Highly Active and Durable Extended Surface Oxygen Reduction Electrocatalysts

Abstract

The amount of platinum (Pt) in the catalyst layer accounts for a significant portion of fuel cell cost and limits the commercial deployment of proton exchange membrane fuel cells.[1] Extended surface nanomaterials have been developed as electrocatalysts in the oxygen reduction reaction for use in fuel cells. Extended surfaces offer key advantages to nanoparticles, including an order of magnitude higher specific activity, long range conductivity, and long term durability, but are traditionally limited by low surface area.[2] Catalyst development has included a variety of approaches, but recently has focused on galvanic displacement. Spontaneous galvanic displacement occurs when a less noble metal template contacts a more noble metal cation and combines aspects of corrosion and electrodeposition. Catalysts formed by spontaneous galvanic displacement are ideally situated, being able to take advantage of the specific activities generally associated with the catalyst type while significantly improving upon the surface area.[3] Recent developments in Pt-nickel (Ni) nanowires have produced materials with surface areas in excess of 90 m2 gPt ‒1.[4] Post-synthesis processing has been used to alloy the Pt-rich and Ni-rich zones, improving activity for oxygen reduction. Exposure to a variety of acid types and concentrations has been used to remove increasing amounts of Ni, creating nanostructures with high Pt surface areas and an array of compositions. Oxidative treatments have also been used to improve catalyst durability in rotating disk electrode (RDE) accelerated stress tests (30,000 potential cycles, 0.6‒1.0 V). By optimizing these techniques, extended surface electrocatalysts have been created with oxygen reduction mass activities 7 times greater than Pt nanoparticles and 5 times greater than the U.S. Department of Energy membrane electrode assembly (MEA) target in RDE half-cells. In accelerated stress tests, these materials lose less than 3% activity and less than 0.5% of their mass due to electrochemical dissolution. Remaining barriers, including synthesis scalability and MEA fabrication, are being addressed. Recent improvements in MEA performance suggest that the promise of extended surface catalysts in half-cells can be realized in the device. [1] D. Papageorgopoulos, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review14/fc000_papageorgopoulos_2014_o.pdf, 2014. [2] M. Debe, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review09/fc_17_debe.pdf, 2009. [3] S.M. Alia, Y.S. Yan, B.S. Pivovar, Catalysis Science & Technology, 4 (2014) 3589-3600. [4] S.M. Alia, B.A. Larsen, S. Pylypenko, D.A. Cullen, D.R. Diercks, K.C. Neyerlin, S.S. Kocha, B.S. Pivovar, ACS Catalysis, 4 (2014) 1114-1119. Figure 1. Surface areas (x-axis) and site-specific oxygen reduction activities (y-axis) of Pt nanoparticles and Pt-Ni nanowires, as-synthesized and modified by post-synthesis processing. The US Department of Energy MEA mass activity target (440 mA mgPt ‒ 1) is included as the solid black line. Figure 1