Water electrolysis method and water electrolysis device

Abstract

Provided are a water electrolysis method and a water electrolysis device in which mixing of the generated hydrogen and oxygen is greatly reduced and which have a high electrolysis efficiency, while being simplified in structure. In the water electrolysis method and water electrolysis device, water is electrolyzed by supplying water to the cathode side of an electrolytic membrane including a solid polymer membrane provided with a catalyst layer on a surface thereof and creating a potential difference between both surfaces of the electrolytic membrane. The temperature-controlled water is supplied only to the cathode side of the electrolytic membrane, while controlling the difference in pressure between both surfaces of the electrolytic membrane to 50 kPa or less.

Claims

1. A water electrolysis method, comprising: providing an electrolytic membrane comprising a solid polymer membrane, and a catalyst layer on the solid polymer membrane, the solid polymer membrane being reinforced with a porous film to prevent the electrolytic membrane from damage caused by a difference in pressure between the first surface and the second surface of the electrolytic membrane, supplying an electric current to the electrolytic membrane, the electric current flowing between a first surface of the electrolytic membrane to a second surface of the electrolytic membrane that is opposite from the first surface, and supplying temperature-controlled water from a water supply unit to the electrolytic membrane, while controlling the difference in pressure between the first surface and the second surface of the electrolytic membrane to 50 kPa or less, wherein the first surface of the electrolytic membrane defines a cathode side of the electrolytic membrane, wherein the second surface of the electrolytic membrane defines an anode side of the electrolytic membrane, wherein the temperature-controlled water is supplied only to the cathode side of the electrolytic membrane, wherein an electrolytic solution is not supplied to the anode side of the electrolytic membrane, and an anode side in a housing defined by the electrolytic membrane is dry, wherein a temperature of the temperature-controlled water in the water supply unit is controlled to be within a range of from room temperature to 60° C., wherein the method is configured to manufacture dry oxygen for human respiration, and wherein the anode side of the electrolytic membrane contains the dry oxygen manufactured by the water electrolysis method.

2. The water electrolysis method according to claim 1, wherein a thickness of the electrolytic membrane is from 5 μm to 200 μm.

3. The water electrolysis method according to claim 1, wherein the electrolytic membrane has a tensile strength of 61 MPa and 56 MPa in a longitudinal direction and transverse direction, respectively, wherein the tensile strength represents a maximum load at the time of rupture.

4. The water electrolysis method according to claim 1, wherein a concentration of hydrogen in the oxygen of the anode side in the housing defined by the electrolytic membrane is 1000 ppm or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an explanatory drawing of a water electrolysis device according to an embodiment of the present invention.

(2) FIG. 2 is a graph representing the amount of hydrogen admixed to the generated oxygen under various conditions.

REFERENCE SIGNS LIST

(3) 100 Electrolysis device 101 Housing 102 Electrolytic membrane 103 Cathode side 104 Anode side 110 Power supply unit 120 Water supply unit 121 Pump 122 Heat exchanger (water temperature control device) 123 Gas-water separator 124 Check valve 125 Water tank 126 Ion exchange membrane 130 Pressure control unit (oxygen side) 131 Flowmeter (oxygen side) 132 Hygrometer (oxygen side) 140 Pressure control unit (hydrogen side) 141 Flowmeter (hydrogen side) 142 Hygrometer (hydrogen side)

DESCRIPTION OF EMBODIMENTS

(4) In the water electrolysis method of the present invention, water is electrolyzed by allowing an electric current to flow between both surfaces of an electrolytic membrane including a solid polymer membrane provided with a catalyst layer on a surface thereof, wherein temperature-controlled water is supplied only to a cathode side of the electrolytic membrane, while controlling a difference in pressure between both surfaces of the electrolytic membrane to 50 kPa or less. Further, the water electrolysis device of the present invention has an electrolytic membrane including a solid polymer membrane provided with a catalyst layer on a surface thereof; a housing having spaces defined by the electrolytic membrane; power supply unit for allowing an electric current to flow between both surfaces of the electrolytic membrane; water supply unit for supplying water to a cathode side of the electrolytic membrane; and pressure control unit for controlling a pressure in both spaces in the housing defined by the electrolytic membrane, wherein the water supply unit has a water temperature control device that controls the temperature of water supplied to the cathode side of the electrolytic membrane, and a difference in pressure between both spaces in the housing defined by the electrolytic membrane is made 50 kPa or less by the pressure control unit. The method and device may be implemented in any specific form, provided that the admixture of the generated hydrogen and oxygen is very small and the electrolysis efficiency is high, while the structure is simplified.

(5) Pt/Ir, Pd/Ir, and the like, are generally used as catalysts on the electrolytic membrane surface, but those examples are not limiting.

(6) Further, it is preferred that an organic polymer or inorganic substance with a low gas permeability that is used for reinforcement provided inside the electrolytic membrane be in the form of a film-like reinforcing membrane, and it may be a single solid polymer membrane or a thin film of a ceramic material, or the like, and may have a single-layer or laminated configuration.

EXAMPLES

(7) An embodiment of the water electrolysis method and water electrolysis device in accordance with the present invention will be explained hereinbelow with reference to the drawings.

(8) As depicted in FIG. 1, a water electrolysis device 100 has an electrolytic membrane 102 including a solid polymer membrane provided with a catalyst layer on a surface thereof; a housing 101 having spaces defined by the electrolytic membrane 102; power supply unit 110 for allowing an electric current to flow between both surfaces of the electrolytic membrane 102; and water supply unit 120 for supplying water to a cathode side 103 of the electrolytic membrane.

(9) The water supply unit 120 is configured to circulate water, which is to be electrolyzed, with a pump 121 and to replenish the amount of water reduced by the electrolysis from a water tank 125.

(10) More specifically, water, which is to be electrolyzed, is supplied from the pump 121 to the cathode side of the housing 101 through a check valve 124, this water together with hydrogen generated on the cathode side pass through a heat exchanger 122 constituting a water temperature control device, hydrogen is separated with a gas-water separator 123, and water is then returned to the pump 121 through the check valve 124 and supplied again.

(11) Water that has passed through an ion exchange membrane 126 from the water tank 125 is replenished on the upstream of the pump 121.

(12) The heat exchanger 122 constituting the water temperature control device is configured, for example, such as to perform heat exchange with the air inside the room and also configured to be capable of controlling the temperature of water supplied to the cathode side within a range of room temperature to 100° C. with any well-known unit, for example, by controlling the revolution speed of an air blowing fan.

(13) Hydrogen generated on the cathode side is separated from the circulating water with the gas-water separator 123 and fed through pressure control unit 140, a flowmeter 141, and a hygrometer 142 to a hydrogen tank or hydrogen-using device (not depicted in the figures) according to the intended use.

(14) A pressure applied to the cathode side of the electrolytic membrane 102 in the housing 101 is regulated by the pressure control unit 140 on the hydrogen side.

(15) Meanwhile, oxygen generated on the anode side 104 in the housing 101 is fed through pressure control unit 130, a flowmeter 131, and a hygrometer 132 to an oxygen tank or oxygen-using device (not depicted in the figures) according to the intended use.

(16) A pressure applied to the anode side of the electrolytic membrane 102 in the housing 101 is regulated by the pressure control unit 130 on the oxygen side, and this unit is configured to operate in coordination with the pressure control unit 140 on the hydrogen side, such as to control the difference in pressure between both spaces of the housing 101 defined by the electrolytic membrane 102 to 50 kPa or less.

(17) The effects obtained with the water electrolysis method and water electrolysis device of the present invention which are configured in the above-described manner will be explained hereinbelow.

(18) FIG. 2 shows the amount of hydrogen admixed to oxygen when the temperature of water supplied to the cathode side and the electric current density applied to the electrolytic membrane 102 are changed, while suppressing the difference in pressure between both spaces of the housing 101 defined by the electrolytic membrane 102 to 50 kPa or less.

(19) The water electrolysis device in the form of a three-cell stack was used for electrolysis in which water was supplied at a rate of 680 ml/min to a cathode as a hydrogen-generating electrode, without supplying water to an oxygen-generating electrode. FIG. 2 illustrates an example in which the operation temperature was 30° C.

(20) In FIG. 2, the results obtained with a cation exchange membrane (Nafion 117 (trade name)) which is usually used in water electrolysis are also presented for comparison. In the test illustrated by FIG. 2, the electrolysis was conducted while raising the current density to a maximum of 0.5 A/cm.sup.2.

(21) As follows from the figure, when the cation exchange membrane (Nafion 117 (trade name)) was used, the concentration of hydrogen in oxygen of 3500 ppm or higher was continuously detected, whereas when a solid polymer membrane having hydrogen ion conductivity and reinforced with a polytetrafluoroethylene porous film with a thickness of 30 μm was used, only the concentration of hydrogen in oxygen of 1000 ppm or less was detected at all times in the same test, and the effect of using the reinforcing membrane could be confirmed.

(22) A 10 mm (width)×10 cm (length) sample of the membrane used herein was prepared for a tensile strength test, tension was applied to the sample at a crosshead speed of 200 mm/min at 25° C. and a relative humidity of 50% by using a gage length (distance between clamps) of 50 mm, and the maximum load at the time of rupture was recorded. The membrane demonstrated strength of 61 MPa and 56 MPa in the longitudinal direction and transverse direction, respectively. When the same test was performed with respect to Nafion 112 (trade name) having a thickness of 50 μm, the measured tensile strength was 30 MPa and 30 MPa in the longitudinal direction and transverse direction, respectively. Thus, it is clear that the reinforced membrane had an about two-fold strength.

(23) Further, a 5 cm×5 cm sample stored for a minimum of 1 day at 25° C. and a relative humidity of 50% was put for 10 min in deionized water at 100° C. The sample was then taken out placed on a rubber mat, and flat spread. The length in the transverse and longitudinal direction after swelling was measured with a JIS first class scale, and a hydration swelling ratio was determined. The change thereof was 3% in the longitudinal direction and 0% in the transverse direction. When the same test was performed with respect to Nafion 112 (trade name) having a thickness of 50 μm, the hydration swelling ratio was 19% in the longitudinal direction and 16% in the transverse direction. It was thus found that the dimensional variability caused by hydration had a value of 1/5 or less.

INDUSTRIAL APPLICABILITY

(24) With the water electrolysis method and water electrolysis device of the present invention, electrolysis efficiency is increased while ensuring the purity of the generated hydrogen and oxygen. Therefore, the method and device are advantageous for generating hydrogen and oxygen in artificial satellites and space stations where strong constraints are placed on weight, space, and equipment.

(25) Further, the present technique which increases the electrolysis efficiency and facilitates gas separation during water electrolysis can be used not only for hydrogen and oxygen generation, but also for regenerative fuel cells and unitized regenerative fuel cells (reversible fuel cells) that require water electrolysis.