WO2012120291A1

WO2012120291A1 - Electrolytic exfoliation of graphite

Info

Publication number: WO2012120291A1
Application number: PCT/GB2012/050487
Authority: WO
Inventors: Christopher Stirling
Original assignee: Morganite Electrical Carbon Limited
Priority date: 2011-03-10
Filing date: 2012-03-05
Publication date: 2012-09-13
Also published as: WO2012120291A4; GB2488825A; GB201317934D0; GB2503838B; GB201104078D0; GB2503838A

Abstract

An electrolytic technique for exfoliation of graphite is presented. The electrolytic technique utilises a previously disclosed electrode material to yield unexpected results. Intercalated graphite particles comprising a gradation of expansion may be produced according to the technique.

Description

Electrolytic Exfoliation of Graphite

This invention relates to graphite and graphene species. Background

Graphene materials and other graphite nanoparticles are of interest for use in a number of applications, including energy storage and conductive materials. A number of methods have been developed to produce graphite nanoparticles, including micromechanical cleavage, epitaxial growth via ultra-high vacuum graphitisation, chemical synthesis through oxidation of graphite, chemical vapour deposition techniques, and solvothermal synthesis.

Wang et al (Carbon 47 (2009) pp. 3242-3246) have reported a technique for producing graphene nanosheets via electrolytic exfoliation in ionic liquids, using high purity graphite rods as electrodes. Amongst the benefits of such methods over the more commonly acid intercalation methods is that they do not necessitate the use of dangerous and environmentally harmful chemicals as part of the process. Another electrolytic technique is disclosed in the material presented by Liua et al at http://practice.nenu.edu.cn/kycgl/lunwen/56.html (website retrieved 22/02/2011). An electrochemical method to produce nano-scaled graphene platelets is disclosed in US patent application no. US2009026086 (Al) where a bed of graphite materials dispersed in electrolyte is electrochemically intercalated to produce a graphite intercalation compound. Geng et al (Journal of Nanoscience and Nanotechnology, Volume 11, No. 2, February 2011, pp. 1084-1091(8)) also cite a method for electrochemically intercalating graphite to produce graphite intercalation compounds. In both US2009026086 and Geng et al the graphite intercalation compounds were subsequently expanded by, for example, thermal or microwave treatments.

An alternative approach to manufacture of graphene is to use kish graphite and mechanically exfoliate it. Kish graphite is a by-product of some iron making processes. Kish graphite typically has high crystallinity but high impurity levels and requires acid leaching to get products with less than 5% impurities. Summary of Invention

There is a recognised need in the art for means of improving the yield and the quality of graphitic nanoplatelets yielded by electrolytic exfoliation methods, preferably without incorporating the use of dangerous or environmentally harmful chemicals.

The present invention makes use of electrode materials comprising aligned graphite flakes (natural or synthetic) bonded with a binder, such as (but not limited to) the carbonised residue of a mesophase pitch, in order to attain an unexpectedly high yield of quality nanoparticulate materials. The PCT application publication no.

WO02/090291A1 discloses such a graphitic body incorporating flakes with a flake size of at least 200 μιη - page 11, lines 21-28 of the publication in particular discloses one method by which alignment may be achieved, though the skilled person will appreciate that it is not the only method for achieving alignment. Likewise, the PCT application publication no. WO2007/063309A1 discloses another such material suitable for use as the electrode material in the present invention. In

WO2007/063309A1 too, the binder may be the carbonised residue of an organic precursor [for example a mesophase pitch]. The skilled person will appreciate that these two examples are not the only electrode materials that could meet the requirements of the present invention.

The scope of the invention is as set out in the appended claims. Description of Figures

Figure 1 is a scanning electron microscopy image of a graphite species produced according to the present invention.

Detailed Description

Electrolytic tests were performed using an electrode material comprising aligned graphite flakes in plate form (so-called "K-plane" or "K-p" plates), using a 0.001M aqueous solution of poly(sodium 4- styrene- sulfonate) as an electrolyte solution and a similar DC voltage to the experiments disclosed by Wang et al (Carbon 47 (2009) 3242-3246). The aligned graphite flakes in the K-plane plates were Graphite V-RFL 99.5 +500 a 99.86% pure natural graphite available from Graphitwerke Kropfmiihl which has a flake size distribution [% by weight > than specified sieve size]: 30%>800μιη, 77%>630μιη, 95%>500μιη. The flakes were bonded with a binder comprising phenolic resin, which after bonding was carbonised or graphitised as indicated.

WO2007/063309A1 contains full details for manufacture of such plates.

The plates had dimensions of 85mm length by 35 mm width and approximately 3 mm thickness. For comparison, tests were also performed using various types high purity (<0.1 wt% ash) graphite rods. Table 1 summarises observations from these experiments; experiments 1, 3, 14 and 15 took place utilising electrode materials comprising aligned graphite flakes in plate form according to the invention, whilst experiments 2, 4, 5, 12, and 13 were comparative experiments utilising graphite rods.

In the tests utilising the electrode material comprising aligned graphite flakes it was observed that gas was evolved at both electrodes with an application of 4-6 Volts, with greater volumes being generated at the negative electrode, unexpectedly considering that this was not mentioned in Wang et al. The gas produced is believed to result from electrolysis of water.

After 10 minutes from the commencement of electrolysis, evidence was observed of particle generation/detachment on the edges of the positive electrode. After approximately 20 minutes from the commencement of electrolysis relatively large particles had detached and floated up to the surface of the liquid, accumulating in a froth around the interface between the electrode, the solution and the air. After an hour from the commencement of electrolysis the surface of the positive electrode exhibited attack and appeared to have expanded. Subsequent examination of the positive electrode under a binocular microscope revealed that there had been actual expansion of graphite flakes at the surface of the positive electrode. Table 1: Electrolysis in ionic liquid electrolyte. Experiment Summary

By comparison, in tests utilising the high-purity graphite rods as electrode materials gas generation was also observed, again with more gas generated at the negative electrode. Furthermore, even at half an hour after the commencement of electrolysis there was no significant detachment of graphite material, as had been observed after 20 minutes with the electrode material comprising aligned graphite flakes.

Therefore, in the experiments taking place with the electrode material comprising aligned graphite flakes, there was an unexpected and different outcome.

The detached, exfoliated material generated by electrolysis utilising the electrode material comprising aligned graphite flakes was ultrasonicated in order to study the particles produced. A cloudy suspension of the floating particles was soon formed. Studied under a binocular microscope it was found that the graphite particles had lateral dimensions ranging from 5 to 50 microns. Compared to commercially-obtained nanoplatelets (15 micron-sized exfoliated graphite nanoplatelets from XG Sciences) the platelets were of a similar size, but those produced via electrolysis utilising the electrode material comprising aligned graphite flakes had more pristine and flatter surfaces than the commercial platelets.

A number of additional comparative trials took place utilising several other grades of commercial graphite in the form of rods as the electrode materials. In all trials attempted no other materials have produced exfoliated graphite to anything like the same extent as the electrode materials comprising aligned graphite flakes. The applicant believes that the key factors contributing to this are the size and shape of the aligned graphite particles comprised in the electrode material, the particles' presentation to the electrolyte, and the relative electrolytic effects at the flake "basal" surface versus the "edge", all of which have an effect on the intercalation process.

The results with the flake graphite composite (K-plane) indicated a significant improvement in rate of exfoliation versus the usual (commercial) graphites, with the potential to generate larger-sized exfoliated graphite particles, using a process that lacks the environmentally harmful components of acid intercalation processes.

The product may also be more chemically pristine and require less subsequent treatment (e.g. reduction). The applicant believes that the alignment of the graphite in an orientation

substantially parallel (within 20°) of the plane of the electrode surface is crucial to the improvement in yield. In the electrode material comprising aligned graphite flakes bonded to a binder, the graphite particles at the electrode surface are (initially) held in place at this advantageous orientation, with electrical contact maintained through the graphite. They can therefore undergo very effective electrochemical intercalation.

During the intercalation process expansion of the natural graphite can occur. Fig. 1 is a scanning electron microscope image of a particle collected from solution following a test with the electrode material comprising aligned graphite flakes bonded to a binder. The applicant believes that this expansion is facilitated by the electrolysis of the solution and generation of oxygen within the particle itself, which would explain why the large detached particles float upwards during the electrolysis process.

Orientation of the graphite flakes substantially parallel to the electrode surface would also facilitate the expansion process whilst the particle is held in place, as is evidenced by vermicular expanded graphite particles observed on the electrode surface following electrolysis. This significant expansion of the intercalated graphite without the application of heat was unexpected, and the applicant believes that this is a phenomenon that has not previously been reported in the literature on

electrochemical intercalation.

Further confirmation that the method produces expanded graphite species directly resulted from attempts to expand the produced graphite species by thermal shock methods or via microwave techniques as described in Geng et al {Journal of

Nanoscience and Nanotechnology, Volume 11, No. 2, February 2011, pp. 1084- 1091(8)). Whilst such methods will expand acid-intercalated graphite, no appreciable expansion occurred in the graphite species yielded by the electrolytic exfoliation in ionic liquids of electrodes comprising aligned Graphite V-RFL 99.5 +500 graphite flakes bonded to a binder. In contrast, electrodes comprising Graphite RFL 99.5 [a 99.86% pure natural graphite available from Graphitwerke Kropfmiihl which has a flake size distribution [% by weight > than specified sieve size]: 50%>200μιη, 25%>355μιη] did show evidence of expansion, but much reduced from the amount shown on acid intercalation as is discussed below. The directly-expanded graphite species exhibit topological features that may be of interest for certain applications. The accordion-like structures (e.g shown in Figure 1) exhibited a 'fan-like' appearances, indicative of a gradation of expansion throughout the intercalated particle. This produces a structure with increased surface area that is accessible to fluids, but still maintaining good connectivity between the expanded sheet. This is expected to offer advantages over graphite materials expanded by other methods, for applications such as energy storage or adsorption.

In addition to the directly-expanded graphite species' own properties, the species may be a useful precursor to other graphite species. As is discussed above, small thin flake-like graphite particles may be produced from the directly-expanded graphite species via ultrasonication and mechanical methods of producing such small thin flake-like particles from the directly-expanded species may also exist.

Subsequent to the above experiments utilising the ionic poly(sodium 4-styrene- sulfonate) as an electrolyte, a new set of experiments took place in which the "K- plane" plate electrodes comprising aligned graphite flakes bonded to a binder were utilised with a 4.5M sulphuric acid solution as electrolyte which yielded surprising results.

The first experiment took place with an applied potential of 6V and resulted in a large amount of gas evolution at the negative electrode. Generation of particles at the positive electrode took place immediately. After 3 minutes the applied potential was reduced to 3V; gas evolution was significantly reduced as a result. After 10 minutes significant attack of the positive electrode was noted, especially at the plate edges, though particles were not observe to lift from the electrode surface. At 10 minutes the voltage was increased to 3.5V, and there was a notable increase in evolution of gas at both electrodes. By 17 minutes particles were free in the solution and were floating to the solution's surface. By 25 minutes the particles had bridged the two electrodes. Particles in a sample taken from the solution at 27 minutes showed clear signs of expansion, and so as in the experiments utilising the ionic solution electrolyte (the poly(sodium 4-styrene sulfonate solution) direct production of expanded graphite species had occurred.

At 45 minutes the test was concluded and the materials washed and dried. It was noted that the positive electrode had lost l.Olg, but the dried powder recovered from the solution weighed 1.75g, indicating that a significant amount of intercalation had taken place. An attempt was made to further expand the graphite species produced by the experiment via thermal methods through the use of a Bunsen burner flame.

Whereas the expanded graphite species produced utilising the ionic liquid electrolyte were not observed to expand further utilising such methods, the expanded graphite species produced during these sulphuric acid electrolyte experiments yielded a surprising amount of expansion. For example some materials produced "worms" of expanded material of the order of 20mm or more from starting graphite particles of 50μιη or less indicating an expansion of the order of 400X.

Three experiments, experiments 21, 22 and 23, took place subsequently to this. These experiments again utilised an aqueous solution of 4.5M sulphuric acid as an electrolyte and the K-plane plate electrodes comprising aligned graphite flakes bonded to a binder. The electrodes were dried at 110°C and weighed prior to the experiment. At the end of the test the plates were washed and loose surface particles were removed. Particles in the electrolyte were washed, filtered, rinsed and dried at 110°C. The results and other parameters of these experiments are given in Table 2 below.

Table 2: Electrolysis in sulphuric acid electrolyte. Experiment Summary

It was observed in the above experiments that the higher voltage (3.5V vs 3V) gave a higher rate of material ejection from the electrode and higher rates of gas evolution. It was also observed that the particles floated almost exclusively to the surface of the solution. The applicant believes that the gas evolution helps detachment of particles from the electrode surface, exposing fresh material for intercalation.

It was also noted that significant attack of the positive electrode occurred on both of the major faces of the plate (the two larger faces). The attack was more pronounced on the "inner" face (the face of the positive electrode facing the negative electrode) than on the "outer" face (the face of the positive electrode facing away from the negative electrode). This was also observed in the tests in the ionic liquid electrolyte, but in those tests the difference in attack between the inner and outer faces was more pronounced, whereas in these tests with the sulphuric acid electrolyte the difference is less pronounced.

It is evident that the intercalation and generation of particles from the K-plane plate electrode comprising aligned graphite flakes bonded to a binder occurred at a very much greater rate in these experiments utilising an acid electrolyte. It will be evident to the skilled person that other electrolytes used in acid intercalation processes may be utilised with the K-plane electrodes as well. It is also notable that the experiments utilising the sulphuric acid electrolyte yielded graphite species which could be expanded subsequently to a significant extent. This is a major difference when compared with the graphite species produced in experiments utilising the ionic liquid electrolyte. (It will be obvious to the skilled person that any method utilised to expand graphite subsequent to acid intercalation could be applied.) This implies a high degree of intercalation, which may be expected to be

advantageous for obtaining high yields of thin graphite nanoplatelets after chemical reduction. The formation of graphite intercalated compounds is also indicated by the overall weight gains found in the materials.

Experiments were further conducted to electrochemically exfoliate materials comprising aligned kish graphite particles in a binder comprising the carbonised residue of a phenolic resin. This material does intercalate and produce flakes which detach from the electrodes, but does not expand as much as materials using aligned natural graphite in a binder comprising the carbonised residue of a phenolic resin. Applicant hypothesises, without wishing to be bound in any way, that this may be due to high crystallinity or aspect ratio or both.

It will be evident to the person skilled in the art that the above examples are illustrative only and variations and modifications may be made while falling within the scope of the claims.

Claims

1: A method of producing intercalating, expanded, and/or exfoliated graphite or graphene species comprising the step of electrolysis utilising electrode materials comprising graphite, characterised in that the graphite layers of the graphite are oriented within 20° to the plane of a surface of the electrode.

2: A method as claimed in Claim 1, in which the graphite layers are oriented substantially parallel to the plane of a surface of the electrode.

3: A method as claimed in Claim 1 or Claim 2, in which the surface of the

electrode which the graphite layers are oriented in respect of is the surface with the largest surface area.

4: A method as claimed in any preceding Claim, in which said surface of the electrode has a diameter of at least 10mm.

5: A method as claimed in any preceding Claim, in which the electrode material comprises aligned graphite flakes bonded by a binding material.

6: A method as claimed in Claim 5, in which the binding material is a mesophase pitch.

7: A method as claimed in Claim 5, in which the binding material is a phenolic resin.

8: A method as claimed in Claim 5, in which the binding material is carbonised or graphitised.

9: A method as claimed in any of claims 5 to 8, in which the flakes have a flake size of no less than 200 μιη. A method as claimed in Claim 1, in which the graphite materials may be selected from natural graphite, synthetic graphite, highly orientated pyrolytic graphite, graphite fibre and highly oriented graphite tape.

A method as claimed in any preceding Claim, comprising the additional step of ultrasonicating the graphite or graphene species produced subsequent to the conclusion of electrolysis.

A method as claimed in any preceding Claim, in which the electrolysis takes place in an electrolyte comprising an ionic liquid.

A method as claimed in Claim 12, in which the ionic liquid is an aqueous solution of poly(sodium 4-styrene-sulfonate).

A method as claimed in any one of Claims 1 to 11, in which the electrolysis takes place in an electrolyte comprising an acid solution.

A method as claimed in Claim 14, in which the acid solution is an aqueous sulphuric acid solution.

A graphite or graphene species produced according to the method of any preceding claim.

A graphite species comprising intercalated graphite particles, characterised in that there exists a gradation of expansion within the intercalated graphite particles.

A graphite species as claimed in Claim 17, further characterised in that the graphite particles possess a fan-like shape.

A graphite or graphene species produced by further treatment from the graphite species of Claims 16 to 18. A graphite or graphene species as in claim 19, in which the graphite or graphene species is produced by exposing the graphite species of claim 17 or 18 to ultrasonic vibration.

A graphite or graphene species as in claim 19, in which the graphite or graphene species is produced by expansion of the graphite species of any of Claims 16 to 18.