Critical Nuclei Size, Rate, and Activation Energy of H2 Gas Nucleation

Electrochemical measurements of the nucleation rate of individual H₂ bubbles at the surface of Pt nanoelectrodes (radius = 7-41 nm) are used to determine the critical size and geometry of H₂ nuclei leading to stable bubbles. Precise knowledge of the H₂ concentration at the electrode surface, C_H2^surf, is obtained by controlled current reduction of H⁺ in a H₂SO₄ solution. Induction times of single-bubble nucleation events are measured by stepping the current, to control C_H2^surf, while monitoring the voltage. We find that gas nucleation follows a first-order rate process; a bubble spontaneously nucleates after a stochastic time delay, as indicated by a sudden voltage spike that results from impeded transport of H⁺ to the electrode. Hundreds of individual induction times, at different applied currents and using different Pt nanoelectrodes, are used to characterize the kinetics of phase nucleation. The rate of bubble nucleation increases by four orders of magnitude (0.3-2000 s^-1) over a very small relative change in C_H2^surf (0.21-0.26 M, corresponding to a ∼0.025 V increase in driving force). Classical nucleation theory yields thermodynamic radii of curvature for critical nuclei of 4.4 to 5.3 nm, corresponding to internal pressures of 330 to 270 atm, and activation energies for nuclei formation of 14 to 26 kT, respectively. The dependence of nucleation rate on H₂ concentration indicates that nucleation occurs by a heterogeneous mechanism, where the nuclei have a contact angle of ∼150° with the electrode surface and contain between 35 and 55 H₂ molecules.