| Summary: | This study investigates the impacts of bismuth and tin on the production of CH<sub>4</sub> and volatile fatty acids in a microbial electrosynthesis cell with a continuous CO<sub>2</sub> supply. First, the impact of several transition metal ions (Ni<sup>2+</sup>, Fe<sup>2+</sup>, Cu<sup>2+</sup>, Sn<sup>2+</sup>, Mn<sup>2+</sup>, MoO<sub>4</sub><sup>2−</sup>, and Bi<sup>3+</sup>) on hydrogenotrophic and acetoclastic methanogenic microbial activity was evaluated in a series of batch bottle tests incubated with anaerobic sludge and a pre-defined concentration of dissolved transition metals. While Cu is considered a promising catalyst for the electrocatalytic conversion of CO<sub>2</sub> to short chain fatty acids such as acetate, its presence as a Cu<sup>2+</sup> ion was demonstrated to significantly inhibit the microbial production of CH<sub>4</sub> and acetate. At the same time, CH<sub>4</sub> production increased in the presence of Bi<sup>3+</sup> (0.1 g L<sup>−1</sup>) and remained unchanged at the same concentration of Sn<sup>2+</sup>. Since Sn is of interest due to its catalytic properties in the electrochemical CO<sub>2</sub> conversion, Bi and Sn were added to the cathode compartment of a laboratory-scale microbial electrosynthesis cell (MESC) to achieve an initial concentration of 0.1 g L<sup>−1</sup>. While an initial increase in CH<sub>4</sub> (and acetate for Sn<sup>2+</sup>) production was observed after the first injection of the metal ions, after the second injection, CH<sub>4</sub> production declined. Acetate accumulation was indicative of the reduced activity of acetoclastic methanogens, likely due to the high partial pressure of H<sub>2</sub>. The modification of a carbon-felt electrode by the electrodeposition of Sn metal on its surface prior to cathode inoculation with anaerobic sludge showed a doubling of CH<sub>4</sub> production in the MESC and a lower concentration of acetate, while the electrodeposition of Bi resulted in a decreased CH<sub>4</sub> production.
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