The chemical that powers most of our cellular processes is produced through something called the electron transport chain. As the name suggests, this system shuffles electrons through a series of chemicals leaving them with a lower energy, while some of the energy difference is harvested to produce ATP.
But the ultimate destination of this electron transport chain need not be a chemical. There are several bacteria that ultimately send the electrons out into the environment. And researchers have figured out how to turn it into a fuel cell, harvesting the electrons to do something useful. While some of these designs were more battery-like than others, they all consumed some sort of material in harvesting the electrons.
A team of researchers in the Netherlands figured out how to close the cycle and create a real bacterial battery. One half of the battery behaves like a bacterial fuel cell. But the second half takes the electrons and uses them to synthesize a small organic molecule that can eat the first. The charge cycle is excruciatingly slow and the energy density atrocious, but the fact that it works at all seems rather remarkable.
While bacterial fuel cells have attracted most of the attention, there is a separate area of research called microbial electrosynthesis. This is exactly what it sounds like: give bacteria energy in the form of electrons and let them synthesize a useful chemical. In this case, the useful chemical was acetate, which is known to most of you as the main ingredient in vinegar. It is a small organic molecule that can be easily connected to the metabolic pathways cells use to metabolize sugars.
In this case, the team already had access to an acetate-producing microbial electrosynthesis system populated by a mixture of bacteria originally from a methane-producing biodigester and cow dung. Separately, they had developed a microbial fuel cell that ran on acetate. These were simply placed in a single container, separated by a membrane that allowed acetate to flow through but kept the bacterial population separate.
All that had to be supplied was carbon dioxide – conveniently, that’s already part of the atmosphere. The system did, however, require a pump to ensure that the materials were mixed into solution and a heater to maintain it at 32 degrees Celsius.
But otherwise all the system needed over the course of two weeks was a supply of electrons while charging. The system was charged for 16 hours, after which the power was shut off; it would then produce electricity for about eight hours. There was no discernible drop in efficiency over 15 days of cycling.
By comparing the electrons sent to the cathode with those at the anode, the authors were able to calculate the efficiency of their system. By this measure, it operated at an efficiency of between 50 and 80 percent over the course of the 15 cycles. Due to several losses, this resulted in an overall efficiency of about one third.
One of the intriguing things about this efficiency is that it is not fully explained by the production of acetate (which the authors also measured). Instead, at the end of microbial electrosynthesis, the bacteria must produce another product that those at the fuel cell portion can digest. In one cycle, this was clearly the chemical formate (an acid based on a single carbon atom, instead of the two of acetate). But the bacteria liked to digest that too, so it didn’t affect the overall efficiency. But that only happened during one cycle, and the authors aren’t sure what was going on the rest of the time.
The system is quite inefficient compared to most batteries and takes 16 hours to charge. Surely it must have some redeeming property, such as high energy density, right? Well no, it falls short on that too. The authors estimate the capacity at about 0.1 kilowatt hours per cubic meter. Existing lithium-ion batteries can achieve more than 500 watt-hours per litre. If you do the math, you’ll find that this messes up the bacterial battery. (There’s 1,000 liters per cubic meter, so the conversion is pretty easy. Why don’t we all go back to the metric system?)
The authors, like any other researcher writing about experimental battery technology, have some ideas on how to improve energy density a bit. But they’ll never improve it by three orders of magnitude, and you wouldn’t expect them to; ions will always be a more compact way of shuffling energy around than microbes. Which could lead to the conclusion that this system is completely useless.
But there is one context where it may not be so. We are probably already building some microbial fuel cells to process our food waste, sewage and other sources of extra organic matter. At the same time, renewable energy plans suggest that we are likely to produce excess electricity during periods of bright sun or high winds. A microbial electrosynthesis system is an option to use that excess electricity to make fuel that can be used when conditions aren’t as favorable for renewable energy production, assuming we have some of the microbial fuel cells around anyway.
And unlike options such as hydrogen or batteries, the raw materials (microbes) are cheap and can be used almost anywhere.
Environmental science and technology letters2016. DOI: 10.1021/acs.estlett.6b00051 (About DOIs).
Frame image by Lawrence Livermore National Lab