A new approach to battery design could hold the key to long-term, low-cost energy storage, according to researchers at Imperial College London.
The team of engineers and chemists created a polysulfide-air redox flow battery (PSA RFB) with not one, but two membranes.
The double-membrane design overcomes the main problems of this type of large-scale battery, opening up its potential to store excess energy from, for example, renewable sources such as wind and solar.
In redox flow batteries, energy is stored in liquid electrolytes that pass through the cells during charging and discharging, which is achieved by chemical reactions. The amount of energy stored is determined by the volume of the electrolyte, making the design potentially easy to scale. However, the electrolyte used in conventional redox flow batteries – vanadium – is expensive and comes mainly from China or Russia.
The Imperial team, led by Professors Nigel Brandon and Anthony Kucernak, worked on alternatives that use widely available lower-cost materials. His approach uses a liquid as the electrolyte and a gas as the other, in this case polysulfide (sulfur dissolved in an alkaline solution) and air.
However, the performance of polysulfide-air batteries is limited because no membrane can allow chemical reactions to take place and at the same time prevent the passage of polysulfide to the other part of the cell.
If the polysulfide passes to the air side, material is lost on one side, reducing the reaction taking place there and inhibiting the activity of the catalyst on the other. This reduces battery performance, so it was an issue we needed to address.
Dr Mengzheng Ouyang, Department of Earth Science and Engineering
The alternative devised by the researchers was use two membranes to separate polysulfide and air, with a solution of sodium hydroxide between them. The advantage of the design is that all materials, including the membranes, are relatively cheap and widely available, and the design offers much more choice in the materials that can be used.
Compared to the best results to date with a polysulfide-air redox flow battery, the new design was able to deliver significantly more power, up to 5.8 milliwatts per square centimeter.
Since cost is a critical factor for large-scale, long-term storage, the team also conducted a cost analysis. They calculated that the energy cost – the price of storage materials relative to the amount of energy stored – was about $2.5 per kilowatt-hour.
The energy cost, the rate of loading and unloading achieved versus the price of the cell’s membranes and catalysts, was found to be around $1,600 per kilowatt. This figure is higher than would be achievable for large-scale energy storage, but the team believes further improvements can be made.
Our double membrane method is very interesting because it opens up many possibilities, both for this one and for other batteries. To make it cost-effective for large-scale storage, a relatively modest improvement in performance would be required, which could be achieved by modifications to the catalyst to increase its activity or by further improvements in the membranes used.
Nigel Brandon, Dean of the Faculty of Engineering.
The team is already working in this area, thanks to catalytic insights from Professor Anthony Kucernak, Department of Chemistry, and research on membrane technology from Dr. Qilei Song, Department of Chemical Engineering.
Spin-off company RFC Power Ltd, set up to develop long-term renewable energy storage based on the team’s research, is ready to bring the new design to market if improvements are made.
There is a pressing need for new ways to store renewable energy for days, weeks or even months at a reasonable cost. This research shows a way to make this possible through improved performance and low cost materials.
Tim Von Werne, CEO of RFC Power Ltd
More information: www.nature.com (English text).
Going through www.imperial.ac.uk