New rechargeable flow battery generates three times the power density of membraneless systems and ten times better than lithium ion

MIT researchers have engineered a new rechargeable flow battery that doesn’t rely on expensive membranes to generate and store electricity. The palm-sized prototype generates three times as much power per square centimeter as other membraneless systems — a power density that is an order of magnitude higher than that of many lithium-ion batteries and other commercial and experimental energy-storage systems.

The device stores and releases energy in a device that relies on a phenomenon called laminar flow: Two liquids are pumped through a channel, undergoing electrochemical reactions between two electrodes to store or release energy. Under the right conditions, the solutions stream through in parallel, with very little mixing. The flow naturally separates the liquids, without requiring a costly membrane.

The reactants in the battery consist of a liquid bromine solution and hydrogen fuel. The group chose to work with bromine because the chemical is relatively inexpensive and available in large quantities, with more than 243,000 tons produced each year in the United States.

In addition to bromine’s low cost and abundance, the chemical reaction between hydrogen and bromine holds great potential for energy storage. But fuel-cell designs based on hydrogen and bromine have largely had mixed results: Hydrobromic acid tends to eat away at a battery’s membrane, effectively slowing the energy-storing reaction and reducing the battery’s lifetime.

“Here, we have a system where performance is just as good as previous systems, and now we don’t have to worry about issues of the membrane,” Bazant says. “This is something that can be a quantum leap in energy-storage technology.”

Possible boost for solar and wind energy

Low-cost energy storage has the potential to foster widespread use of renewable energy, such as solar and wind power. To date, such energy sources have been unreliable: Winds can be capricious, and cloudless days are never guaranteed. With cheap energy-storage technologies, renewable energy might be stored and then distributed via the electric grid at times of peak power demand.

“Energy storage is the key enabling technology for renewables,” Buie says. “Until you can make [energy storage] reliable and affordable, it doesn’t matter how cheap and efficient you can make wind and solar, because our grid can’t handle the intermittency of those renewable technologies.”

By designing a flow battery without a membrane, Buie says the group was able to remove two large barriers to energy storage: cost and performance. Membranes are often the most costly component of a battery, and the most unreliable, as they can corrode with repeated exposure to certain reactants.

The flow battery worked at room temperature over a range of flow rates and reactant concentrations. They found that the battery produced a maximum power density of 0.795 watts of stored energy per square centimeter.

The membraneless flow battery may produce energy costing as little as $100 per kilowatt-hour.

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