At the University of Southern California, Professor Sri Narayan is working on a battery that uses iron for one electrode and draws another out of thin air. "The other electrode operates on oxygen you can get directly from air," Narayan says. "That's as clean as you can get."
The advantages of rechargeable iron-air batteries are numerous, he continues. Iron is inexpensive and abundant, relatively nontoxic and cheap to recycle. The battery mass is determined largely by just the iron electrode, making it lighter. Inherent safety concerns of the now-ubiquitous lithium-ion battery are eliminated with the subtraction of a flammable electrolyte.
While Narayan initially developed the technology for use in the electrical grid — energy storage is crucial if variable wind and solar power are to take on a larger role in fulfilling demand — his research has so improved the efficiency of the iron electrode that the battery is now a viable option for electric vehicles.
"It's not that batteries don't work," he says, referring to today's commercially available batteries. "The argument is not about which battery system is better; it's more about which one is economically feasible and durable."
While Narayan sees a few issues with adapting iron-air technology to power mobile technology — both because the battery would likely be too large compared to other components, and because it's an open system, unlike the closed-system batteries generally used in portable electronics. He sees great promise for larger-scale applications, such as providing backup power in data centers.
"Anywhere in the range of one kilowatt-hour and upward will scale reasonably well," Narayan says. "We think iron-air has a really bright future."
Improving on What's There
Dr. Ping Liu, a program director at the Advanced Research Projects Agency-Energy (ARPA-E), has seen metal-air battery technology improve substantially in recent years. Metal-air batteries are not new, Liu says. Hearing aid batteries, for example, operate on a zinc-air system, but currently are not rechargeable.
ARPA-E is providing funding for numerous rechargeable metal-air battery projects, including Narayan's work and that of private-sector companies such as Fluidic Energy and PolyPlus.
"We don't necessarily pick and choose battery chemistries; we use a set of metrics to derive technology," Liu says. ARPA-E has funded metal-air battery research because it meets the agency's cost, weight and volume requirements.
"We focus on the most plausible approaches to get to our desired baseline," he says, adding that grid storage and electric vehicles are only reasonable first markets, and that other applications of the technology likely will be found.
More Ways to Grow
Dr. Yiying Wu, an associate professor of chemistry at The Ohio State University and an associate editor for the American Chemical Society's Applied Materials & Interfaces journal, has worked on a potassium-air battery. It's an exciting area of research, he says, because there are so many combinations and possibilities.
But batteries are also a system, "and you need all of the components to be compatible — electrodes, the electrolyte, the membranes," he says. "It's optimization — identifying the compatible component is the challenging part."
As the technology improves, Wu sees the potential beyond electric-grid energy storage and electric vehicles. The batteries could be used in medical equipment to replace older nonrechargeable zinc-air batteries, as well as in military communications. "So many chemical reactions can be used to build batteries," Wu says. "We just need to find the right one."