Field of the Invention

The invention provides an electrolyte composition that is useful in the exfoliation of graphite, a method of exfoliating graphite, a graphene product, conductive ink comprising the graphene, and supercapacitor comprising an electrode comprising the graphene.

Background

Due to its high conductivity, chemical stability and intrinsic flexibility, graphene is a prominent contender as a conductive medium for its application in energy storage devices, electronic displays and health diagnostics.

However, a single flake of graphene is generally fragile, and must be handled by a supporting substrate. This hampers its practical application in 3D structures. 3D printing technology provides a versatile and cost effective route to assemble graphene flakes into digitally designed architectures with inherited excellent electrical and mechanical properties.

The large-scale synthesis of high quality solution processable graphene flakes for the formation of conductive inks at low cost is one of the major roadblocks towards its commercialisation. Liquid-phase exfoliation of graphite suffers from low exfoliation efficiency. The chemical reduction of graphene oxide usually results in the production of graphene with low crystallinity, and poorer electronic conductivity than that of pristine graphene. Moreover, energy-intensive and violent exfoliation procedures including long-time sonication, high- oxidation voltage or reactive regents are generally adopted to achieve a high exfoliation rate of graphene (e.g. gram scale per hour). Unfortunately, the lattice of graphene can be damaged under these harsh conditions, which degrades its crystallinity and conductivity.

In contrast, a gentle exfoliation approach with stimuli of a low cathodic voltage provides an alternative route to drive intercalants into the graphite interlayer space. However, the relatively slow exfoliation rate (generally below 1 g/h) drastically hinders the practical application of this technology. Summary of the Invention

To tackle these critical issues, the inventors have developed an electrolyte that is useful in the exfoliation of graphite to graphene. The electrolyte of the invention provides an improved exfoliation rate of graphite to provide graphene with high crystallinity and conductivity at high yields. In contrast to the conventional technologies used for graphene production, the electrolyte of the invention allows a reliable and rapid exfoliation of high conductive graphene at an economically competitive cost.

The graphene produced using the electrolyte may be useful in conductive inks for 3D printing, which allows the production of an electrode useful in supercapacitors.

Thus, the invention provides the following numbered clauses.

1. An electrolyte composition for exfoliation of graphite, comprising: a first quaternary ammonium salt; a second quaternary ammonium salt; and an electrolyte solvent, wherein: the first quaternary ammonium salt has the formula (R ¹ ) ₄ N ⁺ X- where each R ¹ independently represents a linear C1.2 alkyl group and X- is a counterion; and the second quaternary ammonium species has the formula (R ² )4N ⁺ Y; where each R ² independently represents a linear or branched C5-24 hydrocarbyl group and Y ^_ is a counterion.

2. The electrolyte composition according to Clause 1 , wherein the first quaternary ammonium salt comprises tetramethylammonium with a counterion.

3. The electrolyte composition according to Clause 1 or Clause 2, wherein each R ² group independently represents (e.g. one R ² group represents) a linear or branched C5-18 alkyl group that is optionally interrupted by a phenylene group, which phenylene group is optionally substituted by one, two, or three C1.2 alkyl groups, optionally wherein each R ² independently represents a linear or branched C5-8 alkyl group.

4. The electrolyte composition according to any one of the preceding clauses, wherein the second quaternary ammonium salt comprises tetrahexylammonium with a counterion.

5. The electrolyte composition according to any one of the preceding clauses, wherein: (a) the second quaternary ammonium salt is present at a concentration of from 0.01 to 1 M, optionally from 0.05 to 0.5 M, more optionally from 0.1 to 0.8 M, such as from 0.15 to 0.6 M, for example about 0.2 M; and/or

(b) the molar ratio of the first quaternary ammonium salt to the second quaternary ammonium salt is from 1 :30 to 1:1, optionally from 1:20 to 1 :5, more optionally from 1:15 to 1:7, such as from 1 :13 to 1 :8, e.g. about 1:10.

6. The electrolyte composition according to any one of the preceding clauses, wherein the electrolyte solvent comprises one or more of the group consisting of dimethylformamide, a glyme solvent, a cyclic carbonate, a linear carbonate, a cyclic ester, a linear ester, a cyclic or linear ether other than a glyme, a nitrile, dioxolane or a derivative thereof, ethylene sulfide, sulfolane, and sultone or a derivative thereof.

7. The electrolyte composition according to any one of the preceding clauses, wherein the electrolyte solvent comprises a polar aprotic solvent, optionally wherein the polar aprotic solvent has a dielectric constant of from 30 to 100.

8. The electrolyte composition according to any one of the preceding clauses, wherein the electrolyte solvent comprises one or more selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, acetonitrile, dimethylformamide, dimethyl sulfoxide, and N-Methyl-2- pyrrolidone, optionally wherein the electrolyte solvent comprises one or more selected from the group consisting of propylene carbonate and dimethylformamide.

9. A method of exfoliating graphite, comprising the steps:

(a) providing a system comprising: a working electrode comprising graphite; a counter electrode; and an electrolyte composition as defined in any one of Clauses 1 to 7, where the working electrode and counter electrode are at least partly immersed in the electrolyte composition; and

(b) applying a potential difference across the working electrode and counter electrode to generate graphene from the working electrode.

10. The method according to Clause 9, wherein the potential difference is from about 0.5 to 20 V, optionally from about 0.5 to 10 V, such as about 1 to 7 V, for example about 1.5 to 6 V (e.g. about 2 to 5 V). 11. The method according to Clause 9 or 10, which method provides over 70% exfoliation of the graphite in the working electrode to graphene within 2 minutes from the application of the potential difference, optionally over 75%, such as over 80% (e.g. over 90%) exfoliation of the graphite in the working electrode to graphene within 2 minutes from the application of the potential difference, optionally wherein the method provides 70% exfoliation of the graphite in the working electrode to graphene within 20 seconds from the application of the potential difference.

12. The method according to any one of Clauses 9 to 11, wherein the counter electrode comprises one or more selected from the group consisting of a metal and graphite, optionally wherein the counter electrode comprises one or more selected from the group consisting of stainless steel, titanium, nickel, platinum and graphite.

13. The method according to any one of Clauses 9 to 12, wherein the counter electrode comprises graphite and applying a potential difference across the working electrode and counter electrode comprises applying an alternating current, optionally wherein the counter electrode comprises graphite foil.

14. The method according to any one of Clauses 8 to 13, wherein the working electrode comprises graphite foil.

15. A graphene obtainable by the method according to any one of Clauses 8 to 14.

16. A graphene, having a carrier mobility above 1000 cm ² V' ¹ s’ ¹ , optionally having a carrier mobility above 1100 cm ² V' ¹ s’ ¹ , such as above 1200 cm ² V' ¹ s’ ¹ .

17. The graphene according to Clause 16, having an average area of from 2 to 2.5 pm ² , such as about 2.3 pm ² .

18. The graphene according to Clause 16 or Clause 17, having an average thickness of from 2 to 3 nm, such as about 2.6 nm.

19. The graphene according to any one of Clauses 16 to 18, comprising less than 3 molar% oxygen. 20. A conductive ink for 3D printing, comprising a graphene according to any one of Clauses 15 to 19.

21. The conductive ink according to Clause 20, comprising from 1 to 10 wt. % graphene according to any one of Clauses 15 to 19, optionally from 3 to 7 wt. %, such as about 5 wt. %.

22. A supercapacitor comprising an electrode comprising a composite material formed from a graphene according to any one of Clauses 15 to 19 and MnC>2.

Brief Description of the Figures

Figure 1 shows the electrochemical exfoliation of graphite foil with various substituted ammonium cations, (a) Schematic illustration of the cathodic exfoliation process, (b) The exfoliation rate for different sized cations, (c-i) SEM morphology and (j) X-ray diffraction spectra of exfoliated graphene via ammonium cations (concentration 0.1 mol/L) with different substituted alkyl chains.

Figure 2 shows the exfoliation of graphite in a propylene carbonate electrolyte with mixed ammonium cations, (a) l-V curve of graphite foil with different cathodic voltages applied, (b) The exfoliation yield of graphite foil as a function of tetrahexylammonium (THA) concentration in the electrolyte, (c-d) Photographs of graphite foil after a cathodic exfoliation at -5 V vs. Pt in propylene carbonate electrolyte containing 0.01 M tetramethylammonium (TMA) and 0.2 M tetrahexylammonium (THA).

Figure 3 shows the structure characterization of exfoliated graphene, (a) Scanning transmission electron microscopic and (b) AFM images of as exfoliated graphene. Inset in panel (a) shows the selected area electron diffraction pattern of graphene lattice, (c) The cross-sectional height profile and (d) Raman spectrum of graphene flakes as denoted in panel (b). The average lateral size and vertical thickness of as exfoliated graphene based on the AFM statistics, (g) FET device and (h) transport measurement of graphene flakes.

Figure 4 shows the C-C bond peak in the X-ray photoelectron spectroscopy spectrum for the exfoliated graphene.

Figure 5 shows the exfoliated graphene added into ink for 3D printing, (a) Conceptual demonstration for the large scalability of this exfoliation method, in which ~6.5 g graphene can be exfoliated from 9 g graphite foil in 20 seconds, (b) Schematic illustration of printing graphene into 3D architecture, (c) Photograph and (d) SEM image of printed 3D graphene framework, (e) Galvanostatic charge-discharge curves, (f) rate performance, (j) cyclic voltammogram profiles and (h) electrochemical impedance spectroscopic plots of the MnO2- Graphene electrode in supercapacitor device.

Detailed Description

The invention provides an electrolyte composition for exfoliation of graphite, comprising: a first quaternary ammonium salt; a second quaternary ammonium salt; and an electrolyte solvent, wherein: the first quaternary ammonium salt has the formula (R ¹ )4N ⁺ X- where each R ¹ independently represents a linear C1.2 alkyl group and X- is a counterion; and the second quaternary ammonium species has the formula (R ² )4N ⁺ Y; where each R ² independently represents a linear or branched C5-24 hydrocarbyl group and Y- is a counterion.

As used herein, an “electrolyte composition” is a composition that may be used as an electrolyte in the electrochemical exfoliation of graphite to produce graphene. The electrolyte composition of the invention comprises a liquid electrolyte solvent in which a first and second quaternary ammonium salt are dissolved.

The counterions X' and Y ^_ are not particularly limited and may be any counterion that is compatible with the electrolyte solvent and which is compatible for electrochemical exfoliation of graphite to produce graphene. Suitable counterions include tetrafluoroborate (BF ₄ -), hexafluorophosphate (PFe-), bromide (Br), chloride (Ck), hydrogen sulfate (HSO4'), hydroxide (OFT), nitrate (NOs-), perchlorate (CIO4-), phosphate (PO4 ^3- ), sulfate (SO4 ^2- )-

The first quaternary ammonium salt generally comprises a small cation, e.g. of the formula (R ¹ ) ₄ N ⁺ where each R ¹ independently represents a linear C1.2 alkyl group. While the first quaternary ammonium salt comprises a small cation, this cation is highly solvated and so the overall solvated species is large. In contrast, the second quaternary ammonium salt comprises a large cation, e.g. of the formula (R ² )4N ⁺ , where each R ² independently represents a linear or branched C5-24 hydrocarbyl group and Y ^_ is a counterion. However, the second quaternary ammonium salt is less highly solvated than the first quaternary ammonium salt, and so the overall solvated species is smaller than that of the first quaternary ammonium salt. Without being bound by theory, this difference in size of the solvates species is believed to result in different intercalation behaviour of the solvated species with the graphite that is to be exfoliated. The smaller sized solvated species (i.e. the larger cations, e.g. tetrahexylammonium) preferentially intercalate into the interlayer galleries of graphite, while the larger solvated species (i.e. the smaller cations, e.g. tetramethylammonium) synergistically peel off thin flakes from the graphite electrode. These exfoliated graphene flakes have excellent crystallinity, very high electrical conductivity and superb solution processability.

In some embodiments of the invention, the first quaternary ammonium salt comprises tetramethylammonium with a counterion.

The second quaternary ammonium salt has the formula (R ² ) ₄ N ⁺ Y; where each R ² independently represents a linear or branched C5-24 hydrocarbyl group and Y- is a counterion. In some embodiments of the invention each R ² group independently a linear or branched C5- 18 alkyl group that is optionally interrupted by a phenylene group, which phenylene group is optionally substituted by one, two, or three C1.2 alkyl groups. For example, one or two (e.g. one) R ² group may represent a linear or branched C5-18 alkyl group that is optionally interrupted by a phenylene group, which phenylene group is optionally substituted by one, two, or three C1.2 alkyl groups. As used in this context, “interrupted” means that a C-C covalent bond in the alkyl chain is replaced by a phenylene group. The phenylene group may be located at any position in the alkyl chain, and the connectivity of the alkyl chain on the phenylene group may be at any position (e.g. 1 ,2 [o/YPo]; 1 ,3 [meta]; or 1 ,4 [para]).

In some embodiments of the invention the linear or branched C5-18 alkyl group may be a linear or branched C5-14 alkyl group, such as a linear or branched C5-12 alkyl group, linear or branched C5-10 alkyl group, or linear or branched C5-8 alkyl group, all of which may be optionally interrupted by a phenylene group, which phenylene group is optionally substituted by one, two, or three C1.2 alkyl groups.

In some embodiments of the invention, each R ² independently represents a linear or branched C5-8 alkyl group. In some embodiments of the invention the second quaternary ammonium salt comprises tetrahexylammonium with a counterion.

In some embodiments of the invention, the second quaternary ammonium salt may be present in the electrolyte composition at a concentration of from 0.01 to 1 M, optionally from 0.05 to 0.5 M, more optionally from 0.1 to 0.8 M, such as from 0.15 to 0.6 M, for example about 0.2 M.

In some embodiments of the invention, the molar ratio of the first quaternary ammonium salt to the second quaternary ammonium salt is from 1:30 to 1 :1 , optionally from 1 :20 to 1:5, more optionally from 1:15 to 1 :7, such as from 1 :13 to 1 :8, e.g. about 1 :10. Without being bound by theory it is believed that this ratio provides an optimised balance between the two intercalation behaviours of the different quaternary ammonium salts discussed above, leading to improved exfoliation rates and high graphene quality/yields.

Any appropriate solvent may be used in the electrolyte composition. Suitable solvents are well known to a person skilled in the art and include polar aprotic solvents, especially polar aprotic solvents having a dielectric constant of from about 30 to about 100. Suitable solvents that may be used in some embodiments of the invention include one or more of the group consisting of a glyme solvent, a cyclic carbonate solvent, a linear carbonate solvent, a cyclic ester solvent, a linear ester solvent, a cyclic or linear ether solvent other than a glyme, a nitrile solvent, dioxolane or a derivative thereof, ethylene sulfide, sulfolane, and sultone or a derivative thereof. Additionally, a solvent that may be used in some embodiments of the invention is dimethylformamide. Particular solvents that may be useful in embodiments of the invention disclosed herein include linear carbonate solvents and dimethylformamide.

In some embodiments of the invention the electrolyte solvent comprises one or more selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, acetonitrile, dimethylformamide, dimethyl sulfoxide, and N-Methyl-2-pyrrolidone. Particular solvents that may be used in some embodiments of the invention include propylene carbonate and dimethylformamide.

The invention also provides a method of exfoliating graphite to produce graphene, using the electrolyte composition. Thus, the invention provides a method of exfoliating graphite, comprising the steps:

(a) providing a system comprising: a working electrode comprising graphite; a counter electrode; and an electrolyte composition as defined herein, where the working electrode and counter electrode are at least partly immersed in the electrolyte composition; and (b) applying a potential difference across the working electrode and counter electrode to generate graphene from the working electrode.

During the course of the method, the graphite working electrode is exfoliated by the electrolysis reaction and forms flakes of high quality graphene.

In embodiments of the invention, the potential difference may be from about 0.5 to 20 V. In general, a potential difference having a magnitude of below 0.5 V may be too low to result in exfoliation, while a potential difference with a magnitude of above 20 V may produce high amounts of heat and rapid decomposition the electrolyte, possibly leading to partial oxidation of the graphene. In further embodiments of the invention that may be mentioned herein, the potential difference may be about 0.5 to 10 V, such as about 1 to 7 V, for example about 1.5 to 6 V (e.g. about 2 to 5 V). As will be appreciated by a person skilled in the art, the potential differences listed above refer to the magnitude of the potential difference. When the working electrode comprises graphite and the counter electrode comprises a non-graphite conducting material (e.g. a metal), the potential difference may be negative, i.e. a potential difference of -0.5 V to -20 V versus the counter electrode. However, in some embodiments of the invention the counter electrode may also comprise graphite. In such embodiments, an alternating current may be used in order to generate graphene from both electrodes.

The method of the invention rapidly provides high yields of graphene. Thus, in some embodiments of the invention the method provides over 70% exfoliation of the graphite in the working electrode to graphene within 2 minutes from the application of the potential difference. In other words, the method may provide a 70% yield of graphene within 2 minutes from the application of the potential difference. In further embodiments of the invention, the method may provide over 75%, such as over 80% (e.g. over 90%) exfoliation of the graphite in the working electrode to graphene within 2 minutes from the application of the potential difference.

In some embodiments of the invention, the method provides 70% exfoliation of the graphite in the working electrode to graphene within 20 seconds from the application of the potential difference.

As will be appreciated by a person skilled in the art, the working electrode comprises graphite that is to be exfoliated. The counter electrode may comprise any suitable material. For example, in some embodiments of the invention the counter electrode may comprise one or more selected from the group consisting of a metal and graphite. In some embodiments, the counter electrode may comprise one or more selected from the group consisting of stainless steel, titanium, nickel, platinum and graphite.

In particular embodiments of the invention, the graphite in the working/counter electrodes may be graphite foil.

The invention provides a graphene obtainable by the method of the invention.

The invention provides a graphene having a carrier mobility above 1000 cm ² V' ¹ s’ ¹ , for example having a carrier mobility above 1100 cm ² V' ¹ s’ ¹ , such as having a carrier mobility above 1200 cm ² V' ¹ s ^-1

In some embodiments, the graphene of the invention may have an average area of from 2 to 2.5 pm ² , such as about 2.3 pm ² .

In some embodiments, the graphene of the invention may have an average thickness of from 2 to 3 nm, such as about 2.6 nm.

In some embodiments, the graphene of the invention may have less than 3 molar% oxygen.

The invention also provides a conductive ink for 3D printing, comprising a graphene according to the invention. In some embodiments of the invention the conductive ink for 3D printing may comprise from 1 to 10 wt. % graphene according to the invention, optionally from 3 to 7 wt. %, such as about 5 wt. %.

The invention also provides a supercapacitor comprising an electrode comprising a composite material formed from a graphene according to the invention and MnC>2.

Thus, the invention is able to rapidly provide extremely high quality and high value graphene at high yields from very low cost graphite precursors, using simple reagents and apparatus. This is demonstrated further in the below Examples. Examples

Exfoliation of graphite

Where necessary, commercial graphite powders were pressed into foil form to facilitate electrochemical exfoliation.

The electrochemical exfoliation of graphite was conducted using an electrochemical workstation (CHI 760E) consisting of a two electrode system. Graphite foil (e.g. L.T Graphite, Shanghai, China) with thickness of 3 mm was fixed via a metal clip as the working cathode, and a platinum mesh was used as the counter electrode. The electrolyte was a non-aqueous solution consisting of 0.2 M tetrahexyl ammonium and 0.02 M tetramethyl ammonium in propylene carbonate (PC). The electrochemical intercalation of graphite was achieved once a cathodic voltage lower than -0.5 V versus Pt is applied onto the graphite foil. The intercalated graphite can immediately expand and detach into solution, a constant voltage is applied until the thorough detachment of the graphite into electrolyte. The intercalated graphite was collected via filtration, and further washed with water 3 times to eliminate the organic residuals. Finally the intercalated graphite can be further dispersed into desired solvents via bath sonication with a power of 100 W for 0.5 h to provide a uniform graphene dispersion.

To evaluate the intercalation behaviour of various ammonium cations in graphite, the inventors tested a series of tetra-substituted cations with different hydrocarbyl chains (alkyl chains ranging from methyl to octadecyl, and branched/interrupted groups such as methyldodecylbenzyl trimethyl ammonium), dissolved in propylene carbonate (PC) as electrolyte. As shown in Figure 1b, the exfoliation rate of graphite foils show a monotonic decrease as a function of the alkyl length of ammonium cations. Tetra-methyl ammonium cations (TMA, n=1) offer fast intercalation and delamination of graphite foil, which can readily disintegrate graphite foil into powders in 2 minutes with a high yield (approaching 100%). Longer alkyl chains and branched hydrocarbyl groups generally showed slower intercalation kinetics, resulting in thicker flakes and lower yields of graphene. Scanning electron microscopy images in Figure 1c reveal that solvated TMA cations only enter the inter-space between graphite powders rather than graphite interlayers. X-ray diffraction spectra further show that cations with longer alkyl chains (n>5) can intercalate into the graphite interlayer galleries with an intercalation peak at ~23 degree. Therefore, TMA cations with larger solvation size allow for a fast intercalation rate of graphite due to its relatively high solvation effect. In contrast, tetra-hexyl cations (THA, n=6) with a relatively small solvated size can uniformly intercalate into the interlayer galleries of graphite, which can favour the production of atomically thin graphene flakes.

To achieve a fast exfoliation rate and high yield of graphene thin flakes, a mixture of TMA and THA ammonium cations was used to synergize the exfoliation behaviours. The l-V curve of graphite foil was investigated in an electrolyte comprising TMA 0.01 mol/L mixed with THA 0.2 mol/L in PC solvent. A rapid increase of the cathodic current at -0.5 V vs. Pt indicates the onset potential of cations intercalation into graphite. An increase of the cathodic voltage below -2 V vs. Pt can further trigger the decomposition of PC solvent, leading to a fast exfoliation and detachment of thin graphene flakes from the graphite electrode. A detailed study revealed that a high exfoliation yield above 60% can be obtained with the concentration of THA between 0.15-0.6 mol/L (Figure 2b). As shown in Figure 2c, graphite may be readily exfoliated in the minute time scale under (e.g. at -5 V vs. Pt in PC electrolyte including 0.01M TMA and 0.2 M THA).

In addition to the above-described protocol, exfoliation of graphite was also performed using dimethylformamide (DMF) as a solvent. There were no significant differences between the rate, yield and quality of graphene produced in DMF as compared to that produced in PC. As such, it is expected that the results demonstrated herein would be obtained with a variety of polar aprotic solvents.

Since DMF is a cheaper solvent than PC, it may advantageously be used to provide high quality graphene at lower cost.

Characterisation of graphene product

The structure and electrical properties of exfoliated graphene was characterized as shown in Figure 3. Scanning transmission electron microscopy shows that exfoliated graphene reveals an intact atomic lattice in large area. A statistical analysis of multiple atomic force microscopy (AFM) images reveals that exfoliated graphene exhibits an average size of 2.3 pm ² with an average thickness of 2.6 nm. Electrical transport measurement indicates a record high carrier mobility around 1200 cm2 V-1 s-1 , surpassing the best result of solution- exfoliated graphene (405 cm ² V' ¹ S’ ¹ , J. Am. Chem. Soc. 2015, 137, 13927-13932).

As shown by elemental measurements using X-ray photoelectron spectroscopy (Figure 4), the content of oxygen in the exfoliated graphene is lower than 3%, which is substantially lower than that obtained by current solution synthesis methods. Conductive ink, 3D printing and supercapacitor formation

Exfoliated graphene may be formulated into a conductive ink for 3D printing.

The scalability of this method was assessed via increasing the amount graphite foil to 9 g (keeping the other parameters unchanged). As shown in Figure 5a, about 6.5 g graphene can be produced in 20 seconds, suggesting that the method is highly scalable. Exfoliated graphene was mixed with commercial Pluronics F127 (molecular formula: H(OCH2CH2)X(OCH2CHCHS)Y(OCH2CH2)ZOH) to form a continuous ink with a weight ratio of graphene about 5%, which can be directly used for 3D printing in the extrusion mode. Pure graphene framework can be finally obtained after a calcination treatment to eliminate the polymer composite at 500 °C for 1 h. The 3D graphene framework has a highly porous surface as shown in the SEM image of Figure 5d.

An electrochemical deposition of MnC>2 on the graphene framework surface was then performed. As-formed MnO2-graphene was used as the electrode in supercapacitor devices. The MnC>2-graphene electrode reveals a specific capacitance of 0.83 F cm ^-2 at a current density of 1 mA cm ^-2 , and 42.4% capacitance can be well retained even at a large current density of 30 mA cm ^-2 . The small internal resistance ~0.5 ohm (calculated by extrapolating the vertical portion to the real axis in the Nyquist plots in Figure 5h) indicates the excellent electrochemical conductivity of graphene framework, responsible for the outstanding electrochemical performance.

In conclusion, the inventors have developed a cathodic exfoliation method using an electrolyte comprising mixed ammonium cations that is able to rapidly produce high quality graphene flakes. The method provides an easily scalable route for the synthesis of highly conductive graphene flakes with a high yield (> 70%) and production rate (>1 Kg/h). Atomically thin graphene flakes reveal record high carrier mobility and excellent solution processibility, which can be easily formulated into ink for 3D printing of functional architectures and devices with remarkable conductivity.