A team of researchers at MIT has come up with a membrane-free flow battery, and that could mean that the solar powered future we’ve been looking forward to is even closer than we thought. Flow batteries have a huge advantage when it comes to utility-scale energy storage, especially for intermittent sources like solar power and wind, too, for that matter.
The problem has been that conventional flow batteries work by separating two streams of liquid (hence, “flow”) with a membrane, and coming up with an efficient but cost-effective membrane has been one of the obstacles to engineering a commercially viable system.
A Membrane Free Flow Battery
The MIT flow battery solution is elegantly simple: get rid of the membrane! Now, that’s been tried before with mixed results, but the MIT team seems to be on to something. So far, their palm-sized prototypes have achieved a power density significantly higher than other membrane-less designs, and “an order of magnitude” beyond lithium-ion batteries.
To understand how it works, let’s have a brief recap of basic flow battery technology. The idea is to pump a solution of metal ions dissolved in an electrolyte through an electrochemical cell, in which another liquid awaits. Separated by a membrane, the two liquids exchange ions to create an electric current.
The advantages almost hit you over the head. Aside from the membrane, almost the entire system consists of tanks and pumps, which means that you have the potential for an extremely low maintenance, long-lifespan piece of equipment that can be scaled up with a few relatively simple engineering tweaks. In addition, flow batteries can sit idle indefinitely and be called into action quickly when needed.
As described by MIT writer Jennifer Chu, a hydrogen-bromine combination is potentially ideal for flow batteries, since both are relatively cheap and abundant. However, hydrobomic acid likes to munch on membranes, which severely curtails the battery’s lifespan.
To get rid of the membrane, the MIT team relied on a form of parallel flow called laminar flow, in which two liquids stay on their respective courses with little mixing, even though no separating membrane is present.
Basically, the new battery was engineered with a slim channel between two electrodes. As Chu explains:
“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.”
At room temperature, the battery achieved a maximum power density of 0.795 watts of stored energy per square centimeter. That puts the device on track for the golden $100 per kilowatt-hour mark, which is the goal set by the U.S. Department of Energy for commercially viable, utility-scale advanced battery storage.
MIT Is Not The Only Member Of The Membrane Free Club
The membrane free flow battery thing is becoming a bit of a horse race, since a team at Stanford University has come up with an alternate approach, using a single stream of liquid (a lithium polysulfide solution) making contact with a piece of lithium metal.
The Stanford team has been working with the Department of Energy’s SLAC National Acelerator Laboratory on the project, which coincidentally has also resulted in a palm-sized prototype.
Meanwhile, over at Pacific Northwest National Laboratory, a company called UniEnergy Technologies has been working with federal researchers to develop a flow battery that packs more energy into a smaller system, based on vanadium ions and hydrochloric acid.