Although we’ve developed lots of ways to produce electricity, there is no way to store an actual electrical current, which is basically just electrons flowing between atoms. That makes electricity “the ultimate perishable commodity,” Form Energy co-founder and Chief Scientist Yet-Ming Chiang said in an interview for the NOVA documentary “Chasing Carbon Zero.” “If you don’t use it, you have to dissipate it or you have to store it.” And storing it means saving it in a different form. If you’re generating electricity with fossil fuels, you can store them in tanks or stockpiles until you decide to burn them. But excess sunlight and wind can’t be bottled up. One possible solution? Batteries.

With the rise of solar and wind power, the possible structure of a fossil fuel-free electric grid is starting to become clear—and a few key gaps are emerging, Chiang said. In analyzing patterns of electricity use, weather, and availability of renewable energy, Form Energy zeroed in on intervals of up to 100 hours that seemed to appear between days where the weather would allow for solar or wind production. But popular battery materials like lithium are too expensive to be used to bridge such gaps. “In order to do multi-day storage, we have to have batteries that cost about one-tenth or less than today's lithium-ion battery packs,” he said.

And yes, lithium-ion batteries work well for short-term discharge and relatively quick recharge. Plus, they’re light, making them ideal mobile batteries for laptops, cell phones, and electric cars. But for a giant, multi-day battery that is likely to live permanently at a solar or wind-based electricity plant, something heavier would work fine as long as it’s cheaper. “Air is still free,” Chiang said with a laugh, “and iron is one of the most widely produced, lowest cost materials in the world.” That price point will play an important role as Form’s first power plants come online, since the design calls for iron in enormous quantities. Company renderings of a 56 megawatt system show neat rows of hundreds of battery enclosures resembling shipping containers arranged to support a solar panel farm. In a much smaller pilot project in Minnesota, a 1.5 megawatt system will have the capacity to power 400 homes for 100 hours. 

To understand how, it helps to know some basics. A battery is a way to store energy in chemical form and then convert it into an electrical current when needed. Though they take many forms, batteries generally contain a positively charged electrode (a cathode) and a negatively charged electrode (an anode), both immersed in a liquid called an electrolyte. When the battery is supplying power—or discharging—chemical reactions cause the anode to release electrons. They leave the battery through a wire or other conductor to power a device, ultimately flowing back into the battery through the cathode. (The electrolyte allows the reaction to continue by letting ions pass between the anode and cathode.) 

Form’s battery features a thin sheet cathode on one side, an anode made of powdered iron held together with mesh on the other, and a water-based electrolyte in the middle. Air passes through the cathode and reacts with the electrolyte creating negatively charged hydroxide ions on the inside surface of the cathode. Once that surface gets crowded with ions, they move away toward the iron, where they latch on and create iron hydroxide, the first phase of rust. And when they do that, they give up electrons, which can be guided out of the battery in the form of an electrical current. “The battery starts with the anode in a fully metallic form, iron metal,” Vice President of Engineering Zac Judkins says. After 100 hours, that iron is totally rusted through, and the battery runs out.

To recharge the battery—to “unrust” it—a current is sent back through the system, reversing the chemical reactions. “To discharge, we give it oxygen and get out electrons,” Judkins explains. “To charge it, we give it electrons and we get out oxygen.” Introducing electrical current into the system, breaks the rust back down into its oxygen and iron components. The oxygen leaves the cell as bubbles; the iron is left whole and metallic again.

Unrusting might sound strange, but it’s actually a process that’s been harnessed in other arenas as well. Form Energy CEO Mateo Jaramillo points out that occasionally engineers in charge of infrastructure maintenance will apply very light electric current to steel structures like bridges to prevent rust and corrosion. But unrusting is much more difficult to achieve than rusting in a battery, as it can degrade some of the components. Among the company’s technical challenges has been finding a way around such limitations.

Inside Form’s Berkeley warehouse, a lab holds scattered vats of liquid and racks of modules connected to tangles of tubing. The high-pitched whine of battery cyclers against the dull beat of ventilation fans creates an electric symphony. 

A new technology designed for the electric grid requires extensive testing, and this room is devoted to designing tests to identify constraints and answer other questions: Can Form’s batteries function at 50 C? How about -30 C? How much air is best to provide to each cell? How fast do the batteries degrade under various conditions? “Every battery you see here is learning different stuff,” Judkins says. 

Form’s Berkeley facility will remain a center of product testing and engineering, but the plan is to move its manufacturing from the cradle of new technology to a much, well, rustier area. Perhaps unsurprisingly, many of the things you need for an iron-air battery factory are found in steel towns: rivers for moving heavy raw materials, rail lines for shipping finished products, and people with experience working in heavy industry. “This will create real manufacturing jobs in parts of the country that have seen a great loss of jobs from traditional industries and may not have seen themselves as part of this green revolution,” Chiang said. 

That could be good news for Weirton. After considering possible sites all over the U.S., Form Energy broke ground on a production facility in the town in May. It will, somewhat poetically, be situated on the grounds of the old steel mill, the remaining part of which now produces tin plating and employs a much smaller workforce.

“There's a deep sort of cultural knowledge base about working with iron,” Jaramillo says of the town. Plus, Weirton still has access to railroads and a river port. And it helps that ArcelorMittal, now a Form investor, already has roots there.  

All that will come in handy if Form is to meet its goal of delivering batteries to its first clients, utilities providing power in Minnesota, Colorado, and Georgia, starting in 2024. It has gathered over $800 million in funding to make that happen: now it just needs plenty of iron and air.