EFFICIENT ELECTROCATALYST ASSEMBLIES FOR PROTON AND OXYGEN REDUCTION - THE ELECTROSYNTHESIS AND CHARACTERIZATION OF POLYPYRROLE FILMS CONTAINING NANODISPERSED PLATINUM PARTICLES

Polypyrrole films containing nanometer-sized platinum particles (ppy/Pt) were electrosynthesized at glassy carbon and gold electrode surfaces. This was done either by voltammetric cycling between +0.95 and -0.80 V (vs. Ag/AgCl/sat. KCl) or via a potential step technique in solutions containing colloidal platinum particles. A chloroplatinate medium with a citrate reducing/protection agent was employed for generating the colloids. An "electrotrapping" mechanism is proposed wherein the (negatively charged) platinum colloidal particles form a three-dimensional array within a growing polypyrrole matrix. The growth of the ppy/Pt films was studied by combined voltammetry-electrochemical quartz crystal microgravimetry. Quantitative analyses of these data afforded estimates of the platinum loading within the ppy matrix and the polymerization efficiency. The former are compared with assays via X-ray photoelectron spectroscopy. The ppy/Pt films exhibited high catalytic activity towards the hydrogen evolution reaction (HER) and O2 reduction. The enhanced activity is attributed to the fine (nanometer-size range) dispersion of the platinum catalyst particles within the ppy matrix. Comparisons with bulk platinum and platinum electrodeposited on the surface of ppy films are provided. Thus, for the HER at a potential of -400 mV (vs. a saturated calomel electrode (SCE)), the ppy/Pt samples show comparable catalytic activity with bulk platinum and an order of magnitude rate enhancement relative to the surface platinum(0)/ppy case. Correspondingly, the rate enhancement does not saturate with increasing film thickness for our ppy/Pt samples. This behavior contrasts with that characteristic of surface-confined catalyst situations. That the platinum particles within the ppy matrix truly are in the catalytically useful nanometer range (less-than-or-equal-to 10 nm) is demonstrated by transmission electron microscopy data.