Production of graphene materials

Abstract

Methods for the production in an electrochemical cell of metal oxide deposited graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, in a cell having a positive electrode which is graphitic and an electrolyte comprising an intercalating anion and a metal cation, wherein the metal is selected from ruthenium, manganese, iridium, tin, and silver. The methods comprising the step of passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode and such that the metal ion undergoes electrodeposition in the form of the corresponding metal oxide to produce the metal oxide deposited graphene and/or graphite nanoplatelet structures.

Claims

1. A method for the production in an electrochemical cell of metal oxide deposited graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the cell comprises: (a) a positive electrode which is graphitic; (b) a negative electrode; and (c) an electrolyte comprising an intercalating anion and a metal cation, wherein the metal is selected from ruthenium, manganese, iridium, tin, and silver; and wherein the method comprises the step of passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode and such that the metal ion undergoes electrodeposition in the form of the corresponding metal oxide to produce the metal oxide deposited graphene and/or graphite nanoplatelet structures.

2. The method of claim 1, wherein said metal cation is selected from ruthenium, manganese and iridium.

3. The method of claim 1, wherein more than one metal cation is used so as to produce mixed-metal oxide deposited graphene and/or graphite nanoplatelet structures.

4. The method of claim 3, wherein ruthenium and manganese are used.

5. The method of claim 1, wherein the intercalating anion is sulfate.

6. The method of claim 1, wherein the metal oxide deposited graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm is substantially free of graphene oxide.

7. A composition comprising graphene and/or graphite nanoplatelet structures, wherein said graphene and/or graphite nanoplatelet structures have metal oxide nanostructures deposited on the basal surface; wherein the metal oxide deposited graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm is substantially free of graphene oxide.

8. The composition of claim 7, wherein the metal oxide is selected from ruthenium oxide, manganese oxide, iridium oxide, tin oxide, and silver oxide.

9. The composition of claim 7, wherein the metal oxide is selected from ruthenium oxide, manganese oxide and iridium oxide.

10. The composition of claim 8, wherein more than one metal oxide is deposited.

11. A supercapacitor having an electrode comprising a composition according to claim 7.

12. The supercapacitor of claim 11, wherein the electrode comprises metal oxide deposited graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the metal oxide comprises ruthenium oxide.

13. The supercapacitor of claim 12, wherein the electrode comprises metal oxide deposited graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the metal oxide comprises ruthenium oxide and manganese oxide.

14. The supercapacitor of claim 12, wherein the cell has an electrolyte comprising diethylmethylammonium bisulfate.

Description

SUMMARY OF THE FIGURES

(1) Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

(2) FIG. 1 shows (A) Survey-scan XP spectra obtained for graphene samples that were obtained by electrochemical exfoliation of graphite in 20 mM RuCl.sub.3 in 0.5 M Na.sub.2SO.sub.4(aq), 20 mM Mn(NO.sub.3).sub.2 in 0.5 M Na.sub.2SO.sub.4(aq) and in a mixture of 10 mM RuCl.sub.3 and 10 mM Mn(NO.sub.3).sub.2 in 0.5 M Na.sub.2SO.sub.4(aq). (B), (C) and (D) High resolution XP spectra in the O1s, Mn3s and C1s region respectively. All peak positions were charge-corrected by setting the binding energy of the C 1s signal to 285 eV.

(3) FIG. 2 shows (A) SEM, (B) AFM, (C) and (D) TEM images of MnO.sub.2 functionalised graphene sample that were obtained by electrochemical exfoliation of graphite in 20 mM Mn(NO.sub.3).sub.2 in 0.5 M Na.sub.2SO.sub.4(aq). The inset in (B) shows height profile for the selected region.

(4) FIG. 3 shows (A) SEM image of RuO.sub.2 functionalised graphene (B) and (C) TEM images of RuO.sub.2 functionalised graphene. The RuO.sub.2 functionalised samples were obtained by electrochemical exfoliation of graphite in 20 mM RuCl.sub.3 in 0.5 M Na.sub.2SO.sub.4(aq).

(5) FIG. 4 shows (A) and (B) SEM image of Mn.sub.3O.sub.4—RuO.sub.2 functionalised graphene (C), (D) and (E) TEM images of Mn.sub.3O.sub.4—RuO.sub.2 functionalised graphene. The graphene-Mn.sub.3O.sub.4—RuO.sub.2 functionalised samples were obtained by electrochemical exfoliation of graphite in 10 mM RuCl.sub.3 and 10 mM Mn(NO.sub.3).sub.2 in 0.5 M Na.sub.2SO.sub.4(aq).

(6) FIG. 5 shows cyclic voltammograms recorded at 20 mV s.sup.−1 in 1.0 M H.sub.2SO.sub.4 (aq) using symmetrical coin cells constructed from indicated electrodes. The voltage was scanned between 0.0 V (initial potential) and 0.8 V, (B) gravimetric capacitance as a function of scan rates at indicated electrodes, (C) TGA traces recorded at indicated samples by ramping the temperature from 40° C. to 1000° C. at a rate of 10° C. min.sup.−1 in air. The inset shows the Nyquist obtained at open circuit potential. (D) Capacitance retention of G-Mn.sub.3O.sub.4—RuO.sub.2 electrodes after 5000 cycles in 1.0 M H.sub.2SO.sub.4. The inset shows the normalised imaginary capacitance across range of frequencies. (E) RDE polarization curve recorded at 1600 rpm in 0.1 M KOH (aq) at indicated electrodes between −0.7 V and 1.9 V at 10 mV s.sup.−1 and (F) Nyquist plots obtained at 1.5 V vs RHE. The measurements were carried out at amplitude of 5 mV in the frequency range of 100 mHz to 100 kHz.

(7) FIG. 6 shows cyclic voltammograms recorded at 20 mV s.sup.−1 in 1.0 M [dema][HSO.sub.4] in acetonitrile (red colour) and 1.0 M [TEA][BF.sub.4] in acetonitrile (black colour) using symmetrical coin cells constructed from G-Mn.sub.3O.sub.4—RuO.sub.2 electrodes. The voltage was scanned between 0.0 V (initial potential) and 2.0 V, (B) CVs recorded at 150, 75, 60, 50, 30, 20, 10 mV s−1 (from top to bottom) in 1.0 M [dema][HSO.sub.4] in acetonitrile at G-Mn.sub.3O.sub.4—RuO.sub.2 electrode. (C) gravimetric capacitance as a function of scan rates at indicated electrodes and (D) Charge-discharge curves obtained at G-Mn.sub.3O.sub.4—RuO.sub.2 in 1.0 M [dema][HSO.sub.4] in acetonitrile.

(8) Note: At the priority date, the inventors believed the graphene to be functionalised with MnO.sub.2—RuO.sub.2. However, the inventors have since found that the graphene is functionalised with Mn.sub.3O.sub.4—RuO.sub.2. The figure legends reflect this improved understanding.

DETAILED DESCRIPTION OF THE INVENTION

(9) Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

(10) The inventors have found that the inclusion of certain metal salts in the exfoliation solution during the electrochemical exfoliation of graphene leads to exfoliated graphene sheets decorated with the oxide of that metal. The following examples describe, without limitation, the use of Mn and Ru salts in this process and the respective metal oxide decorated graphene sheet products. The inventors have also observed simultaneous exfoliation and decoration in additional experiments not presently described. These experiments include Ir, Sn and Ag salts, resulting in IrO.sub.2, SnO.sub.2 and Ag.sub.2O decorated products.

(11) This process can also be used to produce mixed-metal oxide decorated graphene sheets. Described below, without limitation, is the production of a bimetal oxide (the example is Mn.sub.3O.sub.4—RuO.sub.2) electrodeposited on conducting graphene support.

(12) The loading of the desired metal oxides (mono- or bi-oxides) can be controlled by varying the concentration of the salt during electrochemical exfoliation.

(13) Advantageously, the product may have lower graphene oxide character than convention anodically produced “graphene” products. For example, the inventors have demonstrated low levels of oxidation when ruthenium or iridium is used. In other words, in some embodiments the invention may provide a method for the production in an electrochemical cell of graphene and/or graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the cell comprises (a) a positive electrode which is graphitic; (b) a negative electrode; and (c) an electrolyte comprising an intercalating anion and a cation of a metal selected from ruthenium and iridium; and wherein the method comprises the step of passing a current through the cell to intercalate anions into the graphitic positive electrode so as to exfoliate the graphitic positive electrode to produce the graphene and/or graphite nanoplatelet structures. As described herein, the product is decorated with metal oxide deposits. Metal oxide nanostructures are deposited on the surface. The product may be used in decorated form as obtained, or further processed.

(14) The materials offer utility in electrochemical applications, for example for use as electrodes or as electrocatalysts. To that end, the inventors made a supercapacitor coin cell, the details of which are described below. The materials may be useful as an electrocatalyst for water splitting.

(15) Materials and Reagents

(16) Anhydrous sodium sulfate, ruthenium (III) chloride (99.9%) and manganese (II) nitrate hydrate (99.9%) were obtained from Sigma-Aldrich. All electrochemical measurements were performed either using an Autolab potentiostat model (PGSTAT302N, Metrohm Autolab, The Netherlands) or power source. Graphite foil (>99%) was obtained from Gee Graphite Ltd (UK). Omnipore membrane filters made of poly(tetrafluoroethylene) (JVWP01300) were used, pore size of 0.1 μm. Ultra-pure water (18.2 MΩ cm resistivity) was obtained from a Milli-Q water purification system.

(17) Characterisation of the Exfoliated Product

(18) Raman spectra were obtained using a Renishaw inVia microscope with a 532 nm excitation laser operated at a power of 3.32 mW with a grating of 1800 lines/mm and 100× objective. The samples for Raman measurement were prepared by drop coating the dispersion (in DMF) onto a Si/SiO.sub.2 wafer and then dried on a hot plate at 150° C. to evaporate the solvent. For AFM analysis, the composite (e.g. graphene-MnO.sub.2) dispersion was spray-coated onto a Si/SiO.sub.2 substrate which was dried in a vacuum oven at 80° C. SEM analysis was carried out using an FEI Quanta 650 FEG environmental scanning electron microscope. (S)TEM was carried out by FEI Talos F200X operated at 200 kV and FEI Titan.sup.3 G2 60-300 operated at 80 kV. A sample for (S)TEM was prepared by dispersing dried composite sample into DMF solution for few sec and then drop casted over TEM grid. The concentration of the graphene dispersion was measured with UV-vis spectroscopy using a model DH-2000-BAL (Ocean Optics). X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD spectrometer with a monochromated Al Kα X-ray source (E=1486.6 eV, 10 mA emission). X-ray Diffraction (XRD) was performed on a Philips X'pert PRO diffractometer with Cu Kα radiation (λ=0.154 nm) operating at 40 kV and 30 mA.

(19) Electrochemical Exfoliation of Graphite in the Presence of Metal Cations

(20) An H-type electrochemical cell consisting of a graphite foil working electrode (pre-expanded by immersing in liquid nitrogen for 30 s followed by transferring into absolute ethanol) in the anode compartment and a graphite rod counter electrode in the cathode compartment was used. The electrochemical cell was separated by porous glass frit and the compartment was separated by 7 cm (inset of FIG. 1B). The electrolyte was prepared by dissolving 0.5 M Na.sub.2SO.sub.4 in ultra-pure water and various concentrations of Ru (III) salt or Mn (II) salt (7, 20, 30 mM). In some cases, the exfoliation solution contains 10 mM RuCl.sub.3 (aq), 10 mM Mn(NO.sub.3).sub.2(aq) and 0.5 M Na.sub.2SO.sub.4 (aq). Electrochemical exfoliation and functionalisation of graphite carried out by applying +20 V to graphite foil (anode compartment) using power source. 20.0 V was the optimum voltage. Below this value the exfoliation yield was lower. The exfoliated product was washed several times with water and then re-dispersed in DMF by sonicating for 20 min.

(21) FIG. 1A shows the survey scan XP spectra obtained for graphene samples exfoliated in the presence of 20 mM Mn(NO.sub.3).sub.2, 20 mM RuCl.sub.3 or in the mixture of the two salts (10 mM Mn(NO.sub.3).sub.2 and 10 mM RuCl.sub.3). The presence of either Mn or Ru (or both) signals along with C 1s and O 1s in the wide scan confirms the functionalisation of graphene with their respective metal species. Close inspection of 01s obtained from graphite sample exfoliated in the presence of 20 mM Mn(NO.sub.3).sub.2 shows the formation of a new oxygen peak at the characteristic binding energy position of Mn oxide. Moreover, the splitting width of Mn 3s is 4.9 eV which is consistent with MnO.sub.2 structure.

(22) In similar manner, EDX analysis of the graphene sample exfoliated in the presence of 20 mM RuCl.sub.3 shows the formation of RuO.sub.2. The atomic concertation ratio of Ru (5.2%) to O (12%) from the XPS analysis also supports the conclusion drawn from EDX data. It is interesting to note that the high resolution C 1s spectra of graphene functionalised with RuO.sub.2 shows the absence of any surface oxidation when compared to graphene functionalised with MnO.sub.2 or control samples (FIG. 1D). This suggests that RuO.sub.2 acts as a protective layer against graphite oxidation during electrochemical exfoliation. Since RuO.sub.2 is demonstrably the best electrocatalyst for oxygen evolution, it is likely that water oxidation at RuO.sub.2 surface occurs without significant formation of hydroxyl radical that attack carbon surfaces. A similar effect was noted when iridium was used.

(23) Without wishing to be bound by any particular theory, the inventors attribute the lack of oxidation of the graphene to the use of metal cations having metal oxides that are a good electrocatalyst for water oxidation.

(24) Analysis of XPS data for graphene sample that was exfoliated in the presence of 10 mM Mn(NO.sub.3).sub.2 and 10 mM RuCl.sub.3 showed that the incorporation of both Ru and Mn oxides (FIG. 1A black solid colour). The atomic concentrations of C, O, Ru and Mn from the survey scan were 65, 19, 7.7 and 8.3% respectively. This indicates that the amount of RuO.sub.2 in the composite structure was in a molar ratio to that of MnO.sub.2. Powder X-ray diffraction data showed the absence of diffraction patterns that are associated either with MnO.sub.2 or RuO.sub.2 structures indicating the oxides were highly amorphous structure. At the priority date, the inventors believed that the Mn oxide was MnO.sub.2. However, the inventors have since determined the oxidation state of Mn for a graphene sample that was exfoliated in the presence of 10 mM Mn(NO.sub.3).sub.2 and 10 mM RuCl.sub.3 from the splitting width of Mn 3S high resolution XPS spectrum. The splitting width gives 5.4 eV, indicating that the Mn oxide structure is actually Mn.sub.3O.sub.4..sup.[3] Examination of the bulk chemical composition of the hybrid graphene-oxide structure by EDX gives 6%, 9%, 29.6% and 55.4% of Mn, Ru, O and C respectively. This shows that the chemical structure of the composite oxide is Mn.sub.3O.sub.4—RuO.sub.2. The EDX composition also suggests the amount of RuO.sub.2 in the composite is 50% greater than that of Mn.sub.3O.sub.4, although the composition obtained from XPS analysis indicated that the composite structure contained twice the amount of Ru when compared to Mn.

(25) FIG. 2 shows the SEM, AFM and TEM images of graphene samples that were obtained in the presence of 20 mM of Mn(NO.sub.3).sub.2 and 0.5 M of Na.sub.2SO.sub.4. The majority of the graphene surface was coated with thin layer of inter-connected honey-comb lamellar like structures (FIG. 2A, 2C-D). AFM showed that the thickness of the lamellar honey-comb MnO.sub.2 structure was varies between 1 and 2 nm while that of graphene was also between 1 nm and 2.5 nm (FIG. 2B). In addition, some large hemispherical nanowhisker structures also covered the graphene surface. These large structures were curly flower-like and consisted of petal shaped layers of MnO.sub.2.

(26) By contrast, RuO.sub.2 forms individual thick nanoparticles and many of the nanostructures show an aggregation on graphene sheets (FIG. 3A-B). In some places, the RuO.sub.2 nanostructures are sparsely distributed over the graphene sheets with an average particle size of ˜2 nm (FIG. 3C).

(27) The graphene decorated with a combination of RuO.sub.2 and MnO.sub.2 showed different morphology to that of the graphene decorated with the individual nanostructures. The SEM and TEM images show that the RuO.sub.2 nanoarchitecture grows inside MnO.sub.2 honey comb structures with rod and plate-shaped structure (FIG. 4). The average length of the nanorods is 0.5 μm and its diameter is 0.1 μm. EDX mapping indicated that the oxides structures are uniformly distributed across graphene sheets. The local structure, for example the Mn/Ru ratio, may vary depending on location.

(28) Raman spectroscopy showed the formation of functionalised graphene with MnO.sub.2 nanostructure. The typical D-band, G-band and 2D-band of graphene was seen along with the Mn—O stretching mode at ˜630 cm.sup.−1. Powder X-ray diffraction (XRD) data showed weak feature besides diffraction peaks from graphitic structure. In the case of Mn(NO.sub.3).sub.2, a set of prominent peaks was found at 37.3, 43.0, and 57.0° which corresponds to (101), (111), (211) plane of β-MnO.sub.2 structure. In addition to these peaks, there is broad feature at the lower angle tail of graphite (002) peak. This newly appeared peak corresponds to (002) plane of δ-MnO.sub.2 structure. By contrast to MnO.sub.2, only broad feature appeared at 34.9° which corresponds to the (101) plane of RuO.sub.2 for graphene exfoliated in the presence of RuCl.sub.3. For graphene obtained from a mixture of Mn(NO.sub.3).sub.2 and RuCl.sub.3, similar diffraction pattern as RuO.sub.2 was found, but the most prominent peaks appeared at 36.7°. While the position of the peak is in between RuO.sub.2 (101) and β-MnO.sub.2 (101) and these structures possess same symmetry (P4.sub.2/mnm), this indicates the presence of mixed oxide phase in the sample.

(29) Capacitance of Metal Oxide-Decorated Graphene

(30) The capacitive behaviour of functionalised graphene was assessed using cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge curve and compared to the non-functionalised electrochemically exfoliated graphene (control). FIG. 5A shows the CV obtained at control, graphene functionalised with 20 mM Mn(NO.sub.3).sub.2 (G-MnO.sub.2), 20 mM RuCl.sub.3 (G-RuO.sub.2) and in a mixture of 10 mM Mn(NO.sub.3).sub.2 and 10 mM RuCl.sub.3 (G-MnO.sub.2/RuO.sub.2) in deoxygenated 1.0 M H.sub.2SO.sub.4 (aq). The rectangular shape in the CV curve and triangular curve in glavanostatic charge-discharge at graphene-metal oxide composite electrode shows their fast and reversible pseudo-capacitive nature.

(31) As shown, the width of the CV increased considerably as the electrode changed from EEG (electronically exfoliated graphene) to functionalised graphene. The average capacitance of EEG was 40 F g.sup.−1 and this increased to about 100 F g.sup.−1 for G-MnO.sub.2 and 210 F g.sup.−1 for G-RuO.sub.2 demonstrating the metal oxides contributing to the overall capacitance through their pseudocapacitive behaviour. The capacitance increased with increasing the loading of RuO.sub.2 on graphene. TGA showed that the use of 7 mM, 20 mM and 30 mM RuCl.sub.3 during exfoliation produced 16%, 27% and 38% of RuO.sub.2 loading respectively (weight loss between ˜300° C. and 750° C. in the TGA curve was attributed to the decomposition of graphitic carbon).

(32) The corresponding capacitance value almost doubled as the loading of RuO.sub.2 increased from 16% to 27% (140 F g.sup.−1 in 7 mM RuCl.sub.3 and 230 F g.sup.−1 in 20 mM RuCl.sub.3). At high loading of RuO.sub.2 (38%), its specific capacitance was slightly higher than the 27% loading at lower scan rates but its capacitance becomes the same at high scan rates (v). This demonstrates that 20 mM RuCl.sub.3 produces the optimum RuO.sub.2 loading for supercapacitor application during simultaneous electrochemical exfoliation and functionalisation of graphene.

(33) The composite sample consisting of graphene, RuO.sub.2 and Mn.sub.3O.sub.4 (G-Mn.sub.3O.sub.4—RuO.sub.2) however, showed an extraordinary capacitance of over 520 F g.sup.−1, about two times the best performing G-RuO.sub.2 electrodes. The inventors postulate synergistic effects between the three components, in which graphene probably contributes to the overall enhancement in conductivity of the film while the electrodeposition process might produce high surface area Mn and Ru oxide depositions.

(34) The metal oxides loading of G-Mn.sub.3O.sub.4—RuO.sub.2 (total metal oxide loading was ˜42%) is comparable to that of G-RuO.sub.2 (30 mM RuCl.sub.3). However, the seral resistance of the G-MnO.sub.2—RuO.sub.2 electrode was lower than the G-RuO.sub.2 by more than ten factors (G-Mn.sub.3O.sub.4—RuO.sub.2 was 0.8Ω compared to 4.5Ω for 20 mM RuCl.sub.3 and 10Ω for 30 mM RuCl.sub.3). The decrease in the internal resistance of the bi-metal oxide is reflected by the decrease in ohmic drop (0.05 V in G-MnO.sub.2—RuO.sub.2vs 0.15 V in G-RuO.sub.2) in discharge curve obtained at 10 A g.sup.−1.

(35) Furthermore, the charge transfer resistance at G-Mn.sub.3O.sub.4—RuO.sub.2was 0.3Ω compared to 17Ω at the electrode obtained using 30 mM RuCl.sub.3, which indicates that the redox reactions that contribute to the overall pseudocapacitance is much faster at G-Mn.sub.3O.sub.4—RuO.sub.2. This also the case when analysing the relaxation time constant (τ), a constant which tells the time required to deliver the stored energy and power efficiently. τ may also used to identify the frequency range when the device is dominated by resistive or capacitive behaviour. τ of 1 sec for G-Mn.sub.3O.sub.4—RuO.sub.2and 6 sec for G-RuO.sub.2 electrode was obtained indicating that the resistive loss at G-MnO.sub.2—RuO.sub.2 is minimal compared to G-RuO.sub.2. The synergetic interaction between each component was also more evident when comparing their electrocatalytic activity towards hydrogen and oxygen evolution reactions.

(36) The G-Mn.sub.3O.sub.4—RuO.sub.2 electrode showed a much enhanced catalytic activity for both reactions in alkaline media when compared to G-RuO.sub.2 or G-MnO.sub.2: the H.sub.2 evolution reaction (HER) occurs at −0.06 V and O.sub.2 evolution reaction (OER) occurs at 1.37 V. This gives an overall water splitting potential of 1.43 V (which is close to the thermodynamic potential, 1.23 V) by contrast to 1.6 V at G-RuO.sub.2. The R.sub.CT of OER at G-Mn.sub.3O.sub.4—RuO.sub.2 was also ten times lower than the ROT of G-RuO.sub.2 which demonstrates that the kinetics of the OER is much faster at the bi-metal oxide-graphene composite.

(37) Use of the Material as an Electrode

(38) Electrode Preparation

(39) Prior to dispersing the exfoliated product in DMF, the powder was annealed under air at 250° C. for graphene functionalised with MnO.sub.2 (G-MnO.sub.2) and at 120° C. for graphene functionalised with RuO.sub.2 (G-RuO.sub.2) for 2 h. The dispersed ink was then filtered over polytetrafluroethylene (PTFE) membrane using a syringe pump dispenser (New Era Pump Systems, Inc, NY) at a rate of 10 mL hr.sup.−1. The membrane was then dried in vacuum oven at 100° C. for overnight. Coin cell assembly was prepared in standard CR2032 coin cell hardware with symmetrical active materials. The cells were assembled by stacking the two symmetrical membranes back-to-back with the active material contacting the current collector. A few drops of deoxygenated 1 M H.sub.2SO.sub.4 (aq) were added to fill the electrode before the coin cell was sealed using a hydraulic crimping machine (MSK-160D). The specific capacitance was calculated using the best practice methods established by Stoller and Ruoff..sup.[4]

(40) A three-electrode cell consisting of a 5 mm-diameter glassy carbon (GC) rotating disk working electrode, a saturated calomel reference electrode, and a Pt mesh counter electrode (area of 5 cm.sup.2) was used for hydrogen and oxygen evolution reaction measurements. The desired electrocatalyst ink was prepared by sonicating a mixture of 5 mg of the desired powder (G-MnO.sub.2, G-RuO.sub.2 or G-Mn.sub.3O.sub.4—RuO.sub.2 in 1 mL of N,N′-dimethylformamide and 50 μL of Nafion® (5%, Sigma-Aldrich) for 20 min. The GC electrode was modified by drop coating 10 μL of the above solution, which was then dried at room temperature in air. Polarization curves were obtained while rotating the GC electrode at 1600 rpm at 10 mV s.sup.−1 using deoxygenated 0.1 M KOH (aq) under N.sub.2 atmosphere.

(41) Electrolyte

(42) The charge storage mechanism of ruthenium oxide is based on rapid protonation of the oxide. The successive electron transfer at the metal centres (Ru.sup.4+, Ru.sup.3+, Ru.sup.2+) is balanced by proton transfer for the interconversion of O.sub.2 to OH in the oxides. RuO.sub.2 generally has much higher capacitance than most high surface area carbon-based electrodes when operating in an aqueous acidic media. However, in the absence of proton source such as in organic electrolytes its capacitance is significantly lower than most carbon-based electrodes. This is due to the inaccessibility of its pseudocapacitance by lack of proton source; and the overall capacitance is only accounted from the electrical double layer. As a result, a supercapacitor device that employs RuO.sub.2 as electrode material suffers from low energy density.

(43) The present inventors addressed this problem in their design of a cell using the ruthenium mono- and mixed oxide-decorated materials described here. They did this through design of a protic ionic liquid (PIL) to provide a proton source in the electrolyte. The PIL may be provided in a solvent for use as an electrolyte. It may be used in cells having electrodes comprising RuO.sub.2, for example the RuO.sub.2decorated materials of the present invention, and in other cells where proton transfer mechanisms are implicated.

(44) This ionic liquid is diethylmethylammonium bisulfate ([dema][HSO.sub.4]). It is formed through proton transfer from a Brønsted acid (diethylmethylamine) to a Brønsted base (sulfuric acid). It is a viscous liquid at room temperature.

(45) ##STR00001##

(46) Diethylmethylammonium bisulfate ([dema][HSO.sub.4]) may be synthesised using literature methods,.sup.[5] in which sulfuric acid were added to diethylmethylamine drop wise (1.05 to 1.0 molar ratio of base to acid respectively). During addition of the acids to bases, the mixtures were stirred under an Ar atmosphere in an ice bath. The resulting colourless viscous liquid was dried at 100° C. under vacuum oven.

(47) Note that this PIL structure contains protons on both the cation and anion so that the pseudocapacitance of RuO.sub.2 can be accessible at negative and positive electrodes during charge-discharge process. This PIL was dissolved in acetonitrile (1.0 M concentration) and used as electrolyte in a cell having G-MnO.sub.2—RuO.sub.2 symmetrical electrodes. The capacitance was compared to 1.0 M [TEA][BF.sub.4] in acetonitrile (FIG. 6). The current measured using the electrolyte containing [dema][HSO.sub.4] was significantly higher than the current measured using [TEA][BF.sub.4] at the same υ indicating that the PIL is providing H.sup.+ for the metal oxide's redox reaction.

(48) This resulted in a specific capacitance which is five times (287 F g.sup.−1 at 5 mV s.sup.−1) higher than the capacitance of the electrolyte that do not contain protons ([TEA][BF.sub.4]50 F g.sup.−1 at 5 mV s.sup.−1). Moreover, the shape of the CV and the charge-discharge curve showed responses that are expected for capacitive behaviour for this novel electrolyte. These data demonstrate that PIL-based electrolytes may be useful as a proton source in metal-oxide electrode-based cells.

(49) The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

(50) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(51) For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

(52) Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

(53) Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

(54) It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/−10%.

REFERENCES

(55) A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein. [1] WO 2015/158711 [2] Parvez, Z. S. Wu, R. J. Li, X. J. Liu, R. Graf, X. L. Feng, K. Mullen, Exfoliation of Graphite into Graphene in Aqueous Solutions of Inorganic Salts, J. Am. Chem. Soc. 136(16) (2014) 6083-6091. [3] Chigane, M.; Ishikawa, M.; Manganese oxide thin film preparation by potentiostatic electrolyses and electrochromism, M. J. Electrochem. Soc. 147(6) (2000), 2246-2251.[4] M. D. Stoller, R. S. Ruoff, Best practice methods for determining an electrode material's performance for ultracapacitors, Energ & Environ Sc. 3(9) (2010) 1294-1301. [5] Zhang, S. G.; Miran, M. S.; Ikoma, A.; Dokko, K.; Watanabe, M. J. Am. Chem. Soc. 136(5) (2014) 1690-1693.

(56) “One-step Electrochemical Synthesis of Graphene/Metal Particle Nanocomposite” Dizaji et al., 6th Int. Conf. Nanostructures March 2016.