Synthesis and In-Situ Characterization of Metastable Metal Oxide Polymorphs for Oxygen Evolution Catalysis

Abstract

The widespread implementation of solar energy at the level needed for global energy demand requires its efficient storage in the form of fuels. The conversion of water to H2 and O2 is one of the most energy dense carbon-neutral fuel schemes to store solar energy. Effective catalysts for the hydrogen evolution reaction and oxygen evolution reaction (OER) require a design that manages the coupling of electrons and protons so as to avoid high energy intermediates. Of these two proton-coupled electron transfer reactions, the OER is more kinetically challenging because it requires the management of four electrons and four protons. The development of superior metal oxide oxygen-evolving catalysts (OECs) requires a fundamental understanding of the ubiquitous factors influencing high performing catalysts. In this thesis, we investigate the synthesis of metal oxide polymorphs, particularly for oxygen evolution catalysis, and how a universal design criteria for optimal OECs may be applied across methods of synthesis. Electrodeposited manganese oxide films are promising catalysts for promoting OER in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn3+. First, we present in situ electrochemical and X-ray absorption spectroscopic studies which reveal that Mn3+ may be introduced into MnO2 by an electrochemically-induced comproportionation reaction with Mn2+, and that Mn3+ persists in OER active films. Next, we introduce EXAFS spectra and Raman microspectroscopy that reveal a decrease in the Mn–O coordination number and the presence of distorted Mn–O environments. Additionally, computational studies show that Mn3+ is kinetically trapped in tetrahedral sites, and in a fully oxidized structure. Finally, acknowledging that the effect of morphology is a valuable contributor to OER activity, we introduce the design and construction of a novel synthetic tool for producing highly porous nanofibers of metal oxides, which may be used to achieve the design criteria for OECs proposed herein.