Microbial electrosynthesis (MES) is a process that can be used for the conversion of CO2 into industrially relevant organic products (Prévoteau et al., 2020). This conversion is catalyzed by microorganisms that uptake electrons from a cathode. The Rnf complex possessed by some acetogens allows them to couple oxidation of ferredoxin to the production of NADH and the creation of proton motive force allowing at the same ATP synthesis by the ATPase (Tremblay et al., 2013). They can thus use electrons at a relatively high potential (-500 mV vs. SHE) to fix CO2 and grow autotrophically through the Wood-Ljungdahl pathway. The aim of this study was to evaluate the potential of a redox mediator [Co-(trans-diammac)3+, E’0 <-550mV vs. SHE] to improve MES from CO2. Clostridium ljungdahlii DSM13528T, and Clostridium autoethanogenum DSM10061T, both known to use the Rnf complex were individually tested. The strains were inoculated in the cathodic compartment of a two chambers reactor containing a buffered mineral medium (pH 6). The compartments were separated with a cation exchange membrane. CO2 was used as substrate and was constantly bubbled and recirculated. A three electrode set-up was used to control the cathode potential using a VMP3 potentiostat. Reactors were operated under three different conditions: 1) Working electrode (WE) poised at -756 mV vs. SHE, potential at which electrochemical H2 production occurs; 2) WE poised at -603 mV vs. SHE (no H2 production); and WE poised at -603 mV vs. SHE with [Co-(trans-diammac)3+] = 1 mM. The microbial CO2 reduction was clearly improved by the addition of the redox mediator. The maximum acetate titer and production rate (15 g/l, at 0.39 g/L/d) were achieved with C. ljungdhahlii with mediator, while no acetate production was observed without mediator. For C. autoethanogenum, acetate was produced (≈7 g/l at 0.17 g/L/d) at -756 mV without mediator, however titer and production rate were higher (11 g/l at 0.37 g/L/d) in presence of the mediator. The results showed that the use of Co-(trans-diammac)3+ as a redox mediator allows to obtain high acetate production rates and titers thus demonstrating its efficiency as electron carrier for MES.