A biofuel cell is a device that uses biological materials to generate electricity in a direct way through redox reactions. This contrasts with conventional use of biofuels to generate electricity from the heat provided by combustion of the material. The principle behind biofuel cell technology is to mimic various natural processes that are used to produce energy within living organisms. In some cases, bacteria may play a role in these fuel cells. As of 2011, biofuel cells show potential as an alternative energy source and in various medical and bioengineering applications.
Living organisms obtain energy from the oxidation of carbohydrates, which are generated by photosynthesis in plants and ingested as food by animals. Enzymes facilitate the reactions, in which carbohydrates are converted into carbon dioxide and water by the removal of electrons, which are then stored in adenosine triphosphate (ATP) molecules. In a biofuel cell, electrons produced by the oxidation of organic molecules — usually carbohydrates, as in living organisms — are used to generate an electrical current. The idea of using these biological processes to generate electricity has been around since the 1960s, but early attempts to construct a practical, working biofuel cell encountered difficulties.
A biofuel cell will typically consist of a container divided into two sections by a permeable barrier. In one section, the oxidation of a carbohydrate — for example, glucose — provides electrons. In the other section, a reduction reaction takes place, which uses these electrons. By connecting the two electrodes, a current can be made from the electrode in the oxidation section — the anode — to the electrode in the reduction section — the cathode.
One of the biggest practical problems hindering the development of biofuel cells has been finding an efficient way of getting the electrons released from the carbohydrate into the anode. The electrons are initially stored in the oxidizing enzyme and would, in the natural process, be chemically transferred into ATP molecules. There are two possible methods of extracting electrons from the enzyme into the anode in a biofuel cell.
In the Direct Electron Transfer (DET) method, the enzyme needs to be bonded onto the anode. This can be done chemically or by other methods, such as constructing the anode from a mesh of carbon nanotubes onto which the enzyme is adsorbed. These methods result in reduced activity in the enzyme and consequent loss of efficiency, but this is, at the time of writing, an area of ongoing research and improved techniques may be developed.
The other method of electron transfer is known as Mediated Electron Transfer (MET). This does not require the enzyme to be in contact with the anode; instead, the electrons are passed to another molecule wth a lower redox potential, which then gives up the electrons to the anode. This compound, known as a mediator, must also have a higher redox potential than the anode. This extra step involves a loss of energy and so the fuel cell is in practice less efficient than it could be in theory.
Biofuel cells are an area of active research and various possible solutions to these problems are being investigated. Among the possibilities is the use of bacteria in microbial fuel cells. Iron reducing bacteria that live in anaerobic conditions show particular promise as they naturally reduce iron in its +3 oxidation state to its +2 oxidation state. The iron can then give up an electron at the anode, returning to its +3 state and acting as a natural mediator molecule by transferring electrons from the bacteria to the anode.
The main advantages of biofuel cells are that they are non-polluting, do not require expensive catalysts and use common, inexpensive and easily renewable raw materials. The main disadvantages of biofuel cells is their inefficiency and low power output. As of 2011, however, there are hopes that these problems can be overcome, opening up a new range of possibilities. These include not only cheap, clean and renewable energy, but also the prospect of implanted biofuel cells, running on substances produced by the body, being used to power medical devices such as pacemakers.