In situ potentiometric, resistance, and dilatometric: Measurements of palladium electrodes during repeated electrochemical hydrogen absorption

The physicochemical properties of the Pd-H system were studied by in situ p otentiometric, resistance, and dilatometric measurements in each of three a pplied pulse modes, A, B, and C, and repented H absorption and desorption. Potential, resistance ratio, and an increase in dilation (Delta l/l(0)) wer e measured simultaneously after H equilibrium was attained with the Pd elec trode. During continous absorption, structural phase transition (alpha-->be ta) and void formation occurred, and the values of the H/Pd ratio in the li miting a phase, in the alpha + beta phase coexistence, and in the transitio n and the beta + voids coexistence regions are consistent with those obtain ed from the Pd-H isotherm at 40 degrees C. Hydrogen absorption caused the d ilation, from whose slope the molar volume was obtained as 0.64 (alpha phas e) and 0.40 (alpha + beta phase) cm(3)/mol. The resistance increased in pro portion to the H/Pd ratio and was kept constant at 1.7 to 1.8 over R-tr. For the first absorption through the beta phase (>beta(min)), the electrode potential shifted with an increase in dilution, which suggests nonequilibr ium PdH2-x precipitation followed by conversion to the beta phase and void formation. Although there was a remarkable lack of any dependence on the nu mber of repetitions of the I,values of the limiting resistance and potentia l corresponding to the alpha + beta and beta + vain coexistence, the onset of the beta phase, beta(min), increased as the number of repetitions increa sed. The volumetric ratio for an increase in the H/Pd ratio corresponds to the absorption in high-density defect areas surrounding voids. During repea ted absorption and desorption in the C applied pulse mode, the apparent mol al volumes of the alpha + beta phase coexistence show that absorption proce eds in homogenously, in contrast to the first absorption in the A applied p ulse mode.