Influence of rhodium additive on hydrogen electrosorption in palladium-rich Pd-Rh alloys

Hydrogen electrosorption into Pd-rich (> 80 at.% Pd in the bulk) Pd-Rh alloys has been studied in acidic solutions (0.5 M H2SO4) using cyclic voltammetry and chronoamperometry. The influence of temperature (in the range between 283 and 328 K), electrode potential and alloy bulk composition on hydrogen electrosorption properties of Pd-Rh alloys is presented. It has been found that the additive of Rh to Pd-Rh alloys increases the maximum hydrogen solubility (for Rh bulk content below 10 at.%), decreases the potential of absorbed hydrogen oxidation peak and decreases the potential of the alpha -> aEuro parts per thousand beta phase transition. Increasing temperature decreases the potential of absorbed hydrogen oxidation peak, the maximum hydrogen solubility, and the potential of the alpha -> aEuro parts per thousand beta phase transition. The amounts of electrosorbed hydrogen for alpha- and beta-phase boundaries, i.e., alpha(max) and beta(min), have been determined from the integration of the initial parts of current-time responses in hydrogen absorption and desorption processes. The H/M ratio corresponding to alpha(max) increases with increasing Rh content, while for beta(min) a maximum of H/M ratio is observed for the alloys containing ca. 95% Rh.