Nanostructured Bi electrocatalysts for CO2 reduction have attracted much attention because of their many advantages such as low cost, good stability, environmentally friendly nature, and ease of synthesis. However, it is critical to further enhance its electrocatalytic performance from the perspective of practical application. Considering that grain boundaries (GBs) possess highly active sites and can tune the selectivity of the products through stabilizing the reaction intermediates, here Bi nanoparticles/Bi2O3 nanosheets (Bi NPs/Bi2O3 NSs) with abundant GBs were synthesized through a controlled hydrothermal reduction route. The as-obtained products showed a considerable partial current density (24.4 mA cm(-2)), high selectivity (similar to 100%), and excellent durability (>24 h) for the electrocatalytic CO2 reduction into formate. Our work showed that microstructures of Bi derived from chemical reduction rather than conventional electrochemical reduction played key roles in the aspect of boosting their electrocatalytic CO2 reduction performance. GBs engineering provided a new strategy for designing highly efficient CO2 reduction electrocatalysts. (C) 2018 Elsevier Ltd. All rights reserved.