Electrochemical Applications for Two-Dimensional (2D) Materials: Self-Assembly at Liquid|Liquid Interfaces and Assembled 2D Material Laminates

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Abstract

This thesis develops the use of self-assembled 2D materials as electrocatalysts and permeable membranes for hydrogen production or water purification. Such structures have been formed through self-assembly at liquid|liquid interfaces or through the formation of a laminate on a suitable support material. Novel 2D material based catalysts (MoS2, WS2, graphene) have been studied, following their self-assembly at the water|1,2-dichlorobenzene interfaces, and tested electrochemically in-situ as catalysts for hydrogen evolution. The catalytic efficiency of these materials was determined using voltammetry and in-situ UV-visible spectroscopy. It was found that MoS2 demonstrated the highest catalytic performance due to the abundant sulfur edge sites of MoS2 which are active for hydrogen evolution. The stacking of 2D-materials at the interfaces is transformed as membranes using external supporting materials (PVDF support). This provides laminar stacking membranes, which are shown to be excellent candidate materials for use in water purification. This is due to the network of nanocapillary channels formed between individual 2D nanosheets which exhibit a molecular and ionic sieving effect. Simple chemical functionalisation of the MoS2 membranes with dyes (MoS2/CV (crystal violet) and MoS2/SY (sunset yellow)) resulted in the enhancement of ionic rejection and water permeance compared to those reported for graphene oxide (GO) membranes, with long-term stability (no detectable swelling) in aqueous and organic media. Application of an electric field across the membranes was used to investigate ion transport through the nanocapillary channels. It was found that ion transport through MoS2/SY significantly decreased, by 2 orders of magnitude compared to the bulk ion mobility, and exhibited a 10-fold reduction compared to pristine MoS2 as well as the transport parameters reported for GO, Ti3C2Tx and commercial polymeric membranes. The effect of solute concentration, pH, and ionic charge/size on the ionic selectivity of the MoS2 membranes is also studied. Size-selected graphene membranes have also been demonstrated to be capable of charge- and size-selective ion sieving. It was found that lateral flake length and thickness of the graphene play a crucial role in ion transport through laminar stacked membranes. The membrane with smaller flakes exhibits the most tortuous nanocapillary channels, resulting in the reduction of ion transport with high charge selectivity. The graphene membranes also provided excellent Na+ rejection, combined with a higher water permeance than reported for GO and MoS2 membranes. Therefore, the study of the MoS2 and graphene membranes could be scaled up for potential applications in electro-dialysis and ion-exchange for water purification technologies.

Keyword(s)

2D materials (MoS2, WS2, graphene); electrochemistry; filtration/desalination; functionalization; hydrogen evolution; ion transport; ionic sieving; liquid|liquid interface; membrane; nanomaterials; size exclusion

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