ELECTROCHEMICAL EXFOLIATION OF 2D MATERIALS

Abstract

A method for the production of non-carbon-based 2D materials and/or non-carbon-based 2D material nanoplatelet structures having a thickness of less than 100 nm in an electrochemical cell having a negative electrode which comprises a non-carbon-based bulk 2D material and an electronically conducting material such as a metal.

Description

This application claims priority from GB1521056.0 filed 30 Nov. 2015, the contents of which are incorporated herein in their entirety.

FIELD OF INVENTION

The present invention relates to a method for the production of 2D materials and related nanoplatelet structures.

BACKGROUND

Since the discovery of the remarkable physical properties and chemical stability of graphene, two-dimensional (2D) nanomaterials have attracted significant interest, becoming one of the most active research areas of materials science. The study of 2D materials was originally dominated by research into clays and later oxides but now extends beyond graphene to materials such as hexagonal boron nitride (h-BN), transition metal chalcogenides (TMCs) and carbides (MXene materials that are related to bulk MAX phases).

A common feature of these 2D nanomaterials is that they have strong (in-plane) covalent bonds within each layer but interact with adjacent layers via weak bonds, leading the sheets to stack into 3-dimensional bulk crystals.

Currently, 2D nanomaterials are produced via bottom-up growth by methods such as chemical vapor deposition or by top-down approaches such as micromechanical cleavage or liquid exfoliation of the bulk layered crystals by ultra-sonication. However, these routes are unable to produce large quantities and/or the products suffer from very small sheet size, making them unsuitable for many applications such as energy storage, composites, and catalysis.

DESCRIPTION OF THE INVENTION

Herein, a simple and scalable approach is introduced, in which the exfoliation of the layered compounds is driven by gentle electrochemical reactions. While electrochemical exfoliation of graphite to produce graphene has been reported by the present inventors (including, for example, WO2012/120264, WO2013/132261, and WO2015/019093, each of which is incorporated by reference in its entirety), the special electronic properties of other 2D materials can preclude this type of exfoliation; as the materials are insulators or semi-conductors, electronic intercalation and subsequent exfoliation cannot be readily used.

The present inventors have found that, through the provision of a composite electrode comprising the bulk 2D material to be exfoliated and an electronically conducting material such as a metal, this problem can be overcome. Surprisingly, the presence of the metal in the composite body of the electrode enables efficient intercalation between the stacked 2D layers to provide efficient exfoliation and excellent mono and few layer non-carbon-based 2D materials. This has been demonstrated even with bulk 2D materials which are known to be good insulators such as h-boron nitride.

Without wishing to be bound by any particular theory, the inventors attribute the efficient exfoliation at least in part to the presence of the metal lowering the current needed to effect the intercalation, even in cases where the non-carbon-based 2D material is a semi-conductor. Lower currents reduce electricity consumption, may reduce the occurrence of unwanted side effects and may be safer to scale commercially.

The resultant 2D crystals are significantly larger in size than any reported wet-chemical technology for mass production of 2D materials. Many of these materials are readily dispersible in various organic solvents such as NMP and DMF and offer a variety of potential uses.

The present invention relates to methods for the production of 2D materials and 2D material nanoplatelet structures from non-graphitic sources. The 2D materials produced are not graphene or functionalized graphene.

In a first aspect, the present invention provides a method for the production of non-carbon-based 2D materials and/or non-carbon-based 2D material nanoplatelet structures having a thickness of less than 100 nm in an electrochemical cell, wherein the cell comprises:

(a) a negative electrode which comprises a non-carbon-based bulk 2D material and an electronically conducting material;

(b) a positive electrode; and

(c) an electrolyte;

and wherein the method comprises the step of passing a current through the cell.

During the step of passing the current through the cell, the non-carbon-based bulk 2D material is electrochemically exfoliated to afford non-carbon-based 2D materials and/or non-carbon-based 2D material nanoplatelet structures having a thickness of less than 100 nm.

It will be appreciated that the non-carbon-based bulk 2D material and electronically conducting material are suitably provided as an admixture or composite of flakes or powders.

The negative electrode is the electrode held at the most negative potential out of the two electrodes. The negative electrode is commonly referred to in electrochemistry as the cathode.

A reference electrode may also be used.

Suitably, the electrode comprising a non-carbon-based bulk 2D material and an electronically conducting material (e.g a metal) is provided as a composite comprising a binder. Suitable binders include polymers. For example, polyvinyl alcohol (PVA), cellulose, polyaniline (PANI), and polyvinylidene fluoride (PVDF). It will be appreciated that suitable polymers may include those that are known to be soluble in water or other solvents, which may aid isolated of the final product.

The electrode may be provided as an admixture of a non-carbon-based bulk 2D material and an electronically conducting material (e.g. a metal) in a membrane or mesh, reducing or obviating the inclusion of a binder.

Suitably, the electronically conducting material is a metal. Suitably, the metal is a transition metal.

Preferably, the electronically conducting material is ferromagnetic. This assists removal of after exfoliation (a suspension of the exfoliated material may simply be stirred with a magnetic flea, to which the particles attach). Suitable ferromagnetic materials include iron, nickel and cobalt. Suitable alloys may also be used. In some cases, the metal is nickel.

In some cases, the method includes the step of preparing the electrode comprising a non-carbon-based bulk 2D material and an electronically conducting material such as a metal. The method may comprising forming a pellet comprising a bulk 2D material powder, the electronically conducting material (e.g. metal) and a binder. Suitably, the method comprises:

• • i. forming a slurry of a bulk 2D material, an electronically conducting material (e.g. a metal) and a binder in a solvent (for example, an alcohol such as ethanol);
  • ii. optionally milling the slurry, for example using alumina milling media;
  • iii. drying the slurry to obtain a powder;
  • iv. pressing the powder of step iii to form a pellet, for example, using a hydraulic press;
  • v. sintering the pellet of step iv.

Suitably, the bulk 2D material and electronically conducting material (e.g. metal) are provided as powders.

Suitably, the sintering step takes place at 150° C. or more, preferably at 200° C. or more; more preferably at 250° C. or more. For example, the pellet may be sintered under argon at about 300° C.

In a further aspect, the invention provides a pellet comprising a non-carbon-based bulk 2D material powder, an electronically conducting material (e.g a metal) and a binder, for example, a pellet obtainable from steps i to v.

In a further aspect, the invention provides a method of preparing a pellet comprising a non-carbon-based bulk 2D material and an electronically conducting material (e.g a metal), the method comprising steps i to v detailed above.

In a further aspect, the present invention provides a non-carbon-based 2D material or nanoplatelet structure obtainable by a method as described herein.

It will be appreciated that all options and preferences are combinable, except where context dictates otherwise. For example, options and preferences relating to the method apply equally to pellet and vice versa.

Definitions

2D Material

In the present application, the term “2D material” is used to describe materials consisting of ideally one to ten layers, preferably where the distribution of the number of layers in the product is controlled. The method can also be used to make 2D nanoplatelet structures under 100 nm in thickness, preferably under 10 nm in thickness and more preferably under 1 nm in thickness. The size of the flakes produced can vary from nanometres across to millimetres, depending on the morphology desired.

In some aspects of the invention, the material produced is a 2D material having up to ten layers. The 2D material produced may have one, two, three, four, five, six, seven, eight, nine or ten layers.

In other aspects of the invention, the material produced may comprise at least 10% by weight of 2D material having up to ten layers, preferably at least 25% by weight and more preferably at least 50% by weight of 2D material having up to ten layers.

In some cases, the method produces primarily or only non-carbon-based 2D materials. In other cases, the method produces both non-carbon-based 2D materials and/or non-carbon-based 2D material nanoplatelet structures having a thickness of less than 100 nm. It will be appreciated that, under certain conditions, the method may produce primarily non-carbon-based 2D material nanoplatelet structures having a thickness of less than 100 nm. In other words, the average number of layers of the material produced may vary from mono and few layer to 10 or more layers.

Non-Carbon-Based 2D Material

Carbon-based 2D materials are becoming increasingly well-known. They feature sheets of fused six-membered carbocyclic rings in a honeycomb arrangement. The most famous carbon-based 2D material is graphene, which is composed of planar sheets of unsaturated carbocyclic rings (benzenes) with a delocalised π electron system over the entire sheet. Carbon-based 2D materials may also contain non-carbon groups, and the term carbon-based 2D materials includes these partially saturated and entirely saturated compounds. For example, graphane is a saturated carbon-based 2D material comprising sheets of fused 6-membered carbocycles in which each carbon bears a hydrogen atom. Both 1-sided and 2-sided graphane are known. Other carbon-based 2D materials include, without limitation, graphene oxide and fluorographene.

Non-carbon based 2D materials do not include carbocyclic rings. Examples of non-carbon-based 2D materials include hexagonal boron nitride (h-BN), transition metal chalcogenides (TMCs), and metal carbides.

In some cases, the non-carbon-based 2D material of the present invention is 2D h-BN. In some cases, the non-carbon based 2D material of the present invention is a transition metal dichalcogenide (TMDC). Exemplary TMDCs include, without limitation, molybdenum disulfide (MoS₂), tungsten disulfide (WS₂), molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂). In some cases, the non-carbon based 2D material of the present invention is a disulfide, for example, MoS₂ or WS₂.

Bulk 2D Material

A bulk 2D material comprises many layers of 2D material stacked and held together with weak forces. For example, graphene is a 2D material, while graphite is the corresponding bulk 2D material held together with Van der Walls forces. In the context of the present invention, bulk materials are used in the electrode that undergoes exfoliation. Exfoliation affords 2D materials and/or 2D material nanoplatelet structures. It will be appreciated bulk materials comprise many hundreds of layers, typically many thousands of layers.

Cathode

The term cathode is used to refer to the negative electrode.

The cathode comprises a bulk non-carbon-based 2D material and an electronically conducting material. Suitably, the electronically conducting material is a ferromagnetic metal such as nickel. The w/w ratio of bulk 2D material to electronically conducting material may be 5:1 to 1:1, for example 3:1 to 1:1, for example, 2.5:1 to 1.5:1. In some cases, the w/w ratio is about 2:1.

Suitably, the electronically conducting material is provided as a powder. For example, the powder may be μm-sized or smaller, for example <1 μm.

Electrolyte

Suitable electrolytes are known in the art and include those described in WO2014/191765 (which is herein incorporated by reference in its entirety for all purposes, and in particular the section entitled electrolytes beginning on page 15).

Suitable electrolytes include salt solutions, molten salts, and ionic liquids such as eutectic systems. Salt solutions may be solutions in aqueous solvents, in organic solvents or in eutectic solvents. Eutectic systems are ionic liquids formed of a mixture of compounds having a melting point lower than that of the individual components. In some cases, the melting point of the eutectic system is at least 25° C. lower than that of lowest melting point component, for example at least 50° C. lower, at least 75° C. lower, preferably at least 100° C. lower. Eutectic systems and solvents and may include, for example, choline chloride (ChCl):Urea (1:2 M ratio), ChCl:Ethylene glycol (1:2), ChCl:Glycerol (1:2), ChCl:Malonic acid (1:1), ChCl:CrCl₃.6H2O (1:3), ChCl:ZnCl₂ (1:2), ZnCl₂:Urea (1:3.5), Ethylammonium chloride:Acetamide (1:1.5), EMC:Ethylene glycol (1:3), EMC:Glycerol (1:3), MPB:Ethylene glycol (1:3), MPB:Glycerol (1:3)

In some cases, the electrolyte is a solution of an inorganic salt such as LiCl or an organic salt such as an alkylamine salt, for example trimethylamine hydrochloride or trimethylamine hydrochloride. Naturally, mixtures of salts may be used. Suitable solvents may include DMSO. In some cases, the solvents are eutectic solvents. For example, the electrolyte may be a salt such as LiCl dissolved in a eutectic solvent such as a urea:choline chloride mixture (typically in a 2:1 mole ratio).

In some cases, the electrolyte is a molten salt, for example NaCl or LiCl may be used so that the material can exfoliated at high temperatures of 600° C.+. It will be appreciated that the stability of the 2D material to be produced should be considered in selecting an upper temperature (as some are known to be unstable at high temperatures).

In these cases the electrode's binder may be pyrolised prior to use to increases its structural stability at this elevated temperatures.

In some cases, the electrolyte is a eutectic system. For example, the electrolyte may be a eutectic mixture of a quaternary ammonium salt and a metal chloride. Suitable examples are provided above.

Positive Electrode

This is referred to as the anode.

The positive electrode may consist of any suitable material known to those skilled in the art as it does not play a role in the material production, other than to provide a counter electrode for the anions. Preferably, the positive electrode is made from an inert material such as gold, platinum or carbon. In further embodiments, the positive electrode may be made of a material that oxidises to give the metal ions in the electrolyte, such as lithium.

When the reaction at the positive electrode generates a gas the electrode surface area is as large as possible to prevent gas bubbles wetting it and/or disrupting the process at the negative electrode. The positive and/or reference electrode may also be placed in a membrane or molecule sieve to prevent undesired reactions in the electrolyte or at either electrode. The positive and the negative electrodes could alternatively be placed in a two-compartment cell, wherein each compartment contains one electrode, and the compartments are connected through a channel.

Cell Potential and Current Density

The working potential of the cell will be at least that of the standard potential for reductive intercalation. An overpotential may be used in order to increase the reaction rate and to drive the cations into the galleries of the graphite at the negative electrode. Preferably an overpotential of 1 mV to 10 V is used against a suitable reference as known by those skilled in the art, more preferably 1 mV to 5 V. In cells, with only two terminals, and no reference, a larger potential may be applied across the electrodes but a significant amount of the potential drop will occur over the cell resistance, rather than act as an overpotential at the electrodes. In these cases the potential applied may be up to 20V or 30V.

The voltage applied across the electrodes may be cycled or swept. In one embodiment, both the electrodes comprise a non-carbon-based bulk 2D material and a metal and the potential is swept so that electrodes change from positive to negative and vice versa. In this embodiment the cationic exfoliation would occur at both electrodes, depending on the polarity of the electrode during the voltage cycle. In some embodiments, alternating current can be used to allow for both fast intercalations and de-intercalations.

The current density at the negative electrode may be controlled using methods as are known in the art.

In some methods according to the invention using MoS₂/Ni cathodes, at voltage of 10 V was set. The current range typically varied over the range 2-30 mA. The inventors observed that the current using MoS₂/Ni cathodes was higher than attempts to exfoliate MoS₂ only cathodes. The difference was significant, even at the small experimental scales. This means that, even for certain semi-conducting bulk materials for which exfoliation may be possible, there are economic and environmental benefits to the methods of the invention.

Operating Temperature

The cell is operated at a temperature which allows for production of the desired material.

The cell may be operated at a temperature of at least 10° C., preferably at least 20° C. The maximum cell operating temperature may be 100° C., and more preferably 90° C., 80° C., 70° C. or 50° C. In some embodiments, the cell may be operated at a temperature of at least 30, 40, 50, 75, 100, 150 or even 200° C. The maximum cell operating temperature may, in some cases, be as high as 250° C. The optimum operating temperature will vary with the nature of the electrolyte. Operating the cell up to the boiling point of the electrolyte may be carried out in the present invention, keeping in mind the temperature stability of the desired product.

Recovery of Cations

In one embodiment, the cations used for the exfoliation are recovered after exfoliation. The cations may be recovered by washing and/or heating of the exfoliated material, electrochemical reduction of the cations, ultrasonic energy treatment of the exfoliated material, displacement from the exfoliated material by surfactants or combinations thereof.

The non-carbon-based 2D materials and non-carbon-based 2D material nanoplatelet structures having a thickness of less than 100 nm produced by the method of the invention may be separated from the electrolyte by a number of separation techniques, including:

(a) filtering;

(b) using centrifugal forces to precipitate the non-carbon-based 2D materials and non-carbon-based 2D material nanoplatelet structures;

(c) collecting the non-carbon-based 2D materials and non-carbon-based 2D material nanoplatelet structures at the interface of two immiscible solvents; and

(d) sedimentation.

The electrochemically exfoliated non-carbon-based 2D materials and non-carbon-based 2D material nanoplatelet structures may be further treated after exfoliation. For example, the materials may be further exfoliated using ultrasonic energy and other techniques known to those skilled in the art to decrease the flake size and number of layers.

In some embodiments, the electrochemical intercalation may be repeated in order to achieve full exfoliation.

Analysis of the Material

It is well established in the literature that Raman spectroscopy can be used to measure the number of layers that a graphene flake possesses through the shape, intensity and position of the peaks. In a similar way, non-carbon-based 2D materials may be analysed using Raman spectroscopy. TEM and AFM images may also be used to determine flake size.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a TEM image of the solution produced in example 1 spray-coated on silicon wafer and subjected to AFM analysis to confirm the exfoliation.

FIG. 2 shows the Raman spectra of the bulk (lower) and exfoliated (upper) MoS₂ of Example 1.

FIG. 3 shows an AFM image and measurements of a representative exfoliated flake of Example 2.

FIG. 4 shows the Raman spectra of exfoliated (lower) and bulk (upper) MoS₂ using a 633 nm excitation. The exfoliated material was obtained in example 2.

FIG. 5 shows an AFM image and measurements of a representative exfoliated flake of Example 3.

FIG. 6 shows an AFM image and measurements of a representative exfoliated flake of Example 4.

FIG. 7 shows a TEM image of an exfoliated WS₂ nanosheet of Example 4 and the corresponding selected area electron diffraction (SAED) patterns.

EXAMPLES

The following examples are provided to illustrate the invention and are not intended to limit the invention.

Exemplary Electrode Preparation:

About 2 g of MoS₂ and 1 g of Ni powder (<1 μm, 99.8% trace metals basis) were mixed in ethanol with 0.2 g PVA as a binder in a 500 ml bottle. Alumina milling media was added to the mixture and the slurry was ball milled for 2 hours to achieve good homogeneity. The alumina spheres were then separated from the slurry using metallic sieves. The slurry was then allowed to dry over night until the powder was visibly dry then dried further under vacuum for 4 hours. After drying the powder mixture was pressed into a 20 mm diameter pellet using a uniaxial hydraulic press, and then sintered under argon at 300° C.

Electrodes of WS₂ and other layered materials were prepared using similar protocols.

Electrochemical Exfoliation:

A cell was assembled with the MoS₂ pellets as cathode, and Pt wire as anode. The liquid electrolyte was prepared by dissolving lithium chloride (Sigma Aldrich, 99.9%) and/or triethylamine hydrochloride in dimethyl sulfoxide (DMSO Sigma Aldrich, 99.9%). A potential of 10 V was applied for 10 hours.

After electrolysis, the clear transparent electrolyte changes color to dark green due to suspension of the MoS₂ flakes in the electrolyte. The suspension (about 30 ml) was then mixed with 2 L of water and stirred for 2 hours using a magnetic stirrer. During the stirring process, the Ni powder attached to the magnet, leaving the supernatant free of Ni. The solution was then centrifuged at 5000 rpm and the supernatant was decanted. The remaining powder was dried overnight under vacuum at 60° C.

The powder was then suspended in NMP by mild sonication for 20 minutes and diluted with ×30 isopropanol by volume. The solution was then spray-coated on a silicon wafer and subjected to AFM analysis to confirm the exfoliation. Flakes with a thickness of ˜1-3 nm were observed on the AFM images, indicating MoS₂ was exfoliated down to monolayer and few layers (FIG. 1). The size of the nanosheets ranged from 0.5 to 3 micron, which is 5-10 orders of magnitudes higher than the MoS₂ flakes produced by any previously reported liquid exfoliation method.

Raman analysis also confirmed the exfoliation of the bulk MoS₂. FIG. 2 shows the Raman spectra of the bulk and exfoliated MoS₂. Both materials showed 2 bands at ˜380 (A_{1 g}) and ˜405 (E¹_{2 g}) cm⁻¹. However, the difference between the 2 bands reduced from ˜27 cm⁻¹ for the bulk to ˜23 cm⁻¹ for the exfoliated materials. The intensity of the bands significantly enhanced after the exfoliation. Also full-widths at half-maximum values are obviously increased in the exfoliation products than in the bulk samples, which may be possibly attributed to phonon confinement by facet boundaries.

Example 2

Similar to example 1 but the electrolyte used was 0.5 M LiCl dissolved in a eutectic mixture of urea choline chloride (2:1 mole ratio of urea: choline chloride). FIG. 3 shows an AFM image of a representative example of the exfoliated flake. FIG. 4 shows the Raman spectra of exfoliated (lower) and bulk (higher) MoS₂ using the 633 nm excitation. It is clear that the bands of the exfoliated samples are different to that of the bulk material both in terms of Raman frequency and signal intensity. For the exfoliated sample, two strong Raman bands, deconvoluted by a single Lorentzian centered at 383 cm⁻¹ and 407 cm⁻¹, were assigned to in-plane E¹_{2 g} and out-of-plane A_{1 g} vibrational modes, with no evidence of structural distortion, inferring the absence of structural damage and/or covalent bond formation upon the electrochemical exfoliation. Unlike the bulk MoS₂, the A_{1 g} and E¹_{2 g} modes for exfoliated MoS₂ appeared with equal intensities, indicating weaker coupling between the electronic transition at the K point with the A_{1 g} phonon existing in MoS₂ nanoplatelet. The frequency difference between A_{1 g} and E¹_{2 g} was measured to be 24 cm⁻¹. The value of this difference for the bulk material was found to be 27 cm⁻¹, signifying the success of exfoliation and the existence of MoS₂ in few layers.

Example 3

Similar to example 1, but the electrode was fabricated from h-BN powder. The thickness of the obtained flake as measured by the AFM was below 5 nm for 80% of the flakes measured (FIG. 5).

Example 4

Similar to example 1, but the electrode was fabricated from WS₂ powder. The AFM confirmed the exfoliation of the bulk materials to few layers WS₂ (FIG. 7). The TEM image of exfoliated WS₂ nanosheets and the corresponding selected area electron diffraction (SAED) patterns are shown in FIG. 7 which indicate that the exfoliated TMDC materials still have a hexagonal lattice structure.

Characterisation

Raman spectra were obtained using a Renishaw system 1000 spectrometer coupled to a He—Ne laser (633 nm). The laser spot size was ˜1-2 μm, and the power was about 1 mW when the laser is focused on the sample using an Olympus BH-1 microscope. The SEM images were taken using a Zeiss Leo 1530 FEGSEM. TEM analysis was conducted using FEI Tecnai FZO 200 kv FEGTEM.

All patents, applications, and other publications cited herein are incorporated by reference in their entirety for all purposes.

Claims

1. A method for the production of non-carbon-based 2D materials and/or non-carbon-based 2D material nanoplatelet structures having a thickness of less than 100 nm in an electrochemical cell, wherein the cell comprises:

(a) a negative electrode which comprises a non-carbon-based bulk 2D material and an electronically conducting material;

(b) a positive electrode; and

(c) an electrolyte;

and wherein the method comprises the step of passing a current through the cell.

2. The method of claim 1, wherein the electronically conducting material is a metal.

3. The method of claim 1, wherein the electronically conducting material is ferromagnetic.

4. The method of claim 1, wherein the electronically conducting material is nickel.

5. The method of claim 1, wherein the method produces hexagonal boron nitride and/or hexagonal boron nitride nanoplatelet structures.

6. The method of claim 1, wherein the method produces 2D transition metal dichalcogenide and/or transition metal dichalcogenide nanoplatelet structures.

7. The method of claim 6, wherein the transition metal dichalcogenide is MoS2 or WS2.

8. The method of claim 1, wherein the method includes the step of preparing the electrode comprising a non-carbon-based bulk 2D material and an electronically conducting material, the method comprising:

i. forming a slurry of the bulk 2D material, the electronically conducting material and a binder in a solvent;

ii. optionally milling the slurry;

iii. drying the slurry to obtain a powder;

iv. pressing the powder of step iii to form a pellet;

v. sintering the pellet of step iv.

9. A method of preparing a pellet for use as a cathode comprising a non-carbon-based bulk 2D material and an electronically conducting material, the method comprising:

i. forming a slurry of the bulk 2D material, the electronically conducting material and a binder in a solvent;

ii. optionally milling the slurry;

iii. drying the slurry to obtain a powder;

iv. pressing the powder of step iii to form a pellet;

v. sintering the pellet of step iv.

10. The method of claim 9, wherein the binder is a polymer, optionally wherein the binder is a polyvinylalcohol.

11. The method of claim 9, wherein the electronically conducting material is ferromagnetic.

12. A pellet for use as a cathode comprising a admixture of non-carbon-based bulk 2D material and an electronically conducting material in a binder.

13. The pellet of claim 12, wherein the electronically conducting material is ferromagnetic.

14. A non-carbon-based 2D material or nanoplatelet structure obtainable by a method according to claim 1.

15. The method of claim 8, wherein the binder is a polymer, optionally wherein the binder is a polyvinylalcohol.

16. The method of claim 10, wherein the electronically conducting material is ferromagnetic.