Patent Application
20230416933
The present invention relates to a carbon dioxide gas-phase reduction apparatus and a method for manufacturing a porous reduction electrode-supporting electrolyte membrane.
Conventionally, a technique for reducing carbon dioxide has attracted attention from a viewpoint of prevention of global warming and stable supply of energy. As a reduction apparatus that reduces carbon dioxide, there are a reduction apparatus using an artificial photosynthesis technique for reducing carbon dioxide by applying light energy such as sunlight, and an electrolytic decomposition apparatus that reduces carbon dioxide by applying electric energy from the outside (see Non Patent Literatures 1 to 4).
FIG. 2 of Non Patent Literature 1 illustrates a carbon dioxide gas-phase reduction apparatus using light irradiation. An electrolyte membrane is disposed between a left oxidation tank and a right reduction tank, and each of the oxidation tank and the reduction tank is filled with an aqueous solution. An oxidation electrode of gallium nitride (GaN) is put in the oxidation tank, a reduction electrode of copper (Cu) is put in the reduction tank, and the oxidation electrode and the reduction electrode are connected to each other by a conducting wire. Then, helium (He) is caused to flow into the aqueous solution in the oxidation tank, and carbon dioxide (CO2) is caused to flow into the aqueous solution in the reduction tank.
At this time, when the oxidation electrode is irradiated with light, an electron/hole pair is generated and separated in the oxidation electrode, and oxygen (O2) and protons (H+) are generated by an oxidation reaction of water (H2O). Then, the protons move to the reduction tank through the electrolyte membrane, and electrons (e−) generated in the oxidation electrode move to the reduction electrode through the conducting wire. Thereafter, in the reduction electrode, hydrogen (H2) is generated by bonding between the protons and the electrons, and a carbon dioxide reduction reaction is caused by the protons, the electrons, and carbon dioxide. By this carbon dioxide reduction reaction, carbon monoxide, formic acid, methane, and the like, which are utilized as energy resources, are generated.
In a case of a conventional carbon dioxide gas-phase reduction apparatus, carbon dioxide to be reduced is dissolved in an aqueous solution in a reduction tank, reaches a reduction electrode, and is reduced on a surface of the reduction electrode. However, since the aqueous solution is used as a means for mediating carbon dioxide, there is a limit to the concentration of carbon dioxide that can be dissolved in the aqueous solution, and since diffusion resistance of carbon dioxide in the aqueous solution is large, there is a limit to the amount of carbon dioxide that can be supplied to the reduction electrode.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technique capable of improving efficiency of a carbon dioxide reduction reaction in a carbon dioxide gas-phase reduction apparatus.
A carbon dioxide gas-phase reduction apparatus according to an aspect of the present invention includes: an oxidation tank including an oxidation electrode; a reduction tank which is adjacent to the oxidation tank and into which carbon dioxide is supplied when the inside of the reduction tank is empty; and a porous reduction electrode-supporting electrolyte membrane disposed between the oxidation tank and the reduction tank, in which the porous reduction electrode-supporting electrolyte membrane is a joint body obtained by joining a porous reduction electrode formed by dispersing a first electrolyte membrane inside voids and a second electrolyte membrane, the second electrolyte membrane is disposed on the oxidation tank side, and the porous reduction electrode is disposed on the reduction tank side, connected to the oxidation electrode by a conducting wire, and performs a reduction reaction with the carbon dioxide in the reduction tank by electrons flowing through the conducting wire.
A method for manufacturing a porous reduction electrode-supporting electrolyte membrane according to an aspect of the present invention is a method for manufacturing a porous reduction electrode-supporting electrolyte membrane disposed between an oxidation tank including an oxidation electrode and a reduction tank into which carbon dioxide is supplied when the inside of the reduction tank is empty, the method including: impregnating a porous reduction electrode with an electrolyte dispersion in which a polymer material constituting an electrolyte membrane is dispersed; and superposing the porous reduction electrode impregnated with the electrolyte dispersion and an electrolyte membrane, and joining the porous reduction electrode and the electrolyte membrane by applying pressure while heating.
The present invention can provide a technique capable of improving efficiency of a carbon dioxide reduction reaction in a carbon dioxide gas-phase reduction apparatus.
Hereinafter, Examples of the present invention will be described with reference to the drawings. The present invention is not limited to Examples described later, and can be modified without departing from the gist of the present invention.
An object of the present invention is to provide a technique capable of improving efficiency of a carbon dioxide reduction reaction in a carbon dioxide gas-phase reduction apparatus. In order to achieve this object, the present invention has the following characteristics unlike a conventional carbon dioxide gas-phase reduction apparatus.
A first characteristic is that the inside of a reduction tank is filled with carbon dioxide in gas phase, and the carbon dioxide in gas phase is directly supplied to a reduction electrode. As a result, in the reduction tank, the concentration of carbon dioxide increases, and diffusion resistance of carbon dioxide decreases. As a result, the amount of carbon dioxide supplied to the reduction electrode increases, and efficiency of a carbon dioxide reduction reaction on the reduction electrode can be improved.
In this respect, in order to implement the carbon dioxide reduction reaction, a three-phase interface constituted by [electrolyte membrane-reduction electrode-carbon dioxide in gas phase] is required, but when the first characteristic is adopted, there is no aqueous solution in the reduction tank, and protons cannot move in a gas phase in the reduction tank. In addition, carbon dioxide in gas phase cannot move in the reduction electrode having no pore. As a result, the carbon dioxide reduction reaction cannot be implemented. Therefore, the present invention further has a second characteristic.
The second characteristic is that a porous reduction electrode having pores is used as the reduction electrode, and the porous reduction electrode and an electrolyte membrane are joined. As a result, a three-phase interface constituted by [electrolyte membrane-porous reduction electrode-carbon dioxide in gas phase] is formed, and therefore gas phase reduction of carbon dioxide on the porous reduction electrode can proceed even when the inside of the reduction tank is filled with carbon dioxide in gas phase.
However, only by joining the electrolyte membrane to the porous reduction electrode, the three-phase interface constituted by [electrolyte membrane-porous reduction electrode-carbon dioxide in gas phase] is limited only on a joining surface between the electrolyte membrane and the porous reduction electrode. Therefore, the present invention further has a third characteristic.
The third characteristic is that an electrolyte membrane is formed by being dispersed also inside voids of the porous reduction electrode. As a result, a reaction field of the carbon dioxide gas-phase reduction reaction increases, and therefore efficiency of the carbon dioxide reduction reaction on the porous reduction electrode can be improved.
That is, the present invention is characterized in that carbon dioxide in gas phase is directly supplied to a porous reduction electrode-supporting electrolyte membrane in which an electrolyte membrane is joined to a porous reduction electrode formed by dispersing an electrolyte membrane inside voids. This characteristic makes it possible to improve efficiency of a carbon dioxide reduction reaction on a reduction electrode.
As illustrated in
The oxidation electrode 2 is, for example, a compound exhibiting photoactivity or redox activity, such as a nitride semiconductor, titanium oxide, amorphous silicon, a ruthenium complex, or a rhenium complex. The aqueous solution 3 is, for example, a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, a sodium chloride aqueous solution, a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, or a cesium hydroxide aqueous solution.
Between the oxidation tank 1 and the reduction tank 4, a porous reduction electrode-supporting electrolyte membrane 20 in which a porous reduction electrode 5 formed by dispersing an electrolyte membrane (first electrolyte membrane) inside voids and an electrolyte membrane (second electrolyte membrane) 6 are joined is disposed. The electrolyte membrane 6 is disposed on the oxidation tank 1 side, and the porous reduction electrode 5 is disposed on the reduction tank 4 side. The oxidation electrode 2 and the porous reduction electrode 5 are connected to each other by a conducting wire 7.
A tube 8 is inserted into the oxidation tank 1 in order to cause helium to flow into the aqueous solution 3 in the oxidation tank 1. In the reduction tank 4, a gas input port 9 is formed at a bottom of the reduction tank 4 in order to cause carbon dioxide to flow into the reduction tank 4. Furthermore, a light source 10 is disposed so as to face the oxidation electrode 2 in order to operate the gas-phase reduction apparatus 100. The light source 10 is, for example, a xenon lamp, a pseudo sunlight source, a halogen lamp, a mercury lamp, sunlight, or a combination thereof.
A method for preparing the porous reduction electrode-supporting electrolyte membrane 20 will be described. The porous reduction electrode-supporting electrolyte membrane 20 is formed by joining the porous reduction electrode 5 and the electrolyte membrane 6.
The porous reduction electrode 5 is, for example, a porous body of copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, or an alloy thereof, or a porous body of silver oxide, copper oxide, copper (II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten oxide (VI), or copper oxide. In addition, the porous reduction electrode 5 may be a porous metal complex having a metal ion and an anionic ligand.
The electrolyte membrane 6 is, for example, Nafion (registered trademark), Forblue, or Aquivion which is an electrolyte membrane having a carbon-fluorine skeleton. In addition, the electrolyte membrane 6 may be Selemion or Neosepta which is an electrolyte membrane having a hydrocarbon-based skeleton.
In Example 1, a copper porous body having a thickness of 1 mm and a porosity of 98% was used as the porous reduction electrode 5. Nafion which is a cation exchange membrane was used as the electrolyte membrane 6. As an electrolyte dispersion used at the time of preparation, a Nafion dispersion prepared by diluting Nafion dispersion (registered trademark) (Nafion content: 20 wt. %) manufactured by DuPont with pure water 200 times to adjust a Nafion content (=electrolyte content) to 0.05 wt. % was used. A solvent used for dilution is, for example, pure water, a lower alcohol, or a mixed liquid thereof. In Example 1, pure water was used.
First, the electrolyte membrane 6 is immersed in each of boiling nitric acid and boiling pure water in advance in order to improve proton mobility of the electrolyte membrane 6. Next, as step 1, the porous reduction electrode 5 is impregnated with an electrolyte dispersion (electrolyte content: 0.05 wt. %) in which a polymer material constituting an electrolyte membrane is dispersed. Thereafter, the porous reduction electrode 5 impregnated with the electrolyte dispersion is superposed on the electrolyte membrane 6 immersed in each of boiling nitric acid and boiling pure water, and the sample is disposed between two copper plates 30a and 30b as illustrated in
In step 1, since the porous reduction electrode 5 is impregnated with the electrolyte dispersion, the solution of the electrolyte dispersion 50 adheres to the surface and the inside of the porous reduction electrode 5 as illustrated in the enlarged diagram of
Electrochemical measurement and measurement of gas/liquid generation amount will be described.
The oxidation tank 1 is filled with the aqueous solution 3. As the oxidation electrode 2, a substrate obtained by epitaxially growing a thin film of gallium nitride (GaN) which is an n-type semiconductor and a thin film of aluminum gallium nitride (AlGaN) in this order on a sapphire substrate, and vacuum-depositing nickel (Ni) thereon and performing heat treatment to form a nickel oxide (NiO) co-catalyst thin film was used Then, the oxidation electrode 2 was disposed in the oxidation tank 1 so as to be immersed in the aqueous solution 3. As the aqueous solution 3, a 1.0 mol/L potassium hydroxide aqueous solution was used. As the light source 10, a 300 W high pressure xenon lamp (which cuts light having a wavelength of 450 nm or more, illuminance: 6.6 mW/cm2) was used, and the light source 10 was fixed such that a surface of the semiconductor photoelectrode of the oxidation electrode 2 on which the oxidation co-catalyst was formed (surface on which NiO was formed) was an irradiation surface. A light irradiation area of the oxidation electrode 2 was set to 2.5 cm2.
Helium was caused to flow into the oxidation tank 1 from the tube 8, and carbon dioxide was caused to flow into the reduction tank 4 from the gas input port 9 at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, the carbon dioxide reduction reaction can proceed at the three-phase interface constituted by [electrolyte membrane-copper (porous reduction electrode)-carbon dioxide in gas phase] in the porous reduction electrode-supporting electrolyte membrane 20.
The oxidation tank 1 and the reduction tank 4 were sufficiently replaced with helium and carbon dioxide, respectively. Thereafter, the oxidation electrode 2 was uniformly irradiated with light using the light source 10. When the oxidation electrode 2 is irradiated with light, electrons flow between the oxidation electrode 2 and the porous reduction electrode 5. A current value between the oxidation electrode 2 and the porous reduction electrode 5 at the time of light irradiation was measured with an electrochemical measurement apparatus (Model 1287 Potentiogalvanostat manufactured by Solartron). In addition, gas and liquid in the oxidation tank 1 and the reduction tank 4 were collected at any time during light irradiation, and reaction products were analyzed with a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer. As a result, it was confirmed that oxygen was generated in the oxidation tank 1, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were generated in the reduction tank 4.
In Example 2, the electrolyte content of the electrolyte dispersion in step 1 was set to 0.1 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 1.
In Example 3, the electrolyte content of the electrolyte dispersion in step 1 was set to 0.5 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 1.
In Example 4, the electrolyte content of the electrolyte dispersion in step 1 was set to 1.0 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 1.
In Example 5, the electrolyte content of the electrolyte dispersion in step 1 was set to 5.0 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 1.
In Example 6, a copper porous body having a thickness of 1 mm and a porosity of 90% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 50%. The other conditions are all similar to those in Example 1.
In Example 7, a copper porous body having a thickness of 1 mm and a porosity of 85% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 25%. The other conditions are all similar to those in Example 1.
In Example 8, a copper porous body having a thickness of 1 mm and a porosity of 81% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 5%. The other conditions are all similar to those in Example 1.
As illustrated in
Between the oxidation tank 1 and the reduction tank 4, a porous reduction electrode-supporting electrolyte membrane 20 in which a porous reduction electrode 5 formed by dispersing an electrolyte membrane (first electrolyte membrane) inside voids and an electrolyte membrane (second electrolyte membrane) 6 are joined is disposed. The electrolyte membrane 6 is disposed on the oxidation tank 1 side, and the porous reduction electrode 5 is disposed on the reduction tank 4 side. The oxidation electrode 2 and the porous reduction electrode 5 are connected to each other by a conducting wire 7. Specific examples of the porous reduction electrode 5 and the electrolyte membrane 6 are similar to those in Example 1.
A tube 8 is inserted into the oxidation tank 1 in order to cause helium to flow into the aqueous solution 3 in the oxidation tank 1. In the reduction tank 4, a gas input port 9 is formed at a bottom of the reduction tank 4 in order to cause carbon dioxide to flow into the reduction tank 4. Furthermore, a power supply 11 is connected to the conducting wire 7 in order to operate the gas-phase reduction apparatus 100.
The porous reduction electrode-supporting electrolyte membrane 20 is prepared by a procedure similar to that in Example 1.
Electrochemical measurement and measurement of gas/liquid generation amount will be described.
The oxidation tank 1 is filled with the aqueous solution 3. As the oxidation electrode 2, platinum (manufactured by The Nilaco Corporation) was used. The oxidation electrode 2 was disposed in the oxidation tank 1 such that about 0.55 cm2 of the surface area of the oxidation electrode 2 was immersed in the aqueous solution 3. As the aqueous solution 3, a 1.0 mol/L potassium hydroxide aqueous solution was used.
Helium was caused to flow into the oxidation tank 1 from the tube 8, and carbon dioxide was caused to flow into the reduction tank 4 from the gas input port 9 at a flow rate of 5 ml/min and a pressure of 0.18 MPa. In this system, the carbon dioxide reduction reaction can proceed at the three-phase interface constituted by [electrolyte membrane-copper (porous reduction electrode)-carbon dioxide in gas phase] in the porous reduction electrode-supporting electrolyte membrane 20. The area of the porous reduction electrode 5 to which carbon dioxide is directly supplied is about 6.25 cm2.
The oxidation tank 1 and the reduction tank 4 were sufficiently replaced with helium and carbon dioxide, respectively. Thereafter, the oxidation electrode 2 and the porous reduction electrode 5 were connected to each other by the conducting wire 7 via the power supply 11, and a voltage of 2.5 V was applied to cause electrons to flow. A current value between the oxidation electrode 2 and the porous reduction electrode 5 when a voltage of 2.5 V was applied was measured with an electrochemical measurement apparatus. In addition, gas and liquid in the oxidation tank 1 and the reduction tank 4 were collected at any time during voltage application, and reaction products were analyzed with a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer. As a result, it was confirmed that oxygen was generated in the oxidation tank 1, and hydrogen, carbon monoxide, formic acid, methane, methanol, ethanol, and ethylene were generated in the reduction tank 4.
In Example 10, the electrolyte content of the electrolyte dispersion in step 1 was set to 0.1 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 9.
In Example 11, the electrolyte content of the electrolyte dispersion in step 1 was set to 0.5 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 9.
In Example 12, the electrolyte content of the electrolyte dispersion in step 1 was set to 1.0 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 9.
In Example 13, the electrolyte content of the electrolyte dispersion in step 1 was set to 5.0 wt. % in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The other conditions are all similar to those in Example 9.
In Example 14, a copper porous body having a thickness of 1 mm and a porosity of 90% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 50%. The other conditions are all similar to those in Example 9.
In Example 15, a copper porous body having a thickness of 1 mm and a porosity of 85% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 25%. The other conditions are all similar to those in Example 9.
In Example 16, a copper porous body having a thickness of 1 mm and a porosity of 81% was used in preparation of the porous reduction electrode-supporting electrolyte membrane 20. The porous reduction electrode 5 after thermocompression bonding had a thickness of 0.2 mm and a porosity of 5%. The other conditions are all similar to those in Example 9.
The structure of the reduction tank 4 is different from that in
As the aqueous solution 3 in the oxidation tank 1, a 1 mol/L sodium hydroxide aqueous solution was used. As the aqueous solution 12 in the reduction tank 4, a 0.5 mol/L potassium hydrogen carbonate aqueous solution was used. In addition, the non-porous reduction electrode 5′ was disposed so as to be immersed in the aqueous solution 12 using a copper plate (manufactured by The Nilaco Corporation) having an area of about 6 cm2. The other constituent elements are similar to those in Example 1.
As a result of analyzing reaction products, it was confirmed that hydrogen, carbon monoxide, formic acid, methane, and ethylene were generated.
The structure of the reduction tank 4 is different from that in
As the aqueous solution 3 in the oxidation tank 1, a 1 mol/L sodium hydroxide aqueous solution was used. As the aqueous solution 12 in the reduction tank 4, a 0.5 mol/L potassium hydrogen carbonate aqueous solution was used. In addition, the non-porous reduction electrode 5′ was disposed so as to be immersed in the aqueous solution 12 using a copper plate (manufactured by The Nilaco Corporation) having an area of about 6 cm2. The other constituent elements are similar to those in Example 9.
As a result of analyzing reaction products, it was confirmed that hydrogen, carbon monoxide, methane, ethylene, and formic acid were generated.
Faraday efficiencies of the carbon dioxide reduction reaction in Example 1 to 16 and Comparative Examples 1 and 2 are presented in Table 1.
As indicated in formula (1), the Faraday efficiency is a value indicating a ratio of a current value used in each reduction reaction to a current value flowing between electrodes at the time of light irradiation or voltage application.
Faraday efficiency of each reduction reaction=(current value of each reduction reaction)/(current value between oxidation electrode and reduction electrode) (1)
The “current value of each reduction reaction” in formula (1) can be determined by converting a measured value of the generation amount of each reduction product into the number of electrons required for a generation reaction of each reduction product. The “current value of each reduction reaction” was calculated using formula (2), in which the concentration of a reduction reaction product is represented by A [ppm], a flow rate of a carrier gas is represented by B [L/sec], the number of electrons required for a reduction reaction is represented by Z [mol], the Faraday constant is represented by F [C/mol], and the molar volume of gas is represented by Vm [L/mol].
Current value[A] of each reduction reaction=(A×B×Z×F×10−6)/Vm (2)
From Table 1, it can be understood that selectivity of carbon dioxide reduction in each of Example 1 to 3 is higher than that in each of Examples 4 and 5. It can be understood that selectivity of carbon dioxide reduction in each of Example 9 to 11 is higher than that in each of Examples 12 and 13.
In Examples 5 and 13, no product due to carbon dioxide reduction was detected, and therefore no measurement result of Faraday efficiency was obtained. This is considered to be because in Examples 4, 5, 12, and 13, the electrolyte membrane covered a metal surface of the porous reduction electrode 5, and as a result, supply of carbon dioxide to the porous reduction electrode 5 was significantly insufficient.
In addition, as illustrated in
Meanwhile, when the concentration of the electrolyte dispersion 50 is 0.05 wt. % to 0.5 wt. % as in Examples 1 to 3 and 9 to 11, as illustrated in
Furthermore, Faraday efficiency of carbon dioxide reduction in each of Examples 1 to 3, 6 to 8, 9 to 11, and 14 to 16 is largely improved as compared with that in each of Comparative Examples 1 and 2, and this indicates that the carbon dioxide reduction reaction selectively occurs. This is considered to be because in Examples 1 to 3, 6 to 8, 9 to 11, and 14 to 16, by directly supplying carbon dioxide in gas phase to the porous reduction electrode 5 without interposing an aqueous solution, the amount of carbon dioxide near the surface of the porous reduction electrode 5 increased, diffusion resistance of carbon dioxide decreased, the amount of carbon dioxide supplied to the porous reduction electrode 5 increased, and furthermore, the electrolyte membrane 60 was formed by being dispersed on the surface of the porous reduction electrode 5 to increase a reaction field.
According to the present invention, since carbon dioxide in gas phase is directly supplied to the porous reduction electrode-supporting electrolyte membrane 20 in which the electrolyte membrane 6 is joined to the porous reduction electrode 5, the concentration of carbon dioxide in the reduction tank 4 increases, and diffusion resistance of carbon dioxide near a surface of the porous reduction electrode 5 can be reduced. In addition, since the electrolyte membrane is formed by being dispersed inside the porous reduction electrode 5, a reaction field of a carbon dioxide gas-phase reduction reaction increases, and efficiency of a carbon dioxide reduction reaction in the porous reduction electrode 5 can be improved. Furthermore, in steps 1 and 2, since a void ratio of the porous reduction electrode 5 can be controlled in detail, and furthermore, the amount of the electrolyte dispersion to be dispersed inside the porous reduction electrode 5 can be defined, the reaction field can be easily controlled.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/044254 | 11/27/2020 | WO |
