Studies of transition metal oxides in electrocatalytic oxygen reactions

Abstract

ABSTRACTTransition metal oxides are an important class of materials with many interesting properties such as oxygen electrocatalysis, metal-insulator transition, pseudo-capacitance, and electrochromism. The central aim of this dissertation is to develop a more efficient and robust electrocatalyst for oxygen reactions based on transition metal oxide. The dissertation work consists of three projects. In the first study, a new design strategy for the development of bifunctional electrocatalysts capable of catalyzing both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is proposed. In this strategy, a single MnOx lattice is doped with either electropositive (Sr, Ba) or electronegative (Bi, Pb) elements along with oxygen vacancies to create both electron-rich donor (Mn2+) and electron-poor acceptor (Mn4+) defects in the same parent (Mn3+) lattice. These defects effectively catalyze the reduction (ORR) and oxidation (OER) processes on the same electrode surface. The study is based on the results of a previous study on Mn2O3 that showed Mn2+ and Mn4+ as the active sites for ORR and OER processes, respectively. Our results show that BiMnOx is the most promising bifunctional catalyst with OER/ORR activities that are comparable to the individual activities of state-of-the-art commercial Pt or RuO2 catalysts. Stability tests show the catalyst to be stable for more than 3 h of continuous OER or ORR polarization. This work provides a pathway for the individual tuning of defects to control electrocatalytic activities, which opens up new possibilities for the rational design of many perovskite-based oxides. The second study focuses on the electrical-field induced metal-insulator transition property of transition metal oxides and demonstrates its correlation to bifunctional ability for oxygen reactions. We conducted extensive electrochemical tests on three categories of oxides: bifunctional, single-functional, and pseudo-capacitive oxides. Our results suggest that bifunctional oxides undergo a metal-insulator-metal transition together with a switch in the type of majority charge carrier during the anodic-to-cathodic voltage scan. On the other hand, single-functional oxides undergo a metal-insulator transition during anodic and cathodic scan. Pseudo-capacitive oxides have exceptionally low resistance in capacitive potential ranges. Beyond this potential range, they either show metallic conductivity at potentials of OER/ORR or become an insulator with low activity for OER/ORR. The third study focuses on a special type of transition metal oxide, namely iridium oxide, which is the state-of-the-art-catalyst for OER in acidic medium. However, the nature of Ir intermediates involved in the OER mechanism is not well established. In this study, we prepared various Ir compounds of varying oxidation state to serve as standards for spectroscopic studies. Our in-situ optical absorbance and photoluminescence measurements suggest that not only Ir intermediates but also lattice oxygen intermediates are created during electrochemical polarization that might be responsible for both the OER and the electrochromic processes. More in-depth investigation is still needed to unveil the nature of oxygen intermediates and oxidation states of Ir species involved under a wider range of applied potentials.