Developing enhanced mixed metal oxide catalysts for electrocatalytic water oxidation using insights from X-ray absorption spectroscopy [electronic resource]
Keywords: Chemical Engineering.

Efficient and economic conversion of renewable energy sources is critical for development of technologies that can shift global energy dependence away from fossil fuels. Increased global energy consumption along with heightened awareness of environmental, health, and political issues with fossil fuels are driving the need for alternative technologies. Wind and sun provide more than enough energy to meet the growing energy demand, provided that challenges with intermittency, scale, and cost-effectiveness can be overcome. These obstacles can be mitigated through development of highly active catalysts using abundant and inexpensive materials to convert solar and wind energy into fuels and chemicals. One promising method of converting solar energy into fuel is by splitting water to produce hydrogen and oxygen. This can be achieved using a monolithic photoelectrochemical (PEC) water splitting device which combines photon-absorbing semiconductors with catalysts that drive the respective reactions or a photovoltaic/electrolyzer system which separates these two components. The first part of this thesis presents a model to quantify loss mechanisms in PEC water splitting based on the current state of materials research and calculate maximum solar-to-hydrogen (STH) conversion efficiencies. Results of this model indicate that a major limitation to the efficiency of solar-driven electrochemical water splitting is the oxygen evolution reaction (OER) which requires significant overpotential beyond the thermodynamic redox potential to proceed. The remainder of this dissertation focuses on understanding the interaction between metals in mixed metal oxide catalysts for the OER using electrochemical and advanced spectroscopic techniques towards the development of highly active and stable catalysts. Mixed metal oxide catalysts provide a robust platform for tuning binding energies of OER reaction intermediates to the catalyst surface, thereby affecting activity, through controlled material composition and geometry. We investigate two distinct mixed metal oxide catalyst systems using X-ray absorption spectroscopy (XAS) to probe local geometric and electronic structure and correlate the results with changes in activity. XAS is a synchrotron based technique which provides elemental-specific information by exciting electronic transitions from core to valence orbitals at various elemental edges. XAS studies at the Co K and L edges for a cobalt titanium oxide (CoTiOx) catalyst system identify stabilization effects on the Co oxidation state and overall structure from varying amounts of Ti precursor used during material synthesis. XAS before and after catalyst exposure to OER conditions indicate that catalysts with the least long range order become most oxidized and exhibit the highest activities. Similarly, in situ XAS at the Mn K and Au LIII edges reveal that there is a charge transfer at interfacial sites between manganese oxide (MnOx) and gold (Au) under OER conditions which coincides with significantly increased OER activity compared to MnOx without Au. Our results indicate that Au facilitates stabilization of more oxidized phases of Mn at lower overpotentials, thereby allowing for earlier onset of OER and higher activity. Lastly, we present an investigation of a novel mixed metal oxide catalyst, strontium iridium oxide (SrIrO3) which has the highest reported activity for any known OER catalyst. While it depends on use of Ir, a precious metal, its remarkably high activity compared to rutile IrO2 reduces the Ir loading necessary to achieve similar current densities. In summary, this dissertation explores a broad spectrum of catalysts for the oxygen evolution reaction and uses advanced material characterization methods to draw correlations between the structure, oxidation state, and catalytic activity for these materials. This work provides fundamental insight towards improving efficiency of electrochemical water oxidation processes for the conversion of renewable energy sources to fuels and chemicals.