Thomas Quast

Understanding the competition between the electrochemical CO ₂ reduction reaction (eCO ₂ RR) and the hydrogen evolution reaction (HER) is crucial for efficient CO ₂ conversion, particularly under limited reactant availability using diluted CO ₂ . We investigate the interplay between the operating conditions at various differential backpressures concerning CO ₂ availability, water behavior at the electrified catalyst layer, and how these factors modulate the eCO ₂ RR‐to‐formate selectivity and HER activity. We show that H ₂ formation can be suppressed while maintaining high formate selectivity even using highly diluted CO ₂ (2.25%) by increasing the differential backpressure from the gas side of the gas‐diffusion cathode in the flow‐through electrolyzer, which enhances CO ₂ transport to the catalyst layer through reshaping the triple‐phase boundary. Operando Raman spectroscopy reveals the accumulation and perturbation of water on the catalyst layer at negative potentials, accompanied by weakening of the hydrogen‐bond network. Moreover, even higher differential backpressure minimizes carbonate formation, preserves eCO ₂ RR kinetics, and promotes the displacement of surface water, which in turn minimizes HER.

Multi‐metal catalysts offer opportunities for high activity and selectivity in CO ₂ electroreduction (eCO ₂ RR) through multiple active sites and synergistic effects; however, achieving high current densities remains challenging. Cu _x Ce _y Ag _z nanoparticles (NPs) were synthesized via a modified solvothermal method, with Cu ₉ Ce ₉ Ag ₁ identified as the optimal composition. This catalyst sustained activity up to −1 A cm ⁻² and delivered a partial current density of −707.2 mA cm ⁻² for C ₂₊ products at an iR‐corrected potential of −0.68 V. Ag promotes high local CO generation, while Ce modulates Cu oxidation states, optimizing Cu─O surface chemistry and *CO speciation to favor C─C coupling. In situ mass spectrometry indicated competitive proton utilization among H ₂ , ethylene, and methane, with *CO dimerization governing ethylene production. Operando Raman spectroscopy revealed enhanced *CO coverage, a higher *CO _atop /*CO _bridge ratio, stable Cu─O species, and increased local pH on Cu ₉ Ce ₉ Ag ₁ NPs, collectively suppressing the hydrogen evolution reaction and accelerating CO ₂ RR kinetics. These findings highlight the possibilities to rationally design multi‐metal catalysts to maximize synergetic effects, enabling high C ₂₊ selectivity at industrially relevant current densities.

The discovery of high‐performance electrocatalysts for water electrolysis is highly important. We use a strategy for catalyst discovery based on high‐throughput screening of a broad range of materials compositions on a thin‐film materials library to identify noble‐metal‐lean multi‐metal compositions with high activity towards the hydrogen evolution reaction in alkaline electrolyte. We demonstrate this strategy using a quaternary materials library containing Mo, Ag, Ti, and Ru fabricated by combinatorial magnetron sputtering on a 10 cm diameter wafer providing 342 measurement areas. Surprisingly, binary Mo−Ru‐containing catalyst compositions with comparatively low Ru content exhibited the highest activity. Using the polymer/metal precursor‐based spray technique, Ni foam electrodes were modified with the Mo−Ru hit compositions and evaluated in a model electrolyzer in membrane electrode assembly (MEA) configuration. The electrodes showed a very low overpotential of only 132 mV at a comparatively high current density of −2 A cm⁻² and a 24 h electrolysis stability at −1 A cm⁻² with no observable degradation after the initial electrode conditioning.

Wir schlagen eine hybride elektrokatalytisch‐bioelektrokatalytische Reaktionskaskade vor, die auf einer Gasdiffusionselektrode zur CO₂‐Reduktion unter selektiver Bildung von Methanol integriert ist. Ag‐Bi₂O₃ reduziert gasförmiges CO₂ bei neutralem pH‐Wert selektiv zu Formiat. Eine nachfolgende enzymatische Kaskade aus Formaldehyd‐Dehydrogenase und Alkohol‐Dehydrogenase, die beide von Nikotinamid‐Adenin‐Dinukleotid (NADH) abhängig sind, reduziert Formiat weiter sequentiell zu Formaldehyd und Methanol. Der enzymatisch oxidierte Redoxkofaktor NAD⁺ wird durch das Enzym Diaphorase regeneriert, das elektrochemisch an die Elektrode gekoppelt ist, indem es in ein Redoxpolymer auf Cobaltocen‐Basis eingebettet ist. Die Methanolbildung wird mithilfe eines hochselektiven Biosensors nachgewiesen, der auf Alkoholoxidase in Kombination mit Meerrettichperoxidase basiert, die in ein speziell angepasstes Redoxpolymer integriert sind. Die vorgeschlagene hybride elektrokatalytisch‐bioelektrokatalytische Kaskade ist ein Beispiel für eine allgemeine Strategie zur Verbesserung der Selektivität elektrokatalytischer Reaktionen und/oder zum Ersatz bioelektrochemisch anspruchsvoller Schritte wie der Synthese von Formiat aus CO₂ unter Verwendung sauerstoffempfindlicher Formiatdehydrogenasen durch robustere anorganische Elektrokatalysatoren.

We propose a hybrid electrocatalytic‐bioelectrocatalytic reaction cascade integrated on a gas diffusion electrode for CO₂ reduction under selective formation of methanol. Ag‐Bi₂O₃ selectively reduces gaseous CO₂ to formate at neutral pH conditions. A subsequent enzymatic cascade comprising formaldehyde dehydrogenase and alcohol dehydrogenase, which are both nicotinamide adenine dinucleotide (NAD)‐dependent, further reduce formate sequentially to formaldehyde and methanol. The enzymatically oxidized redox cofactor NAD⁺ is regenerated through the enzyme diaphorase which is electrochemically coupled to the electrode by embedding it into a cobaltocene‐based low‐potential redox polymer. Methanol formation is confirmed using a highly selective biosensor based on alcohol oxidase in combination with horseradish peroxidase integrated into a specifically adapted redox polymer. The proposed hybrid electrocatalytic‐bioelectrocatalytic cascade is an example for a general strategy for enhancing the selectivity of electrocatalytic reactions and/or to replace bioelectrochemically challenging steps such as the synthesis of formate from CO₂ using highly oxygen‐sensitive formate dehydrogenases by more robust inorganic electrocatalysts.

The electroreduction of nitrate offers a promising, sustainable, and decentralized route to generate valuable ammonia. However, a key challenge in the nitrate reduction reaction is the energy efficiency of the reaction, which requires both a high ammonia yield rate and a high Faradaic efficiency of ammonia at a low working potential (≥−0.2 V versus reversible hydrogen electrode). We propose a bimetallic Co–B/Ru₁₂ electrocatalyst which utilizes complementary effects of Co–B and Ru to modulate the quantity of adsorbed hydrogen and to favor the specific hydrogenation for initiating nitrate reduction reaction at a low overpotential. This effect enables the catalyst to achieve a Faradaic efficiency for ammonia of 90.4 ± 9.2% and a remarkable half-cell energy efficiency of 40.9 ± 4% at 0 V versus reversible hydrogen electrode. The in-situ electrochemical reconstruction of the catalyst contributes to boosting the ammonia yield rate to a high level of 15.0 ± 0.7 mg h⁻¹ cm⁻² at −0.2 V versus reversible hydrogen electrode. More importantly, by employing single-entity electrochemistry coupled with identical location transmission electron microscopy, we gain systematic insights into the correlation between the increase in the catalyst’s active sites and its structural transformations during the nitrate reduction reaction.

Mediated processes using a solid material, often called “solid booster”, have been proposed to increase the energy density in redox flow batteries (RFB). The strategy alters the energy storage in the dissolved redox species to a solid active material placed in a compartment of the device. Understanding the reaction kinetics of the dissolved redox mediator and the solid booster is crucial for proposing feasible pairs of solid boosters and dissolved redox mediators. We demonstrate a nanoelectrochemical methodology to monitor the reaction between the dissolved species in solution and the solid active material electrodeposited in recessed carbon nanoelectrodes. Our strategy overcomes issues inherent to standard methodologies, such as mass transport limitation, and evaluation of the intrinsic reactivity of the solid material. As a proof of concept, Prussian blue was electrodeposited in a recessed carbon nanoelectrode and used as a confined‐solid material platform to evaluate the reaction between the reduced form of Prussian blue and triiodide, . A high conversion rate of the solid booster was observed in the presence of μM concentrations of the dissolved redox species. The proposed nanoelectrode was successfully employed as a potentiometric sensor to monitor the evolution of the reaction with the dissolved active species.

Developing a hydrogen evolution reaction (HER) electrocatalyst that can steadily drive a large current density is of great significance for industrial electrochemical water‐splitting technology. Herein, the compositionally complex noble‐metal‐lean system Co–Cu–Mo–Pd–Re is investigated for alkaline HER by a scanning droplet cell‐assisted high‐throughput screening of co‐sputtered thin‐film material libraries. The identified hit compositions are used to prepare a catalyst‐coated Ni foam‐based layer that is hot‐pressed with a specifically designed PEEK‐covered carbon‐based gas diffusion layer to obtain a novel‐design gas diffusion electrode (GDE). H₂ bubbles that collect on the electrode surface are effectively reduced by controlling the hydrophobicity of the catalyst layer of the GDE, hence enabling a high current (−1 A cm⁻²) operation for HER in alkaline water electrolysis.

Die Erhöhung der Widerstandsfähigkeit von Katalysatoren gegen elektrochemische Degradation ist eine der wichtigsten Voraussetzungen für den breiteren Einsatz von Protonenaustausch‐Membran‐Brennstoffzellen (PEMFCs). Hier untersuchen wir die Degradation eines einzelnen Partikels eines hochstabilen Katalysators, Pt@HGS auf einer Nanoelektrode unter nicht‐limitierenden Massentransportbedingungen. Wir stellen fest, dass der Katalysator schneller degradiert als aufgrund früherer Ensemblemessungen erwartet wird. Auf Basis von Transmissionselektronenmikroskopie (TEM) an identischen Stellen nach elektrochemischer Belastung und Experimenten mit einer Katalysatorschicht auf einem TEM‐Träger schließen wir, dass lokal unterschiedliche pH‐Werte wahrscheinlich der Grund für diesen Stabilitätsunterschied sind. Letztendlich bietet diese Arbeit Einblicke in die tatsächlichen Bedingungen in einer PEMFC und wirft Fragen zur Anwendbarkeit von beschleunigten Belastungstests auf, die normalerweise zur Bewertung der Katalysatorstabilität durchgeführt werden, insbesondere wenn sie in Halbzellenaufbauten unter Inertgas durchgeführt werden.

Increasing the resistance of catalysts against electrochemical degradation is one of the key requirements for the wider use of Proton Exchange Membrane Fuel Cells (PEMFCs). Here, we study the degradation of one entity of a highly stable catalyst, Pt@HGS, on a nanoelectrode under accelerated mass transport conditions. We find that the catalyst degrades more rapidly than expected based on previous ensemble measurements. Corroborated by identical location transmission electron microscopy and catalyst layer experiments, we deduce that locally different pH values are likely the reason for this difference in stability. Ultimately, this work provides insights into the actual conditions present in a PEMFC and raises questions about the applicability of accelerated stress tests usually performed to evaluate catalyst stability, particularly when they are performed in half‐cell setups under inert gas.