Technical Field

The present invention relates to a method of forming a stretchable conductive carbon nanofilm.

Background

The excellent physicochemical properties and chemical stability of carbon nanomaterial, and its potential applications as exceptional supercapacitors, batteries, catalyst supports, adsorbents and high strength fillers of composite material, have fuelled much interest. In particular, the promise of higher conductivity and electrochemical activity in nanocarbon with crystalline structures is enticing to a world that is demanding more energy as technology improves the quality of life. However, current methods used for the fabrication of carbon nanomaterials, such as laser ablation, chemical vapour deposition and plasma torch, are intensive in energy consumption and require high temperature treatment such as pyrolysis, which leads to high production cost. Hence, there is a need for an improved and greener processing method.

Summary of the invention

The present invention seeks to address these problems, and/or to provide an improved method of forming a stretchable conductive carbon nanofilm. The present invention relates to a method of producing conductive carbon nanomaterial. In particularly, it relates to the transformation of carbonaceous waste into a stretchable conductive carbon nanocomposite film.

In general terms, the invention relates to a way of forming a conductive carbon nanofilm comprising conductive carbon nanomaterial with highly uniform and small spherical particles and high electrical conductivity, which will reduce the cost of conductive carbon nanomaterial production.

According to a first aspect, the present invention provides a method of forming a stretchable conductive carbon nanofilm, the method comprising: - mixing particles of carbon waste with a polymer in a solvent to form an ink; depositing a layer of the ink on a surface of a substrate; and annealing the layer to form a nanofilm.

According to a particular aspect, the method may further comprise leaching the particles of carbon waste prior to the mixing. The carbon waste may be any suitable carbon waste. For example, the carbon waste may be carbon black.

The carbon waste may comprise particles of a suitable size. According to a particular aspect, the particles of carbon waste may have an average particle size of £ 5 mhi.

According to a particular aspect, the mixing may comprise mixing the particles of carbon waste and polymer in a weight ratio of 1 :1-1 :10. The polymer may be any suitable polymer. In particular, the polymer may be: polyvinyl alcohol (PVA), sodium alginate, polyethylene glycol, polystyrene, polyethylene oxide, polyacrylic acid, polyaniline, poly(3,4-ethylenedioxythiophene), polypropylene, polycarbonate, or a combination thereof. The solvent used in the mixing may be any suitable solvent. For example, the solvent may be a polar organic solvent. In particular, the solvent may be: ethanol, acetone, dichloromethane, chloroform, benzene, isopropyl alcohol, or a combination thereof.

The substrate onto which the layer of ink is deposited may be any suitable substrate. For example, the substrate may be a thermally-responsive substrate. In particular, the substrate may be: polystyrene, polyolefin, or a combination thereof.

The layer of ink deposited on the surface of the substrate may be of a suitable thickness. According to a particular aspect, the thickness of the layer of ink may be 0.1-

20 mGP.

The annealing may be under suitable conditions such as temperature. For example, the annealing may be at a temperature of 100-200°C.

The nanofilm formed from the method of the present invention may have a resistivity of 1000-10000 kQ cm- ¹ .

The method may further comprise the following steps: adding an elastomer to the nanofilm;

curing the elastomer; and

etching the substrate to obtain a free-standing bilayer nanofilm. The etching may comprise etching the substrate in an organic solvent. Any suitable organic solvent may be used for the etching. For example, the organic solvent may be: dichloromethane, chloroform, acetone, or a combination thereof.

The free-standing bilayer nanofilm formed from the method of the present invention may have a resistivity of 1000-10000 kQ cm ^·1 . According to a particular aspect, the method may further comprise surface functionalising the particles of carbon waste prior to the mixing. The surface functionalising may comprise any suitable form of functionalising. For example, the functionalising may be by grafting the particles of carbon waste with fluorocarbons. In particular, the nanofilm formed following the method comprising a step of surface functionalising may have a contact angle of 120-160°.

According to a second aspect, the present invention provides a nanofilm prepared from the method according to the first aspect.

There is also provided a stretchable electrode comprising a nanofilm prepared from the method according to the first aspect. Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings: Figure 1 shows a schematic representation of the method of the present invention according to one embodiment and the scanning electron microscope (SEM) images of the nanofilm formed;

Figure 2(a) shows the transmission electron microscopy (TEM) image and Figure 2(b) shows the size distribution histogram of the leached carbon black waste; Figure 3A shows the field emission scanning electron microscope (FE-SEM) images of unleached carbon black waste, Figure 3B shows the FE-SEM images of leached carbon black waste and Figure 3C shows the FE-SEM images of commercial carbon black; Figure 4 shows the XRD pattern of the leached carbon black waste;

Figure 5 shows the carbon 1s XPS spectrum of the leached carbon black waste;

Figures 6 shows the Raman spectrum of the leached carbon black waste; and

Figures 7A shows the electrical resistivity of planar carbon nanofilm, crumpled carbon nanofilm, and stretchable carbon electrodes, Figure 7B shows the cyclic voltammetry of planar carbon nanofilm, Figure 7C shows the cyclic voltammetry of crumpled carbon nanofilm at different scan rates, and Figure 7D shows the cyclic voltammetry of planar, crumpled, and stretchable electrodes.

Detailed Description

As explained above, there is a need for an improved method of forming a conductive carbon nanomaterial since the conventional fabrication methods, such as chemical vapour deposition, are expensive due to high energy consumption, which makes it both economically and environmentally unsustainable.

The concept of using carbonaceous waste as a precursor for growing carbon nanomaterial has been explored. In the present invention, carbonaceous waste is used for forming a conductive carbon nanofilm, thereby making the fabrication of conductive carbon more sustainable. The carbon nanofilm formed may exhibit multifunctionality in various applications, such as but not limited to, stretchable conductive electrodes, fire- retardant and waterproof nanocoatings.

According to a first aspect, the present invention provides a method of forming a stretchable conductive carbon nanofilm, the method comprising: mixing particles of carbon waste with a polymer in a solvent to form an ink; depositing a layer of the ink on a surface of a substrate; and

annealing the layer to form a nanofilm. The carbon waste may be any suitable carbon waste. For the purposes of the present invention, carbon waste may be defined as any carbon-based waste such as agricultural waste, used plastics and tires, and petroleum coke.

According to a particular aspect, the carbon waste may be carbon black. In particular, the carbon black may be industrial carbon black. Industrial carbon black may comprise hazardous by-product generated from the oil refineries, particularly hazardous solid residue generated from gasification of crude oil bottom in refineries.

The carbon waste may comprise particles of a suitable size. For example, the particles of carbon waste may have an average particle size of £ 5 mhi. For the purposes of the present invention, the average particle size may refer to at least the average diameter of the particles. According to a particular aspect, the carbon waste particles may comprise uniform and small spherical particles with an average diameter of £ 5 mhi. In particular, the average particle size may be 10-5000 nm, 20-4000 nm, 30-3000 nm, 40- 2000 nm, 50-1500 nm, 75-1000 nm, 100-900 nm, 150-800 nm, 200-700 nm, 250-600 nm, 300-400 nm. In particular, the average particle size may be £ 1.5 mhi.

The carbon waste particles may have a suitable surface area. For example, the average surface area of each of the carbon waste particles may be 200-500 m ² /g. In particular, the average surface area may be 250-450 m ² /g, 300-400 m ² /g, 350-375 m ² /g. Even more in particular, the average surface area of each of the carbon waste particles may be about 400-450 m ² /g, particularly about 420 m ² /g.

The carbon waste particles may have a high electrical conductivity. For example, the electrical conductivity may be 10-30 S/cm. In particular, the electrical conductivity may be 12-25 S/cm, 14-20 S/cm, 15-18 S/cm. Even more in particular, the electrical conductivity may be 10-14 S/cm. The high electrical conductivity of the carbon waste particles may be due to the highly-ordered graphitic structure and the relatively high surface area of the particles.

The method may further comprise leaching the particles of carbon waste prior to the mixing. The leaching may be for removing toxic heavy metals comprised in the carbon waste particles. The leaching may comprise removing the heavy metals by any suitable method. For example, the leaching may comprise: adding the carbon waste particles in a suitable solvent under controlled conditions such as temperature and time; filtering the mixture and obtaining the residue. The solvent may be an acid or base solution. The residue may be rinsed with washed to remove any excess solvent. According to a particular aspect, the leaching may comprise adding the carbon waste particles in nitric acid, centrifuging the mixture, filtering the mixture and washing the collected residue with Dl water. The washed residue of the carbon waste particles may be dried prior to the mixing.

The mixing of carbon waste particles with a polymer in a solvent to form an ink may comprise mixing the particles of carbon waste and polymer in a suitable weight ratio. For example, the weight ratio of the carbon waste particles to the polymer may be 1 :1- 1 :10. For example, the weight ratio may be 1 :2-1 :9, 1 :3-1 :8, 1 :4-1 :7, 1 :5-1 :6. Even more in particular, the weight ratio may be 1 :2-1 :3.

The polymer may be any suitable polymer. In particular, the polymer may be: polyvinyl alcohol (PVA), sodium alginate, polyethylene glycol, polystyrene, polyethylene oxide, polyacrylic acid, polyaniline, poly(3,4-ethylenedioxythiophene), polypropylene, polycarbonate, or a combination thereof. Even more in particular, the polymer may be PVA.

The solvent used in the mixing may be any suitable solvent. For example, the solvent may be an organic solvent, particularly a polar organic solvent. In particular, the solvent may be: ethanol, acetone, dichloromethane, chloroform, benzene, isopropyl alcohol, or a combination thereof. Even more in particular, the solvent may be ethanol, preferably a mixture of ethanol and water.

The substrate onto which the layer of ink is deposited may be any suitable substrate. The substrate may be a polymeric substrate. According to a particular aspect, the substrate may be a thermally responsive substrate. In particular, the substrate may be a thermally-responsive polymeric substrate. For example, the substrate may be: polystyrene, polyolefin, or a combination thereof. Even more in particular, the substrate may be polystyrene.

The depositing may be by any suitable method. For example, the depositing may be by drop-casting, doctor blading, spray coating, spin coating, screen printing, or a combination thereof. In particular, the depositing may be by drop-casting the ink onto a surface of the substrate. The layer of ink deposited on the surface of the substrate may be of a suitable thickness. According to a particular aspect, the thickness of the layer of ink may be 0.1- 20 mGP. In particular, the thickness may be 10-20 mhi.

The annealing may be by any suitable method under suitable conditions to form a nanofilm. The nanofilm formed may be a nanofilm with a crumpled texture. The annealing may be without any clamps or constraints for 2D biaxial deformation of the layer. The annealing results in the layer shrinking, thereby inducing the formation of a crumpled texture.

According to a particular aspect, the annealing may comprise thermal actuation. In particular, the annealing may be at a temperature which is above the glass transition temperature (T _g ) of the polymeric substrate. For example, the annealing may be at a temperature of 100-200°C. In particular, the temperature may be 100-150°C. According to a particular aspect, the substrate may be polystyrene and the annealing may be at a temperature of 100-200°C. Even more in particular, the annealing may be at a temperature of about 140°C.

The nanofilm formed following the annealing may have a resistivity of 1000-10000 kQ cm ^·1 .

According to a particular aspect, the method may further comprise surface functionalising the particles of carbon waste prior to the mixing. The surface functionalising may comprise any suitable form of functionalising.

In particular, the functionalising may be by grafting the particles of carbon waste with fluorocarbons. The fluorocarbon may be, but not limited to, trifluorosilane. The nanofilm formed following the method described above and comprising the surface functionalising may be superhydrophobic. Hydrophobicity is the physical property which makes it repel from water, a polar molecule. The contact angle of the nanofilm formed is effectively increased by undergoing mechanical deformations. The planar layer of ink became deformed following the annealing to obtain crumpled microstructures, and the structural complexity of the nanofilm increases after the mechanical deformation. The surface functionalising of the carbon waste particles makes the particles hydrophobic, and following the annealing, the contact angle of the nanofilm progressively increases.

The nanofilm formed may have a contact angle of 120-160°. In particular, the contact angle may be 130-150°, 140-145°. The super-hydrophobicity in the crumpled nanofilm is from the increased structural complexity. In particular, a water droplet on the crumpled nanofilm only remains in contact with the raised portions of the nanofilm, and air becomes trapped in the cavities between micro- and nano-sized features of the crumpled nanofilm. Therefore, the air embedded in the hierarchies and the interface beneath the water droplet may be considered a hydrophobic surface at a Cassie- Baxter state. The high hydrophobicity of the nanofilm is also due to the large amount of air trapped in the voids of the rough surface of the crumpled nanofilm, which acts as a barrier between the water and surface of the nanofilm, resulting in an increase of surface water contact angle.

According to a particular aspect, the method described above may further comprise the following steps: adding an elastomer to the nanofilm;

curing the elastomer; and

etching the substrate to obtain a free-standing bilayer nanofilm. The elastomer may be any suitable elastomer. For example, the elastomer may be polydimethylsiloxane (PDMS), silicone rubber such as Ecoflex™, Dragon Skin™, or a combination thereof.

The adding may be by any suitable method. For example, the adding may comprise infiltrating the elastomer into the microtextual features of the crumpled nanofilm formed. The curing may comprise curing the elastomer under suitable conditions. For example, the curing may be at a temperature of 20-100°C. In particular, the curing may be at a temperature of 25-90°C, 30-75°C, 40-50°C. Even more in particular, the curing may be at room temperature.

The etching may comprise etching the substrate in an organic solvent. Any suitable organic solvent may be used for the etching. For example, the organic solvent may be: dichloromethane (DCM), chloroform, acetone, or a combination thereof. In particular, the organic solvent may be DCM. Even though the etching step may swell the nanofilm formed, the compression-induced texture allows the nanofilm to withstand the swelling and remain intact. The free-standing bilayer nanofilm formed from the method of the present invention may have a resistivity of 1000-10000 kQ cm ¹ .

According to a second aspect, the present invention provides a nanofilm or a free standing bilayer nanofilm prepared from the method described above.

There is also provided a stretchable electrode comprising a nanofilm prepared from the method described above. In particular, the stretchable electrode may comprise the bilayer nanofilm prepared from the method described above.

The present invention provides a facile way to solve not only the waste problem in the petroleum industry, but also high production cost of fabricating carbon nanomaterials in view of the utilization of industrial carbon black waste generated from gasification of crude oil bottom in refineries as the source to fabricate the conductive carbon nanoparticles. The present invention provides a sustainable processing method.

In particular, the leached carbon waste may be incorporated with polymer to prepare a carbon-polymer composite ink which may be further printed out nu a suitable printed to fabricate stretchable electrodes with effective fire-retardant or waterproof properties. Having now generally described the invention, the same will be more readily understood through reference to the following embodiment which is provided by way of illustration, and is not intended to be limiting.

Example

Chemicals and materials Carbon black waste was collected from an oil refinery factory in Singapore. The following chemicals were used as received without further purification: sodium hydroxide (99%, VWR Chemicals) and hydrochloric acid (VWR Chemical, 32%). A commercial carbon black (acetylene, 50% compressed, 99.99%, metals basis) used as reference was purchased from Alfa Aesar and used as received. Deionized water was used for all experiments.

Leaching process

Carbon black waste contained high contents of toxic heavy metals, making it a highly hazardous material. Therefore, the carbon black waste was leached to remove those toxic heavy metals before use. 1.0 g carbon black waste sample was added into 100 ml HNO ₃ solution under a controlled temperature. After leaching, the obtained mixture was centrifuged at 9,000 rpm for 10 minutes and filtered using a 0.2 mhi filter. The collected solid sample was then washed with Dl water several times and dried overnight at 60°C before further use. Table 1 shows the elemental analysis of the carbon waste sample.

Table 1 : CHNS elemental analysis of carbon black waste samples

The elemental analysis result (as presented in Table 1) shows high carbon content in the carbon black waste samples. Moreover, the untreated carbon black waste has 1.08% of sulphur while less than 0.8% of sulphur remained after the leaching to remove the heavy metals.

Fabrication of stretchable conductive carbon nanofilm

A thermally-responsive polystyrene (PS) shrink film was cut into 16 cm ² squares and washed with ethanol. Once dried, samples were treated with pure oxygen plasma for 5 minutes. Carbon black waste nanoparticles were mixed with polyvinyl alcohol (PVA) powders in a solvent mixture of ethanol and water to form a nanocarbon suspension of ink. 150 pl_ of the ink was drop-cast onto the PS shrink films to form a planar film. Once dry, the planar samples were placed in an oven for thermal actuation at 140°C (above the Tg of PS) for 30 minutes. The samples shrunk to 20% of their original area without any clamps or constraints for 2D biaxial deformation, inducing the formation of isotropic crumple texture. Thereafter, the samples were removed from the oven and allowed to cool for approximately 30 minutes on the bench top.

Uncured Ecoflex™ liquid was then infiltrated into the microtextual features of the samples. After degassing, the Ecoflex™ was cured at room temperature for 2-3 hours. The samples were then subjected to an etching step in dichloromethane (DCM) to remove the PS substrate. After rinsing in DCM and ethanol, a freestanding, stretchable, electrically conductive textured-nanocarbon/Ecoflex bilayer film was produced. The final nanocarbon/Ecoflex bilayer films were cut into desired size and shape for further investigation. A schematic representation of the fabrication process and the corresponding SEM image of the conductive carbon nanofilm obtained at each step is shown in Figure 1.

Fabrication of electrodes comprising the nanofilm

Gold electrodes were fabricated via depositing conductive gold paste (Ted Pella) on the nanocarbon/Ecoflex bilayer films. A copper wire was connected with the conductive gold electrode and connected with the electrodes of a portable standard multimeter. After the gold paste was dried, a thin layer of epoxy was covered on the electrodes to stabilize the adhesion.

Characterisation methods

Brunauer-Emmett-Teller (BET) surface area and total pore volume of the samples were measured by using Quantachrome-NOVA-4200e. All samples were first degassed overnight at 80°C before carrying out the BET analysis. Structural properties of the samples were characterized by using a SHIMADZU XRD-6000 diffractometer with Cu Ka radiation (l=0.154 nm), operated at 40 kV and 30 mA.

The structural properties of the samples were further characterized by Transmission Electron Microscopy (TEM, JEOL JEM-2010), operated at an acceleration voltage of 100-150 kV, with magnification in the range of 10,000-100, OOOx. Ultimate analyses of the samples were conducted using an elemental analyzer (EA, Elementarvario MICRO cube). Morphology of the samples was investigated by field-emission scanning electron microscopy (FE-SEM, JEOL-JSM-6700F). The Carbon-Carbon bonds (i.e. sp ² and sp ³ hybridizations) were examined using X-ray photoelectron spectroscopy (on a Kratos AXIS, using a monochromatized Al Ka X-ray source, 1486.6 eV photons). Raman spectroscopy was used to analyze The sp ² and sp ³ hybridized carbon bonds was further examined using RAMAN spectroscopy.

Electrical conductivity measurement The electrical conductivity of the samples was measured using the Lucas Lab four- point probe (FFP) system, connected to a multimeter (2700 Multimeter Data Acquisition system) and Keithley 238 High Current Measurement System. The treated carbon black waste was pressed, using a stainless steel mold (diameter 12 mm), for approximately 1 minute at 20 MPa into a pellet for the conductivity measurement. The conductivity is calculated as below:

where p is resistivity in W.o; V is the measured voltage in mV; I is the current in outer probes (0.1 mA); t is the thickness of the sample pellet; and s is the probe spacing

(0.159 cm). The electrical conductivity s is calculated as— in S/cm.

P Cyclic voltammetry measurement

A three-electrode cell was employed for the electrochemical measurements in the aqueous-cell test, where an Ag/AgCI electrode (BASi) (3 M NaCI) and a Pt mesh were used as the reference and counter electrode, respectively. The nanofilm was used as the working electrode. Cyclic voltammetry (CV) was performed between 0 and 1.0 V (vs. Ag/AgCI) at room temperature in 1.0 M H ₂ SO ₄ solution using a potentiostat/galvanostat (Autolab) The area of nano-carbon films onto the substrate was scanned and measured by ImageJ.

Example 1 This example shows the TEM image of the leached carbon black waste sample (as presented in Figure 2), indicating the agglomeration of small and uniformed carbon black waste particles, and the average particle size was measured to be about 112 nm. Similarly, the SEM images of the carbon black waste samples which are presented in Figure 3 also show that the carbon black waste samples were made up of the uniform spherical particles, which contribute to their high surface area. Table 2 provides the surface area and pore volumes of the samples.

Table 2: Surface area and total pore volume of carbon black waste samples Moreover, it was also observed that the carbon black waste samples had small and much more uniform particle sizes as compared to the commercial carbon black used as a reference.

Example 2

This example shows the XRD pattern of the leached carbon black waste sample (Figure 4). The main characteristic peak at 2Q = 25.0° which can be attributed to the graphitic structure was clearly observed, indicating that the carbon black waste had a highly-ordered graphitic structure. The presence of this graphitic structure significantly affects the conductive properties of the carbon black waste.

Example 3 This example shows the XPS analysis of carbon black waste, which was carried out in order to investigate the surface characteristics of the carbon black waste sample, i.e. types and relative amounts of organic functional groups (e.g., C-C, C-O, C=0, etc.). Figure 5 shows the C 1s XPS spectrum of carbon black waste sample. Four main peaks were observed at 283.9 eV, 284.7 eV, 286.0 eV and 288.7 eV, which attributed to the sp ² carbon bonds, sp ³ carbon bonds, C-0 bonds and C=0 bonds, respectively. Table 3 summaries the atomic percentage of different bonds observed in the XPS spectrum of the carbon black waste sample.

Table 3: Atomic percentage of different carbon bonds of the leached carbon black waste sample

It can be seen that the atomic percentage of the sp ² carbon bonds (46%) was significantly higher than that of the sp ³ carbon bonds (23%). This result indicated that the leached carbon black waste sample contained a high proportion of the graphitic structure, which is in line with the XRD result.

Example 4

Raman spectroscopy, which is an in situ and non-destructive examination technique for the characterization of carbonaceous materials, was used to analyse sp ² and sp ³ hybridized carbon atoms of the carbon black waste sample. This example showed the Raman spectrum of the carbon black waste sample (Figure 6), and the characteristic bands appeared at 1342 cm ^-1 , 1593 cm ^-1 and 2661 cm ^-1 assigned to the D, G and 2D peaks, respectively. The G peak in the Raman spectra arises from the stretching vibration of carbon atoms of sp ² bonds, whereas the D peak is due to the disorder in sp ² hybridized carbon sites. The narrow full width at half maximum (FWHM) of the D and G peaks signifies a high degree of graphitization of the carbon black waste sample. Additionally, the band intensity ratio of D peak to G peak in the Raman spectrum was found to be 1.2, indicating the abundant presence of graphitic structure (sp ² ) in the carbon black waste sample, which was in line with the XPS result. Due to the fact that conductivity of a carbon material highly depends on the graphitic structure (sp ² hybridization) content, therefore the carbon black waste sample with the abundant presence of graphitic structure has a higher electrical conductivity.

Example 5 In order to investigate the conductive properties of the leached carbon black waste samples, the electrical conductivity was measured by using a four-point probe (FFP) method and the obtained results are presented in Table 4.

It was found that the leached carbon black waste had a relatively high electrical conductivity of about 11.1 S/cm. The HR-TEM image (not shown) clearly showed the highly-ordered graphitic structure of the carbon black waste sample. This result is also in line with the XRD and XPS results. Therefore, the high electrical conductivity of carbon black waste can be attributed to its highly-ordered graphitic structure. Moreover, Table 4 also shows that the leached carbon black waste has higher conductivity values as compared to other commercial carbon materials (such as activated carbon, carbon black, carbon aerogels and graphitized materials). Example 6

The resulting carbon/elastomer bilayer nanofilm formed were highly stretchable. The stretchability came from the reversible crumpling/unfolding behaviours to reduce the in plane strains, and the carbon nanofilm remained intact under various stretching (from 0% to 250% areal strains). The electrical resistivity was evaluated via a multimeter, and interestingly the crumpled carbon nanofilms showed a lower resistivity (-300 W) than planar carbon nanofilms (-700 W) (Figure 7a).

The high surface area of these complex textured carbon structures may be advantageous for enhanced electrochemical or catalytic activity. As a proof of concept, the capacitive current densities of crumpled nanocarbon film were evaluated via cyclic voltammetry in H ₂ SO ₄ electrolyte and compared with planar carbon nanofilm. The crumpled carbon nanofilm achieved 300-400% improvement in current density (12 A cm ² ) compared to planar carbon nanofilm (3 A cnr ² ) (Figure 7d). Both the nanocarbon structures remained electromechanically robust under a wide range of scan rates (from 25-100 mV s ¹ ) as shown in Figures 7b and 7c, and showed excellent cycling stability after 300 cycles.

The graphitic structure (sp ² ) of the nanocarbon black provides an efficient route for electron transfer in the nanofilms to obtain highly conductive nature. The p-p interaction between the sp ² carbons and the aromatic rings present in the polymer makes a more ordered arrangement in the nanofilm which plays a vital role for the enhanced electrical conductivity. The sp ³ carbon present in the carbon may hinder the conductivity, but it has a lesser effect due to the low amount present in the carbon. The uniform distribution between the polymer and graphitic carbon reduces the diffusion time of the electrolyte ions to get higher electrochemical performance. The carbon has porous nature, so the diffusion of ions in to the nanofilm is easy and it is an advantage for the potential electrode material. The graphitic carbon present in the nanofilm has high mechanical strength, which prevents mechanical deformation of the nanofilm during the charge-discharge process. Example 7

The mechanically stable, stretchable carbon bilayer nanofilm was then evaluated as fire-retardant barriers under varying degrees of mechanical stretching. It was found that the crumpled carbon films were more fire-retardant than the planar carbon films, since the carbon particles were compressed in-plane, so it led to lower oxygen permeability. A combustibility testing assay was developed to test flammability by cooking the bilayer nanofilm above the flame of ethanol burner (flame temperature ~600-850°C). In a control experiment, bare PS substrates were placed above the flame, and it got ignited immediately (less than 5 seconds) and the samples lost their material integrity. In contrast, the crumpled bilayer nanofilm-protected PS samples were non-flammable under combustion for over 90 seconds which was the duration of the experiment. After the combustibility test, the conformal nanofilm still exhibited similar crumpled topographies.

Further bilayer nanofilms were prepared for the combustion test. For the combustion test, first the elastomer started to expand and then degrade, and the crumpled carbon present in the fire-retardant material reduced the expansion rate of the elastomer layer by strong interaction between the carbon and elastomer structure. The pores of the carbon were densely packed with elastomer to form a rigid structure, which prevented the diffusion of oxygen into the structure for combustion reaction to take place. The sheet morphology of the carbon nanofilm prevented crack formation by heat and acted as an insulative barrier between the flame and the combustible elastomer. The agglomeration of carbon nanoparticles reduced the heat transfer to the elastomer matrix which was thermally more stable to the fire-retardant material. After incorporating the crumpled carbon to the elastomer, it not only enhanced the flame retardancy but also suppressed the smoke while performing the flame test. Generally, fire retardant material produce more smoke due to the incomplete combustion of the organic material. However, in the nanofilm of the present invention, the incomplete combustion was reduced by the high thermally stable crumpled carbon compared to the bare elastomer material and it also reduced the heat release rate of elastomer.

Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.