Electrolyzers and water electrolysis systems

Abstract

To provide an electrolytic tank that can keep an electrolytic voltage low even when an electrolyte with neutral pH range is used.SOLUTION: The present invention is characterized by a porous diaphragm 2 contacting a cathode 1 and an anode 2 while using a buffer electrolyte, in which a pH is 1.5-12.6 when not electrolyzed and the electrolyte concentration is 2 mol / kg or more, as an electrolyte.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an electrolytic cell and a water electrolysis system that operate with high efficiency in a neutral pH range. [Background technology]

[0002] Toward the realization of a sustainable society, the effective use of renewable energy has become an urgent social imperative. However, renewable energy is unevenly distributed by region and fluctuates greatly over time, seasons, and weather, making it difficult to ensure a stable supply, which hinders its large-scale use. From this perspective, a method has been proposed for leveling out energy supplies by producing hydrogen through electrolysis of water using electricity derived from renewable energy sources, storing and transporting that hydrogen, and then using hydrogen power generation equipment or fuel cells to extract energy when and where it is needed.

[0003] The main industrial methods of water electrolysis are alkaline water electrolysis, which uses a concentrated aqueous solution of sodium hydroxide or potassium hydroxide, and proton exchange membrane (PEM) water electrolysis, which uses a proton exchange membrane. These technologies use strongly basic or strongly acidic electrolytes, so the materials used in the electrolysis equipment must have high corrosion resistance.

[0004] For this reason, if water electrolysis could be performed in a neutral range from weakly acidic to weakly basic, it would be possible to construct a water electrolysis device using general-purpose materials and extend the life of the electrolysis device. Therefore, the development of water electrolysis in a neutral pH range is being considered. However, water electrolysis in the neutral pH range tends to be less energy efficient than that under strongly basic or strongly acidic conditions, i.e., the electrolysis voltage tends to be higher.

[0005] For this reason, technologies to improve the energy efficiency of water electrolysis in the neutral pH range have been investigated. For example, Patent Document 1 and Non-Patent Document 1 disclose a technology for performing water electrolysis at a relatively high temperature of 60 to 120°C using a high-concentration buffer solution. The use of a high-concentration buffer solution improves ionic conductivity and reduces the electrical resistance of the electrolytic cell. Furthermore, water electrolysis at a high temperature promotes the cathodic and anodic reactions, thereby reducing the activation overvoltage.

[0006] Furthermore, Patent Document 2 discloses a technology for alkaline water electrolysis in which the electrical resistance of the electrolytic cell is suppressed and the electrolysis voltage is kept low by employing a structure in which a diaphragm is in contact with the cathode and the anode. Patent Document 2 describes that in order to suppress the electrical resistance resulting from the electrolytic solution, it is preferable that there is substantially no gap between the diaphragm and the electrodes, that the porosity of the diaphragm is high, and that the thickness of the diaphragm is thin, while in order to prevent mixing of the gas generated at the cathode and the gas generated at the anode, the porosity of the diaphragm needs to be low and that the thickness of the diaphragm needs to be at least a certain level. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2021-116468 [Patent Document 2] Patent Publication No. 2019-65349 [Non-Patent Document 1] T.Naito,et al.,ChemSusChem 2020,13,5921-5933 Summary of the Invention [Problem to be solved by the invention]

[0008] However, even when water electrolysis is performed using the techniques of Patent Documents 1 and 2 and Non-Patent Document 1, the energy efficiency of water electrolysis in the neutral pH range is still insufficient, and further technological improvements are desired.

[0009] Therefore, an object of the present invention is to provide an electrolytic cell that can suppress electrolysis voltage even when an electrolyte solution in the neutral pH range is used, and a water electrolysis system using the same. [Means for solving the problem]

[0010] As a result of investigations conducted by the present inventors to solve the above problems, they found that by using a buffer solution in the neutral pH range (a pH of 1.5 to 12.6 when not electrolyzed) and having an electrolyte concentration of 2 mol / kg or more as the electrolyte solution and employing an electrolytic cell in which a porous diaphragm is in contact with the cathode and the anode, it is possible to reduce the electrolysis voltage in the neutral pH range while preventing mixing of the gas evolved at the cathode and the gas evolved at the anode, and thus completed the present invention.

[0011] The present invention has been made based on the above findings, and the gist of the present invention is as follows. 1. An electrolytic cell characterized in that a buffer solution having a pH of 1.5 to 12.6 during non-electrolysis and an electrolyte concentration of 2 mol / kg or more is used as an electrolyte, and a porous diaphragm is in contact with the cathode and the anode. 2. The electrolytic cell according to 1 above, wherein the porous diaphragm has a porosity of 40 to 98% and a thickness of 20 to 450 μm. 3. The electrolytic cell according to 1 or 2 above, wherein the porous diaphragm is made of glass fiber. 4. The electrolytic cell according to 3 above, wherein the glass fiber is a hydrophilically treated glass fiber. 5. The electrolytic cell according to any one of 1 to 4 above, wherein the buffer solution is an electrolyte solution containing at least one cation species selected from the group consisting of alkali metal cations and alkaline earth metal cations, and at least one anion species selected from the group consisting of phosphates, borates, and carbonates. 6. A water electrolysis system, characterized in that the electrolytic cell according to any one of 1 to 5 above is operated at 60 to 120°C to perform water electrolysis. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide an electrolytic cell that can suppress electrolysis voltage even when an electrolyte solution in the neutral pH range is used, and a water electrolysis system using the same. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram showing a schematic cross section of an example of an electrolytic cell of the present invention. [Figure 2] 1 is a graph showing the relationship between the electrolyte concentration and the current density (indicators of the amount of hydrogen crossover and the amount of oxygen crossover) of the hydrogen oxidation reaction and the oxygen reduction reaction in Examples 1 and 2 and Comparative Examples 1 and 2. [Figure 3] 1 is a graph showing the change in voltage over time when water electrolysis was carried out at a constant current for 24 hours in Comparative Example 3, Comparative Example 4, and Example 3. [Figure 4] 10 is a graph showing a current-voltage curve in water electrolysis measurement of an electrolytic cell and the results of overpotential analysis in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment for carrying out the composite material processing method of the present invention (hereinafter referred to as "the present embodiment") will be described in detail. Note that the present embodiment is an example for explaining the present invention, and the present invention is not limited to only this embodiment. In other words, the present invention can be modified in various ways without departing from the gist of the present invention.

[0015] FIG. 1 is a schematic cross-sectional view of the electrolytic cell of this embodiment.

[0016] <Electrolytic cell> As shown in FIG. 1 , the electrolytic cell of this embodiment includes a power supply 6, a cathode 1 and an anode 3 connected to the power supply, a porous diaphragm 2 provided between the cathode 1 and the anode 3, a cathode chamber 4 and an anode chamber 5 separated by the porous diaphragm 2, and an electrolytic solution introduced into the cathode chamber 4 and the anode chamber 5.

[0017] (electrolyte) The electrolytic solution of this embodiment is a buffer solution whose pH is in the neutral range (1.5 to 12.6) during non-electrolysis and whose electrolyte concentration is 2 mol / kg or more.

[0018] In water electrolysis, the substrate protons (H + ) or hydroxide ion (OH - ) is supplied to the electrode and the reaction proceeds, but if the substrate concentration is insufficient, the current density is limited by the rate of substrate supply. In this case, the maximum current density that can be operated is called the "diffusion-limited current density." In industrial water electrolysis, from the viewpoint of productivity, it is usually set at 100 mA / cm 2 Since the device is operated at a current density of 100 mA / cm or more, in this embodiment, the diffusion limit current density is 100 mA / cm 2 If it is less than H + Concentration or OH - We defined electrolytes with a pH in the range of 1.5 to 12.6 when not electrolyzed as electrolytes with an insufficient concentration, i.e., in the neutral pH range. For the calculation method of the values, see T. Shinagawa and K. Takanabe, Phys. Chem. Chem. Phys. 2015, 17, 15111-15114.

[0019] As described above, the pH of the electrolytic solution of this embodiment during non-electrolysis is in the range of 1.5 to 12.6, but from the viewpoint of suppressing corrosion of various components of the electrolytic cell, the pH during non-electrolysis is preferably 4 to 11, and more preferably 5 to 10. The pH of the electrolytic solution can be adjusted by appropriately selecting the types and concentrations of the cation species and anion species described below.

[0020] In this embodiment, a buffer solution is used as the electrolyte solution. This makes it possible to maintain a constant pH during electrolysis. The buffer solution is not particularly limited as long as it has a pH in the range of 1.5 to 12.6 during non-electrolysis and has a buffering effect. For example, the buffer solution is preferably an aqueous solution containing at least one cation species selected from the group consisting of alkali metal cations and alkaline earth metal cations, and at least one anion species selected from the group consisting of phosphate anions, borate anions, and carbonate anions.

[0021] The cationic species preferably includes at least one selected from the group consisting of cations of alkali metals (i.e., cations of lithium (Li), sodium (Na), potassium (K), cesium (Cs), and rubidium (Rb)) and cations of alkaline earth metals (i.e., cations of beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba)), and the cations of lithium, sodium, potassium, cesium, and rubidium (i.e., Li + , Na + , K. + , Cs + and Rb + Among these, sodium and potassium cations are more preferred because their saturated solubility increases significantly with increasing temperature, especially in a buffer solution with phosphate.

[0022] The anion species is phosphate anion (H2PO4 - , HPO4 2- , PO4 3- ), the anion of boric acid (B(OH) 4- , B4O7 2- ) and the anion of carbonate (HCO3 - , CO3 2- It is preferable that the compound contains at least one selected from the group consisting of:

[0023] Here, the method for preparing the buffer solution is not particularly limited, but a method of mixing an aqueous solution of phosphoric acid, boric acid, and / or carbonic acid with an aqueous solution of an alkali metal hydroxide (LiOH, NaOH, KOH, CsOH, RbOH), or a method of using an aqueous solution of one or more of phosphates, borates, and / or carbonates (e.g., KH2PO4, K2HPO4, LiH2PO4, Li2HPO4, NaH2PO4, Na2HPO4, Na2B4O7, K2B4O7, LiHCO3, NaHCO3, KHCO3, CsHCO3, etc.) can be used.

[0024] In addition to the salts composed of the cationic and anionic species, the buffer solution may contain other components that do not adversely affect the method of the present invention, such as sulfate, hydrochloride, and perchlorate.

[0025] The electrolyte concentration in the buffer solution is 2 mol / kg or more, preferably 2.5 mol / kg or more, and more preferably 3 mol / kg or more. By setting the electrolyte concentration in the buffer solution to 2 mol / kg or more, the conductivity of the electrolyte is improved, and the boiling point of water is raised, enabling operation at high temperatures. In addition, it is possible to suppress the migration of hydrogen generated in the cathode chamber to the anode chamber (hydrogen crossover) and the migration of oxygen generated in the anode chamber to the cathode chamber (oxygen crossover). While the reason why hydrogen crossover and oxygen crossover can be suppressed by using a high-concentration electrolyte is not entirely clear, it is presumed that the increase in electrolyte concentration reduces the solubility of hydrogen and oxygen in the electrolyte, and the increase in the viscosity of the electrolyte reduces the diffusion rate of hydrogen and oxygen in the electrolyte, thereby suppressing hydrogen and oxygen crossover.

[0026] In this embodiment, in order to uniquely determine the electrolyte concentration, protons (H + ) and hydroxide ions (OH - The electrolyte concentration is defined as the smaller sum of the concentrations of anions other than . For example, when mixing 1 kg of 2 mol / kg sodium dihydrogen phosphate (NaH2PO4) aqueous solution and 1 kg of 2 mol / kg disodium hydrogen phosphate (Na2HPO4) aqueous solution to prepare a buffer solution, + is 3 mol / kg, so the cation concentration is 3 mol / kg and the anion concentration is H2PO4 - is 1 mol / kg, HPO4 2- is 1 mol / kg, so the anion concentration is 2 mol / kg, and the electrolyte concentration is 2 mol / kg. As another example, when mixing 1 kg of 4 mol / kg phosphoric acid (H3PO4) aqueous solution and 1 kg of 6 mol / kg potassium hydroxide (KOH) aqueous solution to make a buffer solution, K + The cation concentration is 3 mol / kg. At this time, the pH of the buffer solution is about 7.2, and almost all of the phosphate has ionized to H2PO4 - and HPO4 2- Therefore, the anion concentration is 2 mol / kg, and the electrolyte concentration is 2 mol / kg.

[0027] In the electrolytic cell of this embodiment, as shown in FIG. 1, a porous diaphragm 2 is in contact with a cathode 1 and an anode 3. Because the porous diaphragm 2 is in contact with the cathode 1 and the anode 3, the distance between the electrodes is reduced, resulting in a shorter ion conduction path for the electrolyte. In addition, the presence of the porous diaphragm suppresses the accumulation of gas bubbles generated between the electrodes, thereby reducing the electrical resistance caused by the electrolyte and enabling a reduction in the electrolysis voltage.

[0028] The means for bringing the porous diaphragm 2 into contact with the cathode 1 and the anode 3 is not particularly limited, and examples thereof include a method in which the porous diaphragm is pressed between a smooth cathode and a smooth anode so as to sandwich the porous diaphragm.

[0029] When performing electrolysis, as shown in FIG. 1, the cathode 1 and the anode 3 must each be electrically connected to a power source 6, and the cathode 1 and the anode 3 must be electrically insulated from each other. In the electrolytic cell of this embodiment, the cathode 1 and the anode 3 are in contact with the porous diaphragm 2, and therefore the porous diaphragm 2 necessarily has insulating properties.

[0030] (Porous diaphragm) In this embodiment, as described above, the porous diaphragm 2 is provided in contact with the cathode 1 and the anode 3. Here, the porous diaphragm 2 is a sheet-like porous membrane having insulating properties. The porous diaphragm 2 does not necessarily need to be composed entirely of an insulator, and may partially contain a conductor or semiconductor as long as it can insulate the cathode 1 from the anode 3. The porous diaphragm 2 may be, for example, an inorganic porous membrane, a polymer porous membrane, a woven fabric, a nonwoven fabric, or the like.

[0031] Examples of the inorganic porous membrane include glass fiber filter paper, sintered glass filter, and asbestos diaphragm. Examples of the polymer porous membrane include those obtained by making porous membranes from polymer materials such as polysulfone, polyethersulfone, polyvinylidene fluoride, polycarbonate, polytetrafluoroethylene, polyethylene, polypropylene, polyphenylene sulfide, and polyimide by methods such as microphase separation, extraction, stretching, and wet gel stretching. Examples of the woven fabric include those obtained by weaving or knitting polymer fibers such as polyamide fiber, polyimide fiber, polyester fiber, and cotton. Examples of nonwoven fabrics include those made by forming films from fibers such as polyamide fibers, polyester fibers, polypropylene fibers, cotton, and cellulose using dry methods such as carding and air-laid methods, or wet methods such as papermaking, and those made by forming films from polymer materials such as polysulfone, polyethersulfone, polyvinylidene fluoride, polycarbonate, polytetrafluoroethylene, polyethylene, polypropylene, polyphenylene sulfide, and polyimide using spunbonding, melt-blowing, electrospinning, etc. From the viewpoints of mechanical strength and chemical durability, glass fiber filter paper can be preferably used.

[0032] The pores in the porous membrane must be filled with an electrolyte (buffer solution) from the viewpoint of improving ion conduction and suppressing permeation of generated oxygen gas and hydrogen gas. Therefore, the porous diaphragm is preferably hydrophilic. In this case, a porous diaphragm made of a hydrophilic material may be used, or a porous diaphragm made of a hydrophobic material may be used and hydrophilicity may be imparted to the porous diaphragm by chemically treating the surface of the porous diaphragm or by supporting a hydrophilic substance on the surface of the porous diaphragm. For example, in the case of glass fiber filter paper, a method of hydrophilizing the surface by immersing the filter paper in a mixture of concentrated sulfuric acid and an aqueous hydrogen peroxide solution (hydrophilization treatment) can be preferably used.

[0033] The porosity of the porous diaphragm can be measured by a common method such as the Archimedes method.

[0034] Here, the porosity of the porous diaphragm is preferably 40 to 98%, more preferably 50 to 97%, even more preferably 60 to 96%, and particularly preferably 70 to 95%. A higher porosity reduces the electrical resistance due to the electrolyte solution, but increases hydrogen and oxygen crossover. Conversely, a lower porosity increases the electrical resistance due to the electrolyte solution, but reduces hydrogen and oxygen crossover. Furthermore, if the porosity is too high, the mechanical strength of the porous diaphragm decreases. By ensuring the porosity of the porous diaphragm within the above-mentioned range, it is possible to balance the electrical resistance due to the electrolyte solution with hydrogen and oxygen crossover while ensuring the mechanical strength of the porous diaphragm.

[0035] The thickness of the porous diaphragm is preferably 20 to 450 μm, more preferably 40 to 400 μm, even more preferably 60 to 350 μm, and particularly preferably 80 to 300 μm. As the thickness of the porous diaphragm becomes thinner, the electrical resistance caused by the electrolyte solution can be reduced, but hydrogen and oxygen crossover increases. Conversely, as the thickness of the porous diaphragm becomes thicker, the electrical resistance caused by the electrolyte solution increases, but hydrogen and oxygen crossover can be reduced. Furthermore, if the thickness of the porous diaphragm is too thin, the mechanical strength of the porous diaphragm decreases. By ensuring the mechanical strength of the porous diaphragm by having the thickness of the porous diaphragm within the above-mentioned range, it is possible to achieve a balance between the electrical resistance caused by the electrolyte solution and hydrogen and oxygen crossover.

[0036] (electrode) Since the electrodes (cathode and anode) of this embodiment are in contact with the porous diaphragm, it is preferable that ions are conducted on the porous diaphragm side of the electrodes and gas is released from the side opposite the porous diaphragm of the electrodes. Therefore, a porous body made of a conductive metal or carbon material is preferable. Examples of porous metal bodies include plain woven mesh, punched metal, expanded metal, and metal foam. Examples of porous carbon material bodies include woven fabrics made of woven carbon fibers, nonwoven fabrics made of carbon fibers bound with a binder, and carbon foam.

[0037] The above-mentioned porous metal body or porous carbon body may be used as an electrode as it is, or a porous metal body or porous carbon body may be used as an electrode substrate with a highly reactive catalyst layer on the surface of the electrode substrate. From the viewpoint of reducing the electrolysis voltage, it is preferable to use a highly reactive catalyst layer on the surface of the electrode substrate as an electrode.

[0038] The material of the electrode substrate is not particularly limited, but mild steel, stainless steel, nickel, nickel alloy, titanium, or titanium alloy is preferably used in view of chemical and electrochemical durability. Carbon materials are susceptible to oxidation degradation and are therefore not suitable for the substrate of the anode, but are preferably used for the substrate of the cathode.

[0039] The catalytic layer of the cathode preferably has a high hydrogen generating capacity, and nickel, cobalt, iron, platinum group elements (e.g., ruthenium, rhodium, palladium, osmium, iridium, etc.), etc., can be used. To achieve the desired activity and durability, the catalytic layer can be formed as a simple metal, a compound such as an oxide, a composite metal oxide or alloy composed of multiple metal elements, or a mixture thereof. Specific examples include Raney nickel, Raney alloys composed of a combination of multiple materials such as nickel and aluminum or nickel and tin, porous coatings prepared by plasma spraying using nickel compounds or cobalt compounds as raw materials, alloys or composite compounds of nickel and an element selected from cobalt, iron, molybdenum, silver, copper, etc., metals or oxides of platinum group elements such as platinum and ruthenium that have high hydrogen generating capacity, and mixtures of metals or oxides of platinum group elements with compounds of other platinum group elements such as iridium and palladium, or compounds of rare earth metals such as lanthanum and cerium, and carbon materials such as graphene. To achieve high catalytic activity and durability, the above materials may be laminated in multiple layers or may be mixed in multiple layers in the catalyst layer. Organic substances such as polymeric materials may also be included to improve durability and contact with the substrate.

[0040] The catalytic layer of the anode preferably has high oxygen generating capacity, and nickel, cobalt, iron, manganese, platinum group elements, titanium, tin, molybdenum, tantalum, niobium, vanadium, etc. can be used. These can be used to form the catalytic layer as simple metals, compounds such as oxides, composite oxides or alloys of multiple metal elements, or mixtures thereof, to achieve the desired activity and durability. Specific examples include nickel plating, alloy plating of nickel and cobalt, nickel and iron, etc., composite oxides containing nickel and cobalt such as LaNiO3, LaCoO3, and NiCo2O4, platinum group element compounds such as iridium oxide, and carbon materials such as graphene. Organic materials such as polymeric materials may also be included to improve durability and contact with the substrate.

[0041] Examples of methods for forming a catalyst layer on the electrode substrate include plating, thermal spraying such as plasma spraying, thermal decomposition in which a precursor solution is applied to a substrate and then heated, a method in which a catalyst substance is mixed with a binder component and then fixed to the substrate, and vacuum film formation methods such as sputtering.

[0042] (Operating conditions of electrolytic cell) In this embodiment, the electrolytic cell is operated in the temperature range of 60°C to 120°C. A temperature range of 70°C to 110°C is more preferable, and a temperature range of 80°C to 100°C is even more preferable. By operating at a temperature near the boiling point of water or higher, the reaction rate of water electrolysis is significantly improved, and the transfer rate of ionic and molecular species is greatly improved, reducing energy loss associated with mass transfer. As a result, the electrolysis voltage can be reduced.

[0043] Furthermore, as shown in FIG. 1, when operating an industrial electrolytic cell, from the viewpoint of replenishing consumed water, a method can be preferably used in which the electrolytic solution is introduced from the lower parts of the cathode chamber 4 and the anode chamber 5, and the electrolytic solution and generated gas (hydrogen at the cathode 1 and oxygen at the anode 3) are extracted from the upper parts.

[0044] <Water electrolysis system> In the water electrolysis system of this embodiment, the electrolytic cell of the present invention described above is operated at 60 to 120°C to perform water electrolysis. According to this water electrolysis system, even when an electrolyte solution in the neutral pH range is used, the electrolysis voltage can be suppressed and efficient electrolysis can be performed.

[0045] The conditions of the electrolytic cell of the water electrolysis system of this embodiment are the same as those of the electrolytic cell of this embodiment described above. [Example]

[0046] The present invention will be described below with reference to specific examples and comparative examples, but the present invention is not limited to the following examples.

[0047] (Measurement of limiting current density of hydrogen oxidation reaction and oxygen reduction reaction) As indicators of the amounts of hydrogen and oxygen crossover in water electrolysis, hydrogen oxidation reaction and oxygen reduction reaction were carried out in a predetermined electrolyte by the rotating disk electrode method, and the respective limiting current densities were determined. The smaller the limiting current density, the slower the diffusion of hydrogen or oxygen in the electrolyte. Therefore, it is considered that the smaller the limiting current density of an electrolyte, the more likely it is that the amount of crossover can be reduced in an actual electrolytic cell.

[0048] In this example, electrochemical measurements were performed using a three-electrode system with a platinum rotating disk electrode as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode as the reference electrode. The electrolyte was prepared by gradually adding potassium hydroxide aqueous solution to phosphoric acid aqueous solution and adjusting the pH to 7.2 at 25°C to prepare a potassium phosphate electrolyte of the desired concentration. The electrolyte was added to an electrochemical cell, and the working electrode, counter electrode, and reference electrode were placed therein. The electrochemical cell was then heated to adjust the electrolyte temperature to the desired temperature. Next, hydrogen gas or oxygen gas was bubbled through the electrolyte to saturate it with hydrogen or oxygen. In this state, the limiting current density of the hydrogen oxidation reaction or oxygen reduction reaction was measured by sweeping at 50 mV / s while rotating the working electrode at 3600 rpm.

[0049] The results of the measured limiting current density are shown in Figure 2. It can be seen that by setting the electrolyte concentration to 2 mol / kg or more, preferably 2.5 mol / kg or more, crossover of hydrogen and oxygen can be significantly suppressed in both the 25°C and 80°C temperature ranges. Furthermore, as described in Non-Patent Document 1, the electrical resistivity of a potassium phosphate electrolyte decreases as the electrolyte concentration increases and as the temperature increases. From these facts, it is clear that in the present invention, it is preferable to use an electrolyte of 2 mol / kg or more and operate at 60 to 120°C.

[0050] [Example 1] 9 mL of 95% sulfuric acid was mixed with 3 mL of 30% hydrogen peroxide, and a glass fiber filter paper (Whatman GF / A, diameter 50 mm) with a porosity of 94% was immersed in this mixed solution for 30 minutes. After immersion, the hydrophilic treated glass fiber filter paper was washed with ultrapure water and air-dried. The thickness of this hydrophilic treated glass fiber filter paper was 260 μm. The hydrophilic glass fiber filter paper was sandwiched between platinum mesh cathode and platinum mesh anode on both sides and placed in an electrochemical cell containing 4.1 mol / kg potassium phosphate electrolyte. The cell was then heated to 100°C, and the electrical resistance was measured using the two-electrode impedance method. The electrical resistance was 0.09 Ω and the electrical resistivity was 0.04 Ωm.

[0051] [Example 2] Except for the fact that the thickness of the hydrophilic treated glass fiber filter paper was 130 μm, the measurement was carried out under the same conditions as in Example 1. At this time, the electrical resistance was 0.05 Ω and the electrical resistivity was 0.04 Ωm.

[0052] [Comparative Example 1] Measurement was carried out under the same conditions as in Example 1, except that Zirfon (manufactured by Agfa Materials Japan, Inc.), which is commonly used as a porous diaphragm for alkaline water electrolysis and has a porosity of 55% and a thickness of 500 μm, was used instead of the hydrophilic treated glass fiber filter paper. At this time, the electrical resistance was 0.48 Ω and the electrical resistivity was 0.09 Ωm.

[0053] Comparative Example 2 Measurement was carried out under the same conditions as in Comparative Example 1, except that a 7 mol / kg potassium hydroxide electrolyte was used instead of a 4.1 mol / kg potassium phosphate electrolyte. At this time, the electrical resistance was 0.09 Ω and the electrical resistivity was 0.02 Ωm.

[0054] Table 1 shows the conditions of the porous diaphragm and electrolyte, and the measured electrical resistance and electrical resistivity for Examples 1 and 2 and Comparative Examples 1 and 2.

[0055] [Table 1]

[0056] The results in Table 1 show that higher porosity and thinner membrane thickness are preferable for reducing electrical resistance. It should be noted that Comparative Example 2 is based on conditions typically used in alkaline water electrolysis, and it can be seen that the electrical resistances of Examples 1 and 2 are comparable to or even lower than those of Comparative Example 2. It should be noted that, because glass fibers dissolve in concentrated alkaline aqueous solutions, the method using glass fiber filter paper cannot be applied to alkaline water electrolysis. This wide degree of freedom in component selection is a major advantage of electrolysis in the neutral pH range.

[0057] [Example 3] According to the method described in Non-Patent Document 1, a platinum mesh carrying platinum was prepared as a cathode, and a titanium mesh carrying iridium oxide was prepared as an anode. The porous diaphragm was made of the hydrophilic treated glass fiber filter paper described above, and was sandwiched between the cathode and anode, which were then sandwiched between two PTFE plates on either side. The two PTFE plates were fastened together with PTFE screws, bringing the porous diaphragm into contact with the cathode and anode.

[0058] Then, the structure in which the porous diaphragm was in contact with the cathode and the anode was immersed in a 4.1 mol / kg potassium phosphate electrolyte, and the cathode and the anode were connected to a potentiostat. After heating the electrolyte to 100°C, an argon atmosphere was created and the current was 100 mA / cm 2 The cell voltage was measured over a 24-hour period during constant current operation. Figure 3 shows the voltage change over a 24-hour period during which water electrolysis was performed.

[0059] Comparative Example 3 As an electrolytic cell in which the porous diaphragm was not in contact with the cathode and anode, a constant current operation was carried out under the same conditions as in Example 3, except that no porous diaphragm was used and the cathode-anode distance was set to 1 cm. The change in cell voltage was measured, and the results are shown in Figure 3.

[0060] The cell voltage at the beginning of operation was about 1.55 V in Example 3, but about 1.90 V in Comparative Example 3, and a significant reduction in cell voltage of about 0.35 V was observed by bringing the cathode and anode into contact with the porous diaphragm. Furthermore, while the cell voltage was stable for 24 hours in Example 3, a sudden increase in cell voltage occurred after 8 hours in Comparative Example 3, making it impossible to continue operation any further. The reason for this sudden increase in cell voltage is unclear, but it is possible that ion conduction is inhibited by the cathode or anode being covered with a large amount of bubbles in a configuration such as Comparative Example 3. This is an example showing that bringing the cathode and anode into contact with a porous diaphragm not only reduces the electrolysis voltage but also improves the stability of electrolysis.

[0061] Comparative Example 4 For alkaline water electrolysis evaluation, constant current operation was performed under the same conditions as in Example 3, except that the nickel iron oxide-supported nickel foam prepared by the method described in Non-Patent Document 1 was used as the anode, Zirfon was used as the porous diaphragm, and a 1 mol / kg aqueous potassium hydroxide solution was used as the electrolyte. The cell voltage was measured over time, and the results are shown in Figure 3. The cell voltage of Comparative Example 4 at the beginning of operation was about 1.53 V, which is slightly lower than that of Example 3. However, in Comparative Example 4, the cell voltage began to increase one hour after operation began, reaching about 1.64 V at 24 hours. The reason for this increase in cell voltage is unclear, but it is possible that the electrode catalyst has changed in quality during operation, resulting in a decrease in activity. This example shows that the stability of electrolysis is improved by using a concentrated buffer solution in the neutral pH range as the electrolyte.

[0062] Furthermore, the current-voltage curve in the water electrolysis measurement of the electrolytic cell of Example 3 and the results of overpotential analysis are shown in Figure 4. From the results of Figure 4, it is clear that the current density was 600 mA / cm 2 The electrolysis voltage at this temperature was 2.2 V, which is extremely low for water electrolysis in a neutral pH range, demonstrating the applicability of the present invention on an industrial scale. [Industrial Applicability]

[0063] The electrolytic cell of the present invention can be used in the neutral pH range and is therefore suitable for use in the field of water electrolysis. [Explanation of symbols]

[0064] 1 cathode 2 Porous diaphragm 3 Anode 4 Cathode Chamber 5 Anode chamber 6 Power supply

Claims

1. An electrolytic cell using, as an electrolyte solution, a buffer solution having a pH of 1.5 to 12.6 during non-electrolysis and an electrolyte concentration of 2 mol / kg or more, wherein a porous diaphragm is in contact with a cathode and an anode, wherein the porous diaphragm is made of glass fiber.

2. 2. The electrolytic cell according to claim 1, wherein the porous diaphragm has a porosity of 40 to 98% and a thickness of 20 to 450 μm.

3. 2. The electrolytic cell according to claim 1, wherein the glass fibers are hydrophilically treated glass fibers.

4. 4. The electrolytic cell according to claim 1 , wherein the buffer solution is an electrolyte solution containing at least one cationic species selected from the group consisting of alkali metal cations and alkaline earth metal cations, and at least one anionic species selected from the group consisting of phosphates, borates, and carbonates.

5. A water electrolysis system comprising the electrolytic cell according to any one of claims 1 to 4, operated at 60 to 120°C, and performing water electrolysis.

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