Challenges in scale-up of electrochemical CO₂ reduction to formate integrated with product extraction using electrodialysis

BACKGROUND: The concept of carbon dioxide (CO₂) conversion to formate has attracted increasing interest in recent years and various small-scale studies are present in the literature. However, upscaling of electrochemical CO₂ reduction comes with many challenges and there are very few reports available on it. In this study, we present a scalable three-chamber reactor system for electrochemical CO₂ reduction to formate, a precursor suitable for the production of fuels, pharmaceuticals and fertilizers and its extraction as pure formic acid by electrodialysis. RESULTS: The reactor produced 11.7 g L^–1 formic acid in 6 h, i.e. 1.95 g L^–1 h^–1 at −1.8 V applied potential, 5 mol L^–1 KOH as an electrolyte, GDE (gas diffusion electrode) cathode with SnO₂ catalyst and Nafion™ 200 membrane. The maximum Faradaic efficiency achieved was 38%. In addition, recovery of the formate is equally important as its production for use as feedstock to form chemicals. We therefore also investigated the extraction of formic acid through conventional electrodialysis (CED) and bipolar membrane electrodialysis (BMED). The formic acid was extracted with 88% recovery using CED and 46% with BMED. Furthermore, BMED resulted in recovery of >95% K⁺ as base and 12 L pure CO₂ for possible recycling to the electrochemical cell. CONCLUSION: We consider this study to provide essential empirical evidence on factors influencing the scale-up and subsequent performance of a liquid electrolyte-based electrochemical CO₂ reduction reaction (CO₂RR) system to formate and its extraction at scale. However, optimized systems and operating strategies still need further investigation, and constituent materials, particularly in terms of membranes and cathode catalyst, need to be developed.