Development and understanding of Pd-based nanoalloys as cathode electrocatalysts for PEMFC
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Proton exchange membrane fuel cells (PEMFC) are attractive power sources as they offer high conversion efficiencies with low or no pollution. However, several challenges, especially the sluggish oxygen reduction reaction (ORR) and the high cost of Pt catalysts, impede their commercialization. With an aim to search for more active, less expensive, and more stable ORR catalysts than Pt, this dissertation focuses on the development of non-platinum or low-platinum Pd-based nanostructured electrocatalysts and a fundamental understanding of their structure-property-performance relationships. Carbon-supported Pd–Ni nanoalloy electrocatalysts with different Pd/Ni atomic ratios have been synthesized by a modified polyol reduction method, followed by heat treatment in a reducing atmosphere at 500–900 oC. The Pd–Ni sample with a Pd:Ni atomic ratio of 4:1 after heat treatment at 500 °C exhibits the highest electrochemical surface area and catalytic activity. The enhanced activity of Pd80Ni20 compared to that of Pd is attributed to Pd enrichment on the surface and the consequent lattice-strain effects. To improve the catalytic activity and long-term durability of the Pd–Ni catalysts, Pd–Pt–Ni nanoalloys have been synthesized by the same method and evaluated in PEMFC. The Pt-based mass activity of the Pd–Pt–Ni catalysts exceeds that of commercial Pt by a factor of 2, and its long-term durability is comparable to commercial Pt within the testing duration of 180 h. Both the favorable and detrimental effects of Pd and Ni dissolution on the performance of the membrane-electrode assembly (MEA) have been investigated by compositional analysis by transmission electron microscopy (TEM) of the MEAs before and after the fuel cell test. The MEAs of the Pd–Pt–Ni catalyst have then been characterized in-situ by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) to better understand the performance changes during cell operation. The surface state change from Pd-enrichment to Pt-enrichment and the consequent decrease in the charge transfer resistance during cell operation is believed to contribute to the activity enhancement. To further improve the MEA performance and durability, the as-synthesized Pd–Pt–Ni catalysts have been pre-leached in acid and Pd–Pt alloy catalysts have been synthesized to alleviate contamination from dissolved metal ions. Compared to the pristine Pd–Pt–Ni catalyst, the preleached catalyst shows improved performance and the Pd–Pt catalyst exhibits similar performance in the entire current density range. Finally, the catalytic activities for ORR obtained from the rotating disk electrode (RDE) and PEMFC single-cell measurements of all the catalysts are compared. The improvement in the activities of the Pd-Pt-based catalysts compared to that of Pt measured by the RDE experiments is much lower than that obtained in single cell test. In other words, RDE tests underestimate the value of the Pd-Pt-based electrocatalysts for real fuel cell applications. Also, based on the RDE data, the Pd–Pt–Cu catalyst exhibits the highest catalytic activity among all the Pd–Pt–M (M = Fe, Ni, Cu) catalysts studied.