Effects of Carbon Structures of Supported Pt Catalysts on the Performance of Polymer Electrolyte Fuel Cells
Authors: Park Young-Chul , Tokiwa Haruki , Kakinuma Katsuyoshi , Watanabe Masahiro , Uchida Makoto 
Published: February 2020

At present, the cathode catalysts of PEFCs require an appreciable amount of costly platinum or its alloys, due to the slow kinetics of the oxygen reduction reaction (ORR) in an acidic environment at low operating temperatures [1]. Because of the high cost and limited availability of Pt resources for the large-scale commercialization, it is very important to reduce the amount of Pt by developing highly efficient cathode catalyst layers (CLs). In the previous works [2-3], we demonstrated the impotance of two factors for the design of high performance cathode CLs. The first is the higher location of Pt particles supported on the exterior surface of carbon supports; the interior/exterior location was estimated from the comparison of transmission electron microscopic (TEM) images and secondary electron microscopic (SEM) images while applying a rotatable specimen holder. The second is the optimized distribution of ionomer on the surfaces both of Pt particles and carbon particles, which was characterized by cold field emission-scanning transmission electron microscopic (STEM) images.
Based on the above background, in this study, we investigate the effects of the pore structures of Pt supports, locations of Pt particles and ionomer (Aquivion™ D70-20BS, IEC 1.43 ± 0.04 meq g-1, Solvay-Solexis) on the cell performances of various carbon-supported Pt catalysts, by using TEM (HT7700S, Hitachi High-Technologies) and electrochemical characterization. Two types of commercial 30 wt% Pt-loaded catalysts (c-Pt/CB, TEC10E30E and c-Pt/GCB, TEC10EA30E,Tanaka Kikinzoku Kogyo K. K.) and 30 wt% acetylene black (AB)-supported Pt catalysts, prepared in house by the nanocapsule method [4] (n-Pt/AB800 and n-Pt/AB250, specific surface area of AB800 and AB250: 800 m2 g-1 and 250 m2 g-1, Denki Kagaku Kogyo K. K.), were used as the cathode catalysts.
The effective Pt surface area (S(e)Pt) that can practically contribute to the ORR was directly calculated by the Pt size distribution from the comparison of TEM images and SEM images with the application of the rotatable specimen holder [2]. The S(e)Pt was increased by using AB supports and our nanocapsule method: 87 m2 gPt
-1 for AB800 and 107 m2 gPt
-1 for AB250, all being higher than that of the CB support (32 m2 gPt
-1). This was in good agreement with the order of their cell performance in the high current density regions. In addition, the ionomers of n-Pt/AB800 and n-Pt/AB250 covered more uniformly and continuously over the surfaces both of the Pt particles and carbon particles, than those of the c-Pt/GCB catalyst, for which the Pt particles were predominantly observed on the exterior surface (S(e)Pt = 68 m2 gPt
-1). Such improvement of the effective Pt surface area and ionomer distribution led to a much higher cell performance of the AB-supported Pt catalysts under high current densities. In particular, the n-Pt/AB250 catalyst, for which all of the Pt particles existed only on the exterior surface (see Figure 1), might establish the best-balanced supply path of protons and oxygen in the high current density regions, in which the reactant feed is rate-limiting.
Figure 2 (a) shows the mass power as a function of relative humidity (RH) for the carbon-supported Pt catalysts. The n-Pt/AB800 and n-Pt/AB250 catalysts were able to achieve high performance over a wide range of RH (53-100%): 12.6-15.1 W mgPt
-1 for the n-Pt/AB800 and 13.2-15.4 W mgPt
-1for the n-Pt/AB250. In contrast, the c-Pt/CB catalyst exhibited a lower cell performance, which might be attributed to the lower effective Pt surface area; the Pt particles in the interior make it difficult to access oxygen and protons sufficiently, thereby leading to larger concentration losses due to the limited supply of oxygen and protons in the high current density regions, as illustrated in Figure 2 (b) and (c). These results indicate that the n-Pt/AB250 catalyst is most useful in order to achieve the high current densities required by actual operating conditions, such as those in automotive applications.

Acknowledgment

This work was supported by funds for the “Research on Nanotechnology for High Performance Fuel Cells” (HiPer-FC)project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors are grateful to Denki Kagaku Kogyo K. K. for kindly providing the experimental AB supports.

References

1. R. O’Hayre, S.W. Cha, W. Colella and F.B. Prinz, Fuel Cell Fundamentals, 2nd ed., Wiley, New York , 2009.
2. M. Uchida, Y.C. Park, K. Kakinuma, H. Yano, D.A. Tryk, T. Kamino, H. Uchida, M. Watanabe, Phys. Chem. Chem. Phys., 15, 11236 (2013).
3. Y.C. Park, K. Kakinuma, H. Uchida, M. Watanabe, M. Uchida, J. Power Sources, 275, 384 (2015).
4. H. Yano, T. Akiyama, H. Uchida and M. Watanabe, Energy Environ. Sci., 3, 1511 (2010).

Figure 1