Durability of Low-Temperature Fuel Cell Electrocatalysts

One of the primary challenges facing the development of polymer electrolyte membrane fuel cells (PEMFCs) for automotive and stationary power applications is the durability of the fuel cell materials.¹ Though significant progress has been made toward achieving the technical target of 5,000 operating hours, especially with unconventional architectures,² the current reported durability status for conventional fuel cell stacks is considerably shorter (~2,500 hours).³ Typical degradation rates for constant load conditions, with adequate humidification and reactant flow, are on the order of 25 µV/h.⁴   Degradation rates are accelerated by approximately an order of magnitude when cells are operated under non-steady-state conditions prevalent in the automotive application, including load and start-stop cycling, and start-up, shut-down cycling.^4,5   Degradation is also increased dramatically at temperatures above 90ºC, under low humidification, humidification cycling, fuel starvation, and with air ingress into the anode.⁵ The observed degradation under these “non-ideal” conditions has reversible and irreversible components: reversible losses attributable to water management (flooding or drying), poisoning, and oxide formation on the cathode catalyst⁵ and the most prevalent irreversible degradation to loss of water removal efficiency and cathode electrocatalyst activity.^4,5

The focus of this talk will be the irreversible losses in PEMFC performance, as these are the most challenging in terms of mitigation strategies.  Specifically, the focus will be on the cathode catalyst degradation, because the degradation of this component has the most profound impact on cell performance.  Though degradation of the anode catalyst has been observed,⁵ the kinetics of the anode reaction are three orders of magnitude faster than the cathode reaction and thus anode catalyst degradation has relatively little overall impact on cell performance.⁶

Current conventional PEMFC cathode catalyst materials typically consist of finely divided Pt or Pt alloy particles (nanometer scale) loaded at 40-70 wt% on high surface area carbon blacks. The Pt particles are highly dispersed on the support materials to maximize the catalyst’s electrochemically-active surface area (ECA) and hence activity per unit mass of Pt. The most dominant cause of degradation of Pt and Pt-based cathode electrocatalytic activity is loss of ECA.⁵  These losses are significant even under steady-state conditions (a 46% loss of ECA was observed after 2,000 hrs at 0.2 A/cm²),⁷ but are exacerbated under load (voltage) cycling conditions.⁵  Numerous studies have focused on elucidating the mechanisms, governing catalyst properties, and cell conditions affecting ECA loss.  For example, ex situ transmission electron microscopy, X-ray diffraction, and X-ray scattering analyses of cathode catalyst layers during and after long-term steady-state and potential cycling operation have shown that there is dramatic coarsening of the platinum and platinum alloy particles and loss of platinum into electrochemically inaccessible regions of the MEA, such as into the membrane, gas diffusion layer, or effluent.^4,5,8  This loss and coarsening is affected by factors such as cell voltage, potential cycling profile, temperature, cathode relative humidity, and type and degree of graphitization of the support.⁵  Recent systematic studies of Pt and Pt₃Co cathode electrocatalysts have shown that the catalyst property that dominates the extent of ECA loss is initial catalyst particle size through its effect on platinum dissolution rates.⁹  These mechanisms and means to mitigate their impact on PEMFC performance loss will be discussed.

Acknowledgements: The authors gratefully acknowledge funding from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office (Nancy Garland, Technology Development Manager) and the U.S. Department of Energy, Office of Basic Energy Sciences for support of the Advanced Photon Source.

References

(1)     M. F. Mathias, R. Makharia, H. A. Gasteiger, J. J. Conley, T. J. Fuller, C. J. Gittleman, S. S. Kocha, D. P. Miller, C. K. Mittelsteadt, T. Xie, S. G. Yan, P. T. Yu, Interface, 14 (2005) 24-35.

(2)     M. Debe, J. Electrochem. Soc., 160 (2013) F522-F534.

(3)     K. Wipke, S. Sprik, J. Kurtz, T. Ramsden, C. Ainscough, G. Saur, “National Fuel Cell Electric Vehicle Learning Demonstration Final Report”, NREL/TP-5600-54860 May 2012. http://www.nrel.gov/hydrogen/pdfs/54860.pdf

(4)  F. A. de Bruijn, V. A. T. Dam, G. J. M. Janssen, Fuel Cells, 8 (2008) 3-22.

(5)  R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. I. Kimijima, N. Iwashita, Chem. Rev., 107 (2007) 3904-51.

(6)  H. A. Gasteiger, J. E. Panels, S. G. Yan, J. Power Sources, 127 (2004) 162-171.

(7)  P. J. Ferreira, G. J. la O', Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha, H. A. Gasteiger, J. Electrochem. Soc., 152 (2005) A2256-A2271.

(8)  J. Gilbert, N. Kariuki, A. J. Kropf, D. Morgan, D.J. Myers, S. Ball, J. Sharman, B. Theobald, and G. Hards, Electrochem. Soc. Trans., Honolulu PRiME 2012 Meeting, October 8, 2012, Volume MA2012-02, Issue 13, p. 1317-1318.; J.A. Gilbert, N.N. Kariuki, R. Subbaraman, A.J. Kropf, M.C. Smith, E.F. Holby, D. Morgan, and D.J. Myers, J. Am. Chem. Soc., 134(36) (2012) 14823-14833.

(9)  R.K. Ahluwalia, S. Arisetty, X. Wang, X. Wang, R. Subbaraman, S.C. Ball, S. DeCrane, and D.J. Myers, J.  Electrochem. Soc., 160 (4) (2013) F447-F455.; D. Myers, 2012 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting, Washington, D.C., 2012. http://www.hydrogen.energy.gov/pdfs/review12/fc012_myers_2012_o.pdf