Exploration of nanostructured high-entropy alloys for key electrochemical reactions: a comparative study for the solid solution systems Cu–Pd–Pt–Ru, Ir–Pd–Pt–Ru and Ni–Pd–Pt–Ru

Electrocatalysis is critical for mitigating climate change by providing green energy solutions, <i>e.g.</i> for hydrogen production by electrolysis of water implying high catalytic activity not only for hydrogen evolution but also for oxygen evolution as the counter reaction. Moreover, reactions such as oxygen reduction and nitrate reduction are of high importance in fuel cells or for environmental remediation. This study focuses on the exploration of electrocatalysts in the enormous composition spaces encountered in multinary materials like high-entropy alloys in the form of compositionally complex solid solutions. These provide paradigm-changing design principles for new electrocatalysts based on their tuneable surface atom arrangements resulting from their multinary composition. However, to master the combinatorial explosion problem of polyelemental catalysts, efficient exploration approaches need to be adapted. For this purpose, we present a comprehensive strategy to compare the electrocatalytic activity for different reactions in alkaline media, namely the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER) and nitrate reduction reaction (NO_<i>x</i>RR) over large compositional spaces in three multinary systems: Cu-Pd-Pt-Ru, Ir-Pd-Pt-Ru and Ni-Pd-Pt-Ru. To generate the necessary large and multidimensional experimental dataset, thin-film materials libraries were synthesised and analysed using high-throughput characterisation methods. This allows for a comparative overview over correlations between composition and electrocatalytic activity, considering also relevant information on crystal structure and surface morphology. Similarities and differences, trends, maxima and minima in electrocatalytic activity are revealed and discussed. Main findings include that for the OER Ir₂₃Pd₃Pt₈Ru₆₆ exhibits the highest activity, exceeding any alloy of the other two systems by 51% (Ni-Pd-Pt-Ru) and 74% (Cu-Pd-Pt-Ru). For HER, Ir₃₆Pd₄Pt₄₈Ru₁₂ surpasses any of its elemental constituents by 26% and maxima in other systems by 5% (Ni-Pd-Pt-Ru) and 23% (Cu-Pd-Pt-Ru). For the NO_<i>x</i>RR, only a marginal increase of 4% was found between the most active measured alloy and the elemental constituent Cu. By comparing activity across systems, we demonstrate the tunability of electrochemical activity on compositionally complex solid solutions, achievable through variations in composition both within and across different material systems for four different reactions.