With increasing global energy demand and energy-related carbon emissions, the transportation sector (accounting for over 25% of total greenhouse gas emissions in the United States) must be pushed towards total decarbonization. The polymer electrolyte membrane fuel cells (PEMFC) are a promising energy conversion technology towards the zero-emission goals in transportation applications. However, the high prices and short life cycles of the electrocatalytic materials hinder the widespread commercialization of PEMFCs. Specifically, the design of the cathode catalyst materials (driving the sluggish cathodic oxygen reduction reaction) is instrumental in the success of PEMFCs. Current industry standards utilize a high platinum content catalyst (in nanoparticle form) supported on carbon blacks. The understanding and development of the ideal catalyst layer is three-fold: (1) the nanoparticle-ionomer interface, (2) the nanoparticle-support interface, and (3) the ionomer-support interface. The first portion of my work outlines the ideal cathodic catalyst layer based on a review of recent works which encompasses all three interfacial interactions. The second portion of my work addresses the solid-state integration challenges (i.e., the nanoparticle-ionomer interactions) for the advanced structured Pt-based nanomaterials. The final portion of my work addresses the next generation design parameters of carbon-based supports with a focus on nanoparticle-support interactions.

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.

Electrification, especially in transportation, is key to objectives mitigating climate change effects. Metal-air batteries, including aluminum-air batteries (AAB), may offer performance and safety improvements over lithium-air batteries by using widely available, inexpensive materials.AAB cell advantages in weight, cost, and safety are countered by high polarization from a persistent oxide film, and by high corrosion rates, most notably in alkaline electrolytes. Much research on these topics has employed pure aluminum or specially prepared alloys to minimize corrosion during experimental evaluation. A key question - could commercial aluminum alloys be used in AAB cells in the immediate future? Thus, twin purposes of this study were to assess the state of current technologies of aluminum-air batteries, and to characterize several commercial aluminum alloys to gauge their suitability for use with common aqueous electrolytes in AAB cells. A comprehensive review of prior AAB works was conducted, including assessments of several other metal-air technologies where common elements, such as catalysts for the oxygen reduction reaction (ORR), are relevant. Effects of anode material composition, and of electrolyte additions as corrosion inhibitors, were considered. Characterization of Al alloy 6061 in alkaline electrolyte was done by weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy. The cathodic effect of second-phase intermetallic particles in Al 6061 to spur galvanic corrosion nearly negated inhibitor benefit in 4M sodium hydroxide electrolyte. Detailed experimental studies were conducted in neutral and acidic aqueous electrolytes. Aluminum alloys 4047 (silicon), 6061 (magnesium/silicon), and 7075 (zinc/copper) were evaluated by half-cell electrochemical techniques, and by use in a flow battery cell coupled to platinum-cobalt or iron-nitrogen-carbon catalysts in air-breathing or oxygen-breathing gas diffusion electrodes (GDE). Post-discharge characterization of anode surfaces was done using scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction. Each alloy manifested different performance, but the strongest influence on cell performance came from ORR kinetics in the GDE. Cell outputs and aluminum utilizations for all materials were similar, regardless of composition. Given the state of the art in ORR catalysts, this work indicates that there appears to be no impediment to using commercial alloys as anode materials in AAB systems at this time.