Every morning, when I arrive at my office in the Botanical Garden, I connect the mobile charger to my fish tank. After a while it is as charged as if you had plugged it into a household outlet. The secret lies in the work of electrogenic bacteria that transform the fish tank into a microbial fuel cell (CCM), whose foundation is the same as conventional cells or batteries.
To move towards the description of a microbial fuel cell it is appropriate to recall the operation of a conventional fuel cell (Figure 1). Most cells (or batteries in mobile devices) have three fundamental components: electrodes, electrolyte, and separator.
Each battery has two electrodes:
The cathode, connected to the positive end of the battery (+), is where the electrical current exits when the battery is used to power something.
The other electrode, the anode, is connected to the negative end of the battery (-) and is where electric current enters (or electrons leave) it during discharge.
Between these electrodes is the electrolyte. It is a gelled liquid substance that contains electrically charged ions or particles. The ions combine with the materials that make up the electrodes producing chemical reactions that allow a battery to generate an electrical current. The function of the separator is to keep the anode and cathode isolated from each other within the battery to prevent the two electrodes from coming into contact and causing a short circuit.
Microbial fuel cell
Let’s now look at a diagram of a microbial fuel cell (Figure 2), a device where certain microorganisms are used to convert the chemical energy present in biodegradable substrates (any form of dissolved organic matter) into electrical energy. The organisms capable of transforming chemical energy into electrical current are electrogenic bacteria.
The ability of bacteria to produce electricity was first described in 1911 by MC Potter, Professor of Botany at Durham University, but its discovery did not attract the attention of other scientists until the 1980s, due to much to the need to find new sources of energy.
In order to advance research in this field, the discoveries of microbial physiology related to the transport of electrons between bacteria and electrodes and, finally, the development of CCM, whose foundation is the same as conventional cells or batteries, were decisive.
Through catalytic oxidation of fuel at the anode and chemical reduction at the cathode, conventional fuel cells are used to electrochemically produce electricity from many different chemicals.
Microbial fuel cells do not require the use of metallic catalysts at the anode. Instead, they use exoelectrogenic bacteria that, by biologically oxidizing organic matter, produce electrons as part of their normal metabolism and transfer them without intermediaries to the anode. These electrons flow through a circuit to the cathode, where they combine with protons and with a catholyte, that is, with a chemical that, like oxygen, is capable of accepting electrons.
These bacteria produce electricity by generating electrons within their cells and then transferring them through tiny channels made of proteins in their cell membranes in a process known as extracellular electron transfer. Bacteria give up the metabolically produced electrons to a conductive material, generating an electrical current.
Converting this natural process into a functional microbial fuel cell is as simple as filling a container like my fish tank with mud, sewage or waste and waiting for the bacteria to grow.
The most widely used electrobacteria are those of the genus Geobacter, very abundant in aquatic sediments, which have such interesting characteristics as the production of electricity, the decontamination of soils and waters, and the production of bacterial nanowires, which can be used as nanoconductors of electrons to lead them to electrodes.
When the billions of bacteria that grow together combine, the nanowires join together to form conductive biofilms that can transfer electrons over considerable distances, opening up the possibility of developing self-healing, organic electrical conductors for use in biocomputers. Furthermore, this direct transmission is favored by a huge stacking of cytochromes, specialized proteins that facilitate the transfer.
These bacteria are anaerobic and in their natural habitat they use metals to breathe and obtain energy. In the same way that they are capable of transferring electrons to many metals, they are also capable of transferring them to electrodes and thus constitute a microbial fuel cell. There are two main types, those that use isolated cultures in controlled devices in the laboratory (Figure 2) and those of sediment, that use the organic matter of aquatic beds (marine and freshwater) to produce electricity.
Mine, based on Geobacter, is simple (Figure 3). In microbial fuel cells, contact of the anode with the anaerobic sediment (surrounded by a biofilm of exoelectrogenic bacteria) is required, while the cathode is exposed on the surface, usually saturated or with high oxygen concentration.
A graphite electrode inserted into the bottom sediment collects the electrons generated by the electrobacteria by breaking down both the dead organic matter deposited in the sediment, as well as the organic acid-rich exudates emitted by the roots of aquatic plants. The electrical circuit started at the anode is closed by the other electrode, the cathode, located on the surface of the water, so that the electrons, oxygen and hydrogen combine to give water.
The degradation of organic materials produces, in addition to C0₂, protons (H +). These cross the medium to the second electrode, while the electrons travel through the external circuit. When oxygen is reduced at the cathode, the end result of the fish tank transformed into an electricity-generating device is that, taking the electrons from the external circuit and the protons formed at the anode, water is formed.
To improve its performance, in my fish tank the cathode is located in a floating bed that houses a consortium of bacteria that capture electrons more efficiently and quickly than oxygen in the water.
Currently, the applications of these devices are limited due to the low power output. The most practical currently used by the United States Navy weighs 16 kilograms and produces the power equivalent to 16 conventional 1.5 V batteries per year. But let’s not forget that when the first computer, the ENIAC, was presented to the public in 1946, it was a monster weighing 27,000 kilos, occupying 167 m² and consuming 160,000 W, which was achieved at the cost of causing frequent blackouts in Philadelphia, where it was located.
And all to not get even a millionth of the performance capacity that the device that I connect to my fish tank offers me today. Matter of time.