JP7763416B2 - Electrolyzers and water electrolysis systems - Google Patents

Description

The present invention relates to an electrolytic cell and water electrolysis system that operates with high efficiency in the neutral pH range.

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 to level out energy supplies by producing hydrogen by electrolyzing water using electricity derived from renewable energy sources, storing and transporting the hydrogen, and then using hydrogen power generation equipment or fuel cells to extract energy when and where it is needed.

The main industrial methods of water electrolysis are alkaline water electrolysis, which uses concentrated aqueous solutions of sodium hydroxide or potassium hydroxide, and proton exchange membrane (PEM) water electrolysis, which uses a proton exchange membrane. Because these technologies use strongly basic or strongly acidic electrolytes, the materials used to make the electrolysis equipment must be highly corrosion-resistant.

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, i.e., the electrolysis voltage tends to be higher, compared to strongly basic or strongly acidic conditions.

For this reason, technologies to improve the energy efficiency of water electrolysis in the neutral pH range are being 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. Using a high-concentration buffer solution improves ionic conductivity and reduces the electrical resistance of the electrolytic cell. Furthermore, water electrolysis at high temperatures promotes the cathode and anode reactions, reducing the activation overvoltage.

Patent Document 2 also 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 the diaphragm is in contact with the cathode and anode. Patent Document 2 also states that in order to suppress the electrical resistance resulting from the electrolyte, 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 diaphragm is thin. However, in order to prevent mixing of the gas generated at the cathode and the gas generated at the anode, the diaphragm needs to have a low porosity and a certain thickness or greater.

Patent Publication No. 2021-116468 Patent Publication No. 2019-65349 T. Naito, et al. , ChemSusChem 2020, 13, 5921-5933

However, even when water electrolysis is performed using the technologies described in 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.

Therefore, the present invention aims to provide an electrolytic cell that can suppress electrolysis voltage even when using an electrolyte solution in the neutral pH range, and a water electrolysis system using the same.

As a result of investigations into solving the above-mentioned problems, the inventors discovered that by using a buffer solution in the neutral pH range (a pH of 1.5 to 12.6 when not electrolyzed) with an electrolyte concentration of 2 mol/kg or more as the electrolyte and employing an electrolytic cell in which a porous diaphragm is in contact with the cathode and anode, it is possible to reduce the electrolysis voltage in the neutral pH range while preventing mixing of the gas generated at the cathode and the gas generated at the anode, leading to the completion of the present invention.

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 solution, 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 glass fiber that has been hydrophilically treated.
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 comprising the electrolytic cell according to any one of 1 to 5 above, operated at 60 to 120°C to perform water electrolysis.

The present invention makes it possible to provide an electrolytic cell that can suppress electrolysis voltage even when using an electrolyte solution in the neutral pH range, and a water electrolysis system that uses the same.

1 is a diagram showing a schematic cross section of an example of an electrolytic cell of the present invention. 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. 1 is a graph showing the change in voltage when water electrolysis was carried out at a constant current for 24 hours in Comparative Example 3, Comparative Example 4, and Example 3. 1 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.

The following provides a detailed description of an embodiment of the composite material processing method of the present invention (hereinafter referred to as the "present embodiment"). Note that this embodiment is an example for explaining the present invention, and the present invention is not limited to this embodiment alone. In other words, the present invention can be modified in various ways without departing from the spirit and scope of the invention.

Figure 1 shows a schematic cross section of the electrolytic cell of this embodiment.

<Electrolytic cell>
As shown in FIG. 1 , the electrolytic cell of the present 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.

(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.

In water electrolysis, the reaction proceeds by supplying the substrate protons (H ⁺ ) or hydroxide ions (OH ⁻ ) to the electrodes. However, if the substrate concentration is insufficient, the current density is limited by the rate-determining substrate supply. In this case, the maximum current density at which operation is possible is called the "diffusion-limited current density." From the viewpoint of productivity, industrial water electrolysis is usually operated at a current density of 100 mA/cm ² or more. Therefore, in this embodiment, when the diffusion-limited current density is less than 100 mA/cm ² , it is considered that the H ⁺ concentration or OH ⁻ concentration is insufficient, i.e., the neutral pH region, and an electrolyte having a pH in the range of 1.5 to 12.6 during non-electrolysis is defined as an electrolyte in the neutral pH region. Regarding the calculation method of the numerical values, see T. Shinagawa, and K. Takanabe, Phys. Chem. Chem. Phys. See 2015, 17, 15111-15114.

As mentioned above, the pH of the electrolyte solution of this embodiment when not electrolyzed is in the range of 1.5 to 12.6. However, from the viewpoint of suppressing corrosion of various components of the electrolytic cell, the pH when not electrolyzed is preferably 4 to 11, and more preferably 5 to 10. The pH of the electrolyte solution can be adjusted by appropriately selecting the types and concentrations of the cation species and anion species described below.

In this embodiment, a buffer solution is used as the electrolyte solution. This allows the pH to be maintained constant during electrolysis. There are no particular limitations on the buffer solution, 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.

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

The anion species preferably contains at least one selected from the group consisting of phosphate anions (H ₂ PO ₄ ⁻ , HPO ₄ ²⁻ , PO ₄ ³⁻ ), boric acid anions (B(OH) ⁴⁻ , B ₄ O ₇ ²⁻ ), and carbonate anions (HCO ₃ ⁻ , CO ₃ ²⁻ ).

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 preparing 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.

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.

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 solution 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). Although the reason why hydrogen crossover and oxygen crossover can be suppressed by using a high-concentration electrolyte solution is not entirely clear, it is presumed that the increase in electrolyte concentration reduces the solubility of hydrogen and oxygen in the electrolyte solution, and the increase in the viscosity of the electrolyte solution reduces the diffusion rate of hydrogen and oxygen in the electrolyte solution, thereby suppressing hydrogen and oxygen crossover.

In this embodiment, in order to uniquely determine the electrolyte concentration, the smaller of the sum of the concentrations of cations other than protons (H ⁺ ) and the sum of the concentrations of anions other than hydroxide ions (OH ⁻ ) is defined as the electrolyte concentration.
For example, if 1 kg of a 2 mol/kg aqueous solution of sodium dihydrogen phosphate (NaH ₂ PO ₄ ) and 1 kg of a 2 mol/kg aqueous solution of disodium hydrogen phosphate (Na ₂ HPO ₄ ) are mixed to make a buffer solution, the cation concentration will be 3 mol/kg because Na ⁺ will be 3 mol/kg, and the anion concentrations will be 1 mol/kg for H ₂ PO ₄ ^- and 1 mol/kg for HPO ₄ ^2- , so the anion concentration will be 2 mol/kg, and the electrolyte concentration will be 2 mol/kg.
As another example, if 1 kg of a 4 mol/kg aqueous solution of phosphoric acid (H ₃ PO ₄ ) and 1 kg of a 6 mol/kg aqueous solution of potassium hydroxide (KOH) are mixed to make a buffer solution, the K ⁺ concentration will be 3 mol/kg, so the cation concentration will be 3 mol/kg. In this case, the pH of the buffer solution will be about 7.2, and almost all of the phosphoric acid will have ionized to H ₂ PO ₄ ^- and HPO ₄ ^2- , so the anion concentration will be 2 mol/kg, and the electrolyte concentration will be 2 mol/kg.

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.

The means for contacting the porous diaphragm 2 with the cathode 1 and the anode 3 is not particularly limited, and examples include a method in which the porous diaphragm is pressed between a smooth cathode and a smooth anode.

When electrolysis is performed, the cathode 1 and the anode 3 must be electrically connected to a power source 6, as shown in FIG. 1, 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.

(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. As the porous diaphragm 2, for example, an inorganic porous membrane, a polymer porous membrane, a woven fabric, a nonwoven fabric, or the like can be used.

Examples of inorganic porous membranes include glass fiber filters, sintered glass filters, and asbestos diaphragms. Examples of polymeric porous membranes include those made by forming porous membranes from polymeric materials such as polysulfone, polyethersulfone, polyvinylidene fluoride, polycarbonate, polytetrafluoroethylene, polyethylene, polypropylene, polyphenylene sulfide, and polyimide using methods such as microphase separation, extraction, stretching, and wet gel stretching. Examples of woven fabrics include those made by weaving or knitting polymeric fibers such as polyamide fibers, polyimide fibers, polyester fibers, and cotton. Nonwoven fabrics include those made by forming films from fibers such as polyamide fibers, polyester fibers, polypropylene fibers, cotton, and cellulose using dry processes such as carding and air-laid processes, or wet processes such as papermaking, and those made by forming films from polymeric materials such as polysulfone, polyethersulfone, polyvinylidene fluoride, polycarbonate, polytetrafluoroethylene, polyethylene, polypropylene, polyphenylene sulfide, and polyimide using methods such as spunbonding, meltblowing, and electrospinning. Glass fiber filter paper is preferred from the standpoint of mechanical strength and chemical durability.

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 (hydrophilization treatment) by immersing the filter paper in a mixture of concentrated sulfuric acid and an aqueous hydrogen peroxide solution can be preferably used.

The porosity of the porous diaphragm can be measured using common methods such as the Archimedes method.

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%. The higher the porosity, the more the electrical resistance caused by the electrolyte solution can be reduced, but hydrogen and oxygen crossover increases. Conversely, the lower the porosity, the more the electrical resistance caused by the electrolyte solution increases, but hydrogen and oxygen crossover can be reduced. 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 caused by the electrolyte solution with hydrogen and oxygen crossover while ensuring the mechanical strength of the porous diaphragm.

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. The thinner the porous diaphragm, the more the electrical resistance due to the electrolyte solution can be reduced, but hydrogen and oxygen crossover increases. Conversely, the thicker the porous diaphragm, the more the electrical resistance due to the electrolyte solution increases, but hydrogen and oxygen crossover can be reduced. Furthermore, if the porous diaphragm is too thin, the mechanical strength of the porous diaphragm decreases. By keeping the thickness of the porous diaphragm within the aforementioned range, it is possible to ensure the mechanical strength of the porous diaphragm and strike a balance between the electrical resistance due to the electrolyte solution and hydrogen and oxygen crossover.

(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 escapes 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 fabric made of woven carbon fibers, nonwoven fabric made of carbon fibers bound with a binder, and carbon foam.

The aforementioned porous metal body or porous carbon body may be used as an electrode directly, 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 perspective 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.

The material for the electrode substrate is not particularly limited, but mild steel, stainless steel, nickel, nickel alloys, titanium, and titanium alloys are preferred due to their chemical and electrochemical durability. Carbon materials are susceptible to oxidation degradation and are therefore not suitable for use as anode substrates, but can be preferably used as cathode substrates.

The cathode catalytic layer preferably has high hydrogen generation 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 single 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 multiple materials such as nickel and aluminum or nickel and tin, porous coatings prepared by plasma spraying using nickel compounds or cobalt compounds, alloys or composite compounds of nickel with an element selected from cobalt, iron, molybdenum, silver, copper, etc., platinum group element metals or oxides such as platinum and ruthenium, which have high hydrogen generation capacity, and mixtures of these platinum group element metals or oxides 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, multiple layers of the above materials may be stacked, or multiple materials may be mixed in the catalyst layer. Organic substances such as polymeric materials may also be included to improve durability and contact with the substrate.

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 a single metal, a compound such as an oxide, a composite oxide or alloy of multiple metal elements, or a mixture 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.

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

(Operating conditions of electrolytic cell)
In this embodiment, the electrolytic cell is operated in a 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 significantly improved, thereby reducing energy loss associated with mass transfer. As a result, the electrolysis voltage can be reduced.

Furthermore, as shown in Figure 1, when operating an industrial electrolytic cell, in order to replenish the water consumed, it is preferable to introduce the electrolyte from the bottom of the cathode chamber 4 and the anode chamber 5, and withdraw the electrolyte and generated gas (hydrogen at the cathode 1 and oxygen at the anode 3) from the top.

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

The conditions for the electrolytic cell of the water electrolysis system of this embodiment are the same as those for the electrolytic cell of this embodiment described above.

The present invention will be explained below using specific examples and comparative examples, but the present invention is not limited to the following examples.

(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.

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, thereby creating a potassium phosphate electrolyte of the specified concentration. The electrolyte was added to an electrochemical cell, and the working electrode, counter electrode, and reference electrode were placed inside. The electrochemical cell was then heated to adjust the electrolyte temperature to the specified temperature. Next, hydrogen gas or oxygen gas was bubbled through the electrolyte, saturating 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.

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 temperature ranges of 25°C and 80°C.
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 can be seen 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.

[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 film thickness of this hydrophilic treated glass fiber filter paper was 260 μm.
The hydrophilic treated glass fiber filter paper was sandwiched between a platinum mesh cathode and a platinum mesh anode, 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 a two-electrode impedance method. The electrical resistance was 0.09 Ω, and the electrical resistivity was 0.04 Ωm.

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

[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. In this case, the electrical resistance was 0.48 Ω and the electrical resistivity was 0.09 Ωm.

[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 the 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.

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

The results in Table 1 show that higher porosity and thinner membrane thickness are preferable for reducing electrical resistance. Comparative Example 2 uses conditions commonly used in alkaline water electrolysis, but it can be seen that the electrical resistance in Examples 1 and 2 is comparable to or even lower than these. Furthermore, because glass fiber dissolves in concentrated alkaline aqueous solutions, the method of using glass fiber filter paper cannot be applied to alkaline water electrolysis. This great degree of freedom in component selection is a major advantage of electrolysis in the neutral pH range.

[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 sandwiched between the cathode and anode using the hydrophilic treated glass fiber filter paper described above, and the porous diaphragm was further sandwiched between two PTFE plates on both sides. The two PTFE plates were fastened together with PTFE screws to bring the porous diaphragm into contact with the cathode and anode.

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 electrolytic solution to 100°C, the cell was operated in an argon atmosphere at a constant current of 100 mA/ ^cm2 for 24 hours, and the change in cell voltage was measured. The change in voltage during 24 hours of water electrolysis is shown in Figure 3.

[Comparative Example 3]
As an electrolytic cell in which the porous diaphragm was not in contact with the cathode and the 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.

The cell voltage at the beginning of operation was about 1.55 V in Example 3, while it was 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 there is a possibility 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.

[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 is an example that shows that the stability of electrolysis is improved by using a concentrated buffer solution in the neutral pH range as the electrolyte.

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. The results in Figure 4 show that the electrolysis voltage at a current density of 600 mA/ ^cm2 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.

The electrolytic cell of the present invention can be used in the neutral pH range, making it suitable for use in the field of water electrolysis.

1 Cathode 2 Porous diaphragm 3 Anode 4 Cathode chamber 5 Anode chamber 6 Power supply

Claims (5)

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 . The electrolytic cell described in claim 1, characterized in that the porous diaphragm has a porosity of 40 to 98% and a thickness of 20 to 450 μm. 2. The electrolytic cell according to claim 1 , wherein the glass fibers are hydrophilically treated glass fibers. 4. The electrolytic cell according to claim 1 , 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. A water electrolysis system, comprising: an electrolytic cell according to any one of claims 1 to 4 , operated at 60 to 120°C to perform water electrolysis.