Supporting PtRh alloy nanoparticle catalysts by electrodeposition on carbon paper for the ethanol electrooxidation in acidic medium

Abstract

Pt80Rh20 and Pt60Rh40 alloy catalysts were electrodeposited at constant current density from different electrolytic baths on commercial carbon paper in order to be tested for the ethanol oxidation reaction (EOR) and as anodes in a direct ethanol fuel cell (DEFC). Pt and Rh anodes prepared in the same form were also examined for comparison. As measured by energy-dispersive X-ray microanalyses, the electrodeposited Pt:Rh atomic ratios were the same as those of the precursors in the bath. X-ray diffraction showed the PtRh alloy formation with mean particle sizes of 8.3 and 7.0 nm for Pt80Rh20 and Pt60Rh40, respectively, and a Pt lattice contraction caused by the Rh addition. The X-ray photoelectron spectroscopy analyses suggested a Pt lattice strain due to Rh alloying because the Pt4f binding energies were shifted to higher values with respect to that of pure Pt. The onset potentials of the alloy oxidation, CO stripping and ethanol oxidation in the cyclic and linear sweep voltammograms indicated that Pt60Rh40 was the most active for the CO and the ethanol electrooxidation. The apparent activation energies for the EOR on that alloy were also the lowest one, in agreement with its highest activity. These results were explained by the bifunctional mechanism, assuming that Rh contributed with hydroxylated species to favor the removal of the CO-type adsorbed species on Pt sites, and by the effect of Rh on the Pt electronic structure, the lattice strain being dominating over the charge transfer between Rh and Pt. Tests carried out in single DEFCs showed the feasibility of using the Pt60Rh40 electrodeposited electrodes on carbon as the anode in a real fuel cell environment.