Surface Properties of Graphite for Electrocatalysis of Vanadium Redox Reactions

Abstract

The interfacial processes controlling charge transfer in vanadium redox reactions at graphite electrodes are still poorly understood. Accurate knowledge of the underlying mechanism is mandatory to improve the efficiency of conversion from electrical to chemical energy and vice versa. Various surface modifications are known to improve the catalytic activity of graphite by introducing active sites. Differentiating between influences on structural properties by activation processes is difficult due to the heterogeneity of the material and the interplay of microstructural changes and surface chemistry. This requires the application of multiple analytical tools and the systematic correlation with electrochemical results. In this way, our group has recently demonstrated that the 30-year-old mechanism involving surface oxygen does not stand up to thorough investigation. Instead, we primarily identified lattice defects such as edges and carbon vacancies as the actual origin of enhanced electron transfer kinetics. We have studied these defects on various scales and introduced an onion-like model to explain why thermal treatment is effective but care must be taken to remove the material from the oxidizing atmosphere at the right time. Otherwise abundant oxygen groups and dull pore edges will counteract the intended activation. Special attention was subsequently paid to the interaction of microstructure and electronic states, establishing a relationship of catalytic activity and the schematic band structure. Whereas edges decrease the work function of graphite, vacancies increase the density of states close to the Fermi level. Using real and model electrodes, we distinguished between the architecture of structural disorder and found zigzag edges to provide superior kinetics over armchair edges. Our experimental studies were complemented by theoretical density functional theory calculations to learn about the adsorption properties of electrolyte species with regard to the orientation of graphite. The potential-dependent structural changes and coordination spheres of the electrode and the near-electrode electrolyte were investigated by operando Raman spectroscopy to elucidate the interaction between vanadium and carbon.