To promote the electrocatalytic transformation from CO

2

to value-added chemicals with boron doped diamond (BDD) electrode

it is critical to make clear that the relationship between the B doping state and the position of B atom in BDD materials and CO

2

reduction performance. Here

a series of BDD electrodes with constant B dopant amount on the surface were prepared by the same process but based on different deposition time (3

6

12

and 24 h) using the heat filament chemical vapor deposition. The results demonstrated that the surface grain size

abundances of B–C relative to B–B bonds of the BDD films increased with increasing the deposition time. Moreover

the formic acid yield and faradaic efficiency also increased as well during electrochemical CO

2

reduction due to more available B atoms doped in crystallinity (B–C bonds) rather than in grain boundary (B–B bonds) of BDD. Finally

electrochemical analysis revealed that the B–C bonds in the crystal of BDD films is the active sites for the reduction of CO

2

. This study provides a simple and convenient path to figure out what is the active site of the

BDDs and its how to impact the CO

2

reduction.