Engineering the Stable BiO_<i>x</i> Species for Efficient Electroreduction of CO₂ into Formic Acid at Ampere-Level Current

BiO_<i>x</i> species have been identified as the most important active species for the electroreduction of CO₂ to HCOOH over Bi-based materials. However, the BiO_<i>x</i> species are unstable under high reduction current/potential, limiting further industrial application. Herein, we constructed robust BiO_<i>x</i> species by incorporating sodion (Na⁺) into Bi nanosheets (denoted as Na-Bi nanosheets). The negatively charged BiO_<i>x</i> species anchored by the stable Na⁺ in Na-Bi nanosheets displayed highly structural stability during CO₂ electroreduction. When the applied current density (<i>j</i>) was set from -200 to -1200 mA cm^-2, all of the faradaic efficiency (FE) of HCOO^- (FE_HCOO^-) for Na-Bi nanosheets was maintained over 90% in the flow-cell device, whereas the FE_HCOO^- for pure Bi nanosheets was dramatically decreased from 90% to 5%. Mechanistic study further revealed that the Na⁺-anchored BiO_<i>x</i> species can not only alter the potential-limiting step (PLS) but also decrease the energy barrier of the PLS for the electroreduction of CO₂ into HCOOH.