Effect of Platinum Loading on Catalyst Stability under Cyclic Potentials

Abstract

The effect of platinum loading on catalyst durability was investigated by subjecting two identical 50-cm2 active area cells with 0.15 and 0.4 mgPt.cm-2 in cathode to 30,000 triangle sweep cycles between 0.6 V and 1 V at 50 mV.s-1 scan rate. The gas flow conditions were 80oC, 1 atm, and 100% relative humidity for H2 (fuel) and N2 (oxidant). Both cells lost about 55% of the initial electrochemically active surface area and showed similar H2 crossover rates and decreases in high frequency resistance. Analysis of the back-scan polarization data indicated that the Tafel slope increased by 5-10% over the initial 5,000 cycles, and then more gradually over the subsequent 25,000 cycles. Consistent with the XRD measurements that showed growth in the average size of Pt particles, the estimated exchange current density (mA.cmPt-2) increased by ~10% over the test period for both cells. Both the activation overpotentials for oxygen reduction and the mass transfer overpotentials increased with cycling. The mass transfer overpotentials were higher in the 0.15-mgPt.cm-2 cell and increased faster with cycling than the overpotentials in the higher Pt-loaded cell.