Paired electrosynthesis of formic acid by CO2 reduction and methanol oxidation. Effect of the anode nature and supporting electrolyte

The paired synthesis of formic acid/formate (FA) by cathodic reduction of CO2 and anodic oxidation of methanol was evaluated using aqueous solutions with different supporting electrolytes (Na2SO4, KHCO3, NaOH) and a Sn cathode combined with various anodic materials (Pt, Cu, Ni(OH)2, Ni NPs, Ti/IrO2–Ta2O5, Ti/RuO2, boron-doped diamond (BDD) and Ti4O7) with the main aims to evaluate what electrolytes are required by the cathodic and anodic processes and the performances of various anodes in different electrolytes. It was found that the cathodic and anodic processes require very different electrolytes: the cathodic reduction of CO2 is strongly affected by the adopted supporting electrolyte, and it is favoured by working in acidic solutions. Conversely, methanol oxidation to FA at the anode is favoured by using NaOH solutions at very high pH. Hence, the best results were achieved using different electrolytes in the anodic and cathodic compartments. Ni-based anodes gave the highest faradaic efficiencies (FEFA = 94-100%) for the FA production in 0.5 M NaOH solutions, especially at low current densities. Just slightly lower FEFA were found under these conditions at BDD anode; moreover, the BDD gave the best results in the other tested electrolytes, although FEFA was significantly lower than in NaOH solutions.