Beneficial Reduction of Surface Compositional Complexity in High-Entropy Oxide Oxygen Evolution Reaction Catalysts into a Ternary Phase

Cost-competitive green hydrogen production via water electrolysis is limited by the scalability and economics of electrolyzers, particularly regarding the oxygen evolution reaction (OER) electrocatalyst. High-entropy oxides hold significant promise, yet the drivers of activity and stability are debated due to challenges in characterizing the catalytically relevant surface. Addressing this, we present the comprehensive surface-specific quantification of chemical and coordination evolution in compositionally complex high-entropy Ni/Co/Fe/Mn/Cr (hydr)oxide electrocatalysts using synchrotron-based X-ray spectroscopies assisted by charge transfer multiplet calculations. An electrochemically driven surface transformation to ternary Ni/Co/Fe or Ni/Co/Mn (oxy)hydroxide in alkaline media is revealed to enable long-term efficient OER performance. This beneficial reduction in compositional complexity, and thus entropy, is induced by leaching of Cr, Mn, and Fe, enriching catalytically active octahedrally coordinated nickel and cobalt species, and is accelerated by a short-range-ordered phase (X-ray amorphous) compared to the crystalline counterpart. These findings highlight the importance of characterizing the surface chemical evolution of high-entropy materials and guide the engineering of next-generation compositionally complex transition-metal-based OER catalysts.