The palm-sized prototype generates three times as much power per cm2 as other membraneless systems – higher than many lithium-ion batteries and other commercial and experimental energy-storage systems, the researchers say.
The battery uses laminar flow, whereby 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, which meanst that a costly membrane is not required.
The MIT researchers built a prototype of a flow battery with a small channel between two electrodes. Through the channel, the group pumped liquid bromine over a graphite cathode and hydrobromic acid under a porous anode. 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.
They were also able to reverse the chemical reaction within the channel to capture electrons and store energy – a first for any membraneless design.
In experiments, the researchers 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.
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. Thus far, 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. However, this does not cause a problem with the membrane removed.
“Contrary to previous opinions that membraneless systems are purely academic, this system could potentially have a large practical impact,” said Cullen Buie, an assistant professor of mechanical engineering at MIT.
“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,” added Martin Bazant, a professor of chemical engineering. “This is something that can be a quantum leap in energy-storage technology.”
Te researchers suggest that this kind of low-cost energy storage could have the potential to foster widespread use of renewable energy, such as solar and wind power. Such energy sources can be unreliable, but 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,” commented Buie. “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.”
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 Bazant sees as promising for the design of future iterations.
“We have a design tool now that gives us confidence that as we try to scale up this system, we can make rational decisions about what the optimal system dimensions should be,” Bazant says. “We believe we can break records of power density with more engineering guided by the model.”
According to preliminary projections, Braff and his colleagues estimate that the membraneless flow battery may produce energy costing as little as US$100 per kilowatt-hour – a goal that the US Department of Energy has estimated would be economically attractive to utility companies.
Buie, Bazant and William Braff, a graduate student in mechanical engineering, published the results in Nature Communications.