Symmetry-Broken Co3O4 via Site-Selective Pd Substitution for Bifunctional Water Electrolysis

Abstract Spinel oxides generally show subpar activity for electrochemical water splitting because of low electronic conductivity, suboptimal adsorption energies, and poor operational stability. To overcome these limitations, Pd2+ is introduced to induce lattice strain in a metastable Co3O4 phase featuring off-centered cobalt and oxygen atoms. While pristine Co3O4 typically crystallizes in the centrosymmetric Fd3̅m space group, 1.2 at % Pd transforms it to the dominant non-centrosymmetric F4̅3m space group. This unconventional lattice contains two non-equivalent tetrahedral sites and one octahedral site. Pd2+ preferentially substitutes the Co1 atom at the (0, 0, 0) 4a tetrahedral position, with a relative stabilization of −1.02 eV. Pd2+-induced enhancement of the density of states near the Fermi level tunes the d-band center, while Co-to-Pd charge redistribution activates the otherwise less active tetrahedral site alongside the intrinsically active octahedral site, thereby enhancing alkaline water electrolysis. For oxygen and hydrogen evolution, this bifunctional catalyst delivers mass activities of 60.6 and 31.0 A/g and turnover frequencies of 0.32 and 0.28 H2 s−1 at 300 and 250 mV overpotentials, respectively. In a two-electrode configuration, it reaches 50 mA/cm2 at an iR-uncorrected cell voltage of 1.85 V, while at 200 mA/cm2, it produces 25.4 mL/h H2 at a cell voltage of 2.31 V.