Polymer electrolyte fuel cells (PEFCs) are suitable for fuel cell vehicles (FCVs) because the PEFCs have high efficiency, high responsibility and high performance, but the critical issues of PEFCs are the cost and the durability of the fuel cell stack. The development of PEFCs requires increased operating temperature, over 100 °C, for high efficiency FCV systems. In addition, the PEFCs of fuel cell buses and trucks need to have higher durability of the fuel cell stack than that of passenger cars. With increasing operating temperature under start-up/shut-down condition, carbon black supports for platinum undergo accelerated corrosion, while niobium-doped tin oxide supports undergo negligible corrosion. We invented a highly durable SnO₂ nanoparticle support with a “fused aggregate network structure” as a new candidate to replace carbon [1-4] and found a novel platinum anti-dissolution mechanism of Pt/Nb-SnO₂ cathode catalyst layers (CLs) during load cycling under an oxygen atmosphere, which was not found in the case of Pt/GCB (graphitized carbon black) CLs. The load cycle durability of Pt/Nb-SnO₂ CLs increased with increasing cathode oxygen concentration, while that of Pt/GCB CLs decreased. When the Pt nanoparticles (NPs) on the Nb-SnO₂ support are oxidized by oxygen in the cathode at the open circuit voltage (OCV), the availability of free electrons in the Pt NPs decrease, and thus the supply of free electrons to the depletion layer on the surface of Nb-SnO₂ becomes difficult. This effect would be particularly evident for the high cathode oxygen concentration [5].

The Pt loading amount for both the Pt/Nb-SnO₂ cathode catalyst and the Pt/CB anode catalyst was 0.1 mg cm^-2. The cell performances were evaluated under H₂/O₂ at 80 °C and 100% RH before and after the durability evaluations. Fig. 1 shows the protocols for the durability evaluation as a function of OCV holding time (2 s, 60 s) using galvanostatic operation under H₂/air conditions, and upper cell potential holding time (2 s, 60 s) using potentiostatic operation under H₂/N₂ conditions. The protocols of Figs. 1(c) and (d) simulate the durability evaluations of Figs. 1(a) and (b). Fig. 2 shows the normalized cell performances at 1.0 A cm^-2 after the durability evaluations under H₂/O₂ conditions at 80 °C and 100% RH. The cell performances were nearly the same before and after the durability evaluations with OCV (upper cell potential) holding time of 2 s during H₂/N₂ and H₂/air operation. In the case of the durability evaluations with OCV (upper cell potential) holding time of 60 s, the cell performance deterioration after the durability evaluations during H₂/air operation was suppressed as compared with that under H₂/N₂ operation. Fig. 3 shows the decrease rate of ohmic resistance during the durability evaluations at lower cell voltage as a function of OCV (upper cell potential) holding time. The decrease rate was the highest of all during the durability evaluations with OCV holding time of 60 s during H₂/air operation. We reported that the degradation of Pt NPs on the Nb-SnO₂ support is suppressed by means of the formation of a depletion layer existing on the surface of the Nb-SnO₂ support with increasing cathode oxygen concentration at the OCV [5]. The OCV holding time in the presence of the oxygen would influence the growth of the depletion layer on the surface of the Nb-SnO₂ support during load cycling. We will discuss in detail the effects of OCV/load holding time, relative humidity, and operating temperature on the load cycle durability of Pt/Nb-SnO₂ cathode catalyst layers.

Acknowledgement

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.

References

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