A fast electrochemical quartz crystal microbalance (EQCM) evidences the presence of nanobubbles in alkaline water splitting

Abstract

A fast electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) was used to study the formation of hydrogen bubbles at a cathode during water splitting. Different metal surfaces behaved similarly. The kinetics revealed two time scales. Upon switching the electrode potential to negative, thereby starting gas evolution, the resonance frequency and the half bandwidth increased and decreased, respectively. They approached a new stationary state over a time scale of about 100 seconds. The reverse process was seen after switching the voltage to the OFF state. Superimposed onto this slow evolution were fast fluctuations with a characteristic time of a few seconds. The amplitudes of the fluctuations were larger for the frequency than for the bandwidth by a factor of about 7. The current fluctuated faster than the frequency and there were no cross-correlations between the fluctuations of the current and the fluctuations of the frequency. Both for the fast fluctuations and for the slower drifts, the overtone-normalized frequency shifts, Δf/n with n the overtone order, were similar at n = 3, 5, 7, and 9. Modeling the effects of nanobubbles with the frequency-domain lattice Boltzmann method (FreqD-LBM) revealed a similar behavior. The experiments can be explained with a combination of, first, air pockets inside crevices of a rough surface, which are not under a large capillary pressure, and second, protruding nanobubbles, which are under such a large pressure. Macrobubbles would be expected to lead to Kanazawa–Gordon behavior (−Δf ≈ ΔΓ, both proportional to n^1/2). The fast fluctuations are interpreted as the consequence of nanobubbles with finite lifetime, coexisting with larger bubbles.