Most of the material we consider to be waste, from sewage to discarded food, actually contains significant amounts of potential energy in the form of carbon-rich chemicals. The problem is that it's generally diffuse and often present in water-saturated materials, making it impossible to combust without extensive processing.

And, if we ever tried, we'd end up competing with microbes that make a living off extracting energy from those same carbon sources. So in recent years, researchers have been experimenting with microbial fuel cells, where bacteria clustered at an electrode extract carbon from waste metabolize it in a way that releases electrons into the device. Researchers have now developed a new device design that merges features of a battery and a fuel cell to extract more electricity from the bacteria.

Most microbial fuel cells have a design where bacteria grow at the anode, which is separated from the cathode by a membrane. Typically, the cathode transfers electrons to reduce oxygen. However, some of that oxygen typically diffuses across to the bacteria, which can use it directly instead of transferring their electrons to the fuel cell itself.

In the new design, researchers have replaced the cathode with a solid piece of silver oxide. As electrons arrive, this gets reduced to silver. Conveniently, silver is toxic to microbes, which means that the bacteria can't grow on the cathode. That eliminates the need for a membrane to separate the two electrodes, cutting down on the complexity and cost. The key difference between this system and normal microbial fuel cells is that enough of the silver oxide is eventually converted to silver that the reaction stalls. At this point, the silver electrode needs to be pulled out and re-oxidized.

On its own, the setup is very efficient. When given a solution of glucose to feed on, the bacteria at the anode happily digested over 90 percent of it, rapidly draining the system of its fuel source. Various fractions of the energy in the glucose went into the bacteria's growth and maintenance, but nearly half of the energy involved ended up being made available as electricity.

That's the good news. The bad news is that as much as half of that went back into the process of re-oxidizing the cathode. That left the final electricity production at somewhere between 20 and 33 percent, depending on factors like the precise method used to reset the cathode to silver oxide. That's not brilliant, but remember, these things can run on things like municipal wastewater, which currently requires energy for processing.

Although the device works on wastewater as it's currently configured, the authors note that silver is far too expensive to be widely deployed as a cathode material, especially given that production versions of these systems would probably need several electrodes to keep them operating while the used ones are getting re-oxidized. The good news is that, if we're redesigning the electrodes and their composition, it might be possible to design ones that would spontaneously re-oxidize in the air.

But that still doesn't handle the complexity issue. If the new cathode isn't toxic to bacteria, then we're back at the point of needing a membrane to keep the two parts of the fuel cell separated. And we'd end up adding a significant amount of complexity in terms of designing a system that allows its cathodes to be swapped out. It'll take a careful design to make sure that the added complexity doesn't swamp the benefits of any improved efficiency that comes out of these microbial batteries.

PNAS, 2013. DOI: 10.1073/pnas.1307327110 (About DOIs).