DEIONIZATION AND METHOD OF FORMING SAME Field of the Invention

The present invention relates to water treatment technology and more particularly to a method of manufacturing a capacitive deionization (CDI) device for treating water, a CDI device for treating water, an apparatus for treating water, a method of forming an electrode for capacitive deionization and an electrode for capacitive deionization.

Background of the Invention

To address global demand for fresh water, many desalination technologies are being developed. Amongst these, capacitive deionization (CDI) technology shows great potential for application in seawater purification. However, desalination capacity is a limitation preventing commercialisation of CDI technology for desalination. There is therefore a need to develop CDI technology with increased desalination capacity.

Summary of the Invention

Accordingly, in a first aspect, the present invention provides a method of manufacturing a capacitive deionization (CDI) device for treating water. The method includes forming a pair of electrosorptive electrodes, forming a spacer compartment including a spacer layer and forming surface plates. The electrosorptive electrodes, the spacer compartment and the surface plates are assembled to form a flow channel for water treatment with the spacer compartment being arranged between the pair of electrosorptive electrodes.

In a second aspect, the present invention provides a CDI device for treating water. The device includes a pair of electrosorptive electrodes, a spacer compartment between the pair of electrosorptive electrodes and including a spacer layer, and a plurality of surface plates. The electrosorptive electrodes, the spacer compartment and the surface plates are assembled to form a flow channel for water treatment.

In a third aspect, the present invention provides an apparatus for treating water. The apparatus includes a plurality of CDI devices for treating water in accordance with the first aspect. The CDI devices are independently replaceable. In a fourth aspect, the present invention provides a method of forming an electrode for capacitive deionization. The method includes providing a MAX phase material having the formula: M _n+1 AX _n , wherein M is an early transition metal, n is an integer from 1 to 3, A is an element from one of groups 12, 13, 14, 15 and 16 of the periodic table of elements, and X is one of carbon and nitrogen. An extraction process is performed on the MAX phase material to form a MXene material having the formula: M _n+1 X _n and an exfoliation process is performed on the MXene material to form a plurality of hydrophobic functional group terminations on a surface of the MXene material. The MXene material having the hydrophobic functional group terminations is mixed with carbon black and a binder to form an electrode slurry. The electrode slurry is cast onto a substrate to form the electrode.

In a fifth aspect, the present invention provides an electrode for capacitive deionization. The electrode includes a substrate and a layer formed on the substrate. The layer includes: a MXene material having the formula: M _n+1 X _n , wherein M is an early transition metal, n is an integer from 1 to 3, and X is one of carbon and nitrogen, and wherein a plurality of hydrophobic functional group terminations is formed on a surface of the MXene material; carbon black; and a binder.

Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is an exploded schematic perspective view of a capacitive deionization (CDI) device for treating water in accordance with an embodiment of the present invention;

FIG. 2 is a schematic flow diagram illustrating a method of manufacturing the CDI device of FIG. 1 in accordance with an embodiment of the present invention;

FIG. 3 is a schematic block diagram illustrating an experimental set-up for testing the CDI device of FIG. 1 ;

FIG. 4 is a graph showing solution conductivity against time for a batch mode test using the experimental set-up of FIG. 3;

FIG. 5 is a photograph of a plurality of electrosorptive electrodes fabricated in accordance with an embodiment of the present invention; FIG. 6 is a schematic flow diagram illustrating a method of forming an electrode for capacitive deionization in accordance with another embodiment of the present invention;

FIG. 7 is a schematic flow diagram illustrating preparation of a two-dimensional (2D) MXene Ti ₃ C ₂ from Ti ₃ AIC ₂ in accordance with one embodiment of the present invention;

FIG. 8 shows X-ray diffraction (XRD) patterns of Ti ₃ AIC ₂ , hydrofluoric acid (HF)- treated Ti ₃ C ₂ T _x and sodium hydroxide (NaOH)-exfoliated Ti ₃ C ₂ T _x in FIG. 7;

FIGS. 9A and 9B are scanning electron microscopy (SEM) images of the Ti ₃ C ₂ T _x in

FIG. 7;

FIGS. 10A through 10C show adsorption/desorption profiles of a pair of two- dimensional (2D) MXene Ti ₃ C ₂ electrodes with applied cell potentials of 1.0 Volt (V), 1.2 V, 1.4 V and 1.6 V in a sodium chloride (NaCI) solution with initial conductivities of (A) 100 micro-Siemens per centimeter (μβ cm ^"1 ), (B) 500 cm ^"1 and (C) 1000 cm ^"1 , respectively;

FIG. 1 1 is a graph of conductivity against time showing cycle reversibility of the two- dimensional (2D) MXene Ti ₃ C ₂ electrodes at a cell voltage of 1.2 V in a sodium chloride (NaCI) solution with an initial conductivity of 100 cm ^"1 ;

FIGS. 12A and 12B are graphs of removal capacity and charge efficiency, respectively, of the two-dimensional (2D) MXene Ti ₃ C ₂ electrodes with respect to a series of cell voltages in sodium chloride (NaCI) solutions having concentrations of 50 milligram per litre (mg/L) and 250 mg/L;

FIG. 13 is a schematic flow diagram illustrating wet chemical modification and argon (Ar) plasma modification of MXene Ti ₃ C ₂ T _x in accordance with one embodiment of the present invention;

FIGS. 14A through 14C are scanning electron microscopy (SEM) images of the MXene Ti ₃ C ₂ T _x of FIG. 13 at different stages: (A) hydrofluoric acid (HF) exfoliation, (B) sodium hydroxide (NaOH) intercalation and (d) argon (Ar) plasma modification;

FIG. 15A shows X-ray diffraction (XRD) patterns of commercial Ti ₃ AIC ₂ and the MXene Ti ₃ C ₂ T _x of FIG. 13 at different stages;

FIG. 15B shows Raman spectra of the MXene Ti ₃ C ₂ T _x of FIG. 13 at different stages; FIGS. 16A through 16F are transmission electron microscope (TEM) images and selected area (electron) diffraction (SAED) patterns of the MXene Ti ₃ C ₂ T _x of FIG. 13;

FIG. 17A is a transmission electron microscope (TEM) image of one (1) nanowire in Ar plasma modified MXene Ti ₃ C ₂ T _x ; FIGS. 17B through 17D are energy-dispersive X-ray spectroscopy (EDS) images of the distribution of C, Ti and O elements of the Ar plasma modified MXene Ti ₃ C ₂ T _x of FIG. 17A;

FIGS. 18A through 18E show X-ray photoelectron spectroscopy (XPS) survey spectrum and narrow scanning of elements of the MXene Ti ₃ C ₂ T _x ;

FIGS. 19A through 19D show electrochemical performance of MXene Ti ₃ C ₂ T _x after wet chemical and Ar plasma modification;

FIG. 20 is a schematic diagram illustrating the mechanism of a CDI device and the electrokinetic potential at the surface;

FIG. 21 is a schematic block diagram of an electrode assembled in a CDI device in accordance with one embodiment of the present invention;

FIGS. 22A and 22B are photographs of a top view and a side view, respectively, of a capacitive desalination battery cell in accordance with one embodiment of the present invention;

FIGS. 23A through 23F are graphs showing the capacitive desalination performance of Ti ₃ C ₂ T _x after wet chemical modification and Ar plasma modification;

FIGS. 24A and 24B are graphs of conductivity against time showing cycle stability of MXene after wet chemical modification and Ar plasma modification; and

FIGS. 25A through 25C are scanning electron microscopy (SEM) images of argon (Ar) plasma modified MXene Ti ₃ C ₂ T _x .

Detailed Description of Exemplary Embodiments

The detailed description set forth below in connection with the appended drawings is intended as a description of presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the scope of the invention.

Referring now to FIG. 1 , a capacitive deionization (CDI) device 10 for treating water is shown. The CDI or water treatment device 10 includes a pair of electrosorptive electrodes 12 and 14, a spacer compartment 16 between the pair of electrosorptive electrodes 12 and 14, and surface plates 18 and 20. The spacer compartment 16 includes a spacer layer 22. The electrosorptive electrodes 12 and 14, the spacer compartment 16 and the surface plates 18 and 20 are assembled to form a flow channel for water treatment. The electrosorptive electrodes 12 and 14 are assembled in close proximity to each other, but are prevented from contact by the spacer compartment 16. The pair of electrosorptive electrodes 12 and 14 in the present embodiment may be a cathode current collector 12 and an anode current collector 14. As the cathode and anode positions in the CDI device 10 are interchangeable, the electrosorptive electrodes 12 and 14 may be interchanged in alternative embodiments.

The spacer compartment 16 in the present embodiment may be integrated with the spacer layer 22 as a single unit. The spacer layer 22 may be formed of nylon. In one embodiment, the spacer layer 22 may be formed of a commercially available nylon netting having a thickness of about 0.2 millimetres (mm).

In the present embodiment, the surface plates 18 and 20 include a first or bottom plate 18 having an inlet (not shown) and a second or top plate 20 having an outlet 26. The surface plates 18 and 20 may be made of acrylic.

In the embodiment shown, a pair of ion-exchange membranes 28 and 30 is provided with each of the ion-exchange membranes 28 and 30 being assembled between the spacer compartment 16 and one of the electrosorptive electrodes 12 and 14. The pair of ion- exchange membranes 28 and 30 in the present embodiment may be a cation-exchange membrane 28 and an anion-exchange membrane 30. Similar to the electrosorptive electrodes 12 and 14, the ion-exchange membranes 28 and 30 may also be interchanged together with the respective cathode and anode, in particular, a cation-exchange membrane should accompany the cathode and an anion-exchange membrane should accompany the anode.

The cation-exchange membrane 28 may be cut out from sheets of an IONTECH cation-membrane and the anion-exchange membrane 30 may be cut out from sheets of an IONTECH anion-membrane. Each of the ion-exchange membranes 28 and 30 may have a thickness of about 0.1 millimetres (mm).

In the present embodiment, the electrosorptive electrodes 12 and 14 and the surface plates 18 and 20 are coupled to a plurality of bus bars 32 and 34. More particularly, the bus bars 32 and 34 extend through the electrosorptive electrodes 12 and 14 and the surface plates 18 and 20. The bus bars 32 and 34 are provided in the present embodiment to supply voltage or current to the electrosorptive electrodes 12 and 14.

A pair of conducting extensions 36 is provided in the present embodiment along one of the bus bars 32 and 34 for each of the electrosorptive electrodes 12 and 14 to improve electrical connection between the electrosorptive electrodes 12 and 14 and the bus bars 32 and 34. Each pair of the conducting extensions 36 sandwiches the respective cathode 12 or anode 14 to ensure proper electrical connection with the bus bars 32 and 34.

The assembly of the electrosorptive electrodes 12 and 14, the spacer layer 22 and the ion-exchange membranes 28 and 30 forms a capacitive deionization (CDI) cell 38. In the present embodiment, the CDI cell 38 includes a complete assembly of the cathode current collector 12, the spacer compartment 16, the cation-exchange membrane 28, the insulating spacer 22, the anion-exchange membrane 30 and the anode current collector 14.

Although the CDI device 10 of the present embodiment is shown as having only a single CDI cell 38, it should be understood by persons of ordinary skill in the art that the present invention is not limited to having only one (1) CDI cell 38. In alternative embodiments, the CDI device 10 may include more than one CDI cell 38. Advantageously, as each CDI cell 38 may be separately assembled, this allows for a modular design. Accordingly, if one of the cells 38 is damaged, the damaged cell 38 may be easily replaced. Furthermore, such a modular design allows for scalability required by commercial applications. In one embodiment, the CDI device 10 may include a modular assembly of stacks of CDI cells 38 with each CDI cell 38 including at least one (1) pair of high surface area, porous, conductive electrodes with a spacer layer in-between. In such an embodiment, the stack of electrodes are assembled in close proximity to each other, but are separated by an insulating spacer layer, and each CDI cell 38 may be held in place by a plurality of screws with alternating layers of cathodes and anodes connected to the bus bars 32 and 34 to be energized during operation of the CDI device 10.

In a further embodiment, an apparatus for treating water may be provided, the apparatus including a plurality of CDI devices 10 for treating water. Advantageously, the CDI devices 10 may be independently replaceable.

In the present embodiment, the CDI device 10 is provided with a polygonal cross- sectional area. Advantageously, the polygonic configuration of the CDI cell 38 helps ensure that no fluid remains trapped within the spacer cavity or at the sides and that the entire surface area of the electrosorptive electrodes 12 and 14 is fully utilized.

In operation, the electrosorptive electrodes 12 and 14 are energized and polarized by a voltage applied to the bus bars 32 and 34 from a power source (not shown) to adsorb ionic material from water by means of a capacitive deionization process. The power source may be either a constant voltage or a constant current power source. The polarized electrosorptive electrodes 12 and 14 form a positively charged anode and a negatively charged cathode configuration with an electric field set up therebetween. Cations and anions in the water are adsorbed onto the negatively charged cathode 12 and the positively charged anode 14, respectively, and concentrate in the pores of the cathode 12 and the anode 14. Once the pores are saturated with ions, a regeneration process is performed by applying a short-circuit across the electrosorptive electrodes 12 and 14 to reset the electrosorptive electrodes 12 and 14 for the next adsorption cycle. When the CDI cell 38 is shorted, the electric field between the electrosorptive electrodes 12 and 14 is removed and the ions are desorbed back into the water.

Water to be treated passes through the flow channel defined by the assembly of the electrosorptive electrodes 12 and 14, the spacer compartment 16 and the surface plates 18 and 20 and in so doing flows into the space bounded by the electrosorptive electrodes 12 and 14 and the spacer compartment 16 where ionic material in the water is electrostatically removed from the water. In the present embodiment, the flow channel follows a planar format and flow of the water to be treated is from the inlet (not shown) to the outlet 26 in a direction perpendicular to the polygonal cross-sectional area of the CDI device 10. Water first enters through the inlet (not shown) at a bottom or base of the CDI device 10, flows upwards to fill the entire cavity of the spacer compartment 16 and exits from the top of the CDI device 10 as treated water. Advantageously, the flow channel configuration allows the water to be treated to achieve maximum contact with the electrosorptive electrodes 12 and 14, thereby enhancing the capacitive deionization process.

Having described the various components and operation of the CDI device 10, a method of manufacturing the CDI device 10 will now be described below with reference to FIG. 2.

Referring now to FIG. 2, a method 50 of manufacturing the CDI device 10 is shown. The method begins at step 52 when a pair of electrosorptive electrodes 12 and 14 is formed.

A spacer compartment 16 having a spacer layer 22 is formed at step 54 and surface plates 18 and 20 are formed at step 56. The spacer compartment 16 may be formed or fabricated by three-dimensional (3D) printing or additive manufacturing. Advantageously, the use of 3D printing technology enables precise control over dimensions of the spacer compartment 16 and this has a direct influence on the performance of the CDI device 10 as larger units may be easily fabricated using 3D printing.

In the present embodiment, a pair of ion-exchange membranes 28 and 30 is formed at step 58. At step 60, the electrosorptive electrodes 12 and 14, the spacer compartment 16 and the surface plates 18 and 20 are assembled to form a flow channel for water treatment with the spacer compartment 16 being arranged between the pair of electrosorptive electrodes 12 and 14. In the present embodiment, the ion-exchange membranes 28 and 30 are assembled with the electrosorptive electrodes 12 and 14, the spacer compartment 16 and the surface plates 18 and 20 with each of the ion-exchange membranes 28 and 30 between the spacer compartment 16 and one of the electrosorptive electrodes 12 and 14.

In the present embodiment, bus bars 32 and 34 are assembled through the electrosorptive electrodes 12 and 14 at step 62 with the bus bars 32 and 34 being configured to supply voltage to the electrosorptive electrodes 12 and 14. A pair of conducting extensions 36 may be provided along one of the bus bars 32 and 34 for each of the electrosorptive electrodes 12 and 14 in the present embodiment to improve electrical connection between the electrosorptive electrodes 12 and 14 and the bus bars 32 and 34.

Referring now to FIG. 3, an experimental set-up 100 for testing the CDI device 10 of FIG. 1 is shown. Apart from the CDI device 10, the experimental set-up 100 also includes a feedwater tank 102, a peristaltic pump 104, a current recorder or sourcemeter 106, a computer 108 and a conductivity meter 110. In the experimental set-up 100 shown, the CDI device 10 that was tested consisted of a 9 centimetre (cm) by 9 cm CDI cell having two (2) parallel electrodes and a nylon spacer. Batch mode experiments were performed with the experimental set-up 100 shown. Water was passed from the feedwater tank 102 into the CDI device 10 for treatment and effluent water exiting the CDI device 10 was fed back into the feedwater tank 102 as part of each batch mode experiment. A conductivity probe of the conductivity meter 110 was placed at an exit of the effluent water from the CDI device 10 to measure solution conductivity and data collected by the conductivity meter 110 was fed back to the computer 108. Different parameters such applied voltages, electrode materials and initial salt concentrations were evaluated with the experimental set-up 100.

Referring now to FIG. 4, a graph showing solution conductivity against time for a batch mode test using the experimental set-up of FIG. 3 is shown. A linear relationship exists between solution conductivity and concentration in parts per million (ppm). The functionality of the CDI device 10 is demonstrated when a voltage is applied and performance of the CDI device 10 is evaluated under different applied voltages. In the batch mode test performed, the concentration of the solution to be treated was 250 ppm. Electrosorption and desorption cycles for the batch mode test involving three (3) different voltages are shown in FIG. 4. During operation of the CDI device 10, the solution conductivity decreases abruptly before reaching equilibrium, indicating saturation of the electrodes. A regeneration cycle is performed by applying a short circuit across the electrodes to restore solution conductivity. As can be seen from FIG. 4, the decrease in conductivity of the effluent water demonstrates that salt adsorption had occurred. On this basis, it is expected that a stack of electrodes may be used to desalinate brackish water having a concentration of 1000 ppm to 2000 ppm.

Various types of electrosorptive electrodes may be used with the CDI device 10 and these will now be described below with reference to FIGS. 5 through 25C.

Referring now to FIG. 5, a plurality of electrosorptive electrodes 150 fabricated in accordance with an embodiment of the present invention is shown. In the present embodiment, the electrosorptive electrodes 150 are prepared using a spray method. More particularly, the electrosorptive electrodes 150 may be formed by loading a solution of electrosorptive material into a dispenser and spraying the solution from the dispenser onto current collectors. The spraying converts the solution into a stream of droplets. The current collectors are then dried to form the electrosorptive electrodes 150.

The spraying may be performed with aerosols or a spray gun. Advantageously, the spray method of fabrication saves on material cost and increases scalability for large scale desalination of seawater. Further advantageously, spray coating ensures a more uniform coating of material even if the material has odd edges. The scale and thickness of deposited films may be easily controlled via pressure and spray duration. This allows for deposition of thin films on different surface architectures, textures and sizes. Spray fabrication of the electrosorptive electrodes 150 also allows for flexible electrode design, decreases interfacial resistance and enhances CDI capacitance.

In the embodiment shown, activated carbon (AC) is mixed with a polyvinylidene fluoride (PVDF) binder and carbon black in a ratio of 8: 1 : 1 and N-methyl-2-pyrrolidone (NMP) is used as a solvent to prepare the solution. The solution is loaded into a spray gun and sprayed onto a graphite current collector. Coated graphite current collectors are then heated at 80 degrees Celsius (°C) for 6 hours to fully evaporate the NMP and the final products are the CDI electrodes 150.

The volume of solution expelled may be controlled by the size of the spray gun nozzle and the speed of expulsion may be controlled either by hand or a computer.

The spray method creates a thin layer of electrosorptive material onto the current collector which allows for further coating with ion-exchange materials. In this manner, the spray method enables hierarchical deposition of films. Accordingly, in a further embodiment, a solution of ion-exchange polymer may be loaded into the spray gun and be used to coat the electrosorptive electrodes 150 to produce a well adhered ion-exchange membrane. In one embodiment, a solution of ion-exchange membrane material may be sprayed onto the electrosorptive electrodes 150 prior to assembling the electrosorptive electrodes, the spacer compartment and the surface plates. In this manner, ion-exchange membranes may be deposited onto carbon electrodes. Membrane capacitive deionization (MCDI) electrodes may be fabricated in this way.

Referring now to FIG. 6, a method 200 of forming an electrode for capacitive deionization in accordance with another embodiment of the present invention is shown. The method 200 begins at step 202 with provision of a MAX phase material having the formula: M _n+1 AX _n , wherein M is an early transition metal, n is an integer from 1 to 3, A is an element from one of groups 12, 13, 14, 15 and 16 of the periodic table of elements, and X is one of carbon and nitrogen. The MAX phase material may be selected from a group consisting of Ti ₃ AIC ₂ , Ti ₂ AIC, V ₂ AIC, Cr ₂ AIC, Sc ₂ AIC and Nb ₂ AIC.

At step 204, an extraction process is performed on the MAX phase material to form a MXene material having the formula: M _n+ iX _n - The MXene material may be chemically derived from layered MAX phases. Advantageously, MXene materials have good conductivities and hydrophilic surfaces. The MXene material may be selected from a group consisting of Ti ₃ C ₂ , Ti ₂ C, V ₂ C, Cr ₂ C, Sc ₂ C and Nb ₂ C and may have a primary particle size of between about 2 micron (μηι) and about 5 μηι. Advantageously, MXene materials such as Ti ₃ C ₂ T _x have been shown to have high volumetric capacitor and cation interactions with Na ⁺ , K+, Mg ²⁺ and Al ³⁺ .

In one embodiment, the extraction process may include adding the MAX phase material to an extraction solution, and immersing the MAX phase material in the extraction solution for a period of between about 20 hours (h) and about 24 h at a temperature of between about 45 degrees Celsius (°C) and about 50 °C. The extraction solution works to selectively extract the element A from the MAX phase material and may be selected from a group consisting of a HF solution and a HCI/NH ₄ F solution.

At step 206, an exfoliation process is performed on the MXene material to form a plurality of hydrophobic functional group terminations on a surface of the MXene material.

In one embodiment, the exfoliation process may include immersing the MXene material in an exfoliation solution for a period of between about 20 hours (h) and about 24 h at a temperature of between about 45 degrees Celsius (°C) and about 50 °C. The exfoliation solution may be selected from a group consisting of sodium hydroxide and potassium hydroxide.

At step 208, the MXene material having the hydrophobic functional group terminations is mixed with carbon black and a binder to form an electrode slurry. The binder may include polyvinylidene fluoride (PVDF).

The electrode slurry is cast onto a substrate to form the electrode at step 210. The substrate may be carbon paper or carbon cloth.

An electrode for capacitive deionization thus formed includes a substrate and a layer formed on the substrate. The layer includes a MXene material having the formula: M _n+1 X _n , wherein M is an early transition metal, n is an integer from 1 to 3, and X is one of carbon and nitrogen, and wherein a plurality of hydrophobic functional group terminations is formed on a surface of the MXene material, carbon black and a binder. The substrate may be carbon paper or carbon cloth. The MXene material may be selected from a group consisting of Ti ₃ C ₂ , Ti ₂ C, V ₂ C, Cr ₂ C, Sc ₂ C and Nb ₂ C. The MXene material may have a primary particle size of between about 2 micron (μηι) and about 5 μηι and/or an interlayer distance of between about 1.0 nanometre (nm) and about 1.2 nm. The binder may include polyvinylidene fluoride (PVDF).

Referring now to FIG. 7, preparation of a two-dimensional (2D) MXene Ti ₃ C ₂ from a mother phase of Ti ₃ AIC ₂ via hydrofluoric acid (HF) treatment at 50°C for 24 hours is shown. As it is dangerous to use hydrofluoric acid (HF) which is toxic, 1 gram (g) of the mother phase Ti ₃ AIC ₂ is added gradually, for example, in quantities of about 0.1 g, into the hydrofluoric acid with vigorous stirring during the process. After being immersed in the hydrofluoric acid solution for 24 hours, the MXene Ti ₃ C ₂ product may be obtained by centrifuging at a rate of 8000 revolutions per minute (rpm) for 5 minutes (min). This is followed by a thorough rinse with deionised (Dl) water. The rinsing may be repeated three (3) times. The MXene Ti ₃ C ₂ product obtained may have a product yield of 75% (0.75 g Ti ₃ C ₂ for 1 g Ti ₃ AIC ₂ ) after the HF treatment. To exfoliate the multilayer structure and remove the -F terminations on the surface, the MXene Ti ₃ C ₂ product is immersed in a 1 Molar (M) sodium hydroxide (NaOH) solution for 24 hours at room temperature. Thereafter, the MXene Ti ₃ C ₂ product may be rinsed with deionised (Dl) water and obtained by centrifuging at a rate of 8000 rmp for 5 min.

Referring now to FIG. 8, X-ray diffraction (XRD) patterns of the mother phase Ti ₃ AIC ₂ , the intermediate hydrofluoric acid (HF)-treated Ti ₃ C ₂ T _x and the sodium hydroxide (NaOH)-exfoliated Ti ₃ C ₂ T _x in FIG 7 are shown. The powder X-ray diffraction (XRD) patterns in FIG. 8 show an evident peak shift of (002) from 2Θ = 9.7° for Ti ₃ AIC ₂ to a lower angle of 8.6°, suggesting successful removal of the Al layers by the hydrofluoric acid (HF) treatment. The lattice parameter c of Ti ₃ C ₂ was calculated to be 19.9 A, indicating expansion of the interlayer distance by the hydrofluoric acid (HF) treatment. After the sodium hydroxide (NaOH) solution treatment, the (002) peak further shifted to a lower angle 7.4° and the lattice parameter c increased to 25.5 A, which demonstrates the intercalation of sodium ions into the layer structures and -OH groups covering the surface, instead of -F terminations.

Referring now to FIGS. 9A and 9B, scanning electron microscopy (SEM) images of the Ti ₃ C ₂ T _x in FIG. 7 are shown. The particle sizes are shown to be in the range of 2-5 microns (μηι) with a multilayer structure.

To evaluate the effectiveness of using two-dimensional (2D) MXene as an electrode material for capacitive desalination of seawater, a CDI device having two-dimensional (2D) MXene Ti ₃ C ₂ capacitive electrodes was investigated via batch-mode experiments in a continuous recycling system driven by a peristaltic pump. The CDI device included a parallel pair of 5 centimetres (cm) by 5 cm electrodes and a nylon spacer. The two- dimensional (2D) MXene Ti ₃ C ₂ was obtained via controlled etching of a commercially available mother phase of Ti ₃ AIC ₂ . By closely monitoring the temperature of and period in the etching solution of hydrofluoric acid (HF), nanosheets of two-dimensional (2D) MXene Ti ₃ C ₂ with desirable morphology and structure were harvested via hydrofluoric acid extraction of Al from the mother phase Ti ₃ AIC ₂ . Subsequent immersion of the two- dimensional (2D) MXene Ti ₃ C ₂ in a sodium hydroxide (NaOH) solution exfoliated the multilayer structure, removed the -F groups and modified the surface with hydrophobic -OH groups. An electrode slurry mix of the two-dimensional (2D) MXene Ti ₃ C ₂ with carbon black and polyvinylidene fluoride (PVDF) was then cast onto carbon paper using a doctor-blade technique to form the electrodes. During the desalination experiments, the two-dimensional (2D) MXene Ti ₃ C ₂ electrodes were energized by a potential power source. When seawater to be treated flows through the electrodes, the ions in the seawater are adsorbed onto the electrodes by means of a CDI process. The results of the experiments performed will now be discussed below with reference to FIGS. 10A through 12B.

Referring now to FIGS. 10A through 10C, adsorption/desorption profiles of the two- dimensional (2D) MXene Ti ₃ C ₂ electrodes with applied cell potentials of 1.0 Volt (V), 1.2 V, 1.4 V and 1.6 V in a sodium chloride (NaCI) solution with initial conductivities of (A) 100 micro-Siemens per centimeter (μβ cm ^"1 ), (B) 500 cm ^"1 and (C) 1000 cm ^"1 , respectively, are shown. In particular, FIGS. 10A through 10C show the electrosorption- desorption response of the two-dimensional (2D) MXene Ti ₃ C ₂ electrodes on applying a cell voltage ranging from 1.0 V to 1.6 V with different initial conductivities of the NaCI solution. As can be seen from FIGS. 10A through 10C, the salt solution conductivity shows immediate decrease upon charging with the external power. For the same initial salt solution conductivity, the CDI device shows a larger decrease in conductivity with a higher external power. This demonstrates a close dependency of the removal capacity of the CDI device on the external power applied. When the two-dimensional (2D) MXene Ti ₃ C ₂ electrodes were fully saturated and the conductivity reached a stable lowest value, the CDI device was then short-circuited for discharge and the adsorbed ions were then released into the salt solution, leading to recovery of the conductivity to the initial value of the salt solution.

Referring now to FIG. 11 , a graph of conductivity against time demonstrating cycle reversibility of the two-dimensional (2D) MXene Ti ₃ C ₂ electrodes at a cell voltage of 1.2 V in a sodium chloride (NaCI) solution with an initial conductivity of 100 cm ^"1 is shown. In FIG. 11 , the conductivity variations showed reproducible results for several cycles of electrosorption and desorption, indicating good regeneration of the Ti ₃ C ₂ based CDI device for deionization. The notable reproducibility of the MXene Ti ₃ C ₂ based CDI device may be ascribed to the flexible shrinkage/expansion of the two-dimensional (2D) electrode materials.

Referring now to FIGS. 12A and 12B, graphs of removal capacity and charge efficiency, respectively, of the two-dimensional (2D) MXene Ti ₃ C ₂ electrodes with respect to a series of cell voltages in sodium chloride (NaCI) solutions having concentrations of 50 milligram per litre (mg/L) (low concentration) and 250 mg/L (high concentration) are shown. As can be seen from FIG. 12A, the removal capacity of the two-dimensional (2D) MXene Ti ₃ C ₂ electrodes is enhanced with increasing cell voltage. The CDI device also displays a higher ion adsorption capacity at a higher initial concentration than at a lower initial concentration at the same cell voltage. The highest removal capacity of about 13.5 milligram per gram (mg/g) was observed at a cell voltage of 1.6 V for an initial sodium chloride (NaCI) solution concentration of 250 mg/L. The charging efficiency observed is greater than 30 percent (%).

As can be seen from the experimental results, the two-dimensional (2D) MXene Ti ₃ C ₂ provides improved capacitive deionization performance with good gravimetric and volumetric salt adsorption capacity. Advantageously, this facilitates optimisation of deionisation equipment, achieving high performance at a low cost. Although the described experiments have been performed in respect of Ti ₃ C ₂ , it should be understood by persons of ordinary skill in the art that MXene family materials for capacitive desalination applications are not limited to Ti ₃ C ₂ . Other MXene materials may be used in alternative embodiments.

Referring now FIG. 13, a surface modification process involving wet chemical modification and argon (Ar) plasma modification performed on MXene Ti ₃ C ₂ T _x is shown.

The two-dimensional (2D) MXene Ti ₃ C ₂ T _x was obtained via a wet chemical etching method involving a commercial TisAIC ₂ phase in 48 percent (%) hydrofluoric (HF) solution at 50 °C. 1 g of Ti ₃ AIC ₂ was slowly added to the HF solution and the resulting mixture was stirred vigorously for 24 hours to exfoliate the Al layer. The resulting product was thoroughly rinsed with deionised (Dl) water until a neutral pH level. The powder was collected by centrifugation at 8000 revolutions per minute (rpm) for 5 minutes (min) and dried at 60 °C for 12 hours. The MXene Ti ₃ C ₂ T _x thus obtained via exfoliation of Ti ₃ AIC ₂ exhibited a surface with multiple complex functional groups of -F, -OH and/or -O terminals.

The exfoliated powder was then immersed into a 1 Molar (M) sodium hydroxide

(NaOH) solution for 24 hours at room temperature to exfoliate the multi-layered Ti ₃ C ₂ T _x . During the exfoliation process, sodium (Na ⁺ ) ions intercalated in-between the Ti ₃ C ₂ T _x nanosheets and this reduced the attractive forces between them. At the same time, the NaOH solution removed any undesirable -F terminations on the surface of Ti ₃ C ₂ T _x . Finally, the product was rinsed with deionized (Dl) water until the pH value reached a neutral level. A sodium hydroxide (NaOH) solution may thus be used to functionalize the surface with hydroxyl groups and to further delaminate the multilayers. The immersion process in the NaOH solution may partially remove the -F terminals and increase the interlayer distance between the nanosheets.

After immersion in the NaOH solution, argon (Ar) plasma treatment was carried out under low pressure conditions to induce the growth of both anatase Ti0 ₂ and an amorphous carbon layer. Argon (Ar) gas was first introduced into the chamber at a flow rate of 400 standard cubic centimeter per minute (seem) when the vacuum reached a level of 10 ^"4 millibar (mbar). A microwave plasma chemical vapour deposition (CVD) chamber may be pumped to a level of 10 ^"4 mbar for low pressure Ar-plasma modification. Next, the sample was irradiated by plasma at 650 °C using a power of 400 watts (W) for 15 minutes (min) before cooling to room temperature. The final Ar-plasma modified product is termed as Ti ₃ C ₂ T _x . The Ar plasma treatment may be performed to modify the surface structures and increase or enlarge the interlayer distance compared to the hydrofluoric acid (HF)-exfoliated Ti ₃ C ₂ T _x . Residual water and oxygen molecules left inside may be simultaneously excited to strong radicals such as superoxide anions (·0 ^2" ), peroxide anions (·0 ₂ ^2" ) and hydroxyl radicals (·ΟΗ). As a result, the -F terminals of the MXene Ti ₃ C ₂ T _x may be replaced with OH/O- after Ar plasma modification.

In order to investigate the electrochemical and desalination properties, Ti ₃ C ₂ T _x with the aforementioned modifications was prepared as a slurry and coated onto graphite electrodes. The electrode slurry is composed of Ti ₃ C ₂ T _x with carbon black (CB) and polyvinylidene difluoride (PVDF) binder in a weight ratio of 8: 1 : 1 using a minimal amount of N-methylpyrrolidone (NMP) as the solvent. The slurry was casted onto graphite electrodes and dried overnight in a vacuum oven at 80°C.

For characterization, the crystal structures were analysed using an X-ray diffractometer (XRD, Rigaku Corporation) operated at 40 milliampere (mA) and 45 kilovolt (kV) using Cu Ka radiation. Room temperature Raman spectra were collected using a Wintech Alpha 300 Raman system with a piezocrystal controlled scanning stage. The frequency-doubled Nd:YAG green laser (k = 532.25 nanometre (nm), E| _aser = 2.33 electronvolt (eV)) was used as an excitation source and the laser power at the sample surface was controlled at 0.85 milliwatt (mW) to avoid unnecessary heating of the sample. An Olympus 100x air objective was used to produce a laser spot size of about 200 nm. The morphology of Ar-Ti ₃ C ₂ T _x was investigated using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7600F). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were performed on a copper mesh TEM grid using a 200 kV JEOL JEM-2100F field emission microscope equipped with a Gatan Ultrascan CCD camera and EDAX Genesis EDS detector.

For electrochemical measurement, cyclic voltammetry (CV) curves were recorded by an electrochemical workstation (VMP3, Bio-logic, France) using a 3-electrode setup. The working electrode was coated with Ar-Ti ₃ C ₂ T _x , whereas a standard calomel electrode (SCE) was used as the reference electrode and a platinum foil as the counter electrode in a 1 M NaCI solution.

For desalination measurement, capacitive desalination tests were performed in a batch-mode using a continuously recycling system that is driven by a peristaltic pump. The influent saline solution flowed through the CDI cell through the spacer layer and a DC voltage was applied to desalinate the water. Once the solution had been desalinated, the effluent flowed out of the cell and returned to the feed tank. All salt solutions used for the CDI tests are adjusted to an initial concentration of 500 milligram per litre (mg/L) and the volume and temperatures were maintained at 40 millilitre (ml_) and 298 Kelvin (K), respectively. The voltage of the cell was varied from 0.8 V to 1.6 V in order to observe any changes in desalination performance.

The results of the investigations performed will now be discussed below with reference to FIGS. 14A through 25C.

Referring now to FIGS. 14A through 14C, morphology evolution of the MXene Ti ₃ C ₂ T _x of FIG. 13 at different stages: (A) hydrofluoric acid (HF) exfoliation, (B) sodium hydroxide (NaOH) intercalation and (d) argon (Ar) plasma modification is shown. The scanning electron microscopy (SEM) image of FIG. 14A shows a single MXene cluster after HF exfoliation and displays a typical multilayer structure with a total thickness in the range of about 2-5 microns (μηι). As can be seen from FIG. 14B, the MXene Ti ₃ C ₂ T _x maintains the same morphology after wet chemical modification in the NaOH solution and as shown in FIG. 14C, the MXene Ti ₃ C ₂ T _x exhibits a nanowire layer wrapping on the overall surface after the Ar plasma treatment. Since the Ar plasma penetrates radically in-between the MXene Ti ₃ C ₂ Tx layers, the layered structure nanosheets in the middle have also been modified by the Ar plasma treatment.

X-ray diffraction (XRD) and Raman measurements were performed to analyse the MXene Ti ₃ C ₂ T _x after wet chemical modification and Ar plasma surface modification and the results are discussed below with reference to FIGS. 15A through 15C.

Referring now to FIG. 15A, X-ray diffraction (XRD) patterns of commercial Ti ₃ AIC ₂ and the MXene Ti ₃ C ₂ T _x of FIG. 13 at different stages are shown. As can be seen from FIG. 15A, the HF-exfoliated Ti ₃ C ₂ T _x shows an evident shift in the peak (002) to a lower angle as compared to the commercial MAX Ti ₃ AIC ₂ phase, suggesting a successful removal of Al layers after HF exfoliation. A summary of the evolution in c-lattice parameter and interlayer distance is provided in Table 1 below. The cell parameters were calculated from the XRD and elemental composition information.

Table 1

The lattice parameter c was calculated to be 19.4 A for the HF exfoliated Ti ₃ C ₂ T _x which indicates a successful expansion of the interlayer distance after the extraction of the Al layer. The (002) peak was further shifted to a much lower angle of 7.2 ° and the c-lattice parameter increased to 23.15 A, which demonstrates the effective intercalation of sodium ions from the NaOH solution into the layered structures. Ar plasma treatment showed a further delamination of the multilayer into fewer layers with a larger c-lattice parameter of 25.30 A indicated by the further shifted (002) peak to the much lower angle. Though products maintain the fingerprint of Ti ₃ C ₂ T _x after exposure to the Ar plasma, the interesting morphology evolution implies the effective surface modification thorough Ar plasma treatment.

Referring now to FIG. 15B, Raman spectra of the MXene Ti ₃ C ₂ T _x of FIG. 13 after hydrofluoric acid (HF), sodium hydroxide (NaOH) and argon (Ar) plasma modification are shown. More particularly, confocal Raman spectroscopy was used for depth profiling and detailed analysis of features below the surface since Raman spectroscopy is very sensitive to tiny changes in the crystal structures. As can be seen from FIG. 15B, MXene Ti ₃ C ₂ T _x shows similar features after HF exfoliation and NaOH intercalation. The dashed lines indicate the main Raman peaks for Ti ₃ C ₂ T _x . Raman spectroscopy has also proven that surface terminal groups may remarkably change the modulation in the surface structures. The similarity in Raman spectra for Ti ₃ C ₂ T _x nanosheets after HF exfoliation and NaOH intercalation can be understood by the interaction of the complicated terminals including -F, -O and -OH groups. No distinguishable differences in Raman spectra were observed for Ti ₃ C ₂ T _x after HF exfoliation and NaOH intercalation even for depth profiling, implying the homogeneous distribution of surface terminals. In contrast, Ar-plasma modified Ti ₃ C ₂ T _x shows depth dependant Raman spectra: the outmost surface shows a clear trace of anatase Ti0 ₂ vibration modes and graphitic G and D (G/D) bands. The intense peak at 149 cm ^"1 and three other peaks at 390 cm ^"1 , 497 cm ^"1 and 624 cm ^"1 which are marked with stars can be assigned to the E _g(1), E _1g(1) , A _1g & B _1g(2) and E _1g(3) of Ti0 ₂ . The broad graphitic G/D modes between 1000 cm ^"1 and 1800 cm ^"1 are related to highly disordered amorphous carbon. Compared to bulk Ti0 ₂ , the absence of E _g(2) around 197 cm ^"1 and the shift in the peaks for Ti0 ₂ may be ascribed to nanosized structures of the Ti0 ₂ . Depth profiling was obtained by adjusting the motorised Z focus control to approximately 200 nanometres (nm) depth from the top surface. The difference in Raman spectra implies a surface reconstruction of terminal groups after Ar plasma modification.

Referring now to FIGS. 16A through 16F, the detailed evolutions in morphologies and structures after modifications were carefully investigated via transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) pattern and the results of these will now be discussed with reference to FIGS. 16A through 16F. FIGS. 16A through 16C are TEM images and SAED of MXene Ti ₃ C ₂ T _x after wet chemical modification, while FIGS. 16D through 16F are TEM images and SAED of MXene Ti ₃ C ₂ T _x after Ar plasma modification. The TEM image of FIG. 16A demonstrates a side view of the nanosheets at the terminals for NaOH modified Ti ₃ C ₂ T _x showing typical laminar structure. The HRTEM image of FIG. 16B showed that the interlayer distance had expanded to 1.1 1 nm from 0.97 nm after wet chemical modification in NaOH solution. This is in agreement with the result calculated from XRD. The interlayer distance was enlarged due to the spontaneous Na ⁺ ions adsorbed/intercalated onto the surface of the nanosheets and the site of nanosheets between each. The corresponding SAED pattern in FIG. 16C revealed a clearly distinguishable diffraction pattern of Ti ₃ C ₂ T _x via wet chemical modification in NaOH solution. The TEM image of FIG. 16D demonstrated that nanowires were observed at the edges and an outermost surface after Ar plasma modification. The HRTEM image of FIG. 16E showed clear nanosheets with an interlayer distance of 1.23 nm, whereas the corresponding SAED pattern in FIG. 16F exhibited diffraction dots consistent with the crystallinity of the two- dimensional (2D) nanosheet Ti ₃ C ₂ T _x .

Referring now to FIGS. 17A through 17D, the detailed composite of the nanowire- like structure and spatial distribution of all elements were investigated via energy dispersive spectroscopy (EDS) mapping and the results will now be discussed with reference to FIGS. 17A through 17D. FIG. 17A is a transmission electron microscope (TEM) image of one (1) nanowire in Ar plasma modified MXene Ti ₃ C ₂ T _x and FIGS. 17B through 17D show the EDS mapping of carbon (C), titanium (Ti) and oxygen (O) element distribution. The element mapping images show a uniform distribution of titanium and oxygen in the same area. However, carbon is distributed at different regions implying a slight decomposition of the MXene upon Ar plasma modification. In addition, no fluorine was detectable in the nanowires indicating a successful removal of the -F terminals via Ar plasma surface modification.

Referring now to FIGS. 18A through 18E, X-ray photoelectron spectroscopy (XPS) surveys were performed to investigate the surface chemistry of the modified Ti ₃ C ₂ T _x . FIG. 18A shows the XPS survey spectrum and FIGS. 18B through 18E show narrow scanning results of Ti 2p, F 1 s, O 1 s and C 1 s of MXene Ti ₃ C ₂ T _x , respectively. As can be seen from FIG. 18A, the wide scan surveys reveal that HF exfoliations induce the surface with evident -F terminal groups together with some oxygen related -0/-OH terminals. Although treatment in the NaOH solution is supposed to replace some -F groups with -OH terminals, there is still some trace of -F terminals residual on the surface, which could be due to the strong electronegativity of fluorine. Significantly, it is noted from the XPS spectra in FIG. 18C that no -F terminals existed after Ar plasma treatment. Moreover, shifts in the 2p spectrum of Ti, 1s spectra of O and C in FIGS. 18B, 18D and 18E, respectively, show good relation to the Raman spectra, demonstrating slight ratio of Ti0 ₂ and carbons. The weight ratios of the respective elements in Table 1 above reveal that Ar plasma may effectively remove the -F groups and enrich the surface with -O related groups.

It follows from the above that the chemical termination and surface modification via low pressure Ar plasma exposure has been carefully confirmed via Raman, TEM and XPS. The major effect of plasma treatment was determined by the chemistry of the gas and samples, the reactor design and the operating parameters. Besides the merits of well- controllable ability, low pressure plasma may render a highly surface specific region of approximately 100 nm a desirable reconstruction without negatively affecting the bulk properties inside. Under electromagnetic fields, the plasma medium such as energetic ions, electrons, neutrons, photons and free radicals may interact strongly with the surface and enable a variety of surface activation via bond breaking, leading to removal of surface terminal or dissociation of outmost layers. This process may be synergistic etching and modification of the surface-chemical structure simultaneously. From the experiments performed, the main reactive plasma radicals Ar ⁺ may etch away surface terminals such as F and break the O-H bands of the hydroxyl groups, while the strong superoxide and peroxide anions may readily mediate the oxidation of the surface Ti atoms to be Ti0 ₂ and amorphous carbon layer on the Ti ₃ C ₂ T _x , which was confirmed by Raman spectra. Since inert Ar gas was introduced to reduce the radical density of oxygen from the residual air trace in the vacuumed chamber, only limited surface depth less than 200 nm was oxide to be Ti0 ₂ and carbon layer. The inside of the nanosheet (> 200 nm) maintained the phase of Ti ₃ C ₂ T _x , which was verified via Raman and HRTEM. It is noted that low pressure Ar plasma was effective for surface modification of less than 100 nm. However, the final modification feature was also significantly affected by the rough structure of the surface and plasma parameters such as power and exposure time.

Referring now to FIGS. 19A through 19D, the electrochemical properties of MXene Ti ₃ C ₂ T _x after wet chemical and Ar plasma modification were investigated in a 1 Molar (M) sodium chloride (NaCI) solution and these will now be discussed with reference to FIGS. 19A through 19D. In particular, FIG. 19A shows cycle voltammetry (CV) curves of Ti ₃ C ₂ T _x within a potential range from -0.6 to 0.3 V versus a saturated calomel electrode (SCE) at a scan rate of 2 millivolts per second (mV/s), FIG. 19B shows Nyquist plots, FIG. 19C shows the gravimetric capacitance performance and FIG. 19D shows the volumetric capacitance at different scan rates of Ti ₃ C ₂ T _x . The CV curves of Ti ₃ C ₂ T _x in FIG. 19A were compared and both of the CV curves display a typical rectangular shape, demonstrating that Ti ₃ C ₂ T _x maintains the capacitive behaviour in the NaCI salt solution. However, the specific capacitances show great difference as can be seen from the area of the CV. An enlarged area in the CV for the Ar plasma modified MXene Ti ₃ C ₂ T _x indicates an increased specific capacitance compared to the NaOH wet chemical modified sample. A high gravimetric capacitance of 196 farad per gram (Fg ^"1 ) and a volumetric capacitance of 560 farad per centimetre (Fern ^"1 ) was obtained for Ar plasma modified Ti ₃ C ₂ T _x at 2 mV s ^' The capacitive storage performance is much better than that of the NaOH modified MXene. Electrochemical impedance spectroscopy (EIS) analysis reveals the intrinsic distinction between Ti ₃ C ₂ T _x after different chemical modifications. The Nyquist plots in FIG. 19B reveal a quasi-semicircle at high frequency and a linear shape at the low frequency range for both electrodes. The equivalent circuit shown in the insert of FIG. 19B was used to fit the spectra, where R _s , R _c t and Z _w represent the series resistance in the cell, charge transfer resistance and Warburg diffusional impedance respectively. The small quasi-semicircle range of the impedance spectra may be assigned to the charge-transfer resistance in parallel with the double-layer capacitance at the electrode interface, while the slope of the low frequency region is governed by the Warburg diffusion of ions from the surface to the inner of nanosheets. The inherent difference in the resistivity may be distinguished by the lower series resistance and charge transfer resistance for the Ar plasma modified product. As can be seen from FIGS. 19C and 19D, the MXene Ti ₃ C ₂ T _x after Ar plasma modification showed an enhanced gravimetric capacitance and volumetric capacitance performance at different scan rates. This may be related to the expanded interlayer distance, providing readily feasible sites for the physical adsorption and intercalation of Na ⁺ ions into the MXene nanosheets. In addition, the removal of -F terminals and the increase of -O related groups lead to higher hydrophilicity after Ar plasma modification. This may benefit fast ion transfer at the interface of the Ar plasma modified Ti ₃ C ₂ T _x surface due to the lower inherent resistance of the partially oxidized MXene Ti ₃ C ₂ T _x surface with a cover layer of Ti0 ₂ and amorphous carbon. Though the capacitance decreases gradually with increasing scanning rate, MXene Ti ₃ C ₂ T _x after Ar plasma modification displayed a higher capacitance. It follows that the volumetric capacitance of the MXene Ti ₃ C ₂ T _x is relatively higher than that of carbon materials such as activated carbon and graphenes. Compared to carbon electrodes, this high volumetric capacitance is a unique advantage related to the compact loading density of MXene materials as electrode. It should be mentioned that the pore architecture including the pore size distribution, porosity surface area and total pore volumes are of critical importance in determining the desalination performance of carbon materials. Pores with uniform distribution in the range of 2-50 nm are attractive for salt ion adsorption carbon based desalination devices. However, the porous carbon materials may hardly make an electrode with a compact loading mass, which results in the lower volumetric capacity and the low efficiency for commercial desalination.

Referring now to FIG. 20, the mechanism of a CDI device and the electrokinetic potential at the surface is shown. As can be seen from FIG. 20, electrochemical double layers may be formed at the interface of MXene Ti ₃ C ₂ T _x and an AC electrode to neutralize the charged surface which was imposed with an external power. This process may simultaneously cause an electrokinetic potential at the surface since the salt ions are electrochemically adsorbed to the electrodes in the CDI cells. The potential also implies the chemical attraction energy required for ion adsorption into the electrode. When the driving force provided by the external power is weaker than that of inner electrokinetic potential, the ions hardly move across the diffusion layer and are adsorbed to the interface. Simultaneously, co-ions may be effectively expulsed near the interface. The conductivity reaches a stable value when the electrode is fully saturated with adsorbed salt ions. The cell may then be short-circuited for discharge and the adsorbed ions are released into the solution with a high flow rate.

Referring now to FIGS. 21 , 22A and 22B, FIG. 21 is a schematic block diagram of an electrode assembled in a CDI device and FIGS. 22A and 22B are photographs of a top view and a side view, respectively, of a capacitive desalination battery cell. For a real desalination application, the CDI device may be assembled using Ti ₃ C ₂ T _x as one side and an over capacitive activate carbon electrode as the counter side. An anionic membrane may be set at the AC electrode side. As can be FIGS. 22A and 22B, all components of the capacitive desalination battery cell may be assembled together to form a packed CDI cell. During a desalination test, the electrodes may be energized by a potential power source and the salt conductivity and current may be recorded to evaluate removal capacity and charge efficiency. The CDI unit cell may be investigated via batch-mode experiments in a continuous recycling system driven by a peristaltic pump. Referring now to FIGS. 23A through 23F, the capacitive desalination performance of Ti ₃ C ₂ T _x after wet chemical modification and Ar plasma modification was investigated and the results will now be discussed with reference to FIGS. 23A through 23F. FIGS. 23A through 23F reveal the electrosorption response of the modified MXene Ti ₃ C ₂ T _x electrodes with an external cell voltage ranging from 0.8 V to 1.4 V in a sodium chloride (NaCI) salt solution. In particular, FIGS. 23A through 23C are graphs showing the capacitive desalination performance of Ti ₃ C ₂ T _x after wet chemical modification, while FIGS. 23D through 23F are graphs showing the capacitive desalination performance of Ti ₃ C ₂ T _x after Ar plasma modification. The maximum removal rate and capacity were investigated as the key criteria to evaluating the performance of the CDI device. Capacitive deionization (CDI) properties were compared in parallel for MXene Ti ₃ C ₂ T _x after wet chemical modification and Ar plasma modification. The salt solution with an initial conductivity of 1000 micro-Siemens per centimetre ^S/cm) shows immediate decrease upon charging with an external power. A sharp increase in the removal rate reveals effective desalination behaviour for both MXene CDI. The maximum removal rate and removal capacity show great difference for the two types of electrode-based CDI systems. The maximum removal capacity of 22.8 milligram per gram (mg/g) was reached for MXene Ti ₃ C ₂ T _x after Ar plasma modification, which is much higher than that of MXene after wet chemical modification. As can be seen from FIGS. 23B and 23D, both CDI units show an evident decrease in the salt conductivity with an increased external power, demonstrating the dependency of the removal capacity on the CDI power. The increased removal capacity may therefore be assigned to the increased external power as the driving force of the CDI unit. The great difference shown in FIGS. 23C and 23E demonstrates the higher removal rate and capacity for MXene Ti ₃ C ₂ T _x after Ar plasma modification. A maximum removal rate of 9.4 milligram per gram per minute (mg/g min) and removal capacity of 26.8 mg/g were achieved for the Ar plasma modified MXene Ti ₃ C ₂ T _x, which exceeded that of the wet chemical modified product at 0.76 mg/g min and 8.9 mg/g, respectively. At a higher external power of 1.4 V, an increased removal rate and capacity may be obtained at the cost of higher energy consumption.

Referring now to FIGS. 24A and 24B, the cycle stability of the CDI unit was also investigated for the two types of MXene and the results will now be discussed with reference to FIGS. 24A and 24B. In particular, FIG. 24A is a graph of conductivity against time showing cycle stability of MXene after wet chemical modification and FIG. 24B is a graph of conductivity against time showing cycle stability of MXene after Ar plasma modification. The salt removal property showed reproducible results for several cycles via electrosorption and desorption, indicating the good regeneration ability of the MXene Ti ₃ C ₂ based CDI system. The notable reproducibility of the MXene Ti ₃ C ₂ based CDI system may be ascribed to the flexible shrinkage/expansion of the 2D MXene electrode materials.

Referring now to FIGS. 25A through 25C, scanning electron microscopy (SEM) images of argon (Ar) plasma modified MXene Ti ₃ C ₂ T _x are shown.

From the experiments performed, it may be concluded that MXene Ti ₃ C ₂ T _x demonstrated unique structural evolution from the smooth surface of nanosheets to the rough nanowire wrapped on the overall surface of MXene Ti ₃ C ₂ T _x after Ar plasma modification, leading to an enlarged interlayer distance between nanosheets for efficient sodium ion transport and a morphology evolution in the surface microstructure. Additionally, Raman observations revealed slight anatase Ti0 ₂ and graphitic carbon at the outmost layer, while the inside nanosheets maintain the MXene Ti ₃ C ₂ T _x nanosheets. It was thus demonstrated that Ar plasma treatment may be used to effectively modify the surface of Ti ₃ C ₂ T _x without disrupting the inner structures. Furthermore, the surface chemistry evolution showed complete removal of -F terminals and a strong increase in -O terminals after Ar plasma modification. Electrochemical tests revealed both high volumetric and gravimetric capacitance for MXene Ti ₃ C ₂ T _x , which is better than commercial carbons. Accordingly, Ar plasma treatment may not only be capable of increasing the interlayer distance for efficient sodium ion transport, but may also induce the evolution of surface microstructures and chemical terminals. Electrochemical investigations also revealed an enhanced volumetric and gravimetric aspect capacitance for the Ar plasma modified MXene Ti ₃ C ₂ T _x . Capacitive desalination experiments also revealed an improved performance with a higher removal rate of 9.4 mg/g min and removal capacity of 26.8 mg/g when an external power of 1.2 V was applied which exceeds that of conventional porous carbon structures. The CDI unit composed of MXene Ti ₃ C ₂ T _x after Ar plasma modification displayed promising ability for salt adsorption/desorption as the Ar plasma modified Ti ₃ C ₂ T _x was enriched with more available sites for ion adsorption. The results demonstrate that the MXene family may via unconventional surface modification be used for capacitive desalination.

As is evident from the foregoing discussion, the present invention provides a scalable capacitive deionization (CDI) device and apparatus for treating water and a method of manufacturing the CDI device. The present invention also provides an electrode for capacitive deionization having increased desalination capacity and a method of forming the same. Advantageously, because the overall construction of the CDI device of the present invention is modular, additional cells may be added either to improve capacitive deionization performance or to replace existing faulty cells. The modular design also affords the advantage of portability as the CDI unit may be assembled and used anywhere. For example, a portable CDI unit may be used as a supporting assembly for an existing home filtration system or a standalone unit for freshwater in parched areas. Further advantageously, the CDI device of the present invention does not require an additional enclosure or gaskets to prevent water leakage as the enclosed cavity comprising the cathode, the anode and the spacer compartment may be tightly pressed together and secured with screws. A further advantage is the use of a spray method to fabricate large scale capacitive deionization (CDI) electrodes for increased desalination capacity. Apart from being environmentally benign and having excellent electrical conductivities and high specific surface areas, the use of MXenes as the electrode material is also advantageous in achieving an increased salt removal capacity.

While preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to the described embodiments only. Apart from being used for seawater desalination, the present invention may also be applied to treat water contaminated by heavy metals and wastewater from industrial processes. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the scope of the invention as described in the claims.

Further, unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising" and the like are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".