Selective Enrichment Establishes a Stable Performing Community for Microbial Electrosynthesis of Acetate from CO₂

The advent of renewable energy conversion systems exacerbates the existing issue of intermittent excess power. Microbial electrosynthesis can use this power to capture CO₂ and produce multicarbon compounds as a form of energy storage. As catalysts, microbial populations can be used, provided side reactions such as methanogenesis are avoided. Here a simple but effective approach is presented based on enrichment of a robust microbial community via several culture transfers with H₂:CO₂ conditions. This culture produced acetate at a concentration of 1.29 ± 0.15 g L^–1 (maximum up to 1.5 g L^–1; 25 mM) from CO₂ at a fixed current of −5 Am^–2 in fed-batch bioelectrochemical reactors at high N₂:CO₂ flow rates. Continuous supply of reducing equivalents enabled acetate production at a rate of 19 ± 2 gm^–2d^–1 (projected cathode area) in several independent experiments. This is a considerably high rate compared with other unmodified carbon-based cathodes. 58 ± 5% of the electrons was recovered in acetate, whereas 30 ± 10% of the electrons was recovered in H₂ as a secondary product. The bioproduction was most likely H₂ based; however, electrochemical, confocal microscopy, and community analyses of the cathodes suggested the possible involvement of the cathodic biofilm. Together, the enrichment approach and galvanostatic operation enabled instant start-up of the electrosynthesis process and reproducible acetate production profiles