EP4692425A1 - Electrode - Google Patents

Electrode

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Publication number
EP4692425A1
EP4692425A1 EP24779181.7A EP24779181A EP4692425A1 EP 4692425 A1 EP4692425 A1 EP 4692425A1 EP 24779181 A EP24779181 A EP 24779181A EP 4692425 A1 EP4692425 A1 EP 4692425A1
Authority
EP
European Patent Office
Prior art keywords
electrode
platinum
paraffin
graphite
amount
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP24779181.7A
Other languages
German (de)
French (fr)
Inventor
Yuko Fukada
Kazumasa Suetsugu
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Tosoh Corp
Original Assignee
Tosoh Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Tosoh Corp filed Critical Tosoh Corp
Publication of EP4692425A1 publication Critical patent/EP4692425A1/en
Pending legal-status Critical Current

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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
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C25B1/01—Products
    • C25B1/21—Manganese oxides
    • 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
    • C25B11/054—Electrodes comprising electrocatalysts supported on a carrier
    • 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
    • C25B11/055—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material
    • C25B11/057—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material consisting of a single element or compound
    • C25B11/065—Carbon
    • 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
    • C25B11/055—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material
    • C25B11/069—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material consisting of at least one single element and at least one compound; consisting of two or more compounds
    • 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
    • C25B11/075—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound
    • C25B11/081—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound the element being a noble metal

Definitions

  • the present disclosure relates to an electrode and more specifically to an electrode used for the production of electrolytic manganese dioxide, such as an electrode used for producing electrolytic manganese dioxide used as a positive electrode active material in manganese dry cells, particularly in alkali manganese dry cells.
  • electrolytic manganese dioxide is produced by passing a current between an anode and a cathode in a sulfuric acid-acidic manganese sulfate electrolyte solution at a temperature close to 100°C and thereby causing electrolytic oxidative deposition on the anode.
  • the cathode, on which a hydrogen evolution reaction occurs, in comparison with the anode on which a deposition reaction of electrolytic manganese dioxide occurs, does not directly affect the quality of the electrolytic manganese dioxide. Therefore, attention has not been paid to the cathode, and there have been few examples examined in the past.
  • the cathode is composed primarily of graphite and also includes copper and steel (Patent Document 1).
  • Patent Document 2 examines a cathode including graphite coated with copper.
  • a cathode containing copper is immersed in a high-temperature electrolyte solution containing sulfuric acid, copper elutes into the electrolyte solution during a non-electrolytic period.
  • metals having corrosion resistance such as titanium and stainless steel, undergo corrosion during a non-electrolytic period.
  • cathodes for electrolysis of brine or water are not designed in consideration of use under sulfuric acid acidity and at a temperature close to 100°C. Therefore, these electrodes contain elements that may affect the quality of manganese dioxide and have high manufacturing costs. This makes it difficult to use them as cathodes for electrolysis of manganese dioxide.
  • electrodes such as cathodes and anodes are taken in and out from upper portions of an electrolytic bath and a cleaning tank during electrolysis and cleaning, and an electrolyte solution and a cleaning solution adhere to the electrodes during withdrawal. Because the electrolyte solution and the like adhering to the electrodes result in loss, a reduction in the amount of water such as the electrolyte solution adhering to the electrodes has been anticipated.
  • the present disclosure provides an electrode for the production of electrolytic manganese dioxide, the electrode being modified for the purpose of reducing the amount of water adhering to the electrode during withdrawal from an aqueous solution and for preventing detachment or elution of the metals even when immersed in a high-temperature and sulfuric acid-acidic electrolyte solution, thereby reducing an electrolytic voltage.
  • the present invention is as defined in the claims, and a gist of the present disclosure is as follows.
  • an electrode for the production of electrolytic manganese dioxide the electrode being modified for the purpose of reducing the amount of water adhering to the electrode during withdrawal from an aqueous solution and for preventing detachment or elution of the metals even when immersed in a high-temperature and sulfuric acid-acidic electrolyte solution, thereby reducing an electrolytic voltage, can be provided.
  • the electrode according to the present disclosure does not cause elution of metals from the electrode plate into an electrolyte solution during a non-electrolytic period of electrolysis for producing manganese dioxide. This further reduces an electrolytic voltage.
  • the amount of liquid adhering to the electrode during withdrawal from an electrolytic bath or a cleaning tank can be reduced. This enables efficient and consistent production of electrolytic manganese dioxide.
  • An electrode according to this embodiment is an electrode comprising a structure including graphite and platinum supported on the graphite and further comprising paraffin, that is, an electrode containing graphite on which platinum is supported and paraffin.
  • the electrode according to this embodiment can also be regarded as an electrode containing a graphite base material on which platinum is supported and paraffin, or even further, as an electrode including a graphite base material on which platinum is supported and further containing paraffin.
  • Examples of the types of the graphite include one or more selected from the group consisting of natural graphite, artificial graphite, carbon black, pyrolytic graphite and carbon fibers. Artificial graphite is preferable from the viewpoint of availability and ease of processing.
  • Platinum serves as a catalyst for the electrolytic oxidation reaction in the production of electrolytic manganese dioxide.
  • Any form of platinum which functions as a catalyst may be used. Examples include one or more selected from the group consisting of metallic platinum, a platinum alloy and a platinum compound.
  • the platinum is preferably metallic platinum because it further reduces the bath voltage during the production of electrolytic manganese dioxide.
  • a platinum alloy is a metal containing platinum and one or more metal elements other than platinum.
  • metal elements include a metal containing platinum (Pt) and one or more elements selected from the group consisting of silver (Ag), gold (Au), cobalt (Co), copper (Cu), iron (Fe), iridium (Ir), manganese (Mn), nickel (Ni), palladium (Pd), ruthenium (Ru), titanium (Ti) and zirconium (Zr).
  • the electrode according to this embodiment has a structure in which platinum is supported on graphite, and in particular, a structure in which platinum is supported on the surface of the graphite. Supporting platinum on the surface of the graphite, that is, in a state in which platinum is supported on the graphite serving as the electrode base material, makes it possible to further reduce the bath voltage during the production of electrolytic manganese dioxide.
  • platinum is supported on a portion of the graphite on which the electrolytic reaction for electrolytic manganese dioxide occurs, and platinum may be supported on either a part or the entirety of the surface of the graphite. It is preferable that platinum be supported on only a part of the graphite surface in order to reduce the amount of platinum used, which is a noble metal. In other words, the graphite may have a region on which platinum is supported and a region on which platinum is not supported.
  • Platinum acts as an electrode catalyst by being present on the graphite and maintaining electrical contact with the graphite. It is known that the catalytic activity of platinum at the hydrogen evolution electrode is superior to that of carbon, which is the material of the graphite. For example, under conditions of a 2N aqueous sulfuric acid solution, the minimum hydrogen overvoltage is 335 mV for carbon, whereas it is 0.002 mV for platinum. Accordingly, platinum exhibits excellent catalytic activity with a lower hydrogen overvoltage than graphite.
  • the amount of platinum supported on the graphite per unit area is preferably 3 ⁇ g/cm 2 or more and 500 ⁇ g/cm 2 or less, more preferably 10 ⁇ g/cm 2 or more and 400 ⁇ g/cm 2 or less, and further preferably 20 ⁇ g/cm 2 or more and 200 ⁇ g/cm 2 or less. Maintaining the amount of platinum supported within the above range prevents detachment of the supported metal (i.e., platinum) even under high-temperature, high-concentration sulfuric acid-acidic conditions, and reduces the bath voltage during electrolysis in the production of electrolytic manganese dioxide.
  • the supported metal i.e., platinum
  • the amount of platinum supported may be determined, according to the following equation, using the composition obtained by compositional analysis, by inductively coupled plasma emission spectroscopy (ICP method), of a solution obtained by pulverizing and acid-dissolving the region of an electrode sample in which platinum has been supported.
  • G M / A
  • G represents the amount of platinum supported ( ⁇ g/cm 2 )
  • M represents the amount of platinum ( ⁇ g) determined from the platinum concentration in the solution obtained by the ICP measurement
  • A represents the area (cm 2 ) of the region of the electrode sample in which platinum has been supported.
  • the ICP method may be conducted using a common inductively coupled plasma emission spectrometer (e.g., trade name: OPTIMA 3000 DV, manufactured by PerkinElmer).
  • a common inductively coupled plasma emission spectrometer e.g., trade name: OPTIMA 3000 DV, manufactured by PerkinElmer.
  • the solution may be prepared by any known method capable of dissolving the electrode sample.
  • An example includes a method in which the region of the electrode in which platinum has been supported is pulverized, the resulting electrode sample is heated at 400°C or more and 700°C under an air atmosphere, a mixed solution of concentrated sulfuric acid and concentrated nitric acid is then added under an air atmosphere, followed by heating to evaporate to dryness, and the residue is dissolved in aqua regia.
  • the electrode according to this embodiment contains paraffin. Because the electrode according to this embodiment contains paraffin, the amount of liquid adhering to the electrode when the electrode is withdrawn from an electrolytic bath or a cleaning tank can be reduced.
  • the paraffin is a mixture of hydrocarbons composed primarily of straight-chain hydrocarbons having 16 to 40 carbon atoms and preferably the same as that used for preventing evaporation in manganese dioxide electrolytic baths (i.e., for preventing evaporation of the electrolyte solution in the production of electrolytic manganese dioxide). In order to prevent evaporation, it is preferable that the paraffin be in a liquid state during electrolysis and in a solid state at normal temperature during recovery.
  • the melting point of the paraffin is 40°C or more and 80°C or less. That is, the paraffin is preferably a paraffin having a melting point of 40°C or more and 80°C or less, further preferably a paraffin having a melting point of 50°C or more and 65°C or less. Specific examples of the paraffin include Paraffin Wax-125 (manufactured by Nippon Seiro Co., Ltd.; melting point: 53°C).
  • the content of paraffin is preferably 1 mg/g or more and 100 mg/g or less, more preferably 3 mg/g or more and 80 mg/g or less, and further preferably 3 mg/g or more and 50 mg/g or less, relative to the mass (unit mass) of a portion of the electrode according to this embodiment which is immersed in an electrolyte solution (hereinafter, the above portion is also referred to as "immersed portion") in the production of electrolytic manganese dioxide using the electrode. Maintaining the paraffin content within the above range reduces the amount of adhering water (amount of adhering liquid).
  • the paraffin may be contained in the graphite prior to the support of platinum, or the electrode may be impregnated with paraffin subsequent to the support of platinum.
  • the unit amount of adhering water of the electrode according to this embodiment is preferably 0 mg/cm 2 or more and 4.0 mg/cm 2 or less, more preferably 0 mg/cm 2 or more and 3.0 mg/cm 2 or less, and further preferably 0 mg/cm 2 or more and 2.5 mg/cm 2 or less. Maintaining the unit amount of adhering water within the above range reduces the amount of liquid adhering to the electrode according to this embodiment when it is withdrawn from an electrolytic bath or a cleaning tank, thereby reducing the loss of the electrolyte solution and the cleaning solution.
  • the contact angle of the electrode according to this embodiment is preferably more than 90° and less than 180°, more preferably 95° or more and less than 180°, and further preferably 100° or more and less than 180°.
  • a contact angle exceeding 90° reduces the amount of liquid adhering to the electrode according to this embodiment when it is withdrawn from an electrolytic bath or a cleaning tank, thereby facilitating a further reduction in the loss of the electrolyte solution and the cleaning solution.
  • the contact angle is a value determined by the following method.
  • a contact angle meter e.g., device name: DMo-501, manufactured by Kyowa Interface Science Co., Ltd.
  • measurement/analysis software e.g., FAMAS1, manufactured by Kyowa Interface Science Co., Ltd.
  • the contact angle (°) of the electrode with respect to pure water is measured according to the ⁇ /2 method, using the following equation. The measurement is conducted three times, and the arithmetic average of the results is used as the contact angle of the electrode according to this embodiment.
  • Contact angle ° 2 ⁇ arctan h / r
  • the electrode according to this embodiment can be used as a cathode for producing electrolytic manganese dioxide. It is preferable that the voltage in electrolysis performed under the following conditions using a plate-shaped electrode according to this embodiment having a height of 250 mm, a width of 200 mm, and a thickness of 10 mm as a cathode (hereinafter, this voltage is also referred to as "bath voltage”) be 1.23 V or more and 2.00 V or less, more preferably 1.23 V or more and 1.80 V or less, and further preferably 1.23 V or more and 1.70 V or less.
  • electrolysis Prior to electrolysis, the electrolyte solution and paraffin are charged into an electrolytic bath, which is then heated to 96°C.
  • the cathode and anodes are installed in the bath in the order of anode, cathode and anode at intervals of 50 mm, with the principal faces (height ⁇ width faces) of the electrodes facing one another. Subsequently, electrolysis may be performed for 24 hours.
  • the concentration of manganese ions decreases as a result of deposition of electrolytic manganese dioxide with the progress of the electrolytic reaction.
  • an aqueous manganese sulfate solution electrolytic feed solution
  • the electrode according to this embodiment is characterized in that, when the electrode is immersed in a mixed aqueous solution of sulfuric acid and manganese sulfate under the following immersion conditions, the platinum content in the mixed aqueous solution of sulfuric acid and manganese sulfate (hereinafter, this platinum content is also referred to as "amount of platinum eluted") after immersion is, for example, 0 mg/L or more and 2 mg/L or less, 0 mg/L or more and 1 mg/L or less, 0 mg/L or more and 0.5 mg/L or less, or 0 mg/L or more and 0.1 mg/L or less.
  • the electrode according to this embodiment be unlikely to cause platinum elution even during a non-electrolytic period after the electrolytic reaction. It is more preferable that, when electrolysis is performed under the following electrolysis conditions using the electrode according to this embodiment, the platinum concentration in the electrolyte solution which is measured 3 hours after the electrolysis is stopped (hereinafter, this platinum concentration is also referred to as "amount of non-electrolytic elution”) be 0 mg/L or more and 2 mg/L or less, 0 mg/L or more and 1 mg/L or less, 0 mg/L or more and 0.5 mg/L or less, or 0 mg/L or more and 0.1 mg/L or less.
  • the electrode according to this embodiment makes it possible to reduce the electrolytic voltage in an electrolytic reaction using the electrode. Accordingly, the electrode according to this embodiment can be used as an electrode (cathode) for producing electrolytic manganese dioxide. This enables efficient production of electrolytic manganese dioxide.
  • a method for producing the electrode according to this embodiment includes, for example, supporting a noble metal by performing electroplating or electroless plating in an electrolyte solution containing noble metal ions using platinum with a graphite plate serving as a working electrode such that the above-described supported amount is achieved.
  • the method for producing the electrode also includes a paraffin-containing step in which paraffin is incorporated into the platinum-modified graphite.
  • a preferable method for producing the electrode according to this embodiment includes a platinum-modifying step in which platinum is supported on graphite to obtain platinum-modified graphite, and a paraffin-containing step in which the platinum-modified graphite is brought into contact with paraffin to incorporate the paraffin into the platinum-modified graphite.
  • the method for supporting platinum on graphite in the platinum-modifying step is not particularly limited and may be any method capable of supporting platinum on graphite.
  • the method is preferably a plating method, more preferably at least one of electroplating and electroless plating, and further preferably electroplating.
  • electroplating may be performed using a titanium-platinum electrode as an anode, graphite as a cathode, and a mixed solution of chloroplatinic acid (VI) and hydrochloric acid as a plating solution, at a temperature of 50°C or more and 100°C or less and a current density of 0.3 A/dm 2 or more and 2.0 A/dm 2 or less. It is preferable to perform the electroplating at a temperature of 60°C or more and 90°C or less and a current density of 0.5 A/dm 2 or more and 1.5 A/dm 2 or less.
  • VI chloroplatinic acid
  • hydrochloric acid as a plating solution
  • the plating time may be set appropriately in accordance with the intended amount of platinum supported and the size of the graphite (electrode base material). For example, the plating time may be 15 seconds or more and 2 minutes or less.
  • the method may further include a cleaning step in which the platinum-modified graphite is cleaned subsequent to the platinum-modifying step and prior to the paraffin-containing step.
  • the cleaning step reduces impurities such as the plating solution remaining on the surface.
  • the cleaning method used in the cleaning step may be any method capable of reducing impurities.
  • An example includes a cleaning method using pure water.
  • a preferable cleaning method includes immersion in warm water at 30°C or more and 100°C or less for 10 minutes or more and 60 minutes or less, followed by cleaning with pure water.
  • drying conditions may be set appropriately. For example, drying may be performed at 30°C or more and 100°C or less under an air atmosphere for 1 hour or more and 24 hours or less.
  • the paraffin-containing step is a step in which the platinum-modified graphite is brought into contact with paraffin to incorporate the paraffin into the platinum-modified graphite.
  • the contact between the platinum-modified graphite and paraffin may be any method capable of causing paraffin to adhere to the surface of the platinum-modified graphite and to permeate into the inside of the platinum-modified graphite.
  • the platinum-modified graphite may be immersed in pure water at a temperature of 60°C or more and less than 100°C, having a paraffin layer on the surface.
  • the pure water is preferably at a temperature of 80°C or more and less than 100°C.
  • the thickness of the paraffin layer may be any thickness that allows paraffin to uniformly adhere to the surface of the platinum-modified graphite, and may be, for example, 1 cm or more and 2 cm or less.
  • the immersion time may be set appropriately and is, for example, 1 hour or more and 24 hours or less, preferably 5 hours or more and 24 hours or less.
  • the platinum-modified graphite is preferably immersed in warm water not containing paraffin, followed by cleaning with warm water and drying. This removes paraffin remaining on the uppermost surface of the platinum-modified graphite.
  • the measurement of the amount of metal supported was conducted by measuring the metal concentration in a measurement solution by inductively coupled plasma emission spectroscopy (ICP method) using a common inductively coupled plasma emission spectrometer (trade name: OPTIMA 3000 DV, manufactured by PerkinElmer).
  • ICP method inductively coupled plasma emission spectroscopy
  • OPTIMA 3000 DV common inductively coupled plasma emission spectrometer
  • the measurement solution was prepared by the following method. First, the region of an electrode sample in which platinum had been supported was pulverized using a stamp mill until it passed through a sieve with an opening of 0.5 mm. Note that, as the electrode sample, a portion immersed in the plating bath described below was cut out.
  • the pulverized electrode sample (fragments) was charged into a magnetic crucible and then heated at 600°C under an air atmosphere for 70 hours. Subsequently, 1 mL of concentrated sulfuric acid and 2 mL of concentrated nitric acid were added under an air atmosphere. The resulting mixture was heated and evaporated to dryness. Subsequently, aqua regia was added to dissolve the residue to form a solution. Pure water was added to the solution until the volume of the solution reached 1 L. Thus, a measurement solution was obtained. The metal concentration in the measurement solution was measured.
  • G represents the amount of metal supported (mg/cm 2 )
  • C represents the metal concentration (mg/L) in the measurement solution obtained by the ICP measurement
  • W1 represents the mass (g) of the electrode sample after cutting out
  • W2 represents the mass (g) of the pulverized electrode sample (fragments)
  • A represents the surface area (cm 2 ) of the region of the platinum-modified graphite which was cut out and in which platinum had been supported.
  • the paraffin content in the electrode sample was determined by gas chromatography using a gas chromatograph (Agilent 7890A GC, manufactured by Agilent Technologies, Inc.).
  • a hexane solution containing paraffin (trade name: Paraffin Wax-125, manufactured by Nippon Seiro Co., Ltd.) at a concentration of 10 mg/L was used.
  • the amount of metal eluted from the electrode sample into the initial electrolyte solution was measured by the ICP method using a common inductively coupled plasma emission spectrometer (trade name: OPTIMA 3000 DV, PerkinElmer).
  • the metal concentration in a diluted electrolyte solution which was obtained by diluting the initial electrolyte solution (described below) tenfold with pure water, was measured by the ICP measurement. The measured metal concentration was multiplied by ten, and the resulting value was used as the metal concentration (mg/L) in the initial electrolyte solution.
  • the amount of water in the electrode in each of the examples and the comparative examples was determined by measuring the mass of the electrode before and after immersion in pure water. Specifically, a screw-cap bottle containing pure water was placed on an electronic balance. A rod-shaped electrode was immersed in the screw-cap bottle such that entire electrode surface was immersed in pure water. After immersion for 10 seconds, the rod-shaped electrode was taken out of the screw-cap bottle. The difference between the values of the electronic balance before and after the immersion of the electrode (i.e., the difference between the mass before immersion of the rod-shaped electrode and the mass after taking out the rod-shaped electrode) was calculated and used as the amount of adhering water (mg).
  • a rod-shaped electrode As the measurement sample, a rod-shaped electrode, a portion located above 5 cm from the bottom face of which was covered with a PTFE tape, was used.
  • the measurement was conducted three times, and the arithmetic average of the results was used as the amount of adhering water (mg).
  • the amount of adhering water (mg) was divided by the surface area (cm 2 ) of the electrode, and the resulting value was used as the unit amount of adhering water (mg/cm 2 ).
  • the surface area (cm 2 ) of the electrode was defined as the surface area (cm 2 ) of the region in which platinum had been supported.
  • the measurement was conducted three times, and the arithmetic average of the results was used as the contact angle (°).
  • the bath voltage and electrode durability of the electrode in each of the examples and the comparative examples were determined by electrolytically synthesizing electrolytic manganese dioxide using an electrolytic bath including the graphite plate of each of the examples and the comparative examples serving as a cathode, a titanium plate serving as an anode, and a mixed aqueous solution of manganese sulfate and sulfuric acid serving as an electrolyte solution.
  • the electrolyte solution used was a mixed aqueous solution of manganese sulfate and sulfuric acid having a manganese sulfate concentration of 85.4 g/L and a sulfuric acid concentration of 27.0 g/L.
  • One plate-shaped electrode (cathode) of each of the examples and the comparative examples and two titanium plate anodes (height: 250 mm, width: 200 mm, thickness: 5 mm) were immersed 160 mm in the electrolyte solution so as to be perpendicular to the liquid surface of the electrolyte solution.
  • the cathode and the anodes were fixed at intervals of 50 mm such that their principal faces (the height ⁇ width faces) faced one another (i.e., in the order of anode, cathode and anode).
  • the temperature of the electrolyte solution was increased to 96°C over 3 hours. After completion of the temperature rise, 50 mL of the electrolyte solution was collected (hereinafter, the electrolyte solution collected after completion of the temperature rise is also referred to as "initial electrolyte solution").
  • a DC stabilized power supply (PAN-18-10A, manufactured by Kikusui Electronics Corporation) was connected, and electrolytic synthesis of electrolytic manganese dioxide was conducted at a constant current of 4.48 A (i.e., 0.65 A/dm 2 with respect to the cathode).
  • an aqueous manganese sulfate solution (electrolyte feed solution) having a manganese sulfate concentration of 118 g/L was continuously supplied to the electrolytic bath.
  • the electrolyte solution was continuously withdrawn from the electrolytic bath such that the amount of the electrolyte solution in the electrolytic bath was maintained at 6 L. 24 hours after the start of electrolysis, the voltage displayed on the DC stabilized power supply was read and used as the bath voltage.
  • Electrolysis was further continued for six days and stopped on the seventh day from the start of electrolysis.
  • 50 mL of the electrolyte solution was collected 3 hours after stopping the electrolysis (hereinafter, the electrolyte solution collected 3 hours after stopping the electrolysis 7 days after the start of electrolysis is referred to as "final electrolyte solution").
  • the amount of metal eluted into the final electrolyte solution that is, the amount of non-electrolytic elution (mg/L) was measured in the same manner as described above in ⁇ Measurement of Amount of Metal Eluted into Initial Electrolyte Solution>.
  • All 6 surfaces of a rectangular parallelepiped (columnar) graphite (PSG322, manufactured by SEC Carbon, Ltd.) having a length (height) of 100 mm, a width of 10 mm, and a thickness of 10 mm were polished with 400-grit abrasive paper (hereinafter, the above-described graphite subjected to the polishing is also referred to as "graphite rod").
  • the portion of the graphite rod extending from 50 mm to 70 mm from the bottom face was covered with a masking tape for plating (manufactured by 3M Japan Limited) (the portion from 50 mm to 70 mm in the longitudinal direction from the bottom face was covered).
  • the portion of the graphite rod extending up to 60 mm from the bottom face (60 mm in the height direction from the bottom face of the graphite rod) and a titanium-platinum electrode were immersed in an electrolyte solution containing 10 g/L of chloroplatinic acid (VI), and 20 g/L of hydrochloric acid, which was maintained at 70°C, to form a plating bath including the titanium-platinum electrode serving as an anode and the graphite rod serving as a cathode.
  • VI chloroplatinic acid
  • electroplating was performed at cathode current density of 1.0 A/dm 2 for 2 minutes to support platinum on the graphite rod (i.e., the portion extending up to 50 mm in the longitudinal direction from the bottom face of the graphite rod).
  • the graphite rod with the masking tape removed was immersed in warm water at 50°C for 30 minutes, washed with running water, and then dried at 50°C under an air atmosphere for 12 hours (hereinafter, the steps from covering with a masking tape to drying at 50°C are also referred to as "platinum plating step").
  • the electrode was immersed overnight up to a height of 6.5 cm from the bottom face in a liquid in which paraffin (trade name: Paraffin Wax-125, manufactured by Nippon Seiro Co., Ltd., melting point: 53°C) melted in warm water at 97°C was floated to a thickness of 1.5 cm.
  • paraffin trade name: Paraffin Wax-125, manufactured by Nippon Seiro Co., Ltd., melting point: 53°C
  • the electrode was transferred to a beaker filled with warm water at 75°C. Then, warm water at 75°C was further poured over it for 10 minutes to remove the paraffin from the surface. Subsequently, the electrode was air-dried at room temperature for one day.
  • the resulting electrode was used as an electrode of Example 1 (hereinafter, the steps from immersion in the warm water with floating paraffin to air-drying are also referred to as "paraffin-containing step").
  • the unit amount of platinum supported was 140 ⁇ g/cm 2 , the paraffin content was 2.9 mg/g, the amount of adhering water was 41 mg, the unit amount of adhering water was 2.0 mg/cm 2 , and the contact angle was 124°.
  • An electrode of Example 2 was obtained in the same manner as in Example 1, except that the electroplating time was changed to 30 seconds.
  • the unit amount of platinum supported was 31 ⁇ g/cm 2 , the paraffin content was 4.5 mg/g, the amount of adhering water was 48 mg, the unit amount of adhering water was 2.3 mg/cm 2 , and the contact angle was 123°.
  • a graphite rod prepared in the same manner as in Example 1 was used as an electrode of Comparative Example 1 (i.e., the same procedure as in Example 1 was conducted, except that the platinum plating step and the paraffin impregnation step were not conducted).
  • the amount of adhering water of the electrode of this comparative example was 95 mg, the unit amount of adhering water was 4.5 mg/cm 2 , and the contact angle was 84°.
  • An electrode of black this comparative example was prepared by conducting the same procedure as in Example 1, except that the platinum plating step was not conducted.
  • An electrode of Comparative Example 3 was prepared by conducting the same procedure as in Example 1, except that the paraffin impregnation step was not conducted.
  • the unit amount of platinum supported was 140 ⁇ g/cm 2 , no paraffin was detected (the paraffin content was 0 mg/g), the amount of adhering water was 95 mg, the unit amount of adhering water was 4.5 mg/cm 2 , and the contact angle was 81°.
  • An electrode of Comparative Example 4 was prepared by conducting the same procedure as in Example 2, except that the paraffin impregnation step was not conducted.
  • the unit amount of platinum supported was 32 ⁇ g/cm 2 , no paraffin was detected, the amount of adhering water was 110 mg, the unit amount of adhering water was 5.2 mg/cm 2 , and the contact angle was 74°.
  • a rectangular parallelepiped graphite (PSG322, manufactured by SEC Carbon, Ltd., hereinafter, also referred to as "graphite plate") having a height of 250 mm, a width of 200 mm, and a thickness of 10 mm was used.
  • the portion of the graphite plate extending from 160 mm to 185 mm from the bottom face was covered with a masking tape for plating (manufactured by 3M Japan Limited).
  • the portion of the graphite plate extending up to 160 mm from the bottom face was immersed in an electrolyte solution containing 10 g/L of chloroplatinic acid (VI) and 20 g/L of hydrochloric acid, which was maintained at 70°C.
  • VI chloroplatinic acid
  • hydrochloric acid hydrochloric acid
  • electroplating was performed at a cathode current density of 1.0 A/dm 2 for 4 minutes to support platinum on the electrode. Then, the electrode with the masking tape removed was immersed in warm water at 50°C for 30 minutes, followed by washing with running water and drying at 50°C for 12 hours. Subsequently, the electrode was immersed overnight up to a height of 15 cm from the bottom face in warm water at 97°C, on which paraffin (trade name: Paraffin Wax-125, manufactured by Nippon Seiro Co., Ltd., melting point: 53°C) was floated to a thickness of 1.5 cm.
  • paraffin trade name: Paraffin Wax-125, manufactured by Nippon Seiro Co., Ltd., melting point: 53°C
  • the electrode After being withdrawn from the warm water with floating paraffin, and before the paraffin solidified, the electrode was immersed in a container filled with warm water at 75°C. Then, warm water at 75°C was further poured over it for 10 minutes to remove the paraffin from the surface. Subsequently, the electrode was air-dried at room temperature for one day. The resulting electrode was used as an electrode of Example 3.
  • the unit amount of platinum supported was 400 ⁇ g/cm 2 , the paraffin content was 44 mg/g, the contact angle was 133°, and the bath voltage was 1.52 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
  • An electrode was prepared in the same manner as in Example 3, except that the electroplating time was changed to 2 minutes, and was used as an electrode of this example.
  • the unit amount of platinum supported was 130 ⁇ g/cm 2 , the paraffin content was 19 mg/g, the contact angle was 114°, and the bath voltage was 1.50 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
  • An electrode was prepared in the same manner as in Example 3, except that the electroplating time was changed to 1 minute, and was used as an electrode of Example 5.
  • the unit amount of platinum supported was 59 ⁇ g/cm 2 , the paraffin content was 9.4 mg/g, the contact angle was 107°, and the bath voltage was 1.54 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
  • An electrode was prepared in the same manner as in Example 3, except that the electroplating time was changed to 30 seconds, and was used as an electrode of Example 6.
  • the unit amount of platinum supported was 21 ⁇ g/cm 2 , the paraffin content was 21 mg/g, the contact angle was 112°, and the bath voltage was 1.62 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
  • An electrode was prepared in the same manner as in Example 3, except that the electroplating time was changed to 15 seconds, and was used as an electrode of Example 7.
  • the unit amount of platinum supported was 3.5 ⁇ g/cm 2 , the paraffin content was 39 mg/g, the contact angle was 115°, and the bath voltage was 1.68 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
  • the graphite plate was used as an electrode of this example (i.e., the same procedure as in Example 3 was conducted, except that the platinum plating step and the paraffin impregnation step were not conducted).
  • the bath voltage of the electrode of this comparative example was measured. Note that the measurement was conducted without adding paraffin to the electrolytic bath, and no analysis of the electrolyte solution was conducted.
  • the contact angle was 84°, and the bath voltage was 1.67 V.
  • An electrode of Comparative Example 6 was prepared by impregnating the graphite plate with paraffin by conducting the same procedure as in Example 3, except that the platinum plating step was not conducted.
  • a graphite plate similar to that used in Example 3 was used.
  • the portion of the graphite plate extending from 160 mm to 180 mm from the bottom face was covered with a masking tape for plating (manufactured by 3M Japan Limited). Subsequently, the portion of the graphite plate extending up to 160 mm from the bottom face was immersed in an electrolyte solution containing 20 g/L of copper (Cu 2+ ) ions and 40 g/L of sulfuric acid, which was maintained at 70°C.
  • Electroplating was performed at a current density of 1.0 A/dm 2 for 10 minutes, using two copper plate electrodes, which were arranged to face the two largest faces of the graphite plate, as anodes and the graphite plate as a cathode, to support copper on both surfaces of the electrode. Then, the electrode with the masking tape removed was immersed in warm water at 50°C for 30 minutes, followed by washing with running water and drying at 50°C for 12 hours. Subsequently, the same paraffin impregnation step as in Example 3 was conducted, and the resulting electrode was used as an electrode of Comparative Example 7.
  • the amount of copper supported per unit area on the graphite was 1900 ⁇ g/cm 2 , the paraffin content was 36 mg/g, the contact angle was 112°, and the bath voltage was 1.74 V. Because copper was detected in the initial electrolyte solution and the final electrolyte solution at 22 mg/L and 27 mg/L, respectively, the amount of copper eluted was 22 mg/L, and the amount of non-electrolytic elution was 27 mg/L.
  • a graphite plate similar to that used in Example 3 was used.
  • the portion of the graphite plate extending from 160 mm to 180 mm from the bottom face was covered with a masking tape for plating (manufactured by 3M Japan Limited). Subsequently, the portion of the graphite plate extending up to 160 mm from the bottom face was immersed in a palladium plating solution (PALLABRIGHT SST-L, manufactured by Japan Pure Chemical Co., Ltd.), which was maintained at 50°C.
  • PALLABRIGHT SST-L manufactured by Japan Pure Chemical Co., Ltd.
  • Electroplating was performed at a current density of 1.0 A/dm 2 for 15 seconds, using two titanium-platinum electrodes, which were arranged to face the two largest faces of the graphite plate, as anodes and the graphite plate as a cathode, to support palladium on both surfaces of the electrode. Then, the electrode with the masking tape removed was immersed in hot water at 50°C for 30 minutes. Subsequently, the electrode was immersed in a 5% aqueous hydrochloric acid solution at 50°C for 5 minutes, and again immersed in hot water at 50°C for 30 minutes, followed by washing with running water and drying at 50°C for 12 hours. Subsequently, the same paraffin impregnation step as in Example 3 was conducted, and the resulting electrode was used as an electrode of Comparative Example 8.
  • the amount of palladium supported per unit area on the graphite was 47 ⁇ g/cm 2 , the paraffin content was 30 mg/g, the contact angle was 126°, and the bath voltage was 1.62 V. Because palladium was not detected in the initial electrolyte solution and was detected in the final electrolyte solution at 2.8 mg/L, the amount of palladium eluted was 0 mg/L, and the amount of non-electrolytic elution was 2.8 mg/L.
  • the electrode of the examples reduces the adhesion of water to the electrode by containing paraffin.
  • the electrode prevents metal elution into an electrolyte solution during a non-electrolytic period and reduces the voltage during electrolysis, even when the electrolyte solution is at a high temperature and contains sulfuric acid.
  • Patent Document 3 discloses that paraffin permeates into graphite and that removing the paraffin improves the performance of a graphite electrode. It has been found that the graphite according to the present invention, by containing paraffin, reduces the amount of liquid adhering to the electrode when it is withdrawn from the liquid.

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Abstract

Provided is an electrode modified for the production of electrolytic manganese dioxide which reduces the amount of liquid adhering to the electrode during withdrawal and prevents detachment or elution of metals even when immersed in a high-temperature and sulfuric acid-acidic electrolyte solution, thereby reducing an electrolytic voltage. An electrode including a structure including graphite and platinum supported on the graphite, the electrode further including paraffin.

Description

    TECHNICAL FIELD
  • The present disclosure relates to an electrode and more specifically to an electrode used for the production of electrolytic manganese dioxide, such as an electrode used for producing electrolytic manganese dioxide used as a positive electrode active material in manganese dry cells, particularly in alkali manganese dry cells.
    • BACKGROUND ART
  • In general, electrolytic manganese dioxide is produced by passing a current between an anode and a cathode in a sulfuric acid-acidic manganese sulfate electrolyte solution at a temperature close to 100°C and thereby causing electrolytic oxidative deposition on the anode. The cathode, on which a hydrogen evolution reaction occurs, in comparison with the anode on which a deposition reaction of electrolytic manganese dioxide occurs, does not directly affect the quality of the electrolytic manganese dioxide. Therefore, attention has not been paid to the cathode, and there have been few examples examined in the past.
  • The cathode is composed primarily of graphite and also includes copper and steel (Patent Document 1). For example, Patent Document 2 examines a cathode including graphite coated with copper. However, when a cathode containing copper is immersed in a high-temperature electrolyte solution containing sulfuric acid, copper elutes into the electrolyte solution during a non-electrolytic period. Furthermore, in electrolysis performed under sulfuric acid acidity and at a temperature close to 100°C, even metals having corrosion resistance, such as titanium and stainless steel, undergo corrosion during a non-electrolytic period. On the other hand, cathodes for electrolysis of brine or water (Patent Documents 3 and 4) are not designed in consideration of use under sulfuric acid acidity and at a temperature close to 100°C. Therefore, these electrodes contain elements that may affect the quality of manganese dioxide and have high manufacturing costs. This makes it difficult to use them as cathodes for electrolysis of manganese dioxide.
  • In the production of electrolytic manganese dioxide, an electrolyte solution is maintained at a high temperature of 93°C to 98°C. Therefore, an oil layer having a high boiling point, such as paraffin, is floated on the electrolyte solution in order to prevent evaporation of the electrolyte solution. However, because paraffin affects the product quality of electrolytic manganese dioxide, removal of paraffin incorporated into electrolytic manganese dioxide and electrolysis methods that do not allow paraffin to be incorporated into electrolytic manganese dioxide have been studied (Patent Document 2).
  • In addition, electrodes such as cathodes and anodes are taken in and out from upper portions of an electrolytic bath and a cleaning tank during electrolysis and cleaning, and an electrolyte solution and a cleaning solution adhere to the electrodes during withdrawal. Because the electrolyte solution and the like adhering to the electrodes result in loss, a reduction in the amount of water such as the electrolyte solution adhering to the electrodes has been anticipated.
    • PRIOR ART DOCUMENTS PATENT DOCUMENTS
    • Patent Document 1: WO2000/037714
    • Patent Document 2: JP-A-2022-7926
    • Patent Document 3: WO2020/110527
    • Patent Document 4: Japanese Patent No. 6837342
    • Patent Document 5: JP-A-2001-247987
    DISCLOSURE OF INVENTION TECHNICAL PROBLEM
  • The present disclosure provides an electrode for the production of electrolytic manganese dioxide, the electrode being modified for the purpose of reducing the amount of water adhering to the electrode during withdrawal from an aqueous solution and for preventing detachment or elution of the metals even when immersed in a high-temperature and sulfuric acid-acidic electrolyte solution, thereby reducing an electrolytic voltage.
    • SOLUTION TO PROBLEM
  • In the present disclosure, attention was paid to expression of performance of the electrode for the production of electrolytic manganese dioxide and to metal elution, and modification of the electrode was examined. As a result, it was found that inclusion of paraffin reduces the amount of water adhering to the electrode during withdrawal and that supporting platinum on the graphite prevents an increase in bath voltage during electrolysis even under high-temperature, high-concentration sulfuric acid-acidic conditions.
  • Specifically, the present invention is as defined in the claims, and a gist of the present disclosure is as follows.
    1. [1] An electrode comprising a structure including graphite and platinum supported on the graphite, the electrode further comprising paraffin.
    2. [2] The electrode according to [1], wherein the electrode is an electrode for production of electrolytic manganese dioxide.
    3. [3] The electrode according to [1] or [2], wherein an amount of the platinum supported on the graphite per unit area is 3 µg/cm2 or more and 500 µg/cm2 or less.
    4. [4] The electrode according to any one of [1] to [3], wherein a content of the paraffin, relative to a weight of a portion of the electrode, which portion is immersed in an electrolyte solution during electrolysis, is 1 mg/g or more and 100 mg/g or less.
    5. [5] The electrode according to any one of [1] to [4], wherein the paraffin has a melting point of 40°C or more and 80°C or less.
    6. [6] The electrode according to any one of [1] to [5], wherein a contact angle of a surface of the electrode with respect to pure water is more than 90° and less than 180°.
    7. [7] A method for producing electrolytic manganese dioxide, the method comprising using the electrode according to any one of [1] to [6].
    8. [8] A method for producing the electrode according to any one of [1] to [6], the method comprising a platinum-modifying step in which platinum is supported on graphite to obtain platinum-modified graphite; and a paraffin-containing step in which the platinum-modified graphite is brought into contact with paraffin so that the paraffin is incorporated into the platinum-modified graphite.
    9. [9] The method for producing the electrode according to [8], wherein, in the platinum-modifying step, a plating method is used to support the platinum on the graphite.
    10. [10] The method for producing the electrode according to [8], wherein the paraffin-containing step includes causing the paraffin to adhere onto a surface of the platinum-modified graphite and then to permeate into an inside of the platinum-modified graphite.
    ADVANTAGEOUS EFFECTS OF INVENTION
  • According to the present disclosure, an electrode for the production of electrolytic manganese dioxide, the electrode being modified for the purpose of reducing the amount of water adhering to the electrode during withdrawal from an aqueous solution and for preventing detachment or elution of the metals even when immersed in a high-temperature and sulfuric acid-acidic electrolyte solution, thereby reducing an electrolytic voltage, can be provided. Furthermore, the electrode according to the present disclosure does not cause elution of metals from the electrode plate into an electrolyte solution during a non-electrolytic period of electrolysis for producing manganese dioxide. This further reduces an electrolytic voltage. In addition, the amount of liquid adhering to the electrode during withdrawal from an electrolytic bath or a cleaning tank can be reduced. This enables efficient and consistent production of electrolytic manganese dioxide.
    • DESCRIPTION OF EMBODIMENTS
  • An embodiment of the present disclosure is described in detail below. Note that, in the present disclosure, any combination of the configurations and parameters disclosed in the present description is included, and any combination of upper and lower limits of the numerical values disclosed in the present description is also included.
    • <Electrode>
  • An electrode according to this embodiment is an electrode comprising a structure including graphite and platinum supported on the graphite and further comprising paraffin, that is, an electrode containing graphite on which platinum is supported and paraffin. The electrode according to this embodiment can also be regarded as an electrode containing a graphite base material on which platinum is supported and paraffin, or even further, as an electrode including a graphite base material on which platinum is supported and further containing paraffin.
  • Graphite serves as the electrode base material of the electrode according to this embodiment.
  • Examples of the types of the graphite include one or more selected from the group consisting of natural graphite, artificial graphite, carbon black, pyrolytic graphite and carbon fibers. Artificial graphite is preferable from the viewpoint of availability and ease of processing.
  • The shape of the graphite may be any shape that can be applied as the electrode for the production of electrolytic manganese dioxide. Examples include a plate shape and a rectangular parallelepiped shape, which are also used in metal refining and plating, but the shape is not essentially limited thereto. Preferable shapes of the graphite include a rectangular parallelepiped (pillar) shape and a plate shape.
  • Platinum serves as a catalyst for the electrolytic oxidation reaction in the production of electrolytic manganese dioxide. Any form of platinum which functions as a catalyst may be used. Examples include one or more selected from the group consisting of metallic platinum, a platinum alloy and a platinum compound. The platinum is preferably metallic platinum because it further reduces the bath voltage during the production of electrolytic manganese dioxide.
  • A platinum alloy is a metal containing platinum and one or more metal elements other than platinum. Examples include a metal containing platinum (Pt) and one or more elements selected from the group consisting of silver (Ag), gold (Au), cobalt (Co), copper (Cu), iron (Fe), iridium (Ir), manganese (Mn), nickel (Ni), palladium (Pd), ruthenium (Ru), titanium (Ti) and zirconium (Zr).
  • A platinum compound is a compound containing platinum and one or more nonmetal elements. Examples include a compound containing platinum (Pt) and one or more elements selected from the group consisting of carbon (C), chlorine (CI) and oxygen (O).
  • The electrode according to this embodiment has a structure in which platinum is supported on graphite, and in particular, a structure in which platinum is supported on the surface of the graphite. Supporting platinum on the surface of the graphite, that is, in a state in which platinum is supported on the graphite serving as the electrode base material, makes it possible to further reduce the bath voltage during the production of electrolytic manganese dioxide.
  • In the electrode according to this embodiment, it is sufficient that platinum is supported on a portion of the graphite on which the electrolytic reaction for electrolytic manganese dioxide occurs, and platinum may be supported on either a part or the entirety of the surface of the graphite. It is preferable that platinum be supported on only a part of the graphite surface in order to reduce the amount of platinum used, which is a noble metal. In other words, the graphite may have a region on which platinum is supported and a region on which platinum is not supported.
  • Platinum acts as an electrode catalyst by being present on the graphite and maintaining electrical contact with the graphite. It is known that the catalytic activity of platinum at the hydrogen evolution electrode is superior to that of carbon, which is the material of the graphite. For example, under conditions of a 2N aqueous sulfuric acid solution, the minimum hydrogen overvoltage is 335 mV for carbon, whereas it is 0.002 mV for platinum. Accordingly, platinum exhibits excellent catalytic activity with a lower hydrogen overvoltage than graphite.
  • In the electrode according to this embodiment, the amount of platinum supported on the graphite per unit area (hereinafter, also referred to simply as "amount of platinum supported") is preferably 3 µg/cm2 or more and 500 µg/cm2 or less, more preferably 10 µg/cm2 or more and 400 µg/cm2 or less, and further preferably 20 µg/cm2 or more and 200 µg/cm2 or less. Maintaining the amount of platinum supported within the above range prevents detachment of the supported metal (i.e., platinum) even under high-temperature, high-concentration sulfuric acid-acidic conditions, and reduces the bath voltage during electrolysis in the production of electrolytic manganese dioxide.
  • In this embodiment, the amount of platinum supported may be determined, according to the following equation, using the composition obtained by compositional analysis, by inductively coupled plasma emission spectroscopy (ICP method), of a solution obtained by pulverizing and acid-dissolving the region of an electrode sample in which platinum has been supported.
    G = M / A where G represents the amount of platinum supported (µg/cm2), M represents the amount of platinum (µg) determined from the platinum concentration in the solution obtained by the ICP measurement, and A represents the area (cm2) of the region of the electrode sample in which platinum has been supported.
  • The ICP method may be conducted using a common inductively coupled plasma emission spectrometer (e.g., trade name: OPTIMA 3000 DV, manufactured by PerkinElmer).
  • The solution may be prepared by any known method capable of dissolving the electrode sample. An example includes a method in which the region of the electrode in which platinum has been supported is pulverized, the resulting electrode sample is heated at 400°C or more and 700°C under an air atmosphere, a mixed solution of concentrated sulfuric acid and concentrated nitric acid is then added under an air atmosphere, followed by heating to evaporate to dryness, and the residue is dissolved in aqua regia.
  • The electrode according to this embodiment contains paraffin. Because the electrode according to this embodiment contains paraffin, the amount of liquid adhering to the electrode when the electrode is withdrawn from an electrolytic bath or a cleaning tank can be reduced. The paraffin is a mixture of hydrocarbons composed primarily of straight-chain hydrocarbons having 16 to 40 carbon atoms and preferably the same as that used for preventing evaporation in manganese dioxide electrolytic baths (i.e., for preventing evaporation of the electrolyte solution in the production of electrolytic manganese dioxide). In order to prevent evaporation, it is preferable that the paraffin be in a liquid state during electrolysis and in a solid state at normal temperature during recovery. Accordingly, the melting point of the paraffin is 40°C or more and 80°C or less. That is, the paraffin is preferably a paraffin having a melting point of 40°C or more and 80°C or less, further preferably a paraffin having a melting point of 50°C or more and 65°C or less. Specific examples of the paraffin include Paraffin Wax-125 (manufactured by Nippon Seiro Co., Ltd.; melting point: 53°C).
  • The content of paraffin is preferably 1 mg/g or more and 100 mg/g or less, more preferably 3 mg/g or more and 80 mg/g or less, and further preferably 3 mg/g or more and 50 mg/g or less, relative to the mass (unit mass) of a portion of the electrode according to this embodiment which is immersed in an electrolyte solution (hereinafter, the above portion is also referred to as "immersed portion") in the production of electrolytic manganese dioxide using the electrode. Maintaining the paraffin content within the above range reduces the amount of adhering water (amount of adhering liquid). As long as it does not interfere with the support of platinum (i.e., within a range in which platinum can be supported), the paraffin may be contained in the graphite prior to the support of platinum, or the electrode may be impregnated with paraffin subsequent to the support of platinum.
  • Note that, in this embodiment, the paraffin content refers to the mass of paraffin relative to the measured mass (g) of the electrode according to this embodiment.
  • The unit amount of adhering water of the electrode according to this embodiment is preferably 0 mg/cm2 or more and 4.0 mg/cm2 or less, more preferably 0 mg/cm2 or more and 3.0 mg/cm2 or less, and further preferably 0 mg/cm2 or more and 2.5 mg/cm2 or less. Maintaining the unit amount of adhering water within the above range reduces the amount of liquid adhering to the electrode according to this embodiment when it is withdrawn from an electrolytic bath or a cleaning tank, thereby reducing the loss of the electrolyte solution and the cleaning solution.
  • In this embodiment, the unit amount of adhering water is a value (mg/cm2) obtained by dividing the mass difference of the electrode before and after immersion in pure water by the surface area (cm2) of the region of the electrode in which platinum has been supported. In the case where a region in which platinum is not supported on the graphite is present, the region may be masked prior to immersion in pure water.
  • The contact angle of the electrode according to this embodiment is preferably more than 90° and less than 180°, more preferably 95° or more and less than 180°, and further preferably 100° or more and less than 180°. A contact angle exceeding 90° reduces the amount of liquid adhering to the electrode according to this embodiment when it is withdrawn from an electrolytic bath or a cleaning tank, thereby facilitating a further reduction in the loss of the electrolyte solution and the cleaning solution.
  • In this embodiment, the contact angle is a value determined by the following method.
  • Specifically, using a contact angle meter (e.g., device name: DMo-501, manufactured by Kyowa Interface Science Co., Ltd.) and measurement/analysis software (e.g., FAMAS1, manufactured by Kyowa Interface Science Co., Ltd.), 2.0 ± 0.1 µL of pure water is dropped onto the region in which platinum has been supported at room temperature (25°C ± 5°C). Assuming that the shape of the droplet 60 seconds after dropping is a part of a perfect circle, the contact angle (°) of the electrode with respect to pure water is measured according to the θ/2 method, using the following equation. The measurement is conducted three times, and the arithmetic average of the results is used as the contact angle of the electrode according to this embodiment.
    Contact angle ° = 2 × arctan h / r
    • h: the distance (mm) from the electrode surface to the apex of the droplet
    • r: the radius (mm) of the contact area between the electrode surface and the droplet
  • The electrode according to this embodiment can be used as a cathode for producing electrolytic manganese dioxide. It is preferable that the voltage in electrolysis performed under the following conditions using a plate-shaped electrode according to this embodiment having a height of 250 mm, a width of 200 mm, and a thickness of 10 mm as a cathode (hereinafter, this voltage is also referred to as "bath voltage") be 1.23 V or more and 2.00 V or less, more preferably 1.23 V or more and 1.80 V or less, and further preferably 1.23 V or more and 1.70 V or less.
    • Anode: titanium electrode
    • (plate-shaped, height: 250 mm, width: 200 mm, thickness: 10 mm)
    • Electrode intervals: 50 mm
    • Electrolyte solution: 6 L of a mixed aqueous solution of 85.4 g/L manganese sulfate and 27.0 g/L sulfuric acid, and 50 g of paraffin
    • Electrolyte feed solution: 118 g/L aqueous manganese sulfate solution
    • Electrolysis temperature: 96°C
    • Electrolysis current density: 0.65 A/dm2
  • Prior to electrolysis, the electrolyte solution and paraffin are charged into an electrolytic bath, which is then heated to 96°C. The cathode and anodes are installed in the bath in the order of anode, cathode and anode at intervals of 50 mm, with the principal faces (height × width faces) of the electrodes facing one another. Subsequently, electrolysis may be performed for 24 hours.
  • During electrolysis, the concentration of manganese ions decreases as a result of deposition of electrolytic manganese dioxide with the progress of the electrolytic reaction. In order to maintain a constant concentration of manganese ions in the electrolyte solution, it is preferable to continuously supply an aqueous manganese sulfate solution (electrolyte feed solution) to the electrolytic bath during electrolysis. It is more preferable to supply the aqueous manganese sulfate solution while discharging an amount of electrolyte solution equal to the amount of the aqueous manganese sulfate solution supplied. This makes it possible to maintain a constant concentration of manganese ions in the electrolyte solution.
  • When the electrode according to this embodiment is immersed in an electrolyte solution for producing electrolytic manganese dioxide, platinum elution from the electrode is unlikely to occur. Accordingly, the electrode according to this embodiment is characterized in that, when the electrode is immersed in a mixed aqueous solution of sulfuric acid and manganese sulfate under the following immersion conditions, the platinum content in the mixed aqueous solution of sulfuric acid and manganese sulfate (hereinafter, this platinum content is also referred to as "amount of platinum eluted") after immersion is, for example, 0 mg/L or more and 2 mg/L or less, 0 mg/L or more and 1 mg/L or less, 0 mg/L or more and 0.5 mg/L or less, or 0 mg/L or more and 0.1 mg/L or less.
    • (Immersion Conditions)
    • Electrode shape: plate-shaped, height: 250 mm, width: 200 mm, thickness: 10 mm Mixed aqueous solution of sulfuric acid and manganese sulfate: 6 L
      • (sulfuric acid concentration: 27.0 g/L)
      • (manganese sulfate concentration: 85.4 g/L)
    • Starting temperature for heating: 86°C
    • Heating rate: 0.1 °C/min
    • Immersion temperature: 96°C
    • Immersion time: 80 min
  • It is also preferable that the electrode according to this embodiment be unlikely to cause platinum elution even during a non-electrolytic period after the electrolytic reaction. It is more preferable that, when electrolysis is performed under the following electrolysis conditions using the electrode according to this embodiment, the platinum concentration in the electrolyte solution which is measured 3 hours after the electrolysis is stopped (hereinafter, this platinum concentration is also referred to as "amount of non-electrolytic elution") be 0 mg/L or more and 2 mg/L or less, 0 mg/L or more and 1 mg/L or less, 0 mg/L or more and 0.5 mg/L or less, or 0 mg/L or more and 0.1 mg/L or less.
    • (Electrolysis Conditions)
    • Anode: titanium electrode
    • (plate-shaped; height: 250 mm, width: 200 mm, thickness: 10 mm)
    • Electrode interval: 50 mm
    • Electrolyte solution: 6 L of mixed aqueous solution of 85.4 g/L manganese sulfate and 27.0 g/L sulfuric acid, and 50 g of paraffin
    • Electrolyte feed solution: 118 g/L aqueous manganese sulfate solution
    • Electrolysis temperature: 96°C
    • Electrolysis current density: 0.65 A/dm2
    • Electrolysis period: 6 days
  • When the amount of non-electrolytic elution falls within the above range, an increase in the bath voltage during electrolysis in the production of electrolytic manganese dioxide can be suppressed. Moreover, when the amount of platinum eluted in the electrolyte solution at the end of electrolysis falls within the above range, the service life of the electrode can be extended, thereby enabling repeated production of electrolytic manganese dioxide.
  • The electrode according to this embodiment makes it possible to reduce the electrolytic voltage in an electrolytic reaction using the electrode. Accordingly, the electrode according to this embodiment can be used as an electrode (cathode) for producing electrolytic manganese dioxide. This enables efficient production of electrolytic manganese dioxide.
    • <Method for Producing Electrode>
  • A method for producing the electrode according to this embodiment includes, for example, supporting a noble metal by performing electroplating or electroless plating in an electrolyte solution containing noble metal ions using platinum with a graphite plate serving as a working electrode such that the above-described supported amount is achieved. The method for producing the electrode also includes a paraffin-containing step in which paraffin is incorporated into the platinum-modified graphite. A preferable method for producing the electrode according to this embodiment includes a platinum-modifying step in which platinum is supported on graphite to obtain platinum-modified graphite, and a paraffin-containing step in which the platinum-modified graphite is brought into contact with paraffin to incorporate the paraffin into the platinum-modified graphite.
  • The method for supporting platinum on graphite in the platinum-modifying step is not particularly limited and may be any method capable of supporting platinum on graphite. The method is preferably a plating method, more preferably at least one of electroplating and electroless plating, and further preferably electroplating.
  • As a preferable electroplating method for supporting platinum on graphite, electroplating may be performed using a titanium-platinum electrode as an anode, graphite as a cathode, and a mixed solution of chloroplatinic acid (VI) and hydrochloric acid as a plating solution, at a temperature of 50°C or more and 100°C or less and a current density of 0.3 A/dm2 or more and 2.0 A/dm2 or less. It is preferable to perform the electroplating at a temperature of 60°C or more and 90°C or less and a current density of 0.5 A/dm2 or more and 1.5 A/dm2 or less.
  • The longer the electroplating time, the greater the amount of platinum supported tends to be. The plating time may be set appropriately in accordance with the intended amount of platinum supported and the size of the graphite (electrode base material). For example, the plating time may be 15 seconds or more and 2 minutes or less.
  • In order to suppress detachment of platinum subsequent to plating, it is preferable to polish the graphite prior to supporting platinum.
  • The method may further include a cleaning step in which the platinum-modified graphite is cleaned subsequent to the platinum-modifying step and prior to the paraffin-containing step. The cleaning step reduces impurities such as the plating solution remaining on the surface. The cleaning method used in the cleaning step may be any method capable of reducing impurities. An example includes a cleaning method using pure water. A preferable cleaning method includes immersion in warm water at 30°C or more and 100°C or less for 10 minutes or more and 60 minutes or less, followed by cleaning with pure water.
  • It is preferable to dry the platinum-modified graphite after cleaning by any suitable method. The drying conditions may be set appropriately. For example, drying may be performed at 30°C or more and 100°C or less under an air atmosphere for 1 hour or more and 24 hours or less.
  • The paraffin-containing step is a step in which the platinum-modified graphite is brought into contact with paraffin to incorporate the paraffin into the platinum-modified graphite.
  • The contact between the platinum-modified graphite and paraffin may be any method capable of causing paraffin to adhere to the surface of the platinum-modified graphite and to permeate into the inside of the platinum-modified graphite. For example, the platinum-modified graphite may be immersed in pure water at a temperature of 60°C or more and less than 100°C, having a paraffin layer on the surface. The pure water is preferably at a temperature of 80°C or more and less than 100°C. When the platinum-modified graphite passes through the paraffin layer, paraffin adheres to the surface of the platinum-modified graphite. Subsequently immersing the platinum-modified graphite in pure water in the above temperature range allows the molten paraffin to permeate into the platinum-modified graphite. The thickness of the paraffin layer may be any thickness that allows paraffin to uniformly adhere to the surface of the platinum-modified graphite, and may be, for example, 1 cm or more and 2 cm or less. The immersion time may be set appropriately and is, for example, 1 hour or more and 24 hours or less, preferably 5 hours or more and 24 hours or less.
  • After immersion, the platinum-modified graphite is preferably immersed in warm water not containing paraffin, followed by cleaning with warm water and drying. This removes paraffin remaining on the uppermost surface of the platinum-modified graphite.
    • EXAMPLES
  • The present disclosure is described with reference to Examples below. It should be noted that the present disclosure is not limited to Examples below.
    • <Measurement of Amount of Metal Supported>
  • The measurement of the amount of metal supported was conducted by measuring the metal concentration in a measurement solution by inductively coupled plasma emission spectroscopy (ICP method) using a common inductively coupled plasma emission spectrometer (trade name: OPTIMA 3000 DV, manufactured by PerkinElmer).
  • The measurement solution was prepared by the following method. First, the region of an electrode sample in which platinum had been supported was pulverized using a stamp mill until it passed through a sieve with an opening of 0.5 mm. Note that, as the electrode sample, a portion immersed in the plating bath described below was cut out. Specifically, a piece prepared by cutting out a portion of the graphite rod (described below) extending up to 5 cm from the bottom face (Examples 1 and 2 and Comparative Examples 1 to 4), and a piece prepared by cutting out a region of the largest face (i.e., the height × width face, hereinafter, also referred to as "principal face") of the graphite plate (described below), the region extending 2 to 6 cm to the right or left and 5 cm upward (i.e., a region having a height of 5 cm and a width of 4 cm) with reference to the widthwise center (the point that bisects a vertical line drawn parallel to the width direction from the left end to the right end of the graphite plate) at a position 4 cm from the lower end of the graphite plate (Example 3 to 8 and Comparative Examples 5 to 8) were used.
  • The pulverized electrode sample (fragments) was charged into a magnetic crucible and then heated at 600°C under an air atmosphere for 70 hours. Subsequently, 1 mL of concentrated sulfuric acid and 2 mL of concentrated nitric acid were added under an air atmosphere. The resulting mixture was heated and evaporated to dryness. Subsequently, aqua regia was added to dissolve the residue to form a solution. Pure water was added to the solution until the volume of the solution reached 1 L. Thus, a measurement solution was obtained. The metal concentration in the measurement solution was measured.
  • On the basis of the metal concentration in the measurement solution obtained by the ICP measurement, the amount of metal supported was determined using the following equation.
    G = C × W 1 / W 2 × V / A
  • In the above equation, G represents the amount of metal supported (mg/cm2), C represents the metal concentration (mg/L) in the measurement solution obtained by the ICP measurement, W1 represents the mass (g) of the electrode sample after cutting out, W2 represents the mass (g) of the pulverized electrode sample (fragments), V represents the volume (= 1 (L); note that, in Example 7 only, 0.1 L) of the measurement solution, and A represents the surface area (cm2) of the region of the platinum-modified graphite which was cut out and in which platinum had been supported.
    • <Paraffin Content>
  • The paraffin content in the electrode sample was determined by gas chromatography using a gas chromatograph (Agilent 7890A GC, manufactured by Agilent Technologies, Inc.).
  • 1.0 g of the electrode sample (fragments) pulverized in the same manner as for the measurement of the amount of metal supported and 5 mL of hexane were added to a screw-neck sample tube, followed by shaking for 15 minutes. The resulting mixture was centrifuged at 3000 rpm for 15 minutes and subsequently filtered using a filter (trade name: Myshori Disk H-13-5, manufactured by Tosoh Corporation). Then, the recovered hexane layer was used as a measurement sample. The measurement sample was analyzed by gas chromatography, and the paraffin content was determined by comparing the signal detected by the FID detector attached to the device with the signal of a standard sample.
  • As the standard sample, a hexane solution containing paraffin (trade name: Paraffin Wax-125, manufactured by Nippon Seiro Co., Ltd.) at a concentration of 10 mg/L was used.
    • <Measurement of Amount of Metal Eluted into Initial Electrolyte Solution>
  • The amount of metal eluted from the electrode sample into the initial electrolyte solution was measured by the ICP method using a common inductively coupled plasma emission spectrometer (trade name: OPTIMA 3000 DV, PerkinElmer).
  • The metal concentration in a diluted electrolyte solution, which was obtained by diluting the initial electrolyte solution (described below) tenfold with pure water, was measured by the ICP measurement. The measured metal concentration was multiplied by ten, and the resulting value was used as the metal concentration (mg/L) in the initial electrolyte solution.
    • <Measurement of Amount of Adhering Water and Unit Amount of Adhering Water>
  • The amount of water in the electrode in each of the examples and the comparative examples was determined by measuring the mass of the electrode before and after immersion in pure water. Specifically, a screw-cap bottle containing pure water was placed on an electronic balance. A rod-shaped electrode was immersed in the screw-cap bottle such that entire electrode surface was immersed in pure water. After immersion for 10 seconds, the rod-shaped electrode was taken out of the screw-cap bottle. The difference between the values of the electronic balance before and after the immersion of the electrode (i.e., the difference between the mass before immersion of the rod-shaped electrode and the mass after taking out the rod-shaped electrode) was calculated and used as the amount of adhering water (mg).
  • As the measurement sample, a rod-shaped electrode, a portion located above 5 cm from the bottom face of which was covered with a PTFE tape, was used.
  • The measurement was conducted three times, and the arithmetic average of the results was used as the amount of adhering water (mg).
  • The amount of adhering water (mg) was divided by the surface area (cm2) of the electrode, and the resulting value was used as the unit amount of adhering water (mg/cm2).
  • The surface area (cm2) of the electrode was defined as the surface area (cm2) of the region in which platinum had been supported.
    • <Measurement of Contact Angle>
  • The contact angle of the electrode was measured using a contact angle meter (device name: DMo-501, manufactured by Kyowa Interface Science Co., Ltd.) and measurement/analysis software attached to the device (FAMAS1, manufactured by Kyowa Interface Science Co., Ltd.). Under an environment at room temperature of 25°C, 2.0 ± 0.1 µL of a droplet of pure water was dropped onto the surface of the electrode. Assuming that the shape of the droplet 60 seconds after dropping was a part of a perfect circle, the contact angle (°) of the electrode with respect to pure water was measured according to the θ/2 method, using the following equation.
    Contact angle ° = 2 × arctan h / r
    • h: the distance (mm) from the electrode surface to the apex of the droplet
    • r: the radius (mm) of the contact area between the electrode surface and the droplet
  • The measurement was conducted three times, and the arithmetic average of the results was used as the contact angle (°).
    • <Measurement of Bath Voltage and Electrode Durability>
  • The bath voltage and electrode durability of the electrode in each of the examples and the comparative examples were determined by electrolytically synthesizing electrolytic manganese dioxide using an electrolytic bath including the graphite plate of each of the examples and the comparative examples serving as a cathode, a titanium plate serving as an anode, and a mixed aqueous solution of manganese sulfate and sulfuric acid serving as an electrolyte solution.
  • 6 L of the electrolyte solution and 50 g of paraffin for preventing evaporation of the electrolyte solution were added to the electrolytic bath. Then, the temperature was increased to 86°C. The electrolyte solution used was a mixed aqueous solution of manganese sulfate and sulfuric acid having a manganese sulfate concentration of 85.4 g/L and a sulfuric acid concentration of 27.0 g/L.
  • One plate-shaped electrode (cathode) of each of the examples and the comparative examples and two titanium plate anodes (height: 250 mm, width: 200 mm, thickness: 5 mm) were immersed 160 mm in the electrolyte solution so as to be perpendicular to the liquid surface of the electrolyte solution. Subsequently, the cathode and the anodes were fixed at intervals of 50 mm such that their principal faces (the height × width faces) faced one another (i.e., in the order of anode, cathode and anode). Then, the temperature of the electrolyte solution was increased to 96°C over 3 hours. After completion of the temperature rise, 50 mL of the electrolyte solution was collected (hereinafter, the electrolyte solution collected after completion of the temperature rise is also referred to as "initial electrolyte solution").
  • A DC stabilized power supply (PAN-18-10A, manufactured by Kikusui Electronics Corporation) was connected, and electrolytic synthesis of electrolytic manganese dioxide was conducted at a constant current of 4.48 A (i.e., 0.65 A/dm2 with respect to the cathode). During electrolysis, in order to prevent changes in the composition of the electrolyte solution, an aqueous manganese sulfate solution (electrolyte feed solution) having a manganese sulfate concentration of 118 g/L was continuously supplied to the electrolytic bath. In addition, the electrolyte solution was continuously withdrawn from the electrolytic bath such that the amount of the electrolyte solution in the electrolytic bath was maintained at 6 L. 24 hours after the start of electrolysis, the voltage displayed on the DC stabilized power supply was read and used as the bath voltage.
  • Electrolysis was further continued for six days and stopped on the seventh day from the start of electrolysis. 50 mL of the electrolyte solution was collected 3 hours after stopping the electrolysis (hereinafter, the electrolyte solution collected 3 hours after stopping the electrolysis 7 days after the start of electrolysis is referred to as "final electrolyte solution"). The amount of metal eluted into the final electrolyte solution, that is, the amount of non-electrolytic elution (mg/L), was measured in the same manner as described above in <Measurement of Amount of Metal Eluted into Initial Electrolyte Solution>.
    • Example 1
  • All 6 surfaces of a rectangular parallelepiped (columnar) graphite (PSG322, manufactured by SEC Carbon, Ltd.) having a length (height) of 100 mm, a width of 10 mm, and a thickness of 10 mm were polished with 400-grit abrasive paper (hereinafter, the above-described graphite subjected to the polishing is also referred to as "graphite rod"). The portion of the graphite rod extending from 50 mm to 70 mm from the bottom face was covered with a masking tape for plating (manufactured by 3M Japan Limited) (the portion from 50 mm to 70 mm in the longitudinal direction from the bottom face was covered).
  • The portion of the graphite rod extending up to 60 mm from the bottom face (60 mm in the height direction from the bottom face of the graphite rod) and a titanium-platinum electrode were immersed in an electrolyte solution containing 10 g/L of chloroplatinic acid (VI), and 20 g/L of hydrochloric acid, which was maintained at 70°C, to form a plating bath including the titanium-platinum electrode serving as an anode and the graphite rod serving as a cathode. Using the plating bath, electroplating was performed at cathode current density of 1.0 A/dm2 for 2 minutes to support platinum on the graphite rod (i.e., the portion extending up to 50 mm in the longitudinal direction from the bottom face of the graphite rod).
  • After supporting platinum, the graphite rod with the masking tape removed was immersed in warm water at 50°C for 30 minutes, washed with running water, and then dried at 50°C under an air atmosphere for 12 hours (hereinafter, the steps from covering with a masking tape to drying at 50°C are also referred to as "platinum plating step").
  • Subsequently, the electrode was immersed overnight up to a height of 6.5 cm from the bottom face in a liquid in which paraffin (trade name: Paraffin Wax-125, manufactured by Nippon Seiro Co., Ltd., melting point: 53°C) melted in warm water at 97°C was floated to a thickness of 1.5 cm. After being withdrawn from the warm water with floating paraffin, and before the paraffin solidified, the electrode was transferred to a beaker filled with warm water at 75°C. Then, warm water at 75°C was further poured over it for 10 minutes to remove the paraffin from the surface. Subsequently, the electrode was air-dried at room temperature for one day. The resulting electrode was used as an electrode of Example 1 (hereinafter, the steps from immersion in the warm water with floating paraffin to air-drying are also referred to as "paraffin-containing step").
  • The unit amount of platinum supported was 140 µg/cm2, the paraffin content was 2.9 mg/g, the amount of adhering water was 41 mg, the unit amount of adhering water was 2.0 mg/cm2, and the contact angle was 124°.
    • Example 2
  • An electrode of Example 2 was obtained in the same manner as in Example 1, except that the electroplating time was changed to 30 seconds.
  • The unit amount of platinum supported was 31 µg/cm2, the paraffin content was 4.5 mg/g,
    the amount of adhering water was 48 mg, the unit amount of adhering water was 2.3 mg/cm2, and the contact angle was 123°.
    • Comparative Example 1
  • A graphite rod prepared in the same manner as in Example 1 was used as an electrode of Comparative Example 1 (i.e., the same procedure as in Example 1 was conducted, except that the platinum plating step and the paraffin impregnation step were not conducted).
  • The amount of adhering water of the electrode of this comparative example was 95 mg, the unit amount of adhering water was 4.5 mg/cm2, and the contact angle was 84°.
    • Comparative Example 2
  • An electrode of black this comparative example was prepared by conducting the same procedure as in Example 1, except that the platinum plating step was not conducted.
  • No platinum was detected in this electrode (the unit platinum content was 0 mg/g), the paraffin content was 18 mg/g,
    the amount of adhering water was 44 mg, the unit amount of adhering water was 2.1 mg/cm2, and the contact angle was 113°.
    • Comparative Example 3
  • An electrode of Comparative Example 3 was prepared by conducting the same procedure as in Example 1, except that the paraffin impregnation step was not conducted.
  • The unit amount of platinum supported was 140 µg/cm2, no paraffin was detected (the paraffin content was 0 mg/g), the amount of adhering water was 95 mg, the unit amount of adhering water was 4.5 mg/cm2, and the contact angle was 81°.
    • Comparative Example 4
  • An electrode of Comparative Example 4 was prepared by conducting the same procedure as in Example 2, except that the paraffin impregnation step was not conducted.
  • The unit amount of platinum supported was 32 µg/cm2, no paraffin was detected, the amount of adhering water was 110 mg, the unit amount of adhering water was 5.2 mg/cm2, and the contact angle was 74°.
  • The results of these examples and comparative examples are shown in the table below. [Table 1]
    Type of metal supported Amount of metal supported on graphite rod [µg/cm2] Paraffin content *1 [mg/g] Contact angle [°] Amount of adhering water [mg] Unit amount of adhering water [mg/cm2]
    Example 1 Pt 140 2.9 124 41 2.0
    Example 2 Pt 31 4.5 123 48 2.3
    Comparative Example 1 - - ND 84 95 4.5
    Comparative Example 2 - - 18 113 44 2.1
    Comparative Example 3 Pt 140 ND 81 95 4.5
    Comparative Example 4 Pt 32 ND 74 110 5.2
    *1: ND indicates a value below the detection limit (0.1 mg/g)
  • From the above table, it was found that, when paraffin was permeated into the graphite, the amount of water adhering to the electrode was reduced. Furthermore, it was found that an electrode containing paraffin exhibited an increased contact angle of pure water and a reduced amount of adhering water, regardless of the presence or absence of metal plating and the amount of metal plating.
    • Example 3
  • A rectangular parallelepiped graphite (PSG322, manufactured by SEC Carbon, Ltd., hereinafter, also referred to as "graphite plate") having a height of 250 mm, a width of 200 mm, and a thickness of 10 mm was used. The portion of the graphite plate extending from 160 mm to 185 mm from the bottom face was covered with a masking tape for plating (manufactured by 3M Japan Limited). Subsequently, the portion of the graphite plate extending up to 160 mm from the bottom face (160 mm in the height direction from the bottom face) was immersed in an electrolyte solution containing 10 g/L of chloroplatinic acid (VI) and 20 g/L of hydrochloric acid, which was maintained at 70°C. Two titanium-platinum electrodes were arranged to face the two largest faces of the graphite plate (the height × width faces; hereinafter, also referred to as "principal faces") to form a plating bath in which the two titanium-platinum electrodes served as anodes and the graphite plate served as a cathode. Using the plating bath, electroplating was performed at a cathode current density of 1.0 A/dm2 for 4 minutes to support platinum on the electrode. Then, the electrode with the masking tape removed was immersed in warm water at 50°C for 30 minutes, followed by washing with running water and drying at 50°C for 12 hours. Subsequently, the electrode was immersed overnight up to a height of 15 cm from the bottom face in warm water at 97°C, on which paraffin (trade name: Paraffin Wax-125, manufactured by Nippon Seiro Co., Ltd., melting point: 53°C) was floated to a thickness of 1.5 cm. After being withdrawn from the warm water with floating paraffin, and before the paraffin solidified, the electrode was immersed in a container filled with warm water at 75°C. Then, warm water at 75°C was further poured over it for 10 minutes to remove the paraffin from the surface. Subsequently, the electrode was air-dried at room temperature for one day. The resulting electrode was used as an electrode of Example 3.
  • The unit amount of platinum supported was 400 µg/cm2, the paraffin content was 44 mg/g, the contact angle was 133°, and the bath voltage was 1.52 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
    • Example 4
  • An electrode was prepared in the same manner as in Example 3, except that the electroplating time was changed to 2 minutes, and was used as an electrode of this example.
  • The unit amount of platinum supported was 130 µg/cm2, the paraffin content was 19 mg/g, the contact angle was 114°, and the bath voltage was 1.50 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
    • Example 5
  • An electrode was prepared in the same manner as in Example 3, except that the electroplating time was changed to 1 minute, and was used as an electrode of Example 5.
  • The unit amount of platinum supported was 59 µg/cm2, the paraffin content was 9.4 mg/g, the contact angle was 107°, and the bath voltage was 1.54 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
    • Example 6
  • An electrode was prepared in the same manner as in Example 3, except that the electroplating time was changed to 30 seconds, and was used as an electrode of Example 6.
  • The unit amount of platinum supported was 21 µg/cm2, the paraffin content was 21 mg/g, the contact angle was 112°, and the bath voltage was 1.62 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
    • Example 7
  • An electrode was prepared in the same manner as in Example 3, except that the electroplating time was changed to 15 seconds, and was used as an electrode of Example 7.
  • The unit amount of platinum supported was 3.5 µg/cm2, the paraffin content was 39 mg/g, the contact angle was 115°, and the bath voltage was 1.68 V. Since no platinum was detected in either the initial electrolyte solution or the final electrolyte solution, both the amount of platinum eluted and the amount of non-electrolytic elution were 0 mg/L.
    • Comparative Example 5
  • The graphite plate was used as an electrode of this example (i.e., the same procedure as in Example 3 was conducted, except that the platinum plating step and the paraffin impregnation step were not conducted).
  • Platinum, copper, palladium and paraffin were not detected from this electrode (the unit amount of platinum supported, the unit amount of copper supported and the unit amount of palladium supported were all 0 µg/cm2, and the paraffin content was 0 mg/g).
  • The bath voltage of the electrode of this comparative example was measured. Note that the measurement was conducted without adding paraffin to the electrolytic bath, and no analysis of the electrolyte solution was conducted.
  • The contact angle was 84°, and the bath voltage was 1.67 V.
    • Comparative Example 6
  • An electrode of Comparative Example 6 was prepared by impregnating the graphite plate with paraffin by conducting the same procedure as in Example 3, except that the platinum plating step was not conducted.
  • Platinum, copper and palladium were not detected from the electrode (the unit amount of platinum supported, the unit amount of copper supported and the unit amount of palladium supported were all 0 µg/cm2). The paraffin content was 48 mg/g, the contact angle was 115°, and the bath voltage was 2.09 V.
    • Comparative Example 7
  • A graphite plate similar to that used in Example 3 was used. The portion of the graphite plate extending from 160 mm to 180 mm from the bottom face was covered with a masking tape for plating (manufactured by 3M Japan Limited). Subsequently, the portion of the graphite plate extending up to 160 mm from the bottom face was immersed in an electrolyte solution containing 20 g/L of copper (Cu2+) ions and 40 g/L of sulfuric acid, which was maintained at 70°C. Electroplating was performed at a current density of 1.0 A/dm2 for 10 minutes, using two copper plate electrodes, which were arranged to face the two largest faces of the graphite plate, as anodes and the graphite plate as a cathode, to support copper on both surfaces of the electrode. Then, the electrode with the masking tape removed was immersed in warm water at 50°C for 30 minutes, followed by washing with running water and drying at 50°C for 12 hours. Subsequently, the same paraffin impregnation step as in Example 3 was conducted, and the resulting electrode was used as an electrode of Comparative Example 7.
  • The amount of copper supported per unit area on the graphite was 1900 µg/cm2, the paraffin content was 36 mg/g, the contact angle was 112°, and the bath voltage was 1.74 V. Because copper was detected in the initial electrolyte solution and the final electrolyte solution at 22 mg/L and 27 mg/L, respectively, the amount of copper eluted was 22 mg/L, and the amount of non-electrolytic elution was 27 mg/L.
    • Comparative Example 8
  • A graphite plate similar to that used in Example 3 was used. The portion of the graphite plate extending from 160 mm to 180 mm from the bottom face was covered with a masking tape for plating (manufactured by 3M Japan Limited). Subsequently, the portion of the graphite plate extending up to 160 mm from the bottom face was immersed in a palladium plating solution (PALLABRIGHT SST-L, manufactured by Japan Pure Chemical Co., Ltd.), which was maintained at 50°C. Electroplating was performed at a current density of 1.0 A/dm2 for 15 seconds, using two titanium-platinum electrodes, which were arranged to face the two largest faces of the graphite plate, as anodes and the graphite plate as a cathode, to support palladium on both surfaces of the electrode. Then, the electrode with the masking tape removed was immersed in hot water at 50°C for 30 minutes. Subsequently, the electrode was immersed in a 5% aqueous hydrochloric acid solution at 50°C for 5 minutes, and again immersed in hot water at 50°C for 30 minutes, followed by washing with running water and drying at 50°C for 12 hours. Subsequently, the same paraffin impregnation step as in Example 3 was conducted, and the resulting electrode was used as an electrode of Comparative Example 8.
  • The amount of palladium supported per unit area on the graphite was 47 µg/cm2, the paraffin content was 30 mg/g, the contact angle was 126°, and the bath voltage was 1.62 V. Because palladium was not detected in the initial electrolyte solution and was detected in the final electrolyte solution at 2.8 mg/L, the amount of palladium eluted was 0 mg/L, and the amount of non-electrolytic elution was 2.8 mg/L.
  • When paraffin is contained, a graphite electrode that does not support a metal exhibits reduced performance. The results of the above examples and comparative examples are shown in the table below. [Table 2]
    Type of metal supported Amount of metal supported on graphite plate *1 [µg/cm2] Paraffin content *2 [mg/g] Contact angle [°] Bath voltage [V] Amount of metal eluted *3 [mg/L]
    In initial electrolyte solution In final electrolyte solution
    Example 3 Pt 400 44 133 1.52 ND ND
    Example 4 Pt 130 19 114 1.50 ND ND
    Example 5 Pt 59 9.4 107 1.54 ND ND
    Example 6 Pt 21 21 112 1.62 ND ND
    Example 7 Pt 3.5 39 115 1.68 ND ND
    Comparative Example 5 - - ND 84 1.67 - -
    Comparative Example 6 - - 48 115 2.09 - -
    Comparative Example 7 Cu 1900 36 122 1.74 22 27
    Comparative Example 8 Pd 47 30 126 1.62 ND 2.8
    *1: Examples 3 to 7 refer to the amount of platinum supported, Comparative Example 7 refers to the amount of copper supported, and Comparative Example 8 refers to the amount of palladium supported
    *2: ND indicates a value below the detection limit (0.1 mg/g)
    *3: ND indicates a value below the detection limit (0.1 mg/L)
  • From Comparative Examples 5 and 6 in Table 2, it was found that a graphite electrode containing paraffin and not plated with a metal (platinum, copper or palladium) exhibited an increased bath voltage and reduced electrode performance.
  • However, it was found that, when the electrode contained paraffin and was plated with a metal (platinum, copper or palladium), the bath voltage was reduced. Furthermore, from Examples 3 to 7, it was found that using platinum as a plating metal suppressed metal elution.
  • As described above, the electrode of the examples reduces the adhesion of water to the electrode by containing paraffin. In addition, by supporting platinum, the electrode prevents metal elution into an electrolyte solution during a non-electrolytic period and reduces the voltage during electrolysis, even when the electrolyte solution is at a high temperature and contains sulfuric acid.
  • Patent Document 3 discloses that paraffin permeates into graphite and that removing the paraffin improves the performance of a graphite electrode. It has been found that the graphite according to the present invention, by containing paraffin, reduces the amount of liquid adhering to the electrode when it is withdrawn from the liquid.
  • The present disclosure has been described in detail with reference to specific embodiments. It will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present disclosure.
  • The entire contents of Japanese Patent Application No. 2023-055509 , filed on March 30, 2023, including the description, claims, drawings and abstract, are hereby incorporated herein by reference as the disclosure of the present description.

Claims (10)

  1. An electrode comprising a structure including graphite and platinum supported on the graphite, the electrode further comprising paraffin.
  2. The electrode according to Claim 1, wherein the electrode is an electrode for production of electrolytic manganese dioxide.
  3. The electrode according to Claim 1 or 2, wherein an amount of the platinum supported on the graphite per unit area is 3 µg/cm2 or more and 500 µg/cm2 or less.
  4. The electrode according to any one of Claims 1 to 3, wherein a content of the paraffin, relative to a weight of a portion of the electrode, which portion is immersed in an electrolyte solution during electrolysis, is 1 mg/g or more and 100 mg/g or less.
  5. The electrode according to any one of Claims 1 to 4, wherein the paraffin has a melting point of 40°C or more and 80°C or less.
  6. The electrode according to any one of Claims 1 to 5, wherein a contact angle of a surface of the electrode with respect to pure water is more than 90° and less than 180°.
  7. A method for producing electrolytic manganese dioxide, the method comprising using the electrode according to any one of Claims 1 to 6.
  8. A method for producing the electrode according to any one of Claims 1 to 6, the method comprising a platinum-modifying step in which platinum is supported on graphite to obtain platinum-modified graphite; and a paraffin-containing step in which the platinum-modified graphite is brought into contact with paraffin so that the paraffin is incorporated into the platinum-modified graphite.
  9. The method for producing the electrode according to Claim 8, wherein, in the platinum-modifying step, a plating method is used to support the platinum on the graphite.
  10. The method for producing the electrode according to Claim 8, wherein the paraffin-containing step includes causing the paraffin to adhere onto a surface of the platinum-modified graphite and then to permeate into an inside of the platinum-modified graphite.
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