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, tind. A large number of measurements (160-200) of tind 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 break deuterium bonds in D2O relative to hydrogen bonds in H2O. We also found that D2 nuclei contain a greater number of molecules (57 to 44) with smaller contact angle (~150o) compared to H2 nuclei (36 to 28 molecules and ~154 o). These findings represent a fundamental discovery in gas phase nucleation and are potentially important for water electrolysis and H2/D2 separations.

Kinetic isotope studies of the electrochemical nucleation of individual H2 and D2 nanobubbles

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