WO2021182385A1 - アルカリ水電解方法及びアルカリ水電解用アノード - Google Patents
アルカリ水電解方法及びアルカリ水電解用アノード Download PDFInfo
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- WO2021182385A1 WO2021182385A1 PCT/JP2021/008958 JP2021008958W WO2021182385A1 WO 2021182385 A1 WO2021182385 A1 WO 2021182385A1 JP 2021008958 W JP2021008958 W JP 2021008958W WO 2021182385 A1 WO2021182385 A1 WO 2021182385A1
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Definitions
- Hydrogen is a secondary energy that is suitable for storage and transportation and has a small environmental load, so there is a lot of interest in hydrogen energy systems that use hydrogen as an energy carrier.
- hydrogen is mainly produced by steam reforming of fossil fuels.
- water electrolysis from renewable energies such as solar power generation and wind power generation has become important among the basic technologies. Water electrolysis is low cost and suitable for large scale, and is a powerful technology for hydrogen production.
- alkaline water electrolysis in which a high-concentration alkaline aqueous solution is used as the electrolyte.
- solid polymer type water electrolysis in which a solid polymer membrane (SPE) is used as the electrolyte.
- SPE solid polymer membrane
- the high-concentration alkaline aqueous solution becomes more conductive as the temperature rises, but also becomes more corrosive. Therefore, the upper limit of the operating temperature is suppressed to about 80 to 90 ° C. Due to the development of constituent materials and various piping materials for electrolytic cells that can withstand high-temperature and high-concentration alkaline aqueous solutions, low-resistance diaphragms, and electrodes with an expanded surface area and a catalyst, the electrolytic cell voltage has a current density of 0. It has improved to 2V or less at 6Acm- 2.
- Non-Patent Documents 1 and 2 A nickel-based material that is stable in a high-concentration alkaline aqueous solution is used as the anode for alkaline water electrolysis.
- Non-Patent Documents 1 and 2 it has been reported that nickel-based anodes have a life of several decades or more.
- Non-Patent Document 3 when renewable energy is used as a power source, severe conditions such as severe start / stop and load fluctuation often occur, and deterioration of the performance of the nickel-based anode has become a problem.
- Both the nickel oxide formation reaction and the produced nickel oxide reduction reaction proceed on the metal surface. Therefore, the desorption of the electrode catalyst formed on the metal surface is promoted along with these reactions.
- the nickel-based anode has a potential lower than the oxygen evolution potential (1.23 V vs. RHE) and is the opposite electrode (cathode) for hydrogen generation (cathode). It is maintained at a potential higher than 0.00V vs. RHE).
- electromotive force is generated by various chemical species, the anode potential is maintained low as the battery reaction progresses, and the reduction reaction of nickel oxide is promoted.
- the above RHE is an abbreviation for Reversible Hydrogen Electrode.
- the current generated by the battery reaction leaks through a pipe connecting the cells.
- a measure to prevent such a current leak for example, there is a method of keeping a minute current flowing at the time of stopping.
- special power supply control is required to keep the minute current flowing when stopped.
- oxygen and hydrogen are constantly generated, there is a problem that excessive labor is required for operation management.
- a catalyst for an anode for oxygen generation used in alkaline water electrolysis a platinum group metal, a platinum group metal oxide, a valve metal oxide, an iron group oxide, or a lanthanide group metal oxide is used. Etc. are used.
- an alkaline water electrolysis anode (Patent Document 1) in which a lithium-containing nickel oxide catalyst layer containing lithium and nickel in a predetermined molar ratio is formed on the surface of a nickel substrate, a nickel cobalt-based oxide, and an iridium oxide or
- An alkaline water electrolysis anode (Patent Document 2) in which a catalyst layer containing a ruthenium oxide is formed on the surface of a nickel substrate has been proposed.
- a catalyst layer containing a hybrid cobalt hydroxide nanosheet (Cons) of a composite of a metal hydroxide and an organic substance is provided on the surface of a conductive substrate whose surface is made of nickel or a nickel-based alloy. It is an provided anode for oxygen generation. Further, using this oxygen generating anode, an electrolytic solution in which the hybrid cobalt hydroxide nanosheet (Cons) which is a forming component of the catalyst layer is dispersed is supplied to the anode chamber and the cathode chamber constituting the electrolytic cell.
- an alkaline water electrolysis method commonly used for electrolysis in each chamber Non-Patent Document 4).
- the present invention has been made in view of the problems of the prior art, and the subject thereof is that even when a power source having a large output fluctuation such as renewable energy is used as a power source. It is an object of the present invention to provide a useful electrode for electrolysis in which electrolysis performance is not easily deteriorated and excellent catalytic activity is stably maintained for a long period of time. Further, the final subject of the present invention is that by using the above-mentioned excellent electrode for electrolysis, the electrolysis performance can be improved even when a power source having a large output fluctuation such as renewable energy is used as a power source. It is an object of the present invention to provide an operation method capable of performing stable alkaline water electrolysis for a long period of time without deterioration.
- An oxygen-evolving anode provided with a catalyst layer containing a hybrid cobalt hydroxide nanosheet (Cons) of a composite of a metal hydroxide and an organic substance, which was proposed by the present inventors as described above, and a new anode using the anode.
- the electrolysis performance is less likely to deteriorate and the catalytic activity is maintained for a long period of time even when the power source is a power source having a large output fluctuation such as renewable energy. Be done.
- An object of the present invention is to further develop the technology developed by the present inventors and to propose a technology that can be used more effectively in industry. Specifically, when power with large output fluctuations such as renewable energy is used as the power source, the electrolytic performance is less likely to deteriorate and excellent catalytic activity is maintained for a longer period of time than when Cons is used. It is to realize the better effect of being maintained stably. Further, an object of the present invention is industrially useful because the catalyst layer of the oxygen-evolving anode capable of obtaining such an excellent effect can be formed with a more versatile material and by a simple electrolysis method. It is in developing technology.
- the present invention provides the following alkaline water electrolysis method.
- An electrolytic solution in which a catalyst containing a hybrid nickel hydroxide / iron nanosheet (NiFe-ns) of a composite of a metal hydroxide and an organic substance is dispersed is placed in an anode chamber and a cathode chamber constituting an electrolytic cell.
- An alkaline water electrolysis method characterized in that it is supplied and commonly used for electrolysis in each chamber.
- An electrolytic solution in which a catalyst containing a hybrid nickel hydroxide / iron nanosheet (NiFe-ns) of a composite of a metal hydroxide and an organic substance is dispersed is placed in an anode chamber and a cathode chamber constituting an electrolytic cell. It is supplied and used in common for electrolysis in each chamber. During operation, the NiFe-ns is electrolyzed and precipitated in the electrolytic cell to form a catalyst layer on the surface constituting the oxygen generation anode.
- An alkaline water electrolysis method characterized by recovering and improving electrolytic performance by electrolytically precipitating the NiFe-ns on the surface of a conductive substrate.
- the condition for electrolytically precipitating the NiFe-ns on the surface of the conductive substrate is that the conductive substrate is 1.2V to 1.8V vs.
- NiFe-ns dispersion As the electrolytic solution in which the NiFe-ns is dispersed, a NiFe-ns dispersion having a concentration of 10 to 100 g / L is used, and the concentration of the NiFe-ns dispersion added to the electrolytic solution is 0.1 to 1.
- the alkaline water electrolysis method according to any one of [1] to [4], which uses a liquid prepared so as to be within the range of 5 mL / L.
- the present invention provides the following anode for alkaline water electrolysis, which is useful when applied to the above alkaline water electrolysis method.
- a conductive substrate whose surface is made of nickel or a nickel-based alloy, An intermediate layer formed on the surface of the conductive substrate and made of a lithium-containing nickel oxide represented by the composition formula Li x Ni 2-x O 2 (0.02 ⁇ x ⁇ 0.5).
- An anode for alkaline water electrolysis which is characterized by producing oxygen.
- the electrolytic performance is less likely to deteriorate during the electrolytic operation, and the excellent catalytic activity is more stable for a long period of time. It becomes possible to provide an anode for alkaline water electrolysis that generates oxygen (also referred to as an anode for oxygen generation in the present specification) that is maintained. Further, according to the present invention, it is possible to stably maintain the catalytic activity of the oxygen-evolving anode for a long period of time by a simple means of supplying a common electrolytic solution to the anode chamber and the cathode chamber.
- the electrolysis performance is less likely to deteriorate, and more stable alkaline water electrolysis can be performed over a long period of time, which is industrially useful. It becomes possible to provide a method.
- the material used in the present invention that constitutes the catalyst layer of the anode for alkaline water electrolysis that can obtain the above-mentioned excellent effects is extremely versatile, and can be easily and quickly electrolyzed with a constant current. Since the catalyst layer can be formed on the surface, the technique of the present invention has excellent industrial utility and its practical value is extremely high.
- FIG. 1 It is sectional drawing which shows typically one Embodiment of the anode for oxygen evolution used in the alkaline water electrolysis method of this invention. It is a figure which shows an example of the layered NiFe-Tris-NH 2 molecular structure which has a tripod type ligand of the catalyst component used in this invention. It is a figure which shows the manufacturing method example, composition, and structural formula of the catalyst layer of a layered structure on the surface of the conductive substrate of the anode for oxygen evolution used in this invention. It is a graph which shows the current-potential change (activity change) of a sample in the potential cycle in Study Example 1. It is a graph which shows the change of the electrolytic characteristic of the study example 1 and the comparative study example 1. It is a graph which shows the change of the electrolytic characteristic of the study example 2 and the comparative study example 2.
- NiFe-LDH NiFe-layered double hydroxide
- Ni x B catalyst nanopowder for the anode and cathode, respectively.
- the cell voltage decreased only when Ni x B was added to the cathode solution.
- a dense particle film was observed on the cathode, but no film formation was observed on the anode.
- Ni x B was only confirmed to have an effect as a cathode catalyst, and had no effect on the anode.
- Non-Patent Document 4 it is reported for the first time in the above-mentioned Non-Patent Document 4 that in the electrolytic solution in which the self-repairing catalyst Cons for the anode is dispersed, the performance of the anode is improved, but there is almost no effect on the cathode electrode. Was done. However, there was still room for improvement in anode performance, and the effect was not sufficient.
- NiFe-ns a hybrid nickel hydroxide / iron nanosheet
- Y. Kuroda et al. Chem. Eur. J. 2017, 23, 5032.
- NiFe-ns a hybrid nickel hydroxide / iron nanosheet
- the present invention has been completed.
- NiFe-ns a hybrid nickel hydroxide / iron nanosheet
- an electrolytic solution in which nanosheets are dispersed is supplied to both the anode chamber and the cathode chamber constituting the electrolytic cell. It was found that it can be commonly used for electrolysis in each chamber.
- FIG. 1 is a cross-sectional view schematically showing an embodiment of an anode 10 for alkaline water electrolysis that generates oxygen, which is used in the alkaline water electrolysis method of the present invention.
- the oxygen-evolving anode of the present embodiment includes a conductive substrate 2, an intermediate layer 4 formed on the surface of the conductive substrate 2, and a catalyst formed on the surface of the intermediate layer 4. It includes a layer 6.
- the details of the oxygen evolving anode used in the alkaline water electrolysis method of the present invention will be described with reference to the drawings.
- the conductive substrate 2 is a conductor for conducting electricity for electrolysis, and is a member having a function as a carrier for supporting the intermediate layer 4 and the catalyst layer 6. At least the surface of the conductive substrate 2 (the surface on which the intermediate layer 4 is formed) is made of nickel or a nickel-based alloy. That is, the conductive substrate 2 may be entirely formed of nickel or a nickel-based alloy, or only the surface may be formed of nickel or a nickel-based alloy. Specifically, the conductive substrate 2 may have a surface of a metal material such as iron, stainless steel, aluminum, or titanium coated with nickel or a nickel-based alloy by plating or the like.
- the thickness of the conductive substrate 2 is preferably 0.05 to 5 mm. Further, the conductive substrate preferably has a shape having an opening for removing bubbles such as oxygen and hydrogen generated by electrolysis. For example, an expanded mesh or a porous expanded mesh can be used as the conductive substrate 2. When the conductive substrate has an opening, the aperture ratio of the conductive substrate is preferably 10 to 95%.
- the oxygen-evolving anode used in the alkaline water electrolysis method of the present invention can be obtained, for example, by forming an intermediate layer 4 and a catalyst layer 6 on the surface of the conductive substrate 2 described above as described below. can.
- Pretreatment process Before performing the steps of forming the intermediate layer 4 and the catalyst layer 6, it is preferable that the conductive substrate 2 is chemically etched in advance in order to remove contaminated particles such as metals and organic substances on the surface.
- the amount of consumption of the conductive substrate 2 by the chemical etching treatment is preferably about 30 g / m 2 or more and 400 g / m 2 or less.
- the surface of the conductive substrate 2 is roughened in advance.
- the roughening treatment means include blasting treatment by spraying powder, etching treatment using a substrate-soluble acid, and plasma spraying.
- the intermediate layer 4 is a layer formed on the surface of the conductive substrate 2.
- the intermediate layer 4 suppresses corrosion of the conductive substrate 2 and stably fixes the catalyst layer 6 to the conductive substrate 2.
- the intermediate layer 4 also plays a role of rapidly supplying an electric current to the catalyst layer 6.
- the intermediate layer 4 may be formed of, for example, a lithium-containing nickel oxide represented by the composition formula Li x Ni 2-x O 2 (0.02 ⁇ x ⁇ 0.5). If x in the above composition formula is less than 0.02, the conductivity becomes insufficient. On the other hand, when x exceeds 0.5, the physical strength and chemical stability decrease.
- the intermediate layer 4 formed of the lithium-containing nickel oxide represented by the above composition formula has sufficient conductivity for electrolysis and exhibits excellent physical strength and chemical stability even when used for a long period of time.
- the thickness of the intermediate layer 4 is preferably 0.01 ⁇ m or more and 100 ⁇ m or less, and more preferably 0.1 ⁇ m or more and 10 ⁇ m or less. If the thickness of the intermediate layer is less than 0.01 ⁇ m, the above-mentioned functions cannot be sufficiently obtained. On the other hand, even if the thickness of the intermediate layer exceeds 100 ⁇ m, the voltage loss due to the resistance in the intermediate layer becomes large and the above-mentioned function is not sufficiently exhibited, and there is a case where it is slightly disadvantageous in terms of manufacturing cost and the like.
- the intermediate layer 4 is formed by a so-called thermal decomposition method.
- an aqueous precursor solution of the intermediate layer is prepared and used.
- Known precursors such as lithium nitrate, lithium carbonate, lithium chloride, lithium hydroxide, and lithium carboxylate can be used to prepare an aqueous solution of a precursor containing a lithium component.
- lithium carboxylate include lithium formate and lithium acetate.
- Known precursors such as nickel nitrate, nickel carbonate, nickel chloride, and nickel carboxylate can be used for the preparation of the precursor aqueous solution containing the nickel component.
- nickel carboxylate include nickel formate and nickel acetate.
- the heat treatment temperature when forming the intermediate tank 4 by the thermal decomposition method using the precursor aqueous solution as described above can be appropriately set.
- the heat treatment temperature is preferably 450 ° C. or higher and 600 ° C. or lower. It is more preferably 450 ° C. or higher and 550 ° C. or lower.
- the decomposition temperature of lithium nitrate is about 430 ° C
- the decomposition temperature of nickel acetate is about 373 ° C.
- the heat treatment temperature is higher than 600 ° C.
- the conductive substrate 2 is likely to be oxidized, the electrode resistance is increased, and the voltage loss may be increased.
- the heat treatment time may be appropriately set in consideration of the reaction rate, productivity, oxidation resistance of the catalyst layer surface, and the like.
- the thickness of the formed intermediate layer 4 can be controlled by appropriately setting the number of times the precursor aqueous solution is applied in the above-mentioned coating step.
- the coating and drying of the precursor aqueous solution may be repeated layer by layer to form the uppermost layer, and then the whole may be heat-treated, or the precursor aqueous solution coating and heat treatment (pretreatment) may be repeated layer by layer to form the uppermost layer.
- the whole may be heat-treated after forming.
- the temperature of the pretreatment and the temperature of the entire heat treatment may be the same or different.
- the pretreatment time is preferably shorter than the total heat treatment time.
- the oxygen-evolving anode used in the alkaline water electrolysis method of the present invention preferably has a form in which a catalyst layer 6 composed of a catalyst component peculiar to the outermost surface of the conductive substrate 2 is formed.
- a hybrid nickel-iron nanosheet (NiFe-ns) of a composite of a metal hydroxide and an organic substance, which is a catalyst component used in the present invention and characterizes the present invention, can be easily produced, for example, as follows. ..
- an aqueous solution of a tripod-type ligand tris (hydroxymethyl) aminomethane (Tris-NH 2 ) and an aqueous solution of Nickel 2 and FeCl 2 are mixed and reacted at 90 ° C. for 24 hours.
- NiFe-ns dispersion liquid The concentration of NiFe-ns in the dispersion was 10 mg / mL. Hereinafter, this is referred to as "NiFe-ns dispersion liquid".
- NiFe-ns dispersion liquid The "NiFe-ns dispersion” obtained by the above production method was added to the NiFe-ns-dispersed electrolytic solution used below to prepare an electrolytic solution having an appropriate addition concentration. ..
- NiFe-ns has a layered NiFe-ns-Tris-NH two- molecular structure having a tripod-type ligand, and the Tris molecule is covalently immobilized in blue. Consists of site layers. Modification with Tris-NH 2 enhances the ability to exfoliate and disperse in layered nickel hydroxide-iron electrolyte. It is confirmed from the TEM image and the AFM image that the molecular structure of NiFe-ns obtained above is in the form of nanosheets having a thickness of about 1.3 nm and a lateral size in the range of 10 to 100 nm. bottom.
- NiFe-ns had a layered structure in which the space between the bottom surfaces was expanded.
- the Ni / Fe ratio of NiFe-ns obtained above was 1.45.
- the ratio of Ni / Fe used in the present invention may be, for example, 1/10 to 10/1.
- the size of the nanosheet is preferably a length (major axis) in the range of 10 to 100 nm. According to the studies by the present inventors, if it is more than this, the efficiency of electrolytic precipitation is lowered, and the improvement of overvoltage and the repair effect may be difficult to be exhibited, which is not preferable.
- a hybrid nickel / iron nanosheet (NiFe-ns) is used as a catalyst component which forms a catalyst layer of an anode and is contained in an electrolytic solution to be used, which is characteristic of the present invention. Therefore, a more excellent effect can be obtained as compared with the technique using the hybrid cobalt hydroxide nanosheet (Cons) previously proposed by the present inventors.
- the above-mentioned nanosheets are used to form an anode catalyst layer, and the anode is used to supply an electrolytic solution containing each of the above-mentioned different nanosheets to an anode chamber and a cathode chamber constituting an electrolytic cell.
- NiFe-ns used as a catalyst component is obtained from an extremely general-purpose material, it has an advantage that it is industrially easy to use.
- a method for forming the catalyst layer 6 containing NiFe-ns will be described.
- a 1.0 M aqueous solution of KOH was used as the electrolytic solution.
- a potential cyclic operation (-0.5 to 0.5 V vs. RHE, 200 mV / s, 200 cycles) is performed.
- a 1.0 M KOH aqueous solution containing an addition concentration of 1 mL / L was prepared from the "NiFe-ns dispersion liquid" obtained as described above, and this was used as an electrolytic solution.
- NiFe-ns In order to deposit NiFe-ns on the surface of the Ni substrate using the electrolytic solution, constant current electrolysis at 800 mA / cm 2 for 30 minutes was performed 8 times. Then, in this electrolysis operation, the dispersibility of NiFe-ns was lowered on the electrode surface by the oxidation of the hydroxide layer and the oxidative decomposition of the surface organic groups, and NiFe-ns was deposited on the electrode surface.
- the concentration of the "NiFe-ns dispersion" added to the electrolytic solution is preferably in the range of 0.1 to 5 mL / L. According to the studies by the present inventors, if the concentration is higher than this, the dispersion of NiFe-ns in the electrolytic solution becomes insufficient, and uniform precipitation may not be obtained in the electrolysis, which is not preferable. Further, if the concentration is lower than this, a sufficient amount cannot be obtained within a practical time in the precipitation by electrolysis.
- the conductive substrate was 1.2V to 1.8V vs. It is preferable to keep it within the potential range of RHE. The precipitation reaction does not proceed at 1.2 V or lower, and if it is 1.8 V or higher, oxygen evolution proceeds at the same time and precipitation is inhibited, which is not preferable.
- the cathode and the diaphragm are not particularly limited, and those used for conventional alkaline water electrolysis may be appropriately used. These will be described below.
- the cathode it is preferable to select and use a substrate made of a material that can withstand alkaline water electrolysis and a catalyst having a small cathode overvoltage.
- a nickel substrate or a nickel substrate coated with an active cathode can be used.
- Examples of the shape of the cathode substrate include a plate shape, an expanded mesh, a porous expanded mesh, and the like.
- the cathode material examples include porous nickel having a large surface area and Ni—Mo-based materials.
- nickel-based materials such as Ni-Al, Ni-Zn, and Ni-Co-Zn
- sulfide-based materials such as Ni-S
- hydrogen storage alloy-based materials such as Ti 2 Ni.
- the catalyst preferably has properties such as low hydrogen overvoltage, high short-circuit stability, and high poisoning resistance.
- metals such as platinum, palladium, ruthenium and iridium, and oxides thereof are preferable.
- any conventionally known membrane such as asbestos, a non-woven fabric, an ion exchange membrane, a polymer porous membrane, and a composite membrane of an inorganic substance and an organic polymer can be used.
- a hydrophilic inorganic material such as a calcium phosphate compound or calcium fluoride
- an organic binding material such as polysulfone, polypropylene, and polyvinylidene fluoride
- a film-forming mixture of granular inorganic hydrophilic substances such as antimony and zirconium oxides and hydroxides with organic binders such as fluorocarbon polymers, polysulfone, polypropylene, polyvinyl chloride and polyvinyl butyral.
- organic binders such as fluorocarbon polymers, polysulfone, polypropylene, polyvinyl chloride and polyvinyl butyral.
- an ion-permeable polymer having a stretched organic fiber cloth embedded therein can be used.
- a high-concentration alkaline aqueous solution can be electrolyzed by using an alkaline water electrolysis cell having an oxygen-evolving anode as a component, which is characteristic of the present invention.
- an aqueous solution of an alkali metal hydroxide such as potassium hydroxide (KOH) and sodium hydroxide (NaOH) is preferable.
- the concentration of the alkaline aqueous solution is preferably 1.5% by mass or more and 40% by mass or less.
- the concentration of the alkaline aqueous solution is 15% by mass or more and 40% by mass or less, the electric conductivity is large and the power consumption can be suppressed, which is preferable. Further, in consideration of cost, corrosiveness, viscosity, operability, etc., the concentration of the alkaline aqueous solution is preferably 20% by mass or more and 30% by mass or less.
- the catalyst layer 6 of the anode can be formed before being incorporated into the electrolytic cell.
- a nanosheet (NiFe-ns) containing the above-mentioned nanosheet (NiFe-ns) as a forming component of the catalyst layer 6 characteristic of the present invention is added to the common electrolytic solution supplied to the anode chamber and the cathode chamber constituting the electrolytic cell.
- the catalyst component is precipitated on the anode. Therefore, by using the technique of alkaline water electrolysis of the present invention, it is possible to recover the performance of the electrolytic cell whose performance has deteriorated due to operation without taking the trouble of disassembling the electrolytic cell, which is practical.
- the industrial benefits are enormous.
- NiFe-ns dispersion having a concentration of 50 g / L obtained by the same method as described above was added to the electrolytic solution used for the pretreatment at a concentration of 0.8 mL.
- a mixture was used so as to have a ratio of / L.
- electrolysis was performed at a constant current of 800 mA / cm 2 for 30 minutes.
- NiFe-ns is oxidized on the electrode surface, the dispersibility is lowered by the oxidation of the hydroxide layer of NiFe-ns and the oxidative decomposition of the surface organic groups, and NiFe-ns is deposited on the electrode surface to deposit the catalyst layer.
- This anode was designated as "Ni-NiFe-ns".
- FIG. 4 shows cyclic voltammetry obtained at 50 mV / s for 4 (a): first time, 4 (b): 2000 times, and 4 (c): 40,000 times.
- the anode (1.41V) and cathode (1.37V) peaks that can be attributed to Ni 2+ / Ni 3+ were observed in the NiFe layered double hydroxide.
- FIG. 5 shows the potential fluctuation cycle dependence of the oxygen evolution overvoltage of the anode Ni—NiFe-ns obtained in the acceleration durability test for the potential fluctuation described above.
- 5 (c) in FIG. 5 when Ni—NiFe-ns was used, the initial overvoltage was 309 mV, but the overvoltage further decreased during the durability test, and after 40,000 cycles of durability. was 276 mV.
- 4 (b) and 4 (c) in FIG. 4 after the durability test of 2000 cycles, the Ni 2+ / Ni 3+ peak in the NiFe layered double hydroxide shifts to 1.43 V, and the potential is increased. Structural changes associated with the change were suggested.
- Ni-NiFe-ns has a self-healing ability under a variable power supply and exhibits high activity.
- Example 1 As an anode substrate, a nickel expanded mesh (10 cm ⁇ 10 cm, LW ⁇ 3.7 SW ⁇ 0.9 ST ⁇ 0.8 T) obtained by immersing in 17.5 mass% hydrochloric acid near the boiling point for 6 minutes and undergoing a chemical etching treatment was used. Using. This expanded mesh was blasted (0.3 MPa) with 60 mesh alumina particles, then immersed in 20% by mass hydrochloric acid, and chemically etched near the boiling point for 6 minutes. An aqueous solution containing a component serving as a precursor of a lithium-containing nickel oxide was applied to the surface of the anode substrate after the chemical etching treatment with a brush, and then dried at 80 ° C. for 15 minutes.
- NiFe-ns dispersion was used, and the dispersion was added to the electrolyte so that the addition concentration was 1 mL / L. Prepared. Then, using the electrolytic solution, from the Ni anode (anode for oxygen generation) in which a catalyst layer made of NiFe-ns was formed on the surface of the above-mentioned intermediate, the diaphragm (Zirphon manufactured by AGFA), Ru and Pr oxide. A small zero-gap type electrolytic cell using a neutral diaphragm was prepared using an active cathode having a catalyst layer formed therein. The electrode area was 19 cm 2 .
- a 25% by mass KOH aqueous solution to which the same NiFe-ns dispersion liquid as above was added at a ratio of 1 mL / L was used as the electrolytic solution. Then, the electrolytic solution was supplied to each of the anode chamber and the cathode chamber constituting the electrolytic cell, and electrolyzed at a current density of 6 kA / m 2 for 6 hours each. Next, the anode and cathode were short-circuited (0 kA / m 2 ) and stopped for 15 hours. A shutdown test was conducted in which the above operation from electrolysis to stop was one cycle. As a result, it was confirmed that the voltage was kept stable in 20 shutdown tests.
- Example 1 As the electrolytic solution supplied to each of the anode chamber and the cathode chamber constituting the electrolytic cell, the same one as used in Example 1 was used except that NiFe-ns was not added. Then, in the same electrolytic cell used in Example 1, the same shutdown test as in Example 1 was performed. As a result, the cell voltage gradually increased as the number of stops increased. From this, the superiority in the configuration using the electrolytic solution to which NiFe-ns was added in Example 1 was confirmed.
- the oxygen-evolving anode that characterizes the present invention has excellent durability, and is suitable as an anode for alkaline water electrolysis that constitutes an electrolysis facility or the like that uses electric power with large output fluctuations such as renewable energy as a power source.
- the hybrid nickel hydroxide / iron nanosheet (NiFe-ns) which is a common anode catalyst component, is dispersed in the anode chamber and the cathode chamber constituting the electrolytic cell according to the present invention.
Abstract
Description
[1]金属水酸化物と有機物との複合体のハイブリッド水酸化ニッケル・鉄ナノシート(NiFe-ns)を含んでなる触媒を分散させた電解液を、電解セルを構成するアノード室とカソード室に供給し、各室での電解に共通して用いることを特徴とするアルカリ水電解方法。
[3]前記NiFe-nsが、10~100nmの大きさの層状の分子構造を有する[1]又は[2]に記載のアルカリ水電解方法。
[4]前記NiFe-nsを導電性基体の表面に電解析出させる条件が、前記導電性基体を、1.2V~1.8V vs.RHEの電位範囲に保持することである[2]又は[3]に記載のアルカリ水電解方法。
[5]前記NiFe-nsを分散させた電解液として、濃度が10~100g/LであるNiFe-ns分散液を用い、該NiFe-ns分散液の電解液への添加濃度が0.1~5mL/Lの範囲内になるように調製したものを用いる[1]~[4]のいずれかに記載のアルカリ水電解方法。
[6]表面がニッケル又はニッケル基合金からなる導電性基体と、
前記導電性基体の表面上に形成された、組成式LixNi2-xO2(0.02≦x≦0.5)で表されるリチウム含有ニッケル酸化物からなる中間層と、
前記中間層の表面上に形成された、金属水酸化物と有機物との複合体のハイブリッド水酸化ニッケル・鉄ナノシート(NiFe-ns)を含んでなる触媒層と、
を備えてなる酸素発生を行うことを特徴とするアルカリ水電解用アノード。
図1は、本発明のアルカリ水電解方法で用いる、酸素発生を行うアルカリ水電解用アノード10の一実施形態を模式的に示す断面図である。図1に示すように、本実施形態の酸素発生用アノードは、導電性基体2と、導電性基体2の表面上に形成された中間層4と、中間層4の表面上に形成された触媒層6とを備える。以下、本発明のアルカリ水電解方法で用いる酸素発生用アノードの詳細につき、図面を参照しつつ説明する。
導電性基体2は、電気分解のための電気を通すための導電体であり、中間層4及び触媒層6を担持する担体としての機能を有する部材である。導電性基体2の少なくとも表面(中間層4が形成される面)は、ニッケル又はニッケル基合金で形成されている。すなわち、導電性基体2は、全体がニッケル又はニッケル基合金で形成されていてもよく、表面のみが、ニッケル又はニッケル基合金で形成されていてもよい。具体的に、導電性基体2は、例えば、鉄、ステンレス、アルミニウム、チタン等の金属材料の表面に、めっき等によりニッケル又はニッケル基合金のコーティングが施されたものであってもよい。
(前処理工程)
中間層4、触媒層6の形成工程を行う前に、表面の金属や有機物などの汚染粒子を除去するために、導電性基体2を予め化学エッチング処理することが好ましい。化学エッチング処理による導電性基体2の消耗量は、30g/m2以上、400g/m2以下程度とすることが好ましい。また、中間層との密着力を高めるために、導電性基体2の表面を予め粗面化処理することが好ましい。粗面化処理の手段としては、粉末を吹き付けるブラスト処理や、基体可溶性の酸を用いたエッチング処理や、プラズマ溶射などが挙げられる。
中間層4は、導電性基体2の表面上に形成される層である。中間層4は、導電性基体2の腐食等を抑制するとともに、触媒層6を導電性基体2に安定的に固着させる。また、中間層4は、触媒層6に電流を速やかに供給する役割も果たす。中間層4は、例えば、組成式LixNi2-xO2(0.02≦x≦0.5)で表されるリチウム含有ニッケル酸化物で形成するとよい。上記組成式中のxが0.02未満であると、導電性が不十分になる。一方、xが0.5を超えると物理的強度及び化学的安定性が低下する。上記組成式で表されるリチウム含有ニッケル酸化物で形成された中間層4は、電解に十分な導電性を有するとともに、長期間使用した場合でも優れた物理的強度及び化学的安定性を示す。
塗布工程では、リチウムイオン及びニッケルイオンを含有する前駆体水溶液を導電性基体2の表面に塗布する。中間層4は、いわゆる熱分解法によって形成される。このように、熱分解法により中間層を形成するに際しては、まず、中間層の前駆体水溶液を調製し、これを用いる。リチウム成分を含む前駆体水溶液の調製には、硝酸リチウム、炭酸リチウム、塩化リチウム、水酸化リチウム、カルボン酸リチウムなど公知の前駆体を使用することができる。カルボン酸リチウムとしては、例えば、ギ酸リチウムや酢酸リチウムなどが挙げられる。ニッケル成分を含む前駆体水溶液の調製には、硝酸ニッケル、炭酸ニッケル、塩化ニッケル、カルボン酸ニッケルなど公知の前駆体を使用することができる。カルボン酸ニッケルとしては、例えば、ギ酸ニッケルや酢酸ニッケルなどが挙げられる。特に、前駆体としてカルボン酸リチウム及びカルボン酸ニッケルの少なくとも一方を用いることにより、後述するように、低温で焼成した場合であっても緻密な中間層を形成することができるので特に好ましい。
本発明のアルカリ水電解方法で用いる酸素発生用アノードは、導電性基体2の最表面に特有の触媒成分からなる触媒層6を形成した形態とすることが好ましい。このように構成し、アルカリ水電解に適用することで、本発明のより優れた効果を発現できる。以下、本発明において効果的で有用な触媒層6の構成について説明する。
本発明で使用し、本発明を特徴づける触媒成分である、金属水酸化物と有機物との複合体のハイブリッドニッケル・鉄ナノシート(NiFe-ns)は、例えば、下記のようにして簡便に製造できる。NiFe-nsの合成のため、三脚型配位子tris(hydroxymethyl)aminomethane(Tris-NH2)水溶液と、NiCl2及びFeCl2水溶液とを混合し、90℃で24時間反応させる。そして、反応生成物を、ろ過、純水洗浄によりゲルとして分離し、これを純水中で超音波処理することで、NiFe-ns分散液を得る。該分散液中のNiFe-nsの濃度は、10mg/mLとした。以下、これを「NiFe-ns分散液」と呼ぶ。以下で用いたNiFe-nsを分散させた電解液には、上記の製造方法で得た「NiFe-ns分散液」を添加して、適宜な添加濃度になるように調製した電解液を用いた。
以下、NiFe-nsを含んでなる触媒層6の形成方法について述べる。電解液として1.0MのKOH水溶液を用いた。触媒層を形成する導電性基体2の表面を清浄化するために、電解液中にて電位操作を行うことが好ましい。例えば、電位サイクリック操作(-0.5~0.5V vs.RHE、200mV/s、200サイクル)を行う。その後、先に述べたようにして得た「NiFe-ns分散液」を、添加濃度1mL/L含む1.0MのKOH水溶液を作製し、これを電解液とした。該電解液を用い、NiFe-nsをNi基材表面に析出させるため、800mA/cm2で30分の定電流電解を8回行なった。そして、この電解操作で、電極表面で、NiFe-nsを水酸化物層の酸化や表面有機基の酸化分解により分散性を低下させ、電極表面にNiFe-nsを堆積させた。
カソードとしては、アルカリ水電解に耐え得る材料製の基体と、陰極過電圧が小さい触媒とを選択して用いることが好ましい。カソード基体としては、ニッケル基体、又はニッケル基体に活性陰極を被覆形成したものを用いることができる。カソード基体の形状としては、板状の他、エクスパンドメッシュや、多孔質エクスパンドメッシュなどを挙げることができる。
電解用隔膜としては、アスベスト、不織布、イオン交換膜、高分子多孔膜、及び、無機物質と有機高分子の複合膜など、従来公知のものをいずれも用いることができる。具体的には、リン酸カルシウム化合物やフッ化カルシウム等の親水性無機材料と、ポリスルホン、ポリプロピレン、及びフッ化ポリビニリデン等の有機結合材料との混合物に、有機繊維布を内在させたイオン透過性隔膜を用いることができる。また、アンチモンやジルコニウムの酸化物及び水酸化物等の粒状の無機性親水性物質と、フルオロカーボン重合体、ポリスルホン、ポリプロピレン、ポリ塩化ビニル及びポリビニルブチラール等の、有機性結合剤とのフィルム形成性混合物に、伸張された有機性繊維布を内在させたイオン透過性隔膜などを用いることができる。
前記アノードの触媒層6は、電解セルに組み込む前に形成することができる。本発明のアルカリ水電解方法では、電解セルを構成するアノード室とカソード室に供給する共通の電解液に、前記した本発明を特徴づける触媒層6の形成成分としたナノシート(NiFe-ns)を懸濁させ、その状態で電解を開始することで、触媒成分をアノードに析出させている。このため、本発明のアルカリ水電解の技術を用いれば、運転によって性能の低下した電解セルの性能回復が、電解セルの解体の手間をかけることなく行うことができるので、実用的であり、その工業上のメリットは極めて大きい。
まず、本発明を特徴づける触媒成分であるNiFe-nsを、電解液に分散させて電解した場合における電解表面への堆積の状態と、その効果についての検討を行った。比較のために、触媒成分にCo-nsを用いた場合についても、同様の試験を行った。
電解操作は、フッ素樹脂であるPFA製の三電極セルを用いて行った。作用極に沸騰塩酸で6分間エッチングしたNiワイヤー、参照極に可逆水素電極(RHE)、対極にNiコイル、電解液に1.0MのKOH水溶液250mLをそれぞれ用いて、30±1℃で実施した。まず、上記電解液にNiFe-ns分散液を加えずに、前処理として、サイクリックボルタンメトリー(0.5~1.5V vs.RHE、200mV/s、200サイクル)を行った。本例では、次に電解液として、先に説明したと同様の方法で得た濃度が50g/LのNiFe-ns分散液を、上記前処理に用いた電解液に、添加濃度が0.8mL/Lの割合となるように混合したものを用いた。そして、800mA/cm2、30分間の定電流の電解を行った。これにより、電極表面でNiFe-nsが酸化され、NiFe-nsの水酸化物層の酸化や表面有機基の酸化分解により分散性を低下させ、電極表面にNiFe-nsを堆積させて、触媒層を形成した。このアノードを「Ni-NiFe-ns」とした。
検討例1で行ったと同様の方法で、Ni表面に、NiFe-nsの代わりに、Co-nsからなる触媒層を形成させたアノードを得た。そして、NiFe-nsを添加しない電解液を用い、検討例1と同様にして、加速劣化試験を実施したときの酸素発生過電圧の電位変動サイクル依存性を調べた。この結果、図5中の5(b)に示したように、初期の過電圧は350mV程度で、その後360mVまで増加し、5(c)に示したアノードNi-NiFe-nsを用いた検討例1で得られた顕著な減少傾向はなかった。この結果は、検討例1で試験したアノードNi-NiFe-nsを用いた場合の方が、耐久性に優れることを示している。
陽極基体として、17.5質量%塩酸中に、沸点近傍で6分間浸漬して化学エッチング処理を行ったニッケルエクスパンドメッシュ(10cm×10cm、LW×3.7SW×0.9ST×0.8T)を用いた。このエクスパンドメッシュを、60メッシュのアルミナ粒子でブラスト処理(0.3MPa)した後、20質量%塩酸に浸漬し、沸点近傍で、6分間化学エッチング処理した。化学エッチング処理後の陽極基体の表面に、リチウム含有ニッケル酸化物の前駆体となる成分を含んだ水溶液を刷毛で塗布した後、80℃で15分間乾燥させた。次いで、大気雰囲気下、600℃で15分間熱処理した。上記した水溶液の塗布から熱処理までの処理を20回繰り返して、陽極基体の表面上に中間層(組成:Li0.5Ni1.5O2)が形成された中間体を得た。
電解セルを構成するアノード室とカソード室の各室に供給する電解液として、NiFe-nsを添加しないこと以外は実施例1で用いたと同様のものを用いた。そして、実施例1で用いたと同様の電解セルで、実施例1で行ったと同様のシャットダウン試験を行った。その結果、停止回数の増加とともにセル電圧も徐々に増加した。このことから、実施例1におけるNiFe-nsを添加した電解液を用いた構成における優位性が確認された。
対極として、RuとPr酸化物からなる触媒層を形成した活性カソードを用いたこと以外は検討例1と同様にして、電位変動の加速試験を実施し、カソードの電位変化を測定した。図6中に、6(a)で示した通り、過電圧は、初期から60mVから80mV程度を維持した。
NiFe-nsを添加しない状態の電解液を用い、それ以外は検討例2と同様にして、電位変動加速試験を行った。図6中に、実線の6(b)で示した通り、過電圧は、初期から60mVから80mV程度を維持した。図6中に、破線の6(a)で示した検討例2の場合との比較から、NiFe-nsの添加によるカソードへの影響はないことが確認された。
4:中間層
6:触媒層
10:アルカリ水電解用アノード
Claims (6)
- 金属水酸化物と有機物との複合体のハイブリッド水酸化ニッケル・鉄ナノシート(NiFe-ns)を含んでなる触媒を分散させた電解液を、電解セルを構成するアノード室とカソード室に供給し、各室での電解に共通して用いることを特徴とするアルカリ水電解方法。
- 金属水酸化物と有機物との複合体のハイブリッド水酸化ニッケル・鉄ナノシート(NiFe-ns)を含んでなる触媒を分散させた電解液を、電解セルを構成するアノード室とカソード室に供給し、各室での電解に共通して用い、運転中に、前記NiFe-nsの電解析出を前記電解セル内にて行い、酸素発生用アノードを構成する表面に触媒層を形成してなる導電性基体の表面に、前記NiFe-nsを電解析出させることで、電解性能を回復、向上させることを特徴とするアルカリ水電解方法。
- 前記NiFe-nsが、10~100nmの大きさの層状の分子構造を有する請求項1又は2に記載のアルカリ水電解方法。
- 前記NiFe-nsを導電性基体の表面に電解析出させる条件が、前記導電性基体を、1.2V~1.8V vs.RHEの電位範囲に保持することである請求項2又は3に記載のアルカリ水電解方法。
- 前記NiFe-nsを分散させた電解液として、濃度が10~100g/LであるNiFe-ns分散液を用い、該NiFe-ns分散液の電解液への添加濃度が0.1~5mL/Lの範囲内になるように調製したものを用いる請求項1~4のいずれか1項に記載のアルカリ水電解方法。
- 表面がニッケル又はニッケル基合金からなる導電性基体と、
前記導電性基体の表面上に形成された、組成式LixNi2-xO2(0.02≦x≦0.5)で表されるリチウム含有ニッケル酸化物からなる中間層と、
前記中間層の表面上に形成された、金属水酸化物と有機物との複合体のハイブリッド水酸化ニッケル・鉄ナノシート(NiFe-ns)を含んでなる触媒層と、
を備えてなる酸素発生を行うことを特徴とするアルカリ水電解用アノード。
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