We have developed the connected platinum-iron (PtFe) nanoparticle catalyst with a porous hollow capsule structure as a carbon-free catalyst-layer material for polymer electrolyte fuel cells (PEFCs). ^[1,2] As shown in Fig. 1A, this catalyst consists of a nanosized beaded network by the fusion of PtFe nanoparticles with a crystallite size of 6~7 nm and an L1₀type chemically-ordered (face centered tetragonal) structure. The beaded metal network is electrically conductive, enabling the removal of carbon supports form a catalyst layer. The carbon-free cathode catalyst layer using a connected PtFe catalyst exhibits high durability against start-stop cycles, because of elimination of carbon corrosion problems. In addition, the specific activity of a connected PtFe catalyst for oxygen-reduction-reaction (ORR) is about 9 times enhanced, compared with that of a commercial Pt-nanoparticle catalyst supported on carbon black (Pt/C). On the other hand, inside a cathode catalyst layer, complex phenomena occur during a fuel-cell operation, such as ORR on catalyst surface, diffusion of oxygen molecules, proton conduction via ionomers, etc. Thus, a high performance of PEFC requires not only high ORR activity and stability but also lower resistance of oxygen mass transport.

In this study, the following structural effects on fuel-cell performances for the carbon-free catalyst layers using connected PtFe catalysts were investigated; (i) inner-space in hollow capsule, and (ii) inter-space between capsules (Fig. 1B). The effect of inner-space in hollow capsule was discussed by the comparison of the connected PtFe catalysts with hollow structure or on solid (silica) support. The inter-space between capsules was controlled by the use of different capsule sizes (capsule size = 300 nm, or 500 nm in diameter). The electrochemical analyses of the prepared catalyst layers revealed that the hollow structure and the enlargement of inter-space between capsules improved the IV performance, especially at the high current density region, implying an improved oxygen mass-transport due to a sufficient space for oxygen diffusion in the catalyst layer and/or effective removal of liquid water from the catalyst layer. Moreover, in this study, the structural effect of gas-diffusion layer on fuel-cell performances was also investigated. It was found that the gas diffusion layer with higher gas permeability would facilitate the removal of liquid water from the capsule catalyst layer, resulting in a lower oxygen-transport resistance. In this presentation, by means of structural and electrochemical analyses of the capsule catalyst layers, the relationship between the catalyst-layer nanostructures and the fuel-cell performances will be discussed in details. The knowledge obtained in this study will provide new insights into the design of a carbon-free catalyst layer to achieve an enhanced PEFC performance.

[1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, Energy Environ. Sci., 8, 3545-3549 (2015).

[2] H. Kuroki, T. Tamaki, and T. Yamaguchi, J. Electrochem. Soc., 163(8), F927-F932 (2016).