Kinetics of hydrogen outgassing and absorption by palladium in molecular driven and electrolysis regimes

Experimental investigations of the kinetics of hydrogen saturation and outgassing from palladium have been carried out in conditions of interaction with molecular and electrolytic hydrogen at near room temperature and the pressure range of 1—2 at. Using the gravimetric method, it was established that the rate of hydrogen saturation during electrolysis is more than two orders higher than in the molecular driven regime. With help of mass­spectrometric method it has been shown that in the course of hydrogen absorption by Pd cathode during electrolysis and subsequent heating process in high vacuum, the ultra­pure hydrogen (higher than 99.999 vol. %) is generated. Based on the measured temperature dependence of hydrogen evolution, the activation energy of hydrogen outgassing from the Pd­cathode after one­hour exposure to electrolysis ion current ≈8 A was calculated to be E ≈ 23 kJ/mol or ≈12 kJ/mol, in dependence on the temperature range (20—100 °C or 300—650 °C). The average value of activation energy for the whole temperature range of the performed measurements was estimated as ≈22.4 kJ/mol. Therefore, hydrogen outgassing from PdHx system could be multi­stage process with ever­changing activation energy on time. The physical­chemical mechanisms and the influence of β ↔ α transition in the PdHx system are further discussed and analyzed to explain such hydrogen behavior during the outgassing process.