Electrochemical CO2 reduction with rationally designed copper-based electrocatalysts is a promising approach to reduce CO2 emission and produce value-added products. Grain boundaries and micronstrains inside catalysts have been proposed as active catalytic sites, while the controlled formation of these sites has remained highly challenging. In this work, we developed a strategy of creating high-density grain boundaries and micron-strains inside CuO electrocatalysts by fast cooling with liquid nitrogen. Compared to samples with slower cooling rates, the fast cooled CuO showed clear difference in their crystal domain sizes, micro-strain densities, and the chemisorption capacities of CO2 and CO. This micro-strain-rich CuO electrocatalyst exhibited a high total current density over 300 mA.cm(-2), and an outstanding Faradaic efficiency for C-2 products (with a majority to ethanol) at similar to 1.0 V vs. reversible hydrogen electrode. Our work suggests a facile approach of tuning grain boundaries and micro-strains inside Cu-based electrocatalysts to scale up electrochemical CO2 reduction for high value-added products. (C) 2020 Elsevier Inc. All rights reserved.