Inhibiting salt precipitation on the gas diffusion electrode surface in gas-phase CO2 electroreduction to formate by using an acidic anolyte

Abstract

The gas-phase CO2 electroreduction to formate represents one of the most promising CO2 conversion processes due to its scalability, as the product concentration surpasses 30 % wt. However, the use of alkaline media anolytes, intended to improve the efficiency and selectivity of formate production, causes the carbonate and bicarbonate salts to precipitate over the Gas Diffusion Electrode (GDE). This precipitation clogs the porous structure, leading to a rapid loss of electrode stability. In this work, we address this issue by proposing the use of acid anolytes, based on K2SO4, to mitigate the precipitation of insoluble salt on the GDE structure, thereby achieving longer and more stable GDE operation times. Various anolyte concentrations and pHs are evaluated, with 0.3 M K2SO4 at pH 1, adjusted using H2SO4, providing the best compromise. This condition inhibited potassium carbonate and bicarbonate precipitation, as observed through XRD, SEM, and EDS analysis, while maintaining high CO2 electroreduction to formate performance, with a concentration of 69 g L−1, and a Faradaic Efficiency of 33 %. Furthermore, the anolyte flowrate per geometric area is optimized to maximize the system performance. At a flowrate of 0.85 mL min−1 cm−2, enhanced concentration of 88 g L−1 and a Faradaic Efficiency of 42 % are reached. Besides, long-term experiments demonstrated that GDEs used with alkaline conditions exhibit a larger deactivation constant (0.7652) compared to the GDEs used with acid anolytes (0.3891). This indicates that salt precipitation more rapidly reduces GDE performance under alkaline conditions. These results represent a promising advance in obtaining longer-lasting GDEs, which are crucial to successfully scaling up the CO2 electroreduction to formate.

Keywords

• Gas-phase CO2 electroreduction
• Formate
• GDE stability
• Acid anolyte
• Salt precipitation