But the ultimate destination of this electron transport chain doesn't have to be a chemical. There are a variety of bacteria that ultimately send the electrons off into the environment instead. And researchers have figured out how to turn these into a fuel cell, harvesting the electrons to do something useful. While some of these designs were closer to a battery than others, all of them consumed some sort of material in harvesting the electrons.
While bacterial fuel cells have attracted the most attention, there's a separate area of research called microbial electrosynthesis. This is exactly what it sounds like: provide bacteria with energy in the form of electrons and get them to synthesize a useful chemical. In this case, the useful chemical was acetate, which is familiar to most of you as the key ingredient in vinegar. It's a small organic molecule that can easily be plugged into the metabolic pathways that 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 that originally came from a methane-producing biodigestor and cow manure. Separately, they had developed a microbial fuel cell that was running on acetate. These were simply placed into a single container, separated by a membrane that would allow acetate to flow through but keep the bacterial population separate.
The only thing that needed to be supplied was carbon dioxide—conveniently, that's already part of the atmosphere. The system did, however, need a pump to ensure mixing of the materials in solution and a heater to keep it at 32 degrees Celsius.
But beyond that, the only thing the system needed over the course of two weeks was a supply of electrons during charging. The system was charged for 16 hours, after which the current was shut off; it would then produce electricity for approximately eight hours. Over 15 days of cycling, there was no detectable drop off in efficiency.
By comparing the electrons sent to the cathode with those harvested at the anode, the authors were able to calculate the efficiency of their system. By this measure, it operated at between 50- to 80-percent efficiency over the course of the 15 cycles. Due to various losses, this resulted in an overall efficiency of about one-third.
One of the intriguing things about this efficiency is that it's not entirely accounted for by the production of acetate (which the authors also measured). Instead, the bacteria at the microbial electrosynthesis end must be producing some other 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 acetate's two). But the bacteria happily digested 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 pretty low-efficiency compared to most batteries, and it takes 16 hours to charge. Surely it must have a redeeming property, such as great energy density, right? Well, no, it comes up short on that as well. The authors rate its capacity at about 0.1 kiloWatt-hours per meter cubed. Existing lithium-ion batteries can get above 500 watt-hours per liter. If you do the math, you'll find that this clobbers the bacterial battery. (There are 1,000 liters per cubic meter, so the conversion's pretty easy. Why don't we all use the metric system again?)
The authors, like every other researcher writing about experimental battery technology, have some ideas about how to improve the energy density a bit. But they're never going to improve it by three orders of magnitude, and you wouldn't expect them to; ions are always going to be a more compact way of shuffling energy around than microbes. Which might lead you to the conclusion that this system is utterly useless.
But there's one context in which it might not be. We're already likely to be making some microbial fuel cells in order to handle our food waste, sewage, and other sources of spare organic material. At the same time, plans for renewable energy suggest we're likely to be producing excess electricity during periods of bright sunshine or high winds. A microbial electrosynthesis system is one option for using that excess electricity to create 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 deployed pretty much anywhere.