Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/052—Electrodes comprising one or more electrocatalytic coatings on a substrate
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C4/00—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge
  • C23C4/18—After-treatment
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F1/00—Etching metallic material by chemical means
  • C23F1/10—Etching compositions
  • C23F1/14—Aqueous compositions
  • C23F1/32—Alkaline compositions
  • C23F1/36—Alkaline compositions for etching aluminium or alloys thereof
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material

Definitions

• This invention relates to a process for making an improved low hydrogen overvoltage cathode of the Raney nickel type for use in chloralkali electrolysis, and to the cathode so made.
• Raney nickel coated cathodes of the prior art while initially exhibiting good performance characteristics, suffer certain defects.
• Such Raney nickel coating on the cathode substrate has been of low durability, i.e., easily mechanically damaged and lost even by, e.g., water jets.
• This progressive spalling and deactivation of Raney nickel after multiple washes and bakes is responsible for a progressive loss of the voltage benefit provided by the Raney nickel, i.e., the operating voltage of the cathode progressively increases and approaches the operating voltage of the cathode substrate.
• a process for making a durable, low-hydrogen-overvoltage cathode for chloralkali use which comprises applying a nickel/aluminum alloy composition containing 56 to 59 % by weight of nickel onto a suitable cathode substrate by plasma spraying, followed by leaching with caustic to remove aluminum.
• Raney nickel coated cathode made by said process, a chloralkali cell containing said cathode, and a process for electrolysis of an alkali metal chloride in said cell.
• a selected nickel/aluminum alloy composition is applied to an electrically conductive cathode substrate by plasma spraying.
• the nickel/aluminum alloy composition employed for deposition on the cathode substrate contain about 56 to 59% by wt. nickel.
• Nickel/aluminum alloys in this composition range contain the intermetallic compound Ni 2 A1 3 as the predominant phase, but also contain minor amounts of NiAl and NiAl 3 , the exact amounts of these varying with the specific overall composition.
• the nickel/aluminum alloy composition contains 56.5 to 57.5% by wt. nickel, most preferably substantially 57% by wt. nickel.
• the nickel/aluminum alloy is suitably employed in the form of particles or granules, preferably 36 to 90 microns (165 to 400 mesh) in size, more preferably 36 to 53 microns (270 to 400 mesh) in size.
• the electrically conductive cathode substrate can have various configurations, e.g., flat, tubular, fingers, etc., but in any case should be foraminous, e.g., woven wire screen, expanded sheet metal or punched sheet metal.
• Iron, ferrous alloys such as mild steel and stainless steel, nickel and nickel alloys are particularly suitable materials for this substrate.
• the nickel/aluminum alloy composition be deposited on the cathode substrate with the use of plasma spraying, also termed plasma arc spraying.
• Plasma spraying is a known technique for depositing metal particles onto a substrate. Briefly, streams of a gas and the metal particles are separately fed to a spray nozzle, (torch), and there is an electrical power input to the gas stream which ionizes the gas; recombination of the positive ions and electrons in the gas stream after mixing with the metal particles releases energy which heats and partially melts the metal particles, and the partially melted metal particles adhere to the substrate on contact with it.
• gases including nitrogen and argon, are suitable gases for this purpose.
• Argon is a preferred gas.
• the power input need only be sufficient to partially melt the metal particles, e.g. 20 to 30 kilowatts in apparatus of ordinary size.
• the fabricated cathode component rather than to coat the foraminous sheet material used to fabricate the cathode, because welding and other assembly techniques are more easily accomplished with the substrate in the uncoated condition. Also, it is best to clean the surface of the substrate before coating, as by sandblasting, or blasting with other grit such as alumina.
• the cathodes of this invention can be either one-side or two-side coated. However if only one side is coated it is preferred to coat the side'which will face the membrane or diaphragm in order to obtain the maximum reduction in cell voltage.
• Thickness of the nickel/aluminum alloy coating applied to the cathode substrate of about 13 to 508 microns (0.5 to 20 mils) is suitable, and is preferably about 127 to 254 microns (5 to 10 mils).
• the Raney nickel surface is developed by contacting the coated article with any strong base such as a caustic solution so as to leach aluminum from the coating.
• a caustic solution containing 5 to 15% by wt. NaOH is suitable for this purpose, and 5 hours to 1 day is adequate time for the leaching. A 10% by wt. aqueous caustic solution for 16 hours is typical.
• the temperature for leaching is not critical; temperatures from room temperature up to the boilihg point of the caustic solution are suitable.
• the aluminum will ordinarily be removed during such leaching; some aluminum appears to remain even after, much longer leaching time, but is believed not to be deleterious to performance of the resulting cathode.
• This leaching step is also referred to as activation, and can be carried out prior to or after assembly of the cell.
• the Raney nickel cathode so made, if permitted to dry, heats up upon contact with air, due to the pyrophoric character of Raney nickel.
• the Raney nickel coated cathode so made not only exhibits low hydrogen overvoltage, but the coating has improved durability against flaking and spalling, as is shown hereinbelow in Example 9 and Comparative Examples B, C and D.
• the nickel/aluminum alloy composition applied as the coating in the process of the present invention provides an activated Raney nickel cathode surface which unexpectedly is much more resistant against spalling and flaking than those derived from alloys which contain 55% by wt. nickel or less, which spall at a commercially unacceptable rate.
• the cathode made by the process herein can be used as the cathode in known types of electrochemical cells which comprise a cathode compartment; a cathode disposed within said cathode compartment, an anode compartment, an anode disposed within said anode compartment, and a separator disposed between the anode and cathode compartments. It is especially useful in chloralkali cells of such description.
• the separator can be, e.g., a liquid permeable diaphragm; such diaphragm can be fabricated of, e.g., asbestos fibers or perfluorinated polymer fibers.
• the separator can alternatively be a membrane, e.g., of highly fluorinated or perfluorinated ion-exchange polymer; membrane of such polymer containing sulfonate and/or carboxylate groups is now well known in the chloralkali art, e.g., U.S. 4,192,725, U.S. 4,065,366 and U.S. 4,178,218.
• membrane cells can be operated in both narrow gap (1-3 mm spacing from anode to cathode) and zero gap (both electrodes in contact with the membrane) configurations.
• an aqueous alkali metal chloride solution is introduced into the anode compartment, an electric current is passed through the cell, an aqueous alkali metal hydroxide solution is removed from the cathode compartment and chlorine is removed from the anode compartment.
• a membrane cell water (or dilute alkali metal hydroxide at startup) is generally introduced into the cathode compartment, and spent alkali metal chloride solution is removed from the anode compartment.
• a diaphragm cell all of the unreacted alkali metal chloride solution percolates through the diaphragm into the cathode compartment, and is removed along with the product caustic.
• the present invention has numerous advantages when compared to the prior art. These include (1) high resistance to flaking, spalling and mechanical damage of the activated coating; (2) resistance to oxidizing atmospheres, such as heating in air, e.g., during baking of a diaphragm thereon; (3) low hydrogen overvoltage compared to mild steel cathodes and Ni cathodes; (4) good tolerance to moderate amounts of contamination by iron overplating; (5) a variety of substrates including mild steel, stainless steel and nickel can be used; (6) low cost and convenient application to preconstructed cathode components; and (7) the substrate is rarely heated above 200°C, so that warping is minimal.
• Spraying conditions were:
• the screen surface was cooled with compressed air jets.
• a sample portion of the screen was cut out, and a 7.62 cm (3 inch) diameter circular section cut from the sample.
• the circle was welded into a stainless steel cathode holder such that the coated side would face toward the anode in the subsequently assembled cell, leached in 10% caustic, and a PTFE fiber-asbestos diaphragm (TAB Diamond Shamrock composition) was deposited on the Raney nickel coated surface of the screen and baked.
• Performance of this cathode was as follows, with typical steel cathode performance given for comparison.
• a cathode finger from a Diamond Shamrock MDC-55 cathode assembly was sprayed on one side with nickel aluminide of the same composition as in Example 1. Before the finger was sprayed, it was degreased and -sandblasted. Spraying conditions were:
• the screen surface was cooled with an air blast.
• Example 2 The advantage for the cathode of Example 2 vs. the steel cathode shown for comparison in Example 1 was 270 mV.
• the fingers for three Diamond Shamrock MDC-55 cathode assemblies were plasma sprayed with nickel aluminide of the same composition as in Example 1 under the conditions outlined under Example 2, only on the sides of the fingers which were subsequently coated with the diaphragm coating and which would face toward the anode. Each can was then leached for about 16 hours in cold 10% caustic solution. The cans were then washed, and a Diamond Shamrock TAB diaphragm deposited and baked on each. Each can was assembled into a chloralkali cell and operated in a cell line. All the cans performed similarly. Performance compared to steel was:
• Samples of Type 304 stainless steel screen of 0.18 cm (0.072 inch) diameter wire spaced on 0.42 cm centers (6 mesh) were welded into stainless steel cathode rings and sprayed with a 0.015 cm (6-mil) thick coating of nickel aluminide of the same composition as in Example 1, only on that side to which the diaphragm coating was subsequently applied and which would face toward the anode in the subsequently assembled cell.
• the coating was leached with 10% caustic.
• a TAB diaphragm was deposited and baked onto the screens and the cathode assembly operated in a 7.62 cm (three-inch) cell. After initial operation, the diaphragms were removed, new diaphragms installed, and a second run made.
• Example 5 Preparation of a cathode for evaluation in a narrow gap membrane cell.
• a cathode for a laboratory membrane cell was prepared by cutting out a 7.62 cm (3-inch) diameter disk of a flattene.d expanded metal having dimensions of 2.54 cm LWD (long way diamond) x 0.635 cm SWD (short way diamond) x 0.16 cm strand width x 0.12 cm thick of a type 200 nickel. A length of 0.635 cm nickel tubing was welded to the center of the disk to serve as a current conductor.
• a coating of an alloy of 57% + 0.5% by wt. Ni and 43% + 0.5% by wt. Al was applied to both sides of this cathode by plasma spraying.
• the alloy was in the form of a fine powder having a particle size of 42 to 110 microns (i.e., between 140 mesh and 325 mesh). Conditions of application were as follows.
• the coating thickness was 203 microns (8 mils) on both sides of the expanded metal.
• the coating was activated by immersing the cathode in a solution of 10% NaOH at room temperature for 16 hours. The cathode was then water rinsed and kept wet-until installed in a cell.
• Example 6 Performance of a two-side coated cathode for evaluation in a finite gap membrane cell.
• the cathode of Example 5 was mounted in a small cell having an active area of 45 cm 2 , together with a reinforced fluorinated ion exchange membrane and a dimensionally stable anode.
• the membrane comprised a 38 micron (1.5 mil) layer of TFE/EVE having an equivalent weight of 1080; a 100 micron (4 mil) layer of TFE/PSEPVE having an equivalent weight of 1100, a layer of fabric having both PTFE filaments and polyethylene terephthalate filaments, and a 25 micron (1 mil) layer of TFE/PSEPVE having an equivalent weight of 1100, and was treated with an aqueous bath containing 11% KOH and 30% dimethylsulfoxide to hydrolyze the functional groups to carboxylate and sulfonate groups.
• the membrane was mounted with the carboxylate side toward the cathode.
• the electrodes were positioned so that there was a 3 mm gap between them.
• the cell was constructed so that a hydraulic head corresponding to approximately 25.4 cm (10 inches) of water on the cathode side pressed the membrane against the anode.
• the cell was operated at 90°C with a current density of 31 A/dm 2 .
• a saturated solution of purified sodium chloride was fed to the anolyte chamber at such a rate as to maintain a concentration of 200 g/1 NaCl. Water was added to the catholyte chamber to maintain the concentration of caustic produced at 32 + 1%.
• the cell After 7 days, the cell was performing at 96% current efficiency and 3.17.volts. The performance remained unchanged after 110 days of operation.
• Example 6 was repeated except that the cathode was made from uncoated mild steel expanded mesh. After 7 days, the cell was operating at 96.4% current efficiency and 3.51 volts. This is 340 mV higher than the activated cathode of Example 6.
• Example 7 Performance of a one-side coated cathode in a finite gap membrane cell.
• Example 5 was repeated except that the coating was applied only to the one face of the cathode which faced the membrane in the cell. This cathode was then installed and operated in a test cell under the same conditions as Example 6.
• Example 8 Performance of a two-side coated cathode in a zero gap membrane cell.
• a cathode prepared as per Example 5 was mounted in a small cell having an active area of 4 5 cm 2 .
• the membrane comprised a 38 micron (1.5 mil) layer of TFE/EVE having an equivalent weight of 1080 and a 100 micron (4 mil) layer of TFE/PSEPVE having an equivalent weight of 1100, was coated on both sides with an inorganic nonconductive layer comprised of ZrO 2 particles bonded with a solution in ethanol of a 950 equivalent weight copolymer of TFE/PSEPVE and dried, and was treated with KOH as in Ex. 6 to hydrolyze the functional groups to carboxylate and sulfonate groups.
• a dimensionally stable anode was used and the electrodes were positioned close together with the membrane between them such that there was no gap between the membrane and either electrode. * The membrane was mounted with the carboxylate side toward the cathode. The cell was operated at 90°C with a current density of 31 A/dm 2 . A saturated solution of purified sodium chloride was fed to the anolyte chamber at such a rate as to maintain the concentration at 200 g/l NaCl. Water was added to the catholyte chamber to maintain the concentration of caustic produced at 32 + 1%.
• Example B Four different nickel/aluminum alloy compositions were used, having nickel/aluminum weight ratios of 52/48 (Ex. B), 55/45 (Ex. C), 57/43 (Ex. 9) and 62/38 (Ex. D). All had particle sizes of 42-110 microns (140 to 325 mesh). The coating thickness applied was 152-178 microns (6-7 mils).
• the plasma spray coated cathodes were activated by leaching successively in 2% aqueous caustic at 25°C, 5% aqueous caustic at 25°C, 10% aqueous caustic at 25°C, and 10% aqueous caustic at 80°C, each stage being carried out until hydrogen evolution ceased before proceeding to the next stage.
• Each cathode (circular disk, 8.25 cm or 3.25 inches in diameter) was tested for performance in a small chloralkali cell as follows.
• the cell had anode and cathode compartments separated by a membrane of a perfluorinated ion exchange polymer having sulfonate groups; the area of membrane in use was circular, 3.5 cm in diameter.
• the anode was a platinum screen.
• a reference standard calomel electrode and the cathode to be tested were placed in the cathode compartment, with the calomel electrode as close as possible to, or in contact with, the cathode under test.
• the anode compartment was filled with a sodium chloride solution (250g NaCl/liter of solution); the cathode compartment was filled with a sodium chloride/sodium hydroxide solution containing about 13% sodium hydroxide (3 liters of the above NaCl solution, 1.5 liters water and 585g NaOH).
• Electrolysis was carried out at a current density of 15.5 A/dm at ca. 90°C, until stable electrode potentials were attained.
• the difference in potential between the test cathode and reference calomel electrode was measured with a high impedance digital voltmeter at intervals throughout the electrolysis period. The performance was compared against an uncoated mild steel cathode, and the results given below are reported as the millivolts advantage for the coated cathode vs. mild steel.
• Each test cathode was then subjected to a sequence of water spray at 1.03 x 10 Pa (1500 psi) for 3 minutes on each side of the cathode to simulate the conditions of removal of a spent asbestos diaphragm and baking at 355°C for 4 hours to simulate the baking a newly applied asbestos diaphragm, even though no asbestos was actually removed or deposited.
• the amount of Raney nickel coating which spalled off during the water spraying was collected by filtration and weighed.
• the cell performance of the washed and baked cathode was again evaluated. This sequence was carried out ten times for each test cathode. The results are summarized in Tables I and II.. Table II
• the present invention is useful whenever Raney nickel coated electrodes are desired. It finds use especially in the chloralkali field where low hydrogen overvoltage cathodes are needed.
• the cathodes are more durable than those of the prior art, and have other advantages set forth above.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
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• General Chemical & Material Sciences (AREA)
• Physics & Mathematics (AREA)
• Plasma & Fusion (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
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• 1983
  • 1983-07-28 EP EP83304386A patent/EP0100659A1/en not_active Ceased
  • 1983-07-28 AU AU17384/83A patent/AU559813B2/en not_active Ceased
  • 1983-07-28 ZA ZA835530A patent/ZA835530B/xx unknown
  • 1983-07-28 JP JP58136937A patent/JPS5941486A/ja active Granted
  • 1983-07-28 CA CA000433480A patent/CA1235386A/en not_active Expired
  • 1983-07-28 IN IN933/CAL/83A patent/IN157836B/en unknown
  • 1983-07-29 NZ NZ205086A patent/NZ205086A/en unknown
  • 1983-07-29 KR KR1019830003543A patent/KR840005497A/ko not_active Withdrawn

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