Unconventional High‐Entropy Oxide with Inner Metallic Coordination and Surface Medium‐Entropy Metal‐OH for Durable PEM Water Electrolysis at 2 A cm −2

Abstract The multicomponent synergistic effect has become increasingly important for electrocatalysis; however, there remain large unexplored compositional and structural spaces. Here, an unconventional high‐entropy oxide (HEO) with inner metallic coordination interactions and surface medium‐entropy metal‐OH for durable proton exchange membrane water electrolyzers (PEMWEs) is presented. The metallic clusters inside the HEO particles generate the inner metallic coordination interactions that reserve electrons, with a multicomponent effect, to inhibit Ru/Ir overoxidation and switch reaction mechanisms toward an exclusive adsorbate evolution mechanism for acidic oxygen evolution reaction (OER). Surface medium‐entropy metal‐OH groups enable adaptive interfacial water networks to trap reactive water and promote proton transfer, reducing mass transport resistance at large current densities. Resultantly, this RuIrNiCoCrO 2 ‐incorporated PEMWEs achieve the ultralow voltages of 1.71 V@2.0 A cm −2 and 2.03 V@5.0 A cm −2 at 80 °C, and unprecedented durability >1500 h@2.0 A cm −2 . Thus, the dual engineering of inner multicomponent coordination environments and surface functional groups overcomes the activity‐stability dilemma in the PEMWEs.