Platinum (Pt) alloy nanoparticles are used as catalysts in electrochemical cells to reduce oxygen to water and to oxidize hydrogen; the overall reaction converts chemical energy into electrical energy. These nanocatalysts are deposited on a carbon substrate and their catalytic function takes place in acid medium. This harsh environment causes an undesired reaction, which is the dissolution of the metal atoms into the acid medium; thus affecting the catalyst life. This dissertation aims to investigate the dissolution mechanism of fuel cell cathode catalysts at the atomic level starting from the oxygen reaction intermediates on the cathode catalyst surface and propose guidelines to improve cathode catalysts durability based on our proposed mechanism. Density functional theory is employed to study various possible scenarios with the goals of understanding the mechanism of the metal atom dissolution process and establishing some guidelines that permit a rational design of catalysts with better stability against dissolution. A thermodynamic analysis of potential metal dissolution reactions in acid medium is presented first, using density functional theory calculations to explore the relative stabilities of transition metals in relation to that of Pt. The study is performed by comparing the change in reaction Gibbs free energies for different metals in a given dissolution reaction. Then, a series of density functional theory studies, tending to investigate the adsorbed atomic oxygen absorption process from cathode catalyst surface into its subsurface, includes: 1) the oxygen adsorption on various catalyst surfaces and oxygen absorption in subsurface sites to figure out the minimum energy pathway and energy barrier of on-surface oxygen migration and absorption into subsurface; 2) the oxygen coverage, the other oxygen reduction reaction intermediates, and water effects on the oxygen absorption process according to reaction pathways, energy barriers, and thermodynamic analysis; 3) the oxygen absorption process on several Pt-based alloys with various compositions and components to find out the best alloy to inhibit atomic oxygen absorption including both kinetic and thermodynamic analyses, and the effects of such alloyed species on the inhibition process.

Proton exchange membrane fuel cells (PEMFCs) have attracted great attention due to their high power density, low-temperature operation and high energy conversion efficiency. However, the high cost of Pt catalysts and durability problems hinder their commercialization. So their cost must be lowered drastically and their durability must be extended. In an effort to overcome these problems, there have been intensive efforts to enhance the activity, durability and to lower the price of catalysts by alloying with other less expensive metals. In particular, the sluggish kinetics of ORR caused by Pt oxide at cathode and Pt catalyst degradation by electrochemical surface area (ECSA) loss have been a huge research area where a lot of researchers have paid lots of attention to solve. In this regard, the objective of this dissertation is to evaluate a series of Pt catalyst electrode surface electrochemical reactions on PEMFC electrode in order to help searching new catalysts and enhancing system design, assist in the search for new catalysts and improved system design by suggesting the developed mechanism of electrocatalyst activity and stability (durability). We have been focused on understanding the oxide-mediated dissolution of Pt by using electrochemical experiment methods such as RRDE, EQCN, SECM with a combination of ICP-MS and computational simulation with COMSOL Multiphysics. Firstly, in chapter 3, we showed the oxide-mediated Pt dissolution rate and the influence of hydrogen and cation underpotential deposition on Pt dissolution. In chapter 4, we revealed oxygen reduction reaction (ORR) plays a significant role in Pt oxide formation and reduction that influences the Pt catalyst dissolution, resulting in accelerated Pt dissolution rate at specific potential range. Finally, we found out the nature of mobile species generated during PtO₂ reduction process which have been disputed as Pt ion or other mobile species and fulfilled computational simulation for evaluation of SECM experiment in chapter 5. Based on these experiments and simulation, we were able to explain some mechanism of literature results that already were reported but have not been clearly explained so far. In terms of the purpose of this dissertation, the mechanism of oxide-mediated Pt dissolution, influence of ORR to Pt oxide formation/reduction and Pt dissolution, the nature of mobile species generated during PtO₂ reduction process, are sure to be very helpful in developing new catalysts and enhancing system design and suggested operating conditions. / text

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