a new battery,’ he says.
In spite of this, ‘there’s a big push to try and find alternatives to the vanadium’, says Toghill. Imperial College spin-out RFC Power is using a manganese electrolyte in a hybrid redox flow system that combines a Mn 2+/3+ redox half-cell in an acidic electrolyte, with hydrogen gas. During charging and discharging the hydrogen gas is reduced and oxidised, with protons moving across the membrane.
Using manganese – already commonly found in disposable alkaline batteries – instead of vanadium makes a kilowatt hour of storage at least 10 times cheaper, according to Mezzavilla. When charging the manganese is oxidised and hydrogen gas is produced at the negative electrode, with the reverse reaction occurring on discharge. Using a gas rather than a second liquid electrolyte also eliminates the risk of crossover as the reactants are in two different phases.
The technology could also be future-proofed if a hydrogen economy were to become a reality, with hydrogen widely used as a fuel. ‘It could be very interesting to couple the hydrogen that we are using to the hydrogen infrastructure that could be in place,’ says Mezzavilla. He can see a future scenario where chemical plants could use such batteries to store both energy and hydrogen as a chemical feedstock: ‘using hydrogen opens up a lot of opportunities’.
RFC Power is now moving towards commercialisation. They have been working with Shell to improve their battery performance, and with investment from Imperial fuel-cell spin-out CERES Power are developing a kilowatt-level demonstrator which they expect to be operational by 2024.
Still in the realms of basic research, transition metal oxide cluster complexes are also electrolyte candidates, with high stabilities, solubilities and redox chemistries. Toghill says she favours all-iron batteries because ‘[iron] shifts its redox potential considerably depending on the ligand it’s paired with.’ Her group are researching systems pairing a commonly used non-toxic ferro–ferricyanide electrolyte with iron triethanolamine or triisopropanolamine complexes, although she admits solubility is currently too low.
Others are moving towards organic materials, to create more sustainable metal-free flow batteries. But it isn’t easy to find a system that can compete with the 20 plus years of development that have gone into vanadium flow batteries already. ‘Newer materials that have been produced and explored always have some caveats,’ says Toghill, including the poor long-term stability of the molecules or side reactions that will be detrimental to the cells’ energy capacity.
Finding organic species with solubility in the 1–2M range to provide a high charge density is ‘in itself is a challenge’, says Gray. Adding ionic side chains to molecules can increase solubility, but also risks increasing the viscosity too much. She has been working with Michael Aziz, a chemist at Harvard University in the US, on his quinone-based flow battery which pairs an anthraquinone half-cell with the ferro-ferricyanide couple in basic aqueous solution. His best-performing system so far uses a singly substituted 2,2-propionate ether anthraquinone (2-2PEAQ) whose methyl side chain gives it improved solubility making it a good prospect for a grid-scale organic redox flow battery.
It’s a beautiful hybrid between a lithium-based battery and a traditional flow battery
Not everyone has given up on lithium for grid storage: the lithium–sulfur battery has been around since the 1960s and has a theoretical capacity three times that of the lithium-ion battery. But several factors have hampered their developed as traditional batteries, including the dissolution of intermediate lithium polysulfide species in the electrolyte – known as ‘the polysulfide shuttle’ effect – causes rapid capacity loss and huge volume expansion at the cathode as sulfur reacts. But StorTera are finding ways round these problems with their polysulfide redox single liquid battery.
‘It’s a beautiful hybrid between a lithium-based battery and a traditional flow battery,’ says inventor Palla