DFG-RSF: Doped-graphene for electrochemical energy storage and conversion: Impact of the electronic structure on electrocatalytic activity in oxygen redox reactions

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Storing energy in chemical bonds enables power supply from sub-W to tens of MW. Electrochemical systems that convert energy of chemical bonds directly to electric power can store GWh of energy and utilize a huge variety of chemical systems. Being easily accessible in ambient atmosphere and nearly inexhausable, molecular oxygen is an excellent component for electrochemical energy conversion and storage systems, so that oxygen redox reactions are of decisive importance for many electrochemical devices such as fuel cells or metal-air batteries. In such systems, heterogeneous electron transfer to/from oxygen occurs at the surface of an electrode. Carbon electrodes, being light-weight, cheap and well conductive, are the materials of choice in the most cases. The surface of carbon electrodes consists of a graphene-like domains, various carbons, however, reveal different efficiency for oxygen redox processes. It was recently demonstrated that doping of carbons with light elements like N, B or S enhances the kinetics of electron transfer to/from oxygen remarkably and leads to an electrocatalytic activity comparable to that of noble metals. However, the underlying physics has yet not been fully understood. A number of studies devoted to this topic suffer from high complexity of real carbon electrodes, i.e. individual effects of electronic structure, impurities, microstructure, etc. can be hardly distinguished.We propose to use epitaxial graphene as a model system to elucidate the role of electronic structure and impurities in doped carbons in heterogeneous electron transfer to and from oxygen. We will start from epitaxial graphene as purely chemical model system and are planning to develop functional electrochemical cells with increasing complexity using transferred graphene as electrode.To realize the chemical model systems monolayers of doped graphene will be grown in situ by chemical vapor deposition on Ni(111) and Co(0001) surfaces. The electronic properties of N-, B- and S-graphene will be explored experimentally and theoretically employing angle resolved photoemission spectroscopy (ARPES) and DFT-based calculations. Chemisorption of hydrogen and alkali metals on doped graphene layers will be used to control the Fermi level position under ultra-high vacuum conditions. The evolution of the system during oxygen exposure will then be traced using ARPES, XPS and near ambient pressure XPS (NAP XPS).A similar methodology will be applied to operating electrochemical cells utilizing doped graphene as an electrode. Graphene will be grown on metallic foils and then transferred onto solid electrolytes or onto Si3N4 grids with liquid electrolyte supplied from the back side. The electron transfer between graphene and oxygen as well as the formation of new oxygen-containing species will be followed using photoelectron spectroscopy and X-ray absorption spectroscopy (NEXAFS) under operando conditions.