Spatio-Temporal Electrowetting and Reaction Monitoring in Microfluidic Gas Diffusion Electrode Elucidates Mass Transport Limitations.

The use of gas diffusion electrodes (GDEs) enables efficient electrochemical CO 2 reduction and may be a viable technology in CO 2 utilization after carbon capture. Understanding the spatio-temporal phenomena at the triple-phase boundary formed inside GDEs remains a challenge; yet it is critical to design and optimize industrial electrodes for gas-fed electrolyzers. Thus far, transport and reaction phenomena are not yet fully understood at the microscale, among other factors, due to a lack of experimental analysis methods for porous electrodes under operating conditions. In this work, a realistic microfluidic GDE surrogate is presented. Combined with fluorescence lifetime imaging microscopy (FLIM), the methodology allows monitoring of wetting and local pH, representing the dynamic (in)stability of the triple phase boundary in operando. Upon charging the electrode, immediate wetting leads to an initial flooding of the catalyst layer, followed by spatially oscillating pH changes. The micromodel presented gives an experimental insight into transport phenomena within porous electrodes, which is so far difficult to achieve. The methodology and proof of the spatio-temporal pH and wetting oscillations open new opportunities to further comprehend the relationship between gas diffusion electrode properties and electrical currents originating at a given surface potential.