PROCESS FOR THE PREPARATION OF RETICULATED COPPER OR NICKEL SULFIDE

Abstract: A process for the preparation of reticulated copper sulfide and/or nickel sulfide which process comprises the steps of: (i) subjecting a mixture comprising copper sulfide and/or nickel sulfide powder and a salt to sintering by hot isostatic pressing at elevated temperature and elevated pressure, the said salt being physically and chemically stable under the hot isostatic pressing conditions; and (ii) dissolving the salt from the sintered body by treating the sintered body with a solvent in which the said salt is soluble. The use of reticulated copper sulfide and/or nickel sulfide prepared according to the method above as an electrocatalytic material.

Description: PROCESS FOR THE PREPARATION OF RETICULATED COPPER OR NICKEL SULFIDE The present invention relates to a process for the preparation of reticulated copper sulfide or nickel sulfide, and to reticulated copper sulfide or nickel sulfide for use as an electrocatalytic material. In particular the present invention relates to a process for the preparation of reticulated copper sulfide or nickel sulfide which involves a hot isostatic pressing stage . Electrodes for use in an electrochemical system are known which comprise, as active material, a sulfide, selenide or telluride, or a mixture thereof, of one or more transition metals, copper or lead. Electrodes of this type are described in GB-A-2042250. The preferred active materials are CoS, Cu 2 S, PbSe, RuS 2 , MoS 2 , NiTe and Cu 2 Se and are stated to be of use as electrodes for redox systems such as S/S 2" , Se/Se 2" and Te/Te 2" . However, since the electrocatalytic materials described in GB-A-2042250 are produced by electroplating techniques using a variety of electrolytes or by other surface coating technologies, the electrocatalyst is deposited in a thin layer and therefore does not have a three dimensional structure. Thus, the electrocatalytic materials have a low surface area and are not suitable for manufacturing flow-through electrodes. Furthermore, when a thicker layer is plated on the electrodes it becomes mechanically less stable. US-A-3847674 discloses the use of porous cathodes of fused cupric sulfide having a porosity of at least 40% and a resistivity of less than 0.5 ohm. cm. These electrodes are formed by compacting a powder mixture of finely divided copper and sulfur into a coherent body with mechanical pressure at a temperature below the melting point of sulfur, and then heating the coherent body at about atmospheric pressure to effect reaction of the finely divided copper and sulfur to form a fused cupric sulfide cathode structure. However, the electrodes produced according to the teaching of US-A-3847674 suffer from a number of disadvantages. Firstly, the process does not allow for control of the porosity of the material. The porosity varies from 40% to 80% and 75% of the pores are of a size greater than 4 microns. This makes the majority of the pores macro in size whereas a greater proportion of meso pores is desirable for electrocatalysis since this results in a greater surface area. Secondly, the process is exothermic and difficult to control and can produce non- stoichiometric material and even unreacted material. The possible presence of unreacted copper or sulfur creates chemical, and subsequently mechanical, stability issues. The possible presence of non- stoichiometric material lowers the overall electrocatalytic efficiency of the material. We have now produced an improved method for the preparation of electrocatalytic materials based on copper sulfide and/or nickel sulfide which are in a reticulated form and which are mechanically stable. Accordingly, the present invention provides a process for the preparation of reticulated copper sulfide and/or nickel sulfide, which process comprises the steps of: (i) subjecting a mixture comprising copper sulfide and/or nickel sulfide powder and a salt to sintering by hot isostatic pressing at elevated temperature and elevated pressure, the said salt being physically and chemically stable under the hot isostatic pressing conditions; and (ii) dissolving the salt from the sintered body by treating the sintered body with a solvent in which the said salt is soluble. Preferably, in carrying out the process of the present invention the mixture which is treated in step (i) comprises from 30 to 70 parts by weight, preferably from 60 to 70 parts by weight, of the copper sulfide or nickel sulfide and from 70 to 30 parts by weight, preferably from 40 to 30 parts by weight, of the salt. Mixtures containing from 60 to 70 parts by weight of copper sulfide or nickel sulfide provide a more mechanically stable product. Generally, the copper sulfide or nickel sulfide has a mean particle size in the range of from 40 to 200 micrometres, preferably from 60 to 100 micrometres and more preferably approximately 80 micrometres and the salt has a mean particle size in the range of from 30 to 500 micrometres, preferably from 38 to 420 micrometres. Below 38 micrometres the product tends to disintegrate into a powder. Above 420 micrometres the product tends to be weak and friable. Most preferably, the particle sizes of copper sulfide and salt particles are similar since this is found to give the best mechanical properties in the final product. The preferred form of copper sulfide used in the invention is copper (II) sulfide. The preferred form of nickel sulfide is Ni 3 S 2 . The salt which is used in the process of the present invention is soluble in the solvent which is used in step (ii) of the process. Thus, if the salt is water soluble then the solvent in step (ii) may be water, or a dilute acid or dilute alkaline solution. It will be appreciated that a very wide range of water soluble salts may be used in the method of the present invention. Some examples of water soluble salts which may be used are sodium chloride, sodium bromide, potassium chloride, potassium bromide, barium chloride and potassium carbonate. In a preferred embodiment, the process of the present invention involves a total of five steps. The first three steps being pre-processing steps carried out prior to the process of the present invention. The first step is a mixing step which ensures the production of an intimate mixture of metal sulfide and salt. The mixing step may be performed using, for example, one of two techniques. The materials may be co-ground using a mechanical agate mortar and pestle for 1 hour, alternatively, the materials may be mixed by simply tumbling them together in a screw-topped container placed on revolving mechanical rollers. The second step is a pressing step which produces powder compacts which are more suitable for subsequent handling and encapsulation. The pressing step may be performed, for example, by vacuum die-pressing using a die of suitable diameter at a pressure of from 14 to 21 MPa for approximately 1 minute. The third step involves encapsulation of the pressed pellets within individual mild steel cans. The top of each can is welded on and the cans are then evacuated and sealed off whilst still under vacuum. The evacuation procedure removes gas trapped between powder particles which would otherwise hinder densification. Preferably, a boron nitride lubricant may be used between the pressed pellets and the steel cans to limit the amount of reaction between the two materials. The fourth step is the hot isostatic pressing step. Isostatic pressing is a technique which is well known in the art, particularly in the ceramics field, whereby high pressures may be applied uniformly by using a flexible membrane to transmit hydrostatic pressure from a compressed fluid to a mixture which is to be formed, thereby ensuring densification throughout the body. Generally, a maximum compaction can be achieved using this technique. By carrying out isostatic pressing at elevated temperatures, the particles in the mixture which is being formed agglomerate into bulk materials. The conditions employed in the hot isostatic processing step are selected such that the pressures and temperatures are sufficiently high to cause densification whilst decomposition of the metal sulfide and excessive reaction between the metal sulfide and the mild steel can is avoided. In carrying out the process of the present invention hot isostatic pressing is preferably carried out at a temperature in the range of from 300° to 600°C, more preferably from 300° to 350°C, a pressure in the range of from 100 to 300 MPa, more preferably 100 to 200 MPa, and a period of time of from 60 to 300 minutes, more preferably 100 to 240 minutes. The final step is a leaching step to remove the soluble salt from the compacts to leave a porous metal sulfide structure. This step involves placing the compact in a beaker and immersing it in a solvent for a time sufficient to dissolve all the salt. The leaching step may be accelerated by warming and stirring the solvent. The leaching step can take several days and the solvent may be periodically replaced with fresh solvent as required. The present invention also includes within its scope reticulated copper sulfide or nickel sulfide produced according to the process of the present invention which has a meso/macro porosity (i.e. porosity due to meso and/or macro pores) in the range of from 35 to 55%, preferably 45 to 50%. Macro pores are those having a pore size greater than 50nm in diameter, meso pores have a pore size from 50nm down to 2nm in diameter and micro pores have a pore size below 2nm in diameter. The method of mercury porosimetry used to measure the porosity of reticulated copper sulfide or nickel sulfide produced according to the process of the present invention only detects porosity due to meso and macro pores. It will be appreciated by those skilled in the art that considerable contributions to the total porosity of the material produced by the present invention may also arise from micro pores within the structure. Therefore, the present invention also includes within its scope reticulated copper sulfide or nickel sulfide produced according to the process of the present invention which has a total porosity (i.e. porosity due to micro and meso and/or macro pores) in the range of from 35 to 80%, preferably 45 to 50%. The reticulated copper sulfide or nickel sulfide which is produced by the process of the present invention is of particular use as an electrocatalytic material for a number of reasons. Firstly, the pore sizes may be specifically tailored for use of the material in an aqueous system by altering the size of the soluble salt particles. Optimisation of the pore sizes in this way provides a material with good mass transport properties within the structure and consequently improved electrocatalytic performance. Secondly, the mechanical stability of the material is excellent which is an advantage when it is used in a flow through cell with high flow rates because degradation is not a problem. The present invention thus includes within its scope an electrode which comprises an electrode core and, in electrical contact therewith, a structure comprising an electrocatalytic material consisting of reticulated copper sulfide or nickel sulfide produced by the process of the present invention. The electrode core may be an electrically conductive carbon polymer composite such as high density polyethylene compounded with synthetic graphite powder and carbon black. The electrocatalytic material preferably forms the surface of the electrode and may be in the form of a single sheet, or mosaic of sheets, of the electrocatalytic material which is/are directly attached to the electrode core. Preferably, the sheet will have a thickness of from 1 to 7 mm, more preferably from 2 to 4 mm. If the thickness is less than 1 mm the mechanical stability of the sheet is too low. If the thickness is greater than 7 mm then the resulting cell can suffer from a large potential drop across the width of the electrocatalytic material. The present invention also includes within its scope an electrochemical apparatus which comprises a single cell or an array of cells, each cell with a positive chamber containing a positive electrode and an electrolyte and a negative chamber containing a negative electrode and an electrolyte, the positive and negative chambers being separated from one another by a cation exchange membrane and the negative electrode being an electrode as defined above. The electrochemical apparatus is preferably an apparatus for energy storage and/or power delivery. The electrolyte in the negative chamber of the electrochemical apparatus preferably contains a sulfide, whilst the electrolyte in the positive chamber of the electrochemical apparatus preferably contains bromine, iron, air or oxygen. The chemical reactions which are involved in these three systems are as follows: (1) Br 2 + S 2~ = 2Br " + S The above reaction actually occurs in separate but dependent bromine and sulfur reactions, the bromine reaction taking place on the positive side of the membrane and the sulfur reaction on the negative side of the membrane . ( 2 ) 2 Fe 3+ + S 2 2Fe 2+ + S Once again, this reaction actually occurs in separate but dependent iron and sulfur reactions, the iron reaction taking place on the positive side of the membrane and the sulfur reaction on the negative side of the membrane . (3) 4H 2 0 + 4S 2~ + 20 2 = 80H " + 4S This reaction also actually occurs in separate but dependent oxygen and sulfur reactions, the oxygen reaction taking place on the positive side of the membrane and the sulfur reaction on the negative side of the membrane. Preferably the electrodes used in the cell array as described above will be bipolar electrodes, the negative electrode surface of which is an electrode as defined above. The present invention also includes within its scope the use of an electrode as defined in a process which comprises the electrochemical reduction or oxidation of sulfur-containing species. In particular, the use wherein the process is a process for electrochemical energy storage which comprises the sulfide/polysulfide redox reaction. The present invention will be further described with reference to the following examples: EXAMPLE 1 Reticulated CuS pellets were prepared according to the following procedure. 20g of copper (II) sulfide powder (supplied by Aldrich Chemical Co. as 99%+, 100 mesh) was mixed with 30g of sodium chloride (supplied by Aldrich Chemical Co. As 99%+, 500μm) . The mixing ratio by weight was 40:60. The mixture was formed into a pellet 4cm in diameter and 1.2cm thick by pressing a 50g batch at 3000psi for one minute at room temperature. The pellet was placed in a mild steel can lined with boron nitride (BN) to stop diffusion of iron into the mixture . The can was evacuated and sealed and the mixture sintered by being hot isostatically pressed under the following conditions: Temperature 300°C Pressure : 200 MPa Time : 4 hours After hot isostatic pressing, the pellets were removed from the can and the sodium chloride salt dissolved from the structure by immersing the pellets in water at a temperature of 50°C with stirring for several days. The treated pellets had a reticulated structure that was visible to the naked eye and was further confirmed by electron microscopy. X-ray diffraction analysis prior to leaching (Figure la) only revealed peaks due to copper sulfide and sodium chloride. X-ray diffraction analysis after leaching (Figure lb) only revealed peaks due to copper sulfide. EXAMPLE 2 The procedure of Example 1 was repeated using 25g of copper (II) sulfide and 25g of sodium chloride, i.e. in a mixing ratio by weight of 50:50. EXAMPLE 3 The process of Example 1 was repeated using 30g of copper (II) sulfide and 20g of sodium chloride, i.e. in a mixing ratio of 60:40. X-ray diffraction analysis prior to leaching (Figure 2a) only revealed peaks due to copper sulfide and sodium chloride. X-ray diffraction analysis after leaching (Figure 2b) only revealed peaks due to copper sulfide. EXAMPLES 4 AND 5 The process of Example 1 was repeated using NaCl particles of a defined size and using the following operating conditions: The reticulated copper sulfide structures produced in these examples were analysed by mercury porosimetry with the following results. It should be noted that the method of mercury porosimetry only detects porosity due to meso pores (i.e. 2 to 50nm in diameter) and macro pores (i.e. greater than 50nm in diameter) and not micro pores (i.e. less than 2nm in diameter) : EXAMPLES 6, 7 AND 8 The process of Example 1 was repeated using NaCl particles of various sizes and using the following operating conditions: Examples 6, and 7 (example 8 disintegrated to a powder on leaching) were analysed by mercury porosimetry with the following results: EXAMPLE 9 A 3 kg mixture containing 70% CuS and 30% NaCl was prepared by weighing out 2.1 kg of CuS and 0.9 kg of NaCl and tumbling this mixture in a plastic container. The mixture was divided into three portions and each portion was formed into a bar of dimensions 25x25x230mm by pressing at 3000psi and room temperature. The bars were placed side by side and encapsulated in mild steel before being hot isostatically pressed at 400°C and 140 MPa for 240 minutes in an ASEA 1 unit . A band saw was used to remove the steel can from around the hot isostatically pressed slab. The slab was placed in a large beaker of warm, circulating water to leach out the salt, the water being replaced with fresh water as required. Once leaching was complete the slab was dried in an oven at 100 °C EXAMPLE 10 The slab produced in Example 9 was sectioned into a large number of blocks, each block being approximately 2 to 3 mm thick, approximately 25 mm wide and approximately 30 mm long. These blocks were glued onto a brass backed polyvinylidene : graphite (50:50) electrode in a mosaic fashion using colloidal carbon conducting glue. This electrode was incorporated into a monopolar cell as the negative electrode thereof. The positive electrode was a brass backed polyvinylidene : graphite (50:50) electrode without the mosaic structure of reticulated copper sulfide. The electrodes were separated by a cation exchange membrane. The electrolyte in the negative compartment of the cell was IM Na 2 S 4 and the electrolyte in the positive compartment of the cell was IM bromine in 5M NaBar. When operated at a current density of 40mA/cm 2 the overpotential was found to be lOOmV on charging the cell and 30mV on discharging the cell. When a similar cell using the negative electrode without the mosaic structure of reticulated copper sulfide was operated at a current density of 40mA/cm 2 the overpotential for sulfur reduction was found to be 400-600mV on charging the cell and 300mV on discharging the cell . The observed decrease in overpotential for the cell containing the reticulated copper sulfide coated electrode was maintained for several hundred cycles with no observable degradation of the electrodes. EXAMPLE 11 The process of Example 1 was repeated using nickel sulfide (supplied by INCO as mainly Ni 3 S 2 ) in place of copper sulfide and using the following conditions: A mechanically stable reticulated nickel sulfide compact was obtained. X-ray diffraction analysis prior to leaching (Figure 3a) only revealed peaks due to nickel sulfide and sodium chloride. X-ray diffraction analysis after leaching (Figure 3b) only revealed peaks due to nickel sulfide.

Claims: CLAIMS : 1. A process for the preparation of reticulated copper sulfide and/or nickel sulfide which process comprises the steps of: (i) subjecting a mixture comprising copper sulfide and/or nickel sulfide powder and a salt to sintering by hot isostatic pressing at elevated temperature and elevated pressure, the said salt being physically and chemically stable under the hot isostatic pressing conditions; and (ii) dissolving the salt from the sintered body by treating the sintered body with a solvent in which the said salt is soluble. 2. A process as claimed in claim 1 wherein the mixture comprises from 30 to 70 parts by weight of the copper sulfide and/or nickel sulfide and from 70 to 30 parts by weight of the salt. 3. A process as claimed in claim 1 or claim 2 wherein the copper sulfide and/or nickel sulfide powder has a mean particle size in the range of from 40 to 200 micrometres. 4. A process as claimed in any one of the preceding claims wherein the salt has a mean particle size in the range of from 30 to 500 micrometres. 5. A process as claimed in any one of the preceding claims wherein the copper sulfide is copper (II) sulfide . 6. A process as claimed in any one of the preceding claims wherein the nickel sulfide is Ni 3 S 2 . 7. A process as claimed in any one of the preceding claims wherein the salt is water soluble. 8. A process as claimed in claim 7 wherein the water soluble salt is selected from sodium chloride, sodium bromide, potassium chloride, potassium bromide, barium chloride, and potassium carbonate . 9. A process as claimed in any one of the preceding claims wherein the hot isostatic pressing is carried out at a temperature in the range of from 300° to 600°C, at a pressure in the range of from 100 to 300 MPa and for a time of from 60 to 300 minutes. 10. Reticulated copper sulfide and/or nickel sulfide whenever produced by a process as claimed in any one of the preceding claims. 11. Reticulated copper sulfide and/or nickel sulfide as claimed in claim 10 having a total porosity in the range of from 35 to 80%. 12. Reticulated copper sulfide and/or nickel sulfide as claimed in claim 10 or claim 11 having a meso/macro porosity in the range of from 35 to 55%. 13. An electrocatalytic material which comprises reticulated copper sulfide and/or nickel sulfide as claimed in any one of claims 10 to 12. 14. An electrode which comprises an electrode core and in electrical contact therewith an electrocatalytic material as claimed in claim 13. 15. An electrode as claimed in claim 14 wherein the electrocatalytic material forms the surface of the electrode. 16. An electrode as claimed in claim 15 wherein the electrocatalytic material is in the form of one or more sheets directly attached to the electrode core . 17. An electrode as claimed in claim 16 wherein the sheets have a thickness of from 1 to 7mm. 18. An electrochemical apparatus which comprises a single cell or an array of cells, each cell with a positive chamber containing a positive electrode and an electrolyte and a negative chamber containing a negative electrode and an electrolyte, the positive and negative chambers being separated from one another by a cation exchange membrane and the negative electrode being an electrode as claimed in any one of claims 14 to 17. 19. An electrochemical apparatus as claimed in claim 18 which is an apparatus for energy storage and/or power delivery. 20. The use of an electrode as claimed in any one of claims 14 to 17 in a process which comprises the electrochemical oxidation or reduction of sulfur. 21. The use as claimed in claim 20 wherein the process is a process for electrochemical energy storage which comprises the sulfide/polysulfide redox reaction.

Inventors: COOLEY GRAHAM EDWARD, COX JONATHAN DAVID, CRANSTONE WAYNE ROBERT IAN, MALE STEWART ERNEST, COOLEY, GRAHAM, EDWARD, COX, JONATHAN, DAVID, CRANSTONE, WAYNE, ROBERT, IAN, MALE, STEWART, ERNEST
Applicants: COOLEY GRAHAM EDWARD, COX JONATHAN DAVID, CRANSTONE WAYNE ROBERT IAN, MALE STEWART ERNEST, NAT POWER PLC, COOLEY, GRAHAM, EDWARD, COX, JONATHAN, DAVID, CRANSTONE, WAYNE, ROBERT, IAN, MALE, STEWART, ERNEST, NATIONAL POWER PLC
Application year: 1999
Main classification: H01M4
IPC codes: C01G3, C01G53, C04B35, H01M4, H01M8, H01M10