JP7468888B2 - Neutral pH water electrolysis method and system - Google Patents

Description

The present invention relates to a method, system, and device for operating water electrolysis in the neutral pH range with high efficiency.

Although renewable energies can achieve a sustainable society, their inherent daily, seasonal and meteorological variability prevents their large-scale use. In this respect, the conversion of renewable electricity generation to chemical energy is seen as one solution to this problem.
Hydrogen has an energy density 120 MJkg ⁻¹ higher than that of gasoline (44 MJkg ⁻¹ ) and has attracted much attention as an energy carrier that can be produced via the electrolysis of water (Non-Patent Document 1).

Established industrial hydrogen production using water electrolysis operates at temperatures of around 80°C and at very strong alkaline and acidic pH levels. Due to these pH conditions, the materials that make up the equipment must be highly resistant to corrosion, and there are also significant safety concerns during operation.

Renewable energy-powered processes are believed to be more compatible with near-neutral pH, especially for in-situ applications, are cost-effective reaction media that offer greater material options for process components, and are potentially safer (2).
However, the efficiency of water splitting under such mild conditions is significantly lower than that of conventional methods operating under extreme pH conditions, and further research is needed.

Satyapal, S., Petrovic, J., Read, C., Thomas, G., and Ordaz, G., Catal. Today, 120, 246 (2007). Nocera, D. G., Inorg. Chem. 48, 21, (2009).

The present invention aims to provide a water electrolysis method and a water electrolysis system that can operate water electrolysis at a neutral pH with high efficiency.

After extensive research, the inventors discovered that water electrolysis can be performed with high efficiency in a high-concentration buffer solution with a near-neutral pH at temperatures near or above the boiling point of water, and thus completed the present invention.

That is, the present invention provides:
[1] A method for electrolyzing water in a buffer solution having a near-neutral pH at a temperature in the range of 60°C to 120°C, comprising the steps of:
the buffer solution is comprised of 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 phosphate, borate, and carbonate, the electrolyte solution having a concentration of 3M or greater.
[2] The method according to [1], wherein water is electrolyzed at a temperature in the range of 80°C to 100°C.
[3] The method according to [1] or [2], wherein the concentration of the electrolyte solution is 4 M or more.
[4] The method according to any one of [1] to [3], wherein the cationic species is a cation of lithium, sodium, potassium, cesium or rubidium.
[5] A system for electrolyzing water in a buffer solution having a near-neutral pH at a temperature in the range of 60°C to 120°C, comprising:
The buffer solution is comprised of 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 phosphate, borate, and carbonate, and the electrolyte solution has a concentration of 3M or greater.
[6] A water electrolysis apparatus comprising:
The electrolytic cell includes an electrolytic cell, a power source, an anode, and a cathode;
Here, an aqueous electrolyte solution is stored in the electrolytic cell,
the anode and the cathode are electrically connected to the power source;
the anode and the cathode are in contact with the electrolyte aqueous solution,
The electrolyte aqueous solution has a pH close to neutral,
the aqueous electrolyte solution contains 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, and the concentration of the aqueous electrolyte solution is 3M or more.
This provides:

The present invention provides a water electrolysis method and a water electrolysis system that can operate water electrolysis at a neutral pH with high efficiency.

As described above, the present invention electrolyzes water highly efficiently at neutral pH, and has the following advantages over conventional methods operated under strongly acidic or strongly alkaline conditions:
(1) The required corrosion resistance of materials is relaxed, allowing the use of cheaper materials, thereby reducing capital costs.
(2) The corrosion resistance required of materials is relaxed, so that electrode materials and device components are expected to have longer life.
(3) Because it operates at a neutral pH, human safety concerns are mitigated, expanding the range of potential locations for installing water electrolysis equipment (e.g., homes, etc.).
These will realize the conditions for fully applying water electrolysis devices at low cost, and will enable the expansion of distributed water electrolysis devices. More specifically, the water electrolysis device will be installed adjacent to various renewable energy-driven power generation devices, and surplus electricity generated by solar power or electricity will be used for water splitting on the spot.

FIG. 1 shows a non-limiting schematic diagram of a water electrolysis device of the present invention. 1 shows the results (voltage profiles) of catalytic tests performed at constant current density using model Pt and _IrOx electrodes. The results of water electrolysis in various electrolytes are shown below. The results of water electrolysis in various electrolytes are shown below. The results of a startup-shutdown cycle test are shown below.

1. Method for electrolyzing water One embodiment of the present invention is a method for electrolyzing water in a buffer solution having a near-neutral pH at a temperature in the range of 60° C. to 120° C., comprising the steps of:
The method of the present invention, wherein the buffer solution is composed of 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, and the concentration of the electrolyte solution is 3M or greater.
That is, in the method for electrolyzing water of the present invention (hereinafter also referred to as the "water electrolysis method of the present invention"), the use of the buffer solution at a high concentration, such as 3 M to a saturated concentration, as an electrolyte significantly increases the boiling point of water, and enables open-system water electrolysis to be operated in a temperature range significantly exceeding the conventional operating temperature of about 80°C.

The buffer solution used in the present invention (hereinafter also referred to as the "buffer solution of the present invention") is composed of 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.

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

The electrolyte solution contains at least one anion species selected from the group consisting of phosphates, borates, and carbonates. Specifically, the electrolyte solution contains at least one anion species selected from the group consisting of phosphoric acid 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⁻ ).

The electrolyte solution may contain one type of salt composed of the above-mentioned cation species and anion species, or may contain two or more types.

As the electrolyte solution, for _example , _neutral buffer solutions _{of KH2PO4} , _{K2HPO4 , LiH2PO4 , Li2HPO4 , NaH2PO4 , Na2HPO4 , Na2B4O7 , K2B4O7 , LiHCO3} , _NaHCO3 , _KHCO3 , and _CsHCO3 are preferably used.

The buffer solution of the present invention may contain, in addition to the salts composed of the above-mentioned cationic and anionic species, other components that do not adversely affect the method of the present invention, such as sulfates, hydrochlorides, and perchlorates.

The buffer solution of the present invention has a pH close to neutral, preferably a pH of 4 to 10, more preferably a pH of 5 to 9.

The concentration of the electrolyte solution is 3M or more, and preferably 4M or more.
In the present invention, the use of the buffer solution of the present invention at a high concentration of 3 M or more as an electrolyte significantly increases the boiling point of water, enabling open water electrolysis to be operated in a temperature range significantly exceeding the conventional operating temperature of about 80°C.

The water electrolysis method of the present invention is carried out at a temperature in the range of 60°C to 120°C, preferably 60 to 110°C, and more preferably 80 to 100°C. The water electrolysis method of the present invention can also be carried out under high pressure conditions, which makes it possible to operate at temperatures of around 120°C.

As described above, in the water electrolysis method of the present invention, the operating temperature can be set to be close to or higher than the boiling point of water, which can provide the following effects.
(1) A significant improvement in reaction rate can be achieved under neutral pH conditions. Conventionally, water splitting activity at neutral pH has been compared with activity at strongly acidic or cobasic pH in the temperature range near room temperature. However, at room temperature, the reaction rate is small at neutral pH. This is expressed as "high activation energy", which means that "the reaction rate is highly temperature-dependent". In other words, it is suggested that the reaction rate increase with increasing reaction temperature is larger at neutral pH than at strongly acidic or strongly basic conditions. Therefore, a significant improvement in reaction rate can be achieved by operating under conditions that can realize a temperature environment higher than before (neutral pH high concentration buffer solution).
(2) The transfer speed of ionic species and molecular weights is greatly improved, i.e., the energy loss associated with these mass transfers is reduced.
As a result, when water electrolysis is performed at a temperature near or above the boiling point of water using such a buffer solution with a neutral pH concentration, not only is efficiency improved compared to conventional trials under neutral pH conditions, but efficiency comparable to that of operation under strongly acidic or strongly basic pH conditions is also achieved. Furthermore, because the operating condition is neutral pH, reduced capital costs and a longer life are expected.

Furthermore, in the water electrolysis method of the present invention, by setting the operating temperature near or above the boiling point of water, the viscosity of the electrolyte solution can be reduced and the operating efficiency can be increased. In addition, by increasing the operating temperature, the electrical conductivity of the solution is improved, and the reactivity is improved.

As the electrode material (electrode catalyst) used in the water electrolysis method of the present invention, any electrode material used in conventional water electrolysis methods can be used.
The electrocatalyst may include one or more abundant elements such as, but not limited to, nickel (Ni), iron (Fe), cobalt (Co), manganese (Mn), copper (Cu), titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo), and/or tungsten (W).
Additionally, in some embodiments of the present invention, the electrocatalyst can include one or more metals, such as a precious metal (e.g., ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au)), aluminum (Al), zinc (Zn), cadmium (Cd), gallium (Ga), indium (In), tin (Sn), and bismuth (Bi) and/or other transition metals.
Additionally, in some embodiments of the present invention, the electrocatalyst may include one or more non-metals, such as boron (B), carbon (C), nitrogen (N), oxygen (O), phosphine (P), sulfur (S), etc., that may form metal oxides, metal carbides, metal nitrides, metal sulfides, and/or metal phosphides.

In the water electrolysis method of the present invention, a conventionally known cathode is used as the cathode. Examples of the material of the cathode include platinum group metals such as Pt, Rh, and Ir, Ni, Fe, and alloys thereof. Examples of the cathode form include a flat plate, a mesh, and a film formed by sputtering, etc.

In the water electrolysis method of the present invention, a conventionally known anode is used as the anode. Examples of the material of the anode include Ni, Ru, Ir, Ti, Sn, Mo, Ta, Nb, V, Fe, Mn, and alloys and oxides of these. Examples of the form of the anode include a flat plate, a mesh, and a film formed by sputtering, etc.

2. Water Electrolysis System Methods for electrolyzing water are generally broadly divided into alkaline water electrolysis, solid polymer water electrolysis, and high-temperature water electrolysis. The water electrolysis method of the present invention can be suitably applied to conventional alkaline water electrolysis methods.

That is, another embodiment of the present invention is a system for electrolyzing water in a buffer solution having a near-neutral pH at a temperature in the range of 60° C. to 120° C., comprising:
The buffer solution is composed of 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 phosphate, borate, and carbonate, and the concentration of the electrolyte solution is 3M or greater.
Details of the buffer solution, operating temperature, etc. are the same as those described in the water electrolysis method of the present invention.

3. Water electrolysis device Another embodiment of the present invention is a water electrolysis device comprising an electrolytic cell, a power source, an anode, and a cathode,
Here, an aqueous electrolyte solution is stored in the electrolytic cell,
the anode and the cathode are electrically connected to the power source;
the anode and the cathode are in contact with the electrolyte aqueous solution,
The electrolyte aqueous solution has a pH close to neutral,
The apparatus is characterized in that the aqueous electrolyte solution contains 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, and the concentration of the aqueous electrolyte solution is 3M or more.
Hereinafter, the water electrolysis device of the present invention will also be referred to as the "water electrolysis device of the present invention."

In the water electrolysis device of the present invention, the electrolytic cell may further have a diaphragm. By providing a diaphragm in the electrolytic cell and dividing the inside of the electrolytic cell into a first chamber having an anode and a second chamber having a cathode, it is possible to prevent the generated hydrogen gas and oxygen gas from mixing. When providing a diaphragm, a porous ceramic plate such as a biscuit plate, a porous polymer membrane such as a polypropylene film, or an ion exchange membrane such as Nafion (registered trademark) can be used.

On the other hand, in the concentrated buffer solution used in the present invention, the amount of gas generated that dissolves in the electrolyte solution is quite small, so that it may be possible to operate the water electrolysis apparatus without using a diaphragm. Therefore, in the water electrolysis apparatus of the present invention, the electrolytic cell does not need to have a diaphragm. In this case, there is an advantage that the capital cost can be reduced by simplifying the apparatus.
That is, one aspect of the water electrolysis apparatus of the present invention is a water electrolysis apparatus in which the electrolytic cell does not have a diaphragm.

An aqueous electrolyte solution is stored inside the electrolytic cell.

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

The electrolyte solution contains at least one anion species selected from the group consisting of phosphates, borates, and carbonates. Specifically, the electrolyte solution contains at least one anion species selected from the group consisting of phosphoric acid 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⁻ ).

The electrolyte solution may contain one type of salt composed of the above-mentioned cation species and anion species, or may contain two or more types.

As the electrolyte solution, for _example , _neutral buffer solutions _{of KH2PO4} , _{K2HPO4 , LiH2PO4 , Li2HPO4 , NaH2PO4 , Na2HPO4 , Na2B4O7 , K2B4O7 , LiHCO3} , _NaHCO3 , _KHCO3 , and _CsHCO3 are preferably used.

The buffer solution of the present invention may contain, in addition to the salts composed of the above-mentioned cationic and anionic species, other components that do not adversely affect the method of the present invention, such as sulfates, hydrochlorides, and perchlorates.

The buffer solution of the present invention has a pH close to neutral, preferably a pH of 4 to 10, more preferably a pH of 5 to 9.

The concentration of the electrolyte solution is 3M or more, preferably 4M or more.

The anode and cathode are placed inside the electrolytic cell so that they are in contact with the aqueous electrolyte solution. The anode and cathode are electrically connected to a power source, which will be described later. Oxygen is generated on the anode. Hydrogen is generated on the cathode.

In the water electrolysis device of the present invention, a conventionally known cathode is used as the cathode. Examples of the material of the cathode include platinum group metals such as Pt, Rh, and Ir, Ni, Fe, and alloys thereof. Examples of the cathode form include a flat plate, a mesh, and a film formed by sputtering, etc.

In the water electrolysis device of the present invention, a conventionally known anode is used as the anode. Examples of the material of the anode include Ni, Ru, Ir, Ti, Sn, Mo, Ta, Nb, V, Fe, Mn, and alloys and oxides of these. Examples of the form of the anode include a flat plate, a mesh, and a film formed by sputtering, etc.

The anode and cathode can be formed by supporting the above metals, their alloys, or their oxides on a conductive substrate. The conductive substrate can have various shapes such as a plate, a rod, or a mesh. Examples of materials that can be used for the conductive substrate include conventionally known materials such as titanium, aluminum, chromium, or alloys thereof, and carbon.

The anode is in contact with the aqueous electrolyte solution. The cathode is in contact with the aqueous electrolyte solution.

The power supply is used to apply a predetermined potential difference between the anode and the cathode. A predetermined potential difference is applied between the anode and the cathode using the power supply, and the water contained in the electrolyte aqueous solution is electrolyzed. It is desirable to apply a potential difference of 1.2 volts or more and 4.0 volts or less. Examples of power supplies are a potentiostat or a battery.

The power source may also be a solar cell. When the power source is a solar cell, the solar cell may include, for example, three silicon cells.

When operating the water electrolysis device of the present invention, it can be carried out at a temperature in the range of 60°C to 120°C, preferably 60 to 110°C, and more preferably 80 to 100°C.

The water electrolysis apparatus of the present invention can also be used to perform water electrolysis under high pressure conditions, in which case operation is possible even at a temperature of about 120°C.
When operating under high pressure conditions, conventionally known materials suitable for high pressures can be used.

A non-limiting schematic diagram of a water electrolysis apparatus of the present invention is shown in FIG.
In the figure, 11 denotes an electrolytic cell, 12 denotes an anode, 13 denotes a cathode, 14 denotes a power source, 15 denotes an aqueous electrolyte solution, and 16 denotes a diaphragm.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[experimental method]
The electrochemical measurement in the present invention was carried out in a two-electrode system. An iridium-supported titanium mesh was used as the anode material, and a platinum electrode was used as the cathode. The iridium-supported titanium mesh was prepared by electrochemically depositing iridium on the titanium mesh. Specifically, the titanium mesh was used as the working electrode in an aqueous solution composed of 400 μM Na ₃ Ir ₃ Cl ₆ , 2 mM H ₂ C ₂ O ₄ , and 5 mM Na ₂ CO _{3.H 2} O, and a current density of 140 mA cm ⁻² was applied for 63,000 seconds (MA Petit, V. Plichon, J. Electroanal. Chem. 1998, 444, 247-252.). The platinum, which is the cathode material, was also prepared by electrolytic deposition in the same manner. A platinum mesh was used as the working electrode, and a potential of -0.1 V was applied for 15 minutes in an aqueous solution consisting of ₁₀ mM _H2PtCl6 and ₂₃ mM HClO4 against a saturated calomel electrode used as a reference electrode (I. Lee, K.-Y. Chan, DL Phillips, Appl. Surf. Sci. 1998, 136, 321-330.).
Electrochemical measurements were performed using various electrolyte aqueous solutions prepared using the following reagents: _HClO4 , H2SO4 _{, H3PO4, H3BO3, Li2CO3, Na2CO3, K2CO3 , Cs2CO3 , LiOH , NaOH , KOH , and} /or _CsOH . The electrolyte aqueous solutions were varied in concentration from 0.1 to _{7 M} , and the reaction temperatures were in the range of 25-120°C.
The following reagents were all purchased from Sigma-Aldrich: KOH (99.99%), H ₃ PO ₄ (99.99%), HClO ₄ (ACS reagent).
K-phosphate was prepared by mixing _KOH and _H3PO4 at pH 7.0.

[Example 1]
Catalytic tests were carried out at 25 °C with constant current density using model electrodes of Pt and _IrOx for cathodic and anodic reactions, respectively, and the obtained voltage profiles are shown in Figure 2. The geometric surface area of the electrodes was ^1.0 cm2 and the distance between the electrodes was kept at 1 cm.
The iR-free voltages required in acidic and alkaline solutions of _0.1 M HClO4 and KOH were approximately 1.47 and 1.50 V at 10 mA cm ^- 2, respectively, whereas a voltage of 1.57 V was required to achieve the same current density in a saturated K-phosphate solution at pH 7.

[Example 2]
The results of water electrolysis in various electrolytes are shown in Figure 3. Figure 3 displays the voltage profile results in a two-electrode configuration at a constant current density of 10 mA cm ^-2 using IrO _x /Ti mesh and Pt/Pt mesh as anode and cathode, respectively. Measurements were performed at a reaction temperature of 80 °C using saturated K-phosphate aqueous solution at pH 7, 0.1 M KOH and 0.1 M HClO ₄ as electrolytes. The geometric surface area of the electrodes was 1.0 cm ² and the distance between the electrodes was kept at 1 cm. Note that in the following Figures 3, 4 and 5, the voltage displayed is not iR-free but the total applied voltage.
In order to reach a current density of 10 mA cm ⁻² , approximately 1.58 V and 1.50 V were required immediately after the start of the reaction in the acidic and alkaline pH solutions at 80 ° C., and the required voltage increased with time. In contrast, the voltage required in the saturated K-phosphate solution at 80 ° C. was about 1.47 V immediately after the start of the reaction. The required voltage also increased over time in the saturated K-phosphate solution at 80 ° C., but the increase was slower than in 0.1 M KOH and 0.1 M HClO _4. As a result, in this measurement, 6 hours after the start of the measurement, voltages of more than 1.70 V were required in 0.1 M KOH and 0.1 M HClO ₄ , respectively, while 10 mA cm ⁻² was maintained with only about 1.55 V in the saturated K-phosphate solution.

[Example 3]
The results of water electrolysis in various electrolytes are shown in Figure 4, which displays the voltage profile results in a two-electrode configuration at a constant current density of 10 mA cm ^-2 using _IrOx /Ti mesh and Pt/Pt mesh as anode and cathode, respectively. The measurements were carried out at reaction temperatures of 80 and 100 °C using saturated K-phosphate aqueous solution of pH 7, ₇ M HClO4 as electrolyte. The geometric surface area of the electrodes was ^1.0 cm2 and the distance between the electrodes was kept at 1 cm.
When 7M _HClO4 was used, the voltage required to reach a current density of 10 mA cm ^-2 was about 1.27 V at the beginning of the reaction, but increased with time, reaching about 1.5 V after 6 hours. On the other hand, when a saturated K-phosphate aqueous solution of pH 7 was used, the voltage required was 1.48 V immediately after the start of the measurement, and although it increased with time, the increase was slower than that of _7M HClO4, reaching about 1.55 V after 6 hours.
When a saturated K-phosphate aqueous solution was used, it was possible to reach a reaction temperature of 100° C. due to the rise in the boiling point of the high-density solution. The voltage required to obtain a current density of 10 mA cm ⁻² at that reaction temperature was 1.44 V immediately after the start of the measurement, and although it increased over time, the increase was gradual, reaching approximately 1.5 V after 6 hours.

[Example 4]
Startup-shutdown cycle test
A start-up-shutdown cycle test was performed and the results are shown in Figure 5. Figure 5 shows the voltage profile in a two-electrode configuration at a constant current density of 10 mA cm ^-2 and open circuit conditions (denoted as "10 mA" and "off", respectively), where _IrOx /Ti mesh and Pt/Pt mesh were used as the anode and cathode, respectively. Saturated aqueous K-phosphate solution at pH 7 and 7.0 M _HClO4 at 80 °C were used as electrolytes. The geometric surface area of the electrodes was 1 ^cm2 and the distance between the electrodes was kept at 1 cm.
Time "0" in FIG. 5 was set as the first time the current application was turned off after the chronoamperometry test at 10 mA cm ⁻² (6 hours in 7 M HClO ₄ and 12 hours in K-phosphate saturated solution).
When 10 mA cm ^-2 was applied again one hour after the current application was stopped, the voltage required in ₇ M HClO4 was around 1.5 V, but this increased significantly over time, reaching approximately 1.61 V. When the current was stopped again and 10 mA cm ^-2 was passed again, a voltage of around 1.6 V was initially required, but this increased again over time, reaching approximately 1.67 V. In contrast, in a saturated K-phosphate saturated solution, the potential was constant, at approximately 1.56 V, regardless of the start and stop cycles of current application.

Claims (5)

A method for electrolyzing water in a buffer solution having a pH of 4 to 10 at a temperature in the range of 60°C to 120°C, comprising the steps of:
the buffer solution is comprised of 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 phosphate, borate, and carbonate, the electrolyte solution having a concentration of 3M or greater. The method according to claim 1, in which water is electrolyzed at a temperature in the range of 80°C to 100°C. The method according to claim 1 or 2, wherein the concentration of the electrolyte solution is 4M or more. The method according to any one of claims 1 to 3, wherein the cationic species is a lithium, sodium, potassium, cesium or rubidium cation. A system for electrolyzing water in a buffer solution having a pH of 4 to 10 at a temperature in the range of 60°C to 120°C, comprising:
The buffer solution is comprised of 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 phosphate, borate, and carbonate, and the electrolyte solution has a concentration of 3M or greater.