WO2012013221A1

WO2012013221A1 - Apparatus for water treatment and method of manufacture thereof

Info

Publication number: WO2012013221A1
Application number: PCT/EP2010/060899
Authority: WO
Inventors: Rudolf Gensler; Ai Cheng Lee
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-07-27
Filing date: 2010-07-27
Publication date: 2012-02-02

Abstract

An apparatus (1) for treatment of water comprises an electrochemical cell (2) having at least a pair of electrodes (3,4) for applying an electrical potential difference across a volume of water (11) therebetween for removing ions or ionizable species contained in said water (11). In accordance with the present invention, at least one said electrode (3) is made of a material comprising graphene.

Description

Apparatus for water treatment and method of manufacture thereof

The present invention relates to an apparatus for water treatment, in particular, for electrochemical treatment of water . Electrochemical treatment of water, for example, waste water, involves removal of ions or ionizable species from water in an electrochemical cell, such as an electrodialysis (ED) cell or an electrodeionization (EDI) cell. The standard material for anodes used in ED and EDI systems include titanium (Ti) coated with platinum (Pt) or metal dioxides based on ruthe^¬ nium (Ru) and iridium (Ir) .

The above-mentioned standard anode materials offer several disadvantages. First, the price of precious metal such as Ti, Pt, Ru and Ir are increasing rapidly such that the anode cost is a significant portion of the overall module cost. Sec^¬ ondly, precious metals, particularly Ti are difficult to ma^¬ chine. Further, EDI system costs are still not economical in many instances compared to existing technologies such as chemically regenerated deionization .

The object of the present invention is to provide a means for water treatment that is cost effective and energy efficient. The above object is achieved by the apparatus according to claim 1 and the method according to claim 8.

The underlying idea of the present invention is to improve the energy efficiency and cost-effectiveness of a water treatment apparatus by using a graphene based electrode therein. Graphene is the building block for graphite-like carbon materials of every other dimensionality. It is a one- atom thick sheet of carbon atoms arranged in a honeycomb, hexagonal lattice. The electrons moving around carbon atoms interact with the periodic potential of the honeycomb lattice of graphene, which gives rise to new quasi-particles that have lost their mass, or 'rest mass' (i.e. mass-less Dirac fermions) . That means that graphene never stops conducting. It was also found that electrons in graphene travel far faster than electrons in other semiconductors. Graphene is a low cost material compared to the precious metals such as Pt, Ti, Ru and Ir and is substantially more electrically conduc- tive than these precious metals. The use of a graphene as an electrode material in a water treatment apparatus is shown minimize the operating cost of the apparatus by providing very high current densities at significantly lower energy consumptions and reducing leakage current.

In one embodiment, said at least one electrode is made of graphene particles dispersed in a composite bulk material. This makes it possible to manufacture the electrode in a sin^¬ gle step, for example, by casting or injection molding.

In an alternate embodiment, said at least one electrode com^¬ prises a substrate and a graphene based coating applied on a surface of the substrate. Advantageously, herein, a graphene coating can be applied on an existing electrode, which now becomes the substrate, to increase energy efficiency of an existing water treatment apparatus.

In a further embodiment, said graphene based coating is ap^¬ plied around multiple surfaces of the substrate so as to form a continuous layer of said coating. Advantageously, in this embodiment, the substrate can be made of an electrically non- conductive material or an inexpensive material.

In one embodiment, said coating comprises graphene particles dispersed in a liquid based binder material. The coating can thus be applied using any known liquid coating process such as spray coating, dip coating, spin coating, flow coating and doctor blading, among others. For providing chemical resistance, in one embodiment, said binder material comprises a polymer, or a sol-gel binder, or ceramic particles.

In one embodiment, to prevent oxidation of graphene, said coating is cured in vacuum.

The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompany^¬ ing drawings, in which:

FIG 1 is a schematic diagram of a water treatment apparatus, FIG 2 shows an electrode having graphene particles dispersed in a composite bulk material, according to one embodiment of the present invention

FIG 3 shows an electrode having a graphene based coating ap- plied on a surface of a substrate, according another embodi^¬ ment of the present invention, and

FIG 4 shows an electrode having a graphene based coating ap^¬ plied continuously around the substrate, according yet an- other embodiment of the present invention.

The present invention provides a energy efficient and cost effective water treatment apparatus having an electrochemical cell for removing ions or ionizable species from waste water. Embodiments of the present invention are advantageous for several types of electrochemical cells for water treatment, such as, electrodialysis cells, electrodeionization (EDI) cells, capacitive deionization (CapDI) cells, electro ad^¬ vanced oxidation process (EAOP) cells, electrochorination cells and electro-disinfection cells.

Referring to FIG 1 is illustrated an exemplary water treat^¬ ment apparatus 1 including an electrodialysis cell 2 for re- moving ions from waste water, according to one embodiment of the present invention. The electrodialysis cell 2 illustrated herein is a multi-chamber cell comprising depletion chambers 12 and concentration chambers 13 arranged in alternate order. The chambers 12 and 13 are defined by spaced apart ion ex^¬ change membranes 14 and 15, i.e., membranes that are selec^¬ tively permeable to either positively or negatively charged ions. The membranes are arranged such that positive ion se^¬ lective membranes 14 are interspaced by 15 negative ion se- lective membranes. The electrochemical cell 2 further in^¬ cludes a pair of electrodes, namely an anode 3 disposed in an anode chamber 16 and a cathode 4 disposed in a cathode cham^¬ ber 17. The anode chamber 16 and cathode chamber 17 are lo^¬ cated at opposite ends of the electrochemical cell 2. Anolyte and catholyte solutions 18 and 19 respectively are conducted continuously through the anode chamber 16 and cathode chamber 17 respectively during the operation of the cell 2. The solu^¬ tion 11 through which ions are to be extracted, which is waste water in this example, is made to occupy the volume of the depletion 12 chambers while the solution 20 in which ions are to be collected are made to occupy the volume of the con^¬ centration chambers 13. The ions are transferred, depending on their polarity, through the ion selective membranes 14 and 15, under the influence of an electrical potential applied to the electrodes 3 and 4 between which an electric current passes in series across the membranes 14 and 15 and compart^¬ ments 12 and 13 defined between them.

In accordance with the present invention, one of the elec- trodes, which, in this example, is the anode 3, is made of a material comprising graphene . As mentioned before, graphene is the building block for graphite-like carbon materials of every other dimensionality. It is a one-atom thick sheet of carbon atoms arranged in a honeycomb, hexagonal lattice. The electrons moving around carbon atoms interact with the peri^¬ odic potential of the honeycomb lattice of graphene, which gives rise to new quasi-particles that have lost their mass, or 'rest mass' (i.e. mass-less Dirac fermions) . That means that graphene never stops conducting. It was also found that electrons travel much faster in graphene than in other semiconductors . Graphene is a low cost material compared to the precious met^¬ als such as Pt, Ti, Ru and Ir. Pure and un-doped graphene can carry and sustain an ultra-high current densities of about 10⁸ A/cm², which is two orders of magnitude greater than cop^¬ per, and has an electrical resistivity of about 1.0x10-6 Q.cm at room temperature. Graphene is 35 % more electrically con^¬ ductive than the most electrically conductive metal known to date, i.e. the silver, and is 79 - 97 % more conductive than the precious metals (Pt, Ti, Ru and Ir) used for electrodes in ED or EDI application. This feature enables the reduction of the leakage current (IR drop) due to the internal resis^¬ tance of electrode materials, and the delivery of current density required by the ED or EDI systems at a lower energy consumption. Thus using graphene electrodes provide cost- effectiveness and high energy efficiency, thereby making electrochemical water treatment systems competitive on both the capital and operating costs basis.

A comparison of the electrical properties of graphene with a few known conductive materials is represented in table 1 be- low.

Table 1

Current density Resistivity (mA/cm^"2) (Ω . cm)

Pt/Ir alloys 750 2xl0^"5

TiN 10¹⁰ 1. lxlO^"5 graphite 33-43 5-30 xlO^"4 graphene 10¹¹ 1. OxlO^"6 Referring to FIG 2, in accordance with one embodiment of the present invention, the graphene based electrode 3 may be formed out of graphene particles 5 dispersed in a bulk com^¬ posite material 6. The composite material 6 may include, for example a polymer composite, a ceramic composite or even a metallic composite. The graphene particles 6 may include, ex^¬ ample nano graphene platelets (NGP) , which exhibit electrical properties comparable to carbon nano-tubes (CNT) but are more readily available at much lower costs. Moreover, the platelet shape offers NGP edges that are easier to modify chemically for enhanced dispersion in polymers. Alternately, particles 5 may include exfoliated nano graphite platelets (xNGP) that include small stacks of graphene that are 1 to 15 nanometers thick, with diameters ranging from sub-micrometers to 100 mi- crometers . Exfoliated nano graphite platelets may be obtained by heat expansion of intercalated graphite. The particles 5 are dispersed to be present in an amount so that composite material 5 is electrically conductive. Advantageously, in this embodiment, the electrode 3 may be formed in a single step by casting or injection molding of the graphene based composite material into a suitable shape, such as a plate, a sheet, a foil or a mesh.

In an alternate embodiment shown in FIGS 3 and 4, a graphene based electrode 3 may be formed by applying a graphene based coating 9 on a surface of a substrate 7. In the embodiment of FIG 3, the coating 9 is applied on one surface 8a of the sub^¬ strate 7. Herein, the coating 9 on the surface 8a is exposed to the electrochemical reaction during operation of the appa- ratus 1, while the surface 8b is coupled to an electrical contact 10 connected to a power supply. The substrate 7 may be made of any metals, such as stainless steel, titanium, niobium, tantalum, vanadium and their alloys, as well as conductive ceramics, conductive plastics, conductive carbona- ceous materials.

In the embodiment of FIG 4, the coating 9 is applied on mul^¬ tiple surfaces 8a, 8b, 8c, and 8d to form a continuous layer of coating 9, having a one side exposed to the electrochemical reaction and an opposite side connected to an electrical contact 10. Advantageously, in this embodiment, the substrate 7 need not necessarily be made an electrically conductive ma- terial as long as it is dimensionally & chemically & electro- chemically stable for the target application. The substrate 7 in both embodiments may be made in any suitable shape, such as a plate, a sheet, a mesh, a foil etc. Referring to FIGS 3 and 4, the coating layer 9 may comprise graphene-based materials or composites or paints. For exam^¬ ple, the coating 9 is made of a mixture containing graphene powder and a liquid based binder component. The binder compo^¬ nent may include materials such as a chemically resistant polymer resin such as epoxy resin and silicone resin, with solvent (s) and processing additives. A sol-gel binder may also be used instead of a polymer binder. A binder composed of ceramic particles, including electrically conductive ce^¬ ramic particles, is also a possible embodiment of this inven- tion.

The processes of making the graphene based coating 9 on the substrate 7 may include any liquid coating technique, for ex^¬ ample spray coating, dip coating, spin coating, flow coating, doctor blading, etc. The coating 9 is cured at temperatures and for curing times most appropriate for the respective coating formulations, typically between room temperature and 1000 °C and for 10 minutes - 2 days. For temperatures higher than 400 °C, curing is realized in vacuum to prevent oxida- tion of the graphene.

Several advantages are realized using a graphene based elec^¬ trode in a water treatment apparatus in accordance with em^¬ bodiments of the present invention. For example, graphene exhibits high chemical stability in acidic and alkaline me^¬ dium, therefore capable of serving as electrode materials with excellent corrosion and oxidation resistance. Moreover, graphene electrodes have a wide operating potential window (> 3.5 V) comparable to that of boron-doped diamond (BDD) elec^¬ trodes, but available at much lower costs compared to BDD. Graphene electrodes may hence prove to be alternatives to BDD electrodes [e.g. DIACHEM® and DiaCell® electrodes] in elec- trochemical advanced oxidation process (EAOP) for water treatment processes. Graphene based electrodes proposed herein may hence have several target application areas in wa^¬ ter treatment, including electrodes for ED, EDI, CapDI, EAOP, electrochlorination and electro-disinfection systems.

While this invention has been described in detail with refer^¬ ence to certain preferred embodiments, it should be appreci^¬ ated that the present invention is not limited to those pre^¬ cise embodiments. Rather, in view of the present disclosure which describes the current best mode for practicing the in^¬ vention, many modifications and variations would present themselves, to those of skill in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.

Claims

Patent claims

1. An apparatus (1) for water treatment, comprising an elec^¬ trochemical cell (2) having at least a pair of electrodes (3,4) for applying an electrical potential difference across a volume of water (11) therebetween for removing ions or ionizable species contained in said water (11), characterized in that at least one electrode (3) is made of a material com^¬ prising graphene .

2. The apparatus (1) according to claim 1, characterized in that said at least one electrode (3) is made of graphene par^¬ ticles (5) dispersed in a composite bulk material (6) .

3. The apparatus (1) according to claim 1, characterized in that said at least one electrode (3) comprises a substrate (7) and a graphene based coating (9) applied on a surface (8a) of the substrate (7) .

4. The apparatus (1) according to claim 3, characterized in that said graphene based coating (9) is applied around multi^¬ ple surfaces (8a-d) of the substrate (7) so as to form a con^¬ tinuous layer of said coating (9) .

5. The apparatus (1) according to claim 4, characterized in that said substrate (7) is made of an electrically non- conductive material.

6. The apparatus (1) according to any of claims 3 to 5, char- acterized that said coating (9) comprises graphene particles dispersed in a liquid based binder material.

7. The apparatus (1) according to claim 6, characterized in that said binder material comprises a polymer, or a sol-gel binder, or ceramic particles.

8. A method for manufacture of a water treatment apparatus (1), comprising manufacturing an electrochemical cell (2) having at least a pair of electrodes (3,4) adapted for apply^¬ ing an electrical potential difference across a volume of wa^¬ ter (11) therebetween for removing ions or ionizable species contained in said water (11), said method characterized in that at least one said electrode (3) is formed using a mate^¬ rial comprising graphene.

9. The method according to claim 8, characterized by forming said at least one electrode (3) by dispersing graphene parti- cles (5) in a composite bulk material (6) .

10. The method according to claim 9, characterized by forming said at least one electrode (3) by casting said composite bulk material (6) having said dispersed graphene particles (5) .

11. The method according to claim 9, characterized by forming said at least one electrode (3) by injection molding of said composite bulk material (6) having dispersed graphene parti- cles (5) .

12. The method according to claim 8, characterized by forming said at least one electrode (3) by:

- forming a substrate (7), and

- applying a graphene based coating (9) on a surface (8a) of the substrate (7) .

13. The method according to claim 12, characterized by ap^¬ plying said coating (9) on multiple surfaces (8a-d) of the substrate (7) so as to form a continuous layer of said coat^¬ ing (9) .

14. The method according to claim 13, characterized by form^¬ ing said substrate (7) from an electrically non-conductive material.

15. The method according to any of claims 12 to 14, charac^¬ terized in that said coating (9) comprises graphene particles dispersed in a liquid based binder material.

16. The method according to claim 15, characterized in that said binder material comprises a polymer, or a sol-gel binder, or ceramic particles.

17. The method according to any of claims 12 to 16, charac^¬ terized by curing the applied coating (9) in a vacuum.