Catalysts for hydrogen (H+) and carbon dioxide (CO2) reduction investigated by X-ray absorption spectroscopy and in functional studies

摘要

Catalysts for the hydrogen evolution reaction (HER) and the carbon dioxide reduction reaction (CO2RR) play an important rule for solar and wind energy. These energy sources are subjected to temporal fluctuations and require a technology to store excess energy. A potential technology is the use of excess energy to drive an electrochemical HER or CO2RR to produce fuels in form of molecular hydrogen, formic acid or methanol. These reactions are catalysed via platinum to boost their efficiency. The rare noble metal is expensive and, hence, represents a significant economic disadvantage. An alternative to platinum are synthetic catalysts based on abundant elements. So far, the activities of these catalysts are significantly lower than of platinum. To find effective approaches for optimizing the synthetic catalysts, a deeper understanding of their catalytic mechanisms is necessary. In this doctoral thesis, the atomic structure of synthetic catalysts for HER and CO2RR is studied in the catalytic and the non-catalytic state to provide insights into the catalytic mechanism. X-ray absorption spectroscopy (XAS) was applied to investigate the atomic structure. Studied catalysts for HER were electrodeposited films based on molybdenum sulphide or cobalt, and a molecular nickel catalyst. For CO2RR, an electrodeposited catalyst based on copper was studied. The XAS investigations led to first insights into the atomic structure of the metal centres in the mentioned catalysts. Furthermore, two significant catalytic effects of the molybdenum sulphide catalyst, the increase in their intrinsic activity via doping with other metals as well as their ability for bulk catalysis, could be correlated to features in the atomic structure of their metal centres. In the case of the molecular nickel catalyst, degradation effects were studied occurring in the catalyst under operating conditions in a fuel cell.