The Nucleation Rate of Single O₂ Nanobubbles at Pt Nanoelectrodes

Nanobubble nucleation is a problem that affects efficiency in electrocatalytic reactions since those bubbles can block the surface of the catalytic sites. In this article, we focus on the nucleation rate of O₂ nanobubbles resulting from the electrooxidation of H₂O₂ at Pt disk nanoelectrodes. Bubbles form almost instantaneously when a critical peak current, i_nb^p, is applied, but for lower currents, bubble nucleation is a stochastic process in which the nucleation (induction) time, t_ind, dramatically decreases as the applied current approaches i_nb^p, a consequence of the local supersaturation level, ζ, increasing at high currents. Here, by applying different currents below i_nb^p, nanobubbles take some time to nucleate and block the surface of the Pt electrode at which the reaction occurs, providing a means to measure the stochastic t_ind. We study in detail the different conditions in which nanobubbles appear, concluding that the electrode surface needs to be preconditioned to achieve reproducible results. We also measure the activation energy for bubble nucleation, E_a, which varies in the range from (6 to 30) kT, and assuming a spherically cap-shaped nanobubble nucleus, we determine the footprint diameter L = 8-15 nm, the contact angle to the electrode surface θ = 135-155°, and the number of O₂ molecules contained in the nucleus (50 to 900 molecules).