13c formaldehyde

The ¹³C-formaldehyde (¹³CH₂O, 20 wt% in H₂O, 99 atom% ¹³C) is purchased from Sigma-Aldrich. Due to the scarcity of ¹³C-labeled formaldehdye, it is only available in 1 mL package with a concentration of 20 wt%. The flow-type experiment in this study requires >20 mL electrolyte at a time. Thus, the isotope labeling experiment was carried out in a batch cell with a fixed volume of 5 mL. The electrolyte contained 4 wt% ¹³CH₂O in water diluted from 1 mL 20 wt% ¹³C-formaldehyde. 1 M sodium acetate was added as supporting electrolyte and 5 mL of 1 M sodium acetate aqueous solution was used as an anolyte. Vulcan carbon on porous carbon paper with an active area of 1 cm² is used as the electrode for the cathode and IrO₂ on Ti felt is used as the anode. A constant current of 10 mA cm⁻² is applied for 10 h. After the electrolysis, a rotary evaporator with a dry ice condenser was used to evaporate all the water from the resulting electrolyte. The water bath temperature is set to 40 °C, and the liquid sample is rotavaped for 15 min to remove all the water. 2 mL acetonitrile was added to extract ethylene glycol. The as-prepared liquid sample is analyzed by GC-MS (Agilent 59771 A) focusing on the mass fragment patterns of resulting ethylene glycol.