Toward the widespread application of polymer electrolyte fuel cells (PEFCs) in electric vehicles, the enhancement of the cathode catalyst activity for the oxygen reduction reaction (ORR) and the improvement of the durability are needed. Pt catalysts loaded on graphitized carbon black (Pt/GCB) have been confirmed to moderate degradation during startup and shutdown, compared with that of Pt/CB, but these carbons have not been able to completely overcome corrosion at high potential. Electronically conducting oxides, without the use of carbon, such as the Magneli phase titanium oxides and tin dioxide, are promising candidate supports [1,2]. Our group also has reported that Pt catalysts loaded on Nb-doped SnO₂ (Pt/Nb-SnO₂) and Ta-doped SnO₂ (Pt/Ta-SnO₂), without any carbon additive, showed high durability during startup/shutdown, while maintaining high ORR activity, by means of rotating disk electrode and membrane electrode assembly (MEA) measurements [3-9]. The high ORR activity of Pt/Nb-SnO₂ and Pt/Ta-SnO₂ is dependent on a strong interaction between the Pt catalyst and the tin oxide. 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 a unique microstructure involving a fused aggregate network structure, which constructs effective gas diffusion pathways, electronically conducting pathways and high surface area in the catalyst layer (CL). For the further improvement of the cell performance, the optimization of the volume and dispersion of the ionomer in the CL is important. In this research, we have evaluated the cell performance and the load cycle durability of single cells using a Pt/Nb-SnO₂ CL with a range of volume ratios of Nafion^® ionomer and the support material (I/S).

The current-voltage performance and resistivity of the cell using Pt/Nb-SnO₂ CL (I/S = 0.24) were equal to those using Pt/GCB CL (I/S = 0.67, optimized I/S ratio, Fig. 1).The apparent mass activity at 0.80 V (MA_app@0.80 V) of the cell using Pt/Nb-SnO₂ CL improved with decreasing I/S, and, at I/S = 0.12, approached a value a factor of 2 higher than that using a commercial Pt/GCB CL with an optimized I/S ratio (Fig. 2). The current density at 0.60 V of the cell using Pt/Nb-SnO₂ (I/S = 0.12) CL approached the same value of the cell using Pt/GCB CL. The electrochemically active surface area (ECA) of the Pt/Nb-SnO₂ CL was twice as large at the initial stage than that of Pt/GCB CL and continued to maintain higher values during a load cycle durability test (0.6 - 1.0 V, 3 s, Fig. 3). We consider that the utilization of Pt and the load-cycle durability of Pt/Nb-SnO₂ are higher than those of Pt/GCB. The Nafion^® ionomer covered uniformly on the hydrophilic surface of the Pt/Nb-SnO₂ (Fig. 4), 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. We suggest that the thin, uniform coverage of the Nafion^® ionomer on the Pt/Nb-SnO₂ surface, due to the appropriate low level of ionomer, moderates both the oxygen diffusion overpotential in the Nafion^® ionomer and the Pt catalyst degradation.

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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