Platinum-Nickel Nanowires As Electrocatalysts in Alkaline Hydrogen Oxidation and Evolution

Abstract

Anion exchange membrane (AEM) fuel cells and electrolyzers have advanced over recent years to become competitive with proton exchange membrane (PEM) devices. The primary benefit of AEM systems is the potential ability to use less expensive, non-platinum (Pt) group metal (PGM) catalysts, and the non-PGM stability that the alkaline environment allows. While these systems have had polymer limitations, recent improvements in the chemical stability of alkaline membranes suggest that catalysts will become the limiting factor in the near future.[1, 2] Catalyst development in AEM systems typically focuses on oxygen reduction and evolution, since the reactions are kinetically orders of magnitude slower than hydrogen oxidation and evolution. Alternatives, however, exist to PGMs in these reactions: silver in oxygen reduction; and nickel and cobalt in oxygen evolution.[3, 4] While hydrogen oxidation and evolution are kinetically faster reactions, they are roughly two orders of magnitude slower on Pt in base compared to acidic environments. Non-PGM catalyst options are also less clear, and generally struggle to justify the AEM cost benefit, producing activities orders of magnitude lower than PGMs at higher overpotentials.[5] Recently, advanced Pt electrocatalysts have been developed in an effort to thrift the amount of PGMs in AEM fuel cells and electrolyzers. Pt-nickel (Ni) nanowires, previously developed for acidic oxygen reduction, were studied for their activity in hydrogen oxidation and evolution.[6] These materials were formed by spontaneous galvanic displacement, a process that occurs when a metal template contacts a nobler metal cation. At low levels of displacement, small amounts of Pt were deposited to produce high electrochemical surface areas. Subsequent post-synthesis processing was used to integrate the Pt-rich and Ni-rich zones, compressing the Pt lattice and improving its activity for hydrogen oxidation and evolution. Compared to carbon-supported Pt nanoparticles (Pt/HSC), Pt-Ni nanowires produced hydrogen oxidation/evolution exchange current densities 9 times greater. Figure 1. Comparison of the mass (red) and site-specific (blue) exchange current densities of Pt-Ni nanowires, Pt-copper nanowires, and Pt/HSC in the hydrogen oxidation and evolution reactions in a 0.1 m sodium hydroxide electrolyte.[7] [1] B. Pivovar, Alkaline Membrane Fuel Cell Workshop Final Report, in: U.S. Department of Energy (Ed.), http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/amfc_may2011workshop_report.pdf, 2011. [2] K.J.T. Noonan, K.M. Hugar, H.A. Kostalik, E.B. Lobkovsky, H.D. Abruña, G.W. Coates, Journal of the American Chemical Society, 134 (2012) 18161-18164. [3] R. Subbaraman, D. Tripkovic, K.-C. Chang, D. Strmcnik, A.P. Paulikas, P. Hirunsit, M. Chan, J. Greeley, V. Stamenkovic, N.M. Markovic, Nat Mater, 11 (2012) 550-557. [4] J.S. Spendelow, A. Wieckowski, Physical Chemistry Chemical Physics, 9 (2007) 2654-2675. [5] J.K. Nørskov, T. Bligaard, A. Logadottir, J.R. Kitchin, J.G. Chen, S. Pandelov, U. Stimming, Journal of The Electrochemical Society, 152 (2005) J23-J26. [6] 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. [7] S.M. Alia, B.S. Pivovar, Y. Yan, Journal of the American Chemical Society, 135 (2013) 13473-13478. Figure 1