Electrosorption of hydrogen into palladium–rhodium alloys

Hydrogen electrosorption into Pd–Rh alloys has been studied in acidic solutions (0.5 M H2SO4) using cyclic voltammetry and chronoamperometry. Pd–Rh electrodes were prepared as thin alloy electrodeposits on Au wires. The influence of electrode potential on the amount of electrosorbed hydrogen was investigated. One can distinguish potential regions of α- and β-phase existence as well as the region of α–β phase transition. For alloys containing less than 20 at.% Rh in the bulk the hydrogen-to-metal ratio, H/(Pd + Rh), is greater than for pure Pd. The maximum value of the H/(Pd + Pt) ratio is 0.80 for an alloy containing in the bulk 94 at.% Pd and 6 at.% Rh. Due to a smaller lattice parameter of the Pd–Rh alloy with respect to pure Pd the α–β phase transition occurs at lower potentials than in Pd. The hysteresis is observed in chronoamperometric absorption and desorption experiments; the effect of hysteresis decreases with the increase in Rh content. Additionally, preliminary results are presented concerning the electrochemical quartz crystal microbalance measurements of hydrogen absorption/desorption into/from Pd–Rh alloys in comparison with Pd.