Mechanistic studies on electrocatalysts for oxygen evolution and nitrate reduction reaction

Abstract

Mitigating climate change, including global warming, requires a shift from traditional fossil fuel-driven industries emitting carbon dioxide to clean fuel-driven industries. For example, ammonia has traditionally been synthesized by the Haber-Bosch process, but this process has high energy consumption and carbon dioxide emissions. Electrochemical energy conversion, on the other hand, can generate a variety of clean fuels, from hydrogen to ammonia, at room temperature and pressure, without emitting carbon dioxide. This doctoral thesis investigated the catalysts mechanisms on electrochemical reactions of oxygen evolution, a relative reaction of hydrogen evolution, and ammonia synthesis from nitrate. The slow electrochemical reaction rate of oxygen evolution is responsible for the low conversion efficiency of the entire hydrogen generation reaction. In Chapter 2, lithium cobalt oxide with a layered structure was used as a catalyst for the oxygen evolution reaction, and the change in oxygen evolution activity due to the delithiation of lithium ions and the insertion of electrolyte alkali ions was described. In particular, the catalyst crystal and local structure changes induced from the insertion of alkaline ions were analyzed to prove the active mechanism of the oxygen evolution catalyst. In Chapter 3, a copper catalyst was used to convert nitrate, an environmental pollutant, into ammonia. The ammonia generation process involves the formation of various intermediates and competition with hydrogen evolution. The activity and selectivity were improved by controlling the oxide layer on the copper surface. In particular, using in-situ Raman spectroscopy and surface analysis, we demonstrate that the low oxidized water on the copper surface is a key mechanism that promotes the reduction of nitric oxide, an intermediate of nitrate.