Patent Publication Number: US-2021180197-A1

Title: Water electrolysis system and control method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Korean Patent Application No. 10-2019-0166031, filed Dec. 12, 2019, the entire contents of which is incorporated herein for all purposes by this reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a water electrolysis system and a control method thereof, and more particularly, to a system in which electrolytic cells are separated, eliminating the necessity of a membrane, and hydrogen and oxygen are generated alternately in a pair of electrolytic cells through an oxidation/reduction reaction by controlling an application direction of a current, and a control method in which the system is circulated. 
     2. Description of the Related Art 
     Hydrogen, which is very high in energy density and environmentally friendly energy with the highest energy density per unit mass, has come to prominence as a next generation energy source. Methods for producing hydrogen with high energy density include various methods such as fossil fuel reforming, a by-product gas that occurs in industrial processes, biomass gasification, and water electrolysis using renewable energy. 
     Water electrolysis among the hydrogen producing methods is a method of obtaining hydrogen by separating water molecules into hydrogen molecules and oxygen molecules using electricity. Water electrolysis is an environmentally friendly hydrogen producing method and is the most reliable technology among hydrogen producing methods. Water electrolysis is also known as the cheapest hydrogen producing technology which is simple in configuration of a system and stable in operation. 
     A water electrolysis device of a related art includes a membrane as an essential component to produce hydrogen and oxygen in a single electrolytic cell and separate hydrogen and oxygen. Here, since a high-priced membrane is necessary, a price of the water electrolysis device is increased, and when the membrane is operated or pressed at a low load, a cross-over phenomenon that hydrogen and oxygen pass through the membrane occurs. 
     In order to solve the problem, a water electrolysis device using in which electrolytic cells are separated and an auxiliary electrode is used has been developed. However, in the disclosed water electrolysis device, the electrolytic cells are simply separated and an anode continuously produces only hydrogen and a cathode continuously produces oxygen, making it difficult for the water electrolysis device to be commercialized as an automatic circulating system. 
     SUMMARY 
     An object of the present disclosure is to provide a water electrolysis system in which an electrolytic cell is separated, thus eliminating the necessity of a membrane, and hydrogen and oxygen are alternately produced and circulate by controlling a direction of a current applied to the electrolytic cell, and a control method thereof. 
     According to an embodiment of the present disclosure, a water electrolysis system includes a pair of separate electrolytic cells configured to accommodate electrolytic water supplied from an electrolytic water tank and connected to a hydrogen tank and an oxygen tank, a pair of active electrodes including a cathode and an anode, the cathode and the anode being accommodated in the pair of electrolytic cells and connected to electric power by an active electrode lead to electrolyze electrolytic water to produce hydrogen and oxygen, respectively, auxiliary electrodes respectively accommodated in the pair of electrolytic cells and connected to each other by an auxiliary electrode lead to provide electrons to the separated electrolytic cells or receive electrons therefrom, a plurality of sensors configured to measure pressure of hydrogen or oxygen generated in the electrolytic cells and to measure an electrolytic water capacity of the electrolytic cells, and a controller configured to control to selectively discharge a hydrogen gas or oxygen gas upon receiving a measurement value of a sensor, to control to selectively supply electrolytic water to the electrolytic cells from the electrolytic water tank, and to selectively control a current direction of the electric power. 
     The sensor may include a pressure sensor configured to measure a pressure of hydrogen or oxygen generated in each electrolytic cell and an electrolytic water sensor configured to measure an electrolytic water capacity and the pressure sensor and the electrolytic water sensor may be positioned on the electrolytic cells, respectively. 
     The water electrolysis system may further include at least one pipe connected to each electrolytic cell to form a flow path allowing a hydrogen or oxygen gas generated in the active electrodes to be discharged to a hydrogen tank or an oxygen tank and having a gas valve provided at an inlet for discharging a gas from the electrolytic cell to selectively open and close the flow path. 
     The pipe may include a hydrogen pipe connected to the hydrogen tank and having a hydrogen valve and an oxygen pipe connected to the oxygen tank and having an oxygen valve, and the hydrogen valve and the oxygen valve may be controlled to be selectively opened and closed by the controller. 
     The pipe may be configured as a single pipe connected to the electrolytic cells and having a branching point connected to the hydrogen tank or the oxygen tank, and a three-way valve is provided at the branching point. 
     The controller may be configured to control to open the oxygen valve of the electrolytic water accommodating the anode and open the hydrogen valve of the electrolytic water accommodating the cathode to store a gas in the oxygen tank and the hydrogen tank, when a pressure of a gas measured by the pressure sensor reaches a predetermined gas discharge pressure. 
     The predetermined gas discharge pressure may be 20 bar or higher. 
     The controller may control to supply electrolytic water to the electrolytic cell to entirely discharge a remaining gas, when the gas generated in the electrolytic water is discharged and a pressure of the gas measured by the pressure sensor is equal to or lower than a predetermined electrolytic water replenishment pressure. 
     The predetermined electrolytic water replenishment pressure may be 1 bar or lower. 
     The controller may control to open an electrolytic water valve when a capacity of the electrolytic water measured by the electrolytic water sensor is less than or equal to a predetermined capacity, and control to close the electrolytic water valve when the predetermined capacity is reached. 
     When the electrolytic water reaches a predetermined capacity, the controller may control to close both the gas valve and the electrolytic water valve and to apply a current by changing a direction of a current applied from the electric power, and as the direction of the current is reversed, the cathode is changed into the anode and the anode is changed into the cathode and an operation of the system is circulated. 
     In addition, the pair of electrolytic cells may be continuously provided in plurality and operated by the same controller. 
     The electrolytic water may be a NaOH or KOH aqueous solution. 
     According to another embodiment of the present disclosure, a method of controlling a water electrolysis system includes a current applying operation of applying a current to active electrodes in one direction, a gas generating operation of generating hydrogen in the active electrode providing an electron and generating oxygen in the active electrode receiving an electron; a gas storing operation of storing a gas generated in an electrolytic cell, an electrolytic water replenishing operation of replenishing electrolytic water in the electrolytic cell, and a current re-applying operation of re-performing the gas generating operation by applying a current to the active electrode in the other direction. 
     The gas storing operation may include a gas discharging operation of controlling, by a controller, to discharge the gas to a hydrogen tank and an oxygen tank when a pressure of the produced hydrogen or oxygen reaches a predetermined gas discharge pressure, and a discharge stopping operation of blocking the connection between the hydrogen tank, the oxygen tank, and the electrolytic cell to stop discharging of the gas when a pressure of the electrolytic cell is lowered to a predetermined electrolytic water replenishment pressure. 
     The gas discharge pressure may be 20 bar and the electrolytic water replenishment pressure may be 1 bar. 
     The electrolytic water replenishing operation may include: an electrolytic water introducing operation of introducing electrolytic water by controlling, by the controller, to connect the electrolytic cell and the electrolytic water tank, and a current re-application preparing operation of blocking, by the controller, the connection between the electrolytic cell and the electrolytic water and stopping current application when a capacity of the electrolytic water reaches a predetermined capacity. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1A and 1B  are views illustrating a water electrolysis system according to an embodiment of the present disclosure. 
         FIG. 2  is a flowchart of a method of controlling a water electrolysis system according to an embodiment of the present disclosure. 
         FIG. 3  is a view illustrating a current applying operation and a gas generating operation of a water electrolysis system according to an embodiment of the present disclosure. 
         FIG. 4  is a view illustrating a gas storing operation of a water electrolysis system according to an embodiment of the present disclosure. 
         FIGS. 5 and 6  are views illustrating an electrolytic water replenishing operation of a water electrolysis system according to an embodiment of the present disclosure. 
         FIG. 7  is a view illustrating a current re-applying operation of a water electrolysis system according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Specific structural or functional descriptions of the embodiments of the present disclosure disclosed in the specification are exemplified for the purpose of describing the embodiments of the present disclosure only, and the embodiments of the present disclosure may be carried out in various forms and should not be construed to limit the embodiments described herein. 
     The terms used in the application are used to describe specific embodiments only and are not intended to limit the present disclosure. A singular expression includes a plural expression as long as they are clearly distinguished in the context. In the application, it should be understood that the terms such as “comprising”, “including” are intended to express that features, numbers, steps, operations, constituent elements, part, or combinations thereof described in the specification are present and do not exclude existence or additions of one or more other features, numbers, steps, operations, constituent elements, part, or combinations thereof. 
     Unless defined in a different way, all the terms used herein including technical and scientific terms have the same meanings as understood by those skilled in the art to which the present disclosure pertains. Such terms as defined in generally used dictionaries should be construed to have the same meanings as those of the contexts of the related art, and unless clearly defined in the application, they should not be construed to have ideally or excessively formal meanings. 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The same constituent elements in the drawings are denoted by the same reference numerals. 
     The present disclosure relates to a water electrolysis system for producing hydrogen and a control method thereof and relates to a circulation system in which a current is applied to a pair of electrolytic cells  100  which are independently separated to produce oxygen in an active electrode  200  of the electrolytic cell  100  in which an oxidation reaction takes place and produce hydrogen in the active electrode  200  of the electrolytic cell  100  in which a reduction reaction takes place, and here, a direction in which the current is applied is reversed at a specific time point to cause oxidation/reduction reactions to take place conversely. 
       FIGS. 1A and 1B  are views illustrating a water electrolysis system according to an embodiment of the present disclosure. 
     Referring to  FIGS. 1A and 1B , the water electrolysis system according to an embodiment of the present disclosure may include the electrolytic cell  100 , the active electrode  200 , an auxiliary electrode  300 , a sensor, and a controller  400 . The water electrolysis system may further include at least one pipe forming a flow path through which a gas moves. 
     The electrolytic cell  100  may accommodate electrolytic water  101  and the active electrode  200  by which an electrolysis reaction takes place to produce hydrogen or oxygen. The electrolytic cell  100  may be configured as a pair separated independently. In the related art, both the active electrode  200  in which the oxidation reaction takes place and the active electrode  200  in which the reduction reaction takes place are accommodated in one electrolytic cell  100 , but in the present disclosure, the electrolytic cell  100  is separated as a pair and the pair of separated electrolytic cells  100  may accommodate the active electrodes  200  respectively. Since the oxidation or reduction reaction takes place alternately in each active electrode, there is no possibility that gases are mixed, and thus no expensive membrane is required. 
     The electrolytic water  101  supplied from an electrolytic water tank may be accommodated in each electrolytic cell  100 . The electrolytic water  101  may be an alkaline solution such as KOH or NaOH. In addition, each electrolytic cell  100  may be connected to a hydrogen tank for storing hydrogen and an oxygen tank for storing oxygen to store the generated hydrogen or oxygen. 
     The electrolytic cell  100  may have an electrolytic water inlet  140  that receives the electrolytic water  101  from the electrolytic water tank. The electrolytic water  101  is replenished through the electrolytic water inlet  140 , and the electrolytic water inlet  140  may have an electrolytic water valve  141 . The electrolytic water valve  141  may selectively open and close the electrolytic water inlet  140  to control introduction of the electrolytic water  101  from the electrolytic water tank. 
     The active electrode  200  is connected to an electric power P, receives a current, and produces oxygen or hydrogen by electrolyzing the electrolytic water  101 . The active electrode  200  needs to have a low overvoltage and high corrosion resistance when hydrogen or oxygen is generated, and an electrode having low resistance under a condition of the alkaline electrolytic water  101  may be used as the active electrode  200 . 
     The active electrodes  200  accommodated respectively in the electrolytic cells  100  may be connected to the electric power P by an active electrode lead  230 . When a current is applied from the electric power P, one of the active electrodes  200  becomes a positive electrode and the other becomes a negative electrode. The current flows from the positive electrode to the negative electrode, and when a direction of the current is reversed, the positive electrode and negative electrode are interchanged each other. 
     In the present disclosure, the active electrode  200  in which oxygen is generated is defined as an anode  220  and the active electrode  200  in which hydrogen is generated is defined as a cathode  210 . As illustrated in  FIGS. 1A and 1B , the cathode  210  and the anode  220  may be converted to each other according to a direction in which the current is applied. Even the same active electrode  200  may become a cathode  210  or an anode  220  according to the direction in which the current is applied, and hydrogen or oxygen is generated depending on whether the active electrode  200  is the cathode  210  or the anode  220 . 
     The auxiliary electrode  300  may be configured as a pair, and the pair of auxiliary electrodes  300  may be accommodated respectively in the electrolytic cells  100  to provide or receive electrons of the electrolytic water  101  to or from the other electrolytic cell  100 . The auxiliary electrodes  300  are connected to each other by an auxiliary electrode lead  310 . 
     The sensor may perform a sensing function to measure a pressure of a hydrogen or oxygen gas generated in the electrolytic cell  100  and may measure a capacity of the electrolytic water  101 . The sensor may include a pressure sensor  130  provided at the top of the electrolytic cell  100  to measure a pressure of a gas and an electrolytic water sensor  150  provided at the bottom of the electrolytic cell  100  to measure a capacity of the electrolytic water  101 . The pressure sensor  130  and the electrolytic water sensor  150  may be provided at each electrolytic cell  100  to measure a gas pressure generated in each electrolytic cell  100  and a capacity of the electrolytic water  101 . 
     The controller  400  may receive a measurement value from the pressure sensor  130  and control to selectively discharge the hydrogen or oxygen gas according to situations. When the pressure of the gas is higher than or equal to a gas discharge pressure, the controller  400  may control to discharge the hydrogen or oxygen gas, and when the pressure of the gas is lower than an electrolytic water replenishment pressure, the controller  400  may control to replenish the electrolytic water  101  to discharge all the remaining gas. 
     In addition, the controller  400  may receive a measurement value from the electrolytic water sensor  150  and control to supply the electrolytic water  101  from the electrolytic water tank to the electrolytic cell  100 . Here, the electrolytic water valve  141  may be opened to supply the electrolytic water  101  of the electrolytic water tank to the electrolytic cell  100 . 
     In addition, the controller  400  may determine the cathode  210  and the anode  220  by controlling a direction of the current applied from the electric power P to selectively reverse the direction of the current. Details of the function of the controller  400  will be described later. 
     The pipe may be connected to the electrolytic cell  100  to form a flow path in which the hydrogen or oxygen gas generated in the active electrode  200  is discharged to the hydrogen tank or the oxygen tank. Gas valves  111  and  121  selectively opening and closing the flow paths may be provided at inlets through which the gas is discharged to the pipes from the electrolytic cells  100 , respectively. One or a plurality of pipes may be provided at the electrolytic cells  100 . 
     As illustrated in  FIGS. 1A and 1B , the pipe may include a hydrogen pipe  110  connected to a hydrogen tank and an oxygen pipe  120  connected to an oxygen tank. A hydrogen valve  111  may be provided at the hydrogen pipe  110 , and an oxygen valve  121  may be provided at the oxygen pipe  120 . 
     Since the controller  400  may control the direction of the current flowing in the active electrode lead  230  by the electric power P, the controller  400  may selectively open and close the gas valves  111  and  121  depending on whether each active electrode  200  is the cathode  210  or the anode  220 . Details thereof will be described later. 
     Although not shown in the drawings, the electrolytic cell  100  may have only a single pipe, a branching point branched from the pipe to each of the hydrogen tank or oxygen tank, and a three-way valve may be provided at the branching point. The controller  400  may control the three-way valve to selectively connect the pipe to the hydrogen tank or the oxygen tank to form a flow path for the hydrogen gas to move to the hydrogen tank and the oxygen gas to move to the oxygen tank. That is, the controller  400  may control the gas valves  111  and  121  to be connected to the hydrogen tank when the active electrode  200  is the cathode  210  and control the gas valves  111  and  121  to be connected to the oxygen tank when the active electrode  200  is the anode  220 , depending on the direction of the current applied to the active electrode lead  230  from the electric power P. 
       FIG. 2  is a flowchart of a method of controlling a water electrolysis system according to an embodiment of the present disclosure. 
     Referring to  FIG. 2 , the method of controlling a water electrolysis system according to an embodiment of the present disclosure may include a current applying operation S 100 , gas generating operation S 200 , gas storing operation S 300 , electrolytic water replenishing operation S 400 , and a current re-applying operation S 500 . Each operation will be described together with the drawings. 
       FIG. 3  is a view illustrating the current applying operation S 100  and the gas generating operation S 200  of a water electrolysis system according to an embodiment of the present disclosure. 
     Referring to  FIG. 3 , the current applying operation S 100  is an operation in which a current starts to flow in the active electrode lead  230  by the electric power P. Here, the active electrode  200  connected to a positive electrode is the anode  220  in which an oxidation reaction takes place to generate oxygen, and the active electrode  200  connected to a negative electrode is the cathode  210  in which a reduction reaction takes place to generate hydrogen. A current flows from the cathode  210  to the anode  220 . That is, the active electrode  200  that receives electrons becomes the cathode  210 , and the active electrode  200  that provides electrons becomes the anode  220 . Here, the reactions taking place in the cathode  210  and the anode  220  are as follows. 
     [Reaction Formula] 
       Anode: 4OH−→2H 2 0+4 e   − +O 2  
 
       Cathode: 4H 2 O+2 e   − →4OH − +2H 2  
 
     The current applying operation S 100  is performed in a state where the hydrogen valve  111 , the oxygen valve  121 , and the electrolytic water valve  141  are all closed. 
     The gas generating operation S 200  is an operation in which hydrogen or oxygen starts to be generated in each active electrode  200  when a current is applied to the active electrode lead by the electric power P in the preceding current applying operation S 100 . 
     As the reaction of the reaction formula takes place, hydrogen is generated in the cathode  210  and oxygen is generated in the anode  220 . The gas generating operation S 200  is performed in a state where both the hydrogen valve  111  and the oxygen valve  121  are closed. Hydrogen or oxygen is generated in each electrolytic cell  100  in the gas generating operation S 200 , and here, since the hydrogen valve  111  and the oxygen valve  121  are both in a closed state, gases are collected at the top of the electrolytic cell  100 . 
       FIG. 4  is a view illustrating the gas storing operation S 300  of a water electrolysis system according to an embodiment of the present disclosure. 
     The pressure sensor  130  measures a gas pressure of the electrolytic cell  100  and transmits the measured gas pressure to the controller  400 . The controller  400  discharges and stores the gas when the pressure of the gas measured by the pressure sensor  130  reaches a predetermined gas discharge pressure. 
     Specifically, in the case of the electrolytic cell  100  in which the cathode  210  is accommodated, the controller  400  controls to open the hydrogen valve  111  provided at the hydrogen pipe  110  to allow the hydrogen gas to move to the hydrogen tank. Here, the oxygen valve  121  at the oxygen pipe  120  is maintained in the closed state, the hydrogen gas may move only to the hydrogen tank through the hydrogen pipe  110 . 
     In addition, in the case of the electrolytic cell  100  in which the anode  220  is accommodated, the controller  400  controls to open the oxygen valve  121  provided at the oxygen pipe  120  to allow the oxygen gas to move to the hydrogen tank. Here, since the hydrogen valve  111  of the hydrogen pipe  110  is maintained in the closed, the oxygen gas may move only to the oxygen tank through the oxygen pipe  120 . 
     The gas discharge pressure may be set to an appropriate pressure at which the hydrogen or oxygen gas being produced may be discharged through the hydrogen pipe  110  or the oxygen pipe  120 , respectively, and may be preferably set to a pressure of 20 bar or higher. 
     In the gas storing operation S 300 , the current is continuously applied, and thus the oxidation or reduction reaction takes place continuously in the active electrode. In addition, in the gas storing operation S 300 , the water electrolytic valve is maintained in a closed state. 
       FIGS. 5 and 6  are views illustrating the electrolytic water replenishing operation S 400  of a water electrolysis system according to an embodiment of the present disclosure. 
     The electrolytic water replenishing operation S 400  is an operation of replenishing the electrolytic water  101  in the electrolytic cell  100 . 
     When the gas escapes in the gas storing operation S 300 , the pressure of the electrolytic cell  100  is gradually lowered. The controller  400  controls both the hydrogen valve  111  and the oxygen valve  121  to be closed in order to stop discharging of the gas when the pressure of the gas measured by the pressure sensor  130  reaches a predetermined electrolytic water replenishment pressure. The electrolytic water replenishment pressure may be set to a pressure of a state in which gas is sufficiently discharged, and may be preferably set to 1 bar or lower. 
     The electrolytic water sensor  150  may continuously measure a capacity of the electrolytic water  101  accommodated in the electrolytic cell  100  and transmit the measured capacity to the controller  400 . The controller  400  may control to open the electrolytic water valve  141  if the capacity of the electrolytic water  101  measured by the electrolytic water sensor  150  is less than or equal to a predetermined reference capacity. When the electrolytic water valve  141  is opened, electrolytic water  101  flows into the electrolytic cell  100  and a level of the electrolytic water  101  rises. Referring to  FIG. 5 , all of the remaining hydrogen or oxygen gas is discharged as the level of the electrolytic water  101  rises. 
     Referring to  FIG. 6 , when the hydrogen and oxygen gases are discharged and the electrolytic water  101  is sufficiently replenished to reach a predetermined capacity or greater, the controller  400  controls the electrolytic water valve  141  to be closed again. In addition, the controller  400  may close both the oxygen valve  121  and the hydrogen valve  111  to prevent the gases from being mixed and control to temporarily stop current application. That is, an initial state before applying the electric power P is achieved. 
       FIG. 7  is a view illustrating the current re-applying operation S 500  of a water electrolysis system according to an embodiment of the present disclosure. 
     The current re-applying operation S 500  is an operation of applying a current again by reversing the direction of the current after the electrolytic water  101  is replenished. As the direction of the current is reversed, the cathode  210  is changed into the anode  220 , and the anode  220  is changed into the cathode  210 . 
     Referring to  FIG. 7 , the current is applied from the electric power P by changing the direction of the electrode. In other words, the direction of the current is opposite to that of the previous cycle. Accordingly, the anode  220  in which the oxidation reaction occurred to produce oxygen is changed into the anode  210 , and the cathode  210  in which the reduction reaction occurred to produce hydrogen is changed into the anode  220 . In addition, the direction of electrons flowing in the auxiliary electrode lead  310  is also reversed. After the current re-applying operation S 500 , the gas generating operation S 200  is performed again, circulating the cycle to produce hydrogen or oxygen. 
     As the water electrolysis system according to an embodiment of the present disclosure, the system in which hydrogen or oxygen takes place in the pair of electrolytic cells  100  is used as an example, but here, it may be configured as a module type in which the pair of electrolytic cell  100  are continuously provided. In the case of the module type, a large amount of hydrogen may be produced at a time by applying a current. 
     In the water electrolysis system and the control method thereof of the present disclosure, since the cathode in which hydrogen is produced and the anode in which oxygen is produced are accommodated in the independent and separate electrolytic cells, there is no possibility that the gases are mixed with each other, thereby eliminating the necessity of a membrane, and thus hydrogen may be produced economically and efficiently. In addition, since the water electrolysis system is configured as a circulating system by controlling an application direction of a current, hydrogen and oxygen may be continuously produced, facilitating the operation of the system. 
     Although the present disclosure has been illustrated and described with respect to specific embodiments, it will be apparent to those having ordinary skill in the art that the present disclosure may be variously modified and altered without departing from the spirit and scope of the present disclosure as defined by the following claims.