Periodic Reporting for period 4 - CASCAT (Catalytic cascade reactions. From fundamentals of nanozymes to applications based on gas-diffusion electrodes)
Reporting period: 2024-03-01 to 2025-05-31
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.
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.
The overall objectives of CASCAT were to gain a deep understanding of electrocatalysis for CO2 reduction using gas diffusion electrodes. This was achieved by searching and characterizing new catalyst materials, developing new technologies for the fabrication of gas diffusion electrodes by airbrush-type spray coating, assembling the catalyst material(s) at the optimal location inside the gas diffusion electrode, and making use of (nano)electrochemical and operando spectroelectrochemical techniques to elucidate properties of the working gas diffusion electrode.
We designed Ag core/porous Cu shell catalyst particles and could demonstrate synergistic cascade reactions during CO2 reduction with the enhanced formation of ethylene, ethanol and propanol. By careful design of gas-diffusion electrodes, we found measures to mitigate the parasitic hydrogen evolution reaction by managing the electrolyte wetting inside the electrode by adjusting the hydrophobicity and porosity. We developed automatic measurement setups with in-line product analysis, nanoelectrochemical methods for characterizing single electrocatalyst nanoparticles, and in-situ electrochemistry/Raman spectroscopy methods to identify intermediate products during CO2 reduction. Most importantly, we developed and applied a nanoelectrode-based method which allows us to measure the local pH value in the gas diffusion electrode in dependence on the applied current density. The implementation into model flow-through electrolyzer systems in combination with in situ selectivity analysis in combination with operando electrochemistry Raman spectroscopy enabled an in-depth understanding of the complex parameter set determining electrocatalysis of gas diffusion electrodes. Furthermore, we worked on the reduction of NOx to ammonia and could demonstrate that a two-step mechanism is required for high ammonia yield. The oxidation of gaseous ammonia using catalyst-modified gas diffusion electrodes was performed demonstrating the possibility that both hydrogen and nitrite as a basis for fertilizer could be generated simultaneously. We extended CO2 reduction at gas diffusion electrodes to potential sources with substantially lower CO2 concentrations and demonstrated that highly selective CO2 reduction to either CO or to formate is possible using highly active and selective catalysts. This brings CO2 reduction closer to removing CO2 from air.
Our research results were disseminated in 57 scientific publications, numerous oral and poster presentations at scientific symposia, several accomplished PhD theses, Master theses, and student practicals. The contribution to CasCat enabled several of the former coworkers to find attractive positions in industry and academia.
We designed Ag core/porous Cu shell catalyst particles and could demonstrate synergistic cascade reactions during CO2 reduction with the enhanced formation of ethylene, ethanol and propanol. By careful design of gas-diffusion electrodes, we found measures to mitigate the parasitic hydrogen evolution reaction by managing the electrolyte wetting inside the electrode by adjusting the hydrophobicity and porosity. We developed automatic measurement setups with in-line product analysis, nanoelectrochemical methods for characterizing single electrocatalyst nanoparticles, and in-situ electrochemistry/Raman spectroscopy methods to identify intermediate products during CO2 reduction. Most importantly, we developed and applied a nanoelectrode-based method which allows us to measure the local pH value in the gas diffusion electrode in dependence on the applied current density. The implementation into model flow-through electrolyzer systems in combination with in situ selectivity analysis in combination with operando electrochemistry Raman spectroscopy enabled an in-depth understanding of the complex parameter set determining electrocatalysis of gas diffusion electrodes. Furthermore, we worked on the reduction of NOx to ammonia and could demonstrate that a two-step mechanism is required for high ammonia yield. The oxidation of gaseous ammonia using catalyst-modified gas diffusion electrodes was performed demonstrating the possibility that both hydrogen and nitrite as a basis for fertilizer could be generated simultaneously. We extended CO2 reduction at gas diffusion electrodes to potential sources with substantially lower CO2 concentrations and demonstrated that highly selective CO2 reduction to either CO or to formate is possible using highly active and selective catalysts. This brings CO2 reduction closer to removing CO2 from air.
Our research results were disseminated in 57 scientific publications, numerous oral and poster presentations at scientific symposia, several accomplished PhD theses, Master theses, and student practicals. The contribution to CasCat enabled several of the former coworkers to find attractive positions in industry and academia.
We developed a comprehensive insight into the functioning of gas diffusion electrodes for the conversion of CO2 and other gaseous reactants using previously not available micro- and nanoelectrochemical tools. This allowed us for the first time to locally detect the pH changes in front of operating gas diffusion electrodes as a basis for understanding mechanistic switches in terms of selectivity but also concerning the competing hydrogen evolution reaction. We discovered novel catalyst materials with high selectivity for CO, or formate, or C-C coupled products which operated at close to industrial current densities. We also extended the application of gas diffusion electrodes to low-concentration CO2 sources, to the oxidation of ammonia under simultaneous formation of hydrogen and fertilizer. The design of new gas diffusion electrodes with known hydrophobicity, decreased flooding as well as the synthesis of complex catalyst materials were performed always considering scalability and potential future applicability. We will continue to contribute to this research topic beyond the ERC funding due to its importance for humankind and our wish to contribute to a scientific basis for ultimately fighting climate change or at least delaying the CO2 emission into the atmosphere.