Homoacetogenesis was performed in a microbial electrosynthesis single-chamber reactor at open and closed circuits modes. The aim is to investigate how an applied reducing power affects acetic acid synthesis and H2 gas–liquid mass transfer. At a cathode voltage of −175 mV vs. Ag/AgCl (3.0 NaCl), the acetic acid synthesis rate ramped up to 0.225 mmol L-1h−1 due to additional electrons and protons liberation from carbon-free sources such as water and ammonium via anodic oxidation. The study sets a new lowest benchmark that acetic acid can be bioelectrochemical synthesized at − 175 mV. The applied reducing power did not increase the H2 gas–liquid mass transfer because the direct electron transfer from cathode to microorganisms reduced the demand for H2 in the fermentation medium. Microbial analysis shows a high presence of Veillonellaceae spore-forming clostridia, which are identified as homoacetogens.