Influence of Co and Mn Doping on the Surface Reconstruction of Faceted NiO(111) Nanosheets after the Oxygen Evolution Reaction

Abstract

Understanding dynamic surface reconstruction processes on transition metal oxides for the oxygen evolution reaction (OER) in alkaline electrolytes is crucial to the development of more active catalysts in water electrolysis technologies. Effective strategies in material development for activity enhancement include doping with additional transition metals and surface structuring through controlled exposure of defined surface facets. Here, a microwave-assisted synthesis route was used, that resulted in phase-pure Co- and Mn-doped NiO with various doping levels while maintaining the rock salt crystal structure of the pure, faceted NiO(111) nanosheets. X-ray diffraction and transmission electron microscopy showed an unaltered structure and morphology up to doping levels of 10 mol %. The impact of doping levels between 2 and 10% on the electrochemistry and OER overpotential was studied using the rotating disc electrode technique. A modest overpotential reduction of 34 mV was achieved for 5% Co-doping, being the most active material in the comparison, and an increase in overpotential of 56 mV for 10% Mn-doping, being the least active material, compared to the undoped NiO(111) material. Associated changes in the physical surface area and charges associated with surface redox reactions were aligned with detailed X-ray absorption spectroscopy and X-ray photoelectron spectroscopy analysis before and after electrochemical measurements, which showed different extents of surface reconstruction depending on the dopant and doping level. Thus, transformation of the less active rock salt structure to more active NiOOH functionalities was hampered by a low extent of surface reconstruction, explaining the modest activity enhancement after potentiodynamic cycling for 350 scans. The results demonstrate the effective synthesis of facet-controlled doped NiO-based model catalysts to scrutinize the impact of individual dopants on the electrochemical behavior and, thus the OER electrode activity.