Porous media plays a significant role in processes like catalysis, membrane filtration, surface coatings or enhanced oil recovery. Particularly in the field of catalysis, electrochemical processes became increasingly important. The development of gas diffusion electrodes (GDEs) enabled efficient electrochemical conversion of gaseous feedstocks such as CO2 or the generation of hydrogen and oxygen gas from water electrolysis. Within the GDE, a solid, a liquid and a gaseous phase are in contact. To enable long-term stability and large-scale commercialization of gas-fed electrolyzers, a fundamental understanding of processes within the pores of the GDE is crucial. Particularly in electrochemical CO2 reduction (ecCO2RR), it is still unclear where exactly in the catalyst layer (CL) the reaction takes place. It is hypothesized, that the CO2 is mainly reduced at the triple-phase boundary (TPB) where catalyst, electrolyte and CO2 meet directly. The location and morphology of the TPB is defined by the wettability of the CL. Thus, the wettability of the CL has a direct influence on the efficiency and selectivity of the process. Therefore, understanding and controlling fluid distribution in the GDE is highly important to optimize the Faraday efficiency (FE) of the ecCO2RR. Abstract model porous media is used to understand wettability on the pore-scale and a realistic model electrolyzer with a silver-based GDE is developed to understand electrowetting and trace reaction locations. Microfluidics is a powerful tool to investigate the ecCO2RR in operando. The measurements showed, that regardless of the driving force for electrolyte imbibition, in all cases a similar steady state is reached. The importance of efficient mass transport at the triple-phase boundary and control of the electrode potential in contrast to control of liquid saturation was shown. Confocal laser scanning microscopy (CLSM) and fluorescence lifetime imaging microscopy (FLIM) were used to measure electrolyte saturation and trace local pH values and species concentrations. Changes in saturation caused by pressure and voltage were recorded using simple fluorescent dyes. Local changes in pH were detected with fluorescence lifetime monitoring. Carbon monoxide was detected with the help of a ligand exchange reaction, facilitating fluorescence of a transition metal complex. Overall, it could be shown that the primary reaction location in ecCO2RR is in fact at the TPB, however, the active area for CO2 reduction is far larger than previously assumed for GDEs. While the wetting state drastically influences the processes within the CL, it is not the only factor to consider. Both hydrogen evolution reaction (HER) and ecCO2RR occurred for all wetting states depending on the other process conditions such as cell potential and CO2 excess. With the results from this thesis, a tailor-made design of GDEs will be possible by an intricate understanding of the wetting and mass transport mechanisms prevalent during operation of the GDE.
Sebastian Brosch (Thu,) studied this question.