Nano-scale changes in structure can help optimise ion exchange membranes for use in devices such as flow batteries, new research shows.
Research that will help fine-tune a new class of ion exchange membranes has been published in Nature by researchers at Imperial, which were characterised by colleagues across the UK, including the University of Birmingham.
The results will make it possible to build longer lasting and more cost- and energy-efficient devices such as flow batteries, a promising technology for long-duration grid-scale energy storage, by creating an exchange membrane that lets ions cross rapidly, giving the device greater energy efficiency, while stopping electrolyte molecules from leaking out. Once electrolytes start to leak through, the device gradually loses its effectiveness and its lifespan diminishes. “So the membranes need to be very conductive but also very selective, and this is the key challenge in the field,” said Dr Qilei Song from the Department of Chemical Engineering at Imperial.
Characterisation of the dynamics of water and ion transport within these novel membranes is critical to understanding and enhancing their performance. The combination of NMR techniques provided experimental evidence that was key to understanding why these polymers work so well.
Dr Song has been working on a new generation of synthetic polymer membranes for more than a decade. These are based on a class of materials known as polymers of intrinsic microporosity (PIMs), which have rigid and twisted backbones, like fusilli pasta, shot through with tiny pores. These hour-glass shaped micropores provide ordered channels through which small molecules and ions can travel, while the pore gates function like sieves that block large molecules and ions.
The group has developed a series of polymer membranes with a variety of ion-conductive groups, but challenges remain to be solved.
“Studying and controlling membrane swelling has been particularly important, since this can open up the pores and reduce the material’s selectivity,” said Dr Anqi Wang, the lead author of the latest study.
“By adding hydrophobic pendant groups to the PIM polymer backbone, such as oxadiazole and benzene rings, we are able to tune the local hydrophobicity and hence tailor the pore size and ion transport when the membrane is in an aqueous solution,” he explained.
“One scientific question that puzzled us for many years is how the membrane pore structures change at the nano-scale in hydration states, and how ions are transported,” said Dr Song, “It was essential to combine molecular modelling and advanced characterisations in order to understand the membrane structures, as well as the water and ion transport dynamics in these membranes.”
The team used molecular modelling to study how the membranes swell in water, the effect on pore size, and how ions pass through different locations in the membrane.
“This molecular modelling confirms the effectiveness of tuning the local hydrophobicity around the pores,” said Dr Wang. “Now we know our new membrane swells a little, but its pores are still below the one nanometre diameter that we need to maintain selectivity.”
Based on this result, the team further probed the water and ion dynamics in these sub-nanometre pores. The way that water behaves in the membrane was examined at the nano scale with quasi-elastic neutron scattering (QENS) spectroscopy, carried out by Dr Fabrizia Foglia at University College London and the ISIS Neutron and Muon source at the Rutherford Appleton Laboratory in Harwell.
“With neutron scattering, we are able to differentiate the long-range and local water diffusion within these micropores,” said Dr Foglia. “These results suggest a highly interconnected network of water clusters within its large pores, consistent with the molecular dynamics simulation results.”
Further work to characterise the water and ion diffusion came from Nuclear Magnetic Resonance studies carried out by Professor Melanie Britton’s team at the University of Birmingham. “Characterisation of the dynamics of water and ion transport within these novel membranes is critical to understanding and enhancing their performance,” said Professor Britton. “The combination of NMR techniques provided experimental evidence that was key to understanding why these polymers work so well.”
These experimental results were backed up by tests in a redox flow battery, which performed well and proved stable for almost two months. “This is very important, because these batteries are going to be used for large-scale, long-duration green energy storage,” Dr Song explained. “You want to operate these batteries for several hours and at the same time have a lifetime of 10 or 20 years, and if the battery decays quickly then it cannot be operated for a long time.”
Work on the materials is ongoing, with other avenues now being explored for fine-tuning their structure and function. There is also potential to optimise the membrane material for other applications. One line of research focuses on how to further develop these selective membranes for applications such as lithium extraction. This connects with work the group is pursuing on new membrane processes for energy-efficient resource recovery.
We are currently exploring options for commercialising the technology, and seeking industrial partners to help us develop it further,” said Dr Song.
Professor in Chemical Imaging
Staff profile for Dr Melanie Britton, Senior Lecturer in Physical Chemistry at the School of Chemistry.
