An electrolytic bubble column with an external hollow fiber membrane gas–liquid contactor for effective microbial electrosynthesis of acetate from CO2

Currently, almost all microbial electrosynthesis (MES) reactors were biofilm-driven whether the cathode electron transfer was direct or indirect. It has been proven that cathode biofilm formation is time-consuming, and the current density achieved on these biofilms was relatively low. However, the main challenge for the non-biofilm-driven MES is how to maintain a good Columbic efficiency (CE) at high current densities. Here, we report a novel electro-H2 bubble column reactor with an external hollow fiber membrane gas–liquid contactor to tackle the challenge. The electrolyzer was placed at the bottom of the reactor to produce H2 micro-bubbles (mean diameter 169 ± 7 µm) at a current density of 156 A/m2. The bubble column was put on the top of the cathodic chamber of the electrolyzer to extend the H2 retention time, while the hollow fiber membrane gas–liquid contactor in the external loop was used to recover the unused gas. Consequently, the homoacetogens in reactor could efficiently convert the electrolytically-produced H2 and the externally-supplied CO2 into acetate. The volume of the catholyte (5.5 L) in this reactor is one order of magnitude higher than all reported MES reactors. Batch tests with enriched homoacetogens demonstrated a maximum acetate production rate (1.15 g/Lcatholyte/d, 898 g/m2cathode/d), acetate titer (34.5 g/L) could be achieved in this setup, and the average (maximum) CE of the electron to acetate efficiency was 64% (92%). Besides, dissolved H2 measurement and bacteria community analysis indicated the high acetate titers were likely caused by the H2 oversaturation situation and good substrate/product mass transfer rather than the selection of certain types of homoacetogens. The electro-H2 bubble reactor seems to be a promising MES setup for scale-up and practical application.