Model and Experimental Analysis of Transport Resistances of Oxygen in Various Structures of PEFC Catalyst Layer

Abstract

In polymer electrolyte fuel cells (PEFCs), the catalyst layer (CL) of the cathode 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 experimentally evaluated using CLs fabricated under several conditions. Furthermore, the oxygen transport resistance in the CL was evaluated. Pore size distribution (PSD) of the CL was measured by nitrogen physisorption method, and the CL structural parameters were estimated using the estimation model developed by the authors (1). Using the model analysis with the estimated parameters, we evaluate various oxygen transport resistances in the CL and discuss the contributions of these resistances to the cell performance. Figure 1 shows the IV characteristics for different ratios of ionomer to carbon (I/C ratios) in the CL. To investigate the effects of ionomer thickness surrounding the carbon agglomerate, the weights of carbon with Pt were set to be similar. Then the Pt loadings of the CLs were 0.16, 0.20, and 0.20 mg/cm2for the I/C ratios, 0.8, 1.0, and 1.2. The oxygen transport resistances distinguished using limiting current density method (2)(3) are summarized in Table 1. It can be considered that the pressure dependent resistance is the oxygen transport resistance in the GDL and the channel, and the pressure independent resistance is the oxygen transport resistance in the CL. It is confirmed that the pressure dependent resistances are similar in all of the I/C ratios because of using the same GDL and channel, and the pressure independent resistance in the I/C=1.2 is the largest. This result shows that the oxygen transport resistance in the CL depends on the structure of the CL. Figure 2 shows the PSDs in the CLs measured by nitrogen physisorption method for the different I/C ratios, 0.8, 1.0, and 1.2. The mode pore diameter becomes smaller as I/C ratio becomes higher, and the porosities were 0.62, 0.55, and 0.42 for the I/C ratios, 0.8, 1.0, and 1.2. From these results, the structural parameters of the CL were estimated using the estimation model developed by the authors (1). The estimated parameters are summarized in Table 2. It is confirmed that the ionomer thickness with the lower I/C ratio is thinner than with the higher I/C ratio, and the carbon agglomerate radiuses are around 15nm in all of the cases. The oxygen transport resistances in the CL were distinguished in further detail using the estimated parameters and the simplified evaluation formula (4), where the effect of the oxygen dissolution resistance was introduced additionally. The oxygen transport resistance in the CL is assumed to consist of three resistances: oxygen dissolution resistance into ionomer, oxygen diffusion resistances in ionomer, and that in pore. We estimated combinations of the oxygen dissolution rate kdiss , the oxygen diffusion coefficient in polymer Dp O2 , and the effective oxygen diffusion coefficient in pore DCL,eff O2 by fitting them to simulate the values of the total oxygen transport resistance in the CL estimated from experiments. Here, we show an example using values, kdiss = 2.35×10-3 m/s and Dp O2 = 1.36×10-9 m2/s. The results are summarized in Table 3. It is confirmed that the oxygen dissolution resistance is the largest with I/C=0.8 because of the smallest surface area of carbon agglomerate with polymer (as shown in Table 2). On the other hand, the oxygen diffusion resistance in pore with I/C=0.8 is the smallest. This is because the resistance in pore depends on the porosity of the CL and decreases with the higher porosity. Figure 3 shows the I-V characteristics calculated by the model analysis using the estimated parameters in Table 2. The result can simulate the tendency of the measured IV characteristics and the limiting current densities for the different I/C ratios well. In this paper, we evaluated the relationships between the oxygen transport resistances and the structure in the CL for only three I/C ratios. We plan to evaluate these relationships using various structures of the CL, and to elucidate the effective structure to achieve small oxygen transport resistance in the CL. Reference (1) S. Akabori, et al., ECS Trans., 64(3), 305 (2014). (2) D.R. Baker, et al., J. Electrochem. Soc., 156, B991 (2009). (3) J.P. Owejan, et al., J. Electrochem. Soc., 160, F824 (2013) (4) Y. Tabe, et al., J. Electrochem. Soc., 158, B1246 (2011). Figure 1