The volumes of both catholyte and anolyte solutions were 14 mL, and both of them was used as an inner standard to quantify the products. 30.7 μL of DMSO dissolved in 70 mL of deionized water was used as the standard solution (6.17 mmol/L). For each test, 400 μL of the electrolyte for working electrode, 40 μL of DMSO standard solution and 50 μL of deuterium oxide (D2O) were mixed.