As a storage solution for renewable energy, scientists see great potential in what are known as redox flow batteries, which hold energy in large tanks rather than compact electrode materials. A new design from Sweden’s Linköping University is a decidedly green version of this technology, swapping out scarce metals and synthetic polymers for all-natural materials.

The reason redox flow batteries are such a promising alternative to lithium-ion batteries when it comes to the intermittent nature of renewable energy is because they can store vast amounts of energy at relatively low cost. While lithium-ion batteries store energy in their electrodes and the capacity is therefore limited by the size of the device, redox flow batteries can store energy in liquid electrolytes housed in huge external tanks for months at a time.

Something that doesn’t help the eco-credentials of redox flow batteries, however, is their use of a scarce and expensive metal known as vanadium. This metal is the basis of the electrolyte solution and offers great reliability during charging and discharging, but some researchers see a greener alternative in water-based electrolytes, including the team behind the world’s largest redox flow battery in Germany.

Another area with room for improvement is the electrodes of redox flow batteries, which are typically made from a synthetic polymer called carbonized polyacrylonitrile. We’ve seen some inventive approaches to producing these components in more sustainable ways, including an MIT study that aims to craft them from ingredients in shrimp shells, but now the Linköping University team is putting forward another solution while solving the electrolyte issue at the same time, producing what it bills as the first all-organic redox flow battery.

The team’s redox flow battery features electrodes made from PEDOT, which is an organic and conducing polymer we’ve also seen used in advanced lithium-ion battery designs, and even “smart bricks” that store energy. The engineers doped their PEDOT polymer to enable it to transport the battery's positive and negative ions, and work nicely with a water-based electrolyte laden with quinone molecules, which occur naturally in forest-based materials.