Subfreezing Phenomena in Polymer Electrolyte Fuel Cells
One of the most critical aspects of proper fuel cell design is water management: too little water, and the membrane will dry out; too much water, and the catalyst layer will flood and block access of the reactant gases to the electro-catalyst surface. Developers have put considerable effort into the optimization of three-dimensional structures that can accommodate the simultaneous demands of membrane hydration and gas access to the catalyst layer under normal operating conditions, as well as of the power-plant-level designs that can ensure water is properly distributed and managed. In fuel cell power plants that operate intermittently and are exposed to atmospheric conditions, such as in automotive applications, the challenges of water management are complicated by the fact that the system will frequently have to be started from subfreezing conditions. Given the volume change associated with freezing water, one expects that any water retained in the pores of the catalyst layer or at the catalyst layer interface with the gas-diffusion layers will expand and can therefore create considerable stresses on the porous materials, possibly deforming them from their initial state. To design a cell that can accommodate these changes, a thorough understanding of the physics of water movement and freezing is necessary. Researchers have seen degradation associated with freeze/ thaw cycles and, more specifically, with drawing current from the fuel cell when the temperature of the cell itself is below freezing, a procedure that will be frequently experienced for fuel cell power plants deployed in automotive applications. Experiments demonstrate a marked increase in the mean pore size and width of the pore size distribution of the catalyst layer after thermal cycling. Modeling of the phase transition associated with thawing and freezing, however, as well as the coupled phenomena of water management and thermal management under partially frozen conditions is rather limited in the open literature. In this chapter, we examine the limits of current understanding, as well as the data that suggest freezing-point depression in polymer electrolyte fuel cell materials and the implications of lowered temperatures on fundamental kinetic processes.
KeywordsFuel Cell Catalyst Layer Cold Start Membrane Electrode Assembly Fuel Cell Performance
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- Blair, L. (2005) Fundamental Issues in Subzero PEMFC Startup and Operation, Presentation at DOE Workshop on Fuel Cell Operations at Sub-Freezing Temperatures, Phoenix, AZ, February 1–2, 2005, http://www1.eere.energy.gov/hydrogenandfuelcells/fc_freeze_workshop.html
- DOE, U.S. (2007) Multi-Year RD&D Plan http://www1.eere.energy.gov/hydrogenandfuelcells/ mypp/.
- Mathias, M.F., Makharia, R., Gasteiger, H.A., Conley, J.J., Fuller, T.J., Gittleman, C.J., Kocha, S.S., Miller, D.P., Mittelsteadt, C.K., Xie, T., Yan, S.G., and Yu, P.T. (2005) Two fuel cell cars in every garage?, Interface, 14, 24.Google Scholar
- Meyers, J.P. (2005) Fundamental Issues in Subzero PEMFC Startup and Operation, Presentation at DOE Workshop on Fuel Cell Operations at Sub-Freezing Temperatures, Phoenix, AZ, February 1–2, 2005, http://www1.eere.energy.gov/hydrogenandfuelcells/fc_freeze_workshop.html
- NEDO, homepage: http://www.nedo.go.jp/nenryo/gijutsu/index.html.
- Pesaran, A.A., Kim, G.-H., and Gonder, J.D., (2005) PEM fuel cell freeze and rapid startup investigation, in Milestone Report, http://www.nrel.gov/hydrogen/pdfs/pem_fc_freeze_milestone. pdf, NREL Editor, NREL.
- Simpson, S.F., Salinas, C.E., Cisar, A.J., and Murphy, O.J. (1995) Factors affecting the performance of proton exchange membrane fuel cells. In A. Landgrebe, S. Gottesfeld, and G. Halpert (Eds.), Proceedings of the First Symposium of the Electrochemical Society, First International Symposium on Proton Conducting Membrane Fuel Cells, The Electrochemical Society, Pennington, NJ, pp. 182–192.Google Scholar
- Yan, Q., and Wu, J. (2005), 207th Meeting Electrochemical Society, MA2005-01, Abstract # 1514.Google Scholar