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MIT Researchers Develop a Rechargeable Flow Battery Without a Costly Membrane

18 August 2013

MIT researchers have engineered a 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 membrane-less 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.

“This technology has as much promise as anything else being explored for storage, if not more,” says Cullen Buie, an assistant professor of mechanical engineering at MIT, and a member of the research team. “…this system could potentially have a large practical impact.”

Buie, along with Martin Bazant, a professor of chemical engineering, and William Braff, a graduate student in mechanical engineering, have published their results this week in Nature Communications.

The researchers were able to develop a storage device using laminar flow, where two liquids are pumped through a channel, then undergo 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.

By designing a flow battery without a membrane, the researchers eliminated the most costly component of a battery, and the most unreliable, as a membrane can corrode with repeated exposure to certain reactants.

The prototype flow battery has liquid bromine flow over a graphite cathode and hydrobromic acid flow under a porous anode with a small channel between two electrodes. At the same time, the researchers flowed hydrogen gas across the anode. The resulting reactions between hydrogen and bromine produced energy in the form of free electrons that can be discharged or released.

The researchers were also able to reverse the chemical reaction within the channel to capture electrons and store energy — a first for any membrane-less design, according to the researchers.

In experiments, the team operated the flow battery 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.

In addition to conducting experiments, the researchers drew up a mathematical model to describe the chemical reactions in a hydrogen-bromine system. Their predictions from the model agreed with their experimental results — an outcome that the researchers see as promising for the design of future iterations.

According to preliminary projections, the researchers estimate that their membrane-less flow battery may produce energy costing as little as $100 per kilowatt-hour — a goal that the U.S. Department of Energy has estimated would be economically attractive to utility companies.

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