Thesis Defense: Jiachen Li, Dai group

Electroreduction of high-pressure CO2 towards fuels and chemicals

Electrocatalytic CO2 reduction (CO2RR) to useful fuels and chemicals (such as CO, hydrocarbons, alcohols, and carboxylates) is a promising strategy for combating against the climate change caused by excessive emission of CO2(g) into the atmosphere. However, challenging application issues such as product selectivity and system durability remain to be solved, and detailed reaction mechanisms are still unclear. In aqueous solutions that are widely employed as the electrolytes for CO2RR, the solubility of CO2 is as low as ~ 0.03 M at the standard condition, largely limiting the reduction performance. One effective strategy is to apply high CO2(g) partial pressures to the reaction system to increase CO2 availability and facilitate CO2RR by suppressing the undesired hydrogen evolution reaction (HER). This dissertation focuses on CO2RR under high CO2(g) partial pressures. With novel Cu/CuOx electrocatalysts prepared by cyclic square-ware redox treatments, extraordinarily efficient productions of formate (maximum Faradaic efficiency ~ 100%) and acetate (maximum Faradaic efficiency ~ 87%) are achieved in aqueous electrolytes under high CO2(g) pressures up to 58 atm, and further introduction of a thin CO2(l) layer above the electrolyte boosts the direct CO2RR production of potassium acetate to a record yield of 30 mg·h-1·cm-2. By coating the electrodes with an protective alginate layer, the durability of the electrolysis system is significantly prolonged to 20 h.

To elucidate the mechanisms of CO2RR under high pressure conditions, various characterization and speciation techniques are developed. A customized micro-IR spectroscopy is developed for the dynamic measurement of CO2(aq) concentration in electrolyte under high pressures. An electrochemical pH measurement strategy based on Ni and Zn redox reactions is developed for the dynamic electrolyte pH tracking under high pressures. A customized enhanced Raman spectroscopy is developed for the dynamic measurement of HCO3- and CO32- concentrations in electrolyte under high pressures. Combining the CO2RR results with electrolyte speciation, [CO2(aq)] and [CO2(aq)]/[HCO3-] are revealed to be two important and general parameters to determine the CO2RR selectivity among methane, ethylene, ethanol, formate, and acetate on Cu/CuOx catalyst in various electrolyte systems of KOH, KCl, KHCO3, and K2SO4. Further, a customized in-situ Raman spectroscopy is developed for the tracking of catalyst states and screening of involved intermediates during high-pressure CO2RR, and by which the slow reduction of Cu and the emerging of unique O-bound bidentate intermediates at high pressure conditions are observed. Thus, the efficient production of formate and acetate under high pressures is attributed to the dominance of *OC•O* on active Cu(I) sites, whose protonation by HCO3- leads to formate production while C-C coupling leads to acetate production. The synergy of these techniques suggests a new CO2RR pathway to carboxylates under high pressure conditions of high CO2 availability, and indicates the possibility to unify and quantify intercorrelated CO2RR mechanisms by some key parameters.