Analysis of Cathode Catalyst Layer Structure and Oxygen Transport Resistance Depending on Fabrication Condition in PEFC

   In polymer electrolyte fuel cell (PEFC), the cathode catalyst layer (CL) needs a large amount of Pt because of the slow oxygen reduction reaction. Since electron, proton, and oxygen are necessary for the cathode reaction, achieving the optimum structure of electrode CL and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. In this study, the effects of the cathode CL structure on the IV characteristics were evaluated experimentally using catalyst coated membranes (CCMs) fabricated under several conditions. Pore size distribution of the CL was measured by nitrogen physisorption method and the estimation method of agglomerate size distribution from the measured PSD was developed. From the estimated CL structural parameters, the relationship between the CL structure and the cell performance was elucidated using the analysis with an agglomerate model. In particular, we focused on the agglomerate size and the polymer film thickness, and the effects of the oxygen diffusion and dissolution resistances in the polymer electrolyte (ionomer) on the cell performance are discussed.

   Figure 1 shows the I-V characteristics for different ratios of ionomer to carbon in the CL (I/C ratios). To investigate the effects of ionomer thickness surrounding the carbon agglomerate, the weights of the carbon with Pt were set to be similar; decrease in the I/C ratio corresponds to decrease in the amount of ionomer in the CL. The results with the I/C ratio from 2.0 to 0.75 shows the expected tendency of decrease in the concentration overpotential and improvement of the cell performance with the lower I/C ratio. This is considered because the oxygen diffusion resistance in the ionomer decreases with smaller amount and thinner ionomer film. However, an increase in concentration overpotential and a deterioration of cell performance are observed with the I/C ratio from 0.75 to 0.5.

   To evaluate the factors dominating the concentration overpotential using the model analysis developed by the authors (1), the estimation of the CL structure was conducted. Figure 2 shows the pore size distributions measured by nitrogen physisorption method for different I/C ratios, 0.5 and 0.75. The mode pore diameters of the CL are similar, but the porosities are different: 0.67 and 0.58 for the I/C ratios of 0.5 and 0.75. This is estimated to be because of decrease in the amount of the ionomer filling the pores among the carbon frame of the CL. Here, the thickness of the ionomer is expected to become thinner with the lower I/C ratio, 0.5. From the measured pore size distribution in Fig. 2, the structural parameters of the CL were identified using an estimation model of the agglomerate diameter, developed by the authors (2). The estimated parameters are summarized in Table 1. It is confirmed that the ionomer thickness with the I/C ratio of 0.5 is thinner as expected, and the carbon agglomerate diameters are around 30nm.

   Figure 3 shows the I-V characteristics calculated by the model analysis using the estimated parameters in Table 1. The result can simulate the tendency of the measured IV characteristics, the concentration overpotential and the limiting current density. The calculation reveals the reasons why the performance with the I/C ratio of 0.5 is lower than that of 0.75 in spite of the thinner ionomer thickness. In the case with the I/C ratio of 0.5, the surface area of the ionomer surrounding the carbon agglomerate is smaller, and the oxygen flux into the ionomer at the surface is larger at the same current density, compared with that of 0.75. This increases the effect of oxygen dissolution resistance, resulting in deterioration of the cell performance. The result means that the oxygen dissolution resistance into the ionomer is also a dominant factor to the concentration overpotential as well as the oxygen diffusion resistance in the ionomer. It can be concluded that the proper CL structure with smaller oxygen transport resistance is determined by the balance between the oxygen diffusion and dissolution resistances in the ionomer.

Reference
(1) Y. Tabe, M. Nishino, H. Takamatsu, and T. Chikahisa, J. Electrochem. Soc., 158, B1246 (2011).
(2) S. Akabori, K. Suzuki, Y. Tabe, and T. Chikahisa, ECS Trans., 64(3), 305 (2014).