Patent Application
20170029964
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2015-149357, filed on Jul. 29, 2015; the entire contents of which are incorporated herein by reference.
Embodiments disclosed herein generally relate to a method of forming a composite catalyst layer, a structure for an electrochemical reaction device, and an electrochemical reaction device
The development of artificial photosynthesis technology that replicates photosynthesis of plants to electrochemically convert sunlight to a chemical substance has been recently progressing in consideration of an energy problem and an environmental problem. Converting sunlight to a chemical substance to store it in a cylinder or a tank is advantages in that it costs lower for energy storage and has a less storage loss than converting sunlight to electricity to store it in a battery.
An electrochemical reaction device capable of artificial photosynthesis includes, for example, a reduction electrode immersed in a first electrolytic solution containing carbon dioxide, an oxidation electrode immersed in a second electrolytic solution containing water, and a photoelectric conversion layer electrically connected to the reduction electrode and the oxidation electrode. The reduction electrode and the oxidation electrode are each formed through the deposition of a film containing a catalyst on a substrate, for instance.
When light enters the photoelectric conversion layer, the oxidation electrode oxidizes the water through an oxidation reaction to produce oxygen. Further, the reduction electrode reduces the carbon dioxide through a reduction reaction to produce a carbon compound, or produce hydrogen or the like from water. The electrochemical reaction device can thus produce a desired chemical substance through the reduction reaction and the oxidation reaction using light energy. To enhance efficiency of the aforesaid electrochemical reaction, a used catalyst material preferably has high catalytic activity.
A method of forming a composite catalyst layer of an embodiment includes:
repeating a first step of forming a first deposit part by depositing a first catalyst material on a substrate and a second step of forming a second deposit part by depositing a second catalyst material in contact with a surface of the first deposit part on the substrate, to alternately deposit the first and second catalyst materials. At least one effective thickness out of a first effective thickness calculated from a growth rate of the first deposit part and a second effective thickness calculated from a growth rate of the second deposit part is not less than 0.02 nm nor more than 0.5 nm.
Embodiments will be hereinafter described with reference to the drawings. The drawings are schematic, and for example, the sizes such as the thickness and width of each constituent element may differ from the actual sizes of the constituent element. In the embodiments, substantially the same constituent elements are denoted by the same reference signs and a description thereof will be omitted in some case.
The example of the method of forming the composite catalyst layer of this embodiment includes a step of repeating the step of forming the deposit part 302a and the step of forming the deposit part 302b to alternately deposit the first and second catalyst materials. At this time, in the step of forming the deposit part 302a for the second time onward, the first catalyst material is deposited so as to be in contact with the surface of the deposit part 302b. Through the above-described steps, a composite catalyst layer 302 is formed.
The first and second catalyst materials each contain, for example, a metal, a metal oxide, or a metal nitride. Examples of a method of depositing the first and second catalyst materials include an atomic layer deposition (ALD) method. The ALD method is capable of forming a thin film with, for example, 0.1 nm or less and thus is suitable for forming a catalyst layer having a high light transmitting property. Further, with the ALD method, a use amount of the materials to be deposited can be minimized, and thus it is suitable for forming a catalyst layer that is resource saving and costs low. The method of depositing the first and second catalyst materials is not limited to the ALD method, but may be a chemical vapor deposition (CVD) method, a molecular beam epitaxy (MBE) method, a vacuum deposition method, or a sputtering method, for instance.
The ALD method includes: a step of introducing precursor vapor or gas of a material to be deposited into a reaction chamber set to a predetermined temperature and a predetermined degree of vacuum and causing a substrate disposed in the reaction chamber to adsorb the precursor of the material to be deposited; a step of introducing inert gas such as nitrogen or argon into the reaction chamber to purge the reaction chamber of the precursor vapor or gas not having reacted; a step of introducing reactive vapor or gas into the reaction chamber to cause the precursor adsorbed on the surface of the substrate to react with the reactive vapor or gas; and a step of introducing inert gas such as nitrogen or argon into the reaction chamber to purge the reaction chamber of the reactive vapor or gas not having reacted. The ALD method includes a step of repeating one cycle of an operation in which the step of causing the material to be deposited to be adsorbed and the step of purging the reaction chamber of the precursor vapor or gas of the material to be deposited are sequentially performed.
Examples of the reactive vapor or gas include materials having oxidizing ability, such as water, ozone, oxygen, oxygen plasma, and hydrogen peroxide, and materials having reducing ability, such as gases of hydrogen, ammonia, and nitrogen and their plasma.
The deposit part 302a has a discontinuous structure including gap parts 320a. The deposit part 302b has a discontinuous structure including gap parts 320b. The structure in
The ALD method has a difficulty in depositing two kinds or more of catalyst materials in a mixed state in the same step. A possible method to form a composite catalyst layer by using the ALD method is, for example, to form a first catalyst material layer on a substrate, form a second catalyst material layer on the first catalyst material layer, and thereafter perform heat treatment to compound the first and second catalyst material layers. However, in a case where the first and second catalyst material layers each have a continuous structure without a gap part, their surfaces dominantly contribute to catalytic activity, making it difficult to compound the first and second catalyst materials. In addition, if the substrate has poor heat resistance, the heat treatment may deteriorate electrode and device characteristics, and thus a substrate having poor heat resistance cannot be used
By forming at least one deposit part out of the deposit part 302a having a first discontinuous structure and the second deposit part 302b having a second discontinuous structure, it is possible to compound the first and second catalyst materials without heat treatment. The catalyst layer formed of the composite of the first and second catalyst materials has high catalytic activity and can improve efficiency of, for example, an electrochemical reaction.
When the deposit part 302a and the deposit part 302b are alternately deposited, the plural deposit parts 302b come into contact with each other in the gap part 320a, and the plural deposit parts 302a come into contact with each other in the gap part 320b. This enables the second catalyst material to diffuse to the deposit part 302a and the first catalyst material to diffuse to the deposit part 302b. At this time, in the step of forming the deposit part 302b, the deposit part 302a when the second catalyst material is deposited may contain the second catalyst material, and the deposit part 302b when the second catalyst material is deposited may contain the first catalyst material.
The method of forming the composite catalyst layer of this embodiment does not require heat treatment at a temperature higher than a deposition temperature of the deposit part 302a and higher than a deposition temperature of the deposit part 302b. In other words, the highest value of process temperatures in all the steps may be equal to or lower than the deposition temperature of the deposit part 302a and equal to or lower than the deposition temperature of the deposit part 302b. This can facilitate forming the composite catalyst layer having high catalytic activity and allows the use of substrates of more kinds. Further, since the heat treatment is not necessary, the deterioration of the device characteristic can be prevented.
Controlling a formation condition of the deposit part is important to form the deposit part having the discontinuous structure. The method of forming the composite catalyst layer of this embodiment controls an effective thickness of the deposit part to a predetermined value or less. The deposit part whose effective thickness is controlled to the predetermined value or less has the discontinuous structure.
The effective thickness is an apparent film thickness calculated from the growth rate of the deposit part and may differ from the actual thickness of the deposit part. In the ALD method, the growth rate corresponds to a deposition thickness per cycle (Growth Per Cycle: GPC). GPC is calculated as follows, for instance. The thicknesses of three material layers or more deposited in different numbers of (for example, 100, 200, 300) cycles are measured with, for example, an atomic force microscope (AFM), a spectral ellipsometer, or a transmission electron microscope (TEM). A relation between the number of the deposition cycles and the actually measured thickness of each of the samples of the material layers is plotted. A gradient of linear approximation of each of these plots corresponds to GPC.
As is seen in
The in-plane number density of the Fe atoms in the FeOx deposit part is analyzed as follows, for instance. A depth-direction Fe concentration profile of a deposited layer is obtained using the 3-dimension atom probe. In the obtained concentration profile, a region with the peak being the center up to a half value width of the peak is defined as an FeOx region. Next, the number of the Fe atoms (including the FeOx molecules) in the FeOx region seen in a normal direction of the layer is counted. A value equal to the counted number divided by a value of the measured area is the in-plane number density of the Fe atoms. In counting the number of the Fe atoms in the FeOx region, if the Fe atoms overlap with each other along the depth direction at a specific position of the deposited layer, the number of the Fe atoms at this position is counted as 1. Consequently, the in-plane number density can be found, with the number density distribution in the depth direction excluded, and the in-plane distribution (coverage) can be taken into consideration.
As is seen in
Another example of the analysis method for evaluating continuity of the deposit part is TEM analysis. For example, regarding a deposited layer, a layer pattern is observed or atoms are mapped from an upper surface direction using TEM. The continuity of the deposit part is evaluated by image analysis of an obtained image. When a ratio of the deposit part region occupying the whole image is not less than about 10% nor more than about 90%, the deposit part can be determined as having a discontinuous structure.
The substrate 301 is preferably a conductor or a semiconductor, for instance. As the substrate 301, a carbon material, a semiconductor material, a metal material, or a metal oxide is usable, for instance. Examples of the carbon material include carbon black, activated carbon, fullerene, carbon nanotube, graphene, ketjen black, and diamond. Examples of the metal oxide include indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), and antimony-doped tin oxide (ATO). TiO2, WO3, BiVO4, TaON, or SrTiO3 may be used as the metal oxide, for instance. Examples of the metal material include at least one metal out of Cu, Al, Ti, Ni, Ag, W, Co, and Au, and an alloy containing any of the aforesaid metals. A stack of the metal materials listed above may be used. Examples of the semiconductor material include silicon, germanium, silicon germanium, GaAs, GaP, GaInP, AlGaInP, CdTe, and CuInGaSe.
In the method of forming the composite catalyst layer of this embodiment, the heat treatment at a temperature higher than the deposition temperature of the first catalyst material and at a temperature higher than the deposition temperature of the second catalyst material is not required after the deposit part 302b is formed. This allows the use of a substrate whose heatproof temperature is, for example, 200° C. or lower, as the substrate 301.
A material contained in each of the first and second catalyst materials differs depending on required properties. For example, in a case where oxygen is to be produced through the oxidation of water, the first and second catalyst materials each contain, for example, at least one metal out of Co, Fe, Ni, Mn, Ru, and Ir, an oxide of any of the aforesaid metals, or a nitride of any of the aforesaid metals. In a case where hydrogen is to be produced through the reduction of water, the first and second catalyst materials each contain, for example, at least one metal out of Pt, Ni, Co, Mo, and Ir, an oxide of any of the aforesaid metals, or a nitride of any of the aforesaid metals. In a case where carbon monoxide, formic acid, other hydrocarbon, or the like is to be produced through the reduction of carbon dioxide, the first and second catalyst materials each contain, for example, at least one metal out of Au, Ag, Zn, Cu, and In, an oxide of any of the aforesaid metals, or a nitride of any of the aforesaid metals.
The second catalyst material may contain a compound different from that contained in the first catalyst material. Examples of the combination of their materials include a combination of a metal oxide and a metal, a combination of a metal oxide and a metal nitride, and a combination of a metal and a metal.
In a case where oxygen is to be produced through the oxidation of water, a combination of the first catalyst material containing a transition metal element (Co, Ni, Fe, Mn) and the second catalyst material containing a metal element having 2.0 electronegativity or more exhibits high activity. Examples of the metal element having 2.0 electronegativity or more include Mo, W, Ru, Os, Rh, Ir, Pd, Pt, and Au. A ratio of the number of metal atoms having 2.0 electronegativity or more to the total number of metal atoms is preferably not less than 15% nor more than 70%.
The presence of the metal element having 2.0 electronegativity or more improves oxygen adsorptivity of the surface of the transition metal and also enables the transition metal element to be in a higher oxidized state, enabling an improvement of catalytic activity. In a case where the deposit part containing the transition metal is formed on top of the deposit part of the metal having 2.0 electronegativity or more, the thickness of the deposit part containing the transition metal has a great influence on the catalytic activity, and the thinner this deposit part (for example, less than 1 nm), the higher the catalytic activity. However, as the deposit part is thinner, the catalyst layer is likely to be less durable. The composite catalyst layer formed by the formation method in this embodiment has high catalytic activity because the first and second catalyst materials are three dimensionally compounded on an atomic level. The composite catalyst layer can also have high durability because it is free from the aforesaid thickness restriction.
An example of a structure including a composite catalyst layer that can be formed by the above-described method of forming the composite catalyst layer will be described with reference to
The description of the substrate 301 can be assisted by the description of the substrate 301 illustrated in
The ALD method requires a longer deposition time than other film formation methods and thus is likely to be poor in productivity. So, the thickness of the composite catalyst layer 302 is preferably not less than 1 nm nor more than 100 nm, for instance. In a case where the composite catalyst layer 302 is disposed on a light receiving surface of an electrochemical reaction device, the composite catalyst layer 302 preferably has a light transmitting property. In this case, the thickness of the composite catalyst layer 302 is more preferably not less than 1 nm nor more than 30 nm. Light transmittance of the composite catalyst layer 302 is preferably 50% or more, more preferably 70% or more of an amount of irradiating light. The composite catalyst layer 302 may have a plurality of island-shaped catalyst layers.
In a case where light having a wide wavelength region, such as sunlight, enters instead of light having a single wavelength, the light transmittance of the composite catalyst layer 302 is calculated as follows. After transmittances t (λ) of light having 300 nm to 1000 nm wavelengths (λ) are measured with a spectrophotometer, the light transmittance can be calculated through calculation using the known spectrum I (λ) of the sunlight (sunlight transmittance T=Σt(λ)×I(λ)/ΣI(λ)).
An interface between the deposit part 302a and the deposit part 302b cannot be observed in the composite catalyst layer 302 illustrated in
The structure of this embodiment is usable as a structure for an electrochemical reaction device such as an electrochemical reaction device, for instance. The composite catalyst layer of this embodiment is high in light transmitting property, catalytic activity, durability, and other properties. Accordingly, its use as a catalyst layer of, for example, an electrochemical reaction device can enhance conversion efficiency, durability, and the other properties.
The electrolytic solution tank 1 has a storage part 11 storing an electrolytic solution 21 and a storage part 12 storing an electrolytic solution 22. The electrolytic solution tank 1 is not limited to a particular shape and may have any three-dimensional shape having cavities serving as the storage parts. For example, the electrolytic solution tank 1 may have a cylindrical shape or a square shape.
The storage part 11 and the storage part 12 are separated from each other by, for example, an ion exchange membrane 4. The ion exchange membrane 4 is permeable only to specific ions and separates a product of an oxidation reaction and a product of a reduction reaction from each other. Examples of the ion exchange membrane 4 include a cation exchange membrane permeable to hydrogen ions.
The electrolytic solution 21 at least contains a substance to be oxidized. The substance to be oxidized is a substance that is to be oxidized through the oxidation reaction. Examples of the substance to be oxidized include water. Other substances to be oxidized include organic matters such as alcohol and amine.
The electrolytic solution 22 at least contains a substance to be reduced. The substance to be reduced is a substance that is to be reduced through the reduction reaction. Examples of the substance to be reduced include carbon dioxide.
The electrolytic solution 21 and the electrolytic solution 22 may contain the same substance. In this case, the electrolytic solution 21 and the electrolytic solution 22 may be regarded as one electrolytic solution.
The photoelectric conversion layer 31 has a function of separating electric charges using energy of irradiating light such as sunlight. As the photoelectric conversion layer 31, a pn-junction or pin-junction photoelectric conversion layer is usable, for instance.
The photoelectric conversion layer 31 has a face 311 electrically connected to the catalyst layer 32 and a face 312 electrically connected to the catalyst layer 36. The photoelectric conversion layer 31 is immersed in the electrolytic solution 21. The photoelectric conversion layer 31 corresponds to the substrate 301 in the first embodiment.
The catalyst layer 32 is in contact with the face 311. The catalyst layer 32 is immersed in the electrolytic solution 21. The catalyst layer 32 contains an oxidation catalyst for the substance to be oxidized. A compound produced by the oxidation reaction differs depending on, for example, the kind of the oxidation catalyst. Examples of the compound produced by the oxidation reaction include hydrogen ions. The compound produced by the oxidation reaction is recovered through, for example, a recovery path. At this time, the recovery path is connected to, for example, the storage part 11.
The catalyst layer 32 has a light transmitting property. As the catalyst layer 32, the composite catalyst layer 302 in the first embodiment is usable, for instance. The catalyst layer 32 can be regarded as an oxidation electrode layer. The oxidation electrode layer oxidizes the electrolytic solution 21.
The conductive layer 33 is in contact with the face 312. The conductive layer 33 is immersed in the electrolytic solution 21. The conductive layer 35 is immersed in the electrolytic solution 22.
The insulating layer 34 covers the side surface of the photoelectric conversion layer 31, the side surface of the catalyst layer 32, and the side surface of the conductive layer 33. The insulating layer 34 covers the upper surface of the conductive layer 33.
The insulating layer 34 is immersed in the electrolytic solution 21. The presence of the insulating layer 34 can hinder a leakage current of the photoelectric conversion layer 31 and can also hinder the erosion of the photoelectric conversion layer 31 by the electrolytic solution 21.
The catalyst layer 36 is in contact with the conductive layer 35. The catalyst layer 36 is immersed in the electrolytic solution 22. The catalyst layer 36 contains, for example, a reduction catalyst for the substance to be reduced. A compound produced by the reduction reaction differs depending on, for example, the kind of the reduction catalyst. Examples of the compound produced by the reduction reaction include: carbon compounds such as carbon oxide (CO), formic acid (HCOOH), methane (CH4), methanol (CH3OH), ethane (C2H6), ethylene (C2H4), ethanol (C2H5OH), formaldehyde (HCHO), and ethylene glycol; and hydrogen. The compound produced by the reduction reaction is recovered through, for example, a recovery path. At this time, the recovery path is connected to, for example, the storage part 12.
The insulating layer 37 covers the side surface of the conductive layer 35 and the side surface of the catalyst layer 32. The insulating layer 37 covers the lower surface of the conductive layer 35. The insulating layer 37 is immersed in the electrolytic solution 22. The presence of the insulating layer 37 can hinder a leakage current and also hinder the erosion of the conductive layer 35 by the electrolytic solution 22.
The wiring line 38 electrically connects the conductive layer 33 and the conductive layer 35. The wiring line 38 passes through the insulating layer 34 and the insulating layer 37, for instance.
An operation example of the electrochemical reaction device illustrated in
The following describes a case where electrolytic solutions containing water and carbon dioxide are used as the electrolytic solution 21 and the electrolytic solution 22 and carbon monoxide is produced. Around the catalyst layer 32, as expressed by the following equation (1), the water undergoes the oxidation reaction and loses electrons, so that oxygen and hydrogen ions are produced. At least one of the produced hydrogen ions migrates to the storage part 12 through the ion exchange membrane 4.
2H2O→4H++O2+4e− (1)
Around the catalyst layer 36, as expressed by the following equation (2), the carbon dioxide undergoes the reduction reaction and the hydrogen ions react with the carbon dioxide while receiving the electrons, so that carbon monoxide is produced. Further, in addition to the carbon monoxide, hydrogen may be produced by the hydrogen ions receiving the electrons. At this time, the hydrogen may be produced simultaneously with the carbon monoxide.
2CO2+4H++4e−→2CO+H2O (2)
The photoelectric conversion layer 31 needs to have an open-circuit voltage equal to or larger than a potential difference between a standard oxidation-reduction potential of the oxidation reaction and a standard oxidation-reduction potential of the reduction reaction. For example, the standard oxidation-reduction potential of the oxidation reaction in the equation (1) is 1.23 [V/vs. NHE]. The standard oxidation-reduction potential of the reduction reaction in the equation (2) is −0.1 [V/vs. NHE]. At this time, in the reactions of the equation (1) and the equation (2), the open-circuit voltage needs to be 1.33 [V] or higher.
The open-circuit voltage of the photoelectric conversion layer 31 is preferably higher than the potential difference between the standard oxidation-reduction potential of the oxidation reaction and the standard oxidation-reduction potential of the reduction reaction by a value of overvoltage or more. For example, the overvoltages of the oxidation reaction in the equation (1) and the reduction reaction in the equation (2) are both 0.2 [V]. In the reactions of the equation (1) and the equation (2), the open-circuit voltage is preferably 1.73 [V] or higher.
The electrochemical reaction device of this embodiment is not limited to the structure illustrated in
The composite catalyst layer in the first embodiment is usable not only as a catalyst for the electrochemical reaction device of this embodiment but also as a catalyst for an existing electrochemical reaction device such as a battery or an electrolysis cell. Examples of the electrolysis cell include a water electrolysis cell and a CO2 electrolysis cell. Similarly to an alkaline water electrolysis cell, the electrolysis cell may have an anode and a cathode, which are immersed in an electrolytic tank and are separated by a diaphragm. Similarly to a solid polymer electrolyte cell, the electrolysis cell may have a membrane electrode assembly (MEA) structure that is a stack of an anode, a solid polymer membrane, and a cathode. These cells are driven by a system power source, or by an external power source of renewable energy such as sunlight, wind power, or heat of the earth. In a case where sunlight is used, the structure in which the photoelectric conversion layer is outside the electrolysis cell is different from the structure of the electrochemical reaction device.
As the water-containing electrolytic solution usable as the electrolytic solution, an aqueous solution containing a desired electrolyte is usable, for instance. This solution is preferably an aqueous solution that accelerates the oxidation reaction of water. Examples of the aqueous solution containing the electrolyte include aqueous solutions containing phosphate ions (PO42−), boric acid ions (BO33−), sodium ions (Na+), potassium ions (K+), calcium ions (Ca2+), lithium ions (Li+), cesium ions (Cs+), magnesium ions (Mg2+), chloride ions (Cl−), or hydrogen carbonate ions (HCO3).
Examples of the carbon dioxide-containing electrolytic solution usable as the electrolytic solution include aqueous solutions containing LiHCO3, NaHCO3, KHCO3, CsHCO3, phosphoric acid, or boric acid. The carbon dioxide-containing electrolytic solution may contain alcohol such as methanol, ethanol, or acetone. The water-containing electrolytic solution may be the same as the carbon dioxide-containing electrolytic solution. However, an absorption amount of carbon dioxide in the carbon dioxide-containing electrolytic solution is preferably high. So, a solution different from the water-containing electrolytic solution may be used as the carbon dioxide-containing electrolytic solution. The carbon dioxide-containing electrolytic solution is preferably an electrolytic solution that lowers a reduction potential of carbon dioxide, has high ion conductivity, and contains a carbon dioxide absorbent that absorbs carbon dioxide.
As the aforesaid electrolytic solution, an ionic liquid that contains salt of cations such as imidazolium ions or pyridinium ions and anions such as BF4− or PF6− and is in a liquid state in a wide temperature range, or its aqueous solution is usable, for instance. Other examples of the electrolytic solution include solutions of amine such as ethanolamine, imidazole, and pyridine, and aqueous solutions thereof. Examples of the amine include primary amine, secondary amine, and tertiary amine. These electrolytic solutions may be high in ion conductivity, have a property of absorbing carbon dioxide, and have a characteristic of lowering reduction energy.
Examples of the primary amine include methylamine, ethylamine, propylamine, butylamine, pentylamine, and hexylamine. Hydrocarbon of the amine may be replaced with, for example, alcohol or halogen. Examples of the amine whose hydrocarbon is replaced include methanolamine, ethanolamine, and chloromethyl amine. Further, an unsaturated bond may be present. The same thing can be said for hydrocarbons of the secondary amine and the tertiary amine.
Examples of the secondary amine include dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, dimethanolamine, diethanolamine, and dipropanolamine. The replaced hydrocarbons may be different. This is also the same for the tertiary amine. Examples of the amine having different hydrocarbons include methylethylamine and methylpropylamine.
Examples of the tertiary amine include trimethylamine, triethylamine, tripropylamine, tributylamine, trihexylamine, trimethanolamine, triethanolamine, tripropanolamine, tributanolamine, tripropanolamine, triexanolamine, methyldiethylamine, and methyldipropylamine.
Examples of the cation of the ionic liquid include a 1-ethyl-3-methylimidazolium ion, a 1-methyl-3-propylimidazolium ion, a 1-butyl-3-methylimidazole ion, a 1-methyl-3-pentylimidazolium ion, and a 1-hexyl-3-methylimidazolium ion
The position 2 of the imidazolium ion may be replaced. Examples of the cation which is the imidazolium ion having the replaced position 2 include a 1-ethyl-2,3-dimethylimidazolium ion, a 1,2-dimethyl-3-propylimidazolium ion, a 1-butyl-2,3-dimethylimidazolium ion, a 1,2-dimethyl-3-pentylimidazolium ion, and a 1-hexyl-2,3-dimethylimidazolium ion.
Examples of the pyridinium ion include methylpyridinium, ethylpyridinium, propylpyridinium, butylpyridinium, pentylpyridinium, and hexylpyridinium. In the imidazolium ion and the pyridinium ion, an alkyl group may be replaced, and an unsaturated bond may be present.
Examples of the anion include a fluoride ion, a chloride ion, a bromide ion, an iodide ion, BF4−, PF6−, CF3COO−, CF3SO3−, NO3−, SCN−, (CF3SO2)3C−, bis(trifluoromethoxysulfonyl)imide, bis(trifluoromethoxysulfonyl)imide, and bis(perfluoroethylsulfonyl)imide. It may be a dipolar ion in which the cations and the anions of the ionic liquid are coupled by hydrocarbons. Incidentally, a buffer solution such as a potassium phosphate solution may be supplied to the storage parts 11, 12.
The photoelectric conversion layer 31 has a semiconductor layer of, for example, silicon, germanium, silicon germanium, GaAs, GaInP, AlGaInP, CdTe, or CuInGaSe. The semiconductor may have, for example, a monocrystalline, polycrystalline, or amorphous structure. Further, the photoelectric conversion layer 31 is more preferably of a multijunction type in order to obtain a high open-circuit voltage. Between the catalyst layer 32 and the photoelectric conversion layer 31, a transparent conductive film of, for example, ITO may be disposed.
The photoelectric conversion layer 31 need not be of a pn junction type or a pin junction type. The photoelectric conversion layer 31 may be a p-type or n-type semiconductor. In this case, it is possible to cause an oxidation-reduction reaction by light irradiation, using a barrier formed on an interface between the electrolytic solution and the semiconductor. If the open-circuit voltage high enough to cause the oxidation-reduction reaction cannot be obtained, an auxiliary power source may be connected to the wiring line 38 to compensate for a deficient voltage.
The catalyst layer 32 contains an oxidation catalyst. As the oxidation catalyst, a material that decreases activation energy for oxidizing water is usable. In other words, a material that lowers overvoltage when oxygen and hydrogen ions are produced by the oxidation reaction of water is usable.
In a case where the composite catalyst layer 302 in the first embodiment is used as the catalyst layer 32 and oxygen is produced by the catalyst layer 32, the catalyst layer 32 preferably contains two or more metal elements selected from Co, Fe, Ni, Mn, Ru, and Ir. The above metal elements may exist in a state of an oxide or a nitride. Alternatively, the catalyst layer 32 may contain a metal element selected from transition metals such as Co, Ni, Fe, and Mn and a metal element having 2.0 electronegativity or more, such as Mo, W, Ru, Os, Rh, Ir, Pd, Pt, or Au.
The catalyst layer 36 contains a reduction catalyst. As the reduction catalyst, a material that decreases activation energy for hydrogen evolution or reducing carbon dioxide is usable. In other words, a material that lowers overvoltage when a carbon compound is produced by the reduction reaction of carbon dioxide is usable. For example, a metal material is usable. For example, a metal such as gold, aluminum, copper, silver, platinum, palladium, or nickel, or an alloy containing this metal is usable as the metal material.
In a case where the composite catalyst layer 302 in the first embodiment is used as the catalyst layer 36 and hydrogen is produced by the catalyst layer 36, the catalyst layer 36 preferably contains two or more metal elements selected from Pt, Ni, Co, Mo, and Ir. In a case where the composite catalyst layer 302 in the first embodiment is used as the catalyst layer 36 and a carbon compound such as carbon monoxide, formic acid, or hydrocarbon is produced by the catalyst layer 36, the catalyst layer 36 preferably contains two or more metal elements selected from Au, Ag, Zn, Cu, and In. The above metal elements may exist in a state of an oxide or a nitride
The conductive layer 33 and the conductive layer 35 each contain, for example, at least one metal out of Cu, Al, Ti, Ni, Ag, W, Co, and Au, an alloy containing any of the aforesaid metals, a transparent conductive oxide such as ITO, ZnO, FTO, AZO, or ATO, or a carbon material such as carbon black, activated carbon, fullerene, carbon nanotube, graphene, ketjen black, or diamond. The conductive layer 33 and the conductive layer 35 each may have a film stack of the aforesaid materials.
The insulating layer 34 and the insulating layer 37 each contain, for example, a resin material such as epoxy resin, fluorocarbon resin, or cycloolefin resin, a metal oxide, nitride, or oxynitride containing Ti, Zr, Al, Si, or Hf, or a glass material whose main component is silica, boric acid, or phosphoric acid.
The electrochemical reaction device of this embodiment can have higher efficiency of the conversion from light to a chemical substance by including the composite catalyst layer of the first embodiment.
In this example, composite catalyst layers of cobalt oxide (CoOx) and iron oxide (FeOx) were formed by an ALD method and their catalytic activities were evaluated.
A 200 nm thick glass substrate with an ITO film was introduced into a reaction chamber of a deposition apparatus, CoOx was deposited on the substrate to a 1 nm effective thickness by the ALD method at a 150° C. temperature to form a first deposit part. Next, without the substrate taken out of the reaction chamber, FeOx was deposited to a 0.1 nm effective thickness by the ALD method at a 150° C. temperature to form a second deposit part, whereby a sample of an example 1-1 was fabricated. Further, the deposition condition was changed so that the effective thickness of FeOx became 0.25 nm, 0.5 nm, 0.75 nm, and 1.0 nm, and under each of the conditions, the above step was separately performed, whereby samples of an example 1-2 to an example 1-5 were fabricated.
A 200 nm thick glass substrate with an ITO film was introduced into the reaction chamber of the deposition apparatus, and only CoOx was deposited to a 1 nm effective thickness by the ALD method at a 150° C. temperature, whereby a sample of a comparative example 1-1 including a catalyst layer was fabricated.
A 200 nm thick glass substrate with an ITO film was introduced into the reaction chamber of the deposition apparatus, and only FeOx was deposited to a 1 nm effective thickness by the ALD method at a 150° C. temperature, whereby a sample of a comparative example 1-2 including a catalyst layer was fabricated.
Only a prescribed area (8 mm diameter) of each of the samples of the example 1-1 to the example 1-5 and the comparative examples 1-1, 1-2 was exposed using a Kapton tape. Working electrodes including the respective samples were each placed in one compartment of an H-cell, a counter electrode of a Pt wire was placed in the other compartment, and a glass filter was disposed between the two compartments. As a reference electrode, Ag/AgCl was used.
Catalyst properties were evaluated from steady-state polarization curves which are obtained by varying the potential in 30 mV increments or 50 mV increments and reading a current value at each potential five minutes later. Series resistance components R such as a solution resistance and a substrate resistance were measured by AC impedance, and an effective potential (Eappl−IR) applied to each of the electrodes was calculated. Overvoltage η of an oxygen production reaction was estimated by the following equation.
In this example, composite catalyst layers of CoOx and Ru were formed by ALD, and their catalytic activities were evaluated.
A 200 nm thick glass substrate with an ITO film was introduced into a reaction chamber of a deposition apparatus, CoOx was deposited on the substrate to a 0.4 nm effective thickness by the ALD method at a 150° C. temperature to form a first deposit part. Next, without the substrate taken out of the reaction chamber, Ru was deposited to a 0.14 nm effective thickness by the ALD method at a 150° C. temperature. This was repeated a plurality of times, whereby a sample of an example 2-1 having a 5 nm thick catalyst layer was fabricated. A ratio of the total effective thickness of a Ru deposit part to the thickness of the obtained catalyst layer was 25%.
A 200 nm thick glass substrate with an ITO film was introduced into the reaction chamber of the deposition apparatus, CoOx was deposited on the substrate to a 0.4 nm effective thickness by the ALD method at a 150° C. temperature. Next, without the substrate taken out of the reaction chamber, Ru was deposited to a 0.42 nm effective thickness by the ALD method at a 150° C. temperature. This was repeated a plurality of times, whereby a sample of an example 2-2 having a 5 nm thick catalyst layer was fabricated. A ratio of the total effective thickness of a Ru deposit part to the thickness of the obtained catalyst layer was 50%.
A 200 nm thick glass substrate with an ITO film was introduced into the reaction chamber of the deposition apparatus, and CoOx was deposited on the substrate by the ALD method, whereby a sample of a comparative example 2-1 having a 5 nm thick catalyst layer was fabricated.
A 200 nm thick glass substrate with an ITO film was introduced into the reaction chamber of the deposition apparatus, and CoOx was deposited on the substrate by the ALD method, whereby a sample of a comparative example 2-2 having a 5 nm thick catalyst layer was fabricated.
Their catalytic activities were evaluated as in the example 1. As an electrolytic solution, a boric acid buffer solution (about pH9.2) was used.
Pattern observation of the example 2-1 and the example 2-2 was performed with a transmission electron microscope by EDS element analysis, and the observation of an about 1 nm space showed that CoOx and Ru in the composite catalyst were in a uniform dispersion state without segregation.
Table 1 presents composition analysis results by X-ray photoelectron spectroscopy of the example 2-1 and the example 2-2. A ratio of the number of Ru atoms to the total number of Co atoms and the Ru atoms was about 15% and about 70% respectively.
In this example, photoelectrochemical reactivities of electrochemical reaction devices each including a composite catalyst layer of cobalt oxide (CoOx) and iron oxide (FeOx) were evaluated.
Three-junction photoelectric conversion layers each having a first photoelectric conversion layer which absorbs light in a short wavelength region, a second photoelectric conversion layer which absorbs light in a mid wavelength region, and a third photoelectric conversion layer which absorbs light in a long wavelength region were prepared. The first photoelectric conversion layer has a p-type microcrystalline silicon layer, an i-type amorphous silicon layer, and an n-type amorphous silicon layer. The second photoelectric conversion layer has a p-type microcrystalline silicon layer, an i-type amorphous silicon germanium layer, and an n-type amorphous silicon layer. The third photoelectric conversion layer has a p-type microcrystalline silicon germanium layer, an i-type amorphous silicon layer, and an n-type amorphous silicon layer.
A ZnO layer was formed on a first face of the three-junction photoelectric conversion layer, an Ag layer was formed on the ZnO layer, and a SUS substrate was formed on the Ag layer. An ITO layer was formed on a second face of the three-junction photoelectric conversion layer. CoOx was deposited on the ITO layer to a 0.15 nm effective thickness by the ALD method, and thereafter FeOx was deposited to a 0.04 nm effective thickness. This was repeated twelve times, and CoOx was finally deposited to a 0.15 nm effective thickness, whereby a sample of an example 3-1 was fabricated. A ratio of the total effective thickness of an FeOx deposit part to the thickness of the obtained catalyst layer was 24%.
A ZnO layer was formed on a first face of the three-junction photoelectric conversion layer, an Ag layer was formed on the ZnO layer, and a SUS substrate was formed on the Ag layer. An ITO layer was formed on a second face of the three-junction photoelectric conversion layer. CoOx was deposited on the ITO layer to a 0.15 nm effective thickness by the ALD method, and thereafter FeOx was deposited to a 0.15 nm effective thickness. This was repeated eight times, and CoOx was finally deposited to a 0.15 nm effective thickness, whereby a sample of an example 3-2 was fabricated. A ratio of the total effective thickness of an FeOx deposit part to the thickness of the obtained catalyst layer was 47%.
A ZnO layer was formed on a first face of the three-junction photoelectric conversion layer, an Ag layer was formed on the ZnO layer, and a SUS substrate was formed on the Ag layer. An ITO layer was formed on a second face of the three-junction photoelectric conversion layer. CoOx was deposited on the ITO layer to a 0.15 nm effective thickness by the ALD method, and thereafter FeOx was deposited to a 0.625 nm effective thickness. This was repeated eight times, and CoOx was finally deposited to a 0.15 nm effective thickness, whereby a sample of an example 3-3 was fabricated. A ratio of the total effective thickness of an FeOx deposit part to the thickness of the obtained catalyst layer was 76%.
A ZnO layer was formed on a first face of the three-junction photoelectric conversion layer, an Ag layer was formed on the ZnO layer, and a SUS substrate was formed on the Ag layer. An ITO layer was formed on a second face of the three-junction photoelectric conversion layer. Only CoOx was deposited on the ITO layer to a 0.25 nm effective thickness, whereby a sample of a comparative example 3-1 including a catalyst layer was fabricated.
A ZnO layer was formed on a first face of the three-junction photoelectric conversion layer, an Ag layer was formed on the ZnO layer, and a SUS substrate was formed on the Ag layer. An ITO layer was formed on a second face of the three-junction photoelectric conversion layer. Only FeOx was deposited on the ITO layer to a 0.25 nm effective thickness, whereby a sample of a comparative example 3-2 including a catalyst layer was fabricated.
Stainless steel surfaces of the fabricated samples were each electrically connected to a conducting wire using a copper tape. The conducting wire was passed to a glass tube with a 6 mm diameter, and a space between the sample and the glass tube was filled with epoxy resin for sealing. Next, a peripheral portion of the front surface and the entire rear surface of each of the samples were encapsulated with epoxy resin, whereby electrodes were formed.
Anodes of the electrodes having the respective samples with a 1 cm2 area and cathodes of Pt wires were immersed in a boric acid buffer electrolytic solution (pH9.2). The catalyst layer sides of the sample surfaces were irradiated with light using a solar simulator (AM1.5, 1000 W/m2). A value of a current passing between the anode and the cathode was measured under the light irradiation in the absence of the application of a bias across the both electrodes. The measured current value corresponds to a reaction amount of water in an oxidation-reduction reaction. From the obtained current density J (A/cm2), solar-to-hydrogen efficiency (STH) ηSTH (%) was calculated, assuming that Faraday's efficiency of hydrogen production is 100%. The solar-to-hydrogen efficiency ηSTH is calculated by the following equation (2). In the equation (2), P1sun represents radiant power of sunlight, and E0 represents standard voltage of the hydrogen production reaction.
As described hitherto, the electrochemical reaction device including the composite catalyst layer fabricated by the method of forming the composite catalyst layer of the above-described embodiment has high photoelectrochemical reactivity.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-149357 | Jul 2015 | JP | national |
