Imaging local pH in boundary layers at 3D electrodes in electrochemical flow systems

Electrochemical conversion reactors, such as water electrolyzers, CO2 electrolyzers, fuel cells, and flow batteries, will be essential in electrifying industry in the ongoing energy transition. These technologies require intensification to become economically viable. Hydrodynamics and mass transfer at electrode-electrolyte interfaces significantly affect electrochemical conversion reactions by influencing the reactant availability and pH in the local reaction environment. 3D electrodes, such as flow-through foams and suspension electrodes, hold a great advantage over 2D electrodes as they moderate pH changes and reactant depletion by spreading the current over a larger electrode area and electrolyte volume. We study the diffusion boundary layer in operando around a single mm-sized particle, representing an element of a 3D electrode. We visualize the local pH with Fluorescence Lifetime Imaging Microscopy (FLIM) during H2O reduction at various current densities and electrolyte flow velocities. FLIM enables us to map the transient pH profile at a resolution down to 9 μm and 2 Hz. In addition, we apply an intermittent current to investigate how long the capacitive electric double layer of a suspension electrode particle can maintain an ongoing electrochemical reaction during their time of non-contact with a current collector in applications that require Faradaic charge transfer (i.e. flow batteries, microbial fuel cells, CO2 electrolyzers). We demonstrate that the diffusion boundary layer and pH gradients are not symmetrical, but depend on the direction of the electric field, the current density and the flow conditions. The substantial pH gradients and boundary layer formation at the scale of hundreds of micrometers underline the importance of controlling flow in or around electrodes, making 3D electrodes an important asset for creating suitable reaction conditions in mass transport-limited electrochemical conversions.