GB2587333A

GB2587333A - Electrochemical reactor

Info

Publication number: GB2587333A
Application number: GB1913371.9A
Authority: GB
Inventors: Lowe Sean
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-09-16
Filing date: 2019-09-16
Publication date: 2021-03-31
Also published as: WO2021053468A1; GB201913371D0

Abstract

An electrochemical reactor (PBER, packed bed electrochemical reactor) is described; it may take the form of a reactor vessel 101, a circular anode 102, a cathode in the form of a plate 104. A bed of particles 104 with an electrolyte (level noted at 114). The particles are comprised using a porous press 105 which is operated by a pressure pad 106. At least one conduit 107 is immersed in the bed of particles. In use, electrolyte circulates through the pipe and into the bed of particles. The reactor may be used for the production of graphite intercalation compounds (GICs). It may also be used for the production of 2D materials such as; graphene, germanene, silicene: through the electrochemical expansion and exfoliation of the parent layered material.

Description

Electrochemical Reactor This invention relates to an electrochemical reactor and in particular to a reactor for conducting an electrochemical reaction on a particulate material.

Reactant materials, such as graphite, are often used in electrochemical reactors in oxidation, reduction or other reactions. A common feature of such reactors is that the reactant material is used in the form of particles, such as flakes, or powder, which are brought into contact with an electrolyte. Known types of electrochemical reactors suitable for reactant materials include stirred tank reactors, fluidised bed reactors and packed bed reactors. Packed bed reactors are commonly used because of their high conversion rates. In a packed bed reactor, particles of material are packed together in intimate contact. For example, a packed bed reactor is described by Lowe et al in ACS Appl. Nano Mater. 2(2) (2019) 867-878.

A disadvantage of known electrochemical reactors, particularly packed bed reactors, is the fact that compaction of the particulate material can reduce diffusion of the electrolyte through the material. Often, the particulate material expands during the reaction, so that the electrolyte flow rate is further reduced. This in turn reduces the rate of electrochemical reaction and can often leave some parts of the reactant material unreacted.

Even if the electrolyte solution is pumped through the particles, the solution finds discrete pathways through the bed, leaving other parts untouched and therefore unreacted. Indeed, pumping can make the diffusion problem worse.

There is a need to increase the diffusion of electrolyte throughout particulate reactant material in electrochemical reactors, to allow larger volumes of particle substrate to be employed, and hence increase the potential for scale-up of the process.

The present invention is based on the provision of at least one porous conduit within a suspension of particulate reactant material, which conduit carries electrolyte, to enhance electrolyte diffusion throughout the suspension.

US patent 4,517,067 (Byerley et al) relates to an electrochemical bipolar reactor. In one embodiment, there is described a reactor containing particles such as graphite, wherein electrolyte is passed into a manifold at the base of the reactor, from where it is passed upward through the particles. However, the manifold containing electrolyte does not extend within the bed, where compaction can occur. It does not therefore achieve the advantages of the present invention. Furthermore, in any event, the graphite particles described therein are not reactants, but catalysts, so that consistent contact of electrolyte with the particulate material is not as critical.

US patent 5,503,717 (Kang et al) describes an electrochemical process to form a zinc chloride graphite intercalation compound. Although the reactor depicted therein does have electrolyte in a chamber to the sides of the graphite flakes, it does not have a means to carry electrolyte within the bed, where compaction can occur. It does not therefore achieve the advantages of the present invention.

US patent 4,350,576 (Watanabe et al) describes a method of producing a graphite intercalation compound. One embodiment depicts a central cathode cylinder with a net structure containing electrolyte. However, there is no suggestion of any attempt to enhance diffusion of the electrolyte through the particulate bed. Indeed, the disclosure is concerned with applying a load to the graphite particles to press all the graphite particles to the surface of an anode.

Accordingly, the present invention provides an electrochemical reactor for conducting an electrochemical reaction on a particulate material, the reactor comprising an anode, a cathode and a reactor vessel, the reactor vessel containing an electrolyte, a suspension of a reactant material in particulate form, and one or more porous conduits, each of which conduit is situated within the suspension of particulate material, wherein the electrolyte is capable of passing through the porous conduits to distribute the electrolyte through the suspension of particulate material.

Preferably, the reactor further contains means for isolating the product of the electrochemical reaction on the reactant material.

The reactor may be, for example, a packed bed reactor, a fluidised bed reactor, a stirred tank reactor, or a flow reactor. Even in a stirred tank reactor, where the particulate reactant material is in the form of a slurry, the concentration can become sufficiently high that electrolyte diffusion can become limited. Thus, the porous conduits of the present invention would be beneficial to increase the diffusion rate.

The suspension of particulate reactant material may, for example, be in the form of a bed or a slurry. Preferably, the suspension is in the form of a packed bed, in particular a mobilised packed bed. As used herein, the term mobilised packed bed means a suspension of particles that are closely packed together but may be mobilised, for example by agitation or mixing. For the sake of clarity, it is confirmed that in a mobilised packed bed, the particles are not freely dispersed throughout the electrolyte.

The porous conduits of the present invention provide a means to bring electrolyte into contact with the particulate reactant material, by allowing electrolyte to pass through the walls of the conduits to facilitate distribution. Preferably, a porous conduit is adapted to direct the electrolyte through the suspension of particulate material. For example, the conduits may be connected to the external electrolyte source and arranged to introduce electrolyte into the suspension of particulate material. In other embodiments, however, conduits are simply located within the electrolyte to facilitate distribution.

The conduits allow electrolyte to flow through the walls thereof, but hinder passage of the particulate material into the conduit. Even if particulate material becomes lodged in some conduits during the electrolytic reaction, electrolyte can typically still flow through sufficient conduits to facilitate distribution. Suitably, the conduits may be in the form of a pipe or tube; and may be formed of any suitable material which is chemically resistant to the electrolyte used in the reactor, such as a porous plastics pipe. For example, one common thermoplastic polymer is polypropylene. Porous plastics pipes are available commercially constructed from plastics materials such as acrylonitrile butadiene styrene (ABS), polyethylene, especially high-density polyethylene (HDPE), or other resin materials such as polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVDF). Suitably, the average pore size, measured along the longest dimension of a pore, may be from 0.1 microns to 1mm, provided that the pores are sufficiently small to hinder ingress of the particulate material into the conduits. Preferably the porosity is from 5 microns to 250 microns, typically from 20 microns to 40 microns.

Alternatively, the porous conduit may be in the form of a membrane comprising a plastics material, such as mentioned above, wrapped around a skeletal scaffolding, a glass fibre membrane, a ceramic material with pores, a metal which is chemically resistant (e.g. titanium, nickel, or stainless steel), or porous/fritted glass.

Depending on the shape and construction of the reactor, the reactor vessel may contain a single porous conduit. The length of a single conduit may be greater than the height and/or length of the vessel and extend throughout the vessel. For example, a single continuous conduit may be in the form of loops, circular configurations or irregular shapes within the bed of layered material. For reactor vessels which are narrow, or contain a narrow constriction, the single conduit may be configured centrally within the narrow vessel or narrow constriction.

In another embodiment, the porous conduits may be in the form of porous hollow spheres, which allow passage of electrolyte therethrough, thereby distributing the electrolyte through the suspension.

Preferably, the reactor contains a plurality of porous conduits within the suspension of particulate material. The number of conduits will depend on the size and shape of the reactor vessel, so that the conduits are able to distribute the electrolyte throughout the particulate material. For example, there may be from 2 to 10 conduits, suitably 3 to 5 conduits.

For larger commercial reactors, it may be appropriate to provide greater than 10 conduits. When the conduits are in the form of porous spheres, there may be greater than 50 spheres.

The anode and the cathode present in the reactor of this invention may be formed of any suitable conductive material, such as platinum, gold, silver, nickel, titanium, stainless steel, carbon (e.g. carbon black, activated carbon, nnesoporous carbon, and lower dimensional carbon such as carbon nanotubes, graphene, and fullerene) or conductive compounds such as zinc oxide, titanium dioxide, stannic oxide, conducting polymers, or transition metal sulphides/carbides/nitrides/oxides. -6 -

In particular, the electrode may be platinum, or a platinum-coated metal material. Suitable metal materials which may be coated with platinum include titanium, niobium, tantalum and zirconium. A preferred platinum-coated metal electrode is platinum-coated titanium.

The electrodes may also include composite materials, such as noble metals (e.g. platinum) or carbon integrated into substrates such as carbon (optionally of a different type), polymers, metals, or metal oxides.

Alternatively, the electrodes may be a non-metallic conductor; that is, the electrode conducts electricity, but is not a metallic element.

Suitable such non-metallic conductors include conductive compounds that may or may not contain metal atoms. Alternatively, the non-metallic conductor may be a poorly-conductive or non-conductive element or compound in which the conductivity can be enhanced by inclusion of, or 'doping' with, other elements.

Specific examples of electrode materials which may be used in the present invention include lead dioxide (Pb02) and derivatives of Pb02 (such as doped-Pb02), stannic oxide (5n02, preferably with a dopant such as antimony), titanium dioxide (Ti02, preferably with a dopant such as niobium or tantalum), substoichiometric-Ti02/Magneli phase titanium oxides, glassy carbon, and doped diamond, such as boron-doped diamond (BDD).

The working electrode, that is the electrode on which the reaction of interest occurs, may be either the anode or the cathode.

The choice of electrolyte employed in the reactor of this invention will depend on the nature of the electrochemical reaction. Generally, the electrolyte may be an aqueous solution of an inorganic salt, such as -7 -sodium chloride, potassium chloride, lithium chloride, ammonium sulphate, sodium sulphate, or an organic salt such as an alkylannine salt. An alkaline solution may also be employed, including sodium hydroxide or potassium hydroxide in solution. When it is desired to include an oxidation step in the process, for example in the production of graphene oxide from graphite, then it is desirable to use an aqueous acidic electrolyte, such as aqueous sulphuric acid, nitric acid, perchloric acid, phosphoric acid or boric acid.

Other suitable electrolytes include non-aqueous solutions, such as organic solvents, organic acids, ionic liquids or eutectic solvents. Examples of suitable non-aqueous electrolytes include propylene carbonate and dimethyl sulphoxide (DMSO) with dissolved salt species.

The electrolyte may consist of mixtures of any of the above-mentioned electrolytes. Any electrolyte may be employed at a suitable pH level for the reaction. The pH may optionally be adjusted using, for example, hydrochloric acid, ammonia solution or sodium hydroxide Preferably, the electrolyte is an aqueous solvent.

The reactor of this invention may be used for a variety of electrochemical reactions. For example, the reactant material may be coal, which can be electrochemically burned in order to produce hydrogen gas.

The average size of the particles of reactant material for use in the reactor of this invention may suitably be from 5 micron to 1 mm in diameter, for example from 40 microns to 1 mm in diameter.

Preferably, the reactant material for use in the reactor of this invention is a layered material. -8 -

Layered materials are substances having atomically thin layers of atoms or molecules with strong covalent bonds within each layer, but weaker bonds, such as electrostatic bonds, or van der Waals forces, between the layers. Examples of layered materials suitable for use in the reactor of this invention include graphite, hexagonal boron nitride, MXenes, layered metal oxides, including layered double hydroxides, and transition metal dichalcogenides such as molybdenum disulphide and tungsten diselenide. Preferred layered materials are graphite and molybdenum disulphide.

For example, the reactor may be employed for electrochemical intercalation of layered materials, in particular for the formation of graphite intercalation compounds (GICs). Thus, the invention also provides a process for the production of an intercalation compound of a layered material which process comprises using the electrochemical reactor of this invention, wherein the electrolyte contains an intercalating element or molecule.

GICs comprise graphite with various molecules intercalated between its constituent layers. GICs are of significant commercial interest due to their improved electrical and electronic properties relative to graphite and because they are a precursor to thermally expanded graphite. The intercalating molecule can either donate an electron to the graphite (i.e. donor-type GIC), or an electronegative species can accept an electron, forming a charge transfer complex with graphite (i.e. acceptor-type GICs). Typical acceptor intercalants include WS04, HNO3, FeCI3 and H3PO4. Electron donor intercalants include elements such as K, Rb and Li.

The present electrochemical reactor may also be used in a process for the production of two-dimensional materials. Two-dimensional materials, referred to herein as 2D materials, are substances that exist in layers, which may be one or a few atoms thick.

Typically, a 2D material may have from 1 to 10 layers, such as from 2 to 5 layers. The thickness of the 2D material may be less than 100nnn, such as 0.5nm to lOnnn, for example from 0.5nm to mm.

The first atomically thin 2D material to be isolated was graphene, which is an allotrope of carbon. Oxygenated derivatives of graphene are known, such as graphene oxide and reduced graphene oxide. In addition, a number of other carbon-containing and non-carbon-containing compounds have been found to form 2D materials.

2D materials may be produced by electrochemical expansion and exfoliation, by placing the parent layered material within an electrolyte and applying a positive or negative potential to expand and optionally exfoliate the material. The degree of exfoliation which occurs depends upon the reaction conditions, such as voltage, electrolyte and reaction time. Preferably, the material is subject to a further exfoliating means, in order to achieve more complete exfoliation and thereby increase the yield of 2D material.

Suitable exfoliating means include electrochemical, thermal, microwave, solvothermal, sonochennical, mechanical and sonication exfoliating.

Preferred exfoliating means are scalable and solution-based, such as sonication and high-shear mechanical mixing.

Suitable 2D materials which may be produced by electrochemical expansion and exfoliation in the reactor of this invention include graphene, graphene oxide, reduced graphene oxide, hexagonal boron nitride, phosphorene, graphitic carbon nitride, germanene, silicene, antimonene, bisnnuthine, borocarbonitrides, MXenes, layered metal oxides, including layered double hydroxides, and transition metal dichalcogenides such as molybdenum disulphide and tungsten diselenide. -10 -

Preferably, the 2D material is graphene, graphene oxide, molybdenum disulphide, phosphorene, hexagonal boron nitride or a MXene.

MXenes are compounds of formula: Mn+1Xn-rx where n is an integer; x is zero or an integer; M is an early transition metal, such as titanium, vanadium, niobium, molybdenum, zirconium, or hafnium; X is carbon and/or nitrogen; and T is a termination moiety, such as =0, -OH or -F.

Examples of MXenes are Ti2C, V2C, M02C, Zr3C2, Nb4C3, or Ti4N3.

The reactor of this invention is particularly advantageous when the electrolyte is an aqueous solution. In that case, a side reaction is oxygen evolution, which is manifested as gas bubbling on the surface of the layered material. The bubbles act to insulate the layered material from the electrolyte and thus hinder the electrochemical reaction. The present reactor allows the bubbles to escape through the porous conduits. That increases the effective surface area for the reaction and so improves the efficiency of the electrochemical reaction.

Specific embodiments of the present invention will now be described with reference to the accompanying drawings in which Fig.1 to Fig. 6 represent six illustrative embodiments of reactors of the invention.

Referring to Fig.1, the apparatus contains a reactor vessel 101, a circular anode 102 and a cathode in the form of a plate 103. A bed of flake graphite particles 104 is contained within the reactor vessel 101. The electrolyte level is depicted as the line 114. The graphite particles are compressed using a porous press 105, operated by a pressure pad 106. The pad 106 is fixed to both the anode 102 and the porous plate 105A by means of a rod 106A. A conduit in the form of a porous pipe 107 contains vertical and circular pipe elements which are situated throughout the bed of particles 104. In operation, during the electrolysis process, electrolyte is introduced into the pipe 107 through an entry port 108 and circulates through the particle bed 104. Excess electrolyte passes through the exit port 109 and is recycled. At the end of the reaction, the product is flushed out of the reactor with excess electrolyte The embodiment illustrated in Fig.2 comprises a reactor vessel 201, comprising an internal annular anode 202. A cathode 203 is fixed within a vertical cathode compartment 203A situated centrally within the reactor vessel. The reactor vessel contains a bed of graphite particles 204, situated between the central cathode 203 and the annular anode 202. The particles are compressed using an annular porous graphite press 205, which surrounds the cathode compartment 203A. The cathode compartment 203A is connected to a series of horizontal porous conduits 207, which are situated at regular intervals on either side of the cathode compartment 203A and within the particle bed 204. In operation, during the electrolysis process, electrolyte is introduced into the cathode chamber 203A through an entry port 208 and passes into the porous conduits 207 to circulate through the particle bed 204. Excess electrolyte passes through the exit port 209 and is recycled. At the end of the reaction, the product is flushed out of the reactor with excess electrolyte.

Fig. 3 shows a further embodiment of the invention having a rectangular reactor vessel 301 containing a bed of graphite particles 304. The electrolyte level is depicted as the line 314. The reactor vessel also contains a number of porous conduits 307A and 307B arranged vertically within the bed of graphite particles 304. The porous conduits 307A are constructed from boron-doped-diamond-coated niobium and are each connected to positive terminals (not shown), thus acting as anodes. The porous conduits 307B are constructed from plastics material, and each -12 -contains a cathode 303. The cathodes 303 are connected via wires 303A to a negative terminal. In operation, during electrolysis, a press (not shown) compresses the bed of graphite particles 304. Electrolyte is distributed throughout the bed 304 through both the porous anode conduits 307A and the porous cathode conduits 307B. At the end of the reaction, the product is flushed out of the reactor with excess electrolyte.

In the embodiment shown in Fig.4, a reactor vessel 401 contains a slurry of molybdenum disulphide particles 404. The upper level of the slurry is depicted as the line 414. A number of anodes 402 are fixed at intervals inside the reactor vessel 401 in contact with the inside surface thereof. A number of porous conduits 407 are situated vertically between the anodes 402, forming constricted portions 401A. Each porous conduit 407 contains a cathode 403. The anode and cathode are connected to a power supply 405. In operation, during electrolysis, the slurry 404 flows out of the reactor vessel 401 through an exit port 409 and is circulated, by means of a pump 410, to re-enter the reactor vessel through an entry port 408. Electrolyte is passed through the porous conduits 407 and, because of the constricted portions 401A, the slurry has to pass close to the conduits so that the electrolyte is distributed through the particles 404. Furthermore, as the slurry is forced through the constricted portions 401A, some particles are pressed against the anodes 402 thereby enhancing the electrochemical reaction. Once the reaction is complete, the slurry is pumped out of the vessel by means of the pump 410 into a separate exit pipe and filter unit (not shown), where the reacted product, few-layered molybdenum disulphide, is separated from the electrolyte and isolated.

Fig. 5 shows a reactor vessel in the form of a semi-circular trough structure 501. An anode 502 is fixed to the lower portion of the trough 501. A cathode 503 is fixed in position above the trough 501, connected to a static inner ring 505. A mobilised packed bed of flake graphite particles 504 is contained in the trough 501. An outer ring 506 is -13 -rotatable around a central axis 506A. Multiple porous conduits 507 are radially attached to the outer ring 506. In operation, electrolyte and graphite are introduced into the trough 501 through the entry port 508, up to the level shown by the line 514. The outer ring 506 rotates anticlockwise, which causes the porous conduits 507 to pass through the particle bed 504. The conduits 507 are retractable and spring-loaded, so that their outer ends maintain contact with the anode 502, thus ensuring that all the particles are moved through the reactor 501. As the porous conduits 507 move through the bed, electrolyte passes through the conduits and is distributed throughout the particle bed 504. This is a continuous reactor. That is, the graphite is continually introduced and removed after it is reacted. Reaction product is collected as it exits the reactor.

In the embodiment shown in Fig. 6, a reactor vessel is in the form of a cylindrical tank 601. An anode 602 is situated at the base of reactor vessel 601. Within the reactor vessel, there is rotatable perforated plate 605, which is rotated by means of a motor mechanism 605A. The reactor vessel 601 has perforated side walls which communicate with an annular chamber 606, containing an annular cathode 603. The reactor vessel 601 contains a slurry of graphite particles 604 in electrolyte, up to the level shown by the line 614, together with porous conduits in the form of hollow porous spheres 607, shown in more detail in Fig. 6A. In operation, the electrolyte, graphite particles 604 and porous spheres 607 are introduced into the reactor vessel through an entry port 608. The perforated plate 605 is rotated and stirs the slurry. Once the reaction is complete, the slurry is pumped out of the vessel via an exit port 609 by means of a pump 610 and passes into a filter unit 611, where the reacted product is separated, and the spheres recycled back to the entry port 608.

Claims

-14 -CLAIMS1. An electrochemical reactor for conducting an electrochemical reaction on a particulate material, the reactor comprising an anode, a cathode and a reactor vessel, the reactor vessel containing an electrolyte, a suspension of a reactant material in particulate form, and one or more porous conduits, each of which conduit is situated within the suspension of particulate material, wherein the electrolyte is capable of passing through the porous conduits to distribute the electrolyte through the suspension of particulate material.
2. An electrochemical reactor as claimed in claim 1, wherein the conduit comprises a porous plastics pipe.
3. An electrochemical reactor as claimed in claim 1 or claim 2, wherein the reactor vessel contains a single conduit, the length of which is greater than the height and/or length of the vessel and extends throughout the vessel.
4. An electrochemical reactor as claimed in claim 1 or claim 2, wherein the reactor vessel contains a plurality of porous conduits within the suspension of particulate material.
5. An electrochemical reactor as claimed in any one of claims 1 to 4, wherein the reactor comprises a packed bed of reactant material.
6. An electrochemical reactor as claimed in claim 5 wherein the reactor comprises a mobilised packed bed of reactant material.
7. An electrochemical reactor as claimed in any one of claims 1 to 6, wherein the material of at least one electrode is platinum or a platinum-coated metal material. -15 -
8. An electrochemical reactor as claimed in any one of claims 1 to 6, wherein the material of at least one electrode is boron-doped diamond.
9. An electrochemical reactor as claimed in any one of claims 1 to 8, wherein the reactant material is a layered material.
10. An electrochemical reactor as claimed in claim 9 wherein the reactant material is graphite.
11. An electrochemical reactor as claimed in claim 9 wherein the reactant material is molybdenum disulphide.
12. A process for the production of an intercalation compound of a layered material which process comprises using the electrochemical reactor as claimed in claim 9, wherein the electrolyte contains an intercalating element or molecule.
13. A process as claimed in claim 12 wherein the intercalated compound is a graphite intercalation compound.
14. An intercalation compound whenever prepared by a process as claimed in either claim 12 or claim 13.
15. A process for the production of a two-dimensional material, which process comprises: (a) the electrolytic treatment of a parent layered material in a reactor vessel as claimed in claim 9; and (b) simultaneously or subsequently exfoliating the parent layered material.
16. A two-dimensional material whenever prepared by a process as claimed in claim 15.