Inorganic 2D Materials for Selective Separation Membranes

Lead Research Organisation: University of Manchester
Department Name: Materials

Abstract

The proposed project aims to investigate the use of a variety of 2D materials for the formation of membranes that can function as selective and tuneable separators for a wide variety of molecular and ionic species. The potential applications of such membranes are vast and include; water filtration (e.g. sea water desalination and the removal of heavy metals, toxic organic contaminants, radioactive elements in environmental remediation), non-aqueous filtration (e.g. catalyst recovery, solvent recycling, ionic liquids etc.), and even energy generation (e.g. redox flow batteries and osmotic power). These membranes will be constructed from a wide library of predominantly inorganic materials; including transition metal dichalcogenides (e.g. MoS2, WS2, MoSe2, WSe2, etc.), hexagonal boron nitride (hBN), and layered metal oxides (e.g. V2O5, MoO3, WO3). Each of these 2D materials can be exfoliated from a bulk form down to a dispersion of individual or few layered thick flakes. These dispersions can then be used to create 'paper' like membranes consisting of stacked layers of the desired 2D material. As well as single component membranes the formation of composite membranes will be performed which consist of a plurality of various different layered 2D materials, allowing us to tune the desired filtration properties.
Current filtration technology requires the use of extremely energy inefficient membranes which require enormous amounts of energy to achieve the desired solvent flux combined with acceptable rejection of the unwanted species. In the case of water reverse osmosis for desalination or purification a polymer based membranes is typically used. To achieve the desired flux of filtered water very large applied pressures must be applied to force the water through while rejecting the dissolved ions (e.g. Na+, Cl-). This requires very large amounts of electricity and is highly inefficient. Due to the intrinsic properties of membranes consisting of 2D materials they can overcome this drawback and allow for vastly increased efficiency of water filtration, as well as simultaneously generating energy through the created chemical potential gradient. This is achieved through the formation of nanocapillaries of solvent molecules which are formed in between the neighbouring layers of 2D crystal. This allows for extremely efficient transport of solvent molecules but very high rejection of undesired hydrated ions which cannot fit into the capillary channels. The defining feature of 2D material based membranes in the spacing between the layers of exfoliated crystal as this determines what the size of allowed species is.
There is currently a great deal of interest in the use of graphene, and graphene oxide (GO), based materials for use as separation membranes, particularly for water filtration. This is fuelled by results which show that GO-based membranes exhibit extremely high water transport while rejecting larger molecular species making them ideal for possible filtration applications. However, despite the promise of these GO membranes there are several key issues which have so far prevented further progress. This includes issues with long term stability and mechanical strength when high pressures inherent to practical filtration are used. Surprisingly, the use of the vast number of similarly structured inorganic 2D materials remains barely touched with only a handful of existing studies that indicate that these other materials show immense promise with high solvent flux as well as excellent rejection properties. The use of composites of both a selection of inorganic 2D materials as well as graphene would allow them to complement one another to selectively remove desired species from a given solvent.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509565/1 01/10/2016 30/09/2021
2181237 Studentship EP/N509565/1 07/09/2018 01/07/2022 Jonathan Aze
EP/R513131/1 01/10/2018 30/09/2023
2181237 Studentship EP/R513131/1 07/09/2018 01/07/2022 Jonathan Aze
 
Description Physical compaction of laminar membranes of 2D materials, using an applied external pressure of 1-20 bar, can alter the properites of these membranes as filters by improving the salt rejection, at the cost of reduced water flux. Theis compaction also improves the stability of the membranes.

The alignment of the individual flakes in these membranes can be observed using the analytical technique polarised raman spectroscopy.

Composite laminar membranes, made of a mixture of different types of 2D material (e.g. graphene and MoS2) show no synergistic effect and do not appear to show in improvment in water flux or salt rejection compared to single-component membranes made of either of the individual material in the composite membrane.
Exploitation Route Interest in 2D laminar membranes is already widespread, particualrly for desalination of seawater of brackish water, or for removing contaminaints in product or waste streams in industrial processes. The outcomes would help create improved membranes that perform better in these tasks.
Sectors Environment,Manufacturing, including Industrial Biotechology