Controlled and selective electrochemical CO2 reduction to hydrocarbons and oxygenates utilizing energy from renewables such as solar energy is a promising alternative approach to store energy in chemical bonds while simultaneously close the anthropogenic carbon cycle, thus to address the twin problems of fossil fuels depletion and environmental challenges. Copper-based electrocatalysts have been demonstrated promising performance for CO2 reduction. However, Cu usually converts CO2 into a mixture, where more than 16 different species have been identified, and the selective yield of any product is limited by the competing reactions. Other major bottlenecks of Cu electrochemical catalyzed CO2 reduction reaction include the competition of hydrogen evolution reaction (HER), high overpotentials needed towards desired product, and lack of high-value products. In this dissertation, we addressed these three issues via surface modification, sulfurization, and coupling cathodic/anodic reactions, respectively. Specifically, (1) we developed a benzimidazole (BIMH)-modified copper foil catalyst, where the formed Cu(BIM)x complexes on Cu surfaces can enhance the Faradaic efficiency (FE) of C2/C3 products. The overall FE for CO2 reduction reaches 92.1% and the undesired hydrogen evolution reaction (HER) is lowered to 7% at -1.07 VRHE. (2) We demonstrated that Cu2S nanoarrays enable the selective CO2 reduction to formate starting at a very low overpotential (~ 120 mV), with high current density (over -20 mA/cm2 at -0.89 VRHE), and good Faradaic efficiency (>75%) over a broad potential window (-0.7 VRHE to -0.9 VRHE). Further- more, Cu2S catalysts show excellent durability without deactivation following more than 15 cycles (1h per cycle) of operation. The notable reactivity toward CO2 reduction to formate achieved by Cu2S nanoarrays may be ascribed to their ability to facilitate CO2 activation by stabilizing the CO2•− intermediate more effectively than pristine Cu foil. (3) We reported that direct electrochemical conversion of CO2 to 2-bromoethanol, a valuable pharmaceutical intermediate, is enabled by coupling the anodic and cathodic reactions with the presence of potassium bromide electrolyte in a membraneless electrochemical cell. The maximum Faradaic Efficiency of converting CO2 to 2-bromoethanol that we achieved is 40 % at -1.01 VRHE with its partial current density of -19 mA cm-2.
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