MIT researchers have developed a rechargeable flow battery which dispenses completely with the need for expensive membranes, bringing inexpensive, large-scale power storage for renewable energy purposes one step closer to reality.

While electricity storage via membrane-free systems has long been considered a purely theoretical undertaking with little practical viability, MIT researchers have now developed a fully functional battery which not only operates without the need for a membrane to separate reactants but also surpasses many of its conventional peers in terms of efficiency.

Prototypes of the batteries, which are small enough to fit in the palm of one's hand, are capable of generating three times more power with a given surface area than other systems which dispense with the use of membranes. In addition, their power density is an order of magnitude greater than many commercial battery systems currently on the market, which rely on lithium ions.

The device uses a phenomenon called laminar flow in order to store and release energy without relying upon a membrane. This entails the funneling of two separate liquids through a channel, which causes a electrochemical reaction between a pair of electrodes for either the storage or release of energy.

As laminar flow naturally keeps the liquids separate, enabling them to move together in parallel with a minimal amount of intermingling, an expensive membrane is not required to separate the reactants subsequently.

The prototypes make use of liquid bromine solution and hydrogen fuel as reactants, with the former chosen specifically because of its low cost and ready availability, with 243,000 tonnes manufactured each year in the United States.

The absence of a membrane also greatly facilitates the usage of hydrogen and bromine as reactants. While the reaction between hydrogen and bromine has long been considered one of great promise for energy storage applications, its usage has thus far been hampered by the fact that the hydrobromic acid which forms can eat away at the expensive membrane of a battery, impeding its functionality and lifespan.

Martin Bazant, professor of chemical energy who is co-author of a paper on the research which is set for publication in Nature Communications, says the system is nothing short of a "quantum leap in energy-storage technology."

"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," he said.

The development of reliable, large-scale power storage at reduced cost greatly improves the prospects for widespread deployment of renewable energy  given the dependence of solar and wind solutions on fickle weather conditions and the need to ensure that electricity remain on tap even when the sky is overcast or the air is still.

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