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 [1]; for example, the n-Pt/AB250 catalyst, for which all of the Pt particles exist only on the AB250 exterior surface, was shown to be highly 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; for example, short-side-chain (SSC) perfluorosulfonic acid ionomers with high ion-exchange capacity (IEC) have shown better continuity and uniformity on Pt and graphitized carbon black particles than conventional ionomers, and this has led to the improvement of both the mass transport and the proton-conducting network in CLs [3].
In the first approach for this study, [4] we investigated the effects of high oxygen permeability ionomers and IEC on the cathode performance and durability of PEFCs. The high oxygen permeability ionomers are expected to increase the flux of oxygen near the Pt surface of the three-phase boundary, which is critical in the case of extremely low Pt loadings. The low Pt loading cathode CLs are prepared from an AB250-supported Pt catalyst. From the current-voltage (I-E) curve measurements, the high oxygen permeability ionomers improved the cathode performance without any adverse effects (Fig. 1). During durability testing, the high IEC ionomers lead to increased solubility of Pt and caused severe Pt agglomeration and a large amount of redeposition of Pt particles in the membrane. Based on these results, we propose a strategy of ionomer property selection to improve both cell performance and durability.
In the second approach, [5] we focused on the catalyst material and CL fabrication method and prepared low Pt loading CLs with Pt/Ta-SnO₂ by the electrospray (ES) method. Two CLs were prepared by the ES method with different volume ratios of ionomer binder to support material (I/S), I/S = 0.7 and 0.2, and were compared to that prepared by the pulse-swirl-spray (PSS) method. Both ES CLs had higher porosity than that for the PSS CL, and had improved ionomer coverage and increased electrochemically active surface area (ECA) and mass activity at 0.85 V. In particular, that for the ES with I/S = 0.2 had high porosity and remarkably increased cell performance (Fig. 2). The improvement obtained by use of the ES method can be explained on the basis that the coverage and uniformity of ionomer are increased due to the small droplet size. The performance of the ES cells is high, particularly under high back-pressure conditions, because of the improved transport of both O₂ and protons. ES is an attractive method for the reduction of the Pt loading while improving cell performance.

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 and oxygen permeability results. Also, the authors are grateful to Asahi Kasei Corporation and Denki Kagaku Kogyo Co., Ltd. for kindly providing the experimental ionomer and the experimental AB support, respectively. The authors are also grateful to Tanaka Kikinzoku Kogyo K. K. for kindly preparing the supported Pt catalyst.

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