Christina Roth

In order to scale electrochemical CO ₂ reduction (CO ₂ R) from H‐cells to zero‐gap electrolyzers in an industrial setting, the focus must shift from potential‐ to current‐controlled operation. This work studies a two‐gap flow cell in three‐electrode configuration as an intermediate platform that enables high current densities while retaining potential control, using the ethylene‐selective CO ₂ R on copper electrodes as a model reaction. Key limitations of potentiostatic operation are identified, including ohmic losses, gas evolution, and resulting instabilities that alter reaction stoichiometry and thus affect selectivity. Galvanostatic operation mitigates these instabilities and provides more robust and reproducible performance. Pulsed electrolysis protocols, which are traditionally operated under potential control, favor C ₂ H ₄ as a product. To mimic such potentiostatic regeneration pulses in a current‐controlled protocol, we transferred them to a charge‐controlled scheme by implementing additional anodic current steps. This strategy enables the study of pulsed operation under realistic, industrially relevant conditions and demonstrates the versatility of the two‐gap cell bridging the gap between fundamental mechanistic studies and practical zero‐gap electrolyzer applications.

The production of hydrogen via acidic water electrolysis requires efficient oxygen evolution reaction (OER) at high current densities. However, this is often limited by oxygen bubble accumulation, which blocks active sites and restricts water access. Although bubble release and water access are closely linked, these interfacial processes are rarely controlled together. We engineer a ruthenium oxide–cerium oxide (RuOx–CeOx) catalyst where CeOx is observed to modulate surface oxygen vacancies during operation, enhancing water availability and promoting bubble detachment in RuOx. In situ X-ray absorption studies show that CeOx dynamically facilitates the formation of surface oxygen vacancies, and Raman spectroscopy indicates improved electrolyte wetting under applied potentials. Simulations reveal that oxygen-depleted RuOx surfaces favor water accumulation, thereby facilitating the initiation of OER. This results in distinct bubble dynamics with a four-fold reduction in median bubble contact area and lower overpotentials at high current densities in RuOx–CeOx compared to RuOx. Implementation in a proton exchange membrane water electrolyzer achieves a 0.1 V reduction in full-cell potential at 1 A·cm–2, linking surface chemistry to device-level outcomes.

Commercial polyolefin separators for lithium-ion batteries (LIBs) exhibit only inadequate wettability and thermal stability. In large-scale production, high electrolyte uptake and wetting are essential to enable rapid electrolyte filling during battery assembly to reduce costs. In addition, it is imperative to develop separators with enhanced thermal stability for improved performance and safety. The focus of this study is the structure–property–performance relationship of separator coatings. Platelet-shaped glass particles are utilized as inorganic coating material for polyethylene (PE) separators. Styrene–butadiene rubber (SBR) was selected as binder due to its high thermal stability. Hybrid separators are prepared using a colloidal coating technology employing a water-based slurry. As the excessive use of binder in the coating can block pores, precise control of the binder content was essential to maintain battery performance. The resulting separators with an optimized binder content of 1 wt.% in the coating demonstrate high porosity, instantaneous wetting with electrolyte, and a 25 K increase in onset temperature for shrinkage. The utilization of glass platelets with an aspect ratio of 10 as coating material provided the best balance among processability, coating homogeneity, thermal stability, ionic conductivity, and cycling performance under the investigated processing conditions.

This study investigates the influence of flow-through electrode length in redox flow batteries on mass transport and related changes in battery resistance and performance metrics. A modular test cell was used that accommodated electrodes between 3 to 20 cm long. Experiments were carried out at 10 and 50% state of charge and with different electrolyte velocities. Parasitic effects such as varying pumping power and inductive distortions were accounted for, enabling fair comparison of electrochemical results. Electrochemical impedance spectroscopy and pulse tests revealed that for the 20 cm electrode, mass transport can be responsible for over 50% of total cell resistance even with a low current density, and that its influence increases for higher currents and lower velocities. By reducing the electrode length to 3 cm, this value decreased to less than 12% for the same operating conditions. Polarization curves and cycling tests demonstrated the resulting impact on system performance, specifically reaching 30% more discharge capacity, higher discharge current densities up to 560 mA cm ⁻² , as well as a threefold increase in maximum discharge power density. As commercially available graphite felts were employed, this study suggests a scalable path to high-power flow-through redox flow stacks.

Electrochemical energy conversion involves both steady-state and transient phenomena, with the latter being difficult to probe under realistic conditions. Conventional in situ/operando X-ray absorption spectroscopy often lacks time resolution and provides mainly qualitative insights. In this study, we show that the Δμ XANES method, applied to the Pt/H3PO3 system, enables semi-quantitative tracking of adsorption dynamics. It distinguishes H3PO3 concentrations and reveals competition for Pt sites among O-, H-, CO-, H3PO3-, and H3PO4-derived species. Despite XAS being bulk-sensitive, Δμ yields indirect yet reliable surface information. Coupled with time-resolved fixed-energy X-ray absorption voltammetry (FEXRAV), we monitor in real time the removal of CO and its competition with H3PO3, a key impurity in high-temperature polymer electrolyte membrane fuel cells. Machine learning validation confirms predictive power despite low signal-to-noise ratio. This establishes Δμ XANES combined with FEXRAV as a versatile tool for probing interfacial dynamics in fuel cells, electrolyzers, and CO2 reduction technologies.

In the emerging hydrogen energy economy, proton-exchange membrane fuel cells (PEMFCs) serve as a key enabling technology, yet their cost is among other things dominated by platinum group metals-based cathode catalysts. This paper is focused on investigation of intermetallic Pt-Co and Pt-Ni nanoparticles supported on carbon (Ketjen Black, reduced graphene oxide) as low-Pt-load candidates for high-temperature PEMFCs (HT-PEMFCs) operated at elevated temperature ~180 °C in the presence of concentrated phosphoric acid. Catalytic activity toward the oxygen reduction reaction (ORR) was quantified by rotating electrode measurements (exchange current densities, Tafel slopes), and stability was probed by leaching in 97.6 wt.% H₃PO₄ at 180 °C followed by post-exposure characterization. A suite of techniques – XAS, XRD, TEM/EDS, XRF, Raman spectroscopy and ICP-OES – was used to study changes composition and structure during degradation. All alloy catalysts showed in HClO₄ at 25 °C higher ORR activity than commercial Pt/C. However, exposure to concentrated H₃PO₄ at 180 °C caused electrochemically active surface area loss, reduced ORR activity, Pt crystallite growth, Co/Ni dissolution, and surface reorganisation. Comparatively, reduced graphene oxide-supported catalyst was more resistant to ripening and dealloying than its Ketjen Black analogue, and Pt–Ni alloy was more stable than Pt–Co. Overall, the results disentangle the roles of the carbon support and alloy composition and outline activity – stability trade-offs that guide the design of low-Pt loading cathodes for HT-PEMFCs.

The intelligent design of hierarchical metallic structures with optimized performance for targeted applications, such as energy devices, sensors, and catalysis, remains a significant challenge. In this study, electrochemically generated hydrogen bubbles are employed as dynamic negative templates for copper electrodeposition. This so‐called dynamic hydrogen bubble templating approach (DHBT) yields highly porous hierarchical copper foams adorned with surface nano‐structures. A comprehensive investigation of DHBT synthesis parameters is provided, organized into four categories: (1) deposition current density and time; (2) current modes, namely direct, pulsed, reversed, and alternating regimes; (3) physical conditions, including stirring and temperature; and (4) bath composition. The results demonstrate that morphological descriptors, such as pore size and density, foam thickness, electrochemically active surface area (ECSA), and nanoscale surface features, can be systematically and reproducibly tuned by varying these DHBT parameters. As a proof of concept, a simple three‐step protocol for the fabrication of copper foam gas diffusion electrodes (GDEs) is presented. The resulting GDEs show promising CO ₂ reduction performance, achieving C ₂₊ products Faradaic efficiencies of approximately 50% at ‐1.1 V versus reversible hydrogen electrode (RHE) and partial current densities of up to 104 mA cm ⁻² at ‐2.5 V versus RHE, with good operational stability tested for 12 h.

The thermal characteristics and surface properties of battery separators are commonly modified by the incorporation of inorganic particles into a polymeric matrix material. At present, the particles employed are predominantly of an arbitrary shape. Herein, we demonstrate significantly improved battery safety features using a glass-based separator consisting of platelet-shaped particles. Glass is selected due to its temperature stability and the freedom of design that it offers when particles are formed directly from the melt. The influence of the particles’ aspect ratio and layer stacking on the electrochemical properties was analyzed, and a parametric study of glass particle layers as function of edge length and thickness was conducted. Particles with an excessively high aspect ratio impede the Li⁺ diffusion pathway, thereby negatively affecting the performance and stability of the battery cell. Conversely, if the aspect ratio is insufficient, a deterioration in cell performance can be observed, particularly at elevated C-rates, due to the high specific surface area of the particles. Hence, the utilization of particles with a moderate aspect ratio of about 10 and a thickness of around 1 µm is proposed to ensure optimum performance.