Synthesis, characterization and electrocatalytic performance of W surface modified, carbon supported Pt anode catalysts for low-temperature fuel cell applications

Novel, tungsten surface modified Pt catalysts supported on carbon (Pt-W(1 wt%)/C, Pt-W(10 wt%)/C), involving selective surface modification of the active Pt nanoparticles, have been prepared and characterized with respect to their structure, electrochemical properties and their performance as anode fuel cell catalyst, for the oxidation of CO contaminated H2 feed gases. Selective surface modification is achieved by adsorption and thermal decomposition of W(CO)6 on ligand-stabilized Pt nanoparticles and their subsequent deposition on Vulcan XC-72. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements show that the surface modification has little effect on the particle size / particle size distribution and that W deposition leads to islands of tungsten oxides (WOx) on the surface of the Pt nanoparticles rather than to surface alloy formation. This conclusion is supported by electrochemical and mass spectrometric measurements (differential electrochemical mass spectrometry (DEMS)). DEMS measurements on COad pre-covered catalyst electrodes monitoring Faradaic current, CO2 formation and H2 consumption; and quasi-stationary (v = 1 mV⋅s) rotating disk electrode (RDE) measurements in electrolyte saturated with CO/H2 mixtures with different CO concentrations (250 ppm 2% CO) show that compared to unmodified commercial Pt/C catalysts the HOR activity is significantly improved at relevant, low potentials on the largely COad covered, W-modified catalysts, where the CO adlayer either stems from pre-saturation with CO or from reaction in CO containing electrolyte with CO contents <2000 ppm. The high HOR activity under these conditions is explained by a combination of several effects, a high HOR activity on the unmodified Pt surface areas, an improved activity of the WOx deposit for the generation of oxygen species and reaction with COad on neighboring Pt sites, and a low mobility of COad species, leaving COad free Pt areas for the HOR around the deposited WOx islands.