Separation and conversion of CO2 reduction products into high-concentration formic acid using bipolar membrane electrodialysis

Electrochemical CO2 reduction to value-added chemicals such as formate represents a crucial pathway in advancing carbon-neutral fuel production. However, the separation of formate from KHCO3 electrolyte solutions and its conversion to formic acid remain cost intensive steps in this process. This study demonstrated the use of bipolar membrane electrodialysis (BMED) to efficiently convert and concentrate formic acid directly from a cathodic solution for CO2 electroreduction. The migration of formate ions during BMED was optimized under different cell configurations and current densities. Mathematical modelling was developed to predict the concentration evolution curves of formic acid production. The results showed that formic acid diffusion through an anion-exchange membrane was more pronounced than that a bipolar membrane; the former diffusion was a key factor that limited the formic acid concentration and reduced the current efficiency. A formic acid concentration of 3.49 mol/L was obtained at an optimum current density of 40 mA/cm2 and a salt-to-acid compartment volume ratio of 25:1. Thus, BMED has emerged as a viable and effective technique for isolating high-purity formic acid from a cathodic solution for CO2 electroreduction.