Electrochemically Induced Oxide‐to‐Hydroxide Transformation Enables Fast Proton Transport for Enhanced Hydrogen Evolution

Developing earth‐abundant electrocatalysts that rival the commercial platinum/carbon catalyst for the hydrogen evolution reaction (HER) remains a central challenge in renewable‐energy conversion. Here, we reveal an electrochemically induced, in situ phase transformation in a Ru‐MgO catalyst that leads to true active material during operation. Under acidic HER conditions, nominal 20 wt.% Ru nanoparticles supported on polar MgO(111) nanocrystals undergo a topotactic hydrolysis to Ru‐Mg(OH) 2 (001), generating an ordered hydroxide layer that serves as a highly conductive proton‐hopping network. After activation, the catalyst delivers performance comparable to commercial Pt/C under identical conditions, matching the current density of −1.1 V and surpassing it by approximately 10% at −2.3 V. Operando synchrotron X‐ray diffraction combined with ex situ characterization techniques directly captures this transformation, while density‐functional theory calculations reveal that water‐assisted Grotthuss proton transfer across the hydroxide requires only a 0.10 eV energy barrier. These findings establish electrochemically driven oxide‐to‐hydroxide conversion as a new design principle for creating low‐Pt or Pt‐free HER electrocatalysts with intrinsically fast proton transport.