A mesh independence study was conducted, resulting in the choice of a uniformly distributed mesh with mesh element size of 0.66 × 10 -9 m. A smaller mesh size results in mesh elements smaller than the size of the individual ions. A further decrease in the mesh size resulted in a change in current density, electrolyte potential and concentrations of H + and OTf -of less than 3%. The coupled equations were solved with a commercial solver (COMSOL 5.5) utilizing a MUMPS solver with a non-linear automatic Newton method. The calculation was done for 𝜑 cathode varying between -0.5 V to -1.2 V vs PZC. The solution was converged when a relative tolerance of 0.001 was reached. Further increasing the relative tolerance resulted in a difference in the results of less than 1%. Typically, 10 to 20 iterations were needed for convergence. acidic medium (0.1 M H2SO4 + 0.4 M K2SO4) was used and CO2 was supplied behind the cathode. Green curve: acidic medium was used and N2 was supplied behind the cathode. GC samples were taken after electrolysis at 200 mA/cm 2 for 3 h to ensure the system reached a steady state. The small peaks of CO2 from acidic medium were due to the oxidation of carbon cloth. , the plateau current density was about 6% higher than the limiting diffusion current density. This difference was ascribed to that the steric effect of cations was not considered in PNP model. If the steric effect was considered, the repulsion from K + ions to H + ions near OHP should be stronger, leading to lower concentration of H + near cathode and lower HER current density, closer to the HER current density in our experiment observation. In N2 atmosphere, a plateau of current density about 65 mA/cm 2 was observed, corresponding to the diffusion limitation of hydronium ions under this condition. The partial current density of formic acid could be significantly higher than 65 mA/cm 2 , indicating water molecules served as the proton source for CO2 reduction and OH -ions were generated.