CO2 electrolyzers require gaseous CO2 or saturated CO2 solutions to achieve high energy efficiency (EE) in flow reactors. However, CO2 capture and delivery to electrolyzers are in most cases responsible for the inefficiency of the technology. Recently, bicarbonate zero-gap flow electrolyzers have proven to convert CO2 directly from bicarbonate solutions, thus mimicking a CO2 capture medium, obtaining high Faradaic efficiency (FE) and partial current density (CD) toward carbon products. However, since bicarbonate electrolyzers use a bipolar membrane (BPM) as a separator, the cell voltage (VCell) is high, and the system becomes less efficient compared to analogous CO2 electrolyzers. Due to the role of the bicarbonate both as a carbon donor and proton donor (in contrast to gas-fed CO2 electrolyzers), optimization by using know-how from conventional gas-fed CO2 electrolyzers is not valid. In this study, we have investigated how different engineering aspects, widely studied for upscaling gas-fed CO2 electrolyzers, influence the performance of bicarbonate zero-gap flow electrolyzers when converting CO2 to formate. The temperature, flow rate, and concentration of the electrolyte are evaluated in terms of FE, productivity, VCell, and EE in a broad range of current densities (10-400 mA cm-2). A CD of 50 mA cm-2, room temperature, high flow rate (5 mL cm-2) of the electrolyte, and high carbon load (KHCO3 3 M) are proposed as potentially optimal parameters to benchmark a design to achieve the highest EE (27% is obtained this way), one of the most important criteria when upscaling and evaluating carbon capture and conversion technologies. On the other hand, at high CD (>300 mA cm-2), low flow rate (0.5 mL cm-2) has the highest interest for downstream processing (>40 g L-1 formate is obtained this way) at the cost of a low EE (<10%).
Keywords: bicarbonate reduction; bipolar membrane; carbon capture and utilization; electrochemical CO2 reduction; zero-gap flow electrolyzer.