(631d) Optimization Study of Proton Exchange Membrane Fuel Cells (Pemfc) for Platinum Reduction and Performance

Several experimental studies on optimizing the amount of platinum loading and ionomer content in the proton exchange membrane fuel cell catalyst layers exist in the literature [1-11]. Recently, Song et al. [12, 13] have carried out numerical optimization studies of catalyst layers in PEM fuel cell cathodes. In their first study, Song et al. [12] have investigated maximizing the current density at the operating potential of 0.60 V. Nafion content, platinum loading, catalyst layer thickness and void fraction were considered as the design parameters and one-parameter and two-parameter optimization results were presented. In this study, uniform distributions of nafion and catalyst particles were considered throughout the catalyst layer region. In their second study, Song et al. [13] have considered variable loadings for nafion and catalyst across the thickness of the catalyst layer. From their optimization results they conclude that the optimum distribution is a linearly increasing function from the gas diffusion layer to the membrane side for both nafion content and platinum. In our earlier optimization studies carried out using a steady state spherical agglomerate model [14] for platinum reduction, we have observed that the optmization results are sensitive to initial guesses and suggest the existence of local minima. The results also suggested that there might be interesting trade-offs between maximizing current and minimizing platinum loading. Multi-objective optimization techniques that can uncover pareto optimal fronts might generate several acceptable alternate choices in a catalyst layer design. In this talk, we will present optimization studies using a full fuel cell model that includes the manifold, diffusion backing, and the electrode. The design parameters that are considered for optimization are the void fraction, weight fraction of platinum on carbon, weight fraction of ionomer inside the catalyst layer and the density of ionomer. The optimization model includes both the first principles equations and also conditions that reflect the dependencies between the optimization variables and other derived parameters that are used in the model.

The amount of current generated within the PEM fuel cell catalyst layer strongly depends on the local conditions such as the concentration of available oxygen, total surface area available for reaction, and the amount of ionomer. Optimization at the catalyst layer level would involve, in addition to the above parameters, taking into consideration the gas transport effects within the void space. The void fraction (ε_r) in the catalyst layer would depend on design parameters such as, weight fraction of platinum on carbon (f_pt), weight fraction of ionomer inside the catalyst layer (f_mem), and the density of ionomer (ρ_mem). In addition, the final void fraction would strongly depend on the experimental conditions under which the membrane-electrode-assembly is prepared. Hence, performing optimization of a PEM fuel cell model incorporating all these effects would involve solving a highly nonlinear optimization problem with constraints. In this work, we present two different optimization formulations: (i) for minimizing the platinum loading, and (ii) maximizing the performance in terms of power generated. Optimization studies were performed for operating points on the i-v curve one at a time. In addition, a third formulation is also presented, in which the power generated is maximized simultaneously for all the i-v points. With the above results, we highlight the presence of local optima and the multi-objective nature of the fuel cell catalyst design problem. A measure based on the platinum usage is used as a benchmark while analyzing the results.

References:

[1] Mukerjee, S., S. Srinivasan, and A. J. Appleby, ?Effect of sputtered film of platinum on low platinum loading electrodes on electrode kinetics of oxygen reduction in proton exchange membrane fuel cells?. Electrochim. Acta, 38, 1661?1669 (1993).

[2] Passalacqua, E., F. Lufrano, G. Squadrito, A. Patti, and L. Giorgi, ?Influence of the structure in low-pt loading electrdoes for polymer electrolyte fuel cells?. Electrochim. Acta., 23(24), 3665?3673 (1998).

[3] Qi, Z. and A. Kaufman, ?Low pt loading high performance cathodes for PEM fuel cells?. J Power Sources, 113, 37?43 (2003).

[4] Ralph, T. R., G. A. Hards, J. E. Keating, S. A. Campbell, D. P. Wilkinson, M. Davis, J. St-Pierre, and M. C. Johnson, ?Low cost electrodes for proton exchange membrane fuel cells?. J Electrochem. Soc., 144(11), 3845?3857 (1997).

[5] Srinivasan, S., E. A. Ticianelli, C. R. Derouin, and A. Redondo, ?Advances in solid polymer electrolyte fuel cell technology with low platinum loading electrodes?. J Power Sources, 22, 359?372 (1988).

[6] Srinivasan, S., D. J. Manko, H. Koch, M. A. Enayetullah, and A. J. Appleby, ?Recent advances in solid polymer electrolyte fuel cell technology with low platinum loading electrodes?. J Power Sources, 29, 367?387 (1990).

[7] Ticianelli, E. A., C. R. Derouin, A. Redondo, and S. Srinivasan, ?Methods to advance technology of proton exchange membrane fuel cells?. J Electrochem. Soc., 135, 2209 (1988a).

[8] Ticianelli, E. A., C. R. Derouin, and S. Srinivasan, ?Localization of platinum in low catalyst loading electrodes to to attain high power densities in spe fuel cells?. J Electroanal. Chem., 251, 275?295 (1988b).

[9] Uchida, M., Y. Fukuoka, Y. Sugawara, N. Eda, and A. Ohta, ?Effects of microstructure of carbon support in the catalyst layer on the performance of polymer-electrolyte fuel cells?. J Electrochem. Soc., 143(7), 2245?2252 (1996).

[10] Uchida, M., Y. Fukuoka, Y. Sugawara, H. Ohara, and A. Ohta, ?Improved preparation process for verylow-platinum-loading electrodes for polymer-electrolyte fuel cells?. J Electrochem. Soc., 145(11), 3708?3713 (1998).

[11] Wilson, M. S., J. A. Valerio, and S. Gottesfeld, ?Low platinum loading electrodes for polymer electrolyte fuel cells fabricated using thermoplastic ionomers?. Electrchim. Acta, 40(3), 355?363 (1995).

[12] Song, D., Q.Wang, Z. Liu, T. Navessin, M. Eikerling, and S. Holdcroft, ?Numerical optimization study of the catalyst layer of PEM fuel cell cathode?. J Power Sources, 126, 104?111 (2004).

[13] Song, D., Q.Wang, Z. Liu, M. Eikerling, Z. Xie, T. Navessin, and S. Holdcroft, ?A method for optimizing distributions of nafion and pt in cathode catalyst layers of PEM fuel cells?. Electrochim. Acta, 50, 3347?3358 (2005).

[14] R. Madhusudana Rao, Rengaswamy, R., ?Optimization Study of an Agglomerate Model for Platinum Reduction and Performance in PEM Fuel Cell Cathode?, submitted to Trans IChemE, Part A, Chemical Engineering Research and Design. (2005)