PtNi supported on oxygen functionalized carbon nanotubes: In depth structural characterization and activity for methanol electrooxidation

Abstract

Mild-condition nitric acid functionalized carbon nanotubes were used as support for PtNi electrocatalysts with variation of Pt to Ni atomic ratio prepared by a polyol method in which ethylene glycol was used as solvent and reducing agent under conventional reflux conditions. TPD, TGA, N2 adsorption and cyclic voltammetry confirmed the presence of functional groups in the functionalized carbon nanotubes. Supported PtNi nanocatalysts were characterized with ICP-OES, TEM, XRD, XPS and XAFS. Well dispersed particles on the supporting material with particle sizes in the range of 2–3nm were obtained. PtNi alloy formation was concluded from XRD, XPS and XAFS results, while the latter two methods point to the formation of Ni oxides as well. The elemental distribution within the catalyst nanoparticles is inhomogeneous with Ni enrichment close to the support. Heat treatment in inert gas up to 400°C results in restructuring of the catalysts surface which changes the active sites arrangements. CV and CO stripping measurements showed that the PtNi catalysts have a higher electrocatalytic activity toward methanol oxidation in comparison to a commercial Pt/C E-TEK catalyst and the highest activity was found for a Pt to Ni atomic ratio of 3. The prepared catalysts show highly stable mass specific activity over 200 potential cycles. The catalysts treated at higher temperature (400°C) show a surface enriched in Pt and exhibit lower activity for methanol oxidation reaction but higher stability over 200 cycles. The high catalytic activity and durability of the prepared PtNi electrocatalysts render them possible candidate catalysts for methanol oxidation in direct methanol fuel cells (DMFCs).