Although they are significantly less power dense than lithium-ion batteries, redox flow batteries are viewed by many as better suited to large-scale energy storage due to their long lifecycles and better environmental credentials. However, many of today’s flow batteries rely on an expensive vanadium electrolyte, sourced largely from China and Russia.
In light of those challenges, the Imperial team created a polysulfide-air redox flow battery (PSA RFB) with a dual membrane design that uses lower cost materials which are widely available, making it a viable solution for long duration, grid-scale storage of excess energy from renewable sources. The research is published in Nature Communications.
Previously, the performance of polysulfide-air batteries has been limited because no membrane could fully enable the chemical reactions to take place while still preventing polysulfide crossing over into the other part of the cell.
“If the polysulfide crosses over into the air side, then you lose material from one side, which reduces the reaction taking place there and inhibits the activity of the catalyst on the other,” explained Dr Mengzheng Ouyang, from Imperial’s Department of Earth Science and Engineering. “This reduces the performance of the battery – so it was a problem we needed to solve.”
The alternative devised by the researchers was to use two membranes to separate the polysulfide and the air, with a solution of sodium hydroxide between them. According to Imperial, the advantage of the design is that all the materials are relatively cheap and widely available, and that the design also provides a wider choice in the materials that can be used.
Compared with the best results obtained to date from a polysulfide-air redox flow battery, the new design was able to provide up to 5.8 milliwatts per centimetre squared. The team carried out a cost analysis as part of their work, calculating the energy cost – the price of the storage materials in relation to the amount of energy stored – to be around $2.5 per kilowatt hour.
The power cost – the rate of charge and discharge achieved in relation to the price of the membranes and catalysts in the cell – was found to be around $1600 per kilowatt. While this is higher than would be feasible for large-scale energy storage, the team is confident further improvements are achievable to make the system price competitive.
“To make this cost effective for large-scale storage, a relatively modest improvement in performance would be required, which could be achieved by changes to the catalyst to increase its activity or by further improvements in the membranes used,” said Professor Nigel Brandon, Dean of the Faculty of Engineering and co-lead of the research alongside Professor Anthony Kucernak.
A spin-out company, RFC Power Ltd, has been established to commercialise the technology in anticipation that these improvements can be made and the battery design becomes cost-effective.