Effects of High Oxygen Permeability Ionomers on the Cathode Performance of Polymer Electrolyte Fuel Cells

Abstract

For the large-scale commercialization of polymer electrolyte fuel cells (PEFCs), it is very important to reduce the amount of Pt by developing highly efficient cathode catalyst layers (CLs) due to the high cost and limited availability of Pt resources. In previous work, we demonstrated the importance of two factors for the design of high performance cathode CLs. The first is higher effective Pt surface area (S(e)Pt), which practically contributes to the oxygen reduction reaction (ORR) [1]; the n-Pt/AB250 catalyst, for which all of the Pt particles exist only on the AB250 exterior surface, is the most attractive in order to generate the large current densities required by actual fuel cell operation [2]. The second is the optimized distribution of ionomer on the surfaces both of Pt particles and carbon particles; short-side-chain (SSC) perfluorosulfonic acid ionomers with high ion-exchange capacity (IEC) showed better continuity and uniformity on the Pt and graphitized carbon black (GCB) particles than the conventional ionomer, which might have led to the improvement of both the mass transport and the proton-conducting network in CLs [3].In this study, we investigate the effects of high oxygen permeability ionomers on the cathode performance of PEFCs. The high oxygen permeability ionomers are expected to increase the flux of oxygen near the Pt surface of the three-phase boundary, in the case of extremely low Pt loadings [4]. The low Pt loading cathode CLs (ca. 0.05 ± 0.003 mg-Pt cm-2) were prepared from the AB250-supported Pt catalyst, with a higher effective Pt surface area (n-Pt/AB250: S(e)Pt = 107 m2 gPt -1, specific surface area of AB250: 219 m2 g-1, Denka Co., Ltd.) and ionomers developed by Asahi Glass Co., Ltd. (AGC); AGC PFSA (IEC = 1.13 meq g-1), AGC sample A (IEC = 1.17 meq g-1, O2 permeability 1.5 times higher than AGC PFSA) and AGC sample B (IEC = 1.50 meq g-1, O2 permeability 1.5 times higher than AGC PFSA). Figure 1 (a) shows the resistance-corrected (IR-free) polarization curves of the high oxygen permeability ionomers for the ORR at 80 oC fed with ambient pressure air humidified at 80% RH. The mass activity (MA) at 0.85 V and the mass power values, calculated from the maximum outputs of the IR-free polarization curves, are plotted as a function of RH in Figure 1 (b) and (c), respectively. The cathode cell performance was significantly increased by using the high oxygen permeability ionomers, i.e., AGC sample A and AGC sample B. However, the ionomers showed different performance behavior at low and high current densities. In the high potential regions (low current densities), in which there is a smaller effect of mass transport, the AGC sample B with the higher IEC of 1.50 exhibited higher cell performance than the other ionomers, as can be confirmed in the MA results in Figure 1 (b). In contrast, the AGC sample A with the lower IEC of 1.17 led to a significant improvement of the cathode performance in the high current density region, which led to the achievement of very high performance, greater than 22 W mgPt -1at 80-100% RH, as presented in the mass power results in Figure 1 (c). These results show that the improved oxygen permeability of the ionomers could result in highly efficient cathode CLs for PEFCs. Acknowledgment This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cell” (SPer-FC) project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors are grateful to Asahi Glass Co., LTD. for kindly providing the experimental ionomers. Also, the authors are grateful to Denka Co., Ltd. for kindly providing the experimental AB supports.