Improving on their previous design, scientists at Harvard University have developed a cheap and highly adaptable flow battery that could prove ideal for storing renewable energy throughout the day. The battery is made using Earth-abundant materials, is much safer than previous designs, and could reach the market in as little as three years.

Lithium-ion batteries are great for smartphones and electric SUVs, but they may not be the best solution to handle the stresses that solar and wind power put on the electric grid. To get the most out of renewables, we need something more flexible, scalable, and most importantly, cheaper than a solid-state battery (Elon Musk would have you use lithium-ion cells for home storage too, but his entry-level Powerwall goes for US$3,000).

So-called flow batteries could be the perfect solution. Such devices store energy in liquid form, in external tanks, and collect or release power by having those liquids exchange ions through a special membrane. This makes flow batteries extremely flexible: their capacity can be tweaked by modifying the size of the tanks, and their power output can be changed by adjusting the area of the membrane. Flow batteries can also remain idle for long periods of time without losing charge and are not affected by temperature extremes.

Prof. Michael Aziz and team have now built on previous work to design a safer and greener flow battery. Their device relies on cheap and abundant elements like carbon, iron and potassium to form compounds that are nontoxic, fire-resistant and generally safer. As such, the battery might find a place in our homes to store the intermittent energy produced by solar panels (or portable wind turbine, if you've got one).

Flow batteries are a relatively new technology and, to put it mildly, their first generation didn't feature the best choice of materials. Such batteries have commonly used vanadium and bromine dissolved in acid as their electrolytes (energy-carrying liquids). Though effective, this combination is expensive, dangerous and even toxic.

In last year's development Aziz and his team designed a flow battery that dropped the vanadium for the more environmentally friendly quinones naturally occurring chemicals involved in photosynthesis. Their latest battery improves on this by replacing bromine with ferrocyanide, which is commonly added to kitchen salt as an anticaking agent.

The new battery also works in an alkaline, not acidic, solution. The higher pH is less corrosive, allowing the tanks to be built out of plastic instead of heavier and more expensive metals. An alkaline environment creates more electrical resistance in the membrane, but this was compensated by increasing the battery's voltage.

According to the team, the cell's current efficiency exceeded 99 percent, while round-trip efficiency was measured at 84 percent. The data also suggests that the battery will have a lifetime of at least 1,900 cycles, which is much longer than a lithium-ion battery.

The Achilles' heel of the technology appears to be its low energy density, or amount of energy that can be stored inside the battery per unit volume. The researchers achieved 19 Wh per liter in their published study, and slightly higher figures using methods they've developed since.

This falls well short of lithium-ion batteries, which are closer to the 300 Wh/L mark. That could make a home-use flow battery out of the question for some (you'll most likely need a roomy basement). But for large-scale applications at the grid level, where space isn't such a concern and cost is the primary issue, these cheap, highly adaptable cells could prove to be the perfect solution.

The scientists are now working on improvements to increase cycle life and reduce the cost of the membrane. They tell us the battery could become commercially available in about three years' time.

A paper describing the advance appears in the latest edition of the journal Science.

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