Temperature dependence of oxygen reduction activity at Pt/Nb-doped SnO2 catalysts with varied Pt loading

Abstract

We have examined the temperature dependence of the activities for the oxygen reduction reaction (ORR) at Pt catalysts supported on Nb-doped SnO2 (Pt/Nb-SnO2) in 0.1 M HClO4. The apparent rate constants (kapp) for the ORR have been evaluated from 20 to 80 °C. The kapp values increased with increasing temperature, and at each temperature, kapp was also observed to increase with a higher loading amount of Pt on the Nb-SnO2 support (from 17.4 to 34.2 wt %). The Pt particles tended to coalesce and form nanorods with preferential growth of (111) planes when the Pt loading was increased. This is proposed to render the surface geometry favorable for the ORR, as explained by density functional theory (DFT) calculations. The high-weight-percent catalyst also showed higher Pt mass activity, making it possible to reduce the cathode Pt loading while maintaining high fuel cell performance. The apparent activation energy of the ORR was found to be virtually independent of Pt loading on the Nb-SnO2, with values of 15−19 kJ mol−1. The activity enhancement is explained by an increase in the pre-exponential factor of the Arrhenius equation, which could be related to the increased exposure of the (111)-like facets, which provide an optimum platform for O2 adsorption, together with facile desorption of OHad. The durability was assessed in a high-temperature acid electrolyte, similar to that in an actual fuel cell. The test involved potential steps between 0.6 and 1.0 V in O2-saturated 0.1 M HClO4 at 80 °C. The ORR mass activity (@0.9 V) of Pt/Nb-SnO2 (34.2 wt %-Pt) after 3000 cycles showed only a small loss, 13%, and the retained value was 2.6 times higher than that of a commercial Pt/C catalyst, which experienced a much larger loss of 41%.