Microbial fuel cells: electricity from bacteria
Bacteria that feed on organic matter are, in chemical terms, moving electrons around. A microbial fuel cell is a device built to intercept some of those electrons on their way and send them through a wire.
How it works in outline
Wikipedia's article on the microbial fuel cell describes it as a bioelectrochemical system that produces electric current by diverting electrons, released when microbes oxidise reduced compounds, from the anode to an oxidised compound such as oxygen at the cathode, through an external electrical circuit. In plainer terms: at one electrode, bacteria consume their food and hand over electrons; those electrons travel through a wire, where they can do work; at the other electrode, they are accepted, most often by oxygen.
In a typical laboratory design the two electrodes sit in separate chambers joined by a separator, often a membrane, that allows charged particles to pass so that the circuit is completed inside the cell as well as outside it. Many other arrangements exist, including single-chamber cells in which the cathode is exposed to air, but the principle is the same in all of them: keep the bacteria's electron donor and the final electron acceptor apart, and make the electrons take the long way round.
Mediated and unmediated cells
How do electrons get out of a bacterium at all? The Wikipedia article explains that microbial fuel cells fall into two general categories. The first ones, demonstrated in the early twentieth century, used a mediator, a chemical that shuttles electrons from the bacteria to the anode. Unmediated cells emerged in the 1970s; in these, the bacteria typically carry electrochemically active proteins, such as cytochromes, on their outer membrane, and can pass electrons directly to the electrode. The discovery that some bacteria do this unaided made the technology much simpler to imagine at scale, since there is no mediator chemical to add and replace.
The history is older than most readers expect. According to the same article, the subject was begun by Michael Cressé Potter in 1911, who produced electricity from the yeast Saccharomyces cerevisiae, though the work received little attention at the time.
Wastewater as fuel
The food for the bacteria can be almost any organic matter they can oxidise, and wastewater is full of it. That is the source of the technology's appeal. A treatment plant already spends energy removing organic matter from water; a fuel cell that removed the same matter while returning some energy as electricity would turn a cost into a partial credit. The Wikipedia article records that in the twenty-first century microbial fuel cells have started to find commercial use in wastewater treatment.
A 2022 review in the Journal of Environmental Management by Boas and colleagues sets out the case and its difficulties. Its abstract notes that the technology has attracted attention for its potential to obtain energy during wastewater treatment, and that using it in industry is attractive because organic wastes can be converted into energy while reducing disposal costs. It then states that attempts to apply microbial fuel cells at large scale have not succeeded so far, because their lower performance and high costs remain challenging. The review covers applications, configurations, operating conditions, diagnostic techniques, modelling approaches and a cost analysis, and concludes that low-cost materials and more efficient, more durable systems with higher power outputs are crucial for industrial use.
What holds it back
The power from a single cell is small, and the obstacles to making it larger are practical ones. Electrodes must be cheap, yet provide a large surface for bacteria to colonise. Membranes can foul and add cost. Real wastewater varies from hour to hour, and the microbial community in a cell changes with it. As the review by Boas and colleagues makes clear, the question is less whether bacteria can make electricity, which is well established, than whether a system can do it cheaply and reliably enough to be worth building.
A classroom favourite
Microbial fuel cells also have a life outside industry. The Wikipedia article notes that soil-based cells serve as educational tools, because they bring together microbiology, geochemistry and electrical engineering and can be made from commonly available materials. For many students they are a first, very direct demonstration that the microbes in ordinary soil are running chemistry that can be measured with a meter.
Reading about MFC results
Studies report performance in several ways, and they are not interchangeable. Current, voltage and power can each be quoted per cell, per electrode area or per volume of liquid, and a figure that looks large under one normalisation can look modest under another. Checking which is being used, and how long a cell ran before being measured, makes comparisons between studies far more meaningful.