Understanding an MFC
A microbial fuel cell exploits the electrons produced by the bacteria metabolism placed in the anodic chamber to make energy. When building a MFC several parameters must be taken into consideration:
- The anodic chamber must be under anaerobic conditions to favor the transfer of electrons to the electrode.
- The two chambers need to be separated by a proton exchange membrane (Nafion) for the equilibration of the total charges.
- The material of the electrodes needs to be highly conductive (carbon cloths connected with a tinned copper wire).
- In the cathode it needs to be placed an acceptor with a high redox potential (i.e. Ferricyanide, air cathode).
- In the anode it is important to place an electroactive bacteria (i.e. Schewanella oneidensis) or supplement the media with chemical mediators (i.e. methylene blue, neutral red) that can cross the bacterial membrane to steal the electrons and bring them outside.
- Additionally, our Solar pMFC needs to be built with a material that allows the light to go through.
Our goal was to build a functional prototype to:
- Enhance current production. This was achieved by connecting 6 small MFCs in parallel.
- Avoid dispersion of energy due to the different potential of the units MFC connected in parallel. This is possible if the potential values of the single units are identical, which is difficult when working with bacteria. To overcome this problem in our design, the 6 cathode chambers are individually connected with the same anode, so that they share the same homogenous bacterial culture.
- Maximize the surface of contact between the bacteria and the anode, and simultaneously increase light exposure of the bacteria. Exploit the redox potential of oxygen with a cathode partially in contact with air (i.e. the cathodic chambers are open to the air, although filled with water).
- Have a sealed anodic chamber to maintain anaerobic conditions.
The Solar pMFC prototype of iGEM Trento 2015 team The cell is composed of a central 1.5 L chamber (the anode) where the bacteria are placed under anaerobic conditions (1). A carbon cloth electrode is placed inside the central chamber and it is in direct contact with the culture medium (2). On the outer surface, six small chambers (the cathode) are filled with 20 ml of water and a small carbon cloth electrode (3) and are left open to the air. Each cathode is connected in parallel by a tinned copper wire (4). A proton exchange membrane (5) separates each cathode from the bacterial solution . Left: MFC under construction. Right: MFC at work, with bacterial culture in the anodic chamber.
Our solar MFC in action
We tested our prototype with a bacterial strain given to us by Dr. Ajo-Franklin at the National Laboratory of Berkeley. This was an engineered E.coli strain containing the MtrCAB Shewanella oneidensis electron transfer pathway. We were also planning to use the device with our engineered E.coli containing proteorhodopsin. We had to put on hold this test, because our retinal-proteorhodopsin device was not ready in time and the alternative of adding exogenous retinal in a large volume of culture would have been too expensive.
Our prototype reached a maximum power (Pmax) of 109 μW, calculated with an internal resistance of 500 Ω which gave a current of 467 μA. Although this was exciting, and it confirmed that our device functions properly, it was not enough to power up any electrical device. We then connected our designed MFC to 2 additional units and we were able to power up a lab timer!!!
Watch this video to see our MFC in action:
We have more exciting results with the small units and our proteorhodopsin engineered bacteria. Check the MFC results page.
Why is our Solar pMFC better than existing technologies?
What is different from the common microbial fuel cells is that our Solar pMFC needs sunlight to work. For that reason we envision that our device will mimic a photovoltaic (PV) system and could be installed for example on the top of a roof. What makes the Solar pMFC better than the already existing PV systems is that, in a near future, bacteria will completely substitute expensive and not-easy to dispose materials that make up the thin-films on PV cells, such as cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon (a-Si), and gallium arsenide (GaAs).
Energy can be harvested by light also with the help of small molecules (i.e. anthocyanins, chlorophyll). These molecules can be used also in biophotovoltaic panels. These pigments have relative short lifetime and need to function in a non living system. Our system instead will keep producing energy as long as the bacteria are kept alive. That is why we envisioned a self-sustainable system.
Bacteria will be fed using organic house waste, directly transported to the microbial fuel cells. A depuration machinery will assist such systems. Safety is guaranteed because all living organisms are constantly kept inside the anodic chamber and replaced once they are exhausted. Our prototype will have a bacterial refilling system that works like an Espresso machine: dead/old bacteria will be collected on an Espresso-like capsule and discarded, replacing it with a new one containing fresh bacteria.