Oxalic acid used to be obtained via the oxidation of carbohydrates using nitric acid and catalysts. This process produces a variety of nitrogen oxides during oxidation and requires a separation process due to various intermediates. It results in increasing the harmfulness and complexity of the process. Recently, the electrochemical reduction of carbon dioxide to obtain oxalic acid was suggested as an environmentally friendly and efficient technology for oxalic acid production. In this electrochemical conversion system, zinc oxalate (ZnC2O4) was obtained by the reaction of Zn2+ ions produced by Zn oxidation and oxalate ion from CO2 reduction. ZnC2O4 can be converted again to oxalic acid with strong acid and heat. In this study, we proposed a system that can easily convert ZnC2O4 to oxalic acid without using strong acid and can separate easily. In addition, it is also further converted to glycolic acid which is a high-value-added chemical. ZnC2O4 was effectively separated into Zn(OH)2 powder and oxalate solution through a chemical treatment and vacuum filtration process. And Zn(OH)2 and oxalate were electrochemically converted to zinc and glycolic acid, respectively.
In addition, this study also conducted research on the development of electrodes for selective electrochemical conversion from oxalic acid to glycolic acid. The electrochemical reduction of oxalic acid was performed at TiO2 nanotubes (TNT) electrode in an aqueous medium under potentiostatic control in a two-compartment cell. The competing H2 evolution was almost non-existent at an applied potential of -1.0 V vs. Ag/AgCl. Thus, conversion of oxalic acid was achieved with 46.8 % and faradaic (98 %) yield for 3 hours. The selectivity of glycolic acid (GC) formation over that of glyoxylic acid (GO) is controlled by the length of the TNTs. A high selectivity was obtained with H2O2-TNTs (GC/GO ≈ 10). The physical properties of the TNTs, such as length, uniformity, and mechanical strength, were controlled by varying anodization time and the electrolyte composition. In addition, the modification of the TNTs was attempted using surface treatments, metal doping, and so on. With zinc-doped TNT electrode, higher conversion of oxalic acid was achieved with 55 % and Faradaic (95.8 %) yield for 3 hours.