Hydrogen Evolution Electrocatalysts - From Pure Metals to Multicomponent Alloys

The ongoing climate crisis underscores the urgent need for an energy transition to limit greenhouse gas emissions and drastically reduce environmental impacts. During this transition, renewable energy sources should gradually replace fossil fuels, which are often associated with high carbon emissions. However, renewable sources have inherent variability. In this regard, hydrogen has emerged as a flexible and sustainable energy carrier, offering considerable potential for decarbonizing various sectors, including stable generation of electricity, energy supply balance, and applications in transportation and industry. A key aspect of adopting green hydrogen technologies is the advancement of high-performance catalysts that ensure the economic feasibility of electrolysis for green hydrogen production. A key goal in catalyst design is to substitute costly noble-metal catalysts with non-noble transition metal alternatives. To address the limitations in the catalytic performance of transition metal catalysts, alloying is employed as an effective method to boost their activities. In this research, alloying was used to combine various elements, beginning with pure Ni and gradually introducing more complexity to form a multicomponent high-entropy alloy. Multi-component alloys have garnered considerable interest in catalysis because of their potential for superior performance, enhanced stability, and flexible tunability; yet, a comprehensive understanding of the fundamental mechanisms often remains elusive. This work aims to closely investigate catalysts with increasing complexity in the context of the alkaline hydrogen evolution reaction. First, Ni thin film electrodes and their in situ oxidation were examined. The impact of partial surface oxidation on the electrocatalytic activity of the thin films was demonstrated and remains a constant factor when working with transition metal catalysts. In the next step, Mo was selected as the first alloying element. NiMo bimetallic alloys of various compositions were analyzed for their activity and stability during HER. For the first time, Mo leaching was confirmed on a microstructural level, revealing a significant alteration in surface composition and structure that influences the overall catalytic performance. Continuing in this direction towards high-entropy materials, FeCoNi, FeCoNiMo, and FeCoNiMoCu were studied. It was demonstrated that all effects observed and discovered in previous investigations, including surface oxidation, component leaching, and surface restructuring, significantly affect the behavior of FeCoNiMo(Cu) and are crucial to their overall catalytic activity. In addition to alloying, the effects and potentials of increased surface area were carefully monitored as a factor in overall catalyst performance. This work emphasizes the need for a holistic view of multi-component materials when utilized as catalysts. The final high-entropy catalyst activity is determined by a combination of multiple factors, including electronic synergistic effects arising from the alloying of components, as well as structural and stability effects, along with the influence of surface structures and exposed sites. Through a carefully designed experimental approach and multidisciplinary analysis techniques, the structural influences on the final performance were isolated, facilitating the elucidation of the fundamental mechanisms controlling high-entropy catalyst materials.