Proton exchange membrane fuel cells (PEMFCs) are considered to be a promising candidate for the future generation of power solutions for vehicles due to the high power density, high efficiency and low emissions. However, there are many barriers that limit the widespread application of PEMFCs and it is believed that cost and durability are the major two challenges to the commercialization of PEMFC. A major part of the high cost is attributed to the state-of-the-art Pt/C electrocatalysts which are usually in the form of Pt nanoparticles dispersing on high surface area carbon support. Due to the inherent slow oxygen reduction kinetics on Pt nanoparticles, the overpotential in the fuel cell is attributed in large part to the cathode where oxygen reduction reaction (ORR) takes place. In addition, the Pt/C electrocatalysts will degrade in the cathode due to the low pH, high voltage oxidation environment and potential variation conditions like dynamic loading or start-up/shut-down cycles in electronic vehicles. Therefore, both the activity and durability of ORR catalysts must be improved simultaneously.In this work, Pd@Pt nanodendrites were synthesized via a one-step aqueous reduction method. In order to decrease the particle size of Pd@Pt NPs and utilize the advantage of the transformation of core-shell structure to alloy structure, commercial Pd/C catalysts with Pd particle size around 4 nm was adopted as the substrate to prepare a series of Pt modified Pd/C catalysts (Pt/Pd/C) with different Pt/Pd molar ratio (Pt: Pd=1:4, 1:2 and 1:1). Pt was confirmed to be selectively deposited on the Pd nanoparticles. Electrochemical active surface area (ECSA) loss and polarization behavior in the single cell testing also suggest that the Pt1/Pd2/C show a much better durability than Pt/C catalysts. The improved durability is believed to be associated with the dissolution of Pd and the corresponding structure transformation from core-shell structure to Pt-Pd alloy with Pt rich surface.In order to make use of the structure-sensitivity of ORR on the catalyst surface, the author synthesized faceted Pd supported Pt nanoparticles (Pd-s-Pt NPs). The Pd2-s-Pt1 NPs with Pt/Pd molar ratio of 1/2 presented superior mass activity and area-specific activity than that of Pd2@Pt1 NDs and Pt1/Pd2/C, reaching 1.8 and 3.7 times that of Pt/C, respectively. In addition, Pd2-s-Pt1 NPs showed outstanding durability towards potential cycling. The retention percentage of mass activity and ECSA was 74% and 73%, respectively, after cycling the potential between 0.6 and 1.2 V (vs. RHE) in 0.1 M HClO4. In contrast, the corresponding value for Pt/C was only 37% and 31%, respectively. The improved durability of Pd2-s-Pt1 NPs was considered to be caused by the slow oxidation of Pd core and the transformation of solid core-shell structure to hollow alloy structure.In order to further increase the activity of catalysts, sub-10 nm binary Pd@Pt NPs with core-shell structure were synthesized by using Pluronic F127 copolymers as the reductant, stabilizer and capping agent simultaneously. In the single-fuel-cell testing, the fuel cell with Pd2@Pt1 NPs as the cathode catalyst presented similar performance with that of commercial Pt/C although the cathode Pt loading for Pd2@Pt1 NPs was one quarter of that of Pt/C. Moreover, Pd2@Pt1 NPs showed good durability in the single-fuel-cell testing, which made Pd2@Pt1 NPs as a promising cathode catalyst for fuel cell applications.

Durable Pd@Pt core-shell electrocatalyst for proton exchange membrane fuel cells (PEMFCs)

248th National Meeting (2014)

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