Enhanced bio-electrochemical reduction of carbon dioxide using neutral red as redox mediator.

Microbial electrosynthesis cells containing Methylobacterium extorquens were studied for reduction of CO2 to formate via direct electron injection and redox mediator-assisted approaches with CO2 as the sole carbon source. The study revealed the formation of the biofilm on a carbon felt electrode while applying a constant potential of -0.75 V vs. Ag/AgCl under CO2-saturated condition. During the biofilm growth period, continuous H2 evolution was observed. The long-term performance for CO2 reduction of the biofilm with and without neutral red as redox mediator was studied by an applied potential of -0.75 V vs. Ag/AgCl. The neutral red was introduced into the systems in two different ways: homogeneous (dissolved in solution) and heterogeneous (electropolymerized onto the working electrode). The heterogeneous approach was investigated in the microbial system, for the first time, where the carbon felt working electrode was coated with poly(neutral red) via the oxidative electropolymerization thereof. The formation of poly(neutral red) was characterized by spectroscopic techniques. During the long-term electrolysis up to 17 weeks, the formate formation was observed continuously with the average Faradaic efficiency of 4%. With the contribution of neutral red, higher formate accumulation was observed. Moreover, the microbial electrosynthesis cell was characterized by means of electrochemical impedance spectroscopy in order to obtain more information of the CO2 reduction mechanism.