The idea of sewage-powered devices is not new. In fact, it’s existed for more than a century. But finding a particularly efficient (and cost-effective) version of microbial fuel cell technology has been an ongoing challenge for engineers. A new "microbial battery," however, looks like a breakthrough on the efficiency side of the equation.
Researchers at Stanford University say they’ve developed a battery that can convert some 30% of the energy of dissolved organic matter in wastewater into electricity, the same proportion of energy that solar cells can harvest from sunlight. The difference between the battery and a regular microbial fuel cell, researchers say, lies in an electrode made up of silver oxide.
Here's the science: Researchers Yi Cui, Craig Criddle, Xing Xie, and their team realized that the oxygen in their microbial fuel cell design was causing problems. Without oxygen, anaerobic bacteria can feast on dissolved sugars on an anode, then transfer energy (electrons) to a cathode on the other side of a membrane. That cathode is then exposed to oxygen, so the output of the whole process is energy and harmless water. But in Stanford’s MFCs, oxygen kept making it back to the bacteria—and once presented with that option, the bacteria used it instead of sending electrons to the other side. Sometimes, this would result in a fuel cell that only harvested 1% of the energy it could.
So the researchers got rid of the membrane setup. "We're using an electrode that replaces oxygen, and fishes for microbes’ food," Criddle tells Co.Exist.
By taking oxygen out of the equation, and instead using silver oxide in place of the cathode, the team just has to oxidize the electrode (silver oxide) when it gets too loaded with electrons and turns into a hunk of silver. The idea is that while electrons are flowing to the silver oxide, they could be picked up by an external circuit and used to charge something like a phone battery.
And here's what it means: the process can be applied to clean up industrial wastewater from food production, or even pollution-caused "dead zones" where excess nitrogen and phosphorus block out oxygen and marine life.
Still, there’s a couple of caveats. One is that the solar efficiency comparison comes from the proportion of energy harvested from wastewater and sunlight, not the total amount—and wastewater contains significantly less energy overall. The second is that silver oxide remains an expensive component, though the Stanford team is working to hack a stand-in.
"It's a very simple device," Criddle says. "The key, though, is to get materials that replace silver oxide, because they're very expensive to make. We have some ideas, and we are testing some things, and they look promising."