KR20160111256A - Photoelectrochemical artificial photosynthesis device - Google Patents

Abstract

The present inventors have developed a low-cost solution-based Cu (In _x Ga _1-x ) (S _y Se _{1 -x} ) solution using CO ₂ and water as raw materials, _-y ) ₂ (CIGS) module fabrication method, we have developed thin film solar cell technology which is highly applicable to conversion reaction. The photovoltaic device of the present invention comprises a cobalt oxide (Co ₃ O ₄ ) nanoparticle thin film deposited by a low-temperature coating method as a water oxidation catalyst and a gold layer of a nanostructure as a CO ₂ reduction catalyst for producing CO, It is confirmed that the integrated device of the present invention, which operates using only solar light as an energy source in the state where there is no input of energy from the light source, has the following performance.

Description

[0001] Photoelectrochemical artificial photosynthesis device [

The present invention relates to an artificial photosynthesis apparatus for producing high-value compounds directly from water and carbon dioxide as sunlight as an energy source.

Global energy production still depends heavily on conventional fossil fuels, raising global humanitarian concerns about global climate change and fossil fuel depletion due to greenhouse gas emissions. In addition, as fossil fuels are being used as raw materials for many high-value compounds (plastics, fibers, etc.) that we currently use as well as energy, there is a need for a method of producing high-value compounds that can replace them in the future, have.

Of the various future energy and high value compounds production methods, the production of compounds using solar energy is the most promising approach in terms of enormous energy size and persistence of solar energy.

The system that produces sunlight as an energy source from carbon dioxide and water mimics the photosynthesis that takes place mainly in the leaves of plants in nature and is called "artificial photosynthesis" and the compounds produced thereby are generally called "solar-fuel" do.

Various systems have been attempted to artificially produce solar-fuel, and a representative example is a method using a multifunctional photocatalyst or photo-electrochemical device. However, to date, no photovoltaic-fuel production system has shown satisfactory efficiency and even does not exceed the natural photosynthetic efficiency. The efficiency of photosynthesis in nature is very broad, depending on the specific system. For example, the photosynthetic efficiency of the growth phase of C ₃ plants, including algae, producing algae, reaches 3.5% under optimal conditions, and the efficiency of C ₄ plants, including herbaceous plants, producing _quadrants is up to 4.3% You can reach it. However, the photosynthetic efficiency of common plants is known to be less than 1% due to limitations in the amount of sunlight and CO ₂ concentration in the atmosphere.

Among various artificial photosynthesis technologies, photoelectrochemical methods are known as the most effective methods. Photochemical artificial photosynthetic technology refers to a method in which substances that absorb sunlight to make electron-hole pairs and substances that act as catalysts for oxidation and reduction reactions are formed in an electrode form and operated by electrochemical principles.

As shown in the conceptual diagram of FIG. 1, in order to generate an electron-hole pair by absorbing sunlight, at least one of the oxidation and reduction electrodes should be a light electrode capable of absorbing light, It should have catalyst properties to minimize it. Separators are also needed to prevent the chemical species produced in the oxidation and reduction reactions from participating in the reaction of the counter electrode and to facilitate the separation of the products. In order to produce solar-fuel as well as photosynthesis in natural leaves, water decomposition must occur. When the generated hydrogen ions are directly reduced, hydrogen gas is generated. When carbon dioxide is dissolved in the water, hydrogen ions C ₂ compounds such as formic acid, methane, carbon monoxide and methanol or C ₂ and C ₃ compounds such as ethylene and isopropanol are produced through the competitive reduction reaction. Reaction selectivity is a very important factor in this reaction, which is mainly determined by the catalytic properties of the reducing electrode. For example, if a platinum (Pt) catalyst is used as the reducing electrode, the hydrogen ion is selectively reduced and the main product becomes hydrogen gas. However, if a gold (Au) catalyst is used, carbon monoxide and hydrogen are simultaneously produced.

In photoelectrochemical solar-fuel production systems, metal oxide semiconductors such as TiO ₂ are widely used as photoelectrodes, but metal oxide semiconductors such as TiO ₂ have bandgaps too large (> 3.0 eV) It is difficult to absorb the light in the visible light region occupied thereby, which makes it impossible to efficiently utilize the sunlight. Many researchers have tried to utilize visible light by doping specific elements in a metal oxide semiconductor with a large bandgap such as TiO ₂ or by combining a sensitizer such as a dye or a quantum dot (QD). However, not all problems are solved by satisfying the absorption of visible light by the photoelectrode material, and in order for the photoelectrochemical cell to operate efficiently and spontaneously, the oxidation and reduction energy levels of the material forming the photoelectrode are reduced by oxidation of water and reduction energy level of carbon dioxide . In addition, the materials used in the electrodes must have stability in aqueous solution and catalytic properties to reduce the overvoltage of the oxidation and reduction reactions. Among the semiconductor materials known so far, very few substances satisfying all of these conditions are very rare, and even if they are very low in efficiency, they are practically unusable. As an alternative, solar cell technology, photocatalyst or electrochemical catalyst technology In this case, a photovoltaic electrode of the combined type is proposed. At this time, the photovoltaic-fuel generation reaction takes place at the intersection of the IV curve of the solar cell and the IV curve of the catalyst electrode (FIG. 2).

Therefore, in order to increase the efficiency of solar-fuel conversion efficiency (STF), i) a high efficiency electrocatalyst (EC) is used to lower the overvoltage of the overall water decomposition proceeding on the anode or the cathode (see FIG. PC) material is applied to the (photo) anode or (photo) cathode to increase the operating current (maximize the short-circuit current I _sc ) or to decrease the operating voltage by using charge separation at the photocatalytic / electrolyte junction (Minimizing the input energy). It is theoretically possible to decompose the water when the bias voltage is 0 (when there is no PV). Therefore, it is more advantageous to use a PC than to use an EC. Therefore, despite the technical difficulties of the tandem structure, Was tilted.

Since the material having a large bandgap is more advantageous in obtaining a large open-circuit voltage at the PV side, but the light first passes through the PC layer, the structure of FIG. 2b shows that the bandgap of the semiconductor in the PV cell is higher than the PC material for effective light absorption Limited to small systems. Therefore, there is a need to carefully design the structure of an effective photovoltaic fueling system so that it can be applied to both systems (EC or PC-introduced PV cells) in which various materials are applicable.

The stability of the photoelectrode is also a very important factor for the realization of high-performance photo-chemical and artificial photosynthesis systems. In particular, since many semiconductor materials used in solar cells are not stable in the electrolyte, a technique for securing the stability of such a solar cell-catalyst integrated electrode is needed.

In order to fabricate a solar cell-catalyst integrated electrode, the solar cell portion and the catalyst film portion must be ohmic-bonded around the conductive layer. This conductive layer is generally formed as a thin film. However, if a glass substrate having both conductive surfaces is used, the solar cell semiconductor material which is not securely stable in the electrolyte is not placed in the electrolyte but is placed outside the electrolyte as shown in FIG. 3 An ohmic junction can be formed with the catalyst film.

In this case, both the photocatalyst and the electrochemical catalyst can be used as the catalyst, but it is advantageous to use a photocatalyst in order to increase the efficiency through efficient use of light. In order to efficiently separate light absorption from the solar cell, It can be an important technology.

Although various kinds of solar cells can be used for a solar cell with a catalyst-integrated photo-electrode, a photovoltage (> 2 V) that can cover both thermodynamic energy and overvoltage necessary for water decomposition and reduction reaction of carbon dioxide Should occur. A tandem solar cell or a solar cell module can be used because currently known solar cell unit cells can not generate such a light voltage, but a tandem solar cell is more advantageous in terms of efficiency. In addition, solar cells must be able to be fabricated on a glass substrate in order to realize an integrated electrode with an (optical) catalyst to divide and absorb light.

In an optoelectrochemical artificial photosynthesis device, reduction catalyst technology is as important as a light electrode. In photovoltaic-electrochemical cells, there is only a hydrogen ion as a reducing species, but in the photovoltaic-C ₁ compound solar cell, CO ₂ is solubilized at the same time, and a competitive reduction reaction between hydrogen ion and CO ₂ occurs. The degree and performance become more important. Particularly, since the reduction potential of thermodynamic energy, that is, hydrogen and the expected C ₁ product (such as carbon monoxide, methane, formic acid, methanol, etc.) does not greatly differ, the selectivity in the reduction reaction is determined by the type and nature of the reduction catalyst do. Therefore, by simply replacing the catalyst of the reduction electrode, it is possible to selectively produce the desired compound.

1. J. L. White, J. T. Herb, J. J. Kaczur, P. W. Majsztrik and A. B. Bocarsly, J. CO2 Util., 2014, 7, 1-5. 2. Y. Hori, in Modern Aspects of Electrochemistry, eds. C. Vayenas, R. White and M. Gamboa-Aldeco, Springer New York, 2008, vol. 42, ch. 3, pp. 89-189. 3. M. J. Cheah, I. G. Kevrekidis and J. B. Benziger, Langmuir, 2013, 29, 9918-9934

DISCLOSURE Technical Problem The present invention solves the above conventional problems and aims to provide a device capable of recycling solar energy with high efficiency by using CO ₂ and water as raw materials. Specifically, a solar cell and a catalyst are integrated, It is manufactured in the state that the solar cell is placed outside the electrolyte to secure the stability of the photoelectrode, and the solar cell-catalyst is manufactured on the conductive glass substrate to increase the efficiency by reducing the overvoltage, and the artificial To provide a photosynthesis device technology.

According to one aspect of the present invention, at least one of the anode and the cathode includes a double-sided conductive support, a PV photoelectrode formed on one surface of the double-sided conductive support, and a catalyst electrode formed on the other surface of the double- Electrode, and the catalyst electrode is located in the oxidation chamber or the reduction chamber, and the PV photoelectrode is located outside the oxidation chamber or the reduction chamber.

The photoacoustic artificial photosynthesis apparatus of the present invention comprises a photoelectrode where a solar cell and a catalyst film are respectively formed on a double-sided conductive glass substrate, and a reduction (or oxidation) catalyst electrode capable of forming a useful compound by causing a carbon dioxide reduction or water oxidation reaction, The stability of the electrode can be expected and the efficiency can be increased. The desired compound can be selectively produced through the reduction (or oxidation) electrode replacement, and the present invention , Demonstrates excellent fuel efficiency of solar energy.

1 is a conceptual diagram of a photoelectrochemical solar-fuel production reaction.
FIG. 2 shows the principle of efficiency enhancement occurring in a photoelectrode in which a solar cell is combined with an electrochemical catalyst (FIG. 2A) or a photocatalyst (FIG. 2B).
3 is a schematic view of a solar cell-catalyst integrated optical electrode manufactured using a double-sided conductive glass substrate.
4 is an actual photograph of a solar photovoltaic apparatus.
FIG. 5 is a photograph of the optical electrode (front and back) of an integrated CIGS module and a Co ₃ O ₄ anode catalyst as an embodiment, (a) SEM images (b, d)
Fig. 6 shows the SEM image (a) of the nanostructured Au cathode catalyst electrode and the ferrodefficiency (b) and performance evaluation (c) of the Au cathode catalyst for carbon dioxide reduction.
7 shows the performance evaluation (a), the operating point (b) of the solar photovoltaic device and the operating current (c) of the solar photovoltaic device, which are composed of a Co ₃ O ₄ anode catalyst and a nano-structured Au catalyst.

Hereinafter, various aspects and various embodiments of the present invention will be described in more detail.

According to one aspect of the present invention, at least one of the anode and the cathode includes a double-sided conductive support, a PV photoelectrode formed on one surface of the double-sided conductive support, and a catalyst electrode formed on the other surface of the double- Wherein the catalytic electrode is located in the oxidation chamber or the reduction chamber, and the PV photoelectrode is located outside the oxidation chamber or the reduction chamber.

As described above, at least one of the anode and the cathode constitutes a solar cell-catalyst integrated electrode in the optoelectrochemistry artificial photosynthesis apparatus according to an aspect of the present invention, so that the anode alone is made up of a solar cell- And a photo-electrochemical artificial photosynthesis apparatus constituting a battery-catalyst-integrated electrode.

That is, the anode is a solar cell-catalyst integrated electrode including a double-sided conductive support, a PV photoelectrode formed on one surface of the double-sided conductive support, and an oxidation catalyst electrode formed on the other surface of the double-sided conductive support, , The photo-electrochemical artificial photosynthesis apparatus in which the oxidation catalyst electrode is located in the oxidation chamber and the PV photoelectrode is located outside the oxidation chamber is included in the scope of the present invention.

Also, the anode is an oxidation catalyst electrode, and the cathode is a solar cell-catalyst integrated electrode including a double-sided conductive support, a PV photoelectrode formed on one surface of the double-sided conductive support, and a reduction catalyst electrode formed on the other surface of the double- A photoelectrochemical artificial photosynthetic apparatus in which the reduction catalyst electrode is located in the reduction chamber and the PV photoelectrode is located outside the reduction chamber is included in the scope of the present invention.

In addition, in the case of the PV-photocatalyst integrated electrode, the double-sided conductive glass substrate should be used so that the PV and the photocatalytic film can absorb solar light separately, do.

The artificial photosynthesis apparatus according to the present invention is a system in which carbon dioxide can be selectively reduced only by sunlight without being supplied to an external power source.

According to one embodiment, the PV photoelectrode may be a CIGS thin film solar cell module or a tandem type CIGS thin film solar cell. In addition, the photo-electrochemical artificial photosynthesis apparatus may be a panel-type apparatus in which the PV light electrode and the catalyst electrode are provided in parallel. That is, the PV photoelectrode is formed in the form of a CIGS thin film solar cell module or a tandem type to be positioned on the outer surface of the artificial photosynthetic device to secure the stability of photoelectrode, to supply energy required for water decomposition and carbon dioxide reduction reaction, And the catalyst electrode are arranged parallel to each other, the whole structure can be constituted by the panel type reactor structure.

The solar cell-catalyst integrated electrode comprises a solar cell and a catalyst electrode of a catalyst film, and the catalyst electrode may be a photocatalyst layer or an electrocatalyst layer.

As described above, since the solar cell-catalyst integrated electrode can be applied not only to the anode but also to the cathode, the catalyst film layer is not limited to the oxidation catalyst but may be a reduction catalyst. In this case, And oxidation reaction, that is, water decomposition, occurs in the oxidation catalyst composed of only the oxidation catalyst.

In the present invention, the double-sided conductive substrate may be a double-sided conductive substrate. However, it is preferable to select a glass substrate having a double-sided conductive layer formed thereon for the application of the photocatalyst layer. All known conductive films such as Ti / Pt layers are applicable.

The oxidation catalyst layer may be selected from an electrochemical catalyst material having a water-decomposing oxidation catalyst performance, that is, a metal, a metal oxide, and the like. In particular, a metal oxide such as Co ₃ O ₄ , TiO ₂ , RuO ₂ , IrO ₂ , MnO ₂ , Catalysts.

In order to form such an oxidation catalyst layer on the surface opposite to the PV, a vapor deposition and coating method which does not use a high temperature heat treatment process can be used, and preferably, the vapor deposition, sputtering, or the low temperature coating method can be selected.

Further, the catalyst layer can be formed of a photocatalyst material in addition to the electrocatalyst. In this case, since the photocatalyst needs to absorb PV and sunlight on the opposite surface separately, a photocatalyst material having a band gap smaller than that of the light absorbing layer semiconductor material used in PV is used. Preferably, n-type and p-type photocatalysts with a band gap of 1.0-1.5 eV are available.

Examples of such n-type and p-type photocatalysts include, but are not limited to, n-Si, p-Si, p-InP, p-GaAs and the like.

According to another embodiment, the PV photoelectrode is a CIGS photoelectrode, the two-sided conductive support is a glass substrate having a Mo conductive layer on one side and a Ti thin film layer on the other side and a Pt thin film conductive layer formed on the Ti thin film layer, At this time, the Mo conductive layer is brought into contact with the PV photoelectrode. At this time, the oxidation catalyst layer may be Co ₃ O ₄ produced by a low temperature paste coating method, and the cathode may be a nanostructured gold (Au) catalyst.

In the present invention, the low temperature paste coating method refers to a paste coating method performed at a temperature ranging from 50 to 120 ° C.

According to another embodiment, the PV photoelectrode is a CIGS photoelectrode, the double-sided conductive support is a glass substrate having an Mo conductive layer on one side and an FTO conductive layer on the other side, Electrode. At this time, the oxidation catalyst layer may be RuO ₂ produced by a low temperature paste coating method, and the cathode may be an (Ag) catalyst having a nanostructure.

According to another embodiment, the output of the PV photoelectrode is adjusted within the range of the first output value to the second output value. In the present invention, the first output value indicates a PV photoelectron output value at which the Faraday process of the PV photoelectrode proceeds, and the second output value indicates a current (I _H2 ) Means a PV photoelectron output value of 1 mA or more.

According to another embodiment, the output of the PV photoelectrode is between 2 and 2.5 volts. That is, the first output value is 2 V and the second output value is 2.5 V.

When the output value of the PV photoelectrode is less than the lower limit value in the absence of the external bias voltage, the Faraday process does not proceed and the chemical reaction does not occur. When the upper limit value of the range is exceeded, CO ₂ reduction rate, it is preferable to configure the output value of the PV photoelectrode within the above range. In particular, as shown in FIG. 4B, it can be seen that the current (I _H2 ) used for hydrogen production sharply increases to more than 1 mA at an output of the PV photoelectrode of 2.5 V or more.

Another aspect of the present invention relates to an optoelectrochemical artificial photosynthesis device panel in which photo-electro-chemical artificial photosynthesis devices according to various embodiments of the present invention are connected to each other on the same plane.

According to one embodiment, there is provided a carbon dioxide reduction method comprising operating an opto-electrochemical artificial photosynthesis apparatus according to various embodiments of the present invention, wherein the output of the PV light electrode is controlled within a range of a first output value to a second output value do. At this time, the Faraday process of the PV photoelectrode is performed above the first output value, and the current I _H2 used for generating hydrogen at the second output value or more increases greatly and increases greatly to 1 mA or more.

According to another aspect of the present invention, there is provided a method of controlling the amount of carbon dioxide reduction, wherein the amount of carbon dioxide reduction is controlled by controlling the output of the PV photoelectrode in the operation of the photoelectrochemical artificial photosynthesis apparatus according to various embodiments of the present invention .

Hereinafter, the present invention will be described in more detail. However, the scope and content of the present invention are not limited by the following detailed description.

As a result of intensive investigation of the above-mentioned representative solar water decomposition systems, the present inventors have found that the PV side (the pn junction of the semiconductor) and the photocatalyst layer or the electrocatalyst layer are formed by a transparent conductive film (for example, ITO layer) And they are connected by resistive contact. In the case where both sides of the substrate (e.g., glass substrate) are conductive, the substrate itself may be used instead of the thin conductive material layer in resistive contact between PV and the photocatalyst layer or the electrocatalyst layer, Resistive contact with the photocatalyst layer or the electrocatalyst layer is maintained. This structure is advantageous in terms of durability of the PV side since the PV side does not directly contact the electrolyte. In general, semiconducting materials used in PV, such as silicon or GaAs, are vulnerable to aqueous electrolyte solutions. In this case, since the incident light can reach the PV and PC layers without absorption loss or scattering loss by the electrolyte aqueous solution, the efficiency reduction can be minimized.

In order to apply such a structure to a solar photovoltaic device, a model system as shown in FIG. 1, which combines PV technology and EC, was designed. In particular, the present inventors have found that, by using an established module fabrication process, a solar cell fueling apparatus in which single-junction PV cells are connected in series (modularized) has a small photocurrent loss due to a reduction in the active area of the absorption layer , It was confirmed that it is possible to sufficiently supply energy required for water decomposition and CO ₂ reduction in the apparatus.

As a result of constant efforts to develop an inexpensive integrated / stand-alone photovoltaic fueling apparatus with excellent efficiency and stability, the inventors of the present invention have developed a solar photovoltaic power generation apparatus for the first time in the development of a model of a photovoltaic fueling apparatus having a solar- It was successful. In this device, Cu (In _x Ga _1-x ) (S _y Se _1-y ) ₂ (CIGS) thin film solar cell technology was applied to the PV side. Unlike the commercialized CIGS thin film solar cell technology (vacuum based method), the present inventors have fabricated a CIGS absorption layer using a low cost solution-based method and are therefore expected to be more advantageous in terms of cost, productivity and scale. One single junction CIGS cell does not provide the optical voltage needed for thermodynamic energy for water decomposition / CO ₂ reduction overvoltage compensation, so a CIGS module was fabricated on a glass substrate. On the opposite side of the PV module, an electrocatalyst layer (Co ₃ O ₄ ) for water oxidation was formed by low-temperature coating method so that the performance of the PV side was not damaged. As an electrocatalyst for selective reduction of CO ₂ to CO, nanostructured gold having a high Faraday efficiency of CO ₂ reduction in an aqueous solution, which requires a relatively low overvoltage, was used. Each component was successfully integrated into a stand-alone solar fueling unit, and the coupling efficiency loss was only ~ 5%. The solar CO conversion efficiency was 4.23%, which was confirmed to be equal to or higher than that of natural photosynthetic conversion efficiency.

Hereinafter, the present invention will be described in more detail with reference to Examples and the like, but the scope and content of the present invention can not be construed to be limited or limited by the following Examples. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the present invention as set forth in the following claims. It is natural that it belongs to the claims.

Example

Fabrication of CIGS module

A CIGS module was fabricated on the patterned molybdenum (Mo) layer. The Mo layer (~ 1 μm) was sputtered on soda lime glass and the first pattern P1 was formed using a nanosecond pulse laser with a wavelength of 532 nm. The laser beam was incident on the soda lime glass side to neglect the thermal damage of the Mo. Next, the CIGS absorption layer was spin-coated on the patterned Mo by a solution process. Cu (NO ₃₎ an appropriate amount of _{2 · xH 2 O (Aldrich,} 0.82 g), In (NO 3) 3 · xH 2 O (Aldrich, 1.12 g) and _{Ga (NO 3) 3 · xH} 2 O (AlfaAesar, 0.41 g) was dissolved in methanol (10.0 mL) to prepare a precursor mixture. Then, a solution (7.0 mL) of polyvinyl acetate (PVA) (Aldrich, 1.0 g) dissolved in methanol was added. After the mixed solution was vigorously stirred for 30 minutes, the precursor solution was spin-coated and heat-treated at 300 占 폚 for 30 minutes. This process was repeated about six times to obtain a layer of the desired thickness (~ 1.2 μm) (typically ~ 200 nm thick layer was obtained for each spin coating process). The formed layer was then selenized / sulphurized at 470 ° C to form a CIGS absorbing layer. Next, a CdS buffer layer having a thickness of 60 nm was formed on the CIGS layer by a chemical solution deposition (CBD) method, and an i-ZnO (50 nm) / Al-doped n-ZnO Thereby forming a contact layer. The second pattern (P2) and the third pattern (P3) were formed by mechanical methods before and after the formation of the Al-doped n-ZnO layer, respectively.

Co ₃ O ₄  Fabrication of the layer

After the CIGS module was fabricated, a Ti thin film of 30 nm thickness and a Pt thin film of 50 nm thickness were formed by sputtering on the back side of the module for contacting the oxide catalyst particles. Then, a Co ₃ O ₄ (~50 nm, Aldrich) catalyst layer was formed on the Ti / Pt layer by doctor blade coating. The Co ₃ O ₄ catalyst was dispersed in isopropanol (JT Baker) and then added with Nafion solution (20 wt%, DuPont) to make a paste. After the Co ₃ O ₄ layer was formed, the sample was heated on a hot plate at 70 ° C in air.

Fabrication of nanostructured Au

The nanostructured Au was prepared by an electrochemical method introduced in the reference (Chen et al., J. Am. Chem. Soc. , 2012, 134, 19969). Amorphous gold oxide (99.99%, Dasom RMS) was washed with aqua regia and pulsed anodization to produce amorphous Au oxide. A voltage of 1.2-3.5 V versus a Hg / Hg ₂ SO ₄ reference electrode (CHI 151, CH Instruments, Inc.) was applied for 65,000 cycles in a 0.5 MH ₂ SO ₄ electrolyte aqueous solution using an Ivium (Iviumtechnology). A platinum electrode was used as a control electrode. After reducing the gold oxide 15 minutes for gold catalyst at -1.0 V against Ag / AgCl was carried out CO ₂ reduced in the 0.5 M NaHCO ₃ aqueous solution saturated with CO _2.

Electrochemical measurement

The electrocatalytic properties were measured by using an electrostatic crusher (Ivium, Iviumtechnology) in a two-compartment electrochemical cell made of polyetheretherketone (PEEK). Using a proton exchange membrane (Nafion® 117) . 0.5 M KHCO ₃ (≥99.99%, Aldrich) The electrolyte was saturated with CO ₂ . The pH after saturation was 7.03. A three electrode system was constructed for measuring the catalytic activity of the Co ₃ O ₄ nanoparticle layer and the nanostructured Au layer for water oxidation and CO ₂ reduction using a platinum counter electrode and an Ag / AgCl reference electrode. All potential values were converted to values relative to reversible hydrogen electrodes (RHE) using the formula E (relative to RHE) = E (Ag / AgCl) + 0.197 + 0.059 V ㅧ pH. The steady-state current density at each fixed potential was measured by the time-of-day current method.

CO ₂  Analysis of reduced products

Two - electrode and three - electrode configurations were used to conduct CO ₂ reduction experiments with water oxidation. Three electrode experiments were carried out in the above-mentioned two-compartment biodegradable electrochemical cell. Gaseous products (H ₂ and CO) were quantified using a gas chromatography apparatus (Younglin 6500 GC) equipped with a pulse discharge detector (PDD) using ultrahigh purity (UHP) He (99.9999%) as carrier gas. The GC device was connected directly to the electrochemical cell and a gas sample was injected through a 6 port valve. Each compartment of the electrochemical cell consisted of 38 mL of electrolyte and 34 mL of upper space, and CO ₂ gas was bubbled through the electrolyte at an average rate of 120 mL / min. From the area of the GC chromatogram, the Faraday efficiency (FE) of 2-electron reactions, H ₂ and CO production, was calculated by the following equation.

FE _{H2 or CO} = V _{H2 or CO} × flow rate × 2F _p0 / RT ₀ i _total (3)

In the above equation, V _{H2 or CO} is the volume concentration of H ₂ or CO determined by GC correction and the flow rate (mL / min) was measured at the outlet of the electrochemical cell using a multipurpose flow meter (ADM 2000, Agilent Technologies). i _total (mA) is the steady-state current, F = 96,485 A · s / mol, p ₀ = 1.013 bar, T ₀ = 273.15 K, R = 8.314 J · mol ^-1 · K ^-1 . The formate concentration was measured using an ion chromatographic apparatus (IC, DIONEX IC25A) equipped with a conductivity detector. To account for the top space equilibrium time, the FE of the gaseous product was normalized such that the sum of the efficiencies was 100%.

In the case of two-electrode measurement, a nano-structured Au film was connected to the cathode side and a Co ₃ O ₄ nanoparticle layer was connected to the anode side without using the reference electrode. The material produced in the cathode compartment was analyzed in the same manner as described above.

Identification of characteristics of photovoltaic power generation equipment

After the fabrication of the electrodes and the acrylic separator was completed, each component was bonded as shown in FIG. 4 using an epoxy resin. The photocurrent of the two-electrode configuration was measured while irradiating incident light from the outside of the cell toward the CIGS module under AM-1.5G irradiation condition (100 mW / cm ² ). The GC was directly connected to the photovoltaic device and the chemical product was quantified in the same manner as described above. Artificial sunlight was irradiated using a simulated solar device (ABET, Sun 2000) equipped with a 300 W Xe lamp and an air mass (AM) 1.5 filter. ^{The 13} C isotope experiment was performed using a gas chromatographic apparatus (GC, 6890N network, Agilent Technologies) equipped with a mass selective detector (MSD, 5973 network, Agilent Technologies).

Structure verification

The surface structure was photographed using a field emission scanning electron microscope (FEG-SEM, Inspect F, FEI).

Results and discussion

One of the most important components of the photovoltaic apparatus is a photoelectrode capable of absorbing light to generate electric power. In the present invention, a photo electrode in which PV and an electrocatalyst layer are formed on one conductive glass substrate is used. A CIGS module for absorbing light was formed on one side of the substrate, and a Co ₃ O ₄ nanoparticle catalyst layer for oxidation of water was formed on the other side as an anode (see FIG. 5A). The catalyst layer was formed by a paste coating method. In order to obtain a high CIGS thin film solar cell voltage, the bandgap of the CIGS absorption layer was increased by adding sulfur (S / Se + S = 40%) in the selenization process. The average open-circuit voltage (V _oc ) of a single solar cell was measured at 550 mV, and when five cells were connected in parallel, V _oc was greater than 2.70 V. The CIGS module was fabricated on a laser patterned Mo substrate.

CIGS layer with Mo / CIGS / CdS / i-ZnO / Al-ZnO structure and very low porosity was formed through the solution-based process as shown in the SEM cross-sectional photograph (FIG. Interestingly, the CIGS layer consisted of two layers, a layer of small grain size formed in the interface with the Mo substrate and a layer of large grain size formed thereon. The reason why the two layers are formed is thought to be the difference in distribution of sulfur and selenium in the selenization process. Of CIGS modules current-voltage (IV) characteristic efficiency 8.58%, the open-circuit voltage (V _oc) 2.77 V, the short-circuit current (I _sc) 9.74 mA, (Figure 5c) in fill factor 66.9%, the V _oc of the single CIGS cell It is confirmed that the module is successfully maintained.

Next, an electrocatalyst layer for water oxidation was formed integrally with the CIGS module. Since the overvoltage of the water oxidation process involving four electrons is large, the catalyst function on the anode side for generating oxygen is important in order to increase the reaction efficiency. In particular, in order to protect the CIGS module formed on the surface, it is necessary to form the catalyst layer on the opposite side by the low-temperature coating method. The present inventors formed a catalyst layer on a glass / Ti / Pt substrate by a doctor blade coating method using Co ₃ O ₄ nanoparticles as an oxygen generating reaction (OER) catalyst and a Nafion solution as a binder material, Heat treated. A layer of about 2 탆 thick, with agglomerated Co ₃ O ₄ nanoparticles of size ~ 50 nm, was observed in the SEM cross-sectional photograph (see Figure 5d) and had a porous structure with a large surface area. To investigate the performance of the OER catalyst, a linear scan voltage and current method (LSV) was performed in a 0.5 M KHCO ₃ electrolyte saturated with CO ₂ (see FIG. 5 e). When the Co ₃ O ₄ catalyst was introduced, the water oxidation activity was significantly increased over the entire voltage range as compared with the conductive substrate (Ti / Pt thin film). An overvoltage of 5 mA / cm ² was reduced by 360 mV in the presence of a Co ₃ O ₄ nanoparticle catalyst (from 2.30 V to 1.94 V versus RHE). In addition, it was confirmed that the performance of the CIGS module did not decrease after Co ₃ O ₄ catalyst deposition.

Based on the method developed by Chen et al., A cathode for CO ₂ reduction was fabricated using a nanostructured gold-plated workpiece synthesized. 6A is an SEM image of synthesized nanostructured Au. It can be seen that Au particles of several tens of nanometers in size are aggregated to form a surface layer. Figure 6b and Figure 6b is a 0.5 M KHCO ₃ in the electrolyte in a voltage range of -0.39 V to -0.84 V RHE than Au of H _2, CO and HCOO nanostructured saturated with CO ₂ ^- faraday efficiency for the normal state Current density. The Faraday efficiency for CO showed successive optimum values of about 90 to 94% in the range of -0.44 V to -0.69 V, but the efficiency started to decrease at voltages higher than -0.70 V. Considering that the thermodynamic equilibrium potential when CO ₂ is reduced to -0.10 V versus RHE, the overvoltage at 5 mA / cm ² was 400 mV. The Faraday efficiency for H ₂ showed the opposite trend. As a result of measuring the portion of the current density on H ₂ and CO (see Fig. 6c), the production rate in the case the bias voltage is high (RHE than -0.7 V or less), the production rate of H ₂ is abruptly increased, while CO is substantially the same I did. The reason for this is thought to be that CO ₂ migration is restricted in the high voltage range. In addition, the present inventors confirmed that the FE of less than 1% in most of the voltage range, except that the FE of formate ion was 5.8% at 0.39 V relative to RHE.

Then, a combined anode (CIGS PV + Co ₃ O ₄ OER catalyst) and a cathode (nano-structured Au) were combined to complete an integral solar photovoltaic apparatus (see FIG. 4). A Nafion membrane was disposed between the anode and the cathode so that the material generated at both electrodes was separated and the proton was transferred from the anode to the cathode. A 0.5 M KHCO ₃ solution saturated with CO ₂ was used as the anode solution and the cathode solution. An acrylic separator was used to hold the solution, and the separator and electrodes were bonded with an epoxy resin to allow the generated gas to be collected. The device was connected directly to gas chromatography (GC) to evaluate the performance of the device associated with product generation. As a result of conducting a two-electrode experiment for confirming the CO ₂ reduction performance, IV characteristics similar to those of the three-electrode experiment using Au of the nanostructure were confirmed (see FIG. 7A). The IV curves obtained from the two electrode experiments were divided into three regions. The first region was below 2.0 V, and the chemical reaction did not occur because the Faraday process did not proceed. The production rate of CO rapidly increased in the range of 2.0 ~ 2.5 V, and the rate of production of H ₂ began to increase at more than 2.5 V. Therefore, in order to increase the CO production efficiency using sunlight, it is necessary to adjust to the narrow CO generation region of the CIGS module. The operating point for the H ₂ O / CO ₂ electrolysis of the fabricated CIGS module was determined to be 7.12 mA at 2.34 V (see FIG. 7B).

In order to verify the performance of the photovoltaic fuel cell, external conductive material was connected to the device in the absence of an external bias voltage and the photocurrent change with time was measured. As shown in FIG. 7C, it was observed that the current increased and then decreased during the light irradiation. This behavior is due to the charging of the bilayer and the conversion of the adsorbate adsorbed on the electrode surface. After the photocurrent is stabilized, the present inventors have observed that the photocurrent is slightly reduced with the generation of bubbles on the EC layer. From these data, the average photocurrent of the device was measured to be about 7.32 mA and the CO 2 FE of 91.2%, which is consistent with the operating current at 2.33 V operating voltage. From this data, the solar-to-fuel conversion efficiency (η _STF ) was calculated by the following equation.

? _STF = (I _op ? DELTA E x?) / (P _in × Area) (1)

? _STF = [( _Voc × _Isc × FF) / (P _in × Area)] × [(ΔE × ξ) / V _op ] × [V _op × I _op / V _oc × I _sc × FF )] (2)

In the above, P _in is the incident solar power (100 mW / cm ² ), Area is the irradiated area of the electrode (2.10 cm ² ), I _op is the operating photocurrent (mA), ΔE is the thermodynamic potential difference (1.33 V) ξ is FE (%) of CO generation, and V _op is operating voltage (V). The STF efficiency can also be calculated by multiplying the efficiency of the PV side, the electrochemical conversion efficiency, and the coupling efficiency as shown in equation (2). The first term on the right side is the power conversion efficiency of the CIGS module, the second term is the efficiency of the electrochemical CO generation process, and the last term is the coupling efficiency between the PV side and the electrochemical system. The power conversion efficiency of the CIGS module was 8.58% as mentioned above and the electrochemical conversion efficiency was 52%. In particular, the efficiency of coupling is as high as 94.5%, resulting in a small efficiency loss due to the combination of PV and electrochemical cell because the operating point of the device is well maintained near the maximum power point of the PV module.

The solar-CO conversion efficiency calculated by the two methods is 4.23%, which means that CO gas is produced at a rate of 0.4 μL / sec · cm ² . Finally, a ¹³ C isotope experiment was conducted to determine if CO was produced by CO ₂ reduction. As a result, it was clearly confirmed that ¹³ CO was reduced from ¹³ CO ₂ .

The solar-fuel conversion efficiency of the artificial photovoltaic fuel producing apparatus according to the present invention is 4.23% in the absence of external bias voltage, which is comparable to the photosynthetic efficiency of microorganisms, and is higher than that of most plants existing in nature. It is also very high compared with the results of the studies reported in the past. For example, Arai et al reported the first 0.2% solar-carbon-based fuel conversion efficiency in 2013 using two semiconductor photoelectrodes, followed by White and others for Si solar panels commercialized in 2014 and In The efficiency of 1.8% was reported using a CO ₂ reduction catalyst.

One of the important advantages of the apparatus of the present invention is that by changing the cathode material, the selectivity of CO ₂ reduction can be changed to obtain the desired type of fuel. For example, by converting the nano-structured Au cathode to Pt, only pure hydrogen can be obtained. In this case, the solar-to-hydrogen conversion efficiency is 4.37%.

Further, the photovoltaic fuel model device of the present invention presents the direction and considerations of future research that are needed to solve the problems of efficiency for improvement of efficiency. From the STF calculation, it can be seen that the final efficiency (4.23%) of the photovoltaic power generation device corresponds to almost half of the power conversion efficiency (8.58%) of the CIGS module, Which means that it can be almost doubled.

The present inventors confirmed that the efficiency loss was mostly due to the electrolysis efficiency (52% in the case of the apparatus according to the present invention) since the reduction in the coupling efficiency was only about 5%. The voltage loss of the device of the present invention is about 1 V when comparing the operating voltage of 2.33 V and the thermodynamic voltage of 1.33 V of the total CO generating reaction. The main causes of voltage loss are overvoltage associated with oxidation and reduction, voltage drop across the membrane, and IR drop with solution resistance. As a result of conducting a three-electrode experiment on the anode (see FIG. 5E) and the cathode (see FIG. 6C), the present inventors confirmed that the voltage loss is mainly due to the overvoltage and the voltage loss due to other causes is not large.

The voltage losses due to overvoltage in the OER and CO ₂ reactions were about 650 mV and 340 mV, respectively. Therefore, in order to obtain a high conversion efficiency, it is very important to develop a catalyst for an oxygen generating reaction which can lower the overvoltage on the anode side, particularly in a neutral pH solution. Thermodynamic reduction potential RHE prepared for CO ₂ reduction is independent of pH, but in acidic solution, the generation of hydrogen prevail over the reduction of CO ₂ and the solubility of CO ₂ is reduced in a basic solution. For this reason, studies related to CO ₂ reduction have been performed primarily in neutral electrolytes. However, most of the excellent performance as an electrocatalyst for water oxidation with low overvoltage has been developed in a basic solution such as 1 M KOH, and their overvoltage increases sharply in the neutral solution. It is also for this reason that the photovoltaic fuel system of the present invention has a large overvoltage. Therefore, the development of an oxidation catalyst optimized for a photovoltaic fuel conversion system is desperately required, which will greatly contribute to improvement of STF efficiency.

In addition to the problem of catalytic activity, gas droplets adhered to the surface of the electrode can be the cause of the increase in overvoltage (the scene of the oxygen drop being removed from the anode), which can be solved through methods such as forming a channel in a solar fuel- It is possible.

As mentioned earlier, the reduction in overvoltage during oxidation and reduction is important in that the conversion efficiency can be increased by reducing the voltage loss. It can be predicted from equation (2) that the overall STF efficiency will increase as the power conversion efficiency of the PV module increases. However, this does not necessarily increase the efficiency of the photovoltaic device because the Faraday efficiency of the CO ₂ conversion depends on the applied voltage (ξ (V _op )) as shown in FIG. 7a. As mentioned earlier, when the overvoltage of water oxidation in a given solar photovoltaic fuel system is reduced, the operating point corresponding to the intersection on the IV curve on the PV side can move towards higher operating current (see FIG. 2a).

In addition, if the operating current exceeds a certain level, the Faraday efficiency of CO ₂ reduction on the cathode can be rather reduced (see FIG. 6b). Specifically, the Faraday efficiency of the CO ₂ reduction of the model device according to the present invention was reduced when the total current density was greater than ~ 10 mA (or when the bias voltage applied to the two electrode system exceeded 2.5 V) The final solar-fuel conversion efficiency also decreased. The reason for this is thought to be that the movement of CO ₂ is restricted under a high current density or that the active sites where hydrogen generation is preferred depend on the applied voltage.

Similarly, when the performance of the PV side is increased and the operating voltage is increased, the applied voltage on the cathode side is increased to affect the Faraday efficiency of CO generation from CO ₂ . Unlike the case of the solar water decomposition system, it is more difficult to implement an efficient photovoltaic fueling apparatus that produces the desired product due to the dependence of Faraday efficiency on the applied voltage of such cathode. In order to minimize this effect and produce the desired fuel, it is important to develop a CO ₂ electrocatalyst in which the Faraday efficiency is less affected by the overvoltage. However, from a different point of view, this dependence may be advantageous in that the ratio of CO / H ₂ can be controlled by changing the photovoltage output of the PV module.

As described above, the inventors of the present invention have found that by using a CIGS thin film solar cell module manufactured through a low-cost solution process, an electrocatalyst layer for water oxidation formed by a low-temperature paste coating method, and an Au cathode having a nanostructure for CO ₂ reduction It succeeded in developing a solar photovoltaic power generation system that operates with solar only and has a solar-fuel conversion efficiency of 4% or more. This achievement is only the first step in developing a high efficiency integrated / stand-alone solar photovoltaic fueling system and it is expected that higher efficiency can be obtained by lowering the overvoltage associated with water oxidation in the electrolyte.

Claims (15)

An optoelectrochemical artificial photosynthesis apparatus comprising an anode / oxidation chamber / separator / reduction chamber / cathode,
Wherein at least one of the anode and the cathode is a solar cell-catalyst integrated electrode including a double-sided conductive support, a PV photoelectrode formed on one surface of the double-sided conductive support, and a catalyst electrode formed on the other surface of the double-
Wherein the catalyst electrode is located in the oxidation chamber or the reduction chamber, and the PV photoelectrode is located outside the oxidation chamber or the reduction chamber. The solar cell-catalyst integrated electrode according to claim 1, wherein the anode comprises a double-sided conductive support, a PV photoelectrode formed on one surface of the double-sided conductive support, and an oxidation catalyst electrode formed on the other surface of the double-
Wherein the cathode is a reduction catalyst electrode,
Wherein the oxidation catalyst electrode is located in the oxidation chamber and the PV photoelectrode is located outside the oxidation chamber. The method of claim 1, wherein the anode is an oxidation catalyst electrode,
The cathode is a solar cell-catalyst integrated electrode including a double-sided conductive support, a PV photoelectrode formed on one surface of the double-sided conductive support, and a reduction catalyst electrode formed on the other surface of the double-
Wherein the reduction catalyst electrode is located within the reduction chamber and the PV photoelectrode is located outside the reduction chamber. 4. The solar cell according to any one of claims 1 to 3, wherein the PV photoelectrode is a CIGS thin film solar cell module or a tandem type CIGS thin film solar cell,
Wherein the catalyst electrode is a PV light electrode or an electrocatalyst electrode,
Wherein the photoelectrochemical artificial photosynthesis apparatus is a panel-type apparatus in which the PV light electrode and the catalyst electrode are provided in parallel. The double-sided conductive substrate according to any one of claims 1 to 3, wherein the conductive support layer is a glass substrate having a conductive layer formed on both surfaces thereof, and the conductive layer is selected from the group consisting of ITO, FTO, AZO, Mo, and Ti /
Wherein the PV photoelectrode is capable of generating a light voltage of 2 V or more. The method as claimed in claim 2, wherein the oxidation catalyst electrode is formed on the opposite side of the double-sided conductive support on which the PV photoelectrode is formed by vapor deposition, sputtering or low-
Wherein the oxidation catalyst electrode is a chemical catalyst electrode or an n-type and a p-type photocatalyst having a band gap of 1 to 1.5 eV. 3. The method of claim 2, wherein the PV photoelectrode is a CIGS photoelectrode,
Wherein the Mo conductive layer is in contact with the PV photoelectrode, and the Pt conductive layer is formed on the Ti thin film layer and the Pt thin film conductive layer on the other side,
Wherein the oxidation catalyst layer is Co ₃ O ₄ ,
Wherein the cathode is gold (Au). 3. The method of claim 2, wherein the PV photoelectrode is a CIGS photoelectrode,
Wherein the double-sided conductive support is a glass substrate having a Mo conductive layer on one side and an FTO conductive layer on the other side, the Mo conductive layer contacting the PV photoelectrode,
Wherein the oxidation catalyst layer is RuO ₂ ,
Wherein the cathode is Ag. ≪ RTI ID = 0.0 > 8. < / RTI > 4. A method according to any one of claims 1 to 3, wherein the output of the PV photoelectrode is adjusted within a range of a first output value to a second output value,
Wherein the first output value is a PV optical electrode output value at which the Faraday process of the PV optical electrode proceeds,
Wherein the second output value is a PV photoelectrode output value having a current (I _H2 ) used for hydrogen generation of 1 mA or more. 4. An optoelectrochemical artificial photosynthesis apparatus according to any one of claims 1 to 3, wherein the output of the PV photoelectrode is 2 to 2.5 V. Wherein at least two of the photo-electrochemical artificial photosynthesis apparatuses according to any one of claims 1 to 3 are connected to one another on the same plane. A method for reducing carbon dioxide comprising the step of operating an opto-electrochemical artificial photosynthesis apparatus according to any one of claims 1 to 3,
The output of the PV photoelectrode is adjusted within a range of a first output value to a second output value;
Wherein the Faraday process of the PV photoelectrode is performed at the first output value or more and the current I _H2 used for generating hydrogen at the second output value or more is 1 mA or more. 13. The method of claim 12, wherein the output of the PV photoelectrode is 2 to 2.5 volts. The method of any one of claims 1 to 3, wherein, in operation of the photoelectrochemical artificial photosynthesis apparatus,
Wherein the amount of carbon dioxide reduction is controlled by adjusting the output of the PV photoelectrode. 15. The method of claim 14, wherein the output of the PV photoelectrode is between 2 and 2.5 volts.

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