Study on nano-catalyst introduced electrode for solid oxide cells

Abstract

Solid oxide cells are highly efficient eco-friendly energy conversion devices that convert the chemical energy of fuel into electrical energy or the remaining electrical energy to fuels such as hydrogen through electrochemical reactions at high temperatures. In particular, owing to the massive movement to replace existing energy sources, fossil fuels, to eco-friendly energy sources because of the serious environmental destruction and global warming, fossil fuels are attracting attention as promising devices to solve the instability of energy supply from renewable energy sources. A key to the commercialization of solid oxide cells is to develop a device with high performance even at a lower temperature, and it is well known that the cathode is the most important component among the various components constituting a cell to determine the overall cell performance at low temperatures. The introduction of nanoparticles into the cathode surface has been widely researched to obtain sufficient electrochemical reactivity of the cathode, even at low operating temperatures. However, there is still no basic research on the mechanism for improving electrochemical reactions through the introduction of nanocatalysts. First, it is owing to the complexity of the electrode structure. The complexity of nanocatalysts with uneven sizes and distributions on porous oxide electrodes with undefined interfaces is a significant factor hindering in-depth research on electrode reactions. Another factor is the complexity of the electrochemical reaction. The reaction of only one oxygen molecule involves the participation of four electrons and two oxygen ions, resulting in complex reaction pathways and steps. In this study, a commercial air electrode material, La$_{0.6}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}O$_{3-d}$ (LSCF)-based model electrochemical cell, was manufactured based on pulsed laser deposition and photolithography, which can apply voltage and observe the oxygen reduction reaction and oxygen evolution reaction. Moreover, nanoparticle catalysts with finely controlled size and distribution were applied to the surface of the electrochemical cell through a nanopatterning process using block copolymer self-assembly. First, we quantitatively evaluated the contribution of the nanocatalyst to the electrochemical reaction without any structural complexity. Various nanocatalysts, including silver, platinum, palladium, and cobalt, have been introduced with uniform sizes and distributions. As a result, the silver nanocatalyst was selected as the best catalyst. Second, the active reaction sites of the electrode were identified by controlling the size and distribution of silver nanoparticles. For the ORR, the interface between silver nanocatalyst and LSCF surface was identified as main active reaction site, and the surface of the silver nanocatalyst was identified as main active reaction site for the oxygen evolution reaction. Finally, to elucidate the detailed mechanism of reaction improvement through the introduction of the nanocatalyst, the current density of each cell with and without the nanocatalyst was measured under various oxygen partial pressures and applied voltage conditions. The reaction orders of the oxygen partial pressure and chemical potential of the electrode surface were obtained. It was confirmed that the reaction-determining step of the oxygen evolution reaction could be a step in the association of oxygen atoms or desorption of oxygen molecules. However, during the ORR, oxygen molecules participate in the rate-determining step with a low applied bias, and oxygen atoms participate in the rate-determining step when the applied voltage is high. In addition, it was confirmed that ions such as electrons and oxygen vacancies are related in both cases. To identify the role of the catalyst in the improved reaction, operando surface measurements were performed under near-ambient pressure conditions with a high temperature and applying bias. It was confirmed that the silver nanocatalyst facilitated the ORR by boosting electron transfer from cobalt to oxygen. Based on the research findings from the basic research on the model electrodes conducted in this study, I hope that more well-designed and effective research on the development of solid oxide cell electrodes could be conducted.