Engineering Aspects for the Design of a Bicarbonate Zero-Gap Flow Electrolyzer for the Conversion of CO 2 to Formate.

CO 2 electrolyzers require gaseous CO 2 or saturated CO 2 solutions to achieve high energy efficiency (EE) in flow reactors. However, CO 2 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 CO 2 directly from bicarbonate solutions, thus mimicking a CO 2 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 (V Cell ) is high, and the system becomes less efficient compared to analogous CO 2 electrolyzers. Due to the role of the bicarbonate both as a carbon donor and proton donor (in contrast to gas-fed CO 2 electrolyzers), optimization by using know-how from conventional gas-fed CO 2 electrolyzers is not valid. In this study, we have investigated how different engineering aspects, widely studied for upscaling gas-fed CO 2 electrolyzers, influence the performance of bicarbonate zero-gap flow electrolyzers when converting CO 2 to formate. The temperature, flow rate, and concentration of the electrolyte are evaluated in terms of FE, productivity, V Cell , 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 (KHCO 3 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%).