Experimental setup of the dual-membrane-structured polysulfide-air redox flow battery. Photo: Imperial College London.
Experimental setup of the dual-membrane-structured polysulfide-air redox flow battery. Photo: Imperial College London.

A new approach to battery design could provide the key to low-cost, long-term energy storage, according to researchers at Imperial College London in the UK.

The team of engineers and chemists have created a polysulfide-air redox flow battery (PSA RFB), with not one but 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 renewable sources such as wind and solar. The researchers describe the new battery in a paper in Nature Communications.

In redox flow batteries, energy is stored in liquid electrolytes that flow through the cells during charge and discharge, as result of chemical reactions. The amount of energy stored is determined by the volume of the electrolyte, making the design potentially easy to scale up. However, the electrolyte used in conventional redox flow batteries – vanadium – is expensive and primarily sourced from either China or Russia.

The Imperial team, led by Nigel Brandon and Anthony Kucernak, have been working on an alternative approach that uses 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 membrane could fully allow 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. This reduces the performance of the battery – so it was a problem we needed to solve,” explained team member Mengzheng Ouyang from Imperial’s Department of Earth Science and Engineering.

Their solution to this problem is to use two membranes, with a solution of sodium hydroxide between them, to separate the polysulfide and the air. The advantage of this design is that all the materials, including the membranes, are relatively cheap and widely available. Furthermore, the design provides far more choice in the materials that can be used.

When compared with the best results obtained to date from a standard polysulfide-air redox flow battery, the new design was able to provide significantly more power, up to 5.8 milliwatts per cm2.

As cost is a critical factor for long-term and large-scale storage, the team also carried out a cost analysis. They calculated that the energy cost – the price of the storage materials in relation to the amount of energy stored – was 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 around $1600 per kilowatt. This is currently higher than would be feasible for large-scale energy storage, but the team believe 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 Brandon, who is dean of the Faculty of Engineering at Imperial. “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.” Work in this area is already underway within the team.

The spin-out company RFC Power Ltd, established to develop long-duration storage of renewable energy based on the team’s research, is set to commercialize this new design should the 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 Ltd. “This research shows a way to make that possible through improved performance and low-cost materials.”

This story is adapted from material from Imperial College London, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.