A new approach to fabricating selective ion transport membranes can reduce the costs and boost the efficiency of water treatment and energy storage systems. The membranes are based on polymers of intrinsic microporosity (PIM) with structures facilitating the transport of molecules and ions based on size.

Amidoxime groups and other hydrophilic functional groups were added to the microporous material to allow small salt ions to pass while retaining large ions and organic molecules. The result is a membrane with a more well-defined structure than that of the costly Nafion membranes commonly used for water purification or in Microporous polymers containing interconnected ionic channels. Source: Imperial College Londonbatteries. The design is also more ion conductive and selective relative to other low-cost polymers deployed for these applications.

Using their new-generation PIMs, the researchers designed cheaper, easily processed membranes with well-defined pores that let specific ions through while keeping others out. They demonstrated the applications of their membranes in organic redox flow batteries using low-cost organic redox-active species such as quinones and potassium ferrocyanide.

Flow battery performance will benefit by use of PIM membranes, as a demonstration unit confirmed high molecular selectivity toward ferrocyanide anions and low crossover of redox species, which undermines battery service life. The membranes also proved highly selective when filtering salt ions from water and in removing organic micropollutants during municipal water treatment. Their capacity to separate lithium ions from magnesium in saltwater could reduce dependence on mined lithium, a major source for lithium-ion batteries.

Researchers from the University of Edinburgh, Imperial College London, the University of Cambridge, the University of Liverpool and Northern Illinois University contributed to this study.