Understanding Bubble-Induced Overpotential Losses in Multiphase Flow Electrochemical Reactors

Green hydrogen production via water electrolysis can play a significant role in decarbonizing energy and multiple industrial processes. In this electrolysis process, water molecules are oxidized to produce oxygen in the anode, while protons are reduced to hydrogen in the cathode. Both of these electrochemical products are gaseous species that lead to bubble nucleation at the surface of electrodes. This bubble evolution phenomena results in substantial energy losses due to the blockage of ion conduction pathways, reduction of the available electrocatalytic area, and disruption of concentration gradients at the electrode-electrolyte interface. In this study, we implement a microfluidic water electrolyzer to elucidate the impacts of electrochemical reaction conditions and convective flows on bubble-induced overpotential losses. We show that high Reynolds (Re) number flows (i.e., Re > 20) mitigate the formation of large bubbles, resulting in minimal bubble-induced overpotential losses. For flows with smaller Re, periodic evolution of large bubbles leads to overpotential fluctuations on the order of ∼100 mV. Furthermore, to understand the impact of bubbles on concentration overpotentials, we use fluorescence microscopy and pH sensitive dyes to capture the spatiotemporal dynamics of pH gradients and correlate the strength and shape of these gradients to the applied potential and convective forces. We find that the presence of large bubbles at low Re can result in more severe concentration gradients that are affected by the hydrodynamic flows around the bubbles. The findings presented in this work provide insights into the effects of convective flows in mitigating bubble-induced overpotential losses.