Non-Monotonic CO ₂ Reduction Selectivity under High Pressure: From Concentration-Driven Regime to Charge-Transfer Regulation

Achieving high electrochemical activity together with precise control over product selectivity remains a central challenge in the electrochemical CO2 reduction reaction (CO2RR). Operating under elevated pressure has been recognized as an effective approach to increase CO2 concentration at the electrode interface and enhance CO2RR rates. However, the intrinsic mechanisms by which pressure governs reaction pathways in CO2 electroreduction remain poorly understood. To clearly highlight the pressure effect, we employ high-pressure conditions up to 20 MPa. High-pressure electrochemical experiments combined with density functional theory (DFT) calculations are employed to systematically investigate the pressure-dependent behavior of CO2RR on Au and Sn catalysts. Experimentally, increasing CO2 pressure from 5 to 15 MPa significantly enhances the overall reaction activity due to improved CO2 solubility and driving the system into a concentration-driven regime. At higher pressure (20 MPa), however, Au exhibits a non-monotonic response with a decline in CO selectivity, whereas Sn maintains a continuous enhancement toward formate (HCOO−). Theoretical analysis reveals that high CO2 surface coverage at elevated pressure induces the reduction of the electron gain per CO2 molecule, leading to the enhancement of the C−O bond strength. Because of coverage-dependent changes in the stability and reaction barriers of key intermediates, the *COOH → *CO pathway on Au becomes unfavorable, while the *HCOO → HCOO–/HCOOH pathway on Sn becomes favorable. These results demonstrate that, beyond concentration-driven enhancement effects, coverage-induced electronic effects play a decisive role in regulating CO2RR selectivity under sufficiently high-pressure conditions.