Electrochemical oxidation of urea on nickel-rhodium nanoparticles/carbon composites

Ni100-xRhx/C composites consisting in bimetallic nanoparticles (10 wt.%) dispersed in high surface area graphite were synthesized and their electrocatalytic performances for urea oxidation tested in alkaline media by cyclic voltammetry using a cavity microelectrode. The composites are synthesized by impregnation of a graphite powder with an aqueous solution of nickel and rhodium salts, followed by drying and reduction under H-2/Ar at 500 degrees C. TEM observations reveal that the Rh fraction influences the average nanoparticle size which varies from 15 nm for pure Ni to 2 nm for pure Rh. XRD analysis indicates that solid solutions are obtained for all compositions. Ni100-xRhx with up to 30 at.% of rhodium have a higher electrocatalytic activity than pure Ni and their voltammograms exhibit two urea oxidation peaks rather than one, the additional peak being at a potential 50 mV less positive than that of pure nickel. When the rhodium content increases above 30 at.%, the current of urea oxidation decreases and becomes almost zero for pure rhodium. These variations in current as the fraction of rhodium increases are only related to the decrease in nanoparticle size and in nickel content (within a surface layer of similar to 1 nm). Hence, there is no synergistic effect between nickel and rhodium for the oxidation of urea in alkaline media. (C) 2018 Elsevier Ltd. All rights reserved.