Studies on Oxophilic Effect in Platinum Nanocatalysts for Aqueous Electrocatalysis

Abstract

Abstract Studies on Oxophilic Effect in Platinum Nanocatalysts for Aqueous Electrocatalysis

Myeong Jae Lee School of Chemical and Biological Engineering The Graduate School Seoul National University

Aqueous electrocatalysis occurs in electrochemical cells that generally comprise electrolyte, an external circuit, electrocatalytic anode and cathode. There are two types of the cells according to the electric flow

(i) polymer electrolyte membrane fuel cell for converting chemical energy to electric energy with spontaneous redox reaction and (ii) water electrolyzer for converting electric energy to chemical energy for fuel production. Hydrogen oxidation/evolution, oxygen reduction/evolution, and carbon monoxide oxidation in acidic and basic media are the most important reactions in the two types of electrochemical cell. In this thesis, the role of platinum nanocatalyst, metal (oxy)hydroxide and metal oxide (hydrate) for those electrocatalytic reactions were investigated. The main issues in the development of catalysts for polymer electrolyte membrane fuel cell, which are mainly platinum-alloy catalysts, are improvements of activity and low usage of the precious metal. In addition, developments of non-platinum catalysts and enhanced activities of platinum-based catalysts have been the main objective of the research works on electrocatalysts for water electrolyzer. Therefore, in this study, state-of-the-art platinum nanocatalysts blended with metal oxide (hydrate) or metal (oxy)hydroxide inducing oxophilic effect for more enhanced activities of carbon monoxide oxidation and hydrogen evolution, and the metal oxide (hydrate) or the metal (oxy)hydroxide for enhanced oxygen evolution were utilized as electrodes. From keen electrochemical and physicochemical observations of the blended catalysts systems, it was confirmed that oxophilic effect without electronic change of platinum surface had a crucial effect on the electrochemical reactions in acidic and basic media. In addition, it was observed that interaction between aqueous electrolytes and the catalytic system facilitated oxophilic effect, and this work could bring the better understanding of oxophilic tuning which generally arises in the platinum-alloy system. In chapter 1, bases to electrocatalysis, polymer electrolyte membrane fuel cell, and water electrolysis cell were introduced. After a brief introduction of kinetics in electrocatalysis and two-types of electrochemical cells according to directions of electric flow, electrochemical reactions in polymer electrolyte membrane fuel cell, fundamentals of platinum alloy catalysts, and previously reported researches of oxophilic effect were introduced. In last, electrocatalysis in water electrolysis cells and possible improvements by oxophilic effect in hydrogen evolution were briefly discussed. In chapter 2, an extended concept of oxophilic effect between platinum nanocatalysts and hydrous ruthenium oxide in acidic media was discussed. The simple model system with blended catalyst was introduced in order to investigate the electrocatalytic carbon monoxide oxidation mechanisms and oxophilic effects in negligible direct-junctions between platinum nanocatalysts and hydrous ruthenium oxide. From this approach, it could be observed the sole oxophilic effect in the electrochemical environment by the unmodified crystallinity and electronic structure of Pt nanoparticles. In addition, significantly enhanced carbon monoxide oxidation was observed in electrochemical measurements and facilitated carbon monoxide oxidation of the blended system in real fuel cell operation conditions was confirmed by single cell experiments. In chapter 3, based on feasibility of the blended system, using thermal annealing at various temperatures, ruthenium and iridium oxide (hydrate) catalysts with different oxygen evolution activities in acid electrolyte were obtained. By physicochemical comparisons between the hydrous oxide catalysts, it was verified that there was a new descriptor for oxygen evolution activity in acid. In addition, blended catalysts were prepared by uniformly blending carbon-supported platinum with oxide hydrate catalysts which had different activities in acidic oxygen evolution. Based on the carbon monoxide oxidation in this set of blended catalysts, a linear correlation between oxygen evolution activity of hydrous oxide catalysts and carbon monoxide oxidation performance in blended catalysts was verified in acidic electrolyte. From these observations, long-range water activation by exchanges between O-H and hydrogen bond in chains of water molecule was suggested. In chapter 4, further studies on oxophilic effect in alkaline media were carried out. From electrochemical observations on carbon monoxide oxidation, oxygen evolution, and hydrogen evolution and physicochemical characterizations, long-range oxophilic interactions between Pt nanoparticles and various water dissociation catalysts through electrolyte were verified in the blended catalysts system. Furthermore, in-depth discussion on the effect of metal-oxygen interaction on oxophilic effects was also performed.

Keywords: electrocatalysis, polymer electrolyte membrane fuel cell, water electrolysis, oxophilic effect

Student number: 2011-22925