All-solid-state batteries (ASSBs) employing sulfide solid electrolytes and lithium metal anodes offer a promising pathway toward high energy density and enhanced safety. However, their practical deployment remains limited by strongly coupled electro-chemo-mechanical instabilities at both cathode and anode interfaces, which evolve across multiple length scales and remain insufficiently understood under realistic operating conditions. Here, we present a comprehensive multimodal operando characterization approach combining X-ray computed tomography (XCT), X-ray diffraction (XRD), and stack pressure monitoring to elucidate the dynamic degradation mechanisms in LiNi0.8Co0.1Mn0.1O2 (NCM811) | Li6PS5Cl (LPSC) | Li cells. By directly correlating particle-level morphological evolution with latticescale structural and strain dynamics, this methodology enables a unified, multiscale description of interfacial failure processes. Operando XCT and XRD jointly reveal reversible volume changes of ~6 % in NCM811 particles during (de-)lithiation. Spatially resolved operando XRD further uncovers pronounced heterogeneity in lithiation fronts and depth-dependent electrochemical activity across the cathode thickness. In parallel, a partial release of interfacial microstrain is identified at the NCM811|LPSC interface, associated with oxidative decomposition of LPSC solid electrolyte. At the LPSC|Li interface, operando XCT directly visualizes, at moderate current density (0.45 mA cm-2) but high areal capacity (4.17 mAh cm-2), crack initiation and propagation within the solid electrolyte, driven by the lithium plating-induced stress, followed by lithium infiltration, and progressive interfacial contact loss. These processes ultimately lead to mechanical disconnection and the emergence of short-circuiting at higher current densities. Notably, these mechanically induced degradation phenomena exhibit partial reversibility during lithium stripping. This multimodal operando framework provides critical insight into degradation pathways and establishes design principles for mechanically robust sulfide-based ASSBs.

The argyrodite-type solid electrolyte (SE) Li6PS5Cl (LPSCl), recognized for its high ionic conductivity and low-temperature processability, offers substantial potential for enabling lithium metal anodes in all-solid-state batteries (ASSBs), promising high energy densities with enhanced safety. However, lithium dendrite penetration and unstable solid electrolyte interphase (SEI) formation hinder stable cycling at high current densities. This work presents a synergistic strategy to address these challenges by combining mild sintering of LPSCl pellets with the deposition of a lithium fluoride (LiF) passivation layer on 50 µm thick lithium metal. Optimized sintering at 80°C improves surface uniformity and densifies the LPSCl pellets, reducing porosity and increasing ionic conductivity. Complementarily, the deposition of a uniform 65 nm LiF layer on lithium via electron beam evaporation, reduces interfacial resistance, and stabilizes SEI formation. This dual modification doubles the critical current density of lithium symmetric cells from 1.1 to 2.2 mA cm-2. In full cells configurations with LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes, remarkable cycling stability is achieved over 2700 cycles (at 1 mA cm-2, 1.5 mAh cm-2), with 75% capacity retained after 1500 cycles. This study provides a practical approach for improving both SE pellet quality and lithium-SE interfacial stability, paving the way for the reliable implementation of thin lithium metal in next-generation ASSBs.

Operando Fourier transform infrared (FTIR) spectroscopy is a powerful technique for investigating the electrocatalytic interface during the oxygen reduction reaction (ORR). This review summarizes the technical challenges and recent developments in applying operando FTIR to the Pt/electrolyte and Pt/ionomer interfaces. The different optical configurations are discussed, and the specific material requirements for internal reflection elements and enhancement layers are outlined. Afterwards, the spectral interpretation of key interactions is carefully examined, distinguishing between reaction intermediates and signals arising from the electrolyte. Particular attention is given to the interactions at the Pt-Nafion interface, where we discuss the identification of vibrational bands related to sulfonate groups and hydration networks. Finally, this review highlights how distinguishing these signals is essential for understanding the mechanisms that govern catalyst activity and durability in fuel cells.

The electrochemical reduction of carbon dioxide (or the CO2-reduction reaction, CO2RR) presents a promising strategy to mitigate CO2 emissions while producing valuable chemical feedstocks. Palladium (Pd) catalysts are particularly interesting for their capacity to selectively produce formate at low overpotentials and carbon monoxide (CO) at higher overpotentials. However, palladium's CO2-to-formate activity is often hindered by the progressive poisoning of its surface with CO. To shed light on the parameters that control this performance-determining process, in this study we employ Operando grazing incidence X-ray absorption spectroscopy and attenuated total reflectance surface-enhanced infrared absorption spectroscopy to investigate the CO2RR mechanism on carbon-supported Pd nanoparticles (Pd/C) and a freestanding Pd aerogel with similar electrochemical surface areas but substantial differences in hydride formation, CO poisoning, and catalytic performance. Pd/C demonstrates rapid hydride formation and constant formate activity at -100 and -200 mV vs the reversible hydrogen electrode, revealing an indirect correlation between activity for formate and hydride stoichiometry that strongly indicates the active involvement of the surface hydride in the CO2RR to formate. In contrast, the Pd aerogel suffers from rapid CO surface poisoning and a concomitantly negligible formate-production activity at the same potentials. These differences in catalytic behavior are linked to an increased presence of grain boundaries in the aerogel's surface that has been tied to a reduction in the activation barrier for CO2 conversion to the surface-adsorbed *COOH and the subsequent formation of strongly adsorbed CO. As such, our findings highlight how optimizing the structural features of Pd-based surfaces can lead to significant enhancements in their efficiency toward formate production.

Thin lithium metal anodes are key to realizing high-energy-density and enhanced safety in all-solid-state batteries (ASSBs). However, extruded lithium foils below 50 µm suffer from poor structural integrity, rough surfaces, and resistive, native passivation layers, which limit cycling stability. In this study, thermally evaporated, high-purity lithium with smooth grain-boundary surface morphology significantly improves interfacial contact and electrochemical performance. Replacing extruded lithium with a 50 µm evaporated lithium anode increases the critical current density (CCD) from 1.6 to 2.1 mA cm-2 and enables 161 cycles at 1.5 mA cm-2 and 1.6 mAh cm-2. A thickness-dependence study reveals that maintaining a sufficient lithium reservoir is crucial to mitigate void formation and compensate for solid electrolyte interphase (SEI) growth. To overcome limitations at reduced thickness, an ultra-thin 65 nm LiF passivation layer was applied to 25 µm evaporated lithium. The LiF coating suppresses chemical degradation during storage, stabilizes lithium/LPSCl interface, limits SEI growth, and mitigates dendrite, increasing the CCD to 2.6 mA cm-2. In full cells, LiF-coated 25 µm lithium delivers over 500 and 300 cycles at current densities of 1.5 and 3 mA cm-2 respectively. These results establish LiF-passivated thermally evaporated lithium as a high-performance anode design for next-generation ASSBs.

Rechargeable lithium (Li) metal batteries, which use Li metal as the negative-electrode, promise energy densities that are two times higher than those achievable with conventional Li-ion batteries. However, the practical application of Li-metal batteries is currently constrained, and where the central challenge is dendritic Li growth, leading to cell failure. To overcome these limitations, electrolytes compatible with Li metal are required, where various electrolyte formulations have been proposed, but a mechanistic understanding of their effects to the kinetics and dynamics of Li deposition remains incomplete. Here, we introduce operando neutron imaging as a versatile modality for observing electrolyte-dependent Li-deposit nucleation, Li-metal plating and stripping behaviour with high temporal and spatial resolution. We tested three different carbonate electrolytes with varying concentrations of fluoroethylene carbonate (FEC) and found that low levels of the FEC additive contribute to better Li cycling reversibility, while higher concentrations lead to adverse effects, revealing an unexpected and critical limitation of FEC-additive application for improving Li-metal cycling. Our imaging methodology can be a starting point for much broader operando studies of the plating and stripping behaviour of Li metal in any electrolyte, potentially making it a key tool for future electrolyte developments.

The alkaline hydrogen reaction on platinum is of great interest for alkaline fuel cell and electrolyzer systems. However, its mechanism is still debated. In this work, we performed extensive rotating disk electrode measurements at different H2-partial pressures and bulk-pH values in highly alkaline electrolytes to determine the reaction parameters and identify the reaction mechanism in different potential regions, namely, at high overpotentials and close to equilibrium, for polycrystalline platinum electrodes. Based on the precise kinetic parameters determined in this study, it was shown that the alkaline H2-evolution/oxidation reaction (HER/HOR) follows different mechanisms in different potential regions: from a Volmer-limited Tafel-Volmer mechanism around equilibrium shifting to a Heyrovsky-limited Heyrovsky-Volmer mechanism at higher HOR overpotentials. This conclusion is supported by the values of the transfer coefficients and H2-reaction orders determined by fitting a generalized electrochemical rate model across the comprehensive set of experimental data under various H2 pressures and solution pH values. The determined reaction orders in OH-, however, do not match any of the alkaline HER/HOR mechanisms and rather suggest an acidic pathway involving H+ instead of OH- as the active species, which is counter-intuitive given the highly alkaline pH of the bulk electrolyte solution. EIS experiments for quantifying the charge-transfer resistance and capacitance of the Hupd process (Volmer reaction) under HOR limiting current conditions support this picture and revealed an acidic reaction environment due to protons generated by the HOR. These findings reconcile previous hypotheses on the mechanism of the HER/HOR in alkaline electrolytes and reveal a surprisingly acidic character of the local reaction environment under reaction conditions.

In this study, the effect of the ionomer loading on the performance of a nickel single-atom catalyst (Ni-SAC) implemented as the cathode catalyst layer (CL) of a forward-bias bipolar membrane (FB-BPM) zero-gap CO2-electrolyzer was systematically investigated. By varying the ionomer-to-catalyst (I/C) mass ratio at a fixed catalyst loading (1.0 mg & centerdot;cm(-2)), we reveal a critical trade-off between ionic conductivity and mass transport within these Ni-SAC catalyst layers. While high ionomer loadings (i.e., I/C values >0.75) improve ionic transport, they simultaneously exacerbate pore blockage and water accumulation within the corresponding CLs, leading to mass transport limitations under full cell operation. As a result, Ni-SAC electrodes with low ionomer loadings (I/C <= 0.75) achieve CO Faradaic efficiencies (FECO) >= 80% at current densities <= 150 mA & centerdot;cm(-2), whereas higher ionomer contents lead to a decrease in FECO accompanied by an increase in cell voltage. Notably, all electrodes exhibit a sharp voltage increase at the latest when reaching a current density of 200 mA & centerdot;cm(-2), highlighting a system-level limitation that we associate with local CO2 depletion. These findings provide initial design guidelines for optimizing Ni-SAC/PiperION cathode CL-microenvironments in FB-BPM CO2-electrolyzers.

Effective water management in polymer electrolyte fuel cells (PEFCs) critically depends on the wettability of gas diffusion layers (GDLs), which controls capillary-driven liquid transport. While hydrophobic coatings (like polytetrafluoroethylene, PTFE) are widely used to tailor wettability, detecting and quantifying their distribution within the complex porous structure remains challenging. Wettability characterization serves as an indirect proxy but is complicated by the coupled effects of pore morphology and surface chemistry, even though effective contact angles remain a required input for capillary transport models. This work summarizes experimental and modeling approaches for GDL wettability assessment. Conventional techniques (sessile drop, Wilhelmy plate) provide surface-level information, whereas bulk methods (Washburn, capillary pressure-saturation (p c –S) curves, capacitance) capture macroscopic behavior but lack spatial resolution. Advanced imaging-based methods, such as environmental scanning electron microscopy and X-ray tomography with image-based contact angle analysis, enable 3D insight yet remain resource-intensive. Furthermore, the widespread concept of “mixed wettability,” attributed to inhomogeneous PTFE coating, is examined and the influence of pore geometry is highlighted, supported by pore-network modeling approaches. Together, these approaches reveal how pore geometry and surface chemistry jointly define GDL wettability.

Cobalt‐based oxides have been investigated as potential alternatives to Ir/Ru‐based oxides for catalyzing the oxygen evolution reaction (OER) in acidic media. Past research, however, is mainly focused on the spinel oxide structure so far. Exploring alternative crystal structures is essential for expanding the material library and developing highly efficient OER catalysts for acidic environments. As a proof of concept, we demonstrate that Co‐based perovskite oxides can drive acidic OER effectively. Appling hard/soft X‐ray absorption spectroscopy (hXAS/sXAS) characterizations, we show that the La and Ce doped SrCoO3 (denoted as LSC and CSC, respectively) have a bulk‐average Co oxidation state close to 3+ and surface‐dominant low‐spin CoIII species. Electrochemical analysis reveals that they only show one Co redox pair, similar to CoOOH in acidic environments. The recorded Tafel slopes are around ∼65 mV dec−1, comparable to the benchmarking Ir/Ru‐based catalysts. The combination of the spectroscopic and electrochemical findings presented here highlights the important role of low‐spin CoIII species in catalyzing OER in acidic environments and contributes to the rational design of non‐noble metal OER catalysts.

Fe‐doped transition metal oxides are leading candidates for noble‐metal‐free oxygen evolution reaction (OER) catalysts. However, distinguishing the commingled roles of Fe incorporation pathways and initial structure in governing catalytic activity of Co‐based materials remains incomplete. In this study, we systematically decouple the effects of bulk versus electrolyte Fe incorporation in crystalline, defective crystalline, and amorphous catalysts derived from the same CoSn(OH)6 precursor, ensuring identical metal compositions throughout. Electrolyte Fe incorporation equalizes the OER activity across all three structural variants, demonstrating that surface Fe chemistry can dominate initial structural effects on the reconstructed catalyst interface. Notably, amorphous CoSnOx benefits more from electrolyte Fe than from bulk Fe incorporation, pointing to the amorphization method as a critical design parameter for high‐performance OER catalysts.

The increasing concentration of CO 2 in the atmosphere has led to severe climate issues, highlighting the need for extensive research on technologies that aid to decrease the atmospheric concentration of this greenhouse gas. Among the approaches envisaged to valorize CO 2 , its electrochemical reduction into value-added products like CO has emerged as a particularly promising approach. However, the catalysis of this reaction is very demanding, since it requires high overpotentials that are often tied to an insufficient selectivity for the product of interest and a poor stability. In this context, Ni-based single atom catalysts (SACs) have gained significant attention owing to their excellent selectivity for CO generation, but the factors that determine their catalytic performance remain poorly understood. More precisely, the active sites in these SACs are believed to consist of atomically dispersed nickel ions coordinated by N-functionalities and embedded in a carbon matrix, and their geometry and electronic properties during the CO 2 -reduction reaction remain under debate. To shed light on this matter, in this study we synthesized a Ni-based SAC that achieved a Faradaic efficiency of ≈ 70 % for CO production at ─ 0.6 V vs. the reversible hydrogen electrode (RHE). Subsequently, operando X-ray absorption spectroscopy (XAS) experiments with a time resolution of 1minute were conducted to observe changes in the catalyst when cycling the potential between the open-circuit voltage (OCV) and -0.6 V vs. RHE. The results revealed potential- and oxidative- vs. reductive-dependent changes in the Ni sites’ structure, and the kinetic behavior of these changes was also analyzed quantitively. Additionally, by performing these operando XAS measurements in 0.5 M KHCO 3 saturated with CO 2 and in a CO 2 -free phosphate-buffer electrolyte with the same pH, we demonstrated how these variations in the Ni sites’ structure are affected by the presence of CO 2 . These findings provide valuable insights into the activation mechanisms of the CO 2 -reduction reaction on Ni-SACs, and offer a promising pathway for the development of more efficient and sustainable electrocatalysts for this reaction.

Bifunctional oxygen electrocatalysts are subjected to stringent performance and stability criteria. The catalyst must achieve high oxygen evolution reaction (OER) activity while in electrolyzer operation, as well as high oxygen reduction reaction (ORR) activity while in fuel cell operation. Additionally, the catalyst must be stable over a wide potential range and withstand alternating reducing and oxidizing potentials. In this work, a composite Ni0.95Fe0.05O1±δ/NiCo2O4 is rigorously tested as a bifunctional catalyst for anion exchange membrane (AEM) fuel cell and electrolyzer operation. An alternating potential stability test is performed, which unveils the areas where the bifunctional catalyst needs improvement. The OER activity of the catalyst is not hindered by the harsh conditions. However, the ORR activity deteriorates. Both the fundamental rotating disk electrode (RDE) methodology and AEM single-cell testing are used to evaluate the electrode activity and stability. The difference in results between the two techniques emphasizes the importance of evaluating the catalyst under applied conditions. The results of this study provide guidance for the development of new high-performing bifunctional catalysts.

Metal‐organic frameworks (MOFs) as electrocatalysts for the alkaline oxygen evolution reaction (OER) show promising catalytic activity by offering great variability and high surface areas, enabling performance optimization and mechanistic studies. However, their stability during reaction and the structure‐performance relationship defining the origin of the high OER activity, are still vigorously debated. Herein, operando X‐ray absorption spectroscopy and operando X‐ray diffraction are applied to unveil the structural and electronic transformations of Ni‐MOF‐74 during OER. The irreversible destruction of the MOF‐74 crystal into a highly OER active, amorphous NiOOH‐metal organic compound is identified. Based on these findings, an amorphous Ni metal organic compound (Ni‐MOC*) is proposed for achieving high current densities both in a three‐electrode cell (14 A g Ni −1 at 1.5 V RHE ) and in an anion exchange membrane water electrolyzer (AEM‐WE) with a stable AEM‐WE performance exceeding 100 h at 500 mA cm −2 .

Zero-gap CO2-electrolyzers using a forward bias bipolar membrane (BPM) are becoming increasingly appealing, since this configuration addresses the issues of CO2 pumping and salt precipitation observed with other approaches. However, such CO2-electrolyzers often suffer from BPM-delamination caused by the generation of water and gaseous CO2 at the junction between cation- and anion-exchange membranes. To circumvent this, in this study we used a rigid titanium porous transport layer (PTL) at the cathode to mechanically suppress the membrane delamination and managed to operate such cells at current densities >100 mAcm(-2). In doing so, we compared the performance differences caused by the implementation of a catalyst-coated membrane (CCM) or a gas diffusion electrode (GDE) at the cell's cathode. These combinations of diffusion media and catalyst layer (CL) deposition approaches result in five different configurations that systematically featured a current-driven rise in high-frequency resistance (HFR) and CO selectivity when operated at current densities <100 mAcm(-2), whereas at current densities >100 mAcm(-2), both HFR and CO selectivity decreased. By determining the water balance at the cathode compartment and BPM-junction, we propose that variations in membrane-CL humidification are tied to this unambiguous correlation between HFR and selectivity across all tested configurations, which we attribute to the concomitant changes in water and ion distribution (and thus pH) along this key operational interface.

The limited choice of oxygen evolution reaction catalysts for proton exchange membrane water electrolyzers hinders their large-scale commercialization. Cobalt-based catalysts are promising candidates and usually undergo surface reconstruction into CoOOH-like structures. However, the directly synthesized CoOOH has not yet been investigated in acidic environments. Here, we show that the CoOOH is active across the whole pH range, while its redox features are pH dependent. Operando hard X-ray absorption spectroscopy characterizations show a pH-induced change in Co oxidation onset, but no change in the coverage of redox-active Co species before the oxygen evolution reaction. The pH-dependent catalytic performance is connected to the interfacial Co oxidative transformations under electrocatalytic conditions. By combining the kinetic isotope effect and the apparent activation energy with theoretical verification, we offer the mechanistic discussion of the possible reaction pathway for CoOOH. In addition, CoOOH demonstrates a stable cell potential of 100 mA cm −2 for 400 h in a proton exchange membrane water electrolyzer. These results shed light on both the fundamental electrochemical properties of CoOOH and its potential for practical device applications.

Water electrolysis shows great promise for clean hydrogen production from renewable energy, particularly through polymer electrolyte water electrolyzers (PEWEs). While these devices can produce high-purity, high-pressure hydrogen, they rely on expensive RuO 2 or IrO 2 catalysts for oxygen evolution. Though RuO 2 is more active, only IrO 2 -based catalysts provide the technically relevant long-term stability. Combined with catalyst activity and stability developments, reduction of the noble-metal loading on the electrolyzer anode is of utmost technical interest. In that quest, the development of the membrane electrode assembly including the proper engineering of the catalyst layer-porous transport layer interface is in the focus of academic and industrial research groups. In this presentation, our efforts to develop and understand both OER catalysts and their catalyst layers as well as the properties of the porous transport layers will be summarized.

Electrochemical urea synthesis (EUS) from CO2 and nitrates has recently emerged as a more sustainable alternative to nitrogen fertilizers derived from fossil fuels. Indeed, using captured CO2 and nitrates from wastewater can offer environmental benefits compared to conventional methods. On the road to EUS technology development, its accurate and reliable quantification is an undeniable cornerstone. As this field is still in its infancy, with very low product concentration and numerous side‐products, EUS product quantification is challenging, with reported false positives and negatives. Despite the consensus that at least two methods ought to be used, the selection of the most suitable methods and quantification protocols is an open topic in the scientific community. This work presents a comparative study of the most common methods, highlighting their advantages, limitations, and recent developments, aiming to provide valuable insights to guide the advancement of this emerging field and facilitate the upscaling of sustainable fertilizer production.

Focusing on five key concepts, we review the roles of cation and oxygen vacancies in determining the surface reconstruction pathway, reaction mechanism, and ultimate activity of cobalt-based oxygen evolution reaction (OER) electrocatalysts. Cation and oxygen vacancies can initiate reactant adsorption, facilitating active surface reconstruction, and can switch the dominant mechanism from the adsorbate evolution mechanism (AEM) to the lattice oxygen evolution mechanism (LOEM). However, these effects are facet-dependent. Rigorous oxygen vacancy quantification promises to identify the OER mechanism steering thresholds and unlock the full potential of vacancy engineering. Finally, oxygen vacancy quantification strategies are critically examined to facilitate this goal.