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.