JP7260791B2 - chemical reaction cell - Google Patents

Description

TECHNICAL FIELD The present invention relates to a chemical reaction cell including an electrochemical reaction cell and a photoelectric conversion element.

Artificial synthesis of hydrogen (H ₂ ) from water (H ₂ O) using solar energy, and carbon monoxide (CO) and formic acid (HCOOH) from water (H ₂ O) and carbon dioxide (CO ₂ ) Photosynthesis has been vigorously studied from the viewpoint of both reduction of CO ₂ emissions and storage of solar energy (see Non-Patent Documents 1 and 2). In particular, a high conversion efficiency (η _STC ) from solar energy to chemical energy has been achieved by combining a photocharge separation device and an electrochemical reaction cell (see Non-Patent Documents 3 to 7).

A photoelectric charge separation device has the same basic structure as a solar cell used for photovoltaic power generation. However, while solar cells are designed to maximize power (the product of current and voltage), photocharge separation devices used in artificial photosynthesis have the maximum current at the voltage required for the reaction. A design that becomes The threshold voltage for the reaction that decomposes H ₂ O to produce H ₂ is 1.23 V, and the threshold voltage for the reaction that produces CO and HCOOH from CO ₂ and H ₂ O is 1.34 V and 1.34 V, respectively. 43V. However, since an overvoltage is necessary to actually advance the reaction, the operating voltage of the electrochemical reaction cell is about 1.5 to 2 V (see Non-Patent Documents 5 to 10). To generate this voltage, a series multi-junction device such as a series 2-junction device or a series 3-junction device is used.

There are two problems with these series multijunction devices. One is that carriers are extracted via two- or three-photon excitation, even though the solar spectrum contains high-energy photons such that single-photon excitation produces the required voltage. photons cannot be used effectively. The other is that current matching between subcells fails when the solar spectrum fluctuates, so the current through the entire device is limited by the minimum of the photocurrents in each subcell.

Sunlight spectra under clear and cloudy weather are shown in Non-Patent Document 14 and are shown in FIG. 5(a). These are the results of actual measurement of the total solar radiation spectrum (sum of direct light and scattered light) on a south facing slope with an inclination angle of 32 degrees in Tsukuba City (36 degrees north latitude, 140 degrees east longitude). FIG. 5(b) shows the result of normalizing each spectrum by the average value from 1020 to 1040 nm in order to characterize the difference in spectrum. Comparing the measured value with AM (air mass) 1.5G, which is the reference spectrum, a clear difference is observed (see Non-Patent Document 15). If the AM (air mass) value is small, the absorption of short wavelength components by the atmosphere is small. While the components are relatively strong, Fine-2 measured at AM (air mass) 6.7 shows the opposite trend. On the other hand, in the case of OC-1 and OC-2, which are the measured values under cloudy weather, the short wavelength components are relatively strong due to scattering by clouds (see Non-Patent Document 16). However, the normalized spectrum at wavelengths 700-1700 nm (photon energy 1.8-0.7 eV) does not change much.

Two problems with the multi-junction photocharge separation device described above are as follows. When subcells constituting a multi-junction device are connected in series, the bandgaps are adjusted so that the photocurrents generated in the subcells are approximately the same, that is, the currents are matched. Therefore, if a series multi-junction device designed for AM1.5G conditions is used with a small air mass or in cloudy weather, current matching will be broken, and only a portion of the relatively large top-cell photocurrent can be extracted. . Conversely, if the air mass is large, the relatively small top cell photocurrent limits the device current. Another problem is that high-energy photons are not effectively used because they only excite carriers in the top cell, even though one photon can generate the voltage required for the reaction. . Since carrier excitation is necessary in the middle cell and the bottom cell for current extraction, in the end, regardless of the photon energy absorbed in the top cell, two (two junctions) or three junctions are required for carrier extraction. (3-junction) photons are absorbed.

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SUMMARY OF THE INVENTION An object of the present invention is to provide a chemical reaction cell including an electrochemical reaction cell and a photoelectric conversion element capable of improving conversion efficiency from solar energy to chemical energy.

（１）

（２）
（ここで、

は、太陽光の光子数スペクトルである。） The present invention is a chemical reaction cell comprising an electrochemical reaction cell and a photoelectric conversion element, wherein an operating voltage (V _op ) of the electrochemical reaction cell is in the range of 1.4 to 2.2 V, and the photoelectric conversion element is The conversion element is a series/parallel three-junction element in which a series-connected middle/bottom cell is connected in parallel with a top cell, and the bandgap (ε _g ^(t) ) of the light-absorbing material of the top cell is 1. 11×V _op +0.21 to 1.11×V _op +0.56 eV and 2.0 to 2.9 eV , and the band gap (ε _g ^(m) ) of the light absorbing material of the middle cell is , 0.55×V _op +0.60 to 0.59×V _op +0.72 eV, and the bandgap (ε _g ^(b) ) of the light absorbing material of the bottom cell is 0.53×V _op +0 .088 to 0.56*V _op +0.21 eV and the minimum value of ε _g ^(m) + ε _g ^(b) is 1.09*V _op +0.74 eV, the AM1.5G solar spectrum The ratio N (m) of the number of absorbed photons N ^(m) of the middle cell represented by the following formula (1) to the number of absorbed photons N ^(b) of the bottom cell represented by the following formula (2 ⁾ when irradiated with /N ^(b) is the chemical reaction cell, which ranges from 0.5 to 1.4.

(1)

(2)
(here,

is the photon number spectrum of sunlight. )

In the chemical reaction cell, the ε _g ^(t) is 1.09×V _op +0.49 eV, the ε _g ^(m) is 0.57×V _op +0.63 eV, and the ε _g ^(b) is preferably 0.55×V _op +0.18 eV .

The present invention is a chemical reaction cell comprising an electrochemical reaction cell and a photoelectric conversion element, wherein an operating voltage (V _op ) of the electrochemical reaction cell is 1.55 V, and the photoelectric conversion elements are connected in series. a series/parallel three-junction device in which a combined middle/bottom cell is connected in parallel with a top cell, the band gap (ε _g ^(t) ) of the light absorbing material of the top cell is 2.17 eV; The bandgap (ε _g ^(m) ) of the light absorbing material in the bottom cell is 1.51 eV, the bandgap (ε _g ^(b) ) of the light absorbing material in the bottom cell is 1.02 eV, and the AM1.5G solar The ratio of the number of absorbed photons N ^(m ) of the middle cell represented by the above formula (1) to the number of absorbed photons N ^(b) of the bottom cell represented by the above formula (2) when a light spectrum is irradiated N ^{( m)} /N ^(b) is in the range of 0.2 to 2.5, and the electrochemical reaction cell is a cell that produces H ₂ and O ₂ from H ₂ O. be.

In the chemical reaction cell, the electrochemical reaction cell is preferably a cell that produces _H2 and _O2 from _H2O .

In the chemical reaction cell, the electrochemical reaction cell is preferably a cell that reduces _CO2 .

In the chemical reaction cell, the electrochemical reaction cell is preferably a cell that produces CO or HCOOH from _CO2 and _H2O .

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a chemical reaction cell including an electrochemical reaction cell and a photoelectric conversion element capable of improving conversion efficiency from solar energy to chemical energy.

1 is a diagram showing a schematic configuration of an example of a facing-type chemical reaction cell according to an embodiment of the present invention; FIG. 1 is a diagram showing a schematic configuration of an example of an integrated chemical reaction cell according to an embodiment of the present invention; FIG. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows schematic structure of an example of the integrated chemical reaction cell using the interpenetrating electrode which concerns on embodiment of this invention. 1 is a diagram showing a schematic configuration of an example of a series/parallel triple-junction photoelectric charge separation device in a chemical reaction cell according to an embodiment of the present invention; FIG. (a) Shows the global solar radiation spectrum, and the measured value in Tsukuba City is compared with the reference spectrum AM1.5G 1 sun (100 mW/cm ² ) (see Non-Patent Documents 14 and 15). Fine-1 (clear weather) June 14, 2015 at 11:40 (solar intensity 92.6 mW/cm ² , air mass 1.03) Fine-2 (clear weather) December 29, 2015 at 15:40 (44.0 mW/cm ² , air mass 6.7), OC-1 (cloudy) at 11:20 on September 21, 2015 (35.2 mW/cm ² , air mass 1.22), OC-2 (cloudy) ) is the spectrum on Sep. 21, 2015 at 11:10 (37.6 mW/cm ² , air mass 1.22). (b) The result of normalizing each spectrum by the average value of 1020 to 1040 nm. Both are cited from Non-Patent Document 14. Fig. 4 is a graph showing the conversion efficiency (η _STC ) of solar energy to chemical energy of _H2 ; Fine-1, Fine-2 (clear weather) and OC-1, OC-2 (cloudy weather) are values obtained when sunlight, whose spectral intensity is shown in FIG. 5, is irradiated. The subcell bandgap of each device was optimized under the conditions of V _op =1.55 V, AM1.5G 1 sun illumination. (a) to (e) Operating current J (V _op ) is a graph. The sub-cell bandgap of each device was optimized under the condition of AM1.5G 1 sun illumination for each operating voltage V _op . (f) Graph showing the bandgap of each subcell of the optimized series/parallel three-junction device. 2 is a graph showing (a) the effect of current matching between a middle cell and a bottom cell, and (b) the effect of voltage matching between a top cell and a middle/bottom series connection in a photoelectric conversion device for H ₂ generation. V _op =1.55V.

An embodiment of the present invention will be described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

（ここで、

は、太陽光の光子数スペクトルである。） A chemical reaction cell according to an embodiment of the present invention is a chemical reaction cell including an electrochemical reaction cell and a photoelectric conversion element, and the operating voltage of the electrochemical reaction cell is in the range of 1.4 to 2.2V. and the photoelectric conversion element is a series/parallel three-junction element in which a series-connected middle/bottom cell is connected in parallel with a top cell, and the bandgap (ε _g ^(t) ) of the light-absorbing material of the top cell is It is in the range of 2.0 to 2.9 eV, and is the sum of the bandgap (ε _g ^(m) ) of the light absorbing material in the middle cell and the bandgap (ε _g ^(b) ) of the light absorbing material in the bottom cell, ε _g ^{(m )} +ε _g ^(b) is 0.25 to 1.1 eV larger than ε _g ^(t) , and the number of absorbed photons N of the middle cell represented by the following formula (1) when irradiated with the AM1.5G sunlight spectrum is The ratio N ^{(m) /} N ^{(b) of (m) to the number of absorbed photons N ( b)} of the bottom cell represented by the following formula (2) is in the range of 0.2 to 2.5. is.

(here,

is the photon number spectrum of sunlight. )

The sum of the bandgap (ε _g ^(m) ) of the light absorbing material in the middle cell and the bandgap (ε _g ^(b) ) of the light absorbing material in the bottom cell, ε _g ^(m) + ε _g ^(b) , is ε _g ^{(t )} and 0.25 to 0.6 eV greater than ε _g ^(t) .

The ratio N (m ⁾ /N (b) of the number of absorbed photons N ^(m) of the middle cell to the number of absorbed photons N ^{(b) of} the bottom cell when irradiated with the AM1.5G sunlight spectrum is 0.2 to 2.5. and preferably in the range of 0.5 to 1.4.

The chemical reaction cell according to the present embodiment can obtain a larger operating current at a predetermined operating voltage than in the case of using conventional serial two-junction photoelectric conversion elements or the like. As a result, the conversion efficiency of solar energy to chemical energy is higher. The chemical reaction cell according to the present embodiment can obtain a large operating current even in cloudy weather other than in fine weather.

The photoelectric conversion element 10, which is a series/parallel three-junction element in which a middle/bottom cell connected in series and a top cell are connected in parallel, emits only high-energy light with relatively large intensity fluctuations, as shown in FIG. A wide-gap top cell 30 for absorption is provided, and this is connected in parallel with two series junctions of a middle cell 32/bottom cell 34 (see Non-Patent Documents 11 to 13). Current matching between the middle/bottom cells is roughly maintained since the normalized spectrum for mid-to-low energies does not change much. On the other hand, no current matching is required between the top and middle/bottom cells. Furthermore, since the carriers generated in the top cell are taken out without passing through the middle/bottom cells, effective utilization of high-energy photons is also possible. Voltage matching between top and middle/bottom cells becomes a new constraint. This condition is almost independent of the solar spectrum.

As materials for constructing a series/ _parallel three-junction photocharge separation device, a CIGS-based material (non-patented References 22 and 29), crystalline silicon, etc., and organic-inorganic hybrid perovskites, etc., for top/middle cells. The electrochemical cell for _CO2 reduction was set to Vop=2. For operation at 0V , organic-inorganic hybrid perovskite or CIGS-based materials are used for middle/bottom cells, and InGaN-based multiple quantum wells are used for top cells. Examples of CIGS-based materials include CuIn _1-x Ga _x Se ₂ and the like. In order to suppress the recombination of electrons and holes and to move them to the cathode and anode quickly, the composition x is often varied in the thickness direction of the CuIn _1-x Ga _x Se ₂ layer. At this time, the range of x may be set to, for example, 0.18 to 0.43 (see Non-Patent Document 35). Organic-inorganic hybrid perovskites include, for example, those having a composition (FA: formamidinium, MA: methylammonium) of (FAPbI ₃ ) _0.95 (MAPbBr ₃ ) _0.05 (see Non-Patent Document 23). Examples of InGaN-based materials include In _0.2 Ga _0.8 N (6 nm)/GaN (10 nm) multiple quantum wells (see Non-Patent Document 33).

The configuration of the chemical reaction cell according to this embodiment is not particularly limited as long as it includes the electrochemical reaction cell and the photoelectric conversion element.

In the chemical reaction cell according to the present embodiment, for example, as shown in FIG. and a photoelectric conversion element 10, which is a series/parallel three-junction element arranged adjacent to a negative electrode (cathode electrode) 14, packaged together with electrical wiring. is. For example, as shown in FIG. 2, the chemical reaction cell according to this embodiment includes a positive electrode (anode electrode) 12 and a negative electrode (cathode electrode) 14 on both sides of a photoelectric conversion element 10, which is a series/parallel three-junction element. An integrated chemical reaction cell 3 may be used by immersing it in the electrolytic solution in the container. Further, in the chemical reaction cell according to the present embodiment, for example, as shown in FIG. It is in the electrolyte (water) in the container 20 or on the electrolyte (water) surface (for example, the cell is floating on the electrolyte (water) surface and the photoelectric conversion element 10 is not immersed in the electrolyte (water)). However, the positive electrode (anode electrode) 12 and the negative electrode (cathode electrode) 14 are in contact with the electrolytic solution (water)).

The chemical reaction cell 1 shown in FIG. 1 will be described below as an example. In the electrochemical reaction cell of the chemical reaction cell 1, for example, a positive electrode (anode electrode) 12 and a negative electrode (cathode electrode) 14, which are, for example, plate-shaped members, are arranged in a housing portion 18 at positions facing each other with a gap therebetween, Between the anode electrode 12 and the cathode electrode 14, there is provided a channel 16 through which an electrolytic solution containing a reaction substrate such as carbon dioxide flows. A photoelectric conversion element 10 is connected to the anode electrode 12 and the cathode electrode 14 via an electric wire or the like.

The electrochemical reaction cell functions by introducing an electrolytic solution containing a reaction substrate such as carbon dioxide into the channel 16 between the anode electrode 12 and the cathode electrode 14 . At the anode electrode 12, for example, water ( _H2O ) is oxidized to obtain oxygen (1/ _2O2 ). At the cathode electrode 14, for example, carbon dioxide is reduced to produce formic acid (HOOOH) or the like.

A negative electrode (cathode electrode) 14, which is a reduction reaction electrode, is an electrode used for reducing a substance by a reduction reaction. The cathode electrode 14 includes, for example, a conductive layer and a conductive layer that are sequentially formed on the substrate.

The substrate is a member that structurally supports the electrodes. The material of the substrate is not particularly limited, but examples thereof include a glass substrate. The substrate may comprise, for example, metals or semiconductors. The metal used as the substrate is not particularly limited, but examples include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin ( Sn), palladium (Pd), lead (Pb), and the like. The semiconductor used as the substrate is not particularly limited, but examples include titanium oxide (TiO ₂ ), tin oxide (SnO ₂ ), silicon (Si), strontium titanate (SrTiO ₃ ), and zinc oxide (ZnO). , tantalum oxide (Ta ₂ O ₅ ), and the like. When the substrate contains a metal or semiconductor, it is preferable to form a conductive layer and an insulating layer between the conductive layer and the substrate. The insulating layer is not particularly limited, but examples thereof include semiconductor oxides, nitrides, and resins. A configuration in which the metal substrate, the conductive layer, and the conductive layer are electrically directly connected may be employed.

The substrate may be configured to contain a carbon material in terms of having a high specific surface area. Examples of the carbon material include, but are not limited to, carbon black, graphene, fullerene, carbon nanotube, carbon cloth, carbon paper, and the like.

The conductive layer is provided to effectively collect current at the cathode electrode 14 . The conductive layer is not particularly limited, but examples thereof include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, considering thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

The conductor layer includes a conductor containing a material having a reduction catalytic function. The conductor may comprise a material containing carbon material (C). It is preferable that the size of a single carbon material structure is 1 nm or more and 1 μm or less. Carbon materials include, for example, at least one of carbon nanotubes, graphene, and graphite. Graphene and graphite preferably have a size of 1 nm or more and 1 μm or less. Carbon nanotubes preferably have a diameter of 1 nm or more and 40 nm or less. The conductor can be formed, for example, by spraying and heating a carbon material mixed with a liquid such as ethanol. It may be applied by spin coating instead of spraying. Alternatively, the solution may be directly dropped and dried without using spin coating.

A complex catalyst or the like can be used as the material having a reduction catalytic function. The complex catalyst is preferably a ruthenium complex, for example. Complex catalysts 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) _2Cl2 ], [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)(CH ₃ CN)Cl ₂ ] etc.

Modification with a complex catalyst can be produced, for example, by dissolving a complex in an acetonitrile (MeCN) solution and applying it on the conductor of the conductor layer. Modification with a complex catalyst can also be performed by an electropolymerization method. For example, a conductor layer containing a conductor as a working electrode (a support substrate may be provided), a glass substrate coated with fluorine-containing tin oxide (FTO) as a counter electrode, and an Ag/Ag ⁺ electrode as a reference electrode, a complex catalyst In ^an electrolytic solution ^containing can be modified with a complex catalyst on the conductor of For the electrolyte solution, for example, acetonitrile (MeCN) can be used, and for the electrolyte, for example, Tetrabutylammonium perchlorate (TBAP) can be used.

The conductive layer formed in this manner is carried, applied or adhered onto the conductive layer constituting the cathode electrode 14 . Thereby, a cathode electrode 14 having a conductive layer and a conductive layer in order is formed on the substrate.

A positive electrode (anode electrode) 12, which is an electrode for oxidation reaction, is an electrode used for oxidizing a substance by an oxidation reaction. The anode electrode 12 includes, for example, a conductive layer and an oxidation catalyst layer that are sequentially formed on a substrate.

The substrate is a member that structurally supports the electrodes. The substrate can be of the same material as the substrate used for cathode electrode 14 .

The conductive layer is provided to effectively collect current at the anode electrode 12 . The conductive layer is not particularly limited, but examples thereof include indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, considering thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

The oxidation catalyst layer includes a material having an oxidation catalyst function. Examples of materials having an oxidation catalytic function include materials containing iridium oxide (IrOx). Iridium oxide can be supported on the surface of the conductive layer as a nanocolloidal solution (see T.Arai et.al, Energy Environ. Sci 8, 1998 (2015)).

For example, iridium oxide (IrOx) nanocolloids are synthesized. First, a yellow solution adjusted to pH 13 by adding 10 wt% sodium hydroxide (NaOH) aqueous solution to 50 mL of 2 mM potassium iridate (IV) chloride (K ₂ IrCl ₆ ) aqueous solution was stirred at 90° C. for 20 minutes using a hot stirrer. heat for 1 minute. The resulting blue solution is cooled with ice water for 1 hour. Then, 3M nitric acid (HNO ₃ ) is added dropwise to the cooled solution (20 mL) to adjust the pH to 1, and the mixture is stirred for 80 minutes to obtain a nanocolloid aqueous solution of iridium oxide (IrOx). Furthermore, a 1.5 wt % NaOH aqueous solution (1 to 2 mL) is added dropwise to this solution to adjust the pH to 12. The nanocolloidal aqueous solution of iridium oxide (IrOx) obtained in this way is applied on the conductive layer at pH 12 and dried by being held at 60° C. for 40 minutes in a drying oven. After drying, the deposited salt can be washed with ultrapure water to form an anode electrode. Note that the application and drying of the iridium oxide (IrOx) nanocolloid aqueous solution may be repeated multiple times.

The reaction substrate contained in the electrolytic solution includes, for example, carbon compounds, such as carbon dioxide (CO ₂ ). 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 carbon dioxide (CO ₂ )-saturated phosphate buffer solution is provided, this solution is supplied to the surfaces of the negative electrode and the positive electrode by a pump, and formic acid (HCOOH) generated by the reduction reaction is removed to the outside. collected in a separate tank.

The housing portion 18 is a member that supports the anode electrode 12 and the cathode electrode 14 and constitutes the flow path 16 through which the electrolytic solution flows. The housing portion 18 is made of a material having mechanical strength required to construct the electrochemical reaction cell as a cell. For example, the housing can be made of metal, plastic, or the like.

A separator may be provided between the anode electrode 12 and the cathode electrode 14 . The separator is not particularly limited as long as it is a material that separates a liquid and a gas and allows protons to move. can.

By connecting the photoelectric conversion element 10 between the anode electrode 12 and the cathode electrode 14, an appropriate bias voltage is applied. By using the photoelectric conversion element 10 as means for applying a bias voltage, an electrochemical reaction cell and the photoelectric conversion element 10 for generating electric power to be supplied to the anode electrode 12 and the cathode electrode 14 of the electrochemical reaction cell. It can be a chemical reaction cell comprising: The photoelectric conversion element 10 can be arranged, for example, adjacent to the anode electrode 12 and the cathode electrode 14 of an electrochemical reaction cell. For example, as shown in FIG. 1, the photoelectric conversion element 10 is arranged behind the cathode electrode 14 of the electrochemical reaction cell, the positive electrode of the photoelectric conversion element 10 is connected to the anode electrode 12, and the negative electrode of the photoelectric conversion element 10 is connected to the cathode. It may be connected to the electrode 14 .

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

The current density-voltage (JV) characteristics of the series/parallel photocharge separation device are modeled, and the characteristics are calculated when illuminated with sunlight having the spectral intensity shown in FIG. Furthermore, η _STC is obtained using the result of this JV characteristic, and compared with the cases of series two-junction and three-junction devices.

The JV characteristics of a single-junction photocharge separation device represented by an equivalent circuit consisting of a combination of a constant current source, a diode, a series resistor, and a parallel resistor are: photocurrent J _ph , diode inverse saturation current J ₀ , series resistance R _s and the parallel resistance R _sh (see Non-Patent Documents 17 to 19).

により決まる。 q, k _B , and T are elementary charge, Boltzmann's constant, and device temperature (300 K), respectively. When approximating the external quantum efficiency η _EQE to be a constant value independent of photon energy, J _ph is the bandgap ε _g of the light absorbing material used in the device and the photon number spectrum of sunlight.

determined by

となる（非特許文献２０参照）。 Using the generalized Planck's law for the radiative recombination rate, the external luminous efficiency η _ERE , and applying the Boltzmann approximation, J ₀ is

(see Non-Patent Document 20).

The JV characteristics of the series/parallel three-junction photocharge separation device are obtained by solving the following simultaneous equations. However, the superscripted J and V are the current density and voltage of the top cell (t), middle cell (m) and bottom cell (b).

The simultaneous equations representing the JV characteristics of a series two-junction device are as follows.

A series 3-junction element can also be found in the same way.

η _STC when a photocharge separation device is combined with an electrochemical reaction cell to generate H ₂ or the like is expressed by the product of power generation efficiency and reaction efficiency. The former is the ratio of the product of the operating current J (V _op ) and the operating voltage V _op to the solar radiation intensity P _sun . The latter is the ratio of the reaction threshold voltages V ₀ and V _op multiplied by the Faraday efficiency η _F (see Non-Patent Document 1).

When numerically calculating the JV characteristics of the photocharge separation device, η _EQE =0.9 and η _ERE =0.01, which are the current values of solar cells or the target values for development, were used (see non-patent document 19, 21). The effect of _Rsh was ignored. R so that the decrease in the form factor (FF) due to R _s is suppressed to 5% (relative value), that is, FF = FF ₀ × 0.95 (FF ₀ is the value when R _s = 0). determined the value of _s . As shown in Table 1, the calculation results using this model roughly match the characteristics of the Cu(In, Ga)Se ₂ (CIGS) single-junction solar cell, which exhibits the highest efficiency at present, and the organic-inorganic hybrid perovskite solar cell. was confirmed to slightly exceed the value of (see Non-Patent Documents 22 and 23).

Table 1 shows the calculated and measured short-circuit current density (JSC), open-circuit voltage (VOC), form factor (FF), conversion efficiency (η), and series resistance (R _s ) of CIGS and perovskite single-junction solar cells. Comparison of values is shown (AM1.5G 1 sun illumination).

Using this model, first, the conversion efficiency η _STC for generating H ₂ using a series/parallel three-junction photocharge separation device is obtained and compared with the case of a series multi-junction device. The operating current J(V _op ) of the electrochemical reaction cell largely depends on the activity of the electrode catalyst and the operating voltage V _op . As will be shown later, the short-circuit current of the series/parallel three-junction photocharge separation device is 20 mA/cm ² or more when irradiated with AM1.5G 1 sun light (see FIG. 8(a)). In order to obtain J(V _op ) of the reaction cell close to this value, it is necessary to apply an overvoltage exceeding the threshold voltage of 1.23V. Operating at V _op = 1.55 V (overvoltage 0.32 V) considering the actual measurement value (1.44 to 1.49 Vat 10 mA/cm ² ) of a cell using a Ni-based electrode catalyst (see Non-Patent Documents 9 and 10) J (V _op ) at that time was calculated on the assumption that Further, η _F =1 was set to obtain η _STC .

When the bandgap of each sub-cell that maximizes η _STC under AM1.5G 1 sun light irradiation was determined, the series/parallel three-junction device yielded ε _g ^(t) /ε _g ^(m) /ε _g ^(b) = 2.17/1.51/1.02 eV, and for series two-junction devices, ε _g ^(t) /ε _g ^(b) = 1.61/0.95 eV. FIG. 6 shows the results of obtaining η _STC when these devices were irradiated with sunlight having the respective spectral intensities shown in FIG. At AM1.5G 1 sun, the η _STC of the series/parallel three-junction device reaches 30.9% (J(V _op )=25.1 mA/cm ² ). In contrast, η _STC for the series two-junction device is 3.7% lower than the series/parallel three-junction device. Moreover, η _STC decreases significantly with the relative increase of the short wavelength component, and at OC-2, when the short wavelength component has the highest relative intensity, η _STC is 16.9% (J(V _op )=5.2 mA /cm ² ). In contrast, the series/parallel three-junction device exhibits a small decrease in η _STC with spectral change, η _STC =24.8% (J(V _op )=7 .6 mA/cm ² ), i.e. 1.5 times higher than for the series two junctions. Therefore, the series two-junction device produces more _H2 , and its superiority becomes greater under cloudy day conditions. That is, it has the advantage of being less susceptible to changes in solar radiation. In the case of the series three-junction element, η _STC =24.6% (J(V _op )=20.0 mA/cm ² ), which is even lower than that of the series two-junction element at AM1.5G 1 sun. Therefore, we omit the details.

Next, consider the case of _CO2 reduction. The threshold voltage for producing CO, HCOOH, etc. from _CO2 and _H2O is higher than that for _H2 production. Furthermore, although research is underway aimed at higher catalytic activity, currently higher overvoltages are required than for _H2 production. Therefore, for each V _op , J(V _op ) when the bandgap is optimized under the AM1.5G 1 sun condition and the influence of solar radiation spectrum fluctuation were examined.

The results are shown in FIG. In the case of AM1.5G 1 sun irradiation, J(V _op ) of the series/parallel three-junction device is the largest up to V _op =1.9 V, but the series three-junction device becomes dominant when V _op is further increased. . However, in Fine-2 (clear weather), the value of the series three-junction element becomes small, so even if V _op ≧2.0 V, the two are in conflict with each other. J (V _op ) of the series/parallel three-junction element, which is less susceptible to fluctuations in the solar radiation spectrum, over the entire range of the calculated V _op at Fine-1 (clear weather), OC-1, and OC-2 (cloudy weather). is the largest, and the value of the series three-junction element having the largest effect is the smallest.

The threshold voltage for the reaction to produce HCOOH from _CO2 is 1.43V. Assuming V _op =2.0 V (overvoltage 0.57 V) and η _F =0.9 (see Non-Patent Document 6), the conversion efficiency η _STC for HCOOH generation using a series/parallel three-junction device is: 22.9% (J(V _op )=17.8 mA/cm ² ) under AM1.5G 1 sun irradiation, and 18.8% (J(V _op )=5.5 mA/cm ² ) even at OC-2. reach. If V _op =2.2 V (overvoltage 0.77 V), then 19.8% (J(V _op )=15.4 mA/cm ² ) and 16.6% (J(V _op )=4.9 mA/cm 2 ), respectively. cm ² ). As described above, in some cases the value of the series 3-junction element competes with the value of the series 3-junction element in fine weather, but it exceeds in cloudy weather, so the series/parallel 3-junction element has a larger amount of reaction products throughout the year.

Finally, an appropriate range of bandgaps for each subcell is determined.

A suitable range for the bandgap ε _g ^(t) of the top cell material is 2.0-2.9 eV, as shown in FIG. 7(f).

Appropriate ranges of the bandgaps ε _g ^(m) and ε _g ^(b) of the middle and bottom cells are considered from the viewpoint of current matching and voltage matching.

When the current matching between the middle cell and the bottom cell is broken, ie, when J _ph ^(m) and J _ph ^(b) are significantly different, J(V _op ) decreases. Therefore, the permissible range for the difference between J _ph ^(m) and J _ph ^(b) was determined. Assume H ₂ production, V _op = 1.55V . The optimal bandgap of each subcell is ε _g ^(t) /ε _g ^(m) /ε _g ^(b) =2.17/1.51/1.02 eV, as mentioned above, so ε _g Set ^(t) = 2.17 eV. Since J _ph ^(m) and J _ph ^(b) are proportional to N ₍ ^{m) and} N ^{(b )} respectively, N ^(m) _/ N ⁽ J(V _op ) was calculated when ^b) was changed. As can be seen from FIG. 8(a), if N ^(m) /N ^(b) is in the range of 0.2 to 2.5, J (V _op ) can be obtained. Therefore, ε _g ^(m) and ε _g ^(b) must be selected so as to satisfy the condition N ^(m) /N ^(b) =0.2 to 2.5. Furthermore, if N ^(m) /N ^(b) is in the range of 0.5 to 1.4, J(V _op ) larger than that of a series double-junction device with an optimum bandgap can be obtained even during AM1.5G 1 sun. .

The series/parallel two-junction device does not require current matching between the top cell and the middle/bottom cell, but requires voltage matching between the two. As before, we assume H ₂ production, V _op =1.55 V , and set ε _g ^(t) =2.17 eV. FIG. 8B shows the calculation results of J(V _op ) when the value of ε _g ^(m) +ε _g ^(b) is changed. From this, if ε _g ^(m) + ε _g ^(b) is in the range of ε _g ^(t) +0.25 to ε _g ^(t) +1.1 eV, the series two junctions with the optimum bandgap for OC-2 It can be seen that a J(V _op ) superior to that of the device is obtained. Furthermore, if ε _g ^(m) + ε _g ^(b) is in the range of ε _g ^(t) +0.25 to ε _g ^(t) +0.6 eV, then even in the case of AM1.5G 1 sun, the series two junctions with the optimum bandgap A larger J(V _op ) than the element is obtained.

Materials for fabricating the series/parallel three-junction photocharge separation device described above are discussed. The optimum bandgap of each subcell for operating the electrochemical reaction cell for H ₂ production at V _op = 1.55 V is, as previously stated, ε _g ^(t) /ε _g ^(m) /ε _g ^(b) = 2.17/1.51/1.02 eV. The first candidate for the multi-junction device is a monolithic device by epitaxial growth of III-V compound semiconductors (see Non-Patent Documents 27 and 28), and solar cells with high conversion efficiency have actually been realized. be. Therefore, devices using thin film materials that can be produced at a lower cost have been developed. is suitable (see Non-Patent Documents 23, 30, 31). Solar cells with middle-cell bandgap have also been realized with CIGS and related materials, but their properties need to be improved (see Non-Patent Document 29). Crystalline silicon is also a candidate material for the bottom cell, although it has a bandgap of 1.12 eV, which is slightly larger than the optimum value [29-32]. The optimum bandgap of the top/middle cell in this case is ε _g ^(t) /ε _g ^(m) =2.16/1.54 eV, and η _STC =30.2% (J( V _op )=24.5 mA/cm ² ), which is comparable to η _STC =30.9% (J(V _op )=25.1 mA/cm ² ), which is the optimum value.

The electrochemical cell for _CO2 reduction is set to V _op =2. The optimum bandgap for operation at 0V is ε _g ^(t) /ε _g ^(m) /ε _g ^(b) =2.66/1.77/1.26 eV. For middle/bottom cells, organic-inorganic hybrid perovskites or CIGS-based materials are candidates. A solar cell with a top-cell bandgap is realized by an InGaN-based multiple quantum well (see Non-Patent Documents 33 and 34).

As described above, it was found that the chemical reaction cell including the electrochemical reaction cell and the photoelectric conversion element, which can improve the conversion efficiency from solar energy to chemical energy, can be obtained according to the examples.

1, 3, 5 chemical reaction cell, 10 photoelectric conversion element, 12 positive electrode (anode electrode), 14 negative electrode (cathode electrode), 16 flow path, 18 container, 20 container, 30 top cell, 32 middle cell, 34 bottom cell.

Claims (6)

A chemical reaction cell comprising an electrochemical reaction cell and a photoelectric conversion element,
The operating voltage (V _op ) of the electrochemical reaction cell is in the range of 1.4 to 2.2 V,
The photoelectric conversion element is a series/parallel three-junction element in which a series-connected middle/bottom cell is connected in parallel with a top cell,
The bandgap (ε _g ^(t) ) of the light absorbing material of the top cell is in the range of 1.11×V _op +0.21 to 1.11×V _op +0.56 eV and 2.0 to 2.9 eV. is the range and
The bandgap (ε _g ^(m) ) of the light absorbing material of the middle cell is in the range of 0.55×V _op +0.60 to 0.59×V _op +0.72 eV,
The bandgap (ε _g ^(b) ) of the light absorbing material of the bottom cell is in the range of 0.53×V _op +0.088 to 0.56×V _op +0.21 eV,
The minimum value of ε _g ^(m) + ε _g ^(b) is 1.09×V _op +0.74 eV,
Number of absorbed photons N (m) of the bottom cell represented by the following formula (2) of the number of absorbed photons N ^(m) of the middle cell represented by the following formula (1 ⁾ when irradiated with the AM1.5G sunlight spectrum A chemical reaction cell, wherein the ratio N ^(m) /N ^(b) to is in the range of 0.5 to 1.4.

(1)

(2)
(here,

is the photon number spectrum of sunlight. ) A chemical reaction cell according to claim 1,
The ε _g ^(t) is 1.09×V _op +0.49 eV, the ε _g ^(m) is 0.57×V _op +0.63 eV, and the ε _g ^(b) is 0.57×V op +0.63 eV. A chemical reaction cell characterized by 55×V _op +0.18 eV. A chemical reaction cell comprising an electrochemical reaction cell and a photoelectric conversion element,
The operating voltage (V _op ) of the electrochemical reaction cell is 1.55 V,
The photoelectric conversion element is a series/parallel three-junction element in which a series-connected middle/bottom cell is connected in parallel with a top cell,
the bandgap (ε _g ^(t) ) of the light absorbing material of the top cell is 2.17 eV;
The bandgap (ε _g ^(m) ) of the light absorbing material of the middle cell is 1.51 eV,
The bandgap (ε _g ^(b) ) of the light absorbing material of the bottom cell is 1.02 eV,
Number of absorbed photons N (m) of the bottom cell represented by the following formula (2) of the number of absorbed photons N ^(m) of the middle cell represented by the following formula (1 ⁾ when irradiated with the AM1.5G sunlight spectrum The ratio N ^(m) /N ^(b) for
A chemical reaction cell, wherein the electrochemical reaction cell is a cell for generating _H2 and O2 _from H2O _.

(1)

(2)
(here,

is the photon number spectrum of sunlight. ) The chemical reaction cell according to claim 1 or 2 ,
A chemical reaction cell, wherein the electrochemical reaction cell is a cell for generating _H2 and _O2 from _H2O . The chemical reaction cell according to claim 1 or 2 ,
A chemical reaction cell, wherein the electrochemical reaction cell is a cell for reducing _CO2 . The chemical reaction cell according to claim 1 or 2 ,
A chemical reaction cell, wherein the electrochemical reaction cell is a cell for producing CO or HCOOH from _CO2 and _H2O .

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