The ultimate objective of the proposed R&D project is to develop a fuel cell which could be operated without external humidification of the reactant gases in a variable power mode and in a variety of ambient conditions. In order to achieve this goal we will apply a novel concept of spatially variable heat removal rate, which establishes a temperature profile along the cathode channel allowing the product water to humidify the air flowing through the cathode up to 100% relative humidity. This concept has already been tested in our lab and preliminary results have been published [1]. However, the concept has been proved only for one current density, given membrane thickness, given flow field configuration, and given set of ambient conditions. We propose to further investigate this promising concept at various current densities, variety of ambient conditions, different membrane thicknesses, and different flow configurations.
One of the most important processes needed to make this concept work is water transfer across the polymer membrane, i.e., its magnitude and net direction. Therefore, one of the objectives of the proposed research is to study thermodynamics of water absorption and desorption and phase change at the polymer membrane surface, as these phenomena are usually neglected in current models. Berning [2] pointed out that the models that assume equilibrium between the membrane and the gas phase inside the catalyst layers neglect important physical effects, i.e, non-equilibrium water absorption/desorption, and can consequently not yield a correct solution.
One of the objectives of this project is also to prove that the proposed concept of fuel cell operation with dry gases employing spatially variable temperature would result in longer life than if the fuel cell is operated at uniform constant temperature. It is well known that decay rates and durability of PEM fuel cells are strong functions of operating conditions [3,4]. It has been reported that the same stacks last longer in one application than in another [5]. Many degradation processes are known, but the understanding of the correlation with the operating conditions is insufficient [6]. The operating parameters that have the highest impact on fuel cell decay rate and durability are cell potential, temperature and humidity. Very often the extreme values of these “stressor” parameters are used in accelerated testing of fuel cells. There are many possible causes of fuel cell performance degradation, such as loss of catalyst area by sintering or dissolution, contamination by adsorption, contamination by foreign cations, drying of ionomer in the catalyst layer (either on the anode or on the cathode side) drying of the membrane, flooding in the catalyst layer, in gas diffusion layer, or in the channels resulting in reactants blockage [7,8]. Of course it is possible to have more than one cause of degradation present which only makes it more difficult to determine the root cause of degradation.
PEM fuel cells operate in a very narrow operating window between drying and flooding. It is even possible to simultaneously have drying on one side and flooding on the other side. Drying or flooding does not have to happen over the entire cell area, it is more likely that it may occur locally, i.e. only in certain regions of the fuel cell active area. Which regions will be affected depends on the cell design particularly on relative flows on the anode and cathode, flow field design, properties of the gas diffusion layer, catalyst layer structure, membrane thickness, temperature distribution which depends on how the heat is being removed from the fuel cell. Operating conditions may also contribute to drying or flooding conditions, particularly operating temperature and how it is being controlled and maintained, and flow rates and humidity of the reactant gases. The consequences of flooding and drying are usually reversible, but long term exposure to these extreme conditions may have detrimental effects on the fuel cell health and durability.
One of the reasons for insufficient durability of PEM fuel cells may be inadequate heat removal (on nanoscale) from the reaction sites, i.e., from the catalyst surface immersed in polymer. This may lead to locally increased temperatures, which could cause local drying of the polymer, which in turn would result in loss of performance and eventually to morphological changes in polymer around the catalyst sites. Therefore, one of the objectives of the proposed research is to pay a closer attention to heat transfer in catalyst layer nanostructures.
[1] I. Tolj, D. Bezmalinovic, F.Barbir, International Journal of Hydrogen Energy 36 (2011) 13105-13113.
[2] T. Berning, International Journal of Hydrogen Energy 36, 15 (2011) 9341-9344.
[3] J. Wu, X.Z. Yuan, J.J. Martin, H.Wang, J. Zhang, J. Shen, S.Wu,W. Merida, Journal of Power Sources 184 (2008) 104–119.
[4] 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. Kimijima, N. Iwashita, Chem. Rev. 107 (2007) 3904.
[5] K. Colbow, Membrane Requirements for Stationary Applications, High Temperature Membrane Working Group Meeting, Palm Springs, CA, November 16, 2009, http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/htmwg_nov09_fuel_cell_growth.pdf
[6] M. Schulze, N. Wagner, T. Kaz, K.A. Friedrich, Electrochimica Acta 52 (2007) 2328–2336.
[7] L. Jörissen, W. Lehnert, J. Garche, and W. Tillmetz, Lifetime of PEMFCs, Materials Science Forum, Vols. 539-543 (2007) pp 1303-1308.
[8] M. L. Perry, Durability of Polymer Electrolyte Fuel Cells, Chapter 11 in F. Barbir (ed.) PEM Fuel Cells Theory and Practice, 2nd Edition, Elsevier/Academic Press, Burlington (2012)