Toward the widespread use of polymer electrolyte fuel cells (PEFCs) in electric vehicles, the improvement of the durability and enhancement of the cathode catalyst activity for the oxygen reduction reaction (ORR) are needed. Pt catalysts loaded on graphitized carbon black (Pt/GCB) are promising candidate catalysts with high durability and activity, but the carbon itself has not been able to completely withstand corrosion at high potential. Our group reported that Pt catalysts loaded on Nb-doped SnO₂ (Pt/Nb-SnO₂), without any carbon additive, showed high electronic conductivity, high ORR activity and high durability (startup/shutdown, load cycling) in membrane electrode assembly (MEA) measurements [1-6].

In the case of a noble metal such as Pt loaded on semiconducting Nb-SnO₂, a Schottky barrier would be constructed at the interface between Pt and Nb-SnO₂, which tends to decrease the electronic conductivity and electrochemical activity. The electronic conductivity of Pt/Nb-SnO₂ with fused-aggregate network structure is enhanced by heat-treatment and high Pt loading (>10 wt%) to equal that of carbon black. XPS spectra for the Pt/Nb-SnO₂ catalyst showed that a slight amount of Sn was detected (Fig. 1). The STEM-EDX elemental line analysis indicated that the Sn diffused into the Pt catalyst. We considered that a PtSn interlayer was inserted in the interface between Pt and Nb-SnO₂, which would mitigate the effect of the Schottky barrier, induce the electron donation from Pt to Nb-SnO₂ and enhance the electronic conductivity. The low cell resistivity of an MEA using our Pt/Nb-SnO₂ cathode, as low as that using a Pt/CB cathode, is ascribed to both the enhancement of the electronic conductivity and the construction of electronic conducting pathways by the fused aggregate network structure.

Nafion^® ionomer film was found to cover uniformly on the hydrophilic surface of the Pt/Nb-SnO₂ (Fig. 2), in contrast to the poor coverage of the ionomer on the hydrophobic surface of the Pt/GCB, based on an evaluation with low acceleration voltage transmission electron microscopy. The thin, uniform coverage of the Nafion^® ionomer on the Pt/Nb-SnO₂ surface helps to construct a membrane electrode assembly (MEA) with low volume ratio of Nafion^® to Nb-SnO₂ (I/S < 0.20), which increases the apparent mass activity (@ 0.80 V) while maintaining a low Tafel slope by mitigation of the oxygen diffusion overpotential in the Nafion^® film with increased porosity in the catalyst layers.

This work was partially 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, and JSPS KAKENHI Grant Number 17H03410 from the Ministry of Education, Culture, Sports, Science and Technology.

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