Abstract: Kinetic isotope studies are frequently used to elucidate the kinetics and mechanisms of chemical reactions. Very few isotope studies have been performed on nucleation kinetics. Those that have been reported indicate no isotope effect on the rates of the nucleation of ice (from liquid water) or water (from water vapor). To the best of our knowledge, there are no reports of kinetic isotope effects on gas phase nucleation.

In this presentation, we describe a kinetic isotope study of the electrochemical nucleation kinetics of individual H₂ and D₂ nanobubbles. Nucleation rates of H₂ and D₂ nanobubbles were quantified using cumulative probability theory based on galvanostatic measurements of the induction time of nucleation, t_ind. A large number of measurements (160-200) of t_ind were obtained at three different Pt nanodisk electrodes (18-28 nm radius) to obtain precise values of H₂ and D₂ gas nucleation rates. As expected, no isotope effects were observed on thermodynamic properties associated with nucleation, including the supersaturation concentration of dissolved H₂ and D₂ molecules and the thermodynamic radius of bubble nuclei. However, we observe a ~40% increase in the activation energy to nucleate D₂ compared to H₂. We speculate the isotope effect arises from the role that the solvent plays in nucleation, where more energy is required to disrupt D₂O solvent due to the presence of a larger number of deuterium bonds than hydrogen bonds in H₂O and greater energy required to expel heaver D₂O molecules for a bubble to nucleate. We also found that D₂ nuclei contain a greater number of molecules (57 to 44) with smaller contact angles (~150^o)  compared to H2 nuclei (36 to 28 molecules and ~154^o).

We conclude that disruption in solvent structure and expulsion of solvent molecules is the rate-limiting step in electrochemical gas nucleation mechanism. These findings represent a fundamental discovery in gas phase nucleation and are potentially important for water electrolysis and H₂/D₂ separations.