In the batteries that power your laptop and cellphone, the charge carrying material is an integral part of the physical structure, moving from one electrode to the other during charge and discharge cycles. When current isn't being input or extracted, the charge carriers stay where they are.

But it doesn't have to be that way. There are alternatives, called flow batteries, where the charge carriers are in a liquid that flows past the electrodes (hence the name). While flow batteries have a much lower energy density than a lithium ion battery, they have a big advantage: the amount of electricity they can store is limited only by the size of the liquid storage tanks. The complicated and expensive parts of the system—the electrodes and membrane separating the two charge carriers—can be relatively compact and still handle a large amount of charge storage.

But flow batteries are pretty bulky, so they haven't received the sort of attention that's been given to the hardware that can power our laptops. But that doesn't mean research isn't ongoing. This week, a team from Harvard described a way of creating a flow battery out of very inexpensive chemicals.

Flow batteries are pretty simple in principle. Two large tanks hold solutions of different charge storage chemical. When charging or discharging is required, pumps force the two solutions into a chamber where they're separated by a membrane. This allows a charge-transfer reaction to take place, with the charges intercepted by the hardware to provide an electrical current. The process can be reversed, and the amount of current that's ultimately produced is limited by the size of the storage tanks.

Aside from the bulk involved in large liquid storage tanks, flow batteries have a variety of additional limitations. They typically operate in extremely acidic solutions, and require metals that aren't as cheap or common as we'd like. So, the researchers tried different formulations, eventually settling on a combination of organic molecules and a common iron-containing compound. Rather than acidic conditions, the batteries work in a basic solution.

(The authors make a big deal about the dangers of the acidic solutions involved in other designs. But their own test battery involved a one Molar solution of potassium hydroxide, which is an extremely strong base that probably requires protective gear to handle. So, in this aspect at least, they appear to have exchanged one challenge for a second.)

One of the charge carriers involved is a solution of ferrocyanide—basically a single iron atom bonded to six cyanide molecules. This molecule can shift between -3 and -4 charge states, providing one means of charge storage. Despite the cyanide, it's completely non-toxic, and has even been approved as a food additive. Salts of this chemical are commonly blue, which came in handy when the test apparatus sprung a leak and the authors had to locate it.

The other chemical is an organic compound called a quinone, which consists of benzene-like rings bonded to oxygen atoms. The modified quinone used here also contained alcohol groups linked to the ring. In basic conditions, these alcohol groups give up their hydrogen and pick up negative charges, which they can then exchange within the flow battery. The charge carrying capacity of a typical quinone molecule is simply set by the number of oxygen's present.

Used within the battery, both of these chemicals—the quinone and the ferrocyanide—are negatively charged, so they don't tend to cross the membrane and undergo chemical reactions with each other. The test battery had decent charge density and operated at roughly room temperature. The only apparent loss of charge after repeated cycling appeared to be due to a small leak in the hardware that allowed some of the solution to escape.

This clearly isn't going to solve all of the limitations of flow batteries—the storage density issue is definitely still there, so you won't hear your laptop's battery sloshing around any time soon. But it does mean that we can potentially build the batteries with relatively cheap and readily available components; ones that are somewhat less toxic, as well. With a number of states pushing for grid-scale electric storage, this might be enough to drag the technology out of the research lab and into the field.

Science, 2015. DOI: 10.1126/science.aab3033 (About DOIs).