Porous polymers provide new ion-transport membrane

Scientists at Imperial College London in the UK have created a new type of membrane that could improve water purification and battery energy storage efforts. Their new approach, reported in a paper in Nature Materials, uses low-cost plastic membranes with many tiny hydrophilic (‘water attracting’) pores.

Currently, ion-exchange membranes known as Nafion are used to purify water and store renewable energy output in fuel cells and batteries. However, the ion transport channels in Nafion membranes are not well defined and the membranes are very expensive. In contrast, low-cost polymer membranes have been widely used in the membrane industry for various applications, from removing salt and pollutants from water to natural gas purification – but these membranes are usually not conductive or selective enough for ion transport.

Now, a multi-institutional team led by Qilei Song in Imperial’s Department of Chemical Engineering and Neil McKeown at the University of Edinburgh in the UK has developed a new ion-transport membrane technology that could reduce the cost of storing energy in batteries and of purifying water. The team developed the new membranes by using computer simulations to build a class of microporous polymers known as polymers of intrinsic microporosity (PIMs) and then alter their building blocks for varying properties.

Their invention could contribute to the use and storage of renewable energy, and boost the availability of clean drinking water in developing nations. “Our design hails a new generation of membranes for a variety of uses – both improving lives and boosting storage of renewable energy such as solar and wind power, which will help combat climate change,” said Song, who is lead author of the paper.

The polymers are made of rigid and twisted backbones, like fusilli pasta. They contain tiny pores known as ‘micropores’ that provide rigid, ordered channels through which molecules and ions travel selectively based on their physical sizes. Furthermore, the polymers are soluble in common solvents so they can be cast into super-thin films, which further speeds up ion transport. These factors mean the new membranes could be used in a wide range of separation processes and electrochemical devices that require fast and selective ion transport.

To make the PIMs more water-friendly, the team incorporated water-attracting functional groups, known as Tröger's base and amidoxime groups. This meant the PIMs could allow small salt ions to pass through while retaining large ions and organic molecules.

The team demonstrated that their membranes were highly selective when filtering small salt ions from water, and when removing organic molecules and organic micropollutants for municipal water treatment. “Such membranes could be used in water nanofiltration systems and produced at a much larger scale to provide drinking water in developing countries,” said Song.

The membranes are also specific enough to filter out lithium ions from magnesium in saltwater – a technique that could reduce the need for expensive mined lithium, which is the major source for lithium-ion batteries.

“Perhaps now we can get sustainable lithium from seawater or brine reservoirs instead of mining under the ground, which would be less expensive, more environmentally friendly, and help the development of electric vehicles and large-scale renewable energy storage,” said Song.

The membranes could also find use in flow batteries, a novel battery technology being developed for large-scale, long-term energy storage, such as required for storing the energy produced by intermittent renewable sources like wind and solar. But current commercial flow batteries use expensive vanadium salts, sulfuric acid and Nafion ion-exchange membranes, which are expensive and limit their large-scale applications.

A typical flow battery consists of two tanks of electrolyte solutions, which are pumped past a membrane held between two electrodes. The membrane separator allows charge-carrying ions to migrate between the tanks while preventing the cross-mixing of the two electrolytes. The cross-mixing of materials can lead to decay in battery performance.

Using their new-generation PIMs, the researchers were able to design cheaper, easily processed membranes with well-defined pores that let specific ions through and kept others out. They demonstrated their membranes in organic redox flow batteries that used low-cost organic redox-active species such as quinones and potassium ferrocyanide. Their PIM membranes showed higher molecular selectivity towards the ferrocyanide anions, and hence a low ‘crossover’ of redox species in the battery, which could lead to a longer lifetime.

“We are looking into a wide range of battery chemistries that can be improved with our new generation of ion-transport membranes, from solid-state lithium-ion batteries to low-cost flow batteries,” said co-first author Rui Tan, a PhD researcher in Imperial’s Department of Chemical Engineering.

The design principles for these ion-selective membranes are generic enough that they could also be extended to many other applications. Examples include industrial separation processes, separators for future generations of batteries such as sodium- and potassium-ion batteries, and electrochemical devices for energy conversion and storage including fuel cells and electrochemical reactors.

“The combination of fast ion transport and selectivity of these new ion-selective membrane makes them attractive for a wide range of industrial applications,” said co-first author Anqi Wang, also a PhD researcher in Imperial’s Department of Chemical Engineering.

Next, the researchers plan to scale up this type of membrane to make filtration membranes. They will also look to commercialize the membrane in collaboration with industry, and are already working with RFC power, a spin-out flow-battery company founded by co-author Nigel Brandon, a professor at Imperial.

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.