synthesis method was scaled up to produce catalyst quantities that enabled the fabrication of membrane–electrode assemblies (MEAs). The results from in situ PEMWE testing confirmed the outstanding performance of the nano-crystalline IrO 2 catalyst. References: [1] S. Cherevko, T. Reier, A. R. Zeradjanin, Z. Pawolek, P. Strasser, K. J. J. Mayrhofer, Electrochem. Commun. 48 , 81–85 (2014). [2] N. Danilovic, R. Subbaraman, K.-C. Chang, S. H. Chang, Y. J. Kang, J. Snyder, A. P. Paulikas, D. Strmcnik, Y.-T. Kim, D. Myers, V. R. Stamenkovic, N. M. Markovic, J. Phys. Chem. Lett. 5 , 2474–2478 (2014). [3] T. Binninger, R. Mohamed, K. Waltar, E. Fabbri, P. Levecque, R. Kötz, T. J. Schmidt, Sci. Rep. 5 , 12167 (2015). [4] T. Binninger, G. C. Moss, Z. S. H. S. Rajan, R. Mohamed, M. H. Eikerling, ChemCatChem 16 , e202400567 (2024). [5] T. Binninger, M.-L. Doublet, Energy Environ. Sci. 15 , 2519–2528 (2022). [6] G. C. Moss, T. Binninger, R. Mohamed, et al. , In preparation .

Oxygen gas generated at the anode of PEM electrolyzers turns the flow in the anodic flow field into two-phase regime. Along with upscaling the electrolyzers and the objective to reach higher current densities, the impact of the two-phase flow regime becomes an issue of major concern for their future design and operating conditions. To observe and quantify gas bubble formation and local gas content in the flow field channels during operation using a high-speed camera, a test rig for a 25 cm2 scaled laboratory electrolyzer cell with a transparent flow field has been set up. Current-voltage curves were recorded, while monitoring at the same time the impedance and the gas evolution in the anode flow field. Gas bubbles were distinguished by a deep learning-based image processing algorithm, revealing enhanced gas area coverage and bubble size with increasing current density. Corresponding calculations from a two-phase flow mixture model show a similar trend for local gas content in the flow field. In the high current density region, a strongly increasing voltage along with changes in the impedance pattern coincide with the transition from the flow regime with individual gas bubbles to an interconnected gas flow regime in the majority of the cell.

Medium-temperature proton exchange membrane fuel cells (PEMFCs) operating between 120–160°C offer significant advantages for water and heat management, reduce platinum sensitivity to impurities, and enhance overall fuel cell efficiency [1,2]. Thanks to wide electrochemical window, high chemical and thermal stability, good proton conductivity, small vapour pressures, and therefore low flammability, Protic ionic liquids (PILs) have emerged as a promising class of electrolytes for these systems [3]. With the aid of molecular simulations we have investigated the critical role of water in PIL-water mixtures. The focus of our studies is on proton transfer mechanisms, water structuring, and the clustering/percolation behavior within the system. We will discuss the effects of varying water content on the formation of hydrogen-bond networks, the dynamic characteristics of the PIL-water system, and the interplay between vehicular and grotthuss-type proton transfer mechanisms. Our results reveal the critical water content required to activate distinct proton transfer pathways. We will show how different PIL acidities and anion types influence the clustering and percolation of water molecules. The results demonstrate that due to the strong electrostatic interactions, high acidity of the cations screen the movement of molecules and hydrogen carriers. On the other hand, by increasing the water content, the chance to activate a grotthus-type mechanism for more efficient proton conduction and mobility in less acidic systems increases. Our findings provide valuable insights for optimization of PIL-based electrolytes for medium-temperature PEMFCs. [1] Q. Li , R. He , J. Jensen and N. Bjerrum , Chem. Mater., 2003, 15 , 4896 [2] S. Peighambardoust , S. Rowshanzamir and M. Amjadi , Int. J. Hydrogen Energy, 2010, 35 , 9349 [3] A. Noda , M. A. B. Hasan Susan , K. Kudo , S. Mitsushima , K. Hayamizu and M. Watanabe , J. Phys. Chem. B, 2003, 107 , 4024 —4033