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] Catalyst studies typically focus on the oxygen reduction reaction (ORR), since the reaction is orders of magnitude slower kinetically than hydrogen oxidation. Extended surface nanomaterials offer key advantages in ORR, including an order of magnitude higher site-specific activity, long range conductivity, and long term durability.[2] Traditionally, however, the catalyst type has been limited by low surface areas. Spontaneous galvanic displacement occurs when a more noble metal cation contacts a less noble metal template, and combines aspects of corrosion and electrodeposition. In ORR, catalysts formed by spontaneous galvanic displacement are ideally situated, taking advantage of the specific activities generally associated with the catalyst type while significantly improving upon their surface area.[3] Galvanic displacement has been used to deposit small amounts of Pt onto extended templates, and in the case of Pt-nickel (Ni) nanowires, has produced materials with surface areas in excess of 90 m2 gPt ‒1.[4] Post-synthesis processing has been used to improve the activity and durability of Pt-Ni nanowires in rotating disk electrode (RDE) half-cells. Thermal annealing integrated Pt-rich and Ni-rich zones, compressing the Pt lattice and improving ORR activity.[5] Acid leaching and oxidation of the Pt-Ni nanowires also removed surface Ni and formed Ni oxides near the nanowire surface, improving catalyst durability and reducing Ni dissolution in durability tests (30,000 cycles, 0.6‒1.0 V). Recently, Pt-Ni nanowires were studied for their ORR activity in RDE half-cells using updated testing protocols. A rotational air drying method formed thin, uniform coatings onto the RDE working electrodes.[6] Thin catalyst ink dispersions also allowed for the ORR diffusion-limited current to be reached while maintaining a low Pt loading. Using these methods improved Pt-Ni nanowire activity in RDE half-cells by 60%. The optimized Pt-Ni nanowires produced an ORR mass activity 9 times greater than Pt/HSC, while losing less than 3% of that activity and less than 0.5% catalyst mass to dissolution in durability testing. Atomic layer deposition (ALD) has also been used in an effort to replicate the Pt-Ni nanowire synthesis by galvanic displacement. Works has included different chemistries (oxygen and hydrogen routes), as well as efforts to disperse the nanowire template without the benefit of a liquid medium. Successful use of ALD can potentially produce catalysts on a much larger, more commercially viable, scale. Figure 1. Surface areas (x-axis) and site-specific oxygen reduction activities at 0.95 V (y-axis) of Pt/HSC and Pt-Ni nanowires, as-synthesized and modified by post-synthesis processing. The solid black line denote constant mass activities of 100, 400, and 800 mA mgPt ‒ 1. [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. [5] B. Pivovar, Extended, Continuous Pt Nanostructures in Thick, Dispersed Electrodes, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review14/fc007_pivovar_2014_o.pdf, 2014. [6] K. Shinozaki, J.W. Zack, R.M. Richards, B.S. Pivovar, S.S. Kocha, Journal of The Electrochemical Society, 162 (2015) F1144-F1158. Figure 1