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# Dual Membrane Redox Flow Batteries for Grid-scale Storage

May 13, 2022 by Darshil Patel

## Researchers at Imperial College came up with a new redox flow battery design for low-cost, long-term energy storage solutions.

With the current trend of the energy transition, there is a need for long-duration grid-scale energy storage technologies. Grid storage solutions require long-life and low-cost batteries. Redox flow batteries (RFBs) fulfill these requirements as they are low-cost and have a longer life cycle than conventional rechargeable batteries. Moreover, they exhibit unique benefits, such as enhanced safety, environmentally friendly, quick response time, and flexibility of power and energy.

RFBs have gained significant attention in recent years, and many RFBs have been proposed and demonstrated recently. Among them, polysulfide-air redox batteries are popular due to their low cost.

Researchers at Imperial College London have created a polysulfide-air redox flow battery (PSA RFB) with two membranes. The dual membrane architecture solves some of the significant problems associated with PSA RFBs.

### Introduction to Redox Flow Batteries

RFBs, like conventional batteries, use chemical energy for energy storage. Here, chemical energy is provided by two chemicals dissolved in liquids that are pumped to the stacks separated by a membrane. As chemicals are pumped into the stacks, ion exchange occurs through the membrane while the liquid continues to circulate. The size of the stack dictates the power of the system, and the amount of electrolyte defines the total energy stored in the battery.

In PSA RFBs, the system comprises an alkaline-based oxygen redox couple and a polysulfide redox couple separated by an anionic exchange membrane (AEM). In the discharge cycle, polysulfides oxidize, and oxygen reduces to OH-. However, the membrane cannot fully enable this reaction to occur while avoiding polysulfide crossing to the other part of the cell.

Dr. Mengzheng Ouyang, from Imperial's Department of Earth Science and Engineering, explained, "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 battery's performance—so it was a problem we needed to solve."

Cationic exchange membranes (CEMs) prevent polysulfides from crossing, but they suffer from poor conductivity.

### The Dual Membrane Architecture

In this recent study, the Imperial researchers demonstrated a PSA RFB with two membranes by combining AEM and a CEM in an individual battery to separate polysulfide and air. The dual membrane architecture reduces the membrane resistance and increases the peak power density. Moreover, the materials used for the battery are relatively cheap and widely available.

##### (a) RFB based on a single AEM, (b) RFB based on a single CEM, and (c) Dual membrane RFB. Image used courtesy of Imperial College London

The researchers cycled the battery 80 times at 1mA per square centimeter and demonstrated a round trip efficiency of around 40%. The battery provided a power of 5.8 mW per square centimeter.

Cost is an essential factor for long-term grid-scale energy storage. The researchers calculated the energy cost of around $2.5/kWh. ### Further Improvements The researchers also calculated the power cost - the rate of charge and discharge achieved to the price of the cell materials - during cycling, and it turned out to be$1600/kW. This price is very high to be feasible for grid-scale energy storage. The research team believes that the design can be further improved to reduce these costs, and they reported that the work is already underway.

Professor Nigel Brandon, who led the research team and is Dean of the Faculty of Engineering, said that a relatively modest performance improvement would make the battery cost-effective for large-scale energy storage.