JP6787289B2 - Redox reaction cell - Google Patents

Description

The present invention relates to a redox reaction cell that performs a redox reaction using light energy.

H ₂ can be produced from H ₂ O using only solar energy, or carbon monoxide (CO), formic acid (HCOOH), formaldehyde (HCHO), and methanol can be produced from water (H ₂ O) and carbon dioxide (CO ₂ ). _(CH 3 OH) oxidation-reduction reaction (hereinafter referred to as artificial photosynthesis) or to synthesize the like are known. For such artificial photosynthesis, it is necessary to apply a potential difference of about 2 V between the oxidation / reduction catalysts.

In order to realize this, Non-Patent Documents 1 and 2 use photoelectrodes in which oxidation / reduction catalysts are supported on both sides of an amorphous silicon-based 3-junction solar cell (a-Si 3J-SC). Further, in Non-Patent Document 3, an amorphous silicon-based 3-junction solar cell is used, a reduction catalyst is supported on the back surface (opposite side of the light incident surface), and oxidation including a member having an oxidation catalyst function so as to face the reduction catalyst. An artificial photosynthesis cell in which a solar cell and an electrochemical cell are integrated is used, in which electrodes are arranged and connected to the surface electrode of the solar cell. Further, in Non-Patent Document 4, a III-V group compound 2-junction solar cell (III-V 2J-SC) is used for higher efficiency.

Japanese Unexamined Patent Publication No. 2004-315942 Japanese Unexamined Patent Publication No. 2006-104571

S. Y. Reece, J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers, and D. G. Nocera, Science 334, 645 (2011) T. Arai, S. Sato, and T. Morikawa, Energy Environ. Sci 8, 1998 (2015) J.-P. Becker, B. Turan, V. Smirnov, K. Welter, F. Urbain, J. Wolff, S. Haas and F. Finger, J. Mater. Chem. A 5, 4818 (2017) G. Peharz, F. Dimroth, and U. Wittstadt, Int. J. Hydrogen Energy 32, 3248 (2007) S. Kedenburg, M. Vieweg, T. Gissibl, and H. Giessen, Opt. Mat. Express 2, 1588 (2012) MOTECH INDUSTRIES, INC. SOLAR DIVISION, XS156b3-200R (Monocrystalline S-Cells), [online], Oct 2012, [2017/09/22 Search], Internet (URL: https://cdn.enfsolar.com/Product/ pdf / Cell / 517cdf0744600.pdf) F. Urbain, V. Smirnov, J.-P. Becker, A. Lambertz, F. Yang, J. Ziegler, B. Kaiser, W. J, U. Rau, and F. Finger, Energy Environ. Sci. 9 , 145 (2016)

Here, in principle, a high conversion efficiency can be obtained with a multi-junction solar cell, but even if the amorphous silicon solar cell has three junctions, the conversion efficiency (14% at the laboratory level) is crystalline silicon consisting of a single pn junction. The current situation is that it is less than solar cells (26% at the lab level). On the other hand, since the cost of a group III-V compound solar cell that can obtain high efficiency is extremely high, its use is limited to the condensing type and is not suitable for artificial photosynthesis.

Further, Patent Documents 1 and ₂ show an example in which a crystalline silicon solar cell composed of a single pn junction and a wide-gap semiconductor such as TiO ₂ are combined to obtain a potential difference of about 2 V required for the reaction. However, since the number of photons absorbed by the wide-gap semiconductor is small, its efficiency remains relatively low.

The redox reaction cell according to the present invention is a 4- to 6-cell crystalline silicon solar cell (heterojunction) in which an oxidation electrode including a member having an oxidation catalyst function and a reduction electrode including a member having a reduction catalyst function are connected in series. (Including mold), including a photoelectric conversion part that gives a potential difference between the oxidizing electrode and the reducing electrode by the electric power obtained by photoelectric conversion, in the order of crystalline silicon solar cell / oxidizing electrode / reducing electrode, or crystalline silicon solar. are arranged in order of the battery / reduction electrode / oxide electrodes, with light is incident on the crystal silicon cell without passing through any of the oxidation electrode and the reduction electrode, the space between the oxidation electrode and the reduction electrode has a reaction chamber, The electrolytic solution is circulated from the outside to this reaction chamber .

Further, it is preferable that the oxidizing electrode has a catalytic function of oxidizing water to generate oxygen, and the reducing electrode has a catalytic function of reducing water to generate hydrogen.

Further, the oxidizing electrode has a catalytic function of oxidizing water to generate oxygen, and the reducing electrode has a catalytic function of reducing carbon monoxide to generate at least one of carbon monoxide, formic acid, formaldehyde, and methanol. It is preferable to have.

According to the present invention, a crystalline silicon solar cell can efficiently generate electric power, and the electric power can be used for photosynthesis.

It is a figure which shows the structure of the artificial photosynthesis cell (circulation type) which concerns on embodiment. It is a diagram showing a configuration of artificial photosynthetic cells (immersion). It is a figure which shows the absorption coefficient of water according to the wavelength of light. It is a figure which shows the comparison of the current-voltage characteristic of the crystalline silicon solar cell 4-cell series connection and the amorphous silicon-based three-junction solar cell used for a circulation type cell. It is a figure which shows the comparison of the current-voltage characteristic of the crystalline silicon solar cell 4-cell series connection and the amorphous silicon-based three-junction solar cell used for the immersion type cell. It is a figure which shows the structure which connected 4 sets of 3 cell parallel | series. It is a figure which shows the structure which connected 3 sets of 4 cell series in parallel. It is a figure which shows the structure which provided 1 set of 4 cell series. It is a figure which shows the case where the cell is arranged according to the shape of the installation place in the structure which connected 3 sets of 4 cells in series in parallel.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the embodiments described herein.

"overall structure"
<Circular cell>
FIG. 1 shows a configuration example of the artificial photosynthesis cell according to the embodiment. An example of this is a circulating cell in which the electrolyte is circulated through the cell.

The substrate 10 is, for example, a quadrangular frame whose top and bottom are open. It is made of an insulator such as plastic. An oxidation electrode 12 having an oxidation catalyst function is arranged on the bottom of the substrate 10, and the bottom of the substrate 10 is closed. Although the lower surface of the oxide electrode 12 is exposed, it may be covered with a protective material or the like.

Above the oxide electrode 12, reduction electrodes 14 having a reduction catalyst function are arranged at predetermined intervals. The oxide electrode 12 and the reduction electrode 14 are watertightly connected to the substrate 10, and the space between the oxidation electrode 12 and the reduction electrode 14 is the reaction chamber 16. Then, an electrolytic solution from the outside (for example, water or an aqueous solution in which a salt is dissolved) is circulated in the reaction chamber 16. The positions of the oxide electrode 12 and the reduction electrode 14 may be opposite to each other.

A crystalline silicon solar cell 18 constituting a photoelectric conversion unit is arranged above the reducing electrode 14, a transparent cover 20 is arranged above the crystalline silicon solar cell 18, and the upper part of the substrate 10 is closed.

Then, the positive electrode of the crystalline silicon solar cell 18 is electrically connected to the oxide electrode 12, and the negative electrode is connected to the reduction electrode 14. Further, the crystalline silicon solar cell 18 is formed by connecting four battery cells 18a in series. The battery cell 18a has an output of about 0.5 V, and an output voltage of about 2 V can be obtained by connecting four in series. Here, the crystalline silicon solar cell 18 includes a heterojunction type silicon solar cell using a heterojunction of crystalline silicon and amorphous silicon. The number of battery cells 18a connected in series is 4 to 6. By using 5 or 6 pieces, a margin can be obtained in the output voltage, and the required voltage can be maintained even if the output voltage drops due to deterioration or the like.

Further, it is also preferable to connect 4 to 6 battery cells 18a connected in parallel in parallel, or to connect a plurality of battery cells 18a connected in series in parallel.

When such an artificial photosynthesis cell is irradiated with light, for example, sunlight, the crystalline silicon solar cell 18 outputs a voltage of about 2 V. As a result, a voltage of about 2 V is applied between the oxide electrode 12 and the reduction electrode 14. The electrolytic solution in the reaction chamber 16 is supplied there between the oxidation electrode 12 and the reduction electrode 14, and an oxidation-reduction reaction occurs here. For example, the oxidation electrode 12, the _{O 2} generated from _{H 2} O, the reduction electrode 14, and generate and _{H 2} from the _{H 2} O, _{H 2} O and CO ₂ from the (carbon dioxide) CO (carbon monoxide) , HCOOH (formic acid), HCHO (formaldehyde), CH ₃ OH (methanol), etc. are synthesized by artificial photosynthesis.

<Immersion cell>
FIG. 2 shows the configuration of an immersion type cell in which an artificial photosynthesis cell is immersed in an electrolytic solution. The electrolytic solution is stored in the electrolytic cell 22.

In the artificial photosynthesis cell, the oxidation electrode 12 is arranged on the top of the substrate 10, and the reduction electrode 14 is arranged on the bottom. The outer surface of the oxidizing electrode 12 and the reducing electrode 14 in contact with the electrolytic solution functions as electrodes.

The crystalline silicon solar cell 18 is arranged in the space between the oxidizing electrode 12 and the reducing electrode 14 inside the substrate 10. In this case, the oxide electrode 12 faces the sun, and the oxide electrode 12 is made of a translucent material such as ITO. The positions of the oxide electrode 12 and the reducing electrode 14 may be opposite to each other, and in that case, the reducing electrode 14 is made of a translucent material.

Then, an artificial photosynthesis reaction occurs by applying the output voltage of the crystalline silicon solar cell 18 between the oxidation electrode 12 and the reduction electrode 14.

<Performance>
The characteristics of the circulating cell of FIG. 1 and the immersion cell of FIG. 2 were calculated and compared with the case where an amorphous silicon-based 3-junction solar cell was used instead of the 4-cell series connection of crystalline silicon solar cells.

For the immersion type cell, the influence of long wavelength light being absorbed by the electrolytic solution (water) was considered. Further, the value shown in Non-Patent Document 5 shown in FIG. 3 was used as the water absorption coefficient.

For the current density (J) -voltage (V) characteristics of the crystalline silicon solar cell, the values shown in the catalog (Non-Patent Document 6) of a commercially available solar cell (MOTEC XS156B3-200R: area 239 cm ² ) were used. .. Further, the current density (J) -voltage (V) characteristics of the amorphous silicon-based 3-junction solar cell are described in FIG. Using the data shown in 6 (green line of a-Si: H / a-Si: H / μc-Si: H), the quantum efficiency spectrum is the same as that of FIG. 6 of Non-Patent Document 6. The data shown in 4 was used.

The state when these cells were irradiated with the AM1.5G spectrum, which is the standard condition of sunlight irradiation, and 1 sun (100 mW / cm ² ) of light was calculated.

For crystalline silicon solar cells, the function J _c-Si-1sun (V) fitted with the JV characteristics in the case of AM1.5G, 1sun, and the function Y fitted with the value obtained by converting the spectral sensitivity spectrum into the quantum efficiency spectrum. _{Using c-Si} (λ), the JV characteristics at the water depth d
Based on this, the JV characteristics of 4-cell series connection were obtained.

Here, n _sun (l) is the photon number spectrum of sunlight, and T _water (λ; d) is the light transmittance up to the water depth d (the light transmittance of the water layer having a thickness d). That is, it is an approximation that the open circuit voltage and the scherrer do not change, and only the short-circuit current density changes according to the water depth d. In reality, since the number of photons absorbed by the solar cell is reduced, the open circuit voltage and the shape factor are also slightly reduced, but the effects are negligibly small if the range is within d = 10 cm, which will be described later.

For an amorphous silicon-based 3-junction solar cell, the JV characteristics for each junction should be obtained, and the characteristics when these are connected in series should be obtained. However, Fig. If the solar cell having the quantum efficiency spectrum shown in No. 4 is used as it is in water, the long wavelength light is weakened and the current matching condition between the cells is not satisfied, so that the efficiency is greatly reduced. However, this decrease can be easily eliminated by adjusting the thickness of each subcell. Therefore, assuming a solar cell optimized for an arbitrary water depth d, and considering only the change in the total number of photons absorbed as in the case of a crystalline silicon solar cell, its JV characteristic _{Ja- Si} (V; d) was determined.

Since the purpose here is to compare 4-cell series connection of crystalline silicon solar cells with 3-junction amorphous silicon solar cells, the effects of light absorption and reflection by members other than water are taken into consideration in order to grasp the general tendency. I didn't.

Figures 4, 5 (a) and (b) show a comparison of the current density (J) -voltage (V) characteristics of the crystalline silicon solar cell 4-cell series connection and the amorphous silicon-based 3-junction solar cell according to the embodiment. .. FIG. 4 is for a circular cell. FIG. 5 shows the case where the immersion cell is installed in a shallow place as much as possible, and the case where a large element of about 1 m is installed so as to be below the water surface even if it is slightly tilted or receives wind. This is the result when d = 1 cm (a) and 10 cm (b). When d = 1 cm in the circulation type and the immersion type, the current density of the four-cell crystalline silicon solar cell connected in series is larger when compared at the same voltage, and higher efficiency can be obtained when used in an artificial photosynthesis cell.

As can be seen from the comparison of the quantum efficiencies of the crystalline silicon solar cell (Non-Patent Document 6) and the amorphous silicon-based 3-junction solar cell (Fig. 4 of Non-Patent Document 7), the amorphous silicon-based 3-junction solar cell is longer. The quantum efficiency of wavelength is low. For this reason, the amorphous silicon-based 3-junction solar cell is less affected by the light absorption of water. Therefore, the current (short-circuit current) of d = 10 cm is larger in the amorphous silicon-based 3-junction solar cell. However, when compared in the vicinity of 2V, which is the operating point when used in an artificial photosynthesis cell, a larger current density can be obtained by connecting four crystalline silicon solar cells in series. The current densities at 1.8, 2.0, and 2.2 V, which are voltages close to the operating point, are shown in Table 1 (4 cell series connection of crystalline silicon solar cells) and Table 2 (amorphous silicon 3-junction solar cells). ..

Next, considering the combination with the electrode including the member having a specific catalytic function, the current density of the operating state when the cell was formed by connecting with the solar cell was obtained for the following three ways.
(I) Ni anode (OER), Ni cathode (HER), 1MKOH electrolytic solution, hydrogen production. The current-voltage characteristics when the anode and cathode are combined are described in Fig. 3 of Non-Patent Document 3. The data of 7 (a) (EC, HER: Ni, OER: Ni, 1M KOH, blue solid line) was used.
(Ii) IrOx anode (OER), Pt cathode (HER), 1 MH2SO4 electrolyte, hydrogen production. The current-voltage characteristics when the anode and cathode are combined are described in Fig. 3 of Non-Patent Document 3. The data of 7 (a) (EC, HER: Pt, OER: IrOx, 1MH ₂ SO ₄ , red dashed line) was used.
(Iii) IrOx anode (OER), carbon cloth / Ru complex cathode (HER), CO ₂ bubbling in 0.1 M phosphate buffer electrolyte, formic acid production. The current-voltage characteristics when the anode and cathode are combined are described in Fig. 2 of Non-Patent Document 2, respectively. S7, Fig. The data of S9 (CC / p-RuCP, solid blue line) was used.

In these cases as well, as shown in Tables 3 and 4, a larger current density can be obtained by using the crystalline silicon solar cell 4-cell series connection when compared under the same conditions. In particular, when there is no or small effect of water absorption, such as in the case of circulating type and immersion type d = 1 cm, and when a relatively high potential difference is required for the reaction, such as (iii). Is more dominant.

<About the number of cells and arrangement>
In the above embodiment, four crystalline silicon solar cells 18 are connected in series. The potential difference required for the reaction can be obtained with 4 cells. However, the voltage may drop due to deterioration of the solar cell and wiring. At that time, if there are 5 cells or more, the required potential difference can be maintained in total even if the voltage of each cell drops. It is also preferable to connect 5 or 6 cells in series.

Further, depending on the area of the cells, 4 to 6 sets of cells in which a plurality of cells are connected in parallel may be connected in series. FIG. 6 shows an example in which 3 cells are connected in parallel and 4 cells are directly connected.

Further, FIG. 7 shows three sets of four cells connected in series and connected in parallel. Depending on the shape of the installation location, it may be arranged in a shape other than a rectangle. In FIG. 8, the four cells are simply arranged linearly. In FIG. 9, these four cells are connected in series with the first row as a 2 × 2 array, and the four cells are linearly connected in series in the second and third rows. Three rows of these four cells connected in series are connected in parallel.

In this way, the degree of freedom of the shape as a whole can be increased. Further, by connecting a plurality of pieces in parallel, the amount of current can be increased and the amount of artificial photosynthesis reaction can be increased.

<Effect of embodiment>
According to this embodiment, a crystalline silicon solar cell is used. As a result, a larger current can be obtained than in the amorphous silicon-based three-junction photocharge separation element, so that the conversion efficiency from solar energy to artificial photosynthesis products (hydrogen, formic acid, etc.) becomes higher. That is, a conversion efficiency close to this can be obtained at a lower cost than the III-V compound multi-junction photocharge separation device.

The potential difference required for the reaction of decomposing water into hydrogen and oxygen and the reaction of synthesizing formic acid and the like from carbon dioxide and water is about 2V. Therefore, this potential difference can be obtained by connecting four crystalline silicon solar cells in series. Further, by connecting 4 to 6 cells in series, it is possible to maintain the reaction even when the cells are deteriorated.

10 substrate, 12 oxide electrode, 14 reduction electrode, 16 reaction chamber, 18 crystalline silicon solar cell, 18a battery cell, 20 transparent cover, 22 electrolytic cell.

Claims (3)

An oxidation electrode containing a member having an oxidation catalyst function,
A reduction electrode containing a member having a reduction catalyst function,
A photoelectric conversion unit that includes 4 to 6 cell crystalline silicon solar cells connected in series and gives a potential difference between the oxidation electrode and the reduction electrode by the electric power obtained by the photoelectric conversion.
Including
Crystalline silicon solar cells / oxide electrodes / reduction electrodes are arranged in this order, or crystalline silicon solar cells / reduction electrodes / oxidation electrodes are arranged in this order.
Causes light to enter the crystalline silicon cell without passing through any of the oxidation electrode and the reduction electrode,
The space between the oxidizing electrode and the reducing electrode is a reaction chamber, and the electrolytic solution is circulated from the outside in this reaction chamber.
Redox reaction cell. The redox reaction cell according to claim 1.
The oxide electrode has a catalytic function of oxidizing water to generate oxygen.
The reducing electrode has a catalytic function of reducing water to generate hydrogen.
Redox reaction cell. The redox reaction cell according to claim 1.
The oxide electrode has a catalytic function of oxidizing water to generate oxygen.
The reducing electrode has a catalytic function of reducing carbon dioxide to generate at least one of carbon monoxide, formic acid, formaldehyde, and methanol.
Redox reaction cell.