Decorating of Graphene-Supported Palladium Nanoparticles with Nanostructured Tungsten Oxide Towards More Efficient Electrocatalytic Oxidation of Formic Acid

By depositing tungsten oxide nanostructures (nanorods) over palladium nanoparticles supported onto graphene nanosheets and dispersed onto glassy carbon substrate, enhancement of the overall electrocatalytic activity towards oxidation of formic acid in acid medium was observed. The effect was particularly evident from the increase of chronoamperometric currents at a fairly low potential of 0.04 V vs. RHE. It is reasonable to expect that introduction of graphene nanosheets to the system facilitates distribution of charge at the electrocatalytic interface.  Under such conditions, tungsten oxide nanorods seem to be partially and reversibly reduced not only to nonstoichiometric hydrogen tungsten(VI,V) oxide bronzes (H_xWO₃, 0<x<1) but also substoichiometric lower tungsten (VI,IV) oxides (WO_3-y, 0<y<1). Mutual interactions between tungsten oxides or graphene and  Pd nanoparticles are likely to affect electrochemical characteristics of all components. The metal-oxide interactions, as well as high population of hydroxyl groups at the electrocatalytic interface (favoring oxidative removal of passivating CO adsorbates), are presumably responsible for the overall enhancement effect. In comparison to conventional electrodeposited microporous tungsten oxides, WO₃ nanorods, despite their small dimensions, are more robust and less hydrated.  Electrochemical diagnostic experiments were supported with microscopic measurements aiming at monitoring morphology of catalytic surfaces with use of transmission (TEM) and scanning (SEM) electron microscopies. The palladium nanoparticles were of the sizes 10 to 20 nm, and the tungsten oxide nanorods had diameters of 50–70 nm while being approximately 5 μm long. Our ultimate goal is to minimize loading of noble metal nanoparticles while keeping the electrocatalytic activity at the high level.