(729c) Platinum Coated Cobalt Nanowires As Oxygen Reducing Electrocatalysts

Platinum Coated Cobalt Nanowires as Oxygen Reducing Electrocatalysts

 Shaun M. Alia¹, Svitlana Pylypenko², K.C.Neyerlin¹, Brian A. Larsen¹, Shyam S. Kocha¹, and Bryan S. Pivovar^1,*

¹ National Renewable Energy Laboratory, ²Colorado School of Mines

National Renewable Energy Laboratory

Hydrogen Technologies and Systems Center

1617 Cole Blvd, Golden, CO 80401-3393

bryan.pivovar@nrel.gov

The commercial deployment of proton exchange membrane fuel cells (PEMFCs) is primarily limited by cost. Carbon supported platinum (Pt) nanoparticles (Pt/C) are typically used in PEMFCs due to their high surface area and thus relatively high ORR mass activity.   A reduction in electrocatalyst cost will enable the rapid commercialization of automotive fuel cell vehicles in the near future.  In order to meet cost targets through  the development of highly active catalysts for ORR, the United States Department of Energy (DOE) has set a 2017 – 2020 mass activity target of 440 mA mg_PGM^—1 for automotive PEMFCs. Pt nanotubes were previously synthesized to improve the durability and ORR activity of PEMFC catalysts.³ Pt nanotubes produced a high specific activity in comparison to Pt/C, but lacked the surface area needed to reach the DOE target. Subsequent efforts have been made to improve Pt utilization by adding nanotube porosity and exploring alternative morphologies; palladium and copper templates have further been used to reduce the Pt content.^4-7

Cobalt (Co) nanowires were used as a template in this case to further improve ORR activity and to avoid template metals that could plate on PEMFC anodes during operation. Pt coated Co nanowires were synthesized in this study by the partial spontaneous galvanic displacement (SGD) of Co nanowires. Pt coated Co nanowires were found to have an outer diameter of 200 – 300 nm and a length of 100 – 200 µm. Electrochemical measurements were conducted in rotating disk electrode (RDE) experiments, with the inks deposited onto the RDE tips with and without the addition of carbon blacks and a Nafion ionomer.⁸

The use of a Co nanowire template increased specific ORR activity to greater than 2500 μA cm_PGM^—2 over a wide range of Pt displacements (5 - 40 wt. % Pt). Decreasing the percentage of Pt displacement also served to increase Pt utilization and surface area, exceeding 30 m² g_PGM^—1. By increasing surface area and maintaining a high specific activity, Pt coated Co nanowires have been able to produce an ORR mass activity approaching 850 900 mA mg_PGM^—1 (in RDEs measured in 0.1 M perchloric acid at 25 °C) that exceeds the DOE target in PEMFCs. Durability testing was further completed to demonstrate the retention of ORR activity following potential cycling. Pt coated Co nanowires were found to exceed the DOE target for ORR following 30,000 potential cycles (0.6 - 1.0 V vs. RHE). The measurements were carried out using protocols similar to that prescribed by the DOE durability working group.^9,10Compared to conventional, supported Pt nanoparticle electrocatalysts and pure Pt nanotubes, Pt coated Co nanowires appear to provide significant activity and durability advantages as measured in RDE half cells.

References 

1. H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Applied Catalysis B-Environmental 2005, 56, 9.
2. P. J. Ferreira, G. J. la O', Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha, H. A. Gasteiger, Journal of the Electrochemical Society 2005, 152, A2256.
3. Z. W. Chen, M. Waje, W. Z. Li, Y. S. Yan, Angewandte Chemie-International Edition 2007, 46, 4060.
4. S. M. Alia, G. Zhang, D. Kisailus, D. S. Li, S. Gu, Y. S. Yan, Advanced Functional Materials 2010, 20, 3742.
5. B. A. Larsen, K. C. Neyerlin, J. B. Bult, C. Bochert, J. L. Blackburn, S. S. Kocha, B. S. Pivovar, Journal of the Electrochemical Society 2012, 159, F622.
6. S. M. Alia, K. O. Jensen, B. S. Pivovar, Y. S. Yan, ACS Catalysis 2012, 2, 858.
7. S.M. Alia, K. Jensen, C. Contreras, F. Garzon, B. Pivovar, Y. Yan, ACS Catalysis 2013, 3, 358.
8. S. S. Kocha, J. W. Zack, S. M. Alia, K. C. Neyerlin, B. S. Pivovar, ECS Transactions 2013, 50, 1475.
9. M. Uchimura, S. Sugawara, Y. Suzuki, J. Zhang, S. S. Kocha, ECS Transactions 2008, 16, 225.
10. 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.