Factors Influencing the Growth of Pt Nanowires via Chemical Self-Assembly and their Fuel Cell Performance.

Abstract

This work reports a detailed investigation of the template-free synthesis of Pt nanowires via the chemical reduction of Pt salt precursors with formic-acid. The results indicate that both the oxidation state of Pt in the salt and the pH value of the aqueous solution comprising the platinum salt and formic acid are critical factors for the formation of Pt nanowires. Nanowires are obtained from platinum atoms in a +IV oxidation state, with ligating chloride anions (H2 PtCl6 and K2 PtCl6 ) or nonligating chloride anions (PtCl4 ). Increasing the pH of the aqueous Pt salt and HCOOH solution leads to a drastic reduction of the nanowires' length between pH 3 and 4.5. A mechanism involving formate as a reducing agent and formic acid as a structure directing agent explains these results. The Pt nanowires are stable up to 200 °C; therefore, these nanowires are suitable for use as catalysts in proton-exchange-membrane fuel cell. The optimized synthesis conditions are then selected for investigating the kinetics of the oxygen reduction reaction (ORR) of such nanowires in a fuel cell. The ORR mass activity of the Pt nanowires is 130 A g(-1) Pt at 0.9 V iR-free potential; significantly higher than that of two commercial Pt/C catalysts tested in the same conditions. The higher mass activity is explained based on a higher surface specific activity. Accelerated degradation tests indicate that Pt nanowires supported on carbon are as stable as Pt nanoparticles supported on carbon.