Cyclic Voltammetric,Chronoamperometric and Quartz Crystal Microbalance Study on Hydrogen Absorption into and Dissolution of Pd and Pd-noble Metal Alloys

摘要

Hydrogen absorption into and electrochemical dissolution of thin Pd and Pd alloy(Pd-Au,Pd-Rh)layers have been studied in acidic solutions(0.5 M H2SO4)with the use of cyclic voltammetry(CV)and chronoamperometry(CA)coupled with the electrochemical quartz crystal microbalance(EQCM).Currents due to hydrogen adsorption and absorption in a-and P-phase are distinguished on CV curves.The amount of absorbed hydrogen depends on electrode potential and increases with the potential decrease.Hydrogen absorbed in Pd-Au and Pd-Rh alloys is oxidized at a lower potential than in the case of pure Pd.In the case of the Pd-Au system the potential region of phase transition is shifted positively in comparison with Pd indicating higher stability of the P-phase,while for the Pd-Rh system the potential region of phase transition is shifted negatively indicating lower stability of the P-phase.Hydrogen absorption is accompanied by stresses inside Pd crystal lattice,both phenomena affecting the EQCM response.EQCM experiments indicate that the magnitude and distribution of stresses are different for absorption and desorption processes,which seems to confirm the important role of the stress effect in the phenomenon of absorption/desorption hysteresis.Pd dissolves electrochemically during polarization to sufficiently high potentials in a CV experiment.The amount of dissolved metal increases with an increase in electrode potential and a decrease in scan rate.Due to Pd preferential dissolution during potential cycling of Pd-Au alloys both cyclic voltam-mogram and frequency-potential response transform towards curves typical of Au electrode.