Regular hexagonal CuBi nanosheets boost highly efficient CO2 reduction to HCOOH in a solid-electrolyte cell

The electrochemical carbon dioxide reduction reaction (CRR) to formic acid has shown great potential in carbon capture, utilization and storage (CCUS). Herein, we synthesized a Cu decorated bismuth nitrate (Cu-BiOON4) composite with regular hexagonal nanosheet morphology. Cu-BiOON4 was in situ reduced to CuBi during the CRR but maintained the hexagonal structure, ensuring the dominant exposure of Bi (012) and Bi (104) planes with high selectivity toward formic acid. The faradaic efficiency of formate reached ∼100% at a low potential of −0.6 V vs. RHE in an H-cell. Fabricated as a membrane electrode, Cu-BiOON4 directly produced ∼0.2 M HCOOH at −80 mA cm−2 in 4 h in a solid electrolyte cell. Further, we found that introducing a small amount of copper into the bismuth precursor can not only control the morphology but also boost the catalytic performance of bismuth by accelerating the electron transfer and reducing the energy barrier of the *OCHO intermediate toward formic acid. This work offered a new insight for improving the catalytic performance of bismuth-based materials for the CRR.