Intermetallic PtCo Catalysts for Fuel Cell Applications

Abstract

Conventional catalysts for the cathode reaction in polymer electrolyte fuel cells (PEFCs) are based on Pt and random alloys of Pt with base metals. Addition of base metals such as Co to nanostructured Pt catalysts is known to enhance oxygen reduction reaction (ORR) activity, but poor stability of Co under the aggressive conditions of high potential and low pH found in the cathode of PEFCs limits the enhancement provided by Co. Recently, we have developed several varieties of L1 0 ordered PtCo nanoparticle catalysts that exhibit remarkably enhanced performance and durability. Several factors, including particle size, Co content, degree of ordering, Pt shell thickness, and nanoparticle-support interactions are critical in determining performance and durability. Synthetic procedures developed at LANL have enabled preparation of L1 0 -PtCo with small particle size, high electrochemical surface area, good dispersion on the carbon-based support, and high performance in PEFC cathode testing under H 2 /air. These intermetallic particles retain high Co content with L1 0 ordering even after 30,000 square wave potential cycles between 0.6 and 0.95 V in membrane electrode assemblies (MEAs). Content of Co is a key factor controlling ORR kinetic performance. Furthermore, leaching of Co into the membrane and electrode ionomer can result in performance degradation due to cationic poisoning. Therefore, any reduction in Co leaching from PtCo catalysts would be helpful. The L1 0 -PtCo structures developed at LANL retain Co content approximately 50% higher than a random PtCo alloy after 30,000 potential cycles in MEA. This excellent resistance to Co leaching is the most important contribution of the L1 0 structure to providing high durability. Testing of new PtCo catalysts in MEAs is particularly critical to validate their performance and durability under real-world conditions. In addition to discussing the L1 0 -PtCo synthesis, this talk will also describe work at LANL on optimizing the integration of these catalysts into high performance, durable MEAs.