(272c) Oxidation of Platinum Nickel Nanowires to Limit Displacement and Improve the Durability Characteristics of Oxygen-Reducing Electrocatalysts

Oxidation of Platinum Nickel Nanowires to Limit Displacement and Improve the Durability Characteristics of Oxygen-Reducing Electrocatalysts

Shaun M. Alia,¹ Svitlana Pylypenko,² Arrelaine Dameron,¹ K. C. Neyerlin,¹ Shyam S. Kocha,¹ and Bryan S. Pivovar^1,*

¹ National Renewable Energy Laboratory, ²Colorado School of Mines

National Renewable Energy Laboratory

Chemical and Materials Science Center

Golden, CO 80401

bryan.pivovar@nrel.gov

The cost of platinum (Pt) catalysts remains a significant barrier in the commercial development of proton exchange membrane fuel cells (PEMFCs). Carbon supported Pt nanoparticles (Pt/HSC) are typically used in PEMFCs due to their high surface area and moderate oxygen reduction reaction (ORR) mass activity. Extended Pt structures have previously been found to have specific activity and durability benefits to Pt nanoparticles.^1-3 Although extended catalysts can produce high specific activities, they generally lack the surface area to produce high mass activity. Catalysts formed by galvanic displacement are poised to produce high mass activities for ORR by maintaining the high specific activities demonstrated by extended Pt surfaces and by allowing for the deposition of thin Pt layers. Nickel (Ni) nanowires were previously used as a template for galvanic displacement, producing Pt coated Ni nanowires with surface areas as high as 91.3 m² g_PGM^‒1 and mass ORR activities as high as 917 mA mg_PGM^‒1, 3.1 times greater than Pt/HSC (in rotating disk electrode, RDE, half-cell measurements).

Ni nanowires and Pt coated Ni nanowires were treated in oxygen at elevated temperatures. By heat treating in oxygen: the effects of an increasing Ni nanowire oxide layer on Pt displacement were examined; and durability losses (surface area and activity) of Pt coated Ni nanowires were reduced. As-received Ni nanowires contain a thin oxide layer and a Ni metal core. Heat treatment of the Ni nanowires with oxygen served to grow the oxide layer, increasing the oxide content near the nanowire surface, and oxidizing the metal nanowire core (to NiO) at higher temperatures. Increasing the oxide layer served to limit and eventually shut off Pt displacement.

Although untreated Pt coated Ni nanowires produce high mass activities, losses (similar to Pt/HSC) were observed following RDE durability testing (30,000 potential cycles, 0.6‒1.0 V vs. RHE, similar protocol to DOE working group).^5,6 While extended Pt structures have previously been found to offer durability benefits to Pt/HSC, the benefit is likely lessened by the presence of Ni metal. By heat treating Pt coated Ni nanowires in oxygen, the oxide layer thickness was increased thereby stabilizing the catalysts in potential cycling. Treated catalyst produced initial surface areas and activities that were lower than the untreated Pt coated Ni nanowires, presumably since Ni near the surface could not be removed electrochemically during break-in. With potential cycling, however, treatment of the nanowires dramatically improved retention of surface area and activity. Following durability testing, treated Pt coated Ni nanowires produced a mass ORR activity of 680 mA mg_PGM^‒1, or a 9 % improvement of initial activity. Compared to conventional Pt electrocatalysts and fully displaced Pt nanotubes, Pt coated Ni nanowires appear to provide significant activity and durability advantages.

References 

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2. Chen, Z.; Waje, M.; Li, W.; Yan, Y. Angew. Chem. Int. Ed. 2007, 46, 4060-4063.

3. Papandrew, A. B.; Atkinson, R. W.; Goenaga, G. A.; Wilson, D. L.; Kocha, S. S.; Neyerlin, K. C.; Zack, J. W.; Pivovar, B. S.; Zawodzinski, T. A. ECS Transactions 2013, 50, 1397-1403.H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Applied Catalysis B-Environmental 2005, 56, 9.

4. Alia, S. M.; Larsen, B. A.; Pylypenko, S.; Cullen, D. A.; Diercks, D. R.; Neyerlin, K. C.; Kocha, S. S.; Pivovar, B. S. ACS Catalysis 2014, 4, 1114-1119.

5. M. Uchimura, S. Sugawara, Y. Suzuki, J. Zhang, S. S. Kocha, ECS Transactions 2008, 16, 225.

6. S. S. Kocha, Electrochemical Degradation: Electrocatalyst and Support Durability. In M. Mench, E. C. Kumbur, T. N. Veziroglu, Polymer Electrolyte Fuel Cell Degradation, pp. 89-185, 2011.