A team of Imperial College London engineers and chemists has adopted a new approach to battery design that they say could offer the key to low-cost, long-term energy storage.
The breakthrough involved the creation of a polysulfide-air redox flow battery that features two membranes. The dual-membrane design overcomes the main problems with this type of large-scale battery, opening up its potential to store excess energy from, for example, renewable sources such as wind and solar.
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In redox flow batteries, energy is stored in liquid electrolytes that flow through the cells during charge and discharge. The amount of energy stored by the battery is determined by the volume of the electrolyte, making it potentially relatively easy to scale up. However, vanadium, the electrolyte used in conventional redox flow batteries, is expensive and primarily sourced from either China or Russia.
The Imperial team has been working on alternatives that use lower-cost materials that are widely available. Their approach uses a liquid as one electrolyte and a gas as the other – in this case polysulfide (sulphur dissolved in an alkaline solution) and air. However, the performance of polysulfide-air batteries is limited because no existing membrane can fully enable the chemical reactions to take place while also preventing polysulfide from crossing over into the other part of the cell.
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‘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 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 solution devised by the researchers was to use two membranes to separate the polysulfide and the air, with a solution of sodium hydroxide between them. The advantage of the new design is that all of the materials, including the membranes, are relatively cheap and widely available. The design also offers far more choice in the materials that can be used.
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The new design was able to provide significantly more power – up to 5.8 milliwatts per square centimetre – than the best results obtained to date from standard polysulfide-air redox flow batteries. The battery’s energy cost – the price of the storage materials in relation to the amount of energy stored – was around US$2.50 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 US$1,600 per kilowatt. This is currently higher than would be feasible for large-scale energy storage, but the team believes that further improvements are readily achievable.
‘Our dual-membrane approach is very exciting as it opens up many new possibilities, for both this and other batteries,’ said Professor Nigel Brandon, Dean of the Faculty of Engineering. ‘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.’
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The spin-out company RFC Power, established to develop long-duration storage of renewable energy based on the team’s research, is set to commercialise the new design should the required improvements be made. ‘There is a pressing need for new ways to store renewable energy over days, weeks or even months at a reasonable cost,’ said Tim Von Werne, CEO of RFC Power. ‘This research shows a way to make that possible through improved performance and low-cost materials.’
The research has been published in Nature Communications.