Hydrogen electrosorption into Pd-rich Pd–Ru alloys

Abstract

Hydrogen electrosorption into Pd-rich (>90% Pd in the bulk) Pd–Ru alloys obtained by electrodeposition was studied at 298K in 0.5M H2SO4 solution using cyclic voltammetry and chronoamperometry. Cyclic voltammograms for pure Pd, pure Ru and a Pd–Ru alloy in a wide range of potentials are compared. Pd–Ru electrodeposits with a small Ru content in the bulk (0.5–2%) can absorb more hydrogen than pure Pd, with the maximum hydrogen-to-metal atomic ratio above 0.80 (as compared with 0.74 for Pd). The maximum absorption capacity of Pd–Ru electrodes containing less than 97% Pd gradually decreases and the H/M ratio drops to ca. 0.40 for electrodes with 90% Pd. The potential of absorbed hydrogen oxidation peak is shifted negatively with decreasing Pd bulk content. The amounts of electrosorbed hydrogen for α and β phase boundaries, i.e. αmax and βmin have been determined from the integration of the initial parts of current–time responses in hydrogen absorption and desorption processes. The influence of electrode potential on the amount of electrosorbed hydrogen is presented. On the plot of H/M vs. potential the regions of α↔β phase transitions are shifted to lower potentials in comparison with pure Pd. The thermodynamic stability of β-phase decreases with decreasing Pd content. The hysteresis effect is observed in chronoamperometric absorption and desorption experiment. The values of maximum, normalized time needed for electrode saturation with hydrogen and its removal with the exception of region containing 97.9–99.6% Pd decrease with increasing Ru bulk content. Hydrogen absorption properties of Pd–Ru electrodes may be explained taking into account changes in electronic and geometric factors probably resulting from alloy formation between Pd and Ru.