Active sites on copper-based catalysts for electrochemical CO2 reduction selectively to C2+ products

Electrochemical CO2 reduction (eCO2RR) towards value-added chemicals, powered by renewable electricity, is a promising technology for storing the intermittent renewable energy in the form of chemical bonds. Among the various products of eCO2RR, multi-carbon (C2+) molecules, such as ethylene and ethanol, are particularly interesting because they can be either important commodity chemicals or liquid fuels compatible with the current storage and transport facilities. Production of C2+ molecules from eCO2RR still suffers from low selectivity, reactivity, and stability, mainly due to the lack of proper catalysts and optimal reaction conditions. The main objective of this thesis is to develop advanced copper catalysts for and understand the eCO2RR process. First, to reveal the active sties on monometallic copper catalysts for eCO2RR to C2+ products, we used copper phthalocyanine as a precursor, which contains a single copper atom in each molecule, allowing us to observe the evolution of single copper atoms to nanoclusters to nanoparticles. We found that the average size of the copper nanoparticles was correlated to the selectivity of the C2+ products and identified the grain boundaries as the active sites for C2+ products. In addition, the reaction intermediates, such as CO, were found to accelerate the reconstruction of the copper nanoparticles.Next, we investigated the active sites of copper-based bimetallic catalysts. We designed and synthesized Ag@Cu and Ag@C@Cu core-shell nanoparticles to control the interface formation between silver and copper, making it possible to distinguish between the tandem and interface effects. By using this specially designed bimetallic catalysts, we found that the high local concentration of CO, generated by the eCO2RR on silver, suppressed the production of ethylene but improved the selectivity of ethanol compared with that on monometallic copper. These results indicate that the tandem effect in bimetallic catalysts for eCO2RR has the potential to switch the dominant C2+ product from ethylene to ethanol.Afterwards, we developed self-supported copper-based gas diffusion electrodes and achieved a current density up to 300 mA cm-2 with a high selectivity for C2+ products (> 40%). The self-supported gas diffusion electrodes also allowed us to compare the selectivity and activity of eCO2RR directly on the same catalysts in the batch and flow cells. This comparison revealed that the copper catalyst in the batch cell was limited by the mass transfer of CO2. However, the Cu catalyst in the flow cell produced a much higher selectivity of C2+ products at a higher current density due to the fast CO2 diffusion and high pH. The electrode design strategies and the experimental findings presented in this section are valuable for the development of other self-supported electrodes for practical applications.Finally, we found that more than 30% of the liquid products from eCO2RR passed through the gas diffusion electrode and anion exchange membrane (AEM) at commercial current densities from 50 mA cm-2 to 300 mA cm-2. Volatile products pass through the gas diffusion electrode mainly by evaporation, and the migration rate increases linearly with the applied current density and CO2 flow rate. Non-volatile products prefer to cross the AEM through electromigration, and the migration rate is affected by the current density and the flow rate of the catholyte.