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

Abstract

We use a high-pressure semicontinuous batch electrochemical reactor with a tin-based cathode to demonstrate that it is possible to efficiently convert CO 2 to formic acid (FA) in low-pH (i.e., pH < pK a ) electrolyte solutions. The effects of CO 2 pressure (up to 50 bar), bipolar membranes, and electrolyte (K 2 SO 4 ) 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 2 . To suppress the hydrogen evolution reaction (HER), the electrochemical reduction of CO 2 in aqueous media is typically performed at alkaline conditions. The consequence of this is that products like formic acid, which has a pK a 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 2 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 2 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.

Electrochemical carbon dioxide capture to close the carbon cycle

“…In a CO 2 electrolyser for methanol production, when the produced CO 2 remains dissolved due to an applied pressure, the avoided typical gas regeneration step and CO(g) compression step save more than 150 kJ mol −1 CO 2 [46]. High pressure electrolyser (up to 40 bar) is also reported to increase the current efficiency of CO 2 reduction to formic acid/formate [351,352]. Moreover, a high pressure CO 2 (g) is required for the transportation in the pipes, enhanced oil recovery or geological CO 2 sequestration for underground injection.…”

Direct Water Injection in Catholyte‐Free Zero‐Gap Carbon Dioxide Electrolyzers

“…on the CO 2 reduction reaction (CO2RR) . The current status of the CO2RR is that only small molecules like carbon monoxide (CO), formate (HCOO − ) and ethylene (C 2 H 4 ) can be obtained with high Faradaic efficiencies (FEs) and current densities (CDs). It is also clear that the CO2RR in the liquid phase is intrinsically limited by mass transfer and one has to rely on gas diffusion electrodes (GDEs) to achieve industrially relevant reaction rates (i. e., CDs >100 mA/cm 2 ) …”

Electrochemical conversion of pressurized CO2 at simple silver-based cathodes in undivided cells: study of the effect of pressure and other operative parameters

“…Indeed, according to several authors, the pressure can affect the selectivity between CO 2 and water reduction, the distribution of the CO 2 reduction products and overpotentials need to drive the reaction [37][38][39][40][41][42][43][44][45][46]. In particular, a dramatic effect of pressure on electrochemical conversion of CO 2 to formic acid in aqueous electrolyte at tin cathodes was observed by some authors [42,44,[56][57][58]. Moreover, among the limited number of studies employing high pressure for the CO 2 reduction, Pardal et al [54] have showed that the combination of high CO 2 pressure with ionic liquidbased electrolyte, as CO 2 adsorption media, can significantly improve the productivity of the process by maintaining high selectivity of the process towards CO (CO productivity of about 0.25 mmol cm −2 coupled with a CE CO ~ 97% after a passed charge of about 50 °C at 30 bar, 45 °C and − 0.8 V vs RHE).…”