Meanwhile, the pH of the solution decreased from 13.5 to 8.1 due to the reaction of OH -with CO2 (fig. ). In near neutral and acidic media, the cell voltages were largely stable (fig. and fig. ), as no net reaction between electrolyte and CO2 occurred in these media. The pHs of the solutions remained stable after 4 h of electrolysis (fig. ). The overall cell voltage with the acidic medium was about 0.3 V lower than that with the near neutral medium for 200 mA/cm 2 at 4 h. The potentials vs standard hydrogen electrode (SHE) for the same partial current density of CO on Au/C were within a range of 100 mV at different pH (fig. ). However, the equilibrium potential of CO2/CO vs SHE shifts with 59.2 mV per pH order at 298 K, leading to a higher overpotential in acidic medium than in near neutral and alkaline media (fig. ). Similar situations were found for formic acid and ethylene formation on SnO2/C and Cu/C, respectively (fig. ). In spite of higher overpotential of CO2 reduction, the overall cell voltage with acidic medium was still lower than that with near neutral medium, due to lower solution resistance (fig. ) and lower overpotential for the OER (fig. ).