Silver, the precious metal once thought to protect against werewolves and make magic mirrors, has a new, real talent: helping microbes turn sewage into power.
Engineers have long used microbes to wring out electrical energy trapped in wastewater, and relied on oxygen to soak up the harvested electrons. But these mini power plants can be finicky and leak oxygen and microbes. When the two mingle, the bugs guzzle the gas and short-circuit the system.
Now researchers at Stanford University have replaced bubbling oxygen with solid silver oxide that gobbles up electrons, making a more reliable, rechargeable bacterial battery. The researchers report the finding September 16 in the Proceedings of the National Academy of Sciences.
All microbe-based batteries and fuel cells need a place to send electrons, says coauthor Craig Criddle, “but putting oxygen in there is a problem.” By using a solid material like silver oxide, which turns into silver and releases hydroxide when it collects electrons, it’s easier to control the battery, he says. And because silver naturally repels microbes, using the metal ensures the two parts of the battery stay separate.
In the new batteries, electron-harvesting microbes grow in a thick tangle around a positively charged electrode, called an anode, made out of carbon cloth. Microbes attached to the anode then start snatching electrons from organic compounds dissolved in the wastewater to produce carbon dioxide and clean water.
The electrons stream through the anode and into a wire, creating current that can be used to power devices. On the other end of the wire is the silver oxide cathode (the battery’s negatively charged electrode), which snags the electrons.
When all of the silver oxide is transformed into silver, scientists can remove the metal, release the electrons and convert the metal back to silver oxide using simple chemical processes. Then the silver oxide can be reused.
“This is much, much simpler than our current system,” says Zhen (Jason) He, an environmental engineer at Virginia Tech. Other microbial batteries being developed for sewage plants and polluted bodies of water use oxygen at the cathode because it’s very good at collecting electrons. But gases are hard to control. The oxygen can bubble over to the anode and the microbes can migrate closer to the cathode to swipe the gas for their own energy production — either case risks a short circuit. To prevent spillover between electrodes in these batteries, engineers use complex membrane barriers.
Despite the perks of silver, He notes that it’s pricy and wastewater plants would need large, and thus very expensive, versions of the battery.
Korneel Rabaey, at Ghent University in Belgium, agrees that cost is a concern. He also cautions that bigger versions of the battery may not be as energy efficient as the setup Criddle and his colleagues describe.
In the study, the battery could net 30 percent of the power contained in wastewater, which is comparable to other energy-harvesting methods. But the experiment was done in favorable conditions that may not occur in sewage plants, Rabaey says.
“It’s still early days for this technology, so it’s hard to calculate which system is best,” he says. “But it’s a nice idea.”
