Potential cycling effects on platinum electrocatalyst surfaces

Abstract

Platinum electrocatalysts with widely divergent initial surface areas were subjected to both triangular and square wave potential cycling. Dissolution rates, surface area changes and surface morphologies were compared for the potential cycling sequences on low surface area Pt sheets, intermediate surface area unsupported Pt blacks and high surface area Pt on graphitized carbon electrocatalysts. On low surface area Pt sheet electrodes, the Pt dissolution rate with slow triangular wave potential cycling through the oxide region amounted to 4.5 μg Pt per real cm2 cycle; the surface area of the Pt increased and the surface morphology, from a comparison of the potentiodynamic adsorbed hydrogen anodic current peak heights and reflection electron diffraction, showed a significant increase in the (111) orientation. With square wave potential cycling, the dissolution rate was lower, the surface area did not increase and the surface morphology changed to an exclusion of the (111) orientation. In contrast to the Pt sheet electrodes, high surface area unsupported Pt crystallites (initially 25 m2/g Pt) and Pt supported on graphitized carbon (initially 93 m2/g Pt) electrodes showed significantly lower apparent dissolution rates with both triangular and square wave potential cycling due to trapping of the soluble Pt complex within the electrode structures, depositing on existing crystallites during the low potential region of the potential cycle. This caused crystallite growth and concomitant surface area losses for the Pt catalysts, with the largest crystallites growing at the expense of the smallest (Ostwald ripening). Even though the surface areas decreased by 70%, the Pt crystallite surface morphologies were unchanged due to rapid equilibration of atoms on the crystallite faces to give the lowest surface energy.