The removal of CO2 from the atmosphere or industrial sources and its conversion into valuable products is highly important for mitigating climate change and its impact on life on our planet. This became even more clear during the past 5 years and consequences of the climate change are increasingly visible throughout the world. Since CO2 is a gaseous compound under atmospheric conditions, its direct conversion using gas-diffusion electrodes was found to be an essential asset to develop systems for CO2 conversion and tremendous progress was made. By investigating novel catalysts integrated in gas diffusion electrodes challenges concerning the necessary locally high concentration of CO2 in the three-phase boundary inside the gas diffusion electrode and the impact of mass transfer limitations could be largely mitigated. We understand now the complex interplay of the multiple parameters determining CO2 reduction in gas diffusion electrodes including the selectivity pathways to form value-added chemicals. These parameters include the location of the three-phase boundary between electrolyte, gas phase and catalyst surface, the local change of the pH value in dependence on the reaction rates of the CO2 reduction and the competitive hydrogen evolution, the hydrophobicity of the reaction environment which determines the wetting and the electrowetting. Moreover, the nature of the catalyst determines the product selectivity to either C1 products such as CO, methane, formate, or to C2+ products including ethylene, ethanol, propanol.
We contributed to this increasing knowledge by acquiring in-depth insight into electrocatalytic reactions in gas diffusion electrodes and the influencing complex parameters. For this, we developed new experimental devices to investigate electrocatalyst-modified gas-diffusion electrodes for CO2 reduction but we also generalized the concept for other processes like oxygen reduction, oxygen evolution, ammonia oxidation, NOx reduction. We employed strategies for the discovery of new complex catalyst materials to convert CO2 into value-added chemicals using combinatorial catalyst libraries in combination with high-throughput catalyst screening and the elucidation of the synergy between catalyst components and their structural modulation during operation. This included the development of micro-electrolyzer systems with in situ product analysis allowing us to reach industrial current densities as well as new micro- and nanoelectrochemical tools for gaining understanding about catalyst restructuring at the nanoscale, selective dissolution of catalyst components during operation, as well as mechanistic details governing selectivity and activity.
Overall, we could contribute to the elucidation of the complex processes of electrocatalytic CO2 reduction reaching industrial current densities with specifically designed catalysts with high selectivities for either CO, formate or C2+ products such as ethylene or ethanol. Overall, we strengthened the fundamental knowledge on which application can be build and we expect that this knowledge will contribute to ultimately mitigate climate change by efficient CO2 removal and electrifying chemical industry to avoid CO2 emissions.
