Water oxidation catalysts

Abstract

Climate change has been widely recognized as a major environmental problem facing the world today. According to IPCC, this is related to the increase in greenhouse gas concentrations, mainly as a result of the combustion of fossil-fuels. To reconcile this, a transition from fossil-fuels to renewable-based energy sources is unavoidable. Solar water splitting has attracted significant attention to convert the abundant solar energy to chemical energy. It uses a semiconductor to convert sunlight into electron-hole pairs, which then split water into hydrogen and oxygen. Hereby metal oxides have emerged as attractive candidates for photoelectrochemical water splitting, mainly due to their good stability in aqueous solutions, easy synthesis, and low cost. However, to achieve a high efficiency, the semiconductor alone is usually not enough; co-catalysts need to be implemented to address the low charge injection efficiency that most metal oxides are suffering from. This thesis consists of three parts: (i) development and characterization of an oxygen evolution reaction (OER) catalyst, (ii) investigations on the fundamental processes involved in the OER, and (iii) studies on the interaction between semiconductor, catalyst and electrolyte. In the first part, the performance of NixMnyOz over a broad stoichiometry range as alternative catalysts for the OER—either as counter electrode or co-catalyst on top of a photoanode in PEC cells—is analyzed. The films were synthesized by pulsed laser deposition using a combinatorial approach that allows scanning over the complete stoichiometry range. Electrochemical as well as spectroscopic methods were used to understand the catalysts’ working principle and the relations between its composition and OER activity. The introduction of Ni into the MnOx catalyst is found to reduce the overpotential significantly. A further increase in Ni concentration in the films results in even further decrease of overpotentials. This shows that pure NiOx still performs best for the OER within this composition range, as well as in terms of stability and transparency. The next paragraph addresses the understanding of the fundamental processes taking place in a cobalt phosphate (CoPi) catalyst during water oxidation. CoPi is a well-known electrocatalyst, which has already been studied extensively in the literature. However, there are still some open questions on the exact processes taking place during the water oxidation reaction. To get a better insight into those mechanisms, in-situ UV-Vis studies were performed as a function of potential to investigate the working principle of a CoPi OER catalyst. Our measurements reveal a sequential oxidation from CoII --> CoIII --> CoIV, and we show that the film does not need to be completely oxidized to CoIII before CoIV can be formed. The oxidation from CoII to CoIII is also found to be the slowest amongst the two oxidation steps. Finally, the processes at the semiconductor/electrolyte and the semiconductor/co-catalyst/electrolyte interface are discussed to understand the role of a co-catalyst layer and its relation to the semiconductor’s limitation. On that account, BiVO4 photoanodes were used as model system and were investigated by intensity modulated photocurrent spectroscopy (IMPS). With this method one can distinguish the recombination and charge transfer at the surface of the working electrode. By comparing the bare and the co-catalyzed photoanode system, it is shown that an effective co-catalyst deposited onto BiVO4 does not enhance the charge transfer, as one would expect intuitively, but reduces the surface recombination instead.