Elucidating the Oxygen Evolution/Reduction Reaction on M-Doped Cobalt Oxide (M=Mn,Fe,Cu) Electrocatalyst for Metal-Air Batteries
Authors: Seth Deepak , Behera Asutosh , Ussama Mohd. , Kala Jyotsana , Agarwal Manish , Bhattacharyya Aninda J , Haider M. Ali 
Published: August 2024

Co3O4, an electrocatalyst with a spinel structure, exhibits promise for a range of applications including fuel cells, water splitting devices, metal-air batteries, and more. Its advantageous characteristics include durability and an extended longevity, among others. By altering the structure and electronic properties, an electrocatalyst makes it suitable for enhanced oxygen evolution reactions (OER). The application of 3d-transition metal dopants (e.g., Fe, Mn, Ni, Zn, Cu) to modify the catalytic properties of a substance has become widespread. In this work, experimental evidence and DFT calculations are combined to provide insight into an OER process in acidic media, thereby enhancing the comprehension and performance of an electrocatalyst. Our investigation is conducted using a Co3O4 (311)-surface model that is derived from our experimental analysis of the principal peaks in our XRD data. For electrocatalytic reactions that are significantly impacted by the solvent (water) molecules' interaction with the solid surface, the surface is additionally represented by an explicit layer of water molecules. The catalytic active site for the intermediate species (O*, OH*, OOH*) utilized in the adsorption reactions has been initially determined in this study. The DFT-derived energies (~25-30 kJ/mol) indicated that the octahedral (Co+3) site on the surface is the most favorable location for the reaction in each N-doped Co3O4 system (N = Mn, Fe, Cu). The electronic characteristics of the surface are altered through doping, leading to distinct changes in the charge density, DOS, band gap, and d-band center. For instance, when Cu+2 is doped at the octahedral site, the d-band center (εd) of pristine Co3O4 is estimated to alter by approximately ~0.7 eV. In acidic environments, OER is typically controlled by a four-electron reaction step/mechanism that ensues after water molecules divide, resulting in the production of O2 gas and the adsorption of O*, OH*, and OOH* species. The adsorption energies for the majority of intermediate species are estimated using this mechanism. It is suggested that the active sites for Mn and Fe on the modeled 311-surface bound the species significantly more strongly than the sites for Co and Cu. The reaction is conducted utilizing DFT, and the enthalpies of each reaction phase in the (Mn, Fe, and Cu)-Co3O4 (311) system are determined. The uphill and downhill slopes in Gibbs free energy diagram facilitate screening and comparison of the OER activity of the N-doped Co3O4 surface. Moreover, an understanding of the mechanism underlying reaction kinetics can be attained through its rate-determining steps (RDS). The transition state (TS) is theoretically determined for each reaction step through the utilization of nudge elastic band (NEB) methods. The RDS of the reaction is anticipated to be the transition from O* to OOH*, given that it exhibits the greater barrier among all the reactions. Furthermore, an additional research is necessary to ascertain the stability and activity of the aforementioned electro-catalyst.