Effect of Added Graphitized Carbon Black on Both Performance and Durability of Pt/Nb-SnO2 Cathodes for PEFCs

The development of cathode catalysts that have both high activity for the oxygen reduction reaction (ORR) and high durability is an important subject for the application of PEFCs in fuel cell vehicles. At present, Pt nanoparticle catalysts supported on carbon black (Pt/CB) are typically being used in PEFC cathodes. While CB supports are essential for the high performance of PEFCs because of their large surface area, high electrical conductivity and well-developed pore structure, they corrode seriously during startup and shutdown of the fuel cell. Graphitized carbon black (GCB), which has a high degree of graphitization, has been used to mitigate such corrosion. According to our previous studies, the high stability of a Pt catalyst supported on GCB (Pt/GCB) was confirmed by use of simulated startup/shutdown evaluation.¹ Electrochemical catalysts for PEFCs are required to have both electrically conductive paths and gas diffusion paths in order to form effective catalyst layers (CL). Therefore well-developed pore structures are needed in addition to high conductivity. We have developed Pt catalysts supported on doped SnO₂ (Pt/Sb-SnO₂, Nb-SnO₂, and Ta-SnO₂) with fused aggregate structures, which are similar to that of CB, constructed by the fusion of nearest-neighbor particles to form a branched structure.^2,3 The Pt/doped SnO₂ catalyst provides high electronic conductivity due to the low grain boundary resistance and also has a large micropore volume. Due to the effect of the doped SnO₂ with fused aggregate structure, the high performance of these cathodes, which was superior to that of Pt/CB, was confirmed by use of single cell measurements.^4,5

In this research,⁶ we report a detailed investigation of a single cell using Pt/Nb-SnO₂ with the fused aggregate structure under actual operating conditions. For instance, we confirmed that the steady-state current-potential (I–E) curves of membrane-electrode assemblies (MEAs) were strongly dependent on the humidity condition of the supplied gases. We also investigated the effects of the addition of GCB into the Pt/Nb-SnO₂cathode for the improvement of the cell performance.

In Fig.1, the single cell performances using Pt/Nb-SnO₂ with/without GCB as function of relative humidity were compared with that of a cell using Pt/GCB at 80^◦C and hydrogen/air under 1 atm. The Pt-mass specific power of the Pt/Nb-SnO₂ cathode under low humidity conditions was superior to that of the Pt/GCB cathode, because the hydrophilic SnO₂ support helped to increase the proton conductivity of the ionomer, which led to high Pt effectiveness. The addition of GCB to the Pt/Nb-SnO₂ cathode improved the cell performance under high humidity, and the Pt-mass specific power value reached more than 10 kW g_Pt⁻¹. The improved performance was attributed to the formation of gas diffusion paths due to the addition of hydrophobic GCB.

Fig. 2 shows that the Pt/Nb-SnO₂ cathodes, with/without GCB, had greater durability than that of the Pt/GCB cathode after 60,000 cycles of potential sweep cycle evaluation (1.0-1.5 V, 0.5 V s^-1). Fig. 3 shows transmission electron microscopic (TEM) images of the Pt/Nb-SnO₂ CL and Pt/GCB CL both before and after durability evaluation. The Pt nanoparticle size on the Pt/Nb-SnO₂ surface increased to 4.9 ± 0.8 nm (after 60000 cycles) from 3.0 ± 0.6 nm (initial state) in diameter during durability evaluation, becoming spherical in shape (Fig. 3(a) and 3(b)). In contrast, it can be observed that the Pt particles on GCB formed elongated clusters, most likely due to the aggregation of spheres, after 60000 cycles in Fig. 3(d). Moreover, we confirmed that the Pt (111) lattice planes were parallel to those of SnO₂ (110), and that the Pt nanoparticles were well oriented on the Nb-SnO₂ surface in the initial state, as shown in Fig. 4. We consider that such interaction between the Pt nanoparticles and the Nb-SnO₂ support could also have suppressed the migration of the Pt nanoparticles during the durability evaluation. We conclude that the Pt/Nb-SnO₂ cathodes, both with and without GCB, exhibited outstanding durability during the startup / shutdown potential sweep evaluation in PEFCs.⁶

Acknowledgement

This research 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, and the JSPS KAKENHI grant Number B24350093.

References

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2. K. Kakinuma et al., Electrochimica Acta, 56, 2881 (2011).

3. Y. Senoo et al., RSC Advances, 4, 32180 (2014).

4. K. Kakinuma et al., Electrochimica Acta, 110, 316 (2013).

5. Y. Senoo et al., Electrochemistry Communications, 51, 37 (2015).

6. Y. Chino et al., Journal of the Electrochemical Society, 162, F736 (2015).