Description Design Model MFCs Software

Microbial Fuel Cell

What are MFCs and what do they do?

A microbial fuel cell (MFC) is a fuel cell which, instead of using conventional chemical reactions to generate electricity, uses microbial metabolic reactions with wastewater organic matter acting as fuel.

MFCs rely on the use of exoelectrogenic bacteria; these are able to undergo (extracellular) electron transfer, meaning electrons are transferred to the outside of the bacterial cell and can then be transported to an electrode and copper wire to generate an electrical current.

What are the components of an MFC?

MFCs typically comprise of 2 chambers, each one having an electrode (either an anode or cathode) with bacteria being cultured within the anode chamber. The chambers are usually separated by a cationic membrane which allows a flow of hydrogen ions, produced when Nicotinamide adenine dinucleotide (NADH) donates an electron to the MTR pathway, to the anode where it binds to oxygen to form water.

How do MFCs work?

The main ‘powerhouse’ of an MFC lies in the microbial cultures found on the anode. The main process of electrical generation ultimately comes down to the conversion of various organic or inorganic substrates by exo-electrogenic microorganisms acting via the Mtr pathway, as explained in detail in the MTR aspect of our model.

MFCs make use of microbial exo-electrogenic abilities by serving electrodes (usually anodes) as final electron acceptors. When receiving electrons, anodes are designed to push the electrons towards the cathode, thus generating an electric current that contains harvestable energy.

Simply put, an MFC works due to the ability of these micro-organisms to utilise extracellular molecules and ions as final electron acceptors. This process is similar to aerobic organisms utilising oxygen (O2) as a final electron acceptor of electron products coming from the electron transport chain (ETC), thus reducing the oxygen into water (H2O).

Bacteria grow on the anode and transfer their electrons to it, flowing through the wire towards the cathode in the other chamber, which is exposed to air. At that point, proton products of NADH electron donation to the Mtr pathway finally combine with oxygen, forming H2O as an end product in the cathode.

It is important to note that most exo-electrogenic microbes are also capable of consuming oxygen as an electron acceptor. This poses an issue if the microbes are planned to be used in MFCs, since some of their electron products will end up binding to oxygen and reduce the utilisation of the MTR pathway and its exogenic electric output onto the anode.

MFC challenges and losses:

The main challenge faced by MFCs is lowering costs of materials which tend to be expensive whilst retaining productivity. Other challenges include maintaining a stable electrogenic bacterial population and reducing the fouling of the cathode.

MFCs can possess efficiency losses such as internal resistance/ohmic losses. Another example is activation losses: the accumulation of gases can interfere with electrode-electrolyte contact. As well as bacterial metabolic losses, there is an efficiency disadvantage from the uneven depletion of reagents in the electrolyte, causing an uneven distribution of those reagents.

Most common applications of MFCs

The long-term goal for MFCs is to implement their use in wastewater treatment plants to generate electricity. This would be achieved by extracting the high (chemical) energy contained within the carbohydrate compounds and micro-plastics found in wastewater.

MFCs do not require using energy to burn anything as the electricity is generated (at room temperature) from the oxidation of organic molecules. Not only is waste being reduced but electricity is also generated thus tapping into an unused source of energy. Furthermore, remote areas with limited electricity may be able to benefit from MFC technology.

Shewanella oneidensis as an MFC and improvements to the MFC system:

To increase the power density, the electron transfer rate will have to increase. This can be done by increasing the potential/efficiency of the bacteria to produce fuel via genetic modification or increasing the efficiency of the MFC architecture (such as increasing the surface area of the anode).

Some bacteria have extensions called pili that provide allow the transport of electrons to the outside of the cell; the electrons may move towards metal oxides in nature but in MFCs, they are artificially influenced towards the anode and cathode. Shewanella have conductive appendages/nanowires that aid the transfer of electrons. Direct contact between the anode and bacteria is another method of electron transfer.

Physical Improvements:

Anode security

As was mentioned, most exo-electrogenic bacteria are aerobic and consume oxygen when it is present. This poses an issue, since the electrons that go on to reduce the oxygen molecules would otherwise reduce the electrode that the bacteria are cultured on. In our case, we used Shewanella oneidensis MR-1 strain which belong to the aerobic family of exo-electrogens, and so to overcome the issue of Shewanella consuming oxygen, we depleted the anode chambers of oxygen and successfully confirmed that this modification works.
Electrode Material

The most expensive part of an MFC is the cathode with a catalyst and binder (e.g. platinum and Naflon). It is therefore important to use cheap alternatives. For example, carbon cloth/carbon paper electrodes are a good (cheaper) alternative to the platinum ones typical used. Activated carbon electrodes can act as a catalyst for oxygen reduction; they are much cheaper and showed improved longevity of performance over time compared to platinum electrodes despite initially performing almost as good as platinum.

According to researchers at Kaunas University of Technology, compared to normal MFCs, MFCs containing modified graphite felt anodes enabled a 20% higher cell voltage.

Using graphite fibre brushes for the anode could be not only cheaper, but they also have a higher surface area to volume ratio (compared to carbon sheet electrodes), are electrically conductive, non-corrosive and relatively easy to manufacture by machines for mass production.

The higher surface area of brushes enables a greater area of contact for the bacteria to transfer electrons. The higher surface area of the brush design is similar to an efficient design found in nature: villi and microvilli, the finger/brush like cellular projections which increase the rate of nutrient absorption in the small intestine by increasing the available area compared to a flat surface.

Image Source: https://radiologykey.com/gastrointestinal-tract-10/

We ourselves found that using activated carbon is the most economically and ecologically sustainable solution as an electrode material, and have thus decided to use it in our experiments.
Reducing Internal Resistance

The anode should be positioned close to the cathode to reduce internal resistance but a barrier should still be present to avoid electrically shorting them. We cannot make the MFCs too big as that would increase the cathode-anode distance and thus increase the internal resistance. Instead, many small MFCs would be better for scaling up (MFC stacking) while using the same amount of space (volume). Another modification proven to be highly effective involves using ceramic materials for chambers of the MFCs that would ultimately be applied (Gajda et al, 2018). In microbial experimentation such as ours, this is unnecessary.
Optimum Electrode Brushes

Smaller anode brushes that are closer to the cathode seem to perform better than bigger and further away brushes because in the big brush, lots of brush fibres/area is wasted, or not doing anything.

Having the cathode exposed to air means bubbling air into the chamber is not necessary thus saving on energy costs. An added diffusion layer would prevent leaks/water getting out, while still allowing air in.

Microbes (efficient in oxidising organic matter) do not tend to foul the anode surface, in fact the new microbes (i.e. recently divided) tend to grow on the anode surface, pushing the old ones away! So the anti-fouling challenge is focused on the cathode, not the anode.

According to Dr. Bruce Logan, fouling causes a power drop of about 15-20% after 1 year. If the cathode is covered with a protective cloth, this may help prevent biological growth/fouling on it and therefore increase performance over time and reduce maintenance costs. A dilute acid wash of the cathode can restore performance/activity to almost the original (e.g. after a year).

During our experimentation, we mainly used carbon sheets for bacterial growth, since those are cheapest and most easy to use. We didn’t end up measuring their efficiency, thus we didn’t have the need to use them.

Microbial improvements:

Further studying of the MTR pathway could help us understand it better and determine how it can be even better utilised for sustainable generation of electricity.

The co-culturing Shewanellas and other exo-electrogenic bacteria with other microbes that assist them in biofilm formation has been found to be extremely beneficial for a higher output (Juliastuti et al, 2017). This could be due to multiple factors, most rational of which is that the co-cultured bacteria release waste metabolic by-products that can be used by exo-electrogens as an energetic resource.

We made some genetic attempts ourselves: indirect genetic modifications of exo-electrogenic abilities such as RhlA (rhamnolipid A) was found to increase biosurfactant production in Pseudomonas aeruginosa which increased MFC output. We tested to see if it does the same in Shewanella. Additionally, we are testing whether the expression of anaerobic transcription factors makes Shewanella oneidensis ‘ignore’ oxygen if it were present and whether the over-expression of specific transcription factors can result in a higher electrical output.

Future outlook:

The IONIS iGEM team is working on this similarly as we are, but they are degrading a plastic that is much more easily degraded: cellulose acetate (https://2019.igem.org/Team:Ionis_Paris). Their idea is much more feasible than ours, although both ideas are interesting. More research is needed on PET degradation and MTR electricity production if we want to see this goal of energy generation from wastewater to become more efficient and practical on a larger scale.

A common source of PET plastic waste is discarded plastic bottles. One idea could be to obtain waste plastic bottles via community collection bins and raising awareness of this project via the media and school education. This would also prevent the bottles ending up in landfill sites and instead be used for energy generation.

iGEM exeter is collecting plastics from washing machines (https://2019.igem.org/Team:Exeter) using specifically designed filters (ingenious!). They also worked on biodegrading PET as a next step.

Another possibility is to form a contract with a company that picks up plastics, such as The Ocean Cleanup Project.

Interdigitated microelectrodes - a potentially major facilitator in MFC research which are essentially tiny MFCs. They contain small anodes and cathodes next to each other on chips that can be set in agar plates and have bacteria directly cultured on them.

They drastically speed up MFC-styled research and although we didn’t end up using them (they’re expensive and we would have gotten them only through sponsorship) we believe they are worth mentioning for any future MFC scientists.

Real life ideas of the applications for our MFC:

More economically developed country: use in charging a mobile phone and to light a Christmas tree.
Less economically developed country: to generate electricity be used to run a motor which pumps water from the ground.

References:

Bruce Logan (2015) Microbial Fuel Technologies. Youtube. Available from https://www.youtube.com/watch?v=su6PfYeMrsI

Waste to Energy Conversion (2017) Introduction to Microbial Fuel Cells. Youtube. Available from https://www.youtube.com/watch?v=YAbrNJ3WwuU

Bruce Logan (2014) Microbial Fuel Cell and Reverse Electrodialysis Technologies. Youtube. Available from https://www.youtube.com/watch?v=_9yPQTDyL3k

https://sites.psu.edu/microbialfuelcells/mfcs/

http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/micvil.html

https://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.highlight/abstract/8957

Kaunas University of Technology (2018) Wastewater treatment plants could generate electricity Researchers are working on improving the efficiency of microbial fuel cells (MFC) by using modified graphite felt Science Daily. 12 February. Available from

https://www.sciencedaily.com/releases/2018/02/180212125816.htm

Juliastuti, S. R., Darmawan, R., Ayuningtyas, A. and Ellyza, N. (2017). The utilization of Escherichia coli and Shewanella oneidensis for microbial fuel cell. Materials Science and Engineering, 334 012067.

Gajda, I., Stinchcombe, A., Merino-Jimenez, I., Pasternak, G., Sanchez-Herranz, D., Greenman, J and Ieropoulos, I. A. (2018). Miniturized Ceramic-Based Microbial Fuel Cell for Efficient Power Generation From Urine and Stack Development. Frontiers in Energy Research.

Team:Westminster UK/MicrobialFuelCell

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