High voltage stability of nanostructured thin film catalysts for PEM fuel cells

This paper provides a comparative evaluation of electrocatalyst surface area stability in PEM fuel cells under accelerated durability testing. The two basic electrocatalyst types are conventional carbon-supported dispersed Pt catalysts (Pt/C), and nanostructured thin film (NSTF) catalysts. Both types of fuel cell electrocatalysts were exposed to continuous cycling between 0.6 and 1.2 V. at various temperatures between 65 and 95 degrees C, with H-2/N-2 on the anode and cathode, while periodic measurements of electrochemical surface area were recorded as a function of the number of cycles. The NSTF electrocatalyst surface areas were observed to be significantly more stable than the Pt/C electrocatalysts. A first order rate kinetic model was applied to the normalized surface area changes as a function of number of cycles and temperature, and two parameters extracted, viz. the minimum stable surface area, S-min, and the activation energy, E-a, for surface area loss in this voltage range. S-min was found to be 10% versus 66%, and E-a 23 kJ mole(-1) versus 52 kJ mole(-1), for Pt/C versus NSTF-Pt, respectively. The loss of surface area in both cases is primarily the result of Pt grain size increases, but the Pt/C XRD grain sizes increase significantly more than the NSTF grain sizes. In addition, substantial peak shifts occur in the Pt/C CVs, which ultimately end up aligning with the NSTF peak positions, which do not change substantially due to the voltage cycling. NSTF catalysts should be more robust against shut down/start-up, operation near OCV and local H-2 Starvation effects. (c) 2006 Elsevier B.V. All rights reserved.