1. MICROBIAL FUEL CELLS
KENAN DALKILIÇ

2. Content A brief presentation of microbial fuel cells (MFCs)
Mechanism of electron transfer
Mediators and mediatorless MFCs
Microorganisms in MFCs
Anode and cathode materials (grafite, cloth coating, metals)
Membranes in MFCs (bipolar, anion, cation and micro, ultra m.) and membraneless MFCs
Operational conditions (pH, temp., OLR, HRT)
MFC designs (single, two chmaber, stack, tubular, upflow)
MFC applications, advantages and disadvantages (cost, treatment, remote sensors)

3. History of MFCs
A microbial fuel cell (MFC) is a bioreactor, that converts chemical energy in the chemical bonds in org. comp. to electrical energy through catalytic reactions of microorg. under anaerobic conditions.
First demonstration of current and voltage generation via microorg. was shown by a botany prof. Michael Cresse Potter, in 1911.
Dependency on energy derived from finite fossil fuel and environmental concerns led researches on MFC accelerate for the last two decades.
(search as: microbial fuel cell in elsevier)

4. Microbial Fuel Cell
Microbes oxidise org. matter and electrons and protons are released.
Electrons flow to the cathode through external circut producing electrical current.
Protons flow through membrane to the cathode.
O2 (or any other electron acceptor) reacts with electrons and protons forming water.
Anode chamber is anaerobic and there should be an electron acceptor in cathode chamber.
Anode
𝐶𝐻3𝐶𝑂𝑂−+2𝐻2𝑂→2𝐶𝑂2+7𝐻++8𝑒−
Cathode
𝑂2+4 e−+4H+→2𝐻2𝑂

5. Mechanism of electron transfer
Basic respiration in the cell, involves the transfer of electrons from a low redox potential electron donor to a electron acceptor at a high redox potential.
Nicotinamide adedine dinucleotide
NAD NADH
Hydrogen atoms removed from carriers (NADH) simultaneously are seperated from electrons.
While electrons transferred to next electron acceptor (carrier), protons are pumped outside the cell.
This is occured by tricarboxylic acid (TCA) cycle in the cell.

6. Electron transfer pathways
Possible microbial interactions with the anode.
Electrons can be transferred through immobilized structures such as membrane-bound proteins (A)
electrically conductive pilus (B)
through mobile components (mediators) that are alternatively oxidized and reduced at the electrode (C).

7. Mediators in MFC
Except the anodophiles, the microbes are incapable of transferring electrons directly to the anode. The outer layers of the majority of microbial species are composed of non-conductive lipid membrane that hinder the electron transfer.
Mediators increase the electron transfer from bacteria to anode by a oxidation-reduction cycle between bacteria and anode.
A good meadiator should be:
(1) able to cross the cell membrane easily
(2) able to grab electrons from the electron carries of the electron transport chains
(3) possessing a high electrode reaction rate
(4) having a good solubility in the anolyte
(5) Nonbiodegradable and non-toxic to microbes
(6) low costed
Dyes and metallorganics such as neutral red (NR), methylene blue (MB), Fe(III)EDTA…
Disadvantages: toxicity and instability of synthetic mediators, additional cost.

8. Mediatorless MFCs
Anodophiles can directly transfer electrons to the anode.
Have electrochemically redox enzymes (cytochrome) on the outer membrane.
They may produce soluble redox mediators (pyocyanin) to transfer the electrons to the anode.
They can built membrane proteins (nanowires-pili) to transfer e-.
Certain mic.org. have two of these abilities.
Mediator electron transfer (a) Non-diffusive mediator electron transfer and (b) Diffusive mediator electron transfer
Product electron transfer
(3) Direct electron transfer. (4) Nano-wires.

9. Microbes used in MFCs
Du, Z. et al., 2007, Biotechnology Advances

10. Microbes used in MFCs
EET (extracellular electron transfer) capacity is widespread in nature and it is found among most of bacteria.
The existence of these microorganisms in nature is also confirmed by the diversity of inocula that can be used to start up lab-scale MFCs.
Raw sewage, activated sludge, anaerobic and methanogenic sludge, river sediment, and seawater
Acetate, propionate, butyrate, ethanol, lactate, glucose, domestic wastewater and beer-processing wastewater.

11. Electrodes (Anodes and Cathodes)
Because different materials have different activation polarization losses, anode and cathode material type is important.
Platinum, platinum coated carbon, graphite, graphite felt, carbon-cloth, carbon paper and some metals
Platinum and carbon based materials
Graphite or carbon based materials are cheaper and have enormous surface area.
Surface area and the roughness of anode material
Rough surfaces decrease the current density
Electrode modification is very useful for increasing MFC performances (Mn graphite anode, mediator )

12. Electrodes (Anodes and Cathodes)
An electron acceptor is needed to maintain the process at the cathode chamber.
Oxygen is widely used as an electron acceptor. Ferricyanide, hydrogen peroxide.
Oxygen has slow reduction kinetic and limited dissolution in water, a catalyst (platinum, cobalt oxide, binded mediators on graphite electrode) is needed.
Air-cathode electrodes and biocatalysts
Ferric sulfate, cobalt and manganese based materials, molybdenum are employed on graphite electrodes.
In biocathode chamber, oxygen, nitrate, sulfate and iron can be used as electron acceptor by microorganisms.

13. Membranes in MFC
The ideal function of an MFC membrane is to seperate the anode and cathode rxn while permitting selective transport of protons from anode to cathode and preventing transport of oxygen into the anode chamber.
good ionic conductor
an insulator
ion selective
durable
chemically stable
biocompatible
unsusceptible to fouling and clogging
inexpensive
Ion EM, cation EM, anion EX, proton EM (Nafion, Ultrex, Hyflon)
Power output is proportional with the membrane`s surface area.
Membrane-less MFCs : cloth cathode assembly (CCA), cloth electrode assembly (CEA)
Cloth assemblies(seperators) reduces the anode-cathode spacing and internal resistance, enhancing power generation.

14. Comparison of studies
200 W/m3 is enough power → to turn on 3 lamps (60 watt) at the same time, to pump 5 L water to a height of 1 m in one second with an appropriate pump.
V.B.Oliveira et al., 2013, Biochemical Engineering Journal

15. Operating Conditions
Power generation of an MFC is affected by many factors:
microbe type
fuel biomass type and concentration
ionic strength
membrane type pH
temperature
reactor configuration (space between electrodes)

16. Operating Conditions pH
pH is tended to due to the production of protons and slow transfer of protons through membrane. (Anode)
pH is tended to due to the consumption of protons by the oxygen reduction rxn and slow transfer of protons from anodic chamber.
pH conc. gradient on both side of membrane
pH changes affects microbial growth and power output
A selectively membrane is needed
To prevent the pH limitations, pH buffers and bipolar membranes can be used.
Microbials cultures could be acclimatized to higher pH values.

17. Operating Conditions Temperature
System kinetics and mass transfer, microbial community
Increase microbial activity and membrane permeability (COD removal and power output)
Optimum 30-45oC
Organic Loading Rate
An optimum OLR
Too high OLR leads to domination of methanogens which compete with electrochemically active microorg.

18. Design of MFCs Upflow mode MFC systems Stacked MFCs
Tubular MFC systems
Two-compartment MFC systems
Single-compartment MFC systems (SMFC)

19. Design of MFCs Stacked MFCs
Several MFCs connected in series and in parallel.
Enhanced current output and COD removal can be achieved especially in parallel stacked MFCs.
Energy autonomous eco-bot 3

20. Applications of MFCs Electricity generation
Since 1999, power production in MFCs has improved 5–6 orders of magnitude (2.7–6.9W/m2 and COD removal as high as %95).
On a volumetric basis, up to 2.2kW/m3 has been reported in the lab. 1kW/m3 is needed to sustain pumping, heating, lighting, aeration, and other energy needs in a wastewater treatment facility.
Although these results are promising, its should be noted that these reported power outputs were achieved at the bench scale (V < 1 L)
Wastewater treatment (50-90% less sludge compared to activated sludge treatment)
Biohydrogen production: instead of electricity production in MFC, H2 is produced with an external potential of 110 mV combining electrons and protons in the cathode.
Biosensor : sensors for pollutant analysis, in situ monitoring and control, ex: BOD-coulumbic yield or maximum current.

21. Pilot scale applications of MFCs
A stackable horizontal MFC (SHMFC) (air-cathode)
The system is the largest single MFC module by far.
Domestic wastewater was fed into SHMFC
in horizontal advection.
During the stable operation period, a maximum current ± A in each module was observed under the external resistance of 1 ohm and the maximum power density was 116 mW.
The COD removal was 79 ± 7% and the TN removal rate was 71 ± 8%.
(Feng et al, 2014)

22. Pilot scale applications of MFCs
Brewery Wastewater Treatment
Fosters, an Australian beer company, built the first pilot scale MFC (combined volume of 1 m3) to clean its wastewater while generating electricity and clean water.
Each long tube (3m) is one large Microbial Fuel Cell.
Downflow MFC (because of the ionic conductivity of wastewater, power: 8 W/m3, must be improved)
Partnered with the University of Queensland, Fosters’ plans to build a 3 m3, 2 Kilowatt MFC that cleans all of the company’s wastewater.
The power generated from cleaning the brewery wastewater is expected to pay for the initial cost of the Microbial Fuel Cell in ten years.
Significant challenges have been observed when these systems are expanded to the pilot scale.
Low power production may result from ohmic losses within the large systems and low conductivity of the wastewater.
No need oxygen to achieve COD removal, and produce little biomass.

23. Limitations / further improving
Energy losses in MFCs
1-mass transport limitations: transfer of reactants and products to/from biofilm (concentration polarization)
2-bacterial metabolic kinetics: microbial consortium and biomass density on the electrode (activation polarization)
3-electron transfer to/from the electrode: limited by the finite rate of electron transfer between microorg. and solid phase of electrode (activation polarization)
4-resistance of the electrolytes including membranes: conductivity, ionic strength, electrode spacing (Ohmic losses-internal resistance)

24. References
Zhuwei Du, Haoran Li, Tingyue Gu, A state of the art review on microbial fuel cells: A promising technology for wastewater treatment and bioenergy, Biotechnology Advances 25 (2007) 464–482
B Virdis, S Freguia, RA Rozendal, K Rabaey, Z Yuan, and J Keller, Microbial Fuel Cells, The University of Queensland, Brisbane, QLD, Australia.
Betül Hande Gürsoy, Kimyasal Arıtma Uygulamalarının Organik Maddelerin Yapıları Üzerindeki Etkisinin Araştırılması: Karasu İle Örnek Çalışma, 2014, Doktora Tezi, İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü.
V.B. Oliveira, M. Simões, L.F. Melo, A.M.F.R. Pinto, Overview on the developments of microbial fuel cells, Biochemical Engineering Journal 73 (2013) 53– 64.
Ashley E. Franks and Kelly P. Nevin, Microbial Fuel Cells, A Current Review, Energies 2010, 3, ; doi: /en
Seokheun Choi, Microscale microbial fuel cells: Advances and challenges,
Minghua Zhou, Jie Yang, Hongyu Wang, Bioelectrochemistry of Microbial Fuel Cells and their Potential Applications in Bioenergy,

25. Hope enjoyed enlightening…