UC Irvine

This work investigates the oxygen reduction reaction (ORR) mechanism on Pt nanoparticles (NPs) dispersed on several carbon blacks with various physicochemical properties (i.e. specific surface area ranging from 80 to 900 m2 g-1, different graphitization degree, etc.). Using the kinetic isotope effect (KIE) along with various electrochemical characterizations, it was determined that the rate determining step (RDS) of the ORR is a proton-independent step when the density of Pt NPs on the surface of the carbon support is high. Upon decrease of the density of Pt NPs on the surface, the RDS of the ORR starts involving a proton, as denoted by an increase of the KIE > 1. This underlined the critical role played by the carbon support in the oxygen reduction reaction electrocatalysis by Pt supported on high surface area carbon. Furthermore, two of these carbon supports were selected and using the same synthesis methods, different loadings of Pt NPs were deposited on them. This allowed us to investigate the effect of catalyst loading, thus, take another approach to assess the particle proximity effect on the ORR. The durability and performance of the selected electrocatalysts were also explored in a membrane electrode assembly (MEA) in real-world operational conditions of automotive fuel cells. The effect of the loading of the platinum nanoparticles (Pt NPs) supported on two of the carbon supports with different morphologies (93 vs. 890 m2 g-1) on the oxygen reduction reaction (ORR) was also investigated. The correlation between their electrochemical performances with their physico-chemical properties was suggested. The experiments were extended from bench-scale lab tests to pilot-size membrane electrode assembly (MEA) fuel cell testing where the performance and durability of the in-house synthesized electrocatalyst was investigated. On the result basis, it was confirmed that low platinum loading on a high surface area carbon support resulted in a contribution of the latter to the ORR (see Chapter II for additional details). Moreover, it was observed that in the MEA systems, when using electrocatalyst with a large loading of Pt on the electrode – and thus a thin catalytic layer on the cathode side – at high current densities, flooding would partially block the Pt active site and thus limits the fuel cell performances. Finally, the heterogeneity of polymer electrolyte fuel cells (PEFCs) catalyst degradation was studied under varied relative humidity and type of feed gas. Accelerated stress tests (AST) were performed on four membrane electrode assemblies (MEAs) under wet and dry conditions in air or nitrogen environment for 30,000 square cycles from 0.60 V to 0.95 V (or open circuit voltage (OCV) for air since during cycling in air the OCV drops to values below 0.9 V). It was observed that the largest electrochemical active area (ECSA) loss during the ASTs was for MEA in wet conditions under nitrogen gas. This was mainly attributed to higher upper potential limit (UPL) of 0.95 V that was maintained during experiment and to higher water content in the MEA enabling higher Pt2+ mobility. In air the OCV was lower than 0.95 V during cycling reducing the overall upper potential limit (UPL) to below 0.95 V and thus reducing the rate of platinum oxide (PtO) formation and dissolution. Micro XRD was performed on end of life (EOL) MEAs to show Pt particle size distribution under lands and channels and in various MEA locations. Largest Pt sizes were observed for wet conditions and under the lands. Both represent locations in MEAs with higher water content. AST in air and wet environment showed the highest Pt particle size growth from inlet to outlet, which is due to larger water content at the outlet compared to the inlet. Micro XRF shows that Pt redistribution is a local phenomenon and Pt loading remains relatively uniform within the MEA. Additional diagnostics were used to understand morphology of the MEA at the EOL and confirmed that catalyst layer support structure does not change during AST. Conclusively, the ORR mechanism is subject to different environmental factors, including the morphological and surface characterization of the carbon support, density of the Pt NPs deposited on the support and operation conditions. A wide range of characterization methods and investigations protocols and procedures, ranging from bench-size lab scale tests to pilot-size automotive operation condition fuel cells was implemented. We hypothesized the ORR mechanism, investigated the effect of surface density and particle proximity of Pt NPs and showed the heterogeneity in the MEAs degradation processes.