High-Pressure Electrochemical Reduction of CO₂ to Formic Acid/Formate: Effect of pH on the Downstream Separation Process and Economics

Abstract

<p>We use a high-pressure semicontinuous batch electrochemical reactor with a tin-based cathode to demonstrate that it is possible to efficiently convert CO<sub>2</sub> to formic acid (FA) in low-pH (i.e., pH < pK<sub>a</sub>) electrolyte solutions. The effects of CO<sub>2</sub> pressure (up to 50 bar), bipolar membranes, and electrolyte (K<sub>2</sub>SO<sub>4</sub>) concentration on the current density (CD) and the Faraday efficiency (FE) of formic acid were investigated. The highest FE (?80%) of FA was achieved at a pressure of around 50 bar at a cell potential of 3.5 V and a CD of ?30 mA/cm<sup>2</sup>. To suppress the hydrogen evolution reaction (HER), the electrochemical reduction of CO<sub>2</sub> in aqueous media is typically performed at alkaline conditions. The consequence of this is that products like formic acid, which has a pK<sub>a</sub> of 3.75, will almost completely dissociate into the formate form. The pH of the electrolyte solution has a strong influence not only on the electrochemical reduction process of CO<sub>2</sub> but also on the downstream separation of (dilute) acid products like formic acid. The selection of separation processes depends on the dissociation state of the acids. A review of separation technologies for formic acid/formate removal from aqueous dilute streams is provided. By applying common separation heuristics, we have selected liquid-liquid extraction and electrodialysis for formic acid and formate separation, respectively. An economic evaluation of both separation processes shows that the formic acid route is more attractive than the formate one. These results urge for a better design of (1) CO<sub>2</sub> electrocatalysts that can operate at low pH without affecting the selectivity of the desired products and (2) technologies for efficient separation of dilute products from (photo)electrochemical reactors.</p>