JP7283455B2 - Reduction reaction electrode - Google Patents

Description

TECHNICAL FIELD The present invention relates to a reduction reaction electrode that uses a carbon-based sheet to reduce carbon dioxide (CO ₂ ).

Conventionally, there have been various proposals for artificial photosynthesis in which formic acid is produced by a reduction reaction of _CO2 . For example, Patent Literature 1 discloses a reduction reaction electrode including a carbon layer made of carbon nanotubes and a complex catalyst covering the surface of the carbon layer on the surface of a substrate such as a metal or semiconductor. According to this proposal, carbon nanotubes are sprayed at a density of about 2 mg/cm ² to form a carbon layer.

JP 2017-125242 A

Here, in the conventional proposals, what kind of material is preferable is experimentally verified, and many problems in constructing an actual device are not verified. When the reduction of _CO2 by artificial photosynthesis is put into practical use, _there is a demand to process a large amount of _CO2 using a large-scale apparatus. must be maintained. For example, a reduction reaction electrode that has a large area (1 m square) specification and can stably obtain a high current density is desired.
In particular, in the prior art, when a high (large negative) potential is applied to the reduction electrode, a large reduction current flows, but the reduction of H ₂ O generates a large amount of hydrogen, resulting in low selectivity and CO ₂ reduction products. There was a problem that the amount of formic acid produced was low. On the other hand, when the potential of the reduction electrode is low (small negative) (for example, -1.2 V vs. Hg/Hg ₂ SO ₄ ), the selectivity is improved, but the current density is low (-2 to -2 .5 mA/cm ² ), there was a problem that the amount of formic acid produced was small.
The present invention provides a reduction electrode with which a high current density can be obtained even with a small negative applied voltage.

The present invention provides a reduction reaction electrode for reducing CO ₂ , comprising a porous carbon-based sheet material, a carbon nanotube layer provided on the surface of the sheet material, and Ru supported on the carbon nanotube layer. and a complex polymer, a bulk density of 0.65 to 0.85 g/cm ³ , and a thickness of the sheet material of 0.3 mm or more .

The sheet material is carbon paper, and the carbon nanotube layer is preferably formed of multi-wall carbon nanotubes.

−1.2 V vs. to the reduction reaction electrode. It is preferable that the current density when a potential of Hg/Hg ₂ SO ₄ is applied is greater than −3.5 mA/cm ² .

According to the present invention, the sheet material can incorporate a relatively large amount of the carbon nanotube layer and the Ru complex polymer to effectively perform the CO ₂ reduction reaction.

It is a diagram showing a comparison of cross-sectional structure images of a carbon paper (CP) impregnated with MWCNTs and a cathode sheet supporting a Ru complex polymer, (a) is a conventional CP thickness of 0.19 mm, (b) is a conventional CP thickness of 0.19 mm. A CP having a CP thickness of 0.35 mm is used. FIG. 10 is a diagram showing the relationship between the number of impregnation coating times of MWCNTs using a CP thickness of 0.35 mm and the weight of a 15×20 mm square sheet. FIG. 10 is a diagram showing a comparison of the MWCNTs coating amount (per area and coating amount) between two-time impregnation coating and three-time impregnation coating with CP thicknesses of 0.19 mm and 0.35 mm. FIG. 10 is a diagram showing SEM images of the sheet surface of MWCNTs with a CP thickness of 0.35 mm, two times and four times of impregnation coating. FIG. 10 is a diagram showing it (current/time) measurement results using cathode sheets with different impregnation coating times of CP/MWCNTs (the amount of Ru complex polymer is constant at 1.33 times). FIG. 6 is a diagram showing the relationship between the number of impregnation coating times of CP/MWCNTs in FIG. 5 and the average value of the current density after 3 hours (3 hours) after it measurement (−1.2 V vs. Hg/Hg ₂ SO ₄ ). FIG. 10 is a diagram showing the results of examining improvement in current density by increasing the amount of Ru complex polymer by increasing the number of impregnation coatings of CP/MWCNTs to 4 times. FIG. 10 is a diagram showing the results of examining improvement in current density by increasing the amount of Ru complex polymer by increasing the number of impregnation coatings of CP/MWCNTs to 4 times. FIG. 4 is a diagram showing a SEM observation photograph of the surface of a cathode sheet with 2.5 times the amount of Ru complex polymer and 4 times of impregnation and coating of CP/MWCNTs, and a cross-sectional image estimated from observation from an oblique direction. Three times of impregnation coating of CP / MWCNTs, i - t measurement (-1.2 V vs. Hg / Hg ₂ SO ₄ ) at 1.33 to 2.5 times the amount of Ru complex polymer, current density results after 3 hours ( n=4). The current density was also compared after 3 hours of it measurement (-1.2 V vs. Hg/Hg ₂ SO ₄ ) in the range of 1.33 to 2.0 times the amount of the Ru complex polymer, with the number of times of impregnation coating twice. It is a figure which shows. FIG. 10 is a diagram showing the results of SEM observation of a cross section of a cathode sheet with a Ru complex polymer amount of 1.5 times cut after three impregnation coatings of CP/MWCNTs that ensured a high current density. FIG. 2 is a diagram showing an image of a cross section of a sheet to which MWCNTs are impregnated and coated; It is a figure which shows the bulk density of CP, CP/MWCNTs, and CP/MWCNTs/RuCP in conventional and this embodiment. −1.2 V vs. bulk density. FIG. 3 shows Hg/Hg ₂ SO ₄ , current after 3 hours. 1 is a diagram showing a schematic configuration of a CO ₂ reduction device 3 using a reduction reaction electrode (cathode electrode) according to an embodiment; FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described based on the drawings. It should be noted that the invention is not limited to the embodiments described herein.

"Overview of Embodiment"
In the present embodiment, the cathode for the reduction reaction (hereinafter referred to as the cathode electrode) is coated with multi-walled carbon nanotubes (MWCNTs) impregnated on carbon paper (CP), which is a carbon-based sheet material, and further Ru complex polymer (RuCP ) is preferably used (CP/MWCNTs/RuCP).

<Carbon paper (CP)>
The description of the cathode electrode should include carbon fiber. For example, it is a composite base material (sheet material) of carbon fiber heat-treated at a high temperature and carbon, and includes carbon paper (CP), carbon cloth, etc. CP is adopted, for example. The CP is formed into a sheet by impregnating organic fibers such as polyacrylonitrile (PAN) fibers with a dispersion of polyvinyl alcohol and an aqueous medium, carbonizing them at about 2000° C. and binding them. The CP may contain about 25% by mass of a fluororesin-based material.

<CP/MWCNTs>
A carbon nanotube layer may be formed by applying carbon nanotubes to this carbon paper (CP). This carbon nanotube layer is preferably formed so as to cling to the periphery of the CP fibers, and is preferably formed in a layered manner covering the front and back of the CP to prevent the opening of the CP from being blocked. For example, by including carbon materials such as multi-wall carbon nanotubes (MWCNTs), a three-dimensional network structure of nanocarbon can be formed. MWCNTs preferably include those having a diameter of at least 1 nm or more and 100 nm or less. Carbon materials such as multi-wall carbon nanotubes (MWCNTs) are prepared by preparing an ink in which the carbon nanotube material is highly dispersed in a solvent such as ethanol, applying it to the CP by a method such as dip coating or impregnation coating, and drying it. can form CP/MWCNTs. In this embodiment, about 1.4 to 1.8 mg/cm ² of MWCNTs were coated by one impregnation coating.

The CP is made into a sheet by impregnating organic fibers such as polyacrylonitrile (PAN) fibers in a dispersion of polyvinyl alcohol and an aqueous medium, carbonizing them at about 2000° C., and binding them. be. The CP may contain about 25% by mass of a fluororesin-based material. MWCNTs, also called multi-walled carbon nanotubes, are composed of a plurality of tubes positioned concentrically, and are believed to be nested with several single-walled tubes.

<CP/MWCNTs/RuCP>
Then, it is preferable to use a sheet-like material obtained by carrying Ru complex polymer (RuCP) nano-dispersed on the base material described above, carrying and drying the same.

Ruthenium complexes (Ru complex monomers) for obtaining Ru complex polymers include, for example, [Ru {4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine} (CO) ( MeCN)Cl ₂ ], [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO) ₂ Cl ₂ ], [Ru{4,4′-di (1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO) ₂ ] _n , [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′- bipyridine}(CO)( _CH3CN ) _Cl2 ] and the like. Ru complex polymer, for example, Ru complex monomer (for example, [Ru {4,4'-di (1-H-1-pyrrolypropyl carbonate) -2,2'-bipyridine} (CO) ₂ Cl ₂ ]) and polymerization initiation It is a Ru complex polymer in a polymerized state using an agent (eg pyrrole) and a polymerization catalyst (eg iron chloride).

The Ru complex polymer is supported, for example, by dissolving a Ru complex monomer, a polymerization initiator (e.g. pyrrole), and a polymerization catalyst (e.g. iron chloride) in a solvent such as acetonitrile (MeCN) (Ru complex polymer solution) with carbon fibers. It can be carried out by coating and drying on a composite substrate with carbon.

As a polymerization initiator (radical initiator), a heterocyclic aromatic having a five-membered ring structure to a nine-membered ring structure containing at least one of N, S, B such as pyrrole, pyridine, thiophene, volepin, azonin, etc. Compounds, azo compounds, organic peroxides, and the like can be mentioned, and pyrrole is preferable because the reaction is easily initiated by oxidation.

Polymerization catalysts (chemical oxidation polymerization catalysts) include iron salts such as iron chloride (FeCl ₃ ), FeCl ₃ ·O ₂ or FeCl ₃ ·O ₂ -ClO ₄ , copper salts of copper chloride, and aluminum salts such as aluminum chloride. Iron chloride (FeCl ₃ ), FeCl ₃ .O ₂ , or FeCl ₃ .O ₂ —ClO ₄ are preferred from the viewpoint of high catalytic activity.

Examples of the solvent include acetonitrile, diethyl ether, N,N-dimethylformamide (DMF), tetrahydrofuran (THF) and the like, and acetonitrile is preferred from the viewpoint that the Ru complex polymer can be highly dispersed.

Here, in order to achieve a large amount of current as a large cathode electrode, the thickness of the CP is increased, and the conditions for coating MWCNTs and supporting the Ru complex polymer are optimized to increase the surface area of the electrode and reduce it. An improvement in reaction can be expected.

For example, as part of further improving the characteristics of the cathode electrode used in a large cell of 1 m square, the thickness of the CP is increased to about twice the conventional thickness, and the conditions for coating MWCNTs and supporting and drying the Ru complex polymer to improve the current density of the electrochemical characteristics.

<MWCNTs coating>
A CP/MWCNTs sheet is prepared by dip-coating carbon paper (CP) with ink prepared by dispersing 5 parts by mass of multi-walled carbon nanotubes (MWCNTs) in a solvent (95 parts by mass of ethanol) and drying at 350°C. . This impregnation application is set to 1 time.

<Amount of Ru complex polymer>
First, the standard supported amount of Ru complex polymer (amount of Ru complex polymer=1 time) is set as follows.

That is, Ru complex monomer ([Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)2Cl2]) 0.0414 g (3.35×10 ⁻⁶ mol/cm ² ), 1.65 mL of 0.22 M iron chloride (FeCl3) ethanol solution (1.32×10 ⁻⁶ mol/cm ² ), 0.33 mL of 0.72 mM pyrrole acetonitrile solution (2.02 ×10 ⁻⁵ mol/cm ² ) and 6.33 mL of acetonitrile (total amount: 8.31 mL) to prepare an Ru complex polymer solution, which is used as a reference load (amount of Ru complex polymer=1 time).

In the following, the amount of the Ru complex polymer is described as an amount corresponding to the above amount being "1 time".

"Image of cathode electrode (cathode sheet)"
In this embodiment, the thickness of the carbon paper (CP) is thicker than that of the conventional one. That is, the conventional CP thickness was about 0.19 mm (reference thickness=0.19 mm), but in the present embodiment, it is approximately doubled to 0.35 mm.

FIG. 1 shows a cross-sectional structure image of a cathode sheet carrying the same MWCNTs impregnation coating and Ru complex polymer on carbon paper (CP) with a conventional CP thickness of 0.19 mm and a CP thickness of 0.35 mm in this embodiment. FIG. 13 shows a comparison;

In the cathode sheet shown in FIG. 1(a), after the CP thickness is 0.19 mm and the MWCNTs are impregnated and coated twice, the Ru complex polymer amount is 1.33 times (the above reference loading amount is 1 time) and the Ru A complex polymer was supported (CP/MWCNTs/RuCP). In the case of this CP/MWCNTs/RuCP, Ru complex polymer layers (RuCP) with a thickness of several μm to several tens of μm are formed on the front and back surfaces of the cathode sheet, as shown in the lower diagram of FIG. 1(a). Therefore, this cathode sheet (CP/MWCNTs/RuCP) loses its porous structure. In the CP/MWCNTs before carrying the Ru complex polymer, the MWCNTs are wrapped around the CP fibers, and the porous structure is maintained as it is, as shown in the photograph of FIG.

On the other hand, in the case of CP/MWCNTs/RuCP with a CP thickness of 0.35 mm and the same number of times of MWCNTs impregnation coating and the amount of Ru complex polymer, a RuCP layer was formed as shown in FIG. Instead, it has a porous structure, as shown in the photograph.

"Measurement of Electrochemical Properties of Sheets"
In this way, when only the thickness of the CP is changed and the manufacturing process conditions are the same, the structural form of the cathode sheet becomes different. Therefore, the electrochemical properties of both cathode sheets were investigated and compared.

Using CP with a CP thickness of 0.35 mm, (Test 1) the amount of Ru complex polymer was kept constant (the amount of Ru complex polymer was 1.33 times), and the current density was maximized by the MWCNTs impregnation coating conditions. Found it. After that, (Test 2) improvement of the current density was investigated by fixing the number of impregnation coating times of CP/MWCNTs and increasing the amount of the Ru complex polymer. Furthermore, (Test 3) Based on the conditions of (Test 2), the conditions under which the MWCNTs impregnation coating and the amount of the Ru complex polymer were reduced while maintaining a high current density were investigated.

By the above method, 15 × 20 mm cathode sheets with various production conditions are produced, and this is used as a cathode to form a ^three -electrode cell. An electrochemical evaluation was performed. The measurement is linear sweep voltammetry (LSV): -0.2 mV→-1.5 V vs. Hg/Hg ₂ SO ₄ ), current/time measurement: it [constant potential: −1.2 V vs. Hg/Hg ₂ SO ₄ ), operated for 3 hours]. A 0.4 M potassium phosphate buffer solution (K ₂ HPO ₄ +KH ₂ PO ₄ ) was used as the electrolyte, and the measurement was performed with CO ₂ bubbling. Sheets manufactured under different conditions were appropriately observed with an SEM to confirm their structural forms.

"Impregnation coating of MWCNTs"
FIG. 2 shows the relationship between the number of impregnation coating times of MWCNTs using a CP thickness of 0.35 mm and the weight of a 15×20 mm square sheet. Based on the weight of CP alone, when the number of coatings of MWCNTs was in the range of 2 to 5 times, the weight tended to increase at a rate of about 10% as the number of coatings increased. In addition, in FIG. 2, the results of two tests (the number of tests n=2) are shown by squares and rhombuses.

FIG. 3 shows a comparison of the amount of MWCNTs applied (per area and applied amount) between two-time impregnation coating and three-time impregnation coating with CP thicknesses of 0.19 mm and 0.35 mm. Thus, it was confirmed that the MWCNTs coating amount was at the same level as the CP thicknesses of 0.19 mm and 0.35 mm.

FIG. 4 shows SEM images of the sheet surface of 0.35 mm CP-thick MWCNTs impregnated and coated twice and four times. When the MWCNTs with a CP thickness of 0.35 mm were impregnated twice, the CP fibers could be clearly confirmed, and it is assumed that the inside of the CP is filled with few MWCNTs. On the other hand, the 4-time impregnation of MWCNTs is sufficiently applied that the CP fibers cannot be identified.

"Application of RuCP"
(Test 1)
FIG. 5 shows it (current/time) measurement results when a constant potential is applied using cathode sheets with different impregnation coating times of CP/MWCNTs (the amount of the Ru complex polymer is constant at 1.33 times). The current density varied depending on the number of impregnation coatings of CP/MWCNTs, and the maximum current density reached 1 mA/cm ² when the number of impregnation coatings was 2 to 5 times. When the number of coatings for impregnation and coating of CP/MWCNTs is small, 2 or 3 times, the variation in current density is large in the measurement of 2 tests, but when the number of coatings is 4 or 5 times, the variation in current density tends to decrease. was taken.

FIG. 6 shows the number of impregnation coating times of CP/MWCNTs in FIG. 5 and the average value of the current density after 3 hours (after 3 hours) of it measurement (−1.2 V vs. Hg/Hg ₂ SO ₄ ). The current density can be increased by up to about 15% when the CP/MWCNTs are impregnated and applied 4 times, compared to 2 times. However, when the CP/MWCNTs are impregnated and applied five times, the current density decreases, indicating that there is an optimum number of times of application.

(Test 2)
Next, improvement of the current density was investigated by increasing the number of impregnation coatings of CP/MWCNTs to 4 times and increasing the amount of the Ru complex polymer. The results are shown in FIGS. FIG. 7 shows it (current/time) measurement results when the amount of the Ru complex polymer was changed from 1.3 to 3.0 times for the cathode sheet impregnated and coated four times with CP/MWCNTs. FIG. 8 is a diagram showing the relationship between the amount of Ru complex polymer and the current value after 3 hours.

When the Ru complex polymer amount is 1.3 to 2.0 times, the current density is −3 to −3.5 mA/cm ² , but when the Ru complex polymer amount is 2.5 times, the current density is −4 mA/cm ² . increased sharply. The current density of −4 to −5 mA/cm ² was maximized when the Ru complex polymer amount was 3.0 times, and the current density tended to decrease when the Ru complex polymer amount was 3.5 times. When the amount of the Ru complex polymer was 3.0 times or more, there was a tendency for the current density measured at n=2 to increase in fluctuation.

FIG. 9 shows a SEM observation photograph of the surface of a cathode sheet with a high current density and a small fluctuation range, which is impregnated and coated four times with CP/MWCNTs and has an Ru complex polymer amount of 2.5 times. It is a figure which shows the cross-sectional image estimated by observation. Since the sheet with this manufacturing process condition has a large amount of MWCNTs applied (see FIG. 4), the MWCNTs network is a random layer (CP) compounded with a Ru complex polymer that has become flat aggregates on the surface. It is characterized in that a Ru complex polymer layer) is formed under the conventional manufacturing process conditions with a thickness of 0.19 mm.

(Test 3)
Next, based on the conditions of Test 2, the fabrication process conditions were investigated in which the amount of MWCNTs impregnated coating and Ru complex polymer was reduced while maintaining a high current density. FIG. 10 shows the current after 3 hours of it measurement (-1.2 V vs. Hg/Hg ₂ SO ₄ ) at 1.33 to 2.5 times the amount of Ru complex polymer with CP/MWCNTs impregnated and applied three times. FIG. 11 shows density results (n=4); Here, FIG. 10 also shows for comparison the result of the same amount of Ru complex polymer when the number of times of impregnation coating was four times, which gave the above-mentioned high current density. A current density of −4 mA/cm ² , which is equivalent to 2.5 times the Ru complex polymer amount when the number of impregnation and coating of CP/MWCNTs is 3 times and the amount of Ru complex polymer is 1.5 times, is obtained when the number of impregnation and coating times is 4 times. I understand.

Furthermore, the current density after 3 hours of it measurement (-1.2 V vs. Hg/Hg ₂ SO ₄ ) in the range of 1.33 to 2.0 times the amount of the Ru complex polymer with the number of times of impregnation coating twice is also compared. The results obtained are shown in FIG. Under the condition of 1.5 times the amount of Ru complex polymer with 2 times of coating, the current density does not increase significantly, and the condition of 1.5 times the amount of Ru complex polymer with 3 times of CP / MWCNTs impregnation coating is specific. It was confirmed that a high current density could be secured in

FIG. 11 shows photographs of SEM observation of the surface of the sheet with 1.5 times the amount of the Ru complex polymer when the CP/MWCNTs were impregnated and coated 2 times, 3 times, and 4 times. When the impregnation coating was applied twice, the CP fibers were exposed and many voids were observed, so it is considered that the interior became porous (see the left side of FIG. 4). On the other hand, when the impregnation coating was applied four times, the CP fibers were not exposed, and the same MWCNT aggregates and Ru complex polymer composite layer as shown in FIG. 9 was covered (see the right side of FIG. 4 ). Three times of impregnation coating was an intermediate form of them, and a layer in which aggregates of MWCNTs and a Ru complex polymer were combined was confirmed on the surface with a thinness that traces of CP fibers could be slightly confirmed. . In addition, depending on the number of times of impregnation coating, there was also a difference in the deposition state of iron oxide on the surface. The amount of iron oxide precipitated was medium in the two-time impregnation coating of CP/MWCNTs, less in the three-time impregnation coating of CP/MWCNTs, and large in the four-time impregnation coating of CP/MWCNTs.

FIG. 12 is a diagram showing the results of SEM observation of a cross section of a cathode sheet with a Ru complex polymer amount 1.5 times cut after three impregnation coatings of CP/MWCNTs that ensured a high current density. The inside of the sheet was filled with the Ru complex polymer together with the MWCNTs in a high-density state with few voids.

From this examination and observation, the sheet cross-sectional shape image and weight ratio of the CP impregnated and coated with MWCNTs when the CP thickness is doubled from the conventional CP thickness have been clarified. FIG. 13 is a diagram showing an image of a cross section. The cross-sectional shape of the CP with a thickness of 0.19 mm and the MWCNTs impregnated and applied twice (the state in which the CP is sufficiently filled with MWCNTs) corresponds to the CP with a thickness of 0.35 mm and the MWCNTs impregnated and applied three times. Similarly, the cross-sectional form of CP with a thickness of 0.19 mm and MWCNTs impregnated and applied three times (a state in which the CP is sufficiently filled with MWCNTs and overflowed on the front and back surfaces) is a CP with a thickness of 0.35 mm and MWCNTs impregnated and applied. Equivalent to 4 times. In addition, the weight increased at a constant rate with respect to the base CP weight, depending on the number of MWCNTs impregnated coatings.

In this way, by increasing the thickness of the CP and optimizing the conditions for coating the MWCNTs and supporting the Ru complex polymer, a cathode sheet with a larger surface area than before can be produced. As described above, the current density in the CO ₂ reduction reaction could be greatly improved at a CP thickness of 0.35 mm. By increasing the thickness of the CP, MWCNTs and RuCP can be incorporated into the interior to improve the bulk density and increase the current density. In order to obtain such effects, the thickness of CP is preferably at least 0.3 mm or more.

"Consideration by Bulk Density"
Based on the above results, attention was paid to the bulk density of the sheet (apparent density; a value obtained by dividing the weight by the volume; voids within the sheet are also considered to be the volume). FIG. 14 is a diagram showing the bulk densities of CP, CP/MWCNTs, and CP/MWCNTs/RuCP in the conventional and the present embodiment. FIG. 15 shows −1.2 V vs. bulk density. Fig. 2 shows the current after Hg/ _{Hg2SO4 3h} ;

The cathode sheet of the present embodiment has a bulk density increased by about 30% compared to the conventional (CP thickness: 0.19 mm, MWCNTs impregnation coating twice, Ru complex polymer amount: 1 time). A bulk density range of 0.65 to 0.85 is suitable for a large current, and a bulk density ^range of 0.75 to 0.80 g/cm A range was found in which the probability of obtaining ² or more was high and the current density was specifically improved. In addition, a layer such as a Ru complex polymer is formed on the surface of the CP, and in order to maintain the porous structure, the thickness of the CP must be . 0.3 mm or more is required.

" _CO2 reduction reactor"
FIG. 16 is a diagram showing a schematic configuration of a CO ₂ reduction device 3 using a reduction reaction electrode (cathode electrode) according to an embodiment. In the CO ₂ reduction device 3 , a reduction reaction electrode (cathode electrode) 10 for advancing a reduction reaction of CO ₂ and an oxidation reaction electrode (anode electrode) 18 for causing an oxidation reaction of water are spaced apart from each other. are positioned opposite each other. The storage unit 28 has an inlet 22 and an outlet 24. An electrolytic solution in which _CO2 is dissolved (normally dissolved to saturation) flows in from the inlet, and an organic substance obtained by reducing _CO2 from the outlet 24, such as formic acid. The electrolytic solution containing the flows out.

A DC voltage is applied between the cathode electrode 10 and the anode electrode 18 from a power source 30 , thereby accelerating the reduction reaction at the cathode electrode 10 and the oxidation reaction at the anode electrode 18 . By using a solar battery as the power source 30, it is possible to obtain an organic substance such as formic acid by reducing _CO2 with solar energy.

The cathode electrode 10 and the anode electrode 18 may each have a structure in which a plurality of electrodes are laminated (stacked), and between the cathode electrode 10 and the anode electrode 18, a liquid and a gas are separated, A separator that allows protons to move may be provided.

Note that the anode electrode 18 is an electrode used for oxidizing a substance through an oxidation reaction. The anode electrode 18 includes, for example, a substrate having a conductive layer formed thereon and an oxidation catalyst layer formed thereon.

Further, the electrolytic solution is preferably an aqueous phosphate buffer solution or an aqueous borate buffer solution. In a specific configuration example, for example, a tank of CO ₂ saturated phosphate buffer solution is provided, this solution is supplied between the cathode electrode 10 and the anode electrode 18 by a pump, and formic acid (HCOOH) or the like produced by the reduction reaction is should be collected in an external tank.

"Effect of embodiment"
Thus, according to the present embodiment, by increasing the thickness in the sheet and increasing the internal bulk density within a specific range, the reduction reaction surface area is increased and the reaction efficiency is also improved. A sword sheet having a region structure can be provided. Therefore, a reduction reaction electrode and a reaction device with high current density can be obtained, and an improvement in the production efficiency of formic acid can be expected.

"Thickness of cathode sheet"
Carbon paper (CP) used in the cathode sheet of the present embodiment can be commercially available from Toray Industries, Inc., for example. Commercially available CPs have various thicknesses, but currently, the maximum thickness that can be purchased is 0.35 mm.

In commercially available CP, the bulk density is adjusted to about 0.45 g/cm ³ (porosity: about 78%) for all thicknesses. As described above, since the optimum bulk density of the cathode sheet is 0.75-0.80 g/cm ³ , the thickness is 0.35 mm (actually, it is believed to be in the range of 0.35-0.40 mm). Assuming that the bulk density is 1.66 to 1.77 times higher than that of CP alone, MWCNTs and Ru complex polymer are highly packed in CP. It should be noted that these are nominal materials, nanomaterials, and are filled with three-dimensional high dispersion.

As a means for further increasing the thickness of the CP, the above-described CP can be subjected to heat pressing with bonding, baking, or the like. If such a thing is done, the cost of raw materials will increase, so it is difficult to implement at present. However, as mass production progresses, it will become possible to use even thicker CPs.

3 _CO2 reducer, 10 cathode electrode, 18 anode electrode, 22 inlet, 24 outlet, 28 housing, 30 power supply.

Claims (3)

A reduction reaction electrode for reducing _CO2 , comprising:
a porous carbon-based sheet material;
a carbon nanotube layer provided on the surface of the sheet material;
a Ru complex polymer supported on a carbon nanotube layer;
including
a bulk density of 0.65 to 0.85 g/cm ³ ;
The thickness of the sheet material is 0.3 mm or more ,
Reduction reaction electrode. The reduction reaction electrode according to claim 1,
The sheet material is carbon paper,
The carbon nanotube layer is formed by multi-wall carbon nanotubes,
Reduction reaction electrode. The reduction reaction electrode according to claim 1 or 2,
-1.2 V vs. a current density of greater than −3.5 mA/cm ² upon application of a potential of Hg/Hg ₂ SO ₄ ;
Reduction reaction electrode.

Images