Surface chemistry and structure of oxides evolve under external driving forces, including temperature, reactant gas pressure, electrochemical potentials, mechanical stresses and strong electric fields. This evolution of this interface affects the charge transfer kinetics in a wide range of technologies, including electrochemical energy conversion, storage, and information processing. I will present two examples where the use of near ambient pressure X-ray photoelectron (XPS) and absorption spectroscopy (XAS) helped uncover the chemistry and electronic structure of surfaces and thin films under working conditions, and the implications of that new knowledge on device performance. First is on the surfaces of perovskite type oxides, which degrade when doped with aliovalent cations. We performed ambient pressure XPS and XAS experiments, and have revealed that by maintaining a more oxidized surface, achieved either by surface doping with more oxidizable cations or by coating, one can improve significantly both the stability and the kinetics of oxygen exchange reactions that are important for the performance of high temperature fuel cells, electrolyzers and thermochemical fuel synthesis. Second is on oxides that can change their phase and their electronic properties upon change of oxygen content. Ability to change the electronic properties by feasible means, such as electrical bias, is promising for electronic device applications. We were able to demonstrate on two different types of oxides, SrCoO<sub>x</sub> and VO<sub>x</sub>, in nm-scale thin film form, that electrical bias to control the oxygen chemical potential in the oxide is a promising way to alter the phase, and the electronic properties drastically. The use of XPS and XAS under controlled oxygen pressure, electrochemical potential and temperature was crucial to reveal the electronic structure evolution taking place during these transitions. The results suggest a new type of metal-insulator-transition that is achievable over a wide range of temperatures, and is solely controlled by electrochemical modulation of oxygen content to switch between two distinct phases.

Structure and chemistry of oxide thin films and surfaces revealed by ambient pressure x-ray photoelectron spectroscopy and absorption spectroscopy: Implications for better electrochemical energy conversion and electronic devices

254th National Meeting (2017)

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