Artificial photosynthesis cells

Abstract

To provide an artificial photosynthesis cell using a perovskite solar cell by which a high conversion efficiency is realized.SOLUTION: An artificial photosynthesis system 100 comprises: a chemical reaction cell 102 for carbon dioxide reduction with an operation voltage of between 1.5 V and 2.0 V, having an oxidation reaction electrode 10 and a reduction reaction electrode 12; and a solar cell 104 for applying a bias voltage to between the oxidation reaction electrode 10 and the reduction reaction electrode 12. The solar cell 104 is configured such that two perovskite solar cells with voltage at the maximum output operation point of 0.8 V or higher are connected in series.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to an artificial photosynthesis cell. [Background technology]

[0002] Research into artificial photosynthesis, which uses sunlight to synthesize formic acid (HCOOH) and carbon monoxide (CO) from carbon dioxide (CO2), is progressing actively. Such artificial photosynthesis technology is an important technology for reducing carbon dioxide (CO2) emissions from fossil fuels.

[0003] A configuration has been disclosed in which an artificial photosynthesis cell using a solar cell and an electrochemical cell comprises a two-junction thin-film solar cell with a band gap of 1.1-2.0 eV / 1.8-2.4 eV, a positive electrode for oxidation reactions, and a negative electrode for reduction reactions (Patent Document 1). It has also been disclosed that a device using a crystalline silicon solar cell achieves a conversion efficiency of 10.5% to formic acid (HCOOH) in an electrochemical reactor for reducing carbon dioxide (CO2) (Non-Patent Document 1). Furthermore, a technology has been disclosed in which three perovskite solar cells in series are electrically connected to an electrochemical cell to synthesize carbon monoxide (CO) from carbon dioxide (CO2) (Non-Patent Document 2). In this technology, the conversion efficiency from carbon dioxide (CO2) to carbon monoxide (CO) is stated to be 8%. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-186454 [Non-patent literature]

[0005] [Non-Patent Document 1] Naohiko Kato, Yasuhiko Takeda, Yasuaki Kawai, Natsumi Nojiri, Masahito Shiozawa, Shintaro Mizuno, Ken-ichi Yamanaka, Takeshi Morikawa, and Tsuyoshi Hamaguchi, ACS Sustainable Chem. Eng, 9, 16031–16037, (2021). [Non-Patent Document 2] Jaehoon Chung, Nam Joong, and Jun Hong Noh, Energies, 15, 270 (2022). [Overview of the project] [Problems that the invention aims to solve]

[0006] The Faraday efficiency for producing formic acid (HCOOH) from carbon dioxide (CO2) using electrochemical cells has already reached 96%. Therefore, even if the Faraday efficiency of an artificial photosynthesis cell powered by a crystalline silicon solar cell with a photoelectric conversion efficiency of 15% were 100%, the conversion efficiency from carbon dioxide (CO2) to formic acid (HCOOH) would only be 12.2%.

[0007] On the other hand, while artificial photosynthesis cells using two-junction perovskite solar cells can theoretically achieve high efficiency, in practice they have challenges such as low voltage, high manufacturing costs, and susceptibility to fluctuations in the solar spectrum.

[0008] Furthermore, in artificial photosynthesis cells using three perovskite solar cells in series, the operating voltage is high, and because three solar cells must be connected in series, the current density is low, resulting in low conversion efficiency. [Means for solving the problem]

[0009] One aspect of the present invention is an artificial photosynthesis cell comprising an electrode for oxidation reaction and an electrode for reduction reaction, an electrochemical cell for carbon dioxide reduction having an operating voltage of 1.5 V or more and 2.0 V or less, and a bias power source for applying a bias voltage between the electrode for oxidation reaction and the electrode for reduction reaction. The bias power source is characterized in that it is configured by connecting two perovskite solar cells in series, the voltage at the maximum output operating point of which is 0.8 V or more.

[0010] Here, the composition of the perovskite layer of the perovskite solar cell is Cs (FA x’ MA 1-x’ ) (1-x) (Pb y Sn 1-y )(I z Br 1-z )3 (where FA is formamidinium, MA is methylammonium, 0 ≦ x ≦ 1, 0 ≦ x' ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1), which is preferable.

[0011] Further, it is preferable that the electrode for oxidation reaction is an electrode coated with an iridium oxide catalyst, and the electrode for reduction reaction is an electrode coated with a ruthenium complex catalyst.

[0012] Further, it is preferable that the electrode for oxidation reaction is an electrode coated with an iridium oxide catalyst on a Ti substrate. Further, it is preferable that the electrode for reduction reaction is an electrode in which a carbon sheet is attached to a Ti substrate, and a multi-walled carbon nanotube and a ruthenium complex catalyst are coated on the carbon sheet.

[0013] Further, it is preferable that the electrolytic solution is a phosphate buffer solution, and the concentration of the electrolytic solution is 0.1 mol / L or more and 0.8 mol / L or less.

Effects of the Invention

[0014] According to the present invention, high conversion efficiency can be achieved by an artificial photosynthesis cell using a perovskite solar cell.

Brief Description of the Drawings

[0015] [Figure 1] This is a diagram showing the configuration of the artificial photosynthesis system in an embodiment of the present invention. [Figure 2] This is a diagram showing the operating characteristics of the perovskite solar cell and the chemical reaction cell in Example 1. [Figure 3] This is a diagram showing the conversion efficiency from sunlight to formic acid in Example 1. [Figure 4] This is a diagram showing the conversion efficiency from sunlight to formic acid in Example 2. [Figure 5] This is a diagram showing the conversion efficiency from sunlight to formic acid in the comparative example.

Mode for Carrying Out the Invention

[0016] As shown in the schematic diagram of FIG. 1, the artificial photosynthesis system 100 in an embodiment of the present invention includes a chemical reaction cell 102, a solar cell 104, an electrolyte supply means 106, and a control unit 108.

[0017] In the artificial photosynthesis system 100, a reaction for generating formic acid (HCOOH) from carbon dioxide (CO2) contained in the electrolyte supplied from the electrolyte supply means 106 is performed in the chemical reaction cell 102. In this embodiment, the product is formic acid (HCOOH), but it is not limited thereto, and other hydrocarbons or the like may be used.

[0018] Power is supplied to the chemical reaction cell 102 from the solar cell 104. Control of each part and data collection in the artificial photosynthesis system 100 are performed by the control unit 108.

[0019] The chemical reaction cell 102 includes an oxidation reaction electrode 10, a reduction reaction electrode 12, a separator 14, a container 16, and an orifice plate 18.

[0020] The oxidation electrode 10 and the reduction electrode 12 are plate-shaped members that extend in the X and Z directions, respectively, and are arranged to face each other along the Y direction. In this embodiment, the oxidation electrode 10 and the reduction electrode 12 are arranged so that their reaction surfaces, which extend in the XZ plane and support each catalyst, face each other with a separator 14 in between. By combining the oxidation electrode 10 and the reduction electrode 12, an electrochemical cell for carbon dioxide reduction with an operating voltage of 1.5V to 2.0V is constructed.

[0021] In this embodiment, the oxidation reaction electrode 10, separator 14, and reduction reaction electrode 12 are arranged along the Y direction, then the reduction reaction electrode 12, separator 14, and oxidation reaction electrode 10 are arranged along the Y direction, then the oxidation reaction electrode 10, separator 14, and reduction reaction electrode 12 are arranged along the Y direction, then the reduction reaction electrode 12, separator 14, and oxidation reaction electrode 10 are arranged along the Y direction, and then the oxidation reaction electrode 10, separator 14, and reduction reaction electrode 12 are arranged along the Y direction. In other words, multiple sets of electrodes consisting of the oxidation reaction electrode 10, separator 14, and reduction reaction electrode 12 are stacked along the Y direction.

[0022] The reduction reaction electrode 12 is an electrode used to reduce a substance through a reduction reaction. The reduction reaction electrode 12 is composed of a conductive layer and a reduction catalyst layer formed on a substrate.

[0023] The substrate is a member that structurally supports the reduction reaction electrode 12. The substrate is not particularly limited in terms of material, but for example, it may be a glass substrate or the like. Further, the substrate may contain, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but it is preferable to contain titanium (Ti), silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb). In particular, it is preferable to use titanium (Ti) which has low electrical resistance and high durability. The semiconductor used as the substrate is not particularly limited, but it is preferable to be titanium dioxide (TiO2), tin dioxide (SnO2), silicon (Si), strontium titanate (SrTiO3), zinc oxide (ZnO), tantalum pentoxide (Ta2O5), etc.

[0024] When the substrate is an insulator, a conductive layer is provided between the substrate and the reduction catalyst layer. The conductive layer is provided to apply a voltage to the reduction catalyst layer of the reduction reaction electrode 12. The conductive layer is not particularly limited, but it is preferable to be a transparent conductive layer such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), zinc oxide (ZnO). In particular, considering thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

[0025] The reduction catalyst layer is composed of a material having a reduction catalyst function. The reduction catalyst layer preferably contains a complex catalyst. The reduction catalyst layer is preferably, for example, a ruthenium complex polymer (RuCP). The complex catalyst is, for example, [Ru{4,4’-di(1-H-1-pyrrolypropyl carbonate)-2,2’-bipyridine}(CO)(MeCN)Cl2], [Ru{4,4’-di(1-H-1-pyrrolypropyl carbonate)-2,2’-bipyridine}(CO)2Cl2], [Ru{4,4’-di(1-H-1-pyrrolypropyl carbonate)-2,2’-bipyridine}(CO)2] nThis can be expressed as [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(CH3CN)Cl2], etc.

[0026] Modification with a complex catalyst can be performed by coating a conductive layer with a solution of the complex dissolved in acetonitrile (MeCN). Alternatively, modification with a complex catalyst can also be carried out by electrolytic polymerization. The working electrode is the conductive layer electrode, the counter electrode is a glass substrate coated with fluorine-containing tin oxide (FTO), and the reference electrode is Ag / Ag + Using electrodes, Ag / Ag in an electrolyte containing a complex catalyst + After applying a cathode current to the electrode to create a negative voltage, Ag / Ag + The surface of the conductive layer can be modified with a complex catalyst by applying an anodic current to the electrode so that it is positively potential. Acetonitrile (MeCN) can be used as the electrolyte solution, and tetrabutylammonium perchlorate (TBAP) can be used as the electrolyte.

[0027] Furthermore, the reduction catalyst layer can be composed of a material containing carbon material (C). Preferably, the maximum length of a single carbon material structure is between 1 nm and 1 μm. The carbon material preferably contains at least one of carbon nanotubes, graphene, and graphite. If graphene and graphite are used, preferably the size of a single particle is between 1 nm and 1 μm. If carbon nanotubes are used, preferably the diameter of the tube is between 1 nm and 40 nm. The conductor can be formed by spraying a carbon material mixed with a liquid such as ethanol and heating it. Instead of spraying, it may be applied by spin coating. Alternatively, without using spin coating, the solution may be directly dropped and dried to coat the conductor.

[0028] Furthermore, the conductive layer and the reduction catalyst layer may be formed on only one side of the substrate, or they may be formed on both sides of the substrate.

[0029] The oxidation reaction electrode 10 is an electrode used to oxidize a substance through an oxidation reaction. The oxidation reaction electrode 10 is composed of a conductive layer and an oxidation catalyst layer formed on a substrate.

[0030] The substrate is a component that structurally supports the oxidation reaction electrode 10. The substrate can be made of the same material as the substrate used for the reduction reaction electrode 12.

[0031] The conductive layer is provided to effectively collect current at the oxidation reaction electrode 10. The conductive layer is not particularly limited, but is preferably made of indium tin oxide (ITO), fluorine-doped tin oxide (FTO), zinc oxide (ZnO), etc. In particular, fluorine-doped tin oxide (FTO) is preferred when considering thermal and chemical stability.

[0032] The oxidation catalyst layer is composed of a material having oxidation catalyst function. Examples of materials having oxidation catalyst function include iridium oxide (IrO2). x The material can contain iridium oxide. Iridium oxide can be supported on the surface of the conductive layer as a nanocolloidal solution (T. Arai et al., Energy Environ. Sci 8, 1998 (2015)).

[0033] For example, iridium oxide (IrO x ) nanocolloids are synthesized. Next, 50 ml of 2 mM potassium iridium(IV) chloride (K2IrCl6) aqueous solution is mixed with 10 wt% sodium hydroxide (NaOH) aqueous solution to adjust the pH to 13, and this yellow solution is heated at 90°C for 20 minutes using a hot stirrer. The resulting blue solution is cooled in ice water for 1 hour. Then, 3 M nitric acid (HNO3) is added dropwise to the cooled solution (20 ml) to adjust the pH to 1, and the mixture is stirred for 80 minutes to form iridium oxide (IrO x Obtain a nanocolloidal aqueous solution of ). Furthermore, adjust the pH to 12 by dropwise adding 1-2 ml of 1.5 wt% NaOH aqueous solution to this solution. Obtain iridium oxide (IrO) obtained in this manner. xA nanocolloidal aqueous solution of iridium oxide (IrO) is applied to a conductive layer at pH 12 and dried in a drying oven at 60°C for 40 minutes. After drying, the precipitated salt can be washed with ultrapure water to form an oxidation reaction electrode 10. x The application and drying of the nanocolloid aqueous solution may be repeated multiple times.

[0034] Furthermore, the conductive layer and the oxidation catalyst layer may be formed on only one side of the substrate, or they may be formed on both sides of the substrate.

[0035] The separator 14 is a component that separates the oxidation reaction electrode 10 and the reduction reaction electrode 12. The separator 14 prevents oxygen (O2) generated by the oxidation reaction of water (H2O) at the oxidation reaction electrode 10 from reaching the opposing reduction reaction electrode 12. This suppresses a decrease in the Faraday efficiency (FE) of formic acid (HCOOH) formation due to the reduction of oxygen (O2) at the reduction reaction electrode 12. The separator 14 can be made of a hydrophilic porous film, which is a porous material that can transfer protons in the electrolyte. For example, the separator 14 can be made of rayon nonwoven fabric, vinylon nonwoven fabric, hydrophilic ultra-high molecular weight polyethylene porous film, hydrophilic polypropylene mesh, or hydrophilic ultra-high molecular weight polyethylene porous film.

[0036] The artificial photosynthesis system 100 functions by introducing an electrolyte between the reduction electrode 12 and the oxidation electrode 10. Specifically, a container 16 is placed so as to surround the reduction electrode 12 and the oxidation electrode 10, and an electrolyte containing dissolved carbon dioxide (CO2), which is a reactant, is supplied to the surfaces of the reduction electrode 12 and the oxidation electrode 10.

[0037] The electrolyte is preferably a phosphate buffer solution or a borate buffer solution. For example, the electrolyte is preferably a phosphate buffer solution with an electrolyte concentration of 0.1 mol / L or more and 0.8 mol / L or less.

[0038] The container 16 supports the oxidation reaction electrode 10, the reduction reaction electrode 12, and the separator 14, and is also a component that forms a channel through which the electrolyte flows. The container 16 is made of a material that has the mechanical strength necessary to constitute the artificial photosynthesis system 100 as a cell. For example, the container 16 can be made of metal, plastic, or the like. The container 16 may be provided with an observation window for observing the reaction in the chemical reaction cell 102.

[0039] An electrolyte supply port is provided at the bottom of the container 16 for supplying electrolyte into the container 16. An electrolyte discharge port is provided at the top of the container 16 for discharging electrolyte. In other words, electrolyte containing the substance to be reacted is supplied into the container 16 from the electrolyte supply port, the electrolyte is circulated in the reaction region between the oxidation reaction electrode 10 and the reduction reaction electrode 12, and then the electrolyte is discharged out of the container 16 from the electrolyte discharge port.

[0040] Furthermore, the container 16 is provided with a gas exhaust port for discharging the gas produced by the reaction. In the container 16, it is preferable to position the gas exhaust port vertically above the reaction region where the oxidation reaction electrode 10 and the reduction reaction electrode 12 are located. In addition, a gas supply port may be provided in the container 16 to supply a purging gas such as nitrogen (N2) to promote discharge from the gas exhaust port. A gas sampling port may be provided at the gas exhaust port so that the concentration of components contained in the exhausted gas can be measured.

[0041] Furthermore, an orifice plate 18 is provided in the chemical reaction cell 102. The orifice plate 18 is a plate-shaped member provided with an orifice hole, which is a through-hole that restricts the flow of electrolyte introduced into the container 16 from the electrolyte supply port. The orifice plate 18 is positioned in the flow path in the container 16 from the electrolyte supply port to the region where the oxidation reaction electrode 10 and the reduction reaction electrode 12 are provided. The orifice plate 18 is made of a material that has the necessary mechanical strength. For example, the orifice plate 18 can be made of metal, plastic, or the like.

[0042] In the artificial photosynthesis system 100 of this embodiment, it is preferable to electrically connect a solar cell 104 between the reduction reaction electrode 12 and the oxidation reaction electrode 10 of the chemical reaction cell 102 and apply an appropriate bias voltage. Here, the positive electrode is connected to the oxidation reaction electrode 10 and the negative electrode is connected to the reduction reaction electrode 12.

[0043] The solar cell 104 is preferably a perovskite solar cell. In this embodiment, it is preferable to have a configuration in which two perovskite solar cells with a voltage of 0.8V or more at the maximum power operating point are connected in series. It is also preferable to use a perovskite solar cell with a photoelectric conversion efficiency of 20% or more. The composition of the perovskite layer in the perovskite solar cell is Cs x (FA x’ MA 1-x’ ) (1-x) (Pb y Sn 1-y )(I z Br 1-z )3 (where FA is formamidinium, MA is methylammonium, 0≦x≦1, 0≦x'≦1, 0≦y≦1, 0≦z≦1) is preferable.

[0044] For example, it is preferable to apply the perovskite solar cell described in "Z. Tang, T. Bessho, F. Awai, T. Kinoshita, MM Maitani, R. Jono, TN Murakami, H. Wang, T. Kubo, S. Uchida, and H. Segawa, Scientific Reports, 7, 12183 (2017)." Alternatively, it is preferable to apply the perovskite solar cell described in "Hanul Min, Do Yoon Lee, Junu Kim, Gwisu Kim, Kyoung Su Lee, Jongbeom Kim, Min Jae Paik, Young Ki Kim, Kwang S. Kim, Min Gyu Kim, Tae Joo Shin & Sang Il Seok, Nature, 598, 444 (2021)."

[0045] The solar cell 104 is preferably placed adjacent to the chemical reaction cell 102.

[0046] The electrolyte supply means 106 comprises a carbon dioxide dissolving device 30, an electrolyte tank 32, a supply pipe 34, a circulation pump 36, a liquid transfer pump 38, a recovery pipe 40, a drain pump 42, a dissolved oxygen sensor 44, a dissolved carbon dioxide sensor 46, a filter 48, a dissolved carbon dioxide sensor 50, and a temperature sensor 52.

[0047] The carbon dioxide dissolution apparatus 30 is a device for dissolving carbon dioxide (CO2), a reactant, in an electrolyte. The carbon dioxide dissolution apparatus 30 is a tank for storing the electrolyte and is equipped with a supply port for supplying carbon dioxide (CO2). By supplying carbon dioxide (CO2) from the supply port to the electrolyte stored in the tank, carbon dioxide (CO2) can be dissolved in the electrolyte.

[0048] The electrolyte tank 32 is a tank that stores the electrolyte that is circulated between the chemical reaction cell 102 and the carbon dioxide dissolution device 30.

[0049] The carbon dioxide dissolving device 30 and the electrolyte tank 32 are connected to the electrolyte supply port of the container 16 of the chemical reaction cell 102 by a supply pipe 34. The carbon dioxide dissolving device 30 and the electrolyte tank 32 are also connected to the electrolyte discharge port of the container 16 of the chemical reaction cell 102 by a recovery pipe 40. A circulation pump 36 is used to circulate the electrolyte between the carbon dioxide dissolving device 30 and the electrolyte tank 32. A liquid transfer pump 38 is used to supply the electrolyte from the electrolyte tank 32 to the container 16 of the chemical reaction cell 102. A drain pump 42 is used to recover the electrolyte from the container 16 of the chemical reaction cell 102 to the electrolyte tank 32.

[0050] The recovery pipe 40 is equipped with a dissolved oxygen sensor 44, a dissolved carbon dioxide sensor 46, and a filter 48. The electrolyte tank 32 is equipped with a dissolved carbon dioxide sensor 50 and a temperature sensor 52. The dissolved oxygen sensor 44 is a sensor that detects the concentration of oxygen (O2) dissolved in the electrolyte recovered from the chemical reaction cell 102. The oxygen (O2) concentration detected by the dissolved oxygen sensor 44 is input to the control unit 108. The dissolved carbon dioxide sensor 46 is a sensor that detects the concentration of carbon dioxide (CO2) dissolved in the electrolyte recovered from the chemical reaction cell 102. The carbon dioxide (CO2) concentration detected by the dissolved carbon dioxide sensor 46 is input to the control unit 108. The filter 48 removes impurities contained in the electrolyte recovered from the chemical reaction cell 102. The dissolved carbon dioxide sensor 50 is a sensor that detects the concentration of carbon dioxide (CO2) contained in the electrolyte stored in the electrolyte tank 32. The concentration of carbon dioxide (CO2) detected by the dissolved carbon dioxide sensor 50 is input to the control unit 108. The temperature sensor 52 is a sensor that detects the temperature of the electrolyte stored in the electrolyte tank 32. The temperature of the electrolyte detected by the temperature sensor 52 is input to the control unit 108.

[0051] The control unit 108 provides power supply and data acquisition and analysis for the artificial photosynthesis system 100. The control unit 108 can be a computer equipped with a control circuit. The control unit 108 also acquires data on the voltage and current supplied from the solar cell 104 to the oxidation reaction electrode 10 and the reduction reaction electrode 12. In addition, the control unit 108 acquires various data detected by the dissolved oxygen sensor 44, dissolved carbon dioxide sensor 46, filter 48, dissolved carbon dioxide sensor 50, and temperature sensor 52.

[0052] <Example 1> The oxidation reaction electrode 10 is made of iridium oxide (IrO) on a mechanically polished Ti substrate. x Apply the colloid 6 times, let it dry, and then apply IrO x The catalyst was immobilized. Four of the oxidation reaction electrodes 10 were fixed to a 1m square Ti plate using Ti bolts / nuts. Eight of these 1m square oxidation reaction electrodes 10 were fabricated.

[0053] Ru-complex polymer (RuCP,[Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2Cl2]) was used as the Ru complex polymer for the reduction reaction electrode 12. A 25cm square Ru complex polymer / multiwall carbon nanotube / carbon sheet (thickness 320μm, hereinafter abbreviated as RuCP / MWCNTs / CS) was prepared by the following procedure. First, the CS was immersed in an ink containing MWCNTs, dried, and heat-treated to support the MWCNTs. Furthermore, a RuCP solution was applied to the MWCNTs-supported CS and vacuum-dried to support the RuCP catalyst on the MWCNTs / CS. The reduction reaction electrode 12 was prepared by attaching four of these RuCP / MWCNTs / CS sheets to the center of a mechanically polished titanium (Ti) substrate using a graphite-based adhesive. Furthermore, four reduction reaction electrodes 12 were fixed to a 1m square Ti plate using Ti bolts / nuts. Eight of these were made as 1m square reduction reaction electrodes 12.

[0054] Furthermore, a crossover reaction occurs where oxygen generated by the oxidation reaction of water at the oxidation reaction electrode 10 reaches the opposing reduction reaction electrode 12 and is reduced, causing a decrease in the Faraday efficiency (FE) of formic acid production. Therefore, in order to suppress this crossover reaction, a separator 14 made of porous ultra-high molecular weight polyethylene film was inserted between the oxidation reaction electrode 10 and the reduction reaction electrode 12.

[0055] 275 liters of 0.4 M potassium phosphate buffer solution (KPi) was used as the electrolyte. The electrolyte tank 32 was filled with the electrolyte, and the electrolyte was circulated between the electrolyte tank 32 and the container 16 using a liquid transfer pump 38 and a drain pump 42. In addition, 100% carbon dioxide (CO2) was supplied to the electrolyte in the carbon dioxide dissolution device 30 to saturate the electrolyte with dissolved carbon dioxide (CO2).

[0056] In this configuration, the Faraday efficiency was measured by producing formic acid in a neutral electrolyte solution with a pH of 6.3 through the oxidation reaction of water at the oxidation reaction electrode 10 and the reduction reaction of carbon dioxide (CO2) at the reduction reaction electrode 12.

[0057] In this configuration, we investigated the case where a perovskite solar cell with a photoelectric conversion efficiency of approximately 20% (Z. Tang, T. Bessho, F. Awai, T. Kinoshita, MM Maitani, R. Jono, TN Murakami, H. Wang, T. Kubo, S. Uchida, and H. Segawa, Scientific Reports, 7, 12183 (2017)) was used as solar cell 104. The current-voltage characteristics of this perovskite solar cell are shown in Figure 2e of "Z. Tang, T. Bessho, F. Awai, T. Kinoshita, MM Maitani, R. Jono, TN Murakami, H. Wang, T. Kubo, S. Uchida, and H. Segawa, Scientific Reports, 7, 12183 (2017)." Example 1 involved connecting two of the perovskite solar cells in series, and the conversion efficiency to formic acid when irradiated with sunlight was estimated.

[0058] Figure 2 shows the operating curve (solid line) when two perovskite solar cells are connected in series, and the operating curve (dashed line) of chemical reaction cell 102. The intersection of the two curves is the operating point of the formic acid production reaction, and the current density at this operating point is 10.4 mA / cm². 2 It was estimated that...

[0059] <Comparative Examples 1-3> Comparative Examples 1 to 3 were created by using the same perovskite solar cell as in Example 1, in combination with three cells connected in series, and in combination with four cells connected in series. The conversion efficiency to formic acid when irradiated with sunlight was estimated.

[0060] Figure 3 shows the conversion efficiency to formic acid for Example 1 and Comparative Examples 1-3, when the operating voltage, current density, and Faraday efficiency were set to 90%, 96%, and 100%. When two perovskite solar cells were connected in series, the conversion efficiency from sunlight to formic acid was highest, at approximately 13.1% when the Faraday efficiency for formic acid production was 90%, approximately 14.0% when the Faraday efficiency was 96%, and approximately 14.6% when the Faraday efficiency was 100%.

[0061] The operating voltage dependence of the Faraday efficiency of formic acid production in chemical reaction cell 102 is shown in Figure S11 of "Naohiko Kato, Shintaro Mizuno, Masahito Shiozawa, Natsumi Nojiri, Yasuaki Kawai, Kazuhiro Fukumoto, Takeshi Morikawa, and Yasuhiko Takeda, Joule, 5, 1-19 (2021)". The Faraday efficiency is high, above 0.96, when the operating voltage is 1.9V or less, but decreases when it exceeds 1.9V. The main reason for this is presumed to be the generation of hydrogen.

[0062] <Example 2> In the same configuration as in Example 1, we investigated the case where a perovskite solar cell with a photoelectric conversion efficiency of approximately 25.6% (Hanul Min, Do Yoon Lee, Junu Kim, Gwisu Kim, Kyoung Su Lee, Jongbeom Kim, Min Jae Paik, Young Ki Kim, Kwang S. Kim, Min Gyu Kim, Tae Joo Shin, and Sang Il Seok, Nature, 598, 444 (2021)) was used as the solar cell 104. The current-voltage characteristics of this perovskite solar cell are shown in Figure 4c of "Hanul Min, Do Yoon Lee, Junu Kim, Gwisu Kim, Kyoung Su Lee, Jongbeom Kim, Min Jae Paik, Young Ki Kim, Kwang S. Kim, Min Gyu Kim, Tae Joo Shin, and Sang Il Seok, Nature, 598, 444 (2021)". Example 1 involved connecting two of the perovskite solar cells in series, and the conversion efficiency to formic acid when irradiated with sunlight was estimated.

[0063] <Comparative Examples 4-6> Comparative Examples 4-6 were created by using the same perovskite solar cell as in Example 2, one cell, three cells connected in series, and four cells connected in series, respectively. The conversion efficiency to formic acid when irradiated with sunlight was estimated.

[0064] Figure 4 shows the conversion efficiency to formic acid for Example 2 and Comparative Examples 4-6 when the operating voltage, current density, and Faraday efficiency were set to 90%, 96%, and 100%. When two perovskite solar cells were connected in series, the conversion efficiency from sunlight to formic acid was highest, at approximately 15.8% when the Faraday efficiency for formic acid production was 90%, approximately 16.8% when the Faraday efficiency was 96%, and approximately 17.5% when the Faraday efficiency was 100%.

[0065] <Comparative Examples 7-12> In the same configuration as in Example 1, the case where a silicon solar cell was used as solar cell 104 was investigated. Comparative Examples 7 to 12 were created using a single silicon solar cell with a photoelectric conversion efficiency of 15%, and 2 to 6 silicon solar cells connected in series, respectively, and the conversion efficiency to formic acid when irradiated with sunlight was estimated. Figure 5 shows the conversion efficiency to formic acid for Comparative Examples 7 to 12 when the operating voltage, current density, and Faraday efficiency were set to 90%, 96%, and 100%. When four silicon solar cells were connected in series, the conversion efficiency from sunlight to formic acid was highest, at approximately 11.0% when the Faraday efficiency of formic acid production was 90%, approximately 11.7% when the Faraday efficiency was 96%, and approximately 12.2% when the Faraday efficiency was 100%. As described above, in both Examples 1 and 2, the conversion efficiency from sunlight to formic acid was higher than that of Comparative Example 1.

[0066] [Structure of the present invention] Configuration 1: An electrochemical cell for carbon dioxide reduction having an electrode for oxidation and an electrode for reduction, with an operating voltage of 1.5V to 2.0V, A bias power supply that applies a bias voltage between the oxidation reaction electrode and the reduction reaction electrode, Equipped with, The aforementioned bias power supply is characterized by a configuration in which two perovskite solar cells, each having a voltage of 0.8V or higher at the maximum output operating point, are connected in series to form an artificial photosynthesis cell. Configuration 2: The artificial photosynthesis cell described in Configuration 1, The artificial photosynthesis cell is characterized in that the composition of the perovskite layer of the perovskite solar cell is Csx(FAx'MA1-x')(1-x)(PbySn1-y)(IzBr1-z)3 (where FA is formamidinium, MA is methylammonium, 0≦x≦1, 0≦x'≦1, 0≦y≦1, 0≦z≦1). Configuration 3: An artificial photosynthesis cell according to configuration 1 or 2, The electrode for the oxidation reaction is an electrode coated with iridium oxide catalyst. The artificial photosynthesis cell is characterized in that the electrode for the reduction reaction is an electrode coated with a ruthenium complex catalyst. Configuration 4: The artificial photosynthesis cell described in configuration 3, The aforementioned oxidation reaction electrode is an electrode coated with an iridium oxide catalyst on a Ti substrate, characterized in that it is an artificial photosynthesis cell. Configuration 5: An artificial photosynthesis cell according to configuration 3 or 4, The reduction reaction electrode is characterized by being an electrode in which a carbon sheet is attached to a Ti substrate, and a multi-wall carbon nanotube and a ruthenium complex catalyst are coated onto the carbon sheet. Configuration 6: An artificial photosynthesis cell described in any one of configurations 1 to 5, An artificial photosynthesis cell characterized in that the electrolyte is a phosphate buffer and the concentration of the electrolyte is 0.1 mol / L or more and 0.8 mol / L or less. [Explanation of symbols]

[0067] 10 Electrode for oxidation reaction, 12 Electrode for reduction reaction, 14 Separator, 16 Container, 18 Orifice plate, 30 Carbon dioxide dissolution device, 32 Electrolyte tank, 34 Supply pipe, 36 Circulation pump, 38 Liquid transfer pump, 40 Recovery pipe, 42 Drainage pump, 44 Dissolved oxygen sensor, 46 Dissolved carbon dioxide sensor, 48 Filter, 50 Dissolved carbon dioxide sensor, 52 Temperature sensor, 100 Artificial photosynthesis system, 102 Chemical reaction cell, 104 Solar cell, 106 Electrolyte supply means, 108 Control unit.

Claims

1. An electrochemical cell for carbon dioxide reduction having an electrode for oxidation and an electrode for reduction, with an operating voltage of 1.5V to 2.0V, A bias power supply that applies a bias voltage between the oxidation reaction electrode and the reduction reaction electrode, Equipped with, The bias power supply has a configuration in which two perovskite solar cells, each having a voltage of 0.8V or more at the maximum output operating point, are connected in series. The perovskite layer of the perovskite solar cell is composed of Cs x (FA x' MA 1-x') (1-x) (Pby y Sn 1-y) (I z Br 1-z) 3 (where FA is formamidinium, MA is methylammonium, 0 ≤ x ≤ 1, 0 ≤ x' ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1), and the photoelectric conversion efficiency of the perovskite solar cell is 20% or more, making it an artificial photosynthesis cell.

2. An artificial photosynthesis cell according to claim 1, The electrode for the oxidation reaction is an electrode coated with iridium oxide catalyst. The artificial photosynthesis cell is characterized in that the electrode for the reduction reaction is an electrode coated with a ruthenium complex catalyst.

3. An artificial photosynthesis cell according to claim 2, The aforementioned oxidation reaction electrode is an electrode coated with an iridium oxide catalyst on a Ti substrate, characterized in that it is an artificial photosynthesis cell.

4. An artificial photosynthesis cell according to claim 3, The reduction reaction electrode is characterized by being an electrode in which a carbon sheet is attached to a Ti substrate, and a multi-wall carbon nanotube and a ruthenium complex catalyst are coated onto the carbon sheet.

5. An artificial photosynthesis cell according to any one of claims 1 to 4, An artificial photosynthesis cell characterized in that the electrolyte is a phosphate buffer solution and the concentration of the electrolyte solution is 0.1 mol / L or more and 0.8 mol / L or less.

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