New Electrochemical Approaches for the Investigation of the Oxygen Evolution and Reduction Reaction of Mesostructured Cobalt-based Transition Metal Oxides

Abstract

Transition metal oxides as abundant materials are used for the electrocatalysis of the oxygen evolution reaction and the oxygen reduction reaction in an alkaline environment and can play an important role in electrolysers and fuel cells. In this thesis, new electrochemical methods were developed on model systems and then used on mesostructured spinel materials Co3O4, NiCo2O4 and CuCo2O4, which were synthesised using a template process and filled into cavity-microelectrodes for the electrochemical investigations. The electrode geometry enables the investigation of the activity of the oxygen reduction reaction under mass transport-limited conditions and to avoid the addition of carbon and binding materials during the characterisation. As a result, the real catalyst surface area, which is relevant for the electrocatalytic reaction, can be determined via the in situ measurement of the double layer capacitance and thus evaluation of the specific activity of the transition metal oxides is achievable. This enables new access to their intrinsic properties. The surface interrogation mode of scanning electrochemical microscopy was used to investigate surface sites that are relevant for the oxygen evolution reaction on the transition metal oxides. For this purpose, this mode was adapted to the investigation of powder materials and verified with model systems made of nanostructured precious metal catalysts. In a second step, the knowledge gained was applied to the Co3O4 spinel and its surface sites were quantified and examined in terms of reactivity for the oxygen evolution reaction. The experiments depending on the applied potential showed a correlation between the oxidation states and the resulting reaction rate during the titration of the Co oxide. Time-dependent studies enabled the determination of rate constants for the reaction of the surface sites with water as the first step of the oxygen evolution reaction. The obtained results and demonstrated methods for powder materials contribute significantly to the understanding of the amount and reactivity of surface sites, which are relevant for the electrocatalysis of the oxygen evolution reaction and the oxygen reduction reaction. This information is important for the development of catalysts for fuel cells and electrolysers.