The role of cation exchange membrane characteristics in CO2 electrolysis to CO using acid anolyte

Abstract

Cation exchange membranes are considered a suitable option for zero-gap CO₂ electrolysis due to their potential to avoid carbonation and improve carbon efficiency. However, the use of acidic anolytes remains an issue due to high hydrogen production. This study investigates Nafion® membranes (111, 112, 115, 211, and 212) with different thicknesses produced by extrusion or solution-cast processes in a zero-gap cell with an acidic anolyte containing K₂SO₄. Faradaic efficiencies for CO production (FE_CO) are higher with thinner membranes, regardless of the manufacturing process, reaching FE_CO around 75 % at 50 mA cm⁻². Additionally, membranes with similar thicknesses (∼50.8 µm) but produced in different ways displayed flow field carbonation after 3 h of electrolysis at 30 °C and 50 mA cm⁻². Linear sweep voltammetry (LSV) in full and half-cell configurations shows limiting diffusion current (i_L) relative to proton transport for all the employed membranes, no matter the thickness. In contrast, the i_L for Nafion® 115, the thicker membrane, is suppressed, indicating that proton depletion is fast and the electrode surface alkalinization primarily results from water reduction in this case. A mechanistic analysis was performed to explain the behavior of the limiting currents in the cell with Ar- and CO₂-feed, indicating that CO₂ reduction aids in the consumption of H⁺ provided by the membrane, increasing the local pH at less negative potentials. Overall, thinner membranes exhibited higher values of FE_CO and energy efficiency for CO (EE^%_CO). However, solution-cast membranes are more prone to provide K⁺, leading to better performance than those prepared by extrusion.