Kinetics of the deuterium and hydrogen evolution reactions at palladium in alkaline solution

The kinetics of the deuterium evolution reaction (DER) at palladium in alkaline solution were investigated by measuring the overpotential decay transients arising from interruption of the polarising current. It was found that the overpotential nη could be time-resolved into two separate components. The initial component η₁, arises from the Volmer reaction, while the longer-lived component η₂ is attributed to the Tafel reaction. The Volmer-Tafel mechanism was also confirmed by examining the dependence of η₂2 on η, although there were some indications that, at high overpotentials, the Heyrovsky reaction is also involved. Studies of the hydrogen evolution reaction (HER) at palladium were also consistent with a Volmer-Tafel process. The rates of the Volmer and Tafel reactions were found to be higher for the HER than the DER, in agreement with known electrolytic H/D separation factors. Measurements of η₂ were used to estimate the stoichiometry of the deuterated palladium cathode using the concept of equivalent D₂ pressure. The D/Pd loading ratios were found to increase with current density, reaching values of between 0.78 and 0.82 at 50 mA cm^-2.