Enhancing Cobalt―Oxygen Bond to Stabilize Defective Co2MnO4 in Acidic Oxygen Evolution

Abstract

Co-based oxides have shown promise as catalysts for the oxygen evolution reaction (OER), as evidenced by experimental and theoretical studies. However, these common Co-based catalysts suffer from poor stability in acidic environments, making them susceptible to corrosion in acid electrolytes. Consequently, developing OER catalysts that can maintain both activity and stability under strongly acidic conditions is a challenging task for large-scale industrial hydrogen production applications. To address this challenge, the incorporation of manganese (Mn) into the spinel lattice of Co₃O₄ (CoMn₁O) has been proposed, resulting in a defect-rich catalyst with improved lifetime in acidic electrolytes. The crystalline phase structures and chemical valence states were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive spectroscopy (EDS) elemental maps. The introduction of Mn led to the generation of a significant number of defects due to changes in the local crystal structure. Additionally, as the amount of Mn atoms increased, a red shift was observed in the Co 2p spectrum, indicating an increase in the overall valence of Co and the formation of more stable Co―O bonds. Moreover, when the Mn-to-Co ratio reached 1 (CoMn₁O), the resulting catalyst exhibited promising OER activity, with overpotentials of 415 and 552 mV at 10 and 50 mA∙cm⁻², respectively. Detailed physical characterization and electrochemical tests demonstrated that CoMn1O exhibited over four times the stability of Mn-free Co₃O₄ (CoMn₀O). This enhanced stability can be attributed to the introduction of Mn, which promotes electron density bias of Co towards O, resulting in the formation of more stable Co―O bonds. Mn also facilitates acidic oxygen evolution by delaying the oxidation rate of the Co active sites, thereby enhancing stability. Density functional theory (DFT) calculations were further employed to analyze the electronic structures of CoMn₁O and CoMn₀O. The d-band center of Co 3d (ε_d) in CoMn₁O shifted closer to the Fermi level (E_F) compared to that of CoMn₀O, indicating a reduced reaction energy barrier for CoMn₁O and enhanced bonding interaction with OER intermediates. Overall, this work presents a promising strategy for achieving highly efficient and stable acidic oxygen evolution using noble-metal-free electrocatalysts.

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