Reduced-Graphene-Oxide-Based Hybrid Supports for Platinum Catalysts Active at Low Loadings during Oxygen Reduction

Abstract

Hybrid systems composed of the reduced-graphene-oxide supported gold, iridium or bimetallic (AuIr) nanoparticles (at loadings typically below 5 µg cm-2) have been considered as active matrices for platinum catalysts utilized at low loadings (<20 µg cm-2) during the reduction of oxygen in mostly acid but also alkaline media. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Gold nanoparticles are prepared both by the electrochemical and chemical reductionapproaches in which the pre-reduced Keggin-type phosphomolybdate heteropolyblue species act as the reducing agent for the HAuCl4 precursor. Polyoxmetallate (PMo12O40 3-) adsorbates or “capping ligands” tend to stabilize gold (iridium or bimetallic) nanoparticle deposits, facilitate their dispersion and attachment to carbon supports. Indeed, it is apparent from the independent diagnostic voltammetric experiments (in 0.5 mol dm-3 H2SO4) that heteropolymolybdates form readily stable adsorbates on nanostructures of gold, irridium and carbon (reduced graphene oxide and Vulcan). It is reasonable to expect that the polyoxometallate-assisted nucleation of gold has occurred in the proximity of oxygenated defects existing on carbon substrates. Under conditions of the electrochemical experiments performed in 0.1 mol dm-3 KOH) the phosphomolybdate adsorbates are removed from the interface because they undergo dissolution and decomposition in alkaline medium. High electrocatalytic activity of the reduced-graphene oxide-supported catalytic systems toward the reduction of oxygen is demonstrated using chronoamperometry and gas-diffusion electrode, in addition to the conventional and rotating ring-disk electrode (RDE) voltammetry. Among important issues is the presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy). When using the reduced graphene oxide carriers for Au (Ir or AuIr) nanostructures together with Pt nanoparticles, the resulting catalytic systems have exhibited typically higher (certainly not lower) O2-reduction currents (relative to those recorded at conventional Vulcan-supported Pt at the same loading) in acid medium (0.5 M H2SO4). The RDE data are consistent with the lower formation of the hydrogen peroxide intermediate (<1% at potentials 0.6 V, or lower, vs RHE). Furthermore, the long-term durability of this family of catalysts should be appreciated.