Peculiarities of the electrolytic hydrogenation of Pd as revealed by resistivity measurements

In this work, the electrochemical hydrogen doping and withdrawal process of Pd metal was examined with respect to the applicability of Faraday's law. It was found that after a suitable electrolytic pre-treatment, the normalized resistivity (R/R0) versus hydrogen concentration (cF = H/M) curves where cF was determined from Faraday's law by assuming that the 100% current efficiency is independent of the value of the applied charging current density in a wide range. In the α-Pd(H) phase (up to c(αmax) = 0.015H/M), a good quantitative agreement was found with previous resistivity studies by both electrolytic and gas-phase charging. However, for c > c(αmax) our R/R0 versus cF data did not agree with the R/R0 versus cH curve from previously reported electrochemical charging experiments of Pd metal where the doped amount of hydrogen, cH, was determined by direct methods. The two sets of resistivity data can be brought into good agreement by assuming that the current efficiency for the absorption process is ηa = 1.55. Similarly, for the electrolytic desorption process, an agreement with the previously reported R/R0 versus cH curve could be achieved by taking ηd = 1.9. These current efficiency values higher than unity would imply that each hydrogen absorbed/desorbed by electrolytic charge/discharge processes is accompanied by the absorption/desorption of some excess H atoms by processes not requiring electric charge transfer (i.e., only neutral H atoms are involved). At present, we do not have a physical/chemical picture about the nature of such processes.