Kinetics of Competing Reactions on Metal Oxide Semiconductors during the Course of Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water splitting is a sustainable and environmentally friendly method for the conversion of solar energy into a portable H2 fuel. However, in order to drive the paradigm shift toward these new sources of energy, the efficiency of this process must be improved. Because the oxygen evolution reaction (OER) is the kinetically slowest step for water splitting, it is imperative to develop materials that demonstrate high efficiencies and turnover frequencies for this reaction. This thesis focuses on developing metal oxides capable of using visible light to drive water oxidation in addition to obtaining a more fundamental understanding of the kinetics of the processes that take place on the electrode surface. The major contribution of the work presented in this thesis focuses on understanding how the catalysis of a few select metal oxides may be affected via doping or through the addition of co-catalysts. Additionally, particular emphasis is placed on understanding how competitive side reactions may affect the OER efficiency of metal oxide photoelectrodes. Improvements in the Faradaic efficiency and aqueous stability of WO3 electrodes were probed via the growth of a surface FeOOH layer. This surface layer increases the Faradaic efficiency for OER to nearly 100% in addition to dramatically improving the aqueous stability. Furthermore, the effects of competing side reactions on WO3 were investigated using a novel rotating ring disk photoelectrode and it was determined that significant competition with OER arises from the oxidation of reduced W sites present in the electrode. Furthermore, visible light absorbing ternary oxide phases were also investigated. Doping CuWO4 with Co was investigated as a means of improving OER kinetics, however, it was observed via a combination of UV-Vis spectroscopy, and electrochemistry that although Co2+ doping improves both dark catalysis and visible light absorption, these impurities produce trap states which quench the photocurrent. Finally, the 2.2 eV band gap material, PbCrO4, was investigated as a “trap-free” metal oxide due to its combination of d10/d0 transition metal cations. Unfortunately, electrodes of this material are unstable during the photoelectrolysis of water and decomposition reactions out compete OER which shuts down the electrode’s reactivity.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137008/1/clhermit_1.pd