Carbon Dioxide Electrolysis Apparatus and Carbon Dioxide Electrolysis Method

This application is based upon and claims the benefits of priority from the prior Japanese Patent Application No. 2024-082760 filed on May 21, 2024, the entire contents of which are incorporated herein by reference.

Embodiments described herein relate generally to a carbon dioxide electrolysis apparatus and a carbon dioxide electrolysis method.

In recent years, depletion of fossil fuels such as oil and coal is concerned, and expectations for renewable energy that can be continuously used are increasing. For example, solar cells that generate power using solar energy, wind power generation that generates power using wind energy, and the like are known. These have a problem that it is difficult to stably supply electric power because the amount of power generation depends on the weather and natural conditions. Therefore, attempts have been made to stabilize electric power by storing electric power generated by renewable energy in a storage battery. However, there are problems that the storage battery requires cost and that loss occurs at the time of discharging and charging the storage battery.

In this regard, there is known a technique for producing hydrogen (H) from water by performing water electrolysis using electric power generated by renewable energy. Alternatively, a technique is also known in which carbon dioxide (CO) is electrochemically reduced using electric power generated by renewable energy and converted into a chemical substance (chemical energy) such as a carbon compound such as carbon monoxide (CO), formic acid (HCOOH), methanol (CHOH), methane (CH), acetic acid (CHCOOH), ethanol (CHOH), ethane (CH), or ethylene (CH). The storage of these chemical substances in a cylinder, a tank, or the like has an advantage that the storage cost of energy can be reduced and the storage loss is also small as compared with the case of storing electric power (electric energy) in a storage battery.

A carbon dioxide electrolysis apparatus that electrochemically reduces carbon dioxide has a stacked structure in which a plurality of electrolysis cells are stacked. When carbon dioxide gas is supplied to the cathode electrode of the electrolysis cell and a current is supplied to the electrolysis cell, carbon dioxide is electrolyzed and reduced to generate carbon monoxide. A cathode flow path serving as a flow path of carbon dioxide gas is adjacent to the cathode electrode, and the carbon dioxide gas is mixed with the electrolytic solution and flows through the cathode flow path while being in contact with the cathode electrode. An anode flow path serving as a flow path of an electrolytic solution is adjacent to the anode electrode, and the electrolytic solution flows through the anode flow path while being in contact with the anode electrode.

Among them, the electrolytic reaction formulas in the cathode electrode are represented by the following Formulas (1) and (2).

A case of operating at a current density at which a theoretical carbon dioxide gas concentration to be described later in the cathode fluid is 100% is considered. In this case, the electrolytic reaction represented by the above-described Formula (1) can proceed. As a result, the generation amount of carbonate ions (CO) increases, and a salt can be precipitated on the inlet side of the cathode flow path. There is a possibility that the precipitated salt could block the cathode flow path.

Meanwhile, when the concentration of the carbon dioxide gas in the cathode fluid is small, the operation mode of the carbon dioxide electrolysis apparatus is the water electrolysis mode. In this case, the electrolytic reaction represented by the above-described Formula (2) can proceed. When the hydrogen gas generated by Formula (2) cross-leaks to the anode electrode, active oxygen species containing OH radicals (·OH) and the like are generated. When the active oxygen species are, for example, OH radicals, the active oxygen species are produced on the outlet side of the cathode electrode as shown in the following Formula (3), and cause deterioration of the electrolyte membrane.

The carbon dioxide electrolysis apparatus according to the embodiment is an apparatus that performs electrolysis of carbon dioxide gas. A carbon dioxide electrolysis apparatus includes an electrolysis cell including a cathode electrode to which carbon dioxide gas is supplied, an anode electrode, and an electrolyte membrane interposed between the cathode electrode and the anode electrode, a current supply unit that supplies a current to the electrolysis cell, and a control unit. A value obtained by dividing the current supplied to the electrolysis cell by the planar effective area of the electrolysis cell is defined as a current density. The control unit controls the current supply unit such that the current density is 10 mA/cmto 1,000 mA/cm.

The carbon dioxide electrolysis apparatus according to the embodiment is an apparatus that performs electrolysis of carbon dioxide gas. A carbon dioxide electrolysis apparatus includes: an electrolysis cell including a cathode electrode to which carbon dioxide gas is supplied, an anode electrode, and an electrolyte membrane interposed between the cathode electrode and the anode electrode; a carbon dioxide gas flow rate adjustment unit that adjusts a supply flow rate of the carbon dioxide gas to the cathode electrode; and a control unit. A value obtained by dividing the current supplied to the electrolysis cell by the planar effective area of the electrolysis cell is defined as a current density. The minimum flow rate of the carbon dioxide gas when the total amount of electricity per unit time at a predetermined current density is used in the electrolytic reaction from carbon dioxide to carbon monoxide is defined as a theoretical carbon dioxide gas flow rate, and a ratio of a supply flow rate of the carbon dioxide gas to the theoretical carbon dioxide gas flow rate is defined as a flow rate ratio. The control unit controls the carbon dioxide gas flow rate adjustment unit such that the flow rate ratio is 50% to 500%.

The carbon dioxide electrolysis method according to the embodiment is a method using a carbon dioxide electrolysis apparatus that performs electrolysis of carbon dioxide gas and includes an electrolysis cell including a cathode electrode to which carbon dioxide gas is supplied, an anode electrode, and an electrolyte membrane interposed between the cathode electrode and the anode electrode. A carbon dioxide electrolysis method includes: a step of supplying carbon dioxide gas to a cathode electrode; a step of supplying a current to an electrolysis cell; and a step of increasing an output of the carbon dioxide electrolysis apparatus. A value obtained by dividing the current supplied to the electrolysis cell by the planar effective area of the electrolysis cell is defined as a current density. In the step of increasing the output, the current value of the current is adjusted such that the current density is 10 mA/cmto 1,000 mA/cm.

The carbon dioxide electrolysis method according to the embodiment is a method using a carbon dioxide electrolysis apparatus that performs electrolysis of carbon dioxide gas and includes an electrolysis cell including a cathode electrode to which carbon dioxide gas is supplied, an anode electrode, and an electrolyte membrane interposed between the cathode electrode and the anode electrode. A carbon dioxide electrolysis method includes: a step of supplying carbon dioxide gas to a cathode electrode; a step of supplying a current to an electrolysis cell; and a step of increasing an output of the carbon dioxide electrolysis apparatus. A value obtained by dividing the current supplied to the electrolysis cell by the planar effective area of the electrolysis cell is defined as a current density. The minimum flow rate of the carbon dioxide gas when the total amount of electricity per unit time at a predetermined current density is used in the electrolytic reaction from carbon dioxide to carbon monoxide is defined as a theoretical carbon dioxide gas flow rate, and the ratio of the supply flow rate of the carbon dioxide gas to the theoretical carbon dioxide gas flow rate is defined as a flow rate ratio. In the step of increasing the output, the supply flow rate of the carbon dioxide gas is adjusted such that the flow rate ratio is 50% to 500%.

Hereinafter, a carbon dioxide electrolysis apparatus and a carbon dioxide electrolysis method according to the present embodiment will be described with reference to the drawings.

As shown in, a carbon dioxide electrolysis apparatusincludes a carbon dioxide electrolysis unit, an electrolytic solution supply unit, a current supply unit, a carbon dioxide gas supply unit, a carbon dioxide gas flow rate adjustment unit, and a control unit. The carbon dioxide electrolysis apparatusperforms electrolysis of carbon dioxide gas supplied to a cathode electrodeto be described later.

As shown in, the carbon dioxide electrolysis unitincludes one or more electrolysis cells. More specifically, carbon dioxide electrolysis unitincludes a pair of current collector plates, a plurality of electrolysis cellsstacked between the pair of current collector plates, and a plurality of separatorsalternately stacked with the electrolysis cells. The electrolysis cell, the separator, and the current collector plateare fastened and pressed by a pair of fastening plates (not shown).

The electrolysis cellincludes the cathode electrode, an anode electrode, and an electrolyte membraneinterposed between the cathode electrodeand the anode electrode. The cathode electrodeis in contact with the electrolyte membraneand the separator. A cathode gas and an electrolytic solution may be supplied to the cathode electrode. A cathode flow paththrough which a cathode gas and an electrolytic solution flow is formed on a surface of the separatorin contact with the cathode electrode. The anode electrodeis in contact with the electrolyte membraneand the separator. An electrolytic solution is supplied to the anode electrode. An anode flow paththrough which an electrolytic solution flows is formed on a surface of the separatorin contact with the anode electrode.

The cathode electrodeand the anode electrodemay be supplied with an electrolytic solution from the electrolytic solution supply unit. The electrolytic solution may be, for example, an aqueous solution of an electrolyte containing potassium element in the composition. Examples of the electrolytic solution include a potassium hydroxide (KOH) aqueous solution, a potassium hydrogen carbonate (KHCO) aqueous solution, and a potassium carbonate (KCO) aqueous solution. The electrolytic solution supplied to the cathode electrodeand the electrolytic solution supplied to the anode electrodemay be the same or different. A cathode gas containing carbon dioxide gas is supplied from the carbon dioxide gas supply unitto the cathode electrode. That is, the carbon dioxide gas supplied to the cathode electrodeis mixed with the electrolytic solution. However, the electrolytic solution may not be supplied to the cathode electrode.

The electrolyte membraneis formed of an electrolyte material. Examples of the electrolyte membraneinclude, but are not limited to, an ion exchange membrane or a porous membrane.

As shown in, electrolytic solution supply unitsupplies an electrolytic solution to carbon dioxide electrolysis unit. The electrolytic solution supply unitmay include, for example, a pump (not shown). The electrolytic solution stored in the storage unit (not shown) may be supplied to the carbon dioxide electrolysis unitby driving the pump.

The current supply unitsupplies a current for performing an electrolytic reaction to the carbon dioxide electrolysis unit. The current supply unitis also referred to as a power supply unit. The current supply unitmay be controlled by the control unitto adjust the current supplied to the carbon dioxide electrolysis unit.

The carbon dioxide gas supply unitsupplies carbon dioxide gas to the cathode electrodeof the carbon dioxide electrolysis unit. The carbon dioxide gas supply unitmay include a cylinder of carbon dioxide gas, or may include a tank storing carbon dioxide gas.

The carbon dioxide gas flow rate adjustment unitadjusts the supply flow rate of the carbon dioxide gas to the cathode electrodeof the carbon dioxide electrolysis unit. The carbon dioxide gas flow rate adjustment unitmay include, for example, a flow rate adjusting valve (not shown). The supply flow rate of the carbon dioxide gas may be adjusted by adjusting the opening degree of the flow rate adjusting valve. The carbon dioxide gas flow rate adjustment unitmay be controlled by the control unitto adjust the supply flow rate of the carbon dioxide gas.

The control unitcontrols the current supply unitand the carbon dioxide gas flow rate adjustment unitdescribed above.

For example, the control unitmay control the current supply unitsuch that the current density of the current supplied to the electrolysis cellbecomes 10 mA/cmto 1,000 mA/cm. The current density is a value obtained by dividing the current supplied to the electrolysis cellby the planar effective area of the electrolysis cell. The planar effective area of the electrolysis cellis the planar area of the electrolysis cellin the region through which the current passes in the electrolysis cell, and is the entire planar area of the electrolysis cellwhen the current passes through the entire region of the electrolysis cell. The control unitmay control the current supply unitsuch that the current density supplied to the electrolysis cellbecomes 100 mA/cmto 1,000 mA/cm.

For example, the control unitmay control the carbon dioxide gas flow rate adjustment unitsuch that the supply flow rate of the carbon dioxide gas is 50% to 500% of the theoretical carbon dioxide gas flow rate. The theoretical carbon dioxide gas flow rate is defined as the minimum flow rate of carbon dioxide gas when the total amount of electricity per unit time at a predetermined current density is used in the electrolytic reaction from carbon dioxide to carbon monoxide. The amount of electricity per unit time is the current. The ratio of the supply flow rate of the carbon dioxide gas to the theoretical carbon dioxide gas flow rate is defined as a flow rate ratio. The flow rate ratio is expressed by supply flow rate of carbon dioxide gas/theoretical carbon dioxide gas flow rate. As an example, a case where the theoretical carbon dioxide gas flow rate when the current density is 1,000 mA/cmis 10 Nm/h is considered. In this case, the supply flow rate of the carbon dioxide gas when the flow rate ratio at the current density of 1,000 mA/cmis 100% is 10 Nm/h, and the supply flow rate of the carbon dioxide gas when the flow rate ratio is 200% is 20 Nm/h. The control unitmay control the carbon dioxide gas flow rate adjustment unitsuch that the flow rate ratio is 100% to 200%.

A carbon dioxide electrolysis method using the carbon dioxide electrolysis apparatus according to the present embodiment configured as described above will be described with reference to.

First, when carbon dioxide gas electrolysis is performed in the carbon dioxide electrolysis apparatusshown in, a current value to be supplied to the electrolysis cellby the current supply unitis set (step S). Specifically, the current value is set in the control unitto obtain a desired current density. For example, the current value may be set such that the current density is 10 mA/cmto 1,000 mA/cm.

Subsequently, carbon dioxide gas is supplied from the carbon dioxide gas supply unitto the carbon dioxide electrolysis unit(step S). At this time, the supply flow rate of the carbon dioxide gas may be smaller than the supply flow rate of the carbon dioxide gas in step Sto be described later.

The carbon dioxide gas is supplied to the cathode electrodeof the electrolysis cellas cathode gas. The cathode gas is mixed with the electrolytic solution, and flows through the cathode flow pathformed in the separatorwhile being in contact with the cathode electrode. The electrolytic solution supplied to the anode electrodeflows through the anode flow pathformed in the separatorwhile being in contact with the anode electrode.

In step S, an inert gas may be mixed with the carbon dioxide gas. Carbon dioxide gas mixed with an inert gas may be supplied to the cathode electrode.

In step S, the electrolytic solution is supplied from electrolytic solution supply unitto the cathode electrodeand the anode electrodeof the electrolysis cell. In step S, after the supply of the carbon dioxide gas to the electrolysis cellis started, the concentration and the flow rate of the carbon dioxide gas supplied from the carbon dioxide gas supply unitto the electrolysis cellmay be measured. The cooling system may be activated after a lapse of a predetermined time from the start of the supply of the carbon dioxide gas. During a period from the start of the supply of the carbon dioxide gas to the activation of the cooling system, a warm-up operation of continuously supplying the carbon dioxide gas to the electrolysis cellmay be performed.

Next, a current is supplied from the current supply unitto the carbon dioxide electrolysis unit(step S). In the cathode electrode, the electrolytic reaction represented by the above-described Formula (1) is performed. The current supply unitsupplies a current at the current value set in the above-described step S. When the current is supplied to the electrolysis cell, electrolysis starts. After the electrolysis starts, it is confirmed that the operation is stabilized. At this time, the current value may be smaller than the current value in step Sto be described later.

Thereafter, the output of the carbon dioxide electrolysis apparatusis increased (step S). In this case, the supply flow rate of the carbon dioxide gas to the carbon dioxide electrolysis unitmay be increased.

The carbon dioxide gas flow rate adjustment unitmay adjust the supply flow rate of the carbon dioxide gas to a desired flow rate by the control unit. For example, the supply flow rate of the carbon dioxide gas may be adjusted such that the flow rate ratio is 50% to 500%.

In step S, the current value supplied to the carbon dioxide electrolysis unitmay be increased. In this case, the current value may be increased to the value set in the above-described step S. For example, the current value in step Smay be adjusted such that the current density is 10 mA/cmto 1,000 mA/cm.

In such step S, the production amount of carbon monoxide increases, and the output of the carbon dioxide electrolysis apparatusincreases.

Thus, the activation of the carbon dioxide electrolysis apparatusis completed (step S). After activation, operation continues and the production of carbon monoxide continues.

When the operation is performed at a current density at which the concentration of the carbon dioxide gas in the cathode gas is 100% and the flow rate ratio is 100%, the generation amount of carbonate ions (CO) increases, and a salt may precipitate on the inlet side of the cathode flow path. When the electrolytic solution is an aqueous solution of an electrolyte containing potassium element in the composition, potassium carbonate (KCO) is generated as a salt. The precipitated salt may block the cathode flow path.

Meanwhile, when the concentration of the carbon dioxide gas in the cathode gas is small and the voltage between the cathode electrodeand the anode electrodeincreases, the electrolytic reaction represented by the above-described Formula (3) proceeds, and active oxygen species including OH radicals and the like can be generated. For example, when the active oxygen species are OH radicals, the OH radicals may be produced on the outlet side of the cathode flow path, but may react with the material of the electrolyte membraneto degrade the electrolyte membrane.

Therefore, in the present embodiment, the control unitcontrols the current supply unit, and the current density of the current supplied to the electrolysis cellis adjusted to a predetermined range. More specifically, the current density is adjusted to 10 mA/cmto 1,000 mA/cm. As a result, the current density can be reduced. In this case, the electrolytic reaction in the cathode electrodeshown in the above-described Formula (1) can be suppressed, and the generation amount of carbonate ions (CO) can be reduced. Therefore, the precipitation amount of salt can be reduced, and blockage of the cathode flow pathcan be suppressed.

More specifically, by setting the current density to 1,000 mA/cmor less, the precipitation amount of salt can be effectively reduced. Meanwhile, by setting the current density to 10 mA/cmor more, the electrolytic reaction (electrolytic reaction Formula (1)) in the cathode electrodecan proceed as shown inwhile reducing the precipitation amount of salt, and the production amount of carbon monoxide can be secured. The cell voltage at a current density of 10 mA/cmis referred to as a theoretical voltage. The theoretical voltage is the minimum voltage for allowing the electrolytic reaction to proceed. A region where the cell voltage is larger than the theoretical voltage is referred to as an overvoltage region.

As shown in, by increasing the voltage (cell voltage) between the cathode electrodeand the anode electrode, the electrolytic reaction Formula (1) becomes the main reaction, and the production amount of carbon monoxide can be increased. When the current density reaches 1,000 mA/cm, the production amount of carbon monoxide does not increase even when the cell voltage is further increased. This current density is referred to as a limit current density.

In addition, as described above, by adjusting the current density to 10 mA/cmto 1,000 mA/cm, the generation amount of OH radicals can be reduced. That is, by reducing the current density, the cell voltage can be reduced, and an increase in overvoltage can be suppressed. Since the generation amount of OH radicals increases as the overvoltage increases, the generation amount of OH radicals can be effectively reduced by decreasing the current density. Therefore, deterioration of the electrolyte membranedue to OH radicals can be suppressed. By setting the current density to 1,000 mA/cmor less, an increase in overvoltage can be effectively suppressed.

In particular, by decreasing the current density in the range of 10 mA/cmto 1,000 mA/cm, an increase in overvoltage can be more effectively suppressed. For example, the current density may be set to 10 mA/cmto 100 mA/cm. Meanwhile, by setting the current density to 10 mA/cmor more, an electrolytic reaction in the cathode electrodecan be performed while suppressing an increase in overvoltage, and a production amount of carbon monoxide can be secured.

Meanwhile, the slope (increase rate) of the current density is larger in the range of 100 mA/cmto 1,000 mA/cmthan in the range of 10 mA/cmto 100 mA/cm. In consideration of more effectively suppressing an increase in overvoltage, the current density is desirably around 100 mA/cm.

During the operation of the carbon dioxide electrolysis apparatus, the control unitcontrols the carbon dioxide gas flow rate adjustment unitto adjust the supply flow rate of the carbon dioxide gas supplied to the electrolysis cellto a predetermined range. More specifically, the flow rate ratio is adjusted to be 50% to 500%. As a result, the supply flow rate of the carbon dioxide gas can be appropriately adjusted. In this case, the supply flow rate of the carbon dioxide gas flowing through the cathode flow pathcan be secured, and the salt precipitated in the cathode flow pathcan be blown off. Therefore, it is possible to suppress blockage of the cathode flow path.

More specifically, by setting the flow rate ratio to 50% or more, the salt precipitated in the cathode flow pathcan be effectively blown off. Meanwhile, by setting the flow rate ratio to 500% or less, it is possible to suppress the discharge amount of the carbon dioxide gas, which is not subjected to the electrolytic reaction, from the cathode flow pathwhile blowing off the salt. The flow rate ratio is more desirably 100% to 200% in consideration of Faraday efficiency. As shown in, by setting the flow rate ratio to 100% or more, the electrolytic reaction can be performed in a region where the Faraday efficiency is high. By setting the flow rate ratio to 200% or less, it is possible to suppress a decrease in the production efficiency of carbon monoxide. That is, when the flow rate ratio reaches 200%, the Faraday efficiency does not increase and the production amount of carbon monoxide does not increase even when the supply flow rate of the carbon dioxide gas is further increased. Therefore, it is effective to set the flow rate ratio to 200% or less. Faraday efficiency refers to the percentage of partial current that contributed to the production of carbon monoxide relative to the total current. By setting the flow rate ratio to 200% or less, the supply flow rate of the carbon dioxide gas can be increased to more effectively blow off the salt precipitated in the cathode flow path, and the discharge amount of the carbon dioxide gas which is not subjected to the electrolytic reaction from the cathode flow pathcan be further suppressed.

In addition, as described above, the flow rate ratio is adjusted to 50% to 500%, and accordingly, the generation amount of OH radicals can be reduced. That is, by appropriately adjusting the supply flow rate of the carbon dioxide gas, the supply flow rate of the carbon dioxide gas to the cathode electrodecan be secured, and the shortage of the carbon dioxide gas in the cathode electrodecan be suppressed. Therefore, the generation amount of OH radicals can be reduced, and deterioration of the electrolyte membranedue to the OH radicals can be suppressed. By setting the flow rate ratio to 50% or more, the generation amount of OH radicals can be effectively reduced. Meanwhile, by setting the flow rate ratio to 500% or less, it is possible to suppress the discharge amount of the carbon dioxide gas, which is not subjected to the electrolytic reaction, from the cathode flow pathwhile reducing the generation amount of OH radicals. As described above, when the flow rate ratio is adjusted to be 100% to 200%, the supply flow rate of the carbon dioxide gas is increased, and accordingly, the generation amount of OH radicals can be more effectively reduced, and the discharge amount of the carbon dioxide gas, which is not subjected to the electrolytic reaction, from the cathode flow pathcan be further suppressed.