WATER ELECTROLYZER AND WATER ELECTROLYSIS METHOD

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a water electrolyzer and a water electrolysis method.

Related Art

Conventionally, upon electrolysis of water, a voltage is applied to an anode and a cathode disposed in an aqueous solution. During this time, oxygen is generated from a side of the anode and hydrogen is generated from a side of the cathode. Here, a theoretical electrolysis voltage of water at room temperature is about 1.23 V. However, considering efficiency of hydrogen production, it is desirable to lower an electrolysis voltage of water, in other words, to bring an electrolysis voltage of water close to the theoretical electrolysis voltage of water.

As a water electrolyzer, a solid-state polymer water electrolyzer having a membrane electrode assembly in which a solid-state polymer electrolyte membrane is sandwiched between an anode and a cathode has been known (see, for example, Patent Document 1).

Patent Document 1: PCT International Publication No. WO2020/116651

SUMMARY OF THE INVENTION

However, because of difficulty in lowering resistance of each portion of the membrane electrode assembly, the electrolysis voltage of water cannot be lower than a certain value.

An object of the present invention is to provide a water electrolyzer and a water electrolysis method that allow an electrolysis voltage of water to be lower.

One aspect of the present invention is a water electrolyzer that includes an anode including a nanodiamond and a light irradiator that irradiates the anode with light.

The anode may further include a divalent or trivalent transition metal.

Another aspect of the present invention is a water electrolysis method in which water is electrolyzed while an anode including a nanodiamond is irradiated with light.

The water may include a divalent or trivalent transition metal.

The present invention can provide a water electrolyzer and a water electrolysis method that allow an electrolysis voltage of water to be lower.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing one exemplary water electrolyzer according to the present embodiment;

FIG. 2 is a flow chart showing one exemplary control of the water electrolyzer shown in FIG. 1;

FIG. 3 is a cross-sectional view showing one exemplary positive electrode to be used in the water electrolyzer shown in FIG. 1;

FIG. 4 is a cross-sectional view showing another exemplary positive electrode to be used in the water electrolyzer shown in FIG. 1; and

FIG. 5 is a cross-sectional view showing another exemplary positive electrode to be used in the water electrolyzer shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

A water electrolyzer according to the present embodiment includes an anode including a nanodiamond and a light irradiator that irradiates the anode with light. The water electrolyzer according to the present embodiment can be used to electrolyze water while the anode is irradiated with light.

When the nanodiamond included in the anode absorbs light, electrons (e−) in a valence band transition to a conduction band, and the electrons that have transitioned to the conduction band transfer to a cathode to thereby cause an electric current to flow. Here, at the anode, vacancies generated in the valence band allow an anode reaction (2H₂O→O₂+4H⁺+4e⁻) to proceed. Thus, use of light energy lowers an electrolysis voltage of water. On the other hand, the electrons transferred to the cathode allow a cathode reaction (2H⁺+2e⁻→H₂) to proceed.

The light with which the anode is irradiated is not particularly limited as long as it is capable of causing the electrons in the valence band in the nanodiamond to transition to the conduction band.

The nanodiamond has an amorphous layer composed of a sp² carbon on a surface of a core part composed of a sp³ carbon, and an oxygen-containing group on a surface. The oxygen-containing group includes, for example, a hydroxyl group, a carboxyl group, a carbonyl group, or an oxy group.

A particle diameter of the nanodiamond is not particularly limited, but is, for example, 3 nm or more and 8 nm or less. Note that, the nanodiamond may be agglomerated.

A method for producing the nanodiamond is not particularly limited, but includes, for example, a chemical vapor deposition (CVD) method.

If an anode further includes a divalent or trivalent transition metal, a transfer of electrons is accelerated, resulting in an even lower electrolysis voltage of water. In this case, the nanodiamond may be doped with the divalent or trivalent transition metal. Furthermore, the divalent or trivalent transition metal may be supported by the nanodiamond. In this case, the divalent or trivalent transition metal may be introduced into the anode by electrolyzing water containing the divalent or trivalent transition metal.

Examples of the divalent or trivalent transition metal include Co(II), Co(III), Mn(II), Mn(III), etc.

FIG. 1 shows one exemplary water electrolyzer according to the present embodiment.

A water electrolyzer 10 includes an electrolytic tank 11, and the electrolytic tank 11 includes an anode 11a including a nanodiamond, a cathode 11b, and a proton-conducting membrane 11c serving as a membrane separating the anode 11a and the cathode 11b. The water electrolyzer 10 includes a light irradiator that irradiates the anode 11a with light (hν), and a power supply 12 that applies an electrolysis voltage between the anode 11a and the cathode 11b. Furthermore, the water electrolyzer 10 is equipped with piping that supplies raw water to the electrolytic tank 11 and that is sequentially provided with a sealing valve 13, a flow meter (FM) 14, and a resistance meter (RS) 15, and a pure water generator 16 that produces pure water from raw water and that is disposed so as to bypass the sealing valve 13. The water electrolyzer 10 is also equipped with piping that discharges hydrogen (H₂) generated at the cathode 11b and that is provided with a flow meter (FM) 17. In addition, the water electrolyzer 10 includes an electronic controller (ECU) 18.

A material constituting the cathode 11b is not particularly limited, but includes, for example, nickel, molybdenum, or the like.

A material constituting the proton-conducting membrane 11c is not particularly limited, but includes, for example, Nafion (registered trademark), or the like.

Note that, an anion-conducting film may be used instead of the proton-conducting film 11c. In this case, piping that discharges oxygen (O₂) generated at the anode 11a is equipped with a flow meter (FM) 17.

A material constituting the anion-conducting film is not particularly limited, but includes, for example, Selemion (registered trademark) (manufactured by AGC Engineering Co., Ltd.), or the like.

A voltage that the power supply 12 applies is not particularly limited.

The pure water generator 16 is not particularly limited, but includes, for example, Elix Essential (manufactured by Merck KGaA).

The electronic controller (ECU) 18 controls a flow rate of raw water to be detected by the flow meter (FM) 14 based on a flow rate of hydrogen to be detected by the flow meter (FM) 17.

Note that, when it is necessary to supply water with high purity to the electrolytic tank 11, the electronic controller (ECU) 18 controls purity of the water to be supplied to the electrolytic tank 11, as shown in FIG. 2.

The sealing valve 13 is opened with the pure water generator 16 stopped, and an electrolysis voltage is applied between the anode 11a and the cathode 11b from the power supply 12 and the anode 11a is irradiated with light (hν) from the light irradiator while raw water is supplied to the electrolytic tank 11. Thus, electrolysis is started (S1). Next, whether a relationship among a flow rate of raw water, resistance of raw water, and a flow rate of hydrogen detected by the flow meter (FM) 14, the resistance meter (RS) 15, and the flow meter (FM) 17, respectively, is normal for a voltage of the power supply 12 is determined (S2). If the relationship among a flow rate of raw water, resistance of raw water, and a flow rate of hydrogen is normal, electrolysis is continued as is. On the other hand, if the relationship among a flow rate of raw water, resistance of raw water, and a flow rate of hydrogen is not normal, purity of the raw water is determined to be low, the sealing valve 13 is closed, and the pure water generator 16 is started to supply pure water to the electrolytic tank 11 (S3). Next, whether the relationship among a flow rate of raw water, resistance of raw water, and a flow rate of hydrogen detected by the flow meter (FM) 14, the resistance meter (RS) 15, and the flow meter (FM) 17, respectively, is normal for a voltage of the power supply 12 is determined (S4). If the relationship among a flow rate of raw water, resistance of raw water, and a flow rate of hydrogen is not normal, the process returns to S3. On the other hand, if the flow rate of raw water, the resistance of raw water, and the flow rate of hydrogen are normal, the pure water generator 16 is stopped and the sealing valve 13 is opened, and then electrolysis is continued.

FIG. 3 shows one exemplary positive electrode to be used in a water electrolyzer 10.

An anode 30 includes a nanodiamond layer 32 formed on one side of a substrate 31.

Examples of a material constituting the substrate 31 include silicon, aluminum, or the like.

A method for producing the anode 30 includes, for example, a method in which one side of the substrate 31 is spray-coated with a coating solution including a nanodiamond and then dried to thereby form a nanodiamond layer 32.

FIG. 4 shows another exemplary positive electrode to be used in a water electrolyzer 10.

An anode 40 includes a nanodiamond layer 42 formed on an entire surface of a substrate 41.

Examples of a material constituting the substrate 41 include silicon, aluminum, or the like.

A method for producing the anode 40 includes, for example, a method in which the substrate 41 is dip-coated with a coating solution including a nanodiamond and then dried to thereby form a nanodiamond layer 42.

FIG. 5 shows another exemplary positive electrode to be used in a water electrolyzer 10.

An anode 50 is made of a nanodiamond.

A method for producing the anode 50 includes, for example, a method in which a nanodiamond is compression molded.

Embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments and the embodiment may be modified as appropriate within the scope of the invention.

EXPLANATION OF REFERENCE NUMERALS

• • 10 Water electrolyzer
  • 11 Electrolytic tank
  • 11a Anode
  • 11b Cathode
  • 11c Proton-conducting membrane
  • 12 Power source
  • 13 Sealing valve
  • 14 Flow meter (FM)
  • 15 Resistance meter (RS)
  • 16 Pure water generator
  • 17 Flow meter (FM)
  • 18 Electronic controller (ECU)
  • 30, 40, 50 Positive electrode
  • 31, 41 Substrate
  • 32, 42 Nanodiamond layer