Cr Doping in Spinel Oxides Coordinates Adsorbate Evolution and Lattice Oxygen Pathways for Efficient Water Oxidation

Water electrolysis is a promising approach for sustainable hydrogen production, yet the oxygen evolution reaction (OER) at the anode─hampered by sluggish kinetics and high overpotentials─remains a major obstacle to achieving cost-effective electrolysis systems. Here, we describe the synthesis and electrocatalytic properties of a new class of 3d transition metal spinel oxides, emphasizing the dual role of chromium (Cr) dopant in synchronically activating metal sites and lattice oxygen redox pairs with lowered energy barriers. The Prussian blue analogue-mediated thermal decomposition method is used to synthesize multicomponent spinel oxide electrocatalysts, ensuring atomic-level homogenization of metal ions. Cr doping reconstructs the electronic structure of active sites by forming magnetic coupling with adjacent ions, thereby optimizing intermediate-sorptive energies and polarizing M-O-M bonds. This modulation reduces the energy barrier of the rate-determining step (RDS) *OH → *O in the adsorbate evolution mechanism. Simultaneously, owing to the addition of Cr, the RDS in the lattice oxygen mechanism is altered to the deprotonation of M-OH in (Ni_0.6Co_0.5Fe_1.3Cr_0.6)O₄ that proceeds with a thermodynamic energy barrier lowered by 0.54 eV. Consequently, our optimal electrocatalyst, (Ni_0.6Co_0.5Fe_1.3Cr_0.6)O₄, shows an ultralow overpotential of 243 mV at 10 mA cm^-2 and excellent operational stability for over 210 h, outperforming most reported spinel oxides and demonstrating great potential for anion exchange membrane water electrolyzer. Our findings challenge the conventional single-pathway optimization paradigm, demonstrating that strategic d-block element engineering can orchestrate complementary OER mechanisms through coupled electronic-lattice modulations.