Molten-Salt Etching Activates Cr-Doped Al2O3 on Graphene for Alkaline Oxygen Evolution

Abstract

Aluminum oxide is an earth-abundant yet catalytically inert p-block oxide, as its lack of accessible d orbitals limits oxygen-intermediate binding in the conventional adsorbate evolution mechanism (AEM). Herein, we activate the intrinsic oxygen evolution reaction (OER) activity of Al₂O₃ by incorporating high-valence Cr species through a one-step molten-salt electrochemical strategy. Using Cr₂AlC MAX phase as an integrated Cr, Al, and C precursor, selective Al extraction, Cr incorporation, and in situ carbon reconstruction are coupled to form Cr-doped Al₂O₃ nanoparticles anchored on a conductive graphene scaffold (Cr-Al₂O₃@G). The integrated route mitigates nanoparticle agglomeration and weak interfacial contact typical of multi-step syntheses. Cr-Al₂O₃@G delivers an overpotential of 310 mV at 10 mA cm⁻², markedly lower than bare Al₂O₃ and comparable to commercial IrO₂, while offering an estimated ∼70,000-fold lower metal cost. It also achieves a metal-mass-normalized activity of 112 A g⁻¹. Mechanistic studies involving pH-dependent kinetics, tetramethylammonium cation inhibition, and in situ ¹⁸O-isotope differential electrochemical mass spectrometry (DEMS) reveal that Cr incorporation reconstructs the Al-O electronic environment and promotes a lattice-oxygen-mediated (LOM) pathway. This work provides a MAX-phase-derived strategy for cost-effective activation of p-block oxides.