Elucidating Heterogeneous Mixed-Metal Synergy Through Electrochemical and Electronic Studies of Earth-Abundant Oxygen Evolving Catalysts
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CitationLi, Nancy. 2018. Elucidating Heterogeneous Mixed-Metal Synergy Through Electrochemical and Electronic Studies of Earth-Abundant Oxygen Evolving Catalysts. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractSolar power is a promising contender to replace fossil fuels with its inexhaustible supply and relatively low cost. Energy storage remains the missing link between intermittent solar supply and on-demand energy output. Hydrogen (H2) storage coupled with fuel cells offers a portable solution to energy storage and electricity production. The implementation of a large-scale hydrogen economy requires the development of cost-effective, clean methods for water splitting to produce H2 and O2. However, artificial photosynthesis remains a challenge due to the kinetically sluggish oxygen evolution reaction (OER) which involves the transfer of four charge equivalents that must be stored and discharged towards O–O bond formation.
The development of superior oxygen-evolving catalysts (OECs) requires the amalgamation of multiple metal centers, each serving a specific role in OER activity and pH stability. This thesis explores structural and electronic changes in mixed-metal oxides with high OER activity under basic conditions and film stability in acidic conditions using a combination of electrochemical and spectroscopic techniques. First, iron doping in nickel oxide films was explored to uncover the underlying source of this OER enhancement. X-ray absorption spectroscopy and coulometry show that Fe acts as a superior Lewis acid, promoting the formation of Ni4+ centers. Next, this concept was expanded to explore additional Lewis acidic metals that could replace Fe and enhance OER in NiOx. Unlike Fe, the incorporation of Al, Ce, La, or Sc yields no observable changes in the electronic structure of NiOx and, hence, effects no change in the Tafel slope for OER. Next, 57Fe Mössbauer spectroscopy was used to identify and track Fe4+ centers in mixed-metal Co-Fe oxides. Additionally, X-ray absorption spectroscopy was used to confirm the Fe4+ oxidation state and further characterize this high-valent species. Finally, PbOx based OECs were studied using X-ray absorption spectroscopy and X-ray photoelectron spectroscopy to elucidate the mechanism of film corrosion in acidic conditions. Factors such as film and buffer composition alter film stability by creating structures that minimize film exposure to acid and/or maximizing self-healing through the availability of soluble metal ions in solution.
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