• Two-dimensional Bi 2 O 2 CO 3 nanosheets synthesized hydrothermally at 90°C and 150°C. • BOC-150 catalyst reached –250 mA cm –2 and >90% Faradaic efficiency for formate. • Flow-cell design enabled stable CO 2 to formate conversion at industrial current densities. • XPS revealed surface reconstruction with stable Bi 3+ species during CO 2 reduction. • NIR spectra showed weakly adsorbed hydroxyls on BOC-150 favoring charge transfer. Electrochemical CO 2 reduction to formate (HCOO - ) represents an efficient strategy to mitigate CO 2 emissions while producing value-added chemicals. However, the reaction typically requires high overpotential and suffers from low selectivity and current density. This work reports the development of Bi 2 O 2 CO 3 (BOC) nanosheets synthesized hydrothermally with urea at 90°C (BOC-90) and 150°C (BOC-150), evaluated in a flow-cell configuration with gas diffusion electrodes. A Bi 2 O 2 CO 3 sample calcined at 400°C (Bi 2 O 3 -400) was also investigated for comparison. Both BOC-90 and BOC-150 outperformed Bi 2 O 3 -400 and a commercial BOC sample in HCOO - production. Notably, BOC-150 achieved a current density of -140 mA cm -2 and a Faradaic efficiency (FE) of 95% for HCOO - at -1.2 V vs. RHE in 0.5 M KOH. Optimizing the electrolyte concentration (1.0 M KOH) and electrode composition (75% BOC-150, 25% carbon black) further enhanced performance, yielding -250 mA cm -2 with FE for HCOO - (FE HCOO- ) above 90%. EIS showed that BOC-150 has the lowest charge-transfer resistance, while ECSA measurements confirmed its highest active surface area, jointly explaining its enhanced ECR performance. NIR and XPS analyses revealed that strongly adsorbed water and Bi dissolution reduced BOC-90 stability, while BOC-150 maintained structural integrity and favorable surface composition. The electrode exhibited stable operation for 6 h, maintaining a Faradaic efficiency above 80% throughout the electrolysis. These findings demonstrate that BOC nanosheets, particularly those obtained under optimized hydrothermal conditions, are highly efficient and durable electrocatalysts for CO 2 reduction to HCOO - under industrially relevant conditions.
Tomas et al. (Sun,) studied this question.