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 potentiometric, resistance, and dilatometric measurements in each of three applied pulse modes, A, B, and C, and repeated H absorption and desorption. Potential, resistance ratio, and an increase in dilation (l/l₀) were measured simultaneously after H equilibrium was attained with the Pd electrode. During continuous absorption, structural phase transition ( [right arrow] ) and void formation occurred, and the values of the H/Pd ratio in the limiting phase, in the + phase coexistence, and in the transition and the +voids coexistence regions are consistent with those obtained from the Pd-H isotherm at 40°C. Hydrogen absorption caused the dilation, from whose slope the molar volume was obtained as 0.64 ( phase) and 0.40 ( + phase) cm³/mol. The resistance increased in proportion to the H/Pd ratio and was kept constant at 1.7 to 1.8 over R_tr.

For the first absorption through the phase (>_min), the electrode potential shifted with an increase in dilation, which suggests nonequilibrium PdH_2-x precipitation followed by conversion to the phase and void formation. Although there was a remarkable lack of any dependence on the number of repetitions of the values of the limiting resistance and potential corresponding to the + and + void coexistence, the onset of the phase, _min, increased as the number of repetitions increased. The volumetric ratio for an increase in the H/Pd ratio corresponds to the absorption in high-density defect areas surrounding voids. During repeated absorption and desorption in the C applied pulse mode, the apparent molar volumes of the + phase coexistence show that absorption proceeds inhomogenously, in contrast to the first absorption in the A applied pulse mode.