Water electrolysis system

Abstract

To provide a water electrolysis system capable of improving water electrolysis reaction efficiency.SOLUTION: A water electrolysis system includes: a water electrolysis device that performs water electrolysis; a water supply device that supplies water to the water electrolysis device; a power source that supplies current to the water electrolysis device; and a control device that adjusts a flow rate of water supplied from the water supply device to the water electrolysis device, wherein, when a product of a current density (A / cm2) of the current supplied to the water electrolysis device and a viscosity (mPa.s) of the water supplied to the water electrolysis device is set as a value X, the water flow rate of the water supplied from the water supply device to the water electrolysis device is reduced when the value X is 1.3 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present application relates to a water electrolysis system. [Background technology]

[0002] In recent years, hydrogen has been attracting attention as a CO2-free energy source. Methods for producing hydrogen include alkaline water electrolysis and water electrolysis. Among these, water electrolysis is attracting attention due to its high efficiency.

[0003] Patent Document 1 discloses the following configuration of a water electrolysis system: that is, the water electrolysis system includes a water electrolysis device that electrolyzes water to generate hydrogen gas, a power supply device that supplies power obtained from natural energy to the water electrolysis device, a water supply device that supplies water to the water electrolysis device, and a control device that adjusts the amount of water supplied by the water supply device in accordance with the power supplied to the water electrolysis device from the power supply device. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-280975 Summary of the Invention [Problem to be solved by the invention]

[0005] In the water electrolysis system of Patent Document 1, the control device adjusts the water supply amount according to the electric power. For example, when the electric power increases, the water supply amount is increased, and conversely, when the electric power decreases, the water supply amount is decreased. This is because the amount of water required for water electrolysis increases as the electric power increases.

[0006] In contrast to this, the present inventors have found that increasing the amount of water flow in a high current density region (high power region) reduces the efficiency of the water electrolysis reaction, contrary to conventional technical common sense.

[0007] In view of the above circumstances, a main object of the present disclosure is to provide a water electrolysis system capable of improving the efficiency of the water electrolysis reaction. [Means for solving the problem]

[0008] In one aspect of the present disclosure to solve the above problems, there is provided a water electrolysis apparatus that performs water electrolysis, a water supply device that supplies water to the water electrolysis apparatus, a power source that supplies current to the water electrolysis apparatus, and a control device that adjusts the amount of water supplied from the water supply device to the water electrolysis apparatus, wherein the control device adjusts the current density (A / cm 2 The present invention provides a water electrolysis system that reduces the amount of water supplied from a water supply device to a water electrolysis device when the product of the viscosity (mPa·s) of the water supplied to the water electrolysis device and the viscosity (mPa·s) of the water supplied to the water electrolysis device is 1.3 or more.

[0009] In the water electrolysis system, the control device may reduce the flow rate of water supplied from the water supply device to the water electrolysis device by 50% or more and 85% or less when the value X is 1.3 or more and less than 2.0, and may reduce the flow rate of water supplied from the water supply device to the water electrolysis device by 33% or more and 90% or less when the value X is 2.0 or more. [Effects of the Invention]

[0010] The water electrolysis system according to the present disclosure can improve the water electrolysis reaction efficiency in a high current density region. Furthermore, since the water electrolysis reaction efficiency can be improved in a high current density region by reducing the amount of water flow, a large water supply device is not required. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a block diagram of a water electrolysis system 100. [Figure 2] FIG. 10 is a diagram showing the relationship between the temperature and viscosity of water. [Figure 3] The results are for Examples 1 to 3. [Figure 4] The results are from Examples 4 to 6. [Figure 5] The results are from Examples 7 to 9. [Figure 6] The results are from Examples 10 to 11. DETAILED DESCRIPTION OF THE INVENTION

[0012] The water electrolysis system of the present disclosure will be described using one embodiment of a water electrolysis system 100. A block diagram of the water electrolysis system 100 is shown in FIG.

[0013] The water electrolysis system 100 includes a water electrolysis device 10, an oxygen electrode side piping section 20 arranged on the oxygen electrode side of the water electrolysis device 10, a hydrogen electrode side piping section 30 arranged on the hydrogen electrode side of the water electrolysis cell, a power supply 40 that supplies current to the water electrolysis device, and a control device 50.

[0014] <Water electrolysis device 10> The water electrolysis device 10 is a device that performs water electrolysis. The configuration of the water electrolysis device 10 is publicly known. An example of the water electrolysis device 10 will be described below.

[0015] The water electrolysis device 10 includes a water electrolysis cell. Typically, the water electrolysis device 10 includes a water electrolysis stack in which a plurality of water electrolysis cells are stacked. The water electrolysis cell 10 also includes a terminal that can be connected to a power source 40.

[0016] A water electrolysis cell is a device for electrolyzing water to produce hydrogen and oxygen. A water electrolysis cell has an oxygen electrode and a hydrogen electrode. By supplying water to the oxygen electrode of the water electrolysis cell and applying a voltage, oxygen is generated from the oxygen electrode and hydrogen is generated from the hydrogen electrode. There are no particular limitations on the type of water electrolysis cell, but a PEM (Polymer Electrolyte Membrane) water electrolysis cell may be used from the perspective of improving water electrolysis efficiency. The configuration of a PEM water electrolysis cell is briefly described below.

[0017] A PEM water electrolysis cell includes a membrane electrode assembly and a pair of separators disposed on both sides of the membrane electrode assembly.

[0018] The membrane electrode assembly has an electrolyte layer, an oxygen electrode catalyst layer disposed on one surface of the electrolyte layer, and a hydrogen electrode catalyst layer disposed on the other surface of the electrolyte layer.

[0019] The electrolyte layer is not particularly limited as long as it has proton conductivity. For example, a proton-conducting polymer may be used. Examples of the proton-conducting polymer include a proton-conducting polymer having a sulfonic acid group, such as a perfluoroalkylsulfonic acid polymer.

[0020] The oxygen electrode catalyst layer includes an oxygen electrode catalyst capable of generating oxygen by water electrolysis. The oxygen electrode catalyst is not particularly limited, but examples thereof include metal catalysts. Examples of the metal catalyst include metal catalysts containing Pt, Ru, Rh, Os, Ir, Pd, or Au in their composition. The metal catalyst may also be an oxide of these metals.

[0021] The hydrogen electrode catalyst layer includes a hydrogen electrode catalyst capable of generating hydrogen by water electrolysis. The hydrogen electrode catalyst is not particularly limited, but examples thereof include metal catalysts. Examples of metal catalysts include metal catalysts containing Pt, Ru, Rh, Os, Ir, Pd, or Au in their composition. The metal catalyst may be an oxide of these metals. The hydrogen electrode catalyst may also be an electrically conductive support (metal-supported catalyst) that supports the metal catalyst.

[0022] Separators are disposed on both sides of the membrane electrode assembly. The separators are made of a conductive material, such as a resin containing a carbon material, or a metal material such as iron, copper, stainless steel, or titanium. A predetermined flow path is formed on the surface of the separator facing the catalyst layer, and the flow path serves to guide water supplied to the water electrolysis cell and oxygen or hydrogen produced by the water electrolysis reaction.

[0023] <Oxygen electrode side piping section 20> The oxygen electrode-side piping section 20 serves to supply water to the oxygen electrode of the water electrolysis apparatus 10. As shown in Fig. 1 , the oxygen electrode-side piping section 20 includes a water supply device 21, a water supply flow path 22, an oxygen electrode-side gas-liquid separator 23, a water discharge flow path 24, a circulation flow path 25, and a discharge flow path 26.

[0024] The water supply device 21 is a device that supplies water to the oxygen electrode of the water electrolysis device 10. In order to circulate the water, the water may be supplied under pressure to the water electrolysis device 10. An example of the water supply device 21 is a pump.

[0025] The water supply flow path 22 connects the water electrolysis apparatus 10 and the water supply device 21, and is a pipe through which water supplied from the water supply device 21 flows.

[0026] The oxygen electrode-side gas-liquid separator 23 is a device that separates water and oxygen discharged from the oxygen electrode of the water electrolysis device 10. The water separated by the oxygen electrode-side gas-liquid separator 23 is sent to the water supply device 21 via a circulation flow path 25 and is reused for the water electrolysis reaction. The oxygen separated by the oxygen electrode-side gas-liquid separator 23 is discharged to the outside via a discharge flow path 26.

[0027] The water discharge flow path 24 connects the water electrolysis device 10 and the oxygen electrode-side gas-liquid separator 23, and is a pipe for passing water and oxygen discharged from the oxygen electrode of the water electrolysis device 10.

[0028] The circulation flow path 25 connects the water supply device 21 and the oxygen electrode side gas-liquid separator 23, and is a pipe for passing water discharged from the oxygen electrode side gas-liquid separator 23. The circulation flow path 25 is used when circulating water in the oxygen electrode side piping section 20, and is therefore not necessary when water is not circulated.

[0029] The discharge flow path 26 is connected to the oxygen electrode side gas-liquid separator 23, and is a pipe for passing oxygen separated by the oxygen electrode side gas-liquid separator 23.

[0030] <Hydrogen electrode side piping section 30> The hydrogen electrode-side piping section 30 has a role of recovering hydrogen generated at the hydrogen electrode of the water electrolysis apparatus 10. As shown in Fig. 1 , the hydrogen electrode-side piping section 30 includes a hydrogen electrode-side gas-liquid separator 31, a hydrogen discharge flow path 32, a hydrogen tank 33, and a hydrogen supply flow path 34.

[0031] The hydrogen electrode-side gas-liquid separator 31 is a device that separates water and hydrogen discharged from the hydrogen electrode of the water electrolysis system 10. As described above, water is supplied to the oxygen electrode of the water electrolysis system 10, but water may permeate the membrane electrode assembly and leak to the hydrogen electrode side. For this reason, a gas-liquid separator is also provided in the hydrogen electrode-side piping section 30. The hydrogen separated by the hydrogen electrode-side gas-liquid separator 31 is sent to the hydrogen tank 33 via a hydrogen supply flow path 34. The water separated by the hydrogen electrode-side gas-liquid separator 31 is discharged as appropriate.

[0032] The hydrogen discharge flow path 32 connects the water electrolysis device 10 and the hydrogen electrode-side gas-liquid separator 31, and is a pipe for passing water and hydrogen discharged from the hydrogen electrode of the water electrolysis device 10.

[0033] The hydrogen tank 33 is for storing the hydrogen separated by the hydrogen electrode side gas-liquid separator 31 .

[0034] The hydrogen supply flow path 34 connects the hydrogen electrode side gas-liquid separator 31 and the hydrogen tank 33, and is a pipe for allowing the hydrogen separated by the hydrogen electrode side gas-liquid separator 31 to flow.

[0035] <Power supply 40> The power supply 40 supplies current to the water electrolysis apparatus 10 and is connected to both the oxygen electrode and the hydrogen electrode of the water electrolysis apparatus 10. Such power supplies 40 are well known. Water electrolysis occurs when a current is applied from the power supply 40 while water is supplied to the water electrolysis apparatus 10.

[0036] <Control device 50> The control device 50 is a computer system equipped with a CPU, RAM, an input / output interface, etc., and adjusts the flow rate of water supplied from the water supply device 21 to the water electrolysis device 10. Specifically, the control device 50 adjusts the current density (A / cm 2 When the product of the current density and the viscosity (mPa·s) of the water supplied to the water electrolysis system 10 is a value X (= [current density] × [water viscosity]), if the value X is 1.3 or greater, control is performed to reduce the amount of water supplied from the water supply system 21 to the water electrolysis system 10.

[0037] The current density (A / cm 2 ) can be measured by a current measuring device provided in the power supply 20. The viscosity (mPa s) of the water supplied to the water electrolysis device 10 can be calculated from the relationship between the temperature and viscosity of the water. For reference, the relationship between the temperature and viscosity of the water is shown in FIG. 2. The water temperature can be the temperature of the water supplied to the oxygen electrode of the water electrolysis device 10, the temperature of the water discharged from the oxygen electrode, or the average value of these temperatures. The water temperature can be measured, for example, by providing a temperature measuring device in the water supply flow path 22 and / or the water discharge flow path 24 in a position close to the water electrolysis device 10.

[0038] The reference amount of water supplied to the water electrolysis apparatus 10 (the amount of water supplied before reduction) is typically adjusted so as to obtain a target current density. The water electrolysis reaction occurring on the oxygen electrode side is as follows. 2H2O→O2+4H + +4e - . From the above formula, the theoretical value of the water flow rate (stoichiometric ratio = 1) is determined according to the stoichiometric ratio. However, in reality, the target current density cannot be obtained with the theoretical value of the water flow rate. Typically, an excessive amount of water is supplied. For example, the water flow rate is set to a stoichiometric ratio of 50 to 100 times the water flow rate (theoretical value) that achieves the target current density. This is set as the reference amount. However, the reference amount is not limited to this and can be determined appropriately based on the configuration of the water electrolysis device 10 or the water electrolysis system 100.

[0039] As described above, the control device 50 reduces the amount of water supplied from the water supply device 21 to the water electrolysis device 10 when the value X, which is the product of the "current density" and the "viscosity of water," is 1.3 or greater. A value X of 1.3 or greater indicates a high current density region. The reduction in the amount of water flow means that the amount of water flow is reduced from a reference amount based on the stoichiometric ratio. In other words, the rate of reduction in the amount of water flow can be rephrased as the rate of reduction in the stoichiometric ratio. In this way, the water electrolysis system 100 can improve the water electrolysis reaction efficiency by reducing the amount of water flow in accordance with the above conditions. Furthermore, because the water electrolysis reaction efficiency can be improved by reducing the amount of water flow, a large water supply device 21 is not required.

[0040] From the viewpoint of further improving the water electrolysis reaction efficiency, when the value X is 1.3 or more and less than 2.0, the amount of water supplied from the water supply device 21 to the water electrolysis device 10 may be reduced to 50% or more and 85% or less. Furthermore, when the value X is 2.0 or more, the amount of water supplied may be reduced to 33% or more and 90% or less, and from the viewpoint of further improving the water electrolysis reaction efficiency, the amount of water supplied may be reduced to 66% or more and 90% or less.

[0041] The estimated mechanism is as follows. First, in the low current density region, the amount of oxygen discharged (amount of bubbles) is small, and permeation of liquid water into the membrane electrode assembly is dominant. Therefore, increasing the water flow rate reduces the voltage and improves the water electrolysis reaction efficiency. In contrast, in the high current density region, bubble discharge becomes rate-limiting. Increasing the water flow rate in this state fills the flow channels in the water electrolysis cell with liquid water, increasing the liquid water pressure. This restricts the movement of gas from the membrane electrode assembly, which includes a catalyst layer for the water electrolysis reaction, to the flow channels, and the movement of water from the flow channels to the membrane electrode assembly. This reduces the water electrolysis reaction efficiency. Furthermore, lowering the water temperature in this state is thought to make this tendency more pronounced. This is because lowering the water temperature significantly increases the viscosity of the water, which is thought to have a negative impact on the permeation of liquid water and the discharge of bubbles.

[0042] Therefore, the inventors conducted experiments focusing on "current density" and "water viscosity," which are factors that significantly affect the above mechanism. They found that the efficiency of the water electrolysis reaction can be improved by reducing the amount of water passing when the value X, which is the product of "current density" and "water viscosity," is 1.3 or greater. The water electrolysis system 100 was invented based on this finding.

[0043] The water electrolysis system according to the present disclosure has been described above using one embodiment. The water electrolysis system according to the present disclosure can improve the water electrolysis reaction efficiency in a high current density region. Furthermore, since the water electrolysis reaction efficiency can be improved in a high current density region by reducing the amount of water flow, a large-scale water supply device is not required. [Example]

[0044] The present disclosure will now be further described using examples.

[0045] A water electrolysis system was constructed following the example shown in Figure 1. Focusing on the value X, which is the product of "current density" and "water viscosity," the relationship between the rate of decrease in water flow rate and the amount of voltage drop was investigated. The results are shown in Tables 1 to 4 and Figures 3 to 6. The rate of decrease in water flow rate is based on the stoichiometric ratio, and a 100% decrease rate represents the theoretical value of the water flow rate at which the target current density is obtained (stoichiometric ratio = 1). Furthermore, a higher voltage drop indicates improved water electrolysis reaction efficiency.

[0046] [Table 1]

[0047] [Table 2]

[0048] [Table 3]

[0049] [Table 4]

[0050] The results of Examples 1 to 7 show that when the value X is 2.0 or more, reducing the water flow rate from 33% to 90% tends to increase the amount of voltage drop, and reducing the water flow rate from 66% to 90% tends to increase the amount of voltage drop even more. Furthermore, the results of Examples 8 to 11 show that when the value X is 1·3 or more and less than 2.0, reducing the water flow rate from 50% to 85% tends to increase the amount of voltage drop.

[0051] From the above, it is thought that when the value X is 1.3 or more, reducing the amount of water flow increases the amount of voltage drop, and therefore when the value X is 1.3 or more, it is thought that reducing the amount of water flow improves the efficiency of the water electrolysis reaction. [Explanation of symbols]

[0052] 10 Water electrolysis equipment 20 Oxygen electrode side piping section 21 Water supply equipment 22 Water supply channel 23 Oxygen electrode side gas-liquid separator 24 Water discharge channel 25 Circulation flow path 26 Discharge flow path 30 Hydrogen electrode side piping section 31 Hydrogen electrode side gas-liquid separator 32 Hydrogen discharge channel 33 Hydrogen Tank 34 Hydrogen supply channel 40 Power supply 50 Control device 100 Water Electrolysis System

Claims

1. a water electrolysis device that performs water electrolysis; a water supply device that supplies water to the water electrolysis device; a power source that supplies current to the water electrolysis device; a control device that adjusts the amount of water supplied from the water supply device to the water electrolysis device, The control device controls the current density (A / cm ２ ) and the viscosity (mPa s) of the water supplied to the water electrolysis apparatus, When the value X is 1.3 or more, the amount of water supplied from the water supply device to the water electrolysis device is reduced based on the amount of water supplied at the time when the current density (A / cm 2 ) and the water temperature for calculating the viscosity of the water are measured. Water electrolysis system.

2. 2. The water electrolysis system according to claim 1, wherein the control device reduces the amount of water supplied from the water supply device to the water electrolysis device by 50% to 85% of the amount of water supplied at the time when the current density (A / cm 2 ) and the water temperature for calculating the water viscosity are measured when the value X is 1.3 or greater and less than 2.0, and reduces the amount of water supplied from the water supply device to the water electrolysis device by 33% to 90% of the amount of water supplied at the time when the current density (A / cm 2 ) and the water temperature for calculating the water viscosity are measured when the value X is 2.0 or greater.

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