Abstract
Microbial electrosynthetic cells containing Methylobacterium extorquens were studied for the reduction of CO2 to formate by direct electron injection and redox mediator‐assisted approaches, with CO2 as the sole carbon source. The formation of a biofilm on a carbon felt (CF) electrode was achieved while applying a constant potential of −0.75 V versus Ag/AgCl under CO2‐saturated conditions. 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 a redox mediator was studied by an applied potential of −0.75 V versus 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 CF working electrode was coated with poly(neutral red) by the oxidative electropolymerization thereof. The formation of poly(neutral red) was characterized by spectroscopic techniques. During long‐term electrolysis up to 17 weeks, the formation of formate was observed continuously with an average Faradaic efficiency of 4 %. With the contribution of neutral red, higher formate accumulation was observed. Moreover, the microbial electrosynthetic cell was characterized by means of electrochemical impedance spectroscopy to obtain more information on the CO2 reduction mechanism.
Highlights
Over the past decades, atmospheric carbon dioxide (CO2) concentration has been increasing continuously and it is regarded as a major greenhouse gas.[1,2,3] the reduction of atmospheric CO2 has attracted lots of interest as the carbon capture and utilization (CCU) processes.[4]
We further investigated the long-term performance of the microbial electrosynthetic cell (MEC) of M. extorquens by monitoring the products produced during the biofilm growth and CO2 reduction period
The cathodic compartment consisted of the medium containing 10 % (v/v) of M. extorquens pure culture suspension equipped with a carbon felt (CF) working electrode and a Ag/AgCl (3 m KCl) reference electrode
Summary
3-amino-7-dimethylamino-2-methylphenazine hydrochloride (neutral red), known as a pH indicator and a staining dye, was used as an efficient redox mediator in several bio-electrochemical systems because its redox potential is close to that of the NAD+/NADH redox couple, which is one of the major electron carrier in the microbial electron transport chain.[52,53,54] it is known that neutral red can be polymerized electrochemically on various substrates, especially carbonbased ones, yielding a poly(neutral red) coating on the electrodes.[55] As reported in previous studies, the resulting poly(neutral red) film was electrochemically active and chemically stable under biological conditions, and it has been widely used for bio-electrochemical applications similar to those of the monomer, such as sensors[56] and NAD+/NADH regeneration.[57] From a practical point of view, a system containing a redox mediator coated or directly deposited onto the electrode is preferred, rather than the homogenous approach (a redox mediator dissolved in the electrolyte) This is firstly because product separation becomes easier and secondly because the electron transfer is greatly enhanced.[58,59,60] such a system will have the advantage of long-term operation and lower mediator costs. In the first approach studied, the cathode electrolyte contained soluble neutral red, whereas in the second approach the poly(neutral red) was directly coated onto the electrode
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