The increasing atmospheric carbon dioxide (CO2) level is calling for more efficient CO2 fixation systems to re-balance the carbon cycle. At the same time, CO2 can be a cheap and abundant carbon source for synthesizing various valuable fuels. The renewable-electricity- powered CO2 reduction reaction (CO2RR) offers a means to synthesize various valuable fuels and store intermittent electricity as stable chemical forms. To date, Copper (Cu) remains the most effective electrocatalyst for CO2RR in producing hydrocarbons and oxygenates. However, Cu-based catalysts usually suffer from insufficient product selectivity. Generating mixed products causes a high economic penalty in post-reaction separation, largely limiting the broad implementation of electrochemical CO2RR. A lack of understanding of the complicated reaction mechanism further hindered the catalyst development. Thus, I have focused on Cu-based electrocatalytic CO2RR catalysts with high efficiency through surface defects and co-catalyst engineering. Three types of Cu-based catalysts have been designed and investigated; Cu foil with rich twin boundaries (tw-Cu), Cu-Ag NWs with the atomically intimate interface, and trimetallic CuPdAg plates, which successfully improved CO2RR performance. Electrochemical and advanced spectroscopies coupling with computational simulation provided mechanistic understandings of the influence of defect engineering on CO2RR selectivity.Overall, these researches demonstrate that tuning surface defect structure and atomic arrangement of Cu-based co-catalysts are effective strategies in improving the activity and selectivity of electrocatalytic CO2RR, thus suggesting a path towards rational designs of Cu-based catalysts for tunable CO2RR.