Simultaneous Electricity Generation and Microbially-Assisted Electrosynthesis in MFCs
Authors: Gajda Iwona , Greenman John , Melhuish Chris , Ieropolous Ioannis 
Published: February 2020

Microbial Fuel Cells (MFCs) are bio-electrochemical transducers that generate electricity as a direct result of microbial metabolism, when breaking down organic matter for continuous growth and maintenance. On the other hand, Microbial Electrolysis Cells (MECs) consume electricity to drive chemical reactions and recover hydrogen or other high value chemicals, at the cathode half-cell [1]. In MFCs, electric current is generated when for every electron donated to the electrode surface, a proton is transferred from the anode to the cathode. Other cations such as Na+ or K+ may be present in significantly higher concentrations and are more likely to be transferred through a cation exchange membrane [2]. This normally results in electro-osmotically dragged water from the anode to the cathode, and often results in a phenomenon known as cathode flooding, which is a problem for MFCs and other chemical fuel cells.
To this day, bio-electrosynthesis has not been reported for energy-generating MFCs, since it is associated with energy-consuming MECs. The main aim of this work was therefore to investigate the effects on MFC performance of low-cost catalyst-free electrode materials, in conjunction with cation and water transport to the cathode half-cell, in the context of beneficial water accumulation and recovery of valuable resources.

Materials and Methods

Twelve dual–chamber MFCs were tested in triplicate groups assembled with carbon veil anode electrodes. Half-cell chambers were 25mL each and activated sewage sludge (Wessex Water) was used as the inoculum. The cathode chambers contained carbon based electrodes mechanically pressed against the CEM separator. The tested cathode electrodes included: Microporous Layer on carbon cloth (MPL), carbon fibre veil (CV), MPL on carbon fibre veil (CV MPL) and activated carbon (AC).

Results and Discussion

Results showed that the range of Pt-free cathodes including plain carbon fibre veil, activated carbon, and microporous layer (MPL) in dual-chamber MFCs generated electric current with simultaneous catholyte generation in the cathode chamber.
During MFC operation, the production of catholyte on the surface of the cathode electrode was a direct result of electricity generation, and power output has been correlated with catholyte volume. Moreover, the pH of the formed catholyte (>13) and conductivity, showed gradual increase with current generation. The MFC system fed with real wastewater supplemented with sodium acetate showed sodium recovery on the cathode in the form of sodium carbonate salts. Similarly, when the anode feedstock was supplemented with potassium acetate, KOH was formed on the cathode half-cell with additional crystallisation of potassium salts.
This paper demonstrates an innovative and energy-efficient system that exploits microbially assisted electrosynthesis for the recovery of valuable elements from wastewater, in the form of chemicals (NaOH, KOH) and electricity.

Conclusions

This approach leads to carbon capture through wet caustic scrubbing on the cathode, which locks the carbon dioxide into carbonate salts.

Acknowledgements

This work has been supported by the Bill & Melinda Gates Foundation, grant no. OPP1094890, and the UK EPSRC, grant numbers EP/I004653/1 and EP/L002132/1.

References

1. Rabaey, K. et al., (2010). High current generation coupled to caustic production using a lamellar bioelectrochemical system. Environ. Sci. Technol, 44, 11:4315–21.
2. Rozendal R. A. et al., (2006). Effects of membrane cation transport on pH and microbial fuel cell performance, Environ. Sci. Technol, 40, 5206-5211.