Patent Application
20240295042
The present invention relates to a solid-electrolyte electrolytic device and a maintenance method thereof.
Studies on carbon dioxide reduction using electrical energy are widely conducted throughout the world. In a solid-electrolyte electrolytic device in which carbon dioxide is reduced, generally, carbon dioxide is dissolved in an aqueous solution containing an electrolyte to be supplied to a cathode, while an anode is supplied with the aqueous solution containing the electrolyte. Generally, the electrolyte for exchanging ions is provided between the cathode and the anode, and an ion exchange membrane may be used as a member of the electrolyte.
Due to its structure, the ion exchange membrane has a property of permeating not only ions but also electrolytes. Thus, for example, a phenomenon in which a small amount of the electrolyte supplied to the anode permeates through the ion exchange membrane and precipitates as salts in the vicinity of the cathode may often be found. The precipitated salts have adverse effects such as hindering the supply of carbon dioxide to the cathode catalyst, and cause a decrease in the electrolytic performance such as current density and selectivity. As a method for removing the precipitated salts, a maintenance method, in which a rinse liquid such as pure water is introduced to the cathode on which the salts are precipitated to directly wash away the salts, has been proposed (Patent Literatures 1 to 4).
However, in the maintenance methods disclosed in Patent Literatures 1 to 4 in which a rinse liquid is introduced to the cathode, the supply of CO2 is interrupted during the maintenance, and thus, there is a problem in that the CO2 reduction reaction has to be stopped during the interruption. In addition, the precipitated salts are directly washed away, and thus, the cathode catalyst supported on the cathode may be caused to flow out, leading to a problem of causing deterioration of the device. Therefore, the present disclosure provides a technology related to a solid-electrolyte electrolytic device which is excellent in operation rate and device life.
As a result of intensive studies aimed at achieving the above object, the present inventors have found that by diluting or substituting the electrolytic solution (electrolyte solution) supplied to the electrode other than the electrode on which the salts are precipitated with a specific recovery liquid, the salts precipitated on the electrode dissolve and move to the other electrode side. The present inventors have found that according to this method, it is possible to remove the salts precipitated on the electrode without directly supplying the rinse liquid to the precipitated salts and the electrode and without stopping the operation (CO2 reduction reaction) of the solid-electrolyte electrolytic device, and that the electrolytic performance of the solid-electrolyte electrolytic device is recovered, and thus have completed the technology according to the present disclosure. That is, the technology according to the present disclosure is as follows.
The embodiment according to the present disclosure is a solid-electrolyte electrolytic device including: a cathode that performs a reduction reaction; an anode that constitutes one pair of electrodes together with the cathode; an electrolytic solution that is in contact with the anode and supports an oxidation-reduction reaction; a solid electrolyte that is disposed between the cathode and the anode; and a refresh unit that supplies a recovery liquid for diluting or replacing the electrolytic solution to the anode to remove a salt precipitated between the cathode and the solid electrolyte, the recovery liquid being a solution with a lower concentration of a cation identical to a cation included in the salt than the electrolytic solution.
According to the present disclosure, it is possible to provide a technology related to a solid-electrolyte electrolytic device that is excellent in operation rate and device life without stopping a CO2 reduction reaction.
Hereinafter, the solid-electrolyte electrolytic device and the method for removing the salts precipitated on the electrodes thereof according to the present disclosure will be specifically described with reference to
First, a solid-electrolyte electrolytic device (also referred to as an electrolytic cell or an electrolytic module) according to the present embodiment will be described with reference to
Although the solid-electrolyte electrolytic device 100 shown in
In addition, the solid-electrolyte electrolytic device 100 according to the present embodiment includes a refresh unit that removes the salts precipitated between the cathode 101 and the solid electrolyte 103 by supplying a recovery liquid which dilutes or replaces the electrolytic solution to the anode 102.
Here, the refresh unit in the technology according to the present disclosure will be described with reference to
The reduction reaction at the cathode 101 varies depending on the type of the solid electrolyte 103 used in the solid-electrolyte electrolytic device 100. When a cation exchange membrane is used as the solid electrolyte 103, reduction reactions represented by the following formulas (1) and (2) occur, and when an anion exchange membrane is used as the solid electrolyte, reduction reactions represented by the following formulas (3) and (4) occur.
[Formula 1]
CO2+2H++2e−→CO+H2O (1)
2H++2e−→H2 (2)
H2O+CO2+2e−→CO+2OH (3)
2H2O+2e−→H2+2OH− (4)
The cathode 101 is a gas diffusion electrode including a gas diffusion layer. The gas diffusion layer includes, for example, a porous material such as paper, non-woven fabric or mesh made of conductive carbon, metal or the like. Examples of the electrode material of the cathode 101 may include graphite carbon, glassy carbon, titanium, and SUS. The cathode catalyst of the cathode 101, which is capable of reducing CO2 (carbon dioxide) to CO (carbon monoxide), includes, for example, a metal selected from silver, gold, copper, or a combination thereof. More specifically, the catalyst includes, for example, gold, a gold alloy, silver, a silver alloy, copper, a copper alloy, or a mixed metal including any one or more thereof. The type of the catalyst is not particularly limited as long as it has a function as a catalyst, and can be determined in consideration of corrosion resistance and the like. For example, when the catalyst does not contain an amphoteric metal such as Al, Sn, or Zn, the corrosion resistance can be improved. The catalyst can be supported on the cathode 101 (or the electrode material) by performing a known method such as vapor deposition, precipitation, adsorption, deposition, adhesion, welding, physical mixing, or spraying.
Here, as shown in
Here, the reason why the use of the solid base 107 increases the efficiency is presumed to be due to the following action mechanism. First, for example, when a low-concentration CO2 gas with a concentration of 10% to 20%, such as an exhaust gas from a factory, is supplied to the solid-electrolyte electrolytic device 100, the CO2 is unlikely to be adsorbed on the surface of the cathode 101 because of the low concentration of CO2. Therefore, as shown in
The oxidation reaction at the anode 102 varies depending on the type of the solid electrolyte 103 used in the solid-electrolyte electrolytic device 100. When a cation exchange membrane is used as the solid electrolyte 103, an oxidation reaction represented by the following formula (5) occurs, and when an anion exchange membrane is used as the solid electrolyte 103, an oxidation reaction represented by the following formula (6) occurs.
[Formula 2]
2H2O→O2+4H++4e− (5)
4OH−→O2+2H2O+4e− (6)
The anode 102 is a gas diffusion electrode including a gas diffusion layer. The gas diffusion layer includes, for example, a porous material such as paper, non-woven fabric or mesh made of a conductive material such as carbon or metal. Examples of the electrode material of the anode 102 include Ir, IrOx, Ru, RuO2, Rh, RhOx, Co, CoOx, Cu, CuOx, Fe, FeOx, FeOOH, FeMn, Ni, NiOx, NiOOH, NiCo, NiCe, NiC, NiFe, NiCeCoCe, NiLa, NiMoFe, NiSn, NiZn, SUS, Au, and Pt.
The solid electrolyte 103 is interposed at least partially in contact with the cathode 101 and the anode 102 between the cathode 101 and the anode 102. Here, the solid electrolyte 103 is not particularly limited to a polymer membrane, but a cation exchange membrane or an anion exchange membrane is preferable, and an anion exchange membrane is more preferable. As the cation exchange membrane, for example, a strongly acidic cation exchange membrane in which a sulfone group is introduced into a fluorine resin matrix, Nafion 117, Nafion115, Nafion212 or Nafion 350 (manufactured by DuPont de Nemours, Inc.), a strongly acidic cation exchange membrane in which a sulfone group is introduced into a styrene-divinylbenzene copolymer matrix, or NEOSEPTA CMX (manufactured by Tokuyama Soda Co., Ltd.), etc. can be used. Examples of the anion exchange membrane include anion exchange membranes having a quaternary ammonium group, a primary amino group, a secondary amino group, or a tertiary amino group, or having a plurality of these ion exchange groups which are mixed together. Specific examples thereof can include NEOSEPTA (registered trademark) ASE, AHA, AMX, ACS, AFN, and AFX (manufactured by Tokuyama Corporation), Selemion (registered trademark) AMV, AMT, DSV, AAV, ASV, AHO, AHT, and APS4 (manufactured by Asahi Glass Co., Ltd.).
Examples of the material of the current collector plate 104 include metal materials such as copper (Cu), nickel (Ni), stainless steel (SUS), nickel-plated steel, and brass, and among these, copper is preferable in terms of ease of processing and cost. When the current collector plate 104 is made of a metal material, examples of the shape of the negative electrode current collector plate include a metal foil, a metal plate, a metal thin film, an expanded metal, a punching metal, and a foamed metal.
Here, as shown in
When the cathode 101 has a role of transmitting electrons, the current collector plate 104 is not necessarily required.
The support plate 105 serves to support the anode. Therefore, the required rigidity of the support plate 105 also varies depending on the thickness, rigidity, etc. of the anode. The support plate 105 needs to have electrical conductivity in order to receive electrons from the anode. Examples of the material of the support plate 105 may include Ti, SUS, and Ni.
Here, as shown in
In this embodiment, the anode 102 and the support plate 105 are described as separate objects, but the anode 102 and the support plate 105 may have an integrated structure (that is, may be configured as an integrated anode having a supporting function).
The electrolyte bath 108 stores an electrolytic solution that supports an oxidation-reduction reaction, and serves as the supply source of the raw material gas to be fed to the anode 102. In addition, by supplying the recovery liquid to the electrolyte bath 108, the electrolytic solution is diluted or replaced, and the precipitated salts which have been precipitated on the cathode 101 are removed. Therefore, the electrolyte bath 108 is connected to the flow channels (pipes 202 and 205) for supplying the recovery liquid or the electrolytic solution respectively from the separately provided recovery liquid tank 201 or electrolytic solution tank 204 and the flow channel (pipe 203) for discharging the diluted electrolytic solution or the replaced recovery liquid. Examples of the material of the electrolyte bath 108 include Ti, SUS, fluororesin, and polypropylene resin.
The electrolytic solution serves as the supply source of the raw material gas to be fed to the anode 102. As the electrolytic solution, a known electrolytic solution can be used. KHCO3, KOH, etc. are preferably used from the viewpoint of pH and ionic conductivity.
The recovery liquid tank 201 stores the recovery liquid. When the precipitated salts which have been precipitated on the cathode 101 are removed, the recovery liquid is sent out to the electrolyte bath 108. Therefore, the recovery liquid tank 201 and the electrolyte bath 108 are connected to each other by the flow channel through which the recovery liquid is supplied from the recovery liquid tank 201 to the electrolyte bath 108. Examples of the material of the recovery liquid tank 201 include Ti, SUS, fluororesin, and polypropylene resin. The recovery liquid tank 201 may be connected to a flow channel that supplies the recovery liquid to the recovery liquid tank 201 from the outside.
The electrolytic solution tank 204 stores an electrolytic solution for temporarily or constantly storing the electrolytic solution to be supplied to the solid-electrolyte electrolytic device 100. The electrolytic solution tank 204 is connected to the electrolyte bath 108 by the flow channel for supplying the electrolytic solution from the electrolytic solution tank 204 to the electrolyte bath 108. The electrolytic solution in the electrolytic solution tank 204 is used to replace the diluted electrolytic solution or the replaced recovery liquid, in which the precipitated salts which are precipitated on the cathode 101 are removed and then the cations of the precipitated salts in the electrolyte bath 108 are dissolved. Examples of the material of the electrolytic solution tank 204 include Ti, SUS, fluororesin, and polypropylene resin. The electrolytic solution tank 204 may be connected to the flow channel through which the electrolytic solution is supplied to the electrolytic solution tank 204 from the outside and may be used as a storage that temporarily or constantly stores the electrolytic solution.
The recovery liquid is used to dilute or replace the electrolytic solution in the electrolyte bath 108 when the precipitated salts which are precipitated on the cathode 101 are removed.
The recovery liquid is mainly selected from any one or a plurality of (1) water; (2) a solution that does not contain the same cation as the cation contained in the precipitated salts; and (3) a solution that contains the same cation as the cation contained in the precipitated salts and has a cation concentration lower than the cation concentration of the electrolytic solution. As the recovery liquid, it is particularly desirable to use a neutral or basic aqueous solution in which CO2 electrolysis is facilitated from the viewpoint of ensuring the electrolysis efficiency during maintenance. A non-aqueous solution can also be used, but is not preferable because it may damage the solid electrolyte 103 when it contains a substance that dissolves the solid electrolyte 103. As the recovery liquid, for example, an aqueous solution containing at least one of KHCO3, NaHCO3, K2CO3, Na2CO3, NaCl, KCl, NaOH, and KOH; water; a phosphate buffer; or a borate buffer can be used. One of these materials may be used alone, or a plurality of these materials may be used in combination.
The cation concentration of the electrolytic solution diluted or replaced by the recovery liquid decreases. For this reason, the cation concentration in the electrolyte bath 108 disposed in the vicinity of the anode becomes lower than that in the vicinity of the cathode where the precipitated salts are present, and a concentration gradient is formed. Due to this concentration gradient, the precipitated salts in the vicinity of the cathode and the cations generated by the re-dissolution of the precipitated salts move (dissolve) into the recovery liquid in the electrolyte bath 108 in order to eliminate the concentration gradient. Thus, the precipitated salts in the vicinity of the cathode can be removed. For example, in the case where the electrolytic solution is KHCO3 of 0.5 M, the precipitated salts are potassium salts, the recovery liquid may be water as the recovery liquid of (1), an aqueous solution of NaOH or an aqueous solution of NaHCO3 (with a concentration of, for example, 0.5 M) as the recovery liquid of (2), or an aqueous solution of KHCO3 of 0.05 M as the recovery liquid of (3). Therefore, it is preferable to use a recovery liquid having a lower concentration of the same cations as those contained in the precipitated salts to increase the concentration gradient, and the ion concentration is preferably 0.05 M or less, more preferably 0.01 M or less, and it is particularly preferable that the recovery liquid is water. Here, tap water, ion-exchanged water, pure water, or the like can be used as the water.
The voltage application unit 106 serves to apply a voltage between the cathode 101 and the anode 102 by applying a voltage to the current collector plate 104 and the support plate 105 as shown in
In the solid-electrolyte electrolytic device 100 according to the present disclosure, a reaction gas supply unit which is not shown may be provided outside the solid-electrolyte electrolytic device 100. That is, as long as CO2 as the reaction gas can be supplied to the surface 101-2, the reaction gas may be supplied from the reaction gas supply unit to the gas supply hole 104-1 through a pipe which is not shown or the like, or the reaction gas supply unit may be provided in a manner that the reaction gas can be blown onto a surface 104-A of the current collector plate 104 opposite to the contact surface 104-B with the cathode 101. In addition, it is preferable to use a factory exhaust gas discharged from a factory as the reaction gas from an environmental point of view. As shown in
The solid-electrolyte electrolytic device 100 according to the present disclosure may include other components such as electrical components, control components, piping components such as valves, pipes, tanks, and pumps which are necessary for the solid-electrolyte electrolytic device.
Next, a method for removing the precipitated salts of the cathode using the above-described solid-electrolyte electrolytic device 100 will be described with reference to
The electrolytic solution in the electrolyte bath 108 is discharged, and at the same time, the recovery liquid is supplied from the recovery liquid tank into the electrolyte bath 108. At this time, the flow rates of discharge and supply are adjusted so that the liquid amounts of the recovery liquid and the electrolytic solution in the electrolyte bath 108 satisfy the liquid amounts required for the device. The supply amount of the recovery liquid is at least 50% by volume or more with respect to the liquid amount required for the device, and the recovery liquid can be supplied until the electrolytic solution is completely replaced with the recovery liquid. Here, the discharged electrolytic solution can be recovered and temporarily or constantly stored in the electrolytic solution tank 204. That is, it is possible to reuse the electrolytic solution diluted by the recovery liquid. The electrolyte concentration of the reusable electrolytic solution is 50% by mass or more.
After a predetermined amount of the recovery liquid is supplied, the discharge of the solution in the electrolyte bath 108 and the supply of the recovery liquid are stopped, and the normal operation of the solid-electrolyte electrolytic device 100 is continued. By continuing the normal operation of the solid-electrolyte electrolytic device 100, the precipitated salts are dissolved in the solution in the electrolyte bath 108, and the precipitated salts in the vicinity of the cathode are removed. The time for removing the precipitated salts varies depending on the size of the electrode and the period of use, and thus is not clearly determined. Thus, the time for removing the precipitated salts can be set to a time until the concentration change gradually decreases or stops by monitoring the concentration of the cations which are included in the precipitated salts and are contained in the solution in the electrolyte bath 108. As a guide, for example, the time can be set to 0.5 hours to 12 hours.
After the removal of the precipitated salt is completed, the solution (the solution obtained by dissolving the precipitated salts in the recovery liquid) in the electrolyte bath 108 is discharged, and at the same time, the electrolytic solution is supplied from the electrolytic solution tank to the electrolyte bath 108. At this time, the flow rates of discharge and supply are adjusted so that the liquid amount of the electrolytic solution in the electrolyte bath 108 satisfies the liquid amount required for the device. The supply amount of the electrolytic solution is at least 100% by volume or more, preferably 200% by volume, and more preferably 400% by volume with respect to the liquid amount required for the device. By supplying such an amount of electrolytic solution, the solution in the electrolyte bath 108 is replaced with the electrolytic solution.
Here, the discharged electrolytic solution can be recovered and temporarily or constantly stored in the electrolytic solution tank 204. That is, it is possible to reuse the electrolytic solution diluted by the recovery liquid as the electrolytic solution. The electrolyte concentration of the reusable electrolytic solution is 50% by mass or more.
Since the discharged electrolytic solution contains the recovery liquid, as another reuse method shown in
As shown in
Examples and comparative examples in which the above-described present embodiment is used will be specifically described below.
The solid-electrolyte electrolytic device is assembled from the following components. A mixture of carbon black having conductivity and a silver nano-catalyst is attached to carbon paper and used as a cathode. A titanium mesh supporting iridium oxide is used as an anode. An ion exchange membrane which is shown in Table 1 and has an aromatic ring in the main chain and a quaternary ammonium group is used as the solid electrolyte, and a KHCO3 aqueous solution of 0.5 M is used as the electrolytic solution. The recovery liquids used in the examples and comparative examples are shown in Table 1. The concentrations of potassium ions in the electrolytic solution and each of the examples and comparative examples are measured using a potassium ion sensor (S030 manufactured by Horiba, Ltd.), and calculated from a calibration curve prepared in advance using a potassium ion calibration solution.
The electrolytic solution is replaced with the recovery liquid of each of the examples and comparative examples, the solid-electrolyte electrolytic device is operated, and the CO2 reduction reaction is continued for 1.5 hours. At this time, the applied potential of the cathode is-1.8 V with respect to a silver/silver chloride reference electrode. Thereafter, the electrolytic solution is replaced, and the CO production current density (JCO), the H2 production current density (JH2), and the CO selectivity are measured. The results are shown in Table 1. The CO production current density (JCO) and the H2 production current density (JH2) are values calculated based on the Faraday constant by obtaining the respective gas concentrations from: the results, which are measured by supplying the gas (CO, H2) generated from the device at the time of evaluating each of the examples and comparative examples to a gas chromatography (GC) measurement device; and the calibration curve prepared in advance. The calibration curve is prepared by supplying CO and H2 whose concentrations are accurately known from gas cylinders of CO and H2 respectively to the gas chromatography (GC) measurement device, detecting CO and H2 with a barrier discharge ionizing detector (BID), and then using the obtained peak areas and concentrations of each gas to create the calibration curve.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-046539 | Mar 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/010132 | 3/8/2022 | WO |
