PROCESS FOR PRODUCING A POROUS CARBON ELECTRODE

Abstract

A method for producing a porous carbon electrode includes preparing a slurry by mixing a porous, particulate, conductive carbon powder with a solution of a polymer binding agent for the particulate carbon powder in a solvent for the polymer binding agent, forming a precursor electrode by casting the slurry as a layer and subjecting the cast layer to a wet phase inversion to realize porosity in the cast layer, subjecting the thus obtained precursor electrode to a thermal treatment to cause oxidative stabilization, carbonization, dehydrogenation or cyclisation of the polymer binding agent or a combination of two or more of the afore mentioned phenomena by heating the precursor electrode and converting the polymer binding agent into a conductive binding agent binding the particles of the conductive carbon powder together.

Claims

1. A method for producing a porous carbon electrode, comprising: preparing a slurry by mixing a porous, particulate, conductive carbon powder with a solution of a polymer binding agent for the carbon powder in a solvent for the polymer binding agent; forming a precursor electrode by casting the slurry as a layer and subjecting the cast layer to a wet phase inversion to realize porosity in the cast layer; and subjecting the thus obtained precursor electrode to a thermal treatment by heating the precursor electrode to a temperature with the purpose of converting the polymer binding agent into a conductive binding agent binding the particles of the conductive carbon powder together, wherein the polymer binding agent is a polymer material having a degradation temperature which is lower than the melting temperature.

2. A method according to claim 1, wherein the thermal treatment comprises a first step of heating the precursor electrode in the presence of an oxidizing agent to a temperature which is equal to or lower than the melting temperature of the polymer binding agent.

3. A method according to claim 2, wherein the thermal treatment comprises a first thermal treatment step which is carried out at a temperature of maximum 300 C., preferably maximum 275 C., in particular maximum 250 C., and at least 50 C., preferably at least 100 C., more preferably at least 150 C., in particular at least 175 C.

4. The method of claim 2, wherein the thermal treatment of the precursor electrode is carried out in the presence of an oxygen containing gas, in particular oxygen or air.

5. A method according to claim 2, wherein the thermal treatment of the precursor electrode comprises a second thermal treatment step of heating the precursor electrode in an inert atmosphere to a temperature of at least 250 C., preferably at least 275 C., and maximum 600 C., preferably maximum 550 C., wherein the second step is carried out after the first thermal treatment step.

6. A method according to claim 3, wherein the first thermal treatment step is carried out for a period of time of at least 20 minutes, preferably at least 30 minutes, more preferably at least 60 minutes, in particular at least 120 minutes, and a period of time of maximum 240 minutes.

7. A method according to claim 1, wherein the polymer binding agent is selected from the group consisting of nitrile polymers, preferably polyacrylonitrile.

8. A method according to claim 1, wherein the polymer binding agent comprises one or more polymers selected from the group consisting of polyacetates, in particular poly(vinylacetate); cellulose compounds, in particular carboxymethyl cellulose.

9. A method according to claim 1, wherein the weight ratio of the polymer binding agent to the conductive carbon powder varies from 70.0:30.0 to 90.0:10.0, preferably from 75.0:25.0 to 85.0:15.0.

10. A method according to claim 1, wherein the solvent for the polymer material is selected from the group consisting of N,N-dimethylformamide (DMF), formamide, dimethylsulphoxide (DMSO), N,N-dimethylacetamide (DMAC), acetonitrile, acetamide, trichloroethylene, chloroform, dichloromethane, N-methyl-pyrrolidinone (NMP), N-ethyl-pyrrolidinone (NEP), methyletherketone, dioxane, triethylphosphate, aceton, diethylenetriamine and mixtures of two or more hereof.

11. A method according to claim 10, wherein the solvent comprises a co-solvent selected from the group consisting of tetrahydrofuran (THF), tetramethyl urea (TMU), N,N-dimethylpropylene urea (DMPU), trimethyl phosphate (TMP), triethyl phosphate (TEP), tri-n-butyl phosphate (TBP), tricresyl phosphate (TCP), acetone, aniline; a ketone, in particular methyl ethyl ketone (MEK); a chlorinated hydrocarbon, in particular methylene chloride, dichloromethane, and trichloroethylene; aromatic fluids and chloroform and a mixture of two or more of the afore-mentioned co-solvents.

12. A method according to claim 1, wherein the cast layer of the slurry after having been subjected to the thermal treatment has a thickness of maximum 500 micron, preferably maximum 250 micron, more preferably maximum 100 micron.

13. A method according to claim 1, wherein the slurry is subjected to degassing before being subjected to the thermal treatment.

14. A method according to claim 1, wherein the slurry is cast on at least one side of an electrically conductive carrier.

15. A method according to claim 1, wherein the porous carbon electrode comprises a current collector and the slurry is applied to both opposite sides of the current collector.

16. A method according to claim 1, wherein the slurry is applied to an electrically conductive carrier using impregnation.

17. A method according to claim 14, wherein as an electrically conductive carrier use is made of a material or a combination of materials selected from the group consisting of sheet material in particular a graphite web, a felt material comprising conductive carbon fibers and reticulated vitreous carbon.

18. A method according to claim 17, wherein a current collector is positioned between two layers of a sheet material impregnated with the slurry.

19. A method according to claim 1, wherein the porous carbon powder has a BET surface area of at least 250 m2/g, preferably at least 300 m2/g, more preferably at least 500 m2/g.

20. A porous carbon electrode, comprising a porous active layer which contains particles of a porous conductive carbon powder, wherein at least part of the particles are connected by a porous residue of a polymer binding agent that has been subjected to a thermal treatment according to claim 1.

21. An electrochemical cell containing at least one porous carbon electrode obtained with the method of claim 1.

22. A method for desalination of water, wherein an aqueous solution containing one or more salts is subjected to desalination in an electrochemical cell according to claim 21.

23. A method for capacitive de-ionization of water, wherein an aqueous solution containing one or more salts is subjected to capacitive de-ionization in an electrochemical cell according to claim 22.

Description

EXAMPLE 1

Production of a Flow by Carbon Based Electrode for Use in Capacitive De-Ionisation

[0072] A suspension was produced which should form a precursor of the active layer, by preparing a polymer solution containing 5 wt % of Dralon X polyacrylonitril (PAN) polymer of Dralon companyDormagen/Lingen Germany, in 95 wt % Dimethylacetamide (DMAc) solvent. The suspension was prepared under continuous cooling to a maximum temperature of 10 C., until a clear solution is obtained.

[0073] To this solution an amount of YP50F active carbon powder from the Kurara company, equal to 9 times the total dissolved amount of PAN polymer present, was gradually added using a high-energy mixer under continuous cooling. When the complete amount of carbon powder had been added the desired suspension was obtained. The suspension contained 90 wt. % of YP50F carbon powder and 10 wt. % of PAN X100 polymer with DENTAc as the solvent,

[0074] The global composition of the precursor-layer suspension is as follows [0075] 3.45 wt % of PAN X100 [0076] 65.52 wt % of DMAc [0077] 31.03 wt % of YP50F activated carbon powder

[0078] This suspension is subsequently degassed by using a vacuum pump under continuous stirring at low temperature (10 C.) As a result a suspension with a viscosity of 200 Pa.s at 20 C. is obtained containing any air bubbles anymore.

[0079] This degassed precursor-layer suspension is then coated horizontally by a doctor knife coating technique onto the graphite support (500 m) which is completely flat-streched, with a wet thickness of about 500 m.

[0080] For obtaining the desired porous structure of the active layer a phase-inversion process was applied upon solidifying the electrode precursor layer from the casting suspension. Thereto the graphite support coated with the coating suspension were immersed together into a water non-solvent bath. By this process the solvent contained inside the coated layer was extracted liquid/liquid extraction by the water of the precipitation bath. After 15 minutes residence in the coagulation/precipitation bath the coated graphite support is put into a hot water bath (70 C.) for another 45 minutes for extracting the solvent remainders completely and to solidify the precursor-layer of the active layer completely.

[0081] A typical property for such layers obtained by this phase-inversion process by water coagulation is that they are substantially porous (50-75 volume percent), next to the porosity of the sorbent carbon powder material suspended in the polymer solution itself.

[0082] Subsequently the water-filled, and highly-porous active layer precursor-layer-onto-graphite-support, is dried for 24 hours into an oven at 80 C. to remove water from the thus formed precursor layer. The thickness of the precursor-layer of the active layer onto graphite support is about 200 m and is now ready for being subjected to a thermal treatment.

[0083] The thermal treatment was carried out by eating the graphite support coated with the precursor-layer active layer in an oven, in air, from room temperature (without any external pressure onto the layer). The temperature of the hot-air oven was gradually raised from room temperature to 230 C. with a heating rate of 100 C. per hour. Once the temperature of the oven reached 230 C., this temperature was maintained for 1 hour. Care was taken that the temperature did not raise above 235 C. Then the oven was cooled to room temperature.

[0084] The residual weight of the active layer after this thermal treatment was at least 85% of the original weight and the layer thickness was increased by 5 to 10%. The active layer had a thickness of 150 m.

[0085] The specific adsorption capacity of the electrode (SAC), expressed as g of salt/m.sup.2 was 1.4. The specific adsorption rate when 50% of the adsorption capacity of the electrode (ASAR 50) was reached was 2.0 mg salt/m.sup.2 electrode/s, whereas the specific adsorption rate when 90% of the adsorption capacity (ASAR 90) was reached was 2.5 mg salt/m.sup.2 electrode/s.

EXAMPLE 2 AND 3

[0086] Example 1 was repeated, now with a suspension containing respectively 80 wt. % and 70 wt. % of YP50F carbon powder, and 20 wt. % and 30 wt. % of PAN X100 polymer with DMAc as the solvent.

[0087] The active layer had a thickness of 150 m.

[0088] The specific adsorption capacity of the electrode (SAC), expressed as g of salt/m.sup.2 of electrode was 0.9. The specific adsorption rate when 50% of the adsorption capacity of the electrode (ASAR 50) was reached was 4.8, respectively 2.5 mg salt/m.sup.2 electrode/s, whereas the specific adsorption rate when 90% of the adsorption capacity (ASAR 90) was reached was 4.2, respectively 2.5 mg salt/m.sup.2 electrode/s.

[0089] The results are summarized in table 1.

EXAMPLE 4 AND 5

[0090] Example 2 and3 were repeated, now with a suspension containing respectively 80 wt. % and 70 wt. % of YP50F carbon powder, and 20 wt. % and 30 wt. % of PAN MOO polymer with DMAc as the solvent, and casting an active layer with a thickness of 500 m.

[0091] The specific adsorption capacity of the electrode (SAC), expressed as g of salt/m.sup.2 of electrode was 3.7, respectively 2.1. The specific adsorption rate when 50% of the adsorption capacity of the electrode (ASAR 50) was reached was 4.6, respectively 3.1 mg salt/m.sup.2 electrode/s, whereas the specific adsorption rate when 90% of the adsorption capacity (ASAR 90) was reached was 3.8, respectively 2.4 mg salt/m.sup.2 electrode/s.

[0092] The results are summarized in table 1.

Comparative Experiment A.

[0093] Example 1 was repeated, now with a suspension containing 90 wt. % of YP50F carbon powder and 10 wt. % of polyvinylidene fluoride (PVDF) polymer with DMAc as the solvent.

[0094] The active layer had a thickness of 150 m. This electrode was not thermally treated.

[0095] The specific adsorption capacity of the electrode (SAC), expressed as g of salt/m.sup.2 was 2.0. The specific adsorption rate when 50% of the adsorption capacity of the electrode (ASAR 50) was reached was 3.1 mg salt/m.sup.2 electrode/s, whereas the specific adsorption rate when 90% of the adsorption capacity (ASAR 90) was reached was 2.9 mg salt/m'electrode/s. The results are summarized in table 1.

EXAMPLE 6-11

[0096] Example 2 was repeated, with varying residence time in the oven, after it had reached a temperature of 230 C.

[0097] The results are summarised in table 2 below.

TABLE-US-00001 TABLE 2 Specific Specific Adsorption Adsorption Specific rate at 50% of rate at 90% of Adsorption adsorption adsorption Time in Capacity capacity capacity oven at (SAC) (ASAR 50) (ASAR 90) 230 C. g salt/m.sup.2 mg salt/m.sup.2 mg salt/m.sup.2 Example nr. Hours electrode electrode/s electrode/s Example 6 0 1.04 3.2 2.8 Example 7 0.25 0.94 2.5 2.2 Example 8 0.5 1.14 3.7 3.3 Example 9 1 0.87 4.8 4.2 Example 10 4 0.87 2.0 2.1 Example 11 16 0.54 1.7 1.7

EXAMPLE 12-13

[0098] Example 3 was repeated, with varying residence ti the oven, after it had reached a temperature of 230 C.

[0099] The results are summarised in table 3 below.

TABLE-US-00002 TABLE 3 Specific Specific Adsorption Specific Adsorption rate Time in Adsorption rate at 50% of at 90% of adsorption oven at Capacity adsorption capacity capacity 230 C. (SAC) (ASAR 50) (ASAR 90) Hours g salt/m.sup.2 mg salt/m.sup.2electrode/s mg salt/m.sup.2electrode/s 1 3.67 4.6 3.8 1.5 2.69 3.7 3.1

EXAMPLE 14

Production of a Full-Carbon Flow-Through Capacitive De-Ionisation Electrode

[0100] The process for the production of a precursor-layer for a full carbon flow-through type of capacitive de-ionization electrode, in particular the used support and the process used, differ somewhat from that of example 1.

[0101] A sheet of a graphite non-woven material (graphite felt) was used as a support for the layer of active material of the electrode, and as current collector. The material of which the graphite felt was made was selected such that it showed good compatibility with the capacitive de-ionization process, in particular the solvent used, the temperature range and current density at which the capacitive de-ionization process was carried out, and a good electron conductivity. Its total porosity was between 10 and 95% and its pore size between 10 and 1000 m. The total thickness was between 0.25 and 10 mm.

[0102] In particular, a graphite felt was used of Baofeng Jinshi New Material Company, Longxing Road nr.10, Baofeng County, Henan Province (China) with a thickness of 3.15 mm with a total porosity of 92%, and an average pore diameter of 2 m.

[0103] The suspension produced in example 1 was degassed and was used for making the precursor of the active layer of the flow through electrode inside the felt. It contained 90 wt % of YP50F carbon powder of the Kuraray company and 10 wt % of polyacrylonitrile (PAN) X100 polymer with DMAc as the solvent. The graphite felt was impregnated with the degassed precursor-layer suspension by bringing the graphite felt in a vertical position, flat-stretching it and using a vertical two-side, simultaneous coating machine for impregnating the graphite felt with the suspension. Use was made of a ribbon of graphite felt, 17 cm wide and 100 cm long. During the impregnation process the felt was transported between the two slot coating heads with a velocity of 0.144 m/min. A total volume of 69.4 cm.sup.3/min of slurry had been applied.

[0104] For obtaining the desired porous structure of the active layer inside the graphite felt, a wet phase-inversion process was applied upon solidifying the precursor layer for the active layer of the electrode inside the felt. Thereto, the graphite support was immersed with the suspension into a water non-solvent bath to perform the wet phase-inversion of the PAN polymer by coagulation. Phase inversion by the water caused the solvent contained inside the felt to be extracted by liquid/liquid extraction. After 15 minutes residence in the coagulation/precipitation bath the impregnated graphite support was put into a hot water bath (70 C.) for another 45 minutes to achieve complete extraction of the solvent remainders and obtain solidification of the precursor-layer of the active layer inside the graphite felt. Thereafter, the water-filled, and highly-porous active layer precursor-layer inside the graphite felt, was dried for 24 hours into an oven 80C. The thickness of the so-obtained precursor for the all carbon flow-through type of electrode was 3.5 mm.

[0105] The thus obtained precursor was then put into an oven with air atmosphere at room temperature, and the oven temperature was gradually raised from room temperature to 230 C. with a rate of 100 C. per hour. Once the temperature of the oven reached 230 C. it was maintained at that temperature for 8 hours, and the temperature was controlled in such a way that it did not rise above 235 C. Then, the oven was cooled down to room temperature.

[0106] The residual weight of the active layer after the thermal treatment was 85% of the original weight. The final thickness of the finished flow-through electrode was 3.5 mm.

TABLE-US-00003 TABLE 1 Avg electrode SAC ASAR50 ASAR90 SAC ASAR50 ASAR90 thickness Per g electrode Per m.sup.2 electrode mm g/g mg/g/min mg/g/min g/m.sup.2 mg/m.sup.2/s mg/m.sup.2/s Example 1 PAN 90/10 150 m 0.167 13.1 1.3 1.2 1.4 2.6 2.5 Example 2 PAN 80/20 150 m 0.145 11.9 3.6 3.1 0.9 4.8 4.2 Example 4 PAN 80/20 500 m 0.511 11.7 0.8 0.6 3.7 4.6 3.8 Example 3 PAN 70/30 150 m 0.149 8.2 1.3 1.3 0.9 2.5 2.5 Example 5 PAN 70/30 500 m 0.451 8.8 0.7 0.5 2.1 3.1 2.4 comparative PVDF 90/10 150 m 0.160 12.5 1.0 1.0 2.0 3.1 2.9 experiment A ASAR 50 = Specific Adsorption rate at 50% of adsorption capacity (mg salt/m.sup.2 electrode/s) ASAR 90 = Specific Adsorption rate at 90% of adsorption capacity (mg salt/m.sup.2 electrode/s) SAC = Specific Adsorption Capacity (g salt/m.sup.2)