JP2009218080A - Photofuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photofuel cell superior in photocurrent conversion efficiency. <P>SOLUTION: In the photofuel cell 40, a cathode 11 and an anode 12 containing a photocatalyst are arranged inside an electrolyte cell 13, the inside of the electrolyte cell 13 is partitioned into a cathode side and an anode side by a proton-permeable membrane 14, a liquid-phase fuel 16 is filled into the cell 13b on the anode side while an electrolytic solution 17 containing redox mediator and oxygen is filled into the vessel 13a on the cathode side, proton formed by photocatalytic oxidation of the liquid-phase fuel moves through the proton-permeable membrane 14, moves to the vessel 13a on the cathode side from the vessel 13b on the anode side, electrons formed by the photocatalytic oxidation of the liquid-phase fuel moves from the anode to the cathode through an exterior circuit 15. In the vessel 13a on the cathode side, under existence of protons, a redox reaction in which oxygen is reduced by electrons and converted into water is formed, and the redox reaction is a reaction via the redox mediator. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a photovoltaic fuel cell.

  Conventionally, as a fuel cell, for example, a polymer electrolyte fuel cell, an alkaline electrolyte fuel cell, a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, etc. have been developed and put into practical use. Yes. Furthermore, recently, a fuel cell (photo fuel cell) using a photocatalyst has been proposed (see, for example, Non-Patent Documents 1 and 2). Photofuel cells require light irradiation to the anode, but organic compounds and nitrogen-containing compounds contained in biomass, waste, etc. can be used as fuel. For this reason, the optical fuel cell has an advantage that the conventional fuel cell can effectively use resources. The photofuel cells described in Non-Patent Documents 1 and 2 use a thin layer of titanium dioxide for the anode, platinum for the cathode, and photocatalytically oxidize liquid phase fuel on the titanium dioxide electrode. Electric power is generated by taking it out to an external circuit. However, the conventional photofuel cell has not had sufficient photocurrent conversion efficiency.

J. et al. Nemoto, M .; Horikawa, K .; Ohnuki, T .; Shibata, H .; Ueno, M.M. Hoshino, M .; Kaneko. J. et al. Appl. Electrochem. 37, 1039 (2007). Kaneko et al., The 86th Annual Meeting of the Chemical Society of Japan, 2A5-51 (2006)

  Accordingly, an object of the present invention is to provide a photovoltaic fuel cell that is excellent in photocurrent conversion efficiency.

To achieve the above object, the photofuel cell of the present invention comprises an anode including a cathode and a photocatalyst, and the cathode and the anode are electrically connected via an external circuit, and by photoirradiation of the photocatalyst, A photofuel cell that generates electric power by extracting electrons generated by photocatalytically oxidizing liquid phase fuel to the external circuit,
Furthermore, an electrolytic cell and a proton permeable membrane are provided,
The cathode and the anode are disposed within the electrolytic cell;
The inside of the electrolytic cell has a two-cell structure divided into the cathode side and the anode side by the proton permeable membrane,
The tank on the anode side is filled with the liquid phase fuel,
The cathode side tank is filled with an electrolyte containing redox mediator and oxygen,
Protons generated by photocatalytic oxidation of the liquid phase fuel pass through the proton permeable membrane and move from the anode side tank to the cathode side tank,
Electrons generated by photocatalytic oxidation of the liquid phase fuel move from the anode to the cathode through the external circuit,
In the tank on the cathode side, in the presence of the proton, an oxidation-reduction reaction occurs in which the oxygen is reduced by electrons transferred from the anode and converted to water, and the oxidation-reduction reaction is a reaction via the redox mediator. It is characterized by being.

  In order to achieve the above-mentioned object, the present inventors have conducted a series of researches. As a result, the photocurrent conversion efficiency of the conventional photofuel cell is not sufficient, and the efficiency of electron transfer on the cathode side is not sufficient. It was found to be due to. Based on this finding, the present inventors have conceived of using a redox mediator to improve the efficiency of electron transfer on the cathode side. Based on this idea, the present inventors have further researched and made a two-cell structure in which the electrolytic cell is divided into an anode side and a cathode side using a proton permeable membrane, and the cathode side cell is filled. It has been found that if a redox mediator is included in the electrolyte solution, the efficiency of electron transfer on the cathode side can be improved, and as a result, the photocurrent conversion efficiency can be greatly improved, leading to the present invention. The mechanism by which the photofuel cell of the present invention is excellent in photocurrent conversion efficiency is presumed as follows. That is, since the redox mediator overvoltage is smaller than the oxygen overvoltage, the redox mediator reduction reaction proceeds more smoothly than the oxygen reduction reaction. Thus, it is estimated that the efficiency of electron transfer can be improved. In addition, the above-mentioned mechanism is speculation and does not limit this invention at all. Further, as will be described later, the photovoltaic fuel cell of the present invention has high photocurrent conversion efficiency even when an inexpensive material such as carbon is used instead of an expensive material such as platinum for the cathode. Obtainable. For this reason, the photovoltaic fuel cell of the present invention can use an inexpensive electrode material and is excellent in terms of cost. In addition, the electrode material used for the photovoltaic fuel cell of this invention is not limited by the above.

  In the present invention, “liquid phase fuel” means a liquid fuel containing at least one of an organic compound and a nitrogen-containing compound as a fuel. The liquid material may be, for example, a solution, a sol, or a gel.

  In the present invention, the “visible light responsive photocatalyst” means a photocatalyst that is excited by visible light and can oxidize liquid phase fuel photocatalytically. In the present invention, “visible light” means light having a wavelength in the range of 400 to 800 nm, for example.

In the photovoltaic fuel cell of the present invention, the redox mediator is preferably an I ₃ ⁻ / I ^- redox mediator.

  In the photovoltaic fuel cell of the present invention, the electrolyte solution is preferably acidic. The pH of the electrolytic solution is more preferably in the range of 0-6, and still more preferably in the range of 1-4.

In the photofuel cell of the present invention, the photocatalyst is preferably a visible light responsive photocatalyst, and more preferably titanium dioxide (TiO ₂ ).

  In the photovoltaic fuel cell of the present invention, the cathode preferably contains carbon. By including carbon in the cathode, it is not necessary to use an expensive material such as platinum, so that the cost can be reduced as described above.

  In the photovoltaic fuel cell of the present invention, it is preferable that dissolved oxygen in the liquid phase fuel is removed.

  In the photofuel cell of the present invention, the wall surface of the anode-side tank is preferably light transmissive.

  Next, the photofuel cell of the present invention will be described in detail. However, the photovoltaic fuel cell of the present invention is not limited by the following description.

  First, an example is given and demonstrated about the composition of the photofuel cell of the present invention.

[1. Photofuel cell)
[1-1. Overall configuration of photovoltaic fuel cell]
The schematic diagram of FIG. 1 shows an example of the configuration of the photofuel cell of the present invention. In the figure, the size, ratio, and the like of each component are different from actual ones for easy understanding. As shown in the figure, the photovoltaic fuel cell 10 includes a cathode 11, an anode 12, an electrolytic cell 13, and a proton permeable membrane 14 as main components. The cathode 11 and the anode 12 are electrically connected via an external circuit 15 and are disposed inside the electrolytic cell 13. The inside of the electrolytic cell 13 has a two-cell structure partitioned by the proton permeable membrane 14 into the cathode 11 side and the anode 12 side. The anode-side tank 13b is filled with liquid phase fuel 16. The tank 13a on the cathode side is filled with an electrolyte solution 17 containing a redox mediator and oxygen. However, the photofuel cell of the present invention is not limited to this example. In this example, the inside of the electrolytic cell 13 is divided into the same size on the cathode 11 side and the anode 12 side by the proton permeable membrane 14 disposed in the center of the electrolytic cell 13. For example, the proton permeable membrane 14 is disposed closer to the cathode 11 or closer to the anode 12, so that the cathode-side tank 13a and the anode-side tank 13b are different in size. Also good. In this example, the number of the cathodes 11 and the number of the anodes 12 is one, but the number of the cathodes 11 and the number of the anodes 12 may be plural, for example.

  Next, each component will be described.

[1-2. Electrolyzer]
The shape of the electrolytic cell may be any shape depending on the form used and its application. Examples of the shape of the electrolytic cell include a quadrangular prism shape, a cylindrical shape, a hemispherical shape, and a combination thereof.

  The material of the electrolytic cell may be any material that can form the shape. From the viewpoint of preventing the liquid phase fuel and the electrolyte from deteriorating moisture, the electrolytic cell may be made of, for example, a laminate material in which a metal layer such as aluminum having low moisture permeability is coated with an insulating polymer, Examples thereof include vinylidene chloride resin, fluorine resin, and glass fiber reinforced plastic material. Further, from the viewpoint of efficiently irradiating the anode with light, the wall surface of the anode-side tank is preferably light transmissive. Examples of the light transmissive material include glass and transparent plastic. Examples of the glass include quartz glass, borosilicate glass, and soda lime glass. Examples of the transparent plastic include acrylic resin, polycarbonate resin, polystyrene resin, hard polyvinyl chloride, and polyphenylene oxide.

  The electrolytic cell can be manufactured, for example, by forming the above-described material using a conventionally known manufacturing method. Moreover, for the electrolytic cell, for example, a commercially available product may be used as it is.

[1-3. Proton permeable membrane]
The proton permeable membrane may be any material that transmits protons. Examples of the material forming the proton permeable membrane include polymer acid, alumina hydrate, solid electrolyte, and the like. Among these, a polymer acid is particularly preferable. Examples of the polymer acid include a condensate of phenolsulfonic acid and formaldehyde, sulfonated polystyrene, trifluorostyrenesulfonic acid, a mixture of fluorocarbonsulfonic acid and polyvinylidene fluoride, and fluorocarbonsulfonic acid.

  As the proton permeable membrane, for example, a commercially available product can be used as it is. Commercially available products of the proton permeable membrane include, for example, a product name “Nafion (registered trademark)” manufactured by DuPont, a product name “Flemion (registered trademark)” manufactured by Asahi Glass Co., Ltd., and a product name manufactured by Asahi Kasei Corporation. “Aciplex” and the like.

  The thickness of the proton permeable membrane is not particularly limited. The thickness of the proton permeable membrane is, for example, in the range of 20 to 300 μm, preferably in the range of 30 to 250 μm, and more preferably in the range of 50 to 200 μm.

  As described above, the electrolytic cell has a two-tank structure divided into the cathode side and the anode side by the proton permeable membrane. The cathode side tank is preferably provided with a stirring means for stirring the electrolytic solution. This is to promote the redox reaction of the redox mediator contained in the electrolytic solution. Examples of the stirring means include a stirring bar such as a magnetic stirrer, a stirring blade such as a stirring nozzle, a rotary blade, and a propeller. It is preferable that the anode side tank is provided with an inert gas introducing means. This is to remove dissolved oxygen from the liquid phase fuel. Examples of the inert gas introduction means include a bubble generator, a bubble injection nozzle, and a foam plate. From the same viewpoint, it is more preferable that the tank on the anode side can be sealed. In this case, it is more preferable that the tank on the anode side includes an exhaust means such as a safety valve for exhausting an inert gas introduced into the liquid phase fuel or a gas generated during power generation (for example, carbon dioxide). .

[1-4. Electrolyte]
As described above, the electrolytic solution includes a redox mediator and oxygen.

  The electrolyte solution may be a solution having electrical conductivity. As the electrolytic solution, a conventionally known electrolytic solution can be used. The electrolytic solution is preferably acidic. The oxidation reaction of the redox mediator can be promoted by making the electrolytic solution acidic. The pH of the electrolytic solution is more preferably in the range of 0-6, and still more preferably in the range of 1-4. The pH of the electrolytic solution can be adjusted by adding, for example, sulfuric acid, chloric acid, perchloric acid or the like.

The redox mediator is not particularly limited. Examples of the redox mediator include transition metal redox mediators, organic redox mediators, and inorganic redox mediators. Examples of the transition metal redox mediator include Cu ²⁺ / Cu redox mediator, Fe ³⁺ / Fe ²⁺ redox mediator, [Fe (CN) ₆ ] ⁴⁺ / [Fe (CN) ₆ ] ³⁺ redox mediator, and the like. Examples of the organic redox mediator include quinone / hydroquinone redox mediator, polyaniline, and the like. The inorganic redox mediator, for _example, ^{I 3 -} / ^I - redox _mediator, ^Br 3 ^- / ^Br - redox mediator, _NO 2 / NO redox mediator, and the like. Among these, those that are easily dissolved or dispersed in the electrolytic solution are preferable. Further, the I ₃ ⁻ / I ^- redox mediator is more preferable because it has a characteristic that it is dissolved in the electrolytic solution and easily spontaneously oxidized under acidic conditions. The I ₃ ^- / I ^- redox mediator, for example, the electrolyte can be prepared by adding an aqueous solution and an iodide iodine at a predetermined rate. Examples of the iodide include sodium iodide, potassium iodide, calcium iodide, lithium iodide and the like.

  The concentration of the redox mediator in the electrolytic solution is, for example, in the range of 0.01 to 0.5 mol / L, preferably in the range of 0.02 to 0.2 mol / L, and more preferably 0.05. It is the range of -0.1 mol / L.

[1-5. Liquid phase fuel]
As described above, the liquid phase fuel is a liquid fuel containing at least one of an organic compound and a nitrogen-containing compound as a fuel. The pH of the liquid phase fuel is not particularly limited, but is preferably adjusted in advance to be the same as that of the electrolytic solution. This is because the pH on the cathode side can be kept constant by adjusting the pH of the liquid phase fuel to be the same as the pH of the electrolyte. The pH of the liquid phase fuel can be adjusted, for example, by adding sulfuric acid, perchloric acid or the like.

  The organic compound is not particularly limited. Examples of the organic compound include alcohol, organic acid, monosaccharide, oligosaccharide, polysaccharide, and polymer compound. Examples of the alcohol include methanol, ethanol, propanol, glycerin and the like. Examples of the organic acid include formic acid, acetic acid, propionic acid, and the like. Examples of the monosaccharide include ribose, glucose, galactose, mannose, and fructose. Examples of the oligosaccharide include lactose, sucrose, maltose and the like. Examples of the polysaccharide include starch, cellulose sulfate, and agarose. Examples of the polymer compound include lignin, polyethylene glycol, polyacrylamide and the like. The said organic compound may be used individually by 1 type, and may use 2 or more types together.

  The nitrogen-containing compound is not particularly limited. Examples of the nitrogen-containing compound include ammonia, urea, amine, amino acid, protein, and the like. Examples of the amino acid include glycine, glutamine, glutamic acid, tyrosine, phenylalanine and the like. Examples of the protein include gelatin and collagen. The said nitrogen containing compound may be used individually by 1 type, and may use 2 or more types together. Further, the nitrogen-containing compound may be used in combination with the organic compound described above, for example.

  When the organic compound and the nitrogen-containing compound are liquid, the concentration of at least one of the organic compound and the nitrogen-containing compound in the liquid phase fuel is not particularly limited, but is, for example, in the range of 1 to 50% by volume. The range is preferably 5 to 30% by volume, and more preferably 10 to 20% by volume.

  When the organic compound and the nitrogen-containing compound are solid, the concentration of at least one of the organic compound and the nitrogen-containing compound in the liquid phase fuel is not particularly limited, but is, for example, in the range of 1 to 50% by weight. The range is preferably 5 to 30% by weight, and more preferably 10 to 20% by weight.

As described above, the dissolved oxygen in the liquid phase fuel is preferably removed. If oxygen (O ₂ ) is present in the liquid phase fuel, for example, the photocatalyst is used for the oxidation of the oxygen (O ₂ ), and the liquid phase fuel is prevented from being oxidized by the photocatalyst. Because. In addition, that the dissolved oxygen is removed includes not only a state in which oxygen (O ₂ ) in the liquid phase fuel is completely removed but also a state in which oxygen is substantially removed.

[1-6. anode]
As described above, the anode includes a photocatalyst. When the photocatalyst is irradiated with light having energy greater than or equal to the band gap of the photocatalyst, electrons in the valence band are excited to the conduction band, and the organic compound and the nitrogen-containing compound contained in the liquid phase fuel At least one of these can be oxidized. Examples of the photocatalyst include metal oxide semiconductors such as alkali metals, alkaline earth metals, transition metals, noble metals, rare earths, and compound semiconductors. Examples of the metal oxide semiconductor include TiO ₂ (rutile type), TiO ₂ (anatase type), FeO ₃ , Cu ₂ O, In ₂ O ₃ , WO ₃ , Fe ₂ TiO ₃ , PbO, and V ₂ O _5. FeTiO ₃ , BiO ₃ , BiVO ₄ , Nb ₂ O ₃ , SrTiO ₃ , ZnO, BaTiO ₃ , CaTiO ₃ , KTaO ₃ , SnO ₂ , ZrO _{2 and the} like. Examples of the compound semiconductor include Si, GaAs, CdSe, GaP, CdS, and ZnS. Among these, titanium dioxide (TiO ₂ ) is preferable. The said photocatalyst may be used individually by 1 type, and may use 2 or more types together.

The photocatalyst can be classified into an ultraviolet light responsive photocatalyst having a band gap energy (E) of 3.2 eV or more and a visible light responsive photocatalyst having a band gap energy (E) of less than 3.2 eV. Examples of the ultraviolet light responsive photocatalyst include TiO ₂ (anatase type), SrTiO ₃ , ZnO, BaTiO ₃ , CaTiO ₃ , KTaO ₃ , SnO ₂ , ZrO ₂ , and ZnS. The band gap energy (E) of the TiO ₂ (anatase type) is 3.2 eV. In TiO ₂ (anatase type), electrons in a valence band are excited by light having a wavelength shorter than 380 nm, which is a wavelength obtained from the following formula (I), that is, ultraviolet light. Examples of the visible light responsive photocatalyst include TiO ₂ (rutile type), FeO ₃ , Cu ₂ O, In ₂ O ₃ , WO ₃ , Fe ₂ TiO ₃ , PbO, V ₂ O ₅ , FeTiO ₃ , and BiO _3. BiVO ₄ , Nb ₂ O ₃ , Si, GaAs, CdSe, GaP, CdS, and the like. The band gap energy (E) of the WO ₃ is 2.7 eV. In WO ₃ , electrons in the valence band are excited by light having a wavelength shorter than 460 nm, which is a wavelength obtained from the following formula (I), that is, part of visible light and ultraviolet light. The photocatalyst is preferably a visible light responsive photocatalyst from the viewpoint of effective use of sunlight. Among these, WO ₃ and BiVO ₄ have strong oxidizing power and are particularly preferable.

E = {(hc / (λ × 10 ⁻⁹ )} × e (I)
E: Band gap energy (eV)
h: Planck's constant (6.63 × 10 ⁻³⁴ J · s)
c: speed of light (3.0 × 10 ⁸ m / s)
λ: Wavelength (nm)
e: Elementary electric quantity (1.602 × 10 ⁻¹⁹ C)

  The visible light responsive photocatalyst may be, for example, a titanium oxide thin film manufactured by depositing titanium oxide on a substrate. The titanium oxide thin film is particularly preferably a nitrogen-substituted titanium oxide thin film produced by an RF magnetron sputtering method under a nitrogen-containing inert gas atmosphere and a substrate temperature of 400 ° C. or higher.

FIG. 2A shows an example of a configuration of an RF magnetron sputtering apparatus. In the figure, the size, ratio, and the like of each component are different from actual ones for easy understanding. As shown in the figure, the RF magnetron sputtering apparatus 20 has a target 22 and a substrate 23 arranged in a vacuum chamber 21. The target 22 is attached to a magnetron portion 24 having a permanent magnet 25 in a circular shape. The substrate 23 is attached to a substrate heater 26. The vacuum chamber 21 includes an inert gas introduction port 27 and an exhaust port 28. A high frequency power supply 29 and a high frequency matching device 30 are connected to the magnetron unit 24. Examples of the RF magnetron sputtering apparatus 20 include a trade name “N-SP-12 type” manufactured by Osaka Vacuum Co., Ltd. Examples of the target 22 include a TiO ₂ sintered body. Examples of the substrate 23 include quartz, indium tin oxide (ITO), and metal Ti.

  With reference to FIG. 2 (B), the manufacturing method of the nitrogen substitution type titanium oxide thin film using the said apparatus is demonstrated. However, the method for producing the nitrogen-substituted titanium oxide thin film is not limited to this example. First, the inside of the vacuum chamber 21 is substantially evacuated, the substrate 23 is heated, and the nitrogen-containing inert gas 31 is introduced from the inert gas inlet 27. Next, a voltage is applied between the substrate 23 and the target 22 to perform high frequency magnetron sputtering. In this way, the nitrogen-substituted titanium oxide thin film 32 can be manufactured on the substrate 23. The temperature of the substrate 23 is, for example, 400 ° C. or higher, preferably 600 ° C. or higher, more preferably 700 ° C. or higher, and still more preferably 800 ° C. or higher.

  In this manufacturing method, the nitrogen-containing inert gas is preferably a mixed gas of nitrogen gas and argon gas. The mixing ratio of the mixed gas is not particularly limited. The mixing ratio of the mixed gas (volume ratio: nitrogen gas / argon gas) is, for example, in the range of 0.02 to 0.7, preferably in the range of 0.03 to 0.6, and more preferably. Is in the range of 0.05 to 0.3.

  In this manufacturing method, it is preferable that the nitrogen-substituted titanium oxide thin film is further baked. A calcination temperature is 200 degreeC or more, for example, Preferably, it is the range of 200-500 degreeC, More preferably, it is the range of 200-400 degreeC.

  In the method for producing a nitrogen-substituted titanium oxide thin film, for example, an ultraviolet light-responsive titanium oxide thin film can be produced by using a mixed gas of oxygen gas and argon gas instead of the mixed gas. The mixing ratio of the mixed gas of oxygen gas and argon gas (volume ratio: oxygen gas / argon gas) is, for example, in the range of 0.02 to 0.7, preferably 0.03 to 0.6. More preferably, it is the range of 0.05-0.3.

  The shape of the anode is not particularly limited. The shape of the anode may be, for example, a flat plate shape, a linear shape, or a rod shape. The anode is preferably porous. If the anode is porous, for example, the contact area with the liquid phase fuel can be increased, and the oxidation reaction on the anode side can be promoted. As a method for forming a porous anode, for example, a method in which an anode is formed in a state where volatile substances are mixed in advance and then the volatile substance is vaporized by heating, or a porous mold is used. A method of forming a cathode by forming a pinhole with a needle after forming the cathode, a method of forming an anode by vapor deposition or the like on an uneven substrate, and a method of forming an anode by vapor deposition And a method of forming by mixing gases.

[1-7. Cathode]
The cathode material may be any material having conductivity. Conventionally known materials can be used for the cathode. Examples of the cathode material include metals, alloys, inorganic compounds, organic compounds, and carbon. Examples of the metal include Pt, Au, Ag, Cu, Fe, Ni, Ti, Pd, Ir, Rh, Os, Ru, Co, Re, and oxides thereof. Examples of the alloy include Pt—Co, Pt—Au, Pt—Sn, Cu—Au, Cu—Ag, Pd—Ni, Pd—Au, Ru—Ta, Ni—Mn, Ni—P, and Ni—oxidation. Examples include cobalt, Pt—Pd—Au, Ag—Ni—P, Raney silver, Raney nickel, and the like. Examples of the inorganic compound include nickel boride, cobalt boride, tungsten carbide, tungsten oxide, tungsten phosphide, niobium phosphide, titanium hydroxide, transition metal carbide, spinel compound, transition metal perovskite ion crystal, etc. Is given. Examples of the organic compound include nonionic activators, phthalocyanines, metal phthalocyanines, quinones, and the like. Examples of the carbon include carbon paper, carbon fiber, a carbon molded body, and a carbon sintered body. Among these, it is preferable to use an inexpensive material such as carbon from the viewpoint of cost. The said material may be used individually by 1 type, and may use 2 or more types together.

  The shape of the cathode is not particularly limited. The shape of the cathode can be, for example, the same shape as the anode.

[1-8. External circuit]
The external circuit is not particularly limited, and a conventionally known circuit can be used.

[1-9. Other components]
The photofuel cell of the present invention may include other constituent members in addition to the main constituent members described above. Examples of other components include a supply port for supplying liquid phase fuel.

[2. Photofuel cell manufacturing method]
The photovoltaic fuel cell of the present invention can be produced, for example, by assembling the above-described main constituent members and, if necessary, other constituent members by a conventionally known method.

[3. Photoelectric fuel cell power generation method]
Next, the power generation method of the photovoltaic fuel cell according to the present invention will be described with an example. However, the power generation method of the photovoltaic fuel cell of the present invention is not limited to this example.

With reference to the schematic diagram of FIG. 3, the power generation method of the photovoltaic fuel cell of this invention is demonstrated. In the figure, the size, ratio, and the like of each component are different from actual ones for easy understanding. Moreover, in the same figure, the same code | symbol is attached | subjected to the same part as FIG. As shown in the figure, in the photovoltaic fuel cell 40, the liquid phase fuel 16 contains methanol as an organic compound. The electrolytic solution 17 includes I ₃ ⁻ / I ^- redox mediator as a redox mediator. The liquid phase fuel 16 and the electrolytic solution 17 are acidic. The photofuel cell of the present invention is not limited to this example.

First, the anode 41 including the photocatalyst is irradiated with light 41a having energy greater than or equal to the band gap of the photocatalyst. Thereby, electrons in the valence band of the photocatalyst contained in the anode 12 are excited to the conduction band, and holes 42 (h ⁺ ) are generated in the valence band. The holes (h ⁺ ) 42 reaching the interface of the liquid phase fuel 16 oxidize methanol into carbon dioxide (arrow A: the following formula (II)) to generate protons (H ⁺ ). The excited electrons 43a move along the potential gradient generated in the conduction band and reach the cathode 11 through the external conductor 18 (arrow B). As a result, a photocurrent (arrow C) is generated in the external circuit 15. In this example, if the wall surface of the anode side tank 13b (for example, the right side surface in the figure) is light transmissive, the light 41b may be irradiated instead of or in addition to the light 41a.

CH ₃ OH + H ₂ O + 6h ⁺ → CO ₂ + 6H ⁺ (II)

  The liquid phase fuel 16 is preferably free from dissolved oxygen. As shown in the figure, the dissolved oxygen is removed by purging with an inert gas 45 using an inert gas introducing means 44 provided in the anode side tank 13b. The inert gas introducing means 44 is as described above. Examples of the inert gas include nitrogen, helium, argon, krypton, methane, and ethane. The said inert gas may be used individually by 1 type, and may use 2 or more types together. The inert gas purge is not limited to this example. The inert gas purge may be performed, for example, before power generation in advance, or may be performed during power generation as in this example after being performed in advance. The photofuel cell of the present invention is not limited to this example. For example, the photovoltaic fuel cell of the present invention may not include the inert gas introducing means.

  The light is preferably sunlight from the viewpoint of effectively using natural energy. The light may be, for example, light from an artificial light source that can irradiate light having energy greater than or equal to the band gap of the photocatalyst. Examples of the artificial light source include a xenon lamp, an ultraviolet light lamp, a mercury lamp (high pressure or ultrahigh pressure), an incandescent lamp, and a fluorescent lamp.

On the other hand, in the electrolytic solution 17, as described above, I ⁻ easily undergoes natural oxidation (arrow D: the following formula (III)) under acidic conditions to become I ₃ ⁻ . After a certain period of time, I ⁻ and I ₃ ⁻ have reached equilibrium. I ₃ ⁻ is reduced to I ⁻ (arrow E: the following formula (IV)) by the electrons 43b reaching the cathode 11. Thereby, the redox cycle on the cathode 11 side is realized. At this time, the proton (H ⁺ ) on the cathode 11 side decreases due to the oxidation reaction (arrow D). However, protons (H ⁺ ) generated on the anode 12 side pass through the proton permeable membrane 14 and move to the cathode 11 side (arrow F), thereby being supplied to the cathode 11 side. Thereby, in the photovoltaic fuel cell of the present invention, a complete circuit is realized, and it is possible to constantly generate a photocurrent (arrow C).

6I ⁻ + O ₂ + 4H ⁺ → 2I ₃ ⁻ + 2H ₂ O (III)

2I ₃ ⁻ + 4e ⁻ → 6I ⁻ (IV)

The electrolytic solution 17 is preferably stirred. As illustrated, the agitation is performed using agitation means 46 provided in the cathode-side tank 13a. By carrying out the stirring, oxygen (O ₂ ) consumed in the oxidation reaction (arrow D) can be supplied from the air to the electrolytic solution 17, and power generation can be continued. The stirring means 46 is as described above. The stirring is not limited to this example. The agitation may be performed before power generation in advance, or may be performed during power generation as in this example after being performed in advance. The photofuel cell of the present invention is not limited to this example. For example, the photovoltaic fuel cell of the present invention may not include the stirring means.

  The operating temperature of the photovoltaic fuel cell of the present invention during power generation is not particularly limited. The photofuel cell can operate sufficiently even at room temperature, but may be operated at a high temperature from the viewpoint of improving the oxidation rate of the liquid phase fuel by the photocatalyst. The said operating temperature is the range of 3-90 degreeC, for example, Preferably it is the range of 10-80 degreeC, More preferably, it is the range of 30-70 degreeC.

  Next, examples of the present invention will be described together with comparative examples. The present invention is not limited or restricted by the following examples and comparative examples. In addition, the measurement of various properties and physical properties and the calculation of numerical values in each example and each comparative example were performed by the following methods.

(A) Photocurrent Photocurrent was measured by constant potential measurement. The constant potential measurement was performed by irradiating light with the potential difference between the anode and the cathode being 0 V, and measuring the photocurrent flowing at this time. The constant potential measurement was performed using a program of an electrochemical measurement system (trade name “HZ-3000” manufactured by Hokuto Denko Co., Ltd.) incorporating a potentiostat / galvanostat, a function generator, and an XY recorder. .

(B) Short-circuit current density (J _SC ) and open circuit voltage (V _OC )
The short circuit current density (J _SC ) and the open circuit voltage (V _OC ) were determined using linear sweep voltammetry (LSV) measurement. The LSV measurement was performed by plotting the photocurrent value when the photovoltaic power value generated during light irradiation was changed from 0V to 0V. From this result, the photocurrent flowing in a state where the resistance was zero was defined as the short-circuit current density (J _SC ). Moreover, the photovoltaic power generated in the open circuit state (resistance is infinite) was defined as the open circuit voltage (V _oc ). The LSV measurement was performed using a program of the above-described electrochemical measurement system (trade name “HZ-3000” manufactured by Hokuto Denko Co., Ltd.).

(C) Photocurrent conversion efficiency (η)
The photocurrent conversion efficiency (η) was calculated from the following formula (V). The formation factor (ff) is a value calculated from the following formula (VI) using the maximum value (P _max ) of electric power generated by the photovoltaic fuel cell. The incident intensity (I ₀ ) was measured using a power meter (trade name “NovaII” manufactured by OPHIR).

η = {(J _SC × V _OC × ff) / I ₀ } × 100 (V)
η: Photocurrent conversion efficiency (%)
J _SC : short circuit current density (mA / cm ² )
V _OC : Open circuit voltage (V)
ff: formation factor
I ₀ : Incident intensity (mW / cm ² )

ff = P _max / (J _SC × V _OC ) (VI)
P _max : Maximum power value (mW)

(D) Absorption spectrum The absorption spectrum of the electrolytic solution was measured using a self-recording spectrophotometer (manufactured by Shimadzu Corporation, trade name “UV-2200A”).

〔anode〕
[Reference Example 1]
Using an RF magnetron sputtering device (trade name “N-SP-12 type” manufactured by Osaka Vacuum Co., Ltd.), visible light responsive titanium dioxide (Vis—TiO ₂ ) was produced under the following conditions.
Target: TiO ₂ (manufactured by Oraltech Co., Ltd., 99.990%)
Substrate: ITO glass substrate (manufactured by Sanyo Vacuum Industry Co., Ltd., size: 0.7 mm × 8 mm × 40 mm)
Substrate temperature: 600 ° C
Distance between target and substrate: 75mm
Ultimate vacuum: 8 × 10 ⁻⁴ Pa
Deposition pressure: 2.0Pa
Sputtering gas: Argon gas (99.995%)
Sputtering gas flow rate: 4.23 × 10 ⁻³ Pa · m ³ / s
RF output: 300W
Deposition time: 300 minutes

[Reference Example 2]
Ultraviolet light-responsive titanium dioxide (UV-TiO ₂ ) was produced under the same conditions and method as in Reference Example 1 except that the substrate temperature was 200 ° C.

[Cathode]
[Reference Example 3]
A platinum wire was produced by joining a platinum wire and a copper wire by soldering. Next, a glass ball of soda glass melted using a burner was attached to one portion of the platinum portion of the platinum wire. Next, the platinum wire with the glass ball attached is inserted into a glass tube (trade name “Pyrex (registered trademark)” manufactured by AGC Techno Glass Co., Ltd.) from the copper wire side of the platinum wire, and the glass ball Was passed to the end of the glass tube. The glass tube having the same diameter as the glass ball or smaller than the glass ball was used. Finally, using a burner, the end of the glass tube and the glass ball were thermally fused to produce a Pt electrode.

[Reference Example 4]
A carbon electrode 072591 95615201 manufactured by Niraco Co., Ltd. was prepared.

[Electrolysis tank]
[Reference Example 5]
A glass container (manufactured by AGC Techno Glass Co., Ltd., trade name “Pyrex (registered trademark)”) having a length of 60 mm × width of 60 mm × height of 60 mm was prepared. The glass container has an opening of 40 mm in length and 40 mm in width at the central portion of one side surface thereof. The two glass containers were joined using a rubber band so that the openings were aligned, thereby producing an electrolytic cell.

[Proton permeable membrane]
[Reference Example 6]
A proton permeable membrane manufactured by DuPont, trade name “Nafion (registered trademark) 117” was prepared.

[Liquid phase fuel]
[Reference Example 7]
Methanol was added to the Na ₂ SO ₄ aqueous solution as an organic compound so as to be 10% by volume. Next, the pH of the Na ₂ SO ₄ aqueous solution containing methanol was adjusted to 2 using 1 mol / L sulfuric acid. Next, a liquid phase fuel was prepared by adjusting the Na ₂ SO ₄ aqueous solution whose pH was adjusted to 2 to a concentration of 0.5 mol / L.

[Reference Example 8]
A liquid-phase fuel was prepared under the same conditions and method as in Reference Example 7 except that ethanol was added so as to be 10% by volume as the organic compound.

[Reference Example 9]
A liquid-phase fuel was prepared under the same conditions and method as in Reference Example 7, except that glycerol was added as an organic compound so as to be 10% by volume.

[Electrolyte]
[Reference Example 10]
In aqueous Na ₂ SO _4, as a redox mediator, a NaI and _{I 2} aqueous solution, respectively, it was added to a 0.5 mol / L and 0.025 mol / L. Next, the pH of the Na ₂ SO ₄ aqueous solution containing the redox mediator was adjusted to 2 using 1 mol / L sulfuric acid. Subsequently, the electrolytic solution was prepared by adjusting the concentration of the Na ₂ SO ₄ aqueous solution whose pH was adjusted to 2 to 0.5 mol / L. The absorption spectrum of the electrolyte solution of this reference example is shown in the graph of FIG. The absorption spectrum of the electrolyte solution of this reference example was measured by measuring the absorption spectrum of a diluted solution obtained by diluting the electrolyte solution of this reference example 250 times. In the electrolytic solution of this reference example, it was considered that I ⁻ and I ₃ ⁻ reached equilibrium from the time of preparation.

[Reference Example 11]
As a redox mediator, an electrolytic solution was prepared under the same conditions and method as in Reference Example 10 except that only NaI was added to 0.5 mol / L. The graph of FIG. 5 shows the relationship between the stirring time and the absorption spectrum when the electrolytic solution of this reference example is stirred in air. In addition, the measurement of the absorption spectrum of the electrolyte solution of this reference example was performed by measuring the absorption spectrum of the diluted solution which diluted the electrolyte solution of this reference example 5000 times. In the figure, 0.5h, 1h, 2h, 3h, 5h, 7h, and 9h are 0.5 hour, 1 hour, 2 hours, 3 hours, 5 hours, and 7 hours from the start of stirring, respectively. , 9 hours have passed. In addition, it was thought that only the I < ^- > existed in the electrolyte solution of this reference example at the time of preparation. As shown in the figure, the absorption spectrum of the electrolytic solution changed with time, and an absorption spectrum almost the same as that shown in FIG. 4 was observed. From this, it was confirmed that I ⁻ was easily spontaneously oxidized to I ₃ ⁻ under acidic conditions.

[Reference Example 12]
An electrolyte solution was prepared under the same conditions and method as in Reference Example 10 except that no redox mediator was added.

[Artificial light source]
[Reference Example 13]
A xenon lamp (500 W, trade name “XENON SHORT ARC LAMP” manufactured by USHIO INC.) That emits light having a wavelength of 300 nm or more was prepared. The xenon lamp was adjusted using a water filter so that light of about 100 mW / cm ² was irradiated on the anode.

[Example 1]
First, the anode of Reference Example 1 and the cathode of Reference Example 3 were placed in the electrolytic cell of Reference Example 5. In the electrolytic cell, the proton permeable membrane of Reference Example 6 was disposed so as to be divided into an anode-side cell and a cathode-side cell. Subsequently, the anode-side tank was filled with the liquid phase fuel of Reference Example 7, and the cathode-side tank was filled with the electrolyte solution of Reference Example 10. In this way, the photofuel cell of this example was produced. The liquid phase fuel was purged with argon gas (99.995%) for about 15 minutes before power generation in order to remove dissolved oxygen. Next, light (incident intensity (I ₀ ): 101.3 mW / cm ² , irradiation area: 1.0 cm ² ) is irradiated on the anode of the photofuel cell using the artificial light source of Reference Example 13 to generate power. went. The purge of the argon gas performed before power generation was continued during power generation. The electrolytic solution was vigorously stirred during power generation using a magnetic stirrer. Table 1 below shows the configuration and power generation conditions of the photovoltaic fuel cell of this example.

[Example 2]
Except for using the cathode of Reference Example 4 as the cathode, the photofuel cell of this example was produced and generated electricity under the same conditions and method as in Example 1. Table 1 below shows the configuration and power generation conditions of the photovoltaic fuel cell of this example.

[Example 3]
Except for using the anode of Reference Example 2 as the anode, a photovoltaic fuel cell of this example was produced and generated electricity under the same conditions and method as in Example 2. Table 1 below shows the configuration and power generation conditions of the photovoltaic fuel cell of this example.

[Example 4]
Except for using the electrolyte solution of Reference Example 11 as the electrolyte solution, the photovoltaic fuel cell of this example was produced and generated under the same conditions and method as in Example 2. Table 1 below shows the configuration and power generation conditions of the photovoltaic fuel cell of this example.

[Example 5]
Except for using the electrolyte solution of Reference Example 11 as the electrolyte solution, the photofuel cell of this example was produced and generated under the same conditions and method as in Example 3. Table 1 below shows the configuration and power generation conditions of the photovoltaic fuel cell of this example.

[Example 6]
Except for using the liquid phase fuel of Reference Example 8 as the liquid phase fuel, a photovoltaic fuel cell of this example was produced and power was generated under the same conditions and method as in Example 2. Table 1 below shows the configuration and power generation conditions of the photovoltaic fuel cell of this example.

[Example 7]
Except for using the liquid phase fuel of Reference Example 9 as the liquid phase fuel, the photovoltaic fuel cell of this example was produced and power was generated under the same conditions and method as in Example 2. Table 1 below shows the configuration and power generation conditions of the photovoltaic fuel cell of this example.

[Comparative Example 1]
Except for using the electrolyte solution of Reference Example 12 as the electrolyte solution, a photofuel cell of this comparative example was produced under the same conditions and method as in Example 1 to generate electric power. Table 1 below shows the configuration and power generation conditions of the photovoltaic fuel cell of this comparative example.

[Comparative Example 2]
Except for using the electrolyte solution of Reference Example 12 as the electrolyte solution, a photofuel cell of this comparative example was produced under the same conditions and method as in Example 2 to generate electric power. Table 1 below shows the configuration and power generation conditions of the photovoltaic fuel cell of this comparative example.

The graph of FIG. 6 shows the LSV measurement results of Example 1 and Comparative Example 1 as a current-voltage curve. From the LSV measurement, the photofuel cell of Example 1 had V _OC = 0.74 V, J _SC = 1.1 mA / cm ² , and ff = 0.25. In addition, η of the photovoltaic fuel cell of Example 1 was 0.19% based on the formula (V). On the other hand, the optical fuel cell of Comparative Example 1 was V _OC = 0.82 V, J _SC = 0.46 mA / cm ² , and ff = 0.12 from LSV measurement. Moreover, (eta) of the photofuel cell of the comparative example 1 was 0.045% from the said Formula (V). From this result, by introducing I ₃ ⁻ / I ^- redox mediator as a redox mediator, the photofuel cell of Example 1 is compared with the photofuel cell of Comparative Example 1 in which no redox mediator is introduced, The short-circuit current density and the formation factor were improved about twice, and the photocurrent conversion efficiency was improved about 4.2 times.

  The graph of FIG. 7 shows the LSV measurement results of Comparative Examples 1 and 2 as current-voltage curves. As shown in the drawing, the photoelectric fuel cell of Comparative Example 2 in which the redox mediator was not introduced and the carbon electrode was used for the cathode had a low photocurrent conversion efficiency.

The graph of FIG. 8 shows the LSV measurement results of Examples 1 and 2 as current-voltage curves. As shown, as a redox mediator, I 3 ^{_- /} I _- by introducing a redox mediator, light fuel cell of Example 2 using the carbon electrode to the cathode, the light of Example 1 using the Pt electrode to the cathode The photocurrent conversion efficiency was similar to that of a fuel cell. That is, by introducing the redox mediator, the photocurrent conversion efficiency of the photofuel cell of Example 2 using carbon for the cathode was significantly improved as compared with the photofuel cell of Comparative Example 2 described above.

The graph of FIG. 9 shows the LSV measurement results of Examples 2 and 3 as current-voltage curves. As shown in the drawing, Example 2 using Vis-TiO ₂ for the anode was able to obtain a higher short-circuit current density than Example 3 using UV-TiO ₂ for the anode. In addition, η of the photovoltaic fuel cell of Example 3 was 0.17%. That is, the photocurrent conversion efficiency of the photofuel cell of Example 3 was improved by about 3.8 times compared to Comparative Example 1 in which no redox mediator was introduced.

In the graph of FIG. 10, the relationship between the short circuit current value of Example 4 and 5 and the stirring time of electrolyte solution is shown. As shown in the figure, the short-circuit current values of both Example 4 using Vis-TiO ₂ for the anode and Example 5 using UV-TiO ₂ for the anode improve with the passage of time from the start of stirring. After a lapse of time, the current value became constant. As described above, it was considered that I ⁻ added to the electrolytic solution was subjected to natural oxidation by stirring of the electrolytic solution to become I ₃ ⁻ .

  The graph of FIG. 11 shows the LSV measurement results at the time when the stirring of the electrolytes of Examples 4 and 5 was started (0h: 0 hour) and when the short-circuit current value became constant (4h: 4 hours). Shown as a current-voltage curve. As shown in the figure, when the short-circuit current value becomes constant (4h: 4 hours), the photocurrent conversion of the photovoltaic fuel cells of Examples 4 and 5 is compared with the time when stirring is started (0h: 0 hours). Efficiency has improved significantly.

  The graph of FIG. 12 shows the LSV measurement results of Examples 2, 6, and 7 as current-voltage curves. Η of the photovoltaic fuel cell of Example 2 was 0.19% as described above. The η of the photovoltaic fuel cell of Example 6 was 0.14%. In addition, η of the photovoltaic fuel cell of Example 7 was 0.25%. In a photofuel cell using any of methanol, ethanol, and glycerin as the organic compound, the photocurrent conversion efficiency was improved as compared with Comparative Example 1 in which no redox mediator was introduced.

  As described above, the photofuel cell of the present invention is excellent in photocurrent conversion efficiency. The photofuel cell of the present invention can generate electric power by irradiating light to the anode and oxidizing liquid phase fuel photocatalytically. For this reason, the photofuel cell of the present invention can be used in a form incorporated in a small electronic device used in an environment where light is irradiated, for example, a mobile phone, an electronic notebook, a notebook personal computer or the like. The photovoltaic fuel cell of the present invention can also be used as a fuel cell for generating power by effectively using resources because organic compounds and nitrogen-containing compounds contained in biomass, waste, etc. can be used as fuel. . However, the photovoltaic fuel cell of the present invention is not limited to the above-mentioned applications, and can be applied to a wide range of fields.

FIG. 1 is a schematic diagram showing an example of the configuration of the photovoltaic fuel cell of the present invention. FIG. 2A is a schematic diagram illustrating an example of a configuration of an RF magnetron sputtering apparatus. FIG. 2B is a schematic diagram for explaining an example of a method for producing a nitrogen-substituted titanium oxide thin film using the apparatus of FIG. FIG. 3 is a schematic diagram for explaining a power generation method using the photovoltaic fuel cell according to the present invention. FIG. 4 is a graph showing an absorption spectrum of an electrolytic solution containing both I ₃ ⁻ and I ⁻ . FIG. 5 is a graph showing the relationship between the stirring time and the absorption spectrum when the electrolyte containing only I ⁻ is stirred in air. FIG. 6 is a graph showing the LSV measurement result of one example of the present invention as a current-voltage curve. FIG. 7 is a graph showing the LSV measurement result of one reference example of the present invention as a current-voltage curve. FIG. 8 is a graph showing LSV measurement results of other examples of the present invention as current-voltage curves. FIG. 9 is a graph showing the LSV measurement results of still another example of the present invention as a current-voltage curve. FIG. 10 is a graph showing the relationship between the short-circuit current value and the electrolytic solution stirring time in one example of the present invention. FIG. 11 is a graph showing the LSV measurement results of still another example of the present invention as a current-voltage curve. FIG. 12 is a graph showing the LSV measurement results of still another example of the present invention as a current-voltage curve.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 40 Photofuel cell 11 Cathode 12 Anode 13 Electrolytic tank 13a Cathode side tank 13b Anode side tank 14 Proton permeable membrane 15 External circuit 16 Liquid phase fuel 17 Electrolyte 18 External conductor 20 RF magnetron sputtering apparatus 21 Vacuum chamber 22 Target 23 Substrate 24 Magnetron unit 25 Permanent magnet 26 Substrate heater 27 Inert gas inlet 28 Inert gas exhaust port 29 High frequency power supply 30 High frequency matching device 31 Nitrogen-containing inert gas 32 Nitrogen-substituted titanium oxide thin films 41a and 41b Light 42 Positive Holes 43a, 43b Electron 44 Inert gas introduction means 45 Inert gas 46 Stirring means A, B, C, D, E, F Arrow

Claims (8)

An anode including a cathode and a photocatalyst, wherein the cathode and the anode are electrically connected via an external circuit, and electrons generated by photocatalytically oxidizing liquid phase fuel by photoirradiation of the photocatalyst A photovoltaic fuel cell that generates electricity by being taken out into a circuit,
Furthermore, an electrolytic cell and a proton permeable membrane are provided,
The cathode and the anode are disposed within the electrolytic cell;
The inside of the electrolytic cell has a two-cell structure divided into the cathode side and the anode side by the proton permeable membrane,
The tank on the anode side is filled with the liquid phase fuel,
The cathode side tank is filled with an electrolyte containing redox mediator and oxygen,
Protons generated by photocatalytic oxidation of the liquid phase fuel pass through the proton permeable membrane and move from the anode side tank to the cathode side tank,
Electrons generated by photocatalytic oxidation of the liquid phase fuel move from the anode to the cathode through the external circuit,
In the tank on the cathode side, in the presence of the proton, an oxidation-reduction reaction occurs in which the oxygen is reduced by electrons transferred from the anode and converted to water, and the oxidation-reduction reaction is a reaction via the redox mediator. A photovoltaic fuel cell characterized by the above. The photovoltaic fuel cell according to claim 1, wherein the redox mediator is an I ₃ ⁻ / I ⁻ redox mediator. The photovoltaic fuel cell according to claim 1, wherein the electrolytic solution is acidic. The photofuel cell according to any one of claims 1 to 3, wherein the photocatalyst is a visible light responsive photocatalyst. The photofuel cell according to any one of claims 1 to 4, wherein the photocatalyst is titanium dioxide. The photovoltaic fuel cell according to claim 1, wherein the cathode contains carbon. The photovoltaic fuel cell according to any one of claims 1 to 6, wherein dissolved oxygen in the liquid phase fuel is removed. The photovoltaic fuel cell according to any one of claims 1 to 7, wherein a wall surface of the anode-side tank is light transmissive.

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