Quantitative Insights into Pressure-Dependent Mass Transport and Reaction Kinetics in Electrochemical CO₂ Reduction

Although the electrochemical CO₂ reduction reaction (CO₂RR) plays a crucial role in achieving carbon neutrality, its practical deployment is still limited by insufficient catalytic activity and product selectivity. Elevating pressure has been recognized as an effective strategy to improve CO₂RR performance, yet the underlying mechanisms remain insufficiently understood. Here, we establish a quantitative framework combining distribution of relaxation times analysis and kinetic modeling to elucidate pressure effects on mass-transport and charge-transfer resistances and competitive coverages of *COOH, *CO, and *H. The results demonstrate that the pressure effect is finite. While increasing pressure initially boosts performance (1-10 bar) by enhancing mass transport and increasing *COOH/*CO coverages, this pressure effect decreases between 20-30 bar. This is because the reaction becomes kinetically controlled and intermediate coverage (the sum of *COOH and *CO) reaches saturation, leading to further pressurization being ineffective. Finally, in situ spectroscopic characterization confirms the increased *CO signal intensity under high pressure, supporting the mechanistic conclusions. The quantitative methods carried out in this work offer fundamental insights into pressure-governed mass transport and reaction kinetics.