JP2016089249A - Utilization method of light energy and utilization device of light energy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a utilization method of light energy, which produces and collects high-purity oxygen at necessary place and time.SOLUTION: The utilization method of light energy comprises: an accumulation step of accumulating in an anode chamber 18 oxidation products such as persulfate ions generated from materials to be oxidized such as sulfate ions by a redox reaction, where the standard redox potential is more positive than +1.23 V (RHE), by applying light on an n-type semiconductor 26 on an anode electrode 22 installed in the anode chamber 18 of an electrolytic tank 12; and a reaction step of transferring the oxidized product accumulated in the anode chamber 18 into a vessel 14 different from the anode chamber 18, thereafter generating oxygen gas by decomposing the oxidized product, and collecting the oxygen gas generated. In a cathode chamber 20, hydrogen gas is generated from water.SELECTED DRAWING: Figure 1

Description

  The present invention uses light energy for irradiating a semiconductor photoelectrode with light to accumulate oxidation products, producing oxygen from the oxidation products, and decomposing organic pollutants using the oxidation products. The present invention relates to a method and an apparatus for utilizing light energy.

In recent years, the decomposition of water into hydrogen and oxygen using a photoelectrode comprising an n-type semiconductor (hereinafter sometimes referred to as “semiconductor photoelectrode”) has been extensively studied for the conversion and storage of solar energy ( (See Patent Document 1, Patent Document 2, and Non-Patent Document 1). Among them, photoelectrodes comprising n-type semiconductors such as oxides such as Fe ₂ O ₃ , WO ₃ and BiVO ₄ , oxynitrides such as TaON, nitrides such as Ta ₃ N ₅ , and sulfides are inexpensive. It is excellent in practical point that it is easy to increase the area. Hydrogen is produced and collected in a concentrated manner on a cathode electrode such as Pt. In the case where oxygen generated on a large-area semiconductor photoelectrode is released into the air as it is, a gas leakage prevention cover is unnecessary in the electrolytic cell. However, there are various problems in practical use of solar energy conversion of these semiconductor photoelectrodes.

When a solar energy conversion device is put into practical use, it is necessary to consider economic efficiency. The production cost of hydrogen produced at the cathode electrode will need to be 30 yen / Nm ³ or less in the future. In order to satisfy this condition, there is a limit to what it is necessary to improve the current solar energy conversion efficiency and reduce the manufacturing cost of the semiconductor photoelectrode. Considering economics, it is desirable to change the concept and the system that produces and sells only hydrogen.

  Many current water splitting systems focus on the recovery of hydrogen produced at the cathode electrode, so there is a low awareness of the recovery and use of oxygen produced on the semiconductor photoelectrode, and oxygen may be released into the atmosphere. Many. However, since oxygen is also a highly versatile material, if oxygen can be produced and collected as needed, it can be an industrially high value-added system from the viewpoint of producing and selling both hydrogen and oxygen products. .

Not only the generation of oxygen but also various oxidation reactions can proceed on the semiconductor photoelectrode. When a reduced form of a redox medium (redox medium) that dissolves in water coexists in an electrolytic cell, the redox medium is oxidized by holes generated on the semiconductor photoelectrode during light irradiation, and an oxidized form can be generated. It has also been reported that peroxides such as persulfuric acid can be generated on a photoelectrode provided with WO ₃ (see Non-Patent Document 2), and oxygen decomposes easily in the presence of a specific catalyst (Ag ⁺ , Pt, etc.). Generate.

Special table 2003-504799 gazette JP 2005-44758 A

Rie Saito, Yugo Miseki, Kazuhiro Sayama, "Highly efficient photoelectrochemical water splitting using a thin film photoanode of BiVO4 / SnO2 / WO3 multi-composite in a carbonate electrolyte", Chemical Communications, 48 (2012), 3833-3835 Qixi Mi, Almagul Zhanaidarova, Bruce S. Brunschwig, Harry B. Gray, Nathan S. Lewis, "A Quantitative assessment of the competition between water and anion oxidation at WO3 photoanodes in acidic aqueous electrolytes", Energy Environ. Sci., 2012, 5, 5694-5700

  From the background as described above, the present invention provides a technique for efficiently collecting not only hydrogen but also useful oxidation products in consideration of economic efficiency in a solar energy conversion system using a semiconductor photoelectrode. It is an issue. If useful oxidation products can be collected efficiently, not only can this be produced and sold, but high-purity oxygen can be produced and recovered as required. High-purity oxygen is a very important material industrially.

  In hydrogen production by water splitting using a semiconductor photoelectrode, the inventors transferred an aqueous solution produced and accumulated an oxidation product out of the semiconductor photoelectrode chamber (anode chamber), and then oxidized in the presence of a catalyst. By decomposing the product, the inventors have intensively studied a technique for producing and collecting high-purity oxygen, which was difficult with the prior art, as necessary, and completed the present invention.

  In the method of using light energy according to the present invention, the standard oxidation-reduction potential is more positive than +1.23 V (RHE) by irradiating light to the n-type semiconductor on the surface of the anode electrode provided in the anode chamber of the electrolytic cell. An accumulation process for accumulating oxidation products generated from the oxide by oxidation-reduction reaction in the anode chamber, and a chemical reaction involving reduction of the oxidation products after transferring the oxidation products accumulated in the anode chamber to the outside of the electrolytic cell And a reaction step of performing. The standard redox potential is +1.23 V (RHE), which is a potential of +1.23 V (NHE, pH = 0) at pH = 0, and in the case of a reaction involving redox water, according to the Nernst equation, the pH = Since it shifts by +0.059 V / pH at 14, this corresponds to +0.404 V (NHE, pH = 14).

In the light energy utilization method of the present invention, the reaction step preferably includes a process of transferring the oxidation product accumulated in the anode chamber to a container different from the electrolytic cell. In the light energy utilization method of the present invention, the reaction step may include a process of decomposing the oxidation product to generate oxygen gas and collecting the generated oxygen gas. In the light energy utilization method of the present invention, the oxide is sulfate ion and the oxidation product is persulfate ion, the oxide is Ce ³⁺ and the oxidation product is Ce ⁴⁺ , and the oxide is oxidized with IO ₃ ⁻ The product may be IO ₄ ⁻ , the oxide may be Cl ⁻ and the oxidation product may be ClO ⁻ , or the oxide may be Br ⁻ and the oxidation product may be BrO ⁻ .

  In the method of using light energy of the present invention, the n-type semiconductor preferably contains one or more elements selected from Ti, W, V, Bi, Fe, Nb, lanthanoid, and Ta. In the light energy utilization method of the present invention, in parallel with the oxidation-reduction reaction in the anode chamber, the reduction product may be generated from the substance to be reduced or the hydrogen gas may be generated from water in the cathode chamber of the electrolytic cell. .

  The light energy utilization apparatus of the present invention is used in a light energy utilization method, and has an electrolytic cell and a container different from the electrolytic cell. The electrolytic cell is divided into an anode chamber and a cathode chamber separated by a diaphragm, and an anode chamber. And an anode electrode having an n-type semiconductor on the surface, and a cathode electrode provided in the cathode chamber and electrically connected to the anode electrode. The light energy utilization apparatus of the present invention may further include a transfer member that transfers the oxidation product accumulated in the anode chamber to the container.

  According to the present invention, after transferring an oxidation product generated by electrolysis of water using a semiconductor photoelectrode to the outside of the semiconductor photoelectrode chamber, the oxidation product is decomposed, so that the purity can be obtained at a place and time as required. Can produce and collect high oxygen.

It is the figure which showed typically manufacture and accumulation | storage of the oxidation product using a semiconductor photoelectrode, and manufacture of oxygen.

  DESCRIPTION OF EMBODIMENTS Hereinafter, a light energy utilization method and a light energy utilization apparatus according to the present invention will be described in detail based on embodiments and examples with reference to the drawings. Note that repeated explanation is omitted as appropriate.

  When putting solar energy conversion devices into practical use, it is desirable to change the system and concept of producing and selling only hydrogen, considering the economy. The present inventors examined a new concept called “photoelectrochemical complex”. It is a concept that not only produces and collects hydrogen by decomposing water, but also simultaneously performs high value-added reactions such as producing useful chemicals by fusing multiple functions. If expensive useful chemicals other than hydrogen can be manufactured and sold, the overall economics of the hydrogen production system will improve, leading to early commercialization.

  A photoelectrochemical complex is a form in which various chemical processes are arranged around a photoelectrochemical apparatus to form a complex. It is possible to carry out a reaction with high added value not only in the anode reaction but also in the cathode reaction. High value-added reactions include waste disposal, toxic substance decomposition (bleaching, washing, sterilization, etc.) and organic synthesis reactions. In the present invention, in particular, a technique for producing and collecting high-purity oxygen as necessary by decomposing these in the presence of a catalyst from an aqueous solution produced and accumulated oxidation products is studied. It came to be completed.

  FIG. 1 schematically shows a light energy utilization apparatus 10 according to an embodiment of the present invention. The light energy utilization apparatus 10 includes an electrolytic cell 12 and a container 14 different from the electrolytic cell 12. The light energy utilization device 10 is used in a light energy utilization method described later. The electrolytic cell 12 is an apparatus for electrolysis, and includes an anode chamber 18 and a cathode chamber 20 separated by a diaphragm 16. The diaphragm 16 allows ions in the anode chamber 18 to pass into the cathode chamber 20 and ions in the cathode chamber 20 to pass into the anode chamber 18.

An anode electrode 22 is provided in the anode chamber 18. The anode electrode 22 includes a conductive substrate 24 and an n-type semiconductor 26 formed on the surface of the conductive substrate 24. As the n-type semiconductor 26, a substance having a valence band level on the positive side of +1.23 V (RHE) and stable to electrolysis in the anode chamber 18 can be used. The n-type semiconductor 26 preferably contains one or more elements selected from Ti, W, V, Bi, Fe, Nb, lanthanoid, and Ta. Specifically, oxides such as TiO ₂ , WO ₃ , BiVO ₄ , Fe ₂ O ₃ , Ta ₂ O ₅ , Bi ₂ WO ₆ , oxynitrides such as TaON, nitrides such as Ta ₃ N ₅ , An oxyhalide such as sulfide, oxysulfide, BiOCl, BiOBr, BiOI, or a material doped with these compounds can be used as the n-type semiconductor 26.

  An electrolytic solution is injected into the anode chamber 18 so that the anode electrode 22 is immersed therein. The oxide is dissolved in the electrolytic solution. A cathode electrode 28 is provided in the cathode chamber 20. The anode electrode 22 and the cathode electrode 28 are electrically connected via a DC power source. An electrolytic solution is injected into the cathode chamber 20 so that the cathode electrode 28 is immersed therein. Further, the light energy utilization apparatus 10 includes a transfer member such as a pump (not shown) that transfers the oxidation product accumulated in the anode chamber 18 to the container 14.

A general operating principle for electrolyzing water using a semiconductor photoelectrode will be described. When the semiconductor photoelectrode is irradiated with light, electrons (e ⁻ ) are generated in the conduction band and holes are generated in the valence band. The holes that have moved to the surface of the semiconductor photoelectrode oxidize water to produce oxygen. On the other hand, the generated electrons move to the conductive substrate of the semiconductor photoelectrode, and then move to the counter electrode through the external short-circuit line. At this time, since the conduction band of the n-type semiconductor is on the positive side with respect to the generation potential of hydrogen, a bias potential is applied between the semiconductor photoelectrode and the counter electrode to increase the energy of electrons. By these electrons, water is reduced on the counter electrode to generate hydrogen.

  Next, a method for using light energy according to an embodiment of the present invention will be described. The method of using light energy includes an accumulation process and a reaction process. In the accumulation step, the n-type semiconductor 26 on the surface of the anode electrode 22 provided in the anode chamber 18 of the electrolytic cell 12 is irradiated with light, and the standard oxidation-reduction potential is an oxidation whose positive side is +1.23 V (RHE). Oxidation products generated from the oxide are accumulated in the anode chamber 18 by the reduction reaction. This oxidation product is a water-soluble and high value-added substance. In particular, the oxidation product is preferably a peroxide, more preferably persulfuric acid.

Peroxide is a substance with an OO bond, such as hydrogen peroxide or persulfuric acid. Persulfuric acid is one of sulfur oxo acids having an OO bond, and persulfate ion is represented by SO ₅ ^2- or S ₂ O ₈ ^2- . Examples of combinations of oxides and oxidation products that can be used in the light energy utilization method of the present embodiment include H ₂ O and H ₂ O ₂ , SO ₄ ^2- and S ₂ O ₈ ^2- , SO ₄ ^2- and SO ₅ ^{2- and the} like. Also, there is no OO bond, but Ce ³⁺ and Ce ⁴⁺ etc., and for ions containing halogen, IO ₃ ⁻ and IO ₄ ⁻ , Cl ⁻ and ClO ⁻ , Cl ⁻ and ClO ₂ ⁻ , Cl ⁻ and ClO ₃ ⁻ , Br ^- and BrO ^-, Br ^- and BrO ₃ ^-, I ^- and IO ₄ ^-, I ^- given as a combination, such as may be oxides and oxidation products ^- and IO. The oxide and the oxidation product are preferably water-soluble.

In the reaction step, the electrolytic solution containing the oxidation product accumulated in the anode chamber 18 is transferred to the outside of the anode chamber 18 and then a chemical reaction involving reduction of the oxidation product is performed. In the reaction step, it is preferable to transfer the electrolytic solution containing the oxidation product accumulated in the anode chamber 18 to a container 14 different from the electrolytic cell 12. This is because a chemical reaction can be performed at a place and time as required. The electrolytic solution can be transferred to the container 14 by either a flow type or a batch type. In the reaction step, the oxidized product in the container 14 is decomposed to generate oxygen gas, and the generated oxygen gas can be collected. Metal catalysts such as Ag ⁺ and Pt, heating, or light irradiation can be used to decompose the oxidation product that generates oxygen gas. In addition to the generation of oxygen gas, chemical reactions involving the reduction of oxidation products include decomposition of organic pollutants using the strong oxidizing power of oxidation products, wastewater treatment, bleaching, disinfection, disinfection, washing, selective organic Examples include synthesis.

On the other hand, in the cathode chamber 20 of the electrolytic cell 12, in parallel with the oxidation-reduction reaction in the anode chamber 18, reduction is performed from the reduction target by an oxidation-reduction reaction in which the standard oxidation-reduction potential is more negative than +1.23 V (RHE). Hydrogen gas is produced from the product or water. ^Examples of combinations of reductants and reduction products that can be used in the light energy utilization method of the present embodiment include Fe ³⁺ and Fe ²⁺ , IO ₃ ⁻ and I ⁻ , and I ₃ ⁻ and I ⁻ . Also, organic redox such as methyl viologen can be used. If an electrolytic cell in which a plurality of anode electrodes are separated by partition walls is used, a plurality of different oxidation reactions can be performed simultaneously.

A cocatalyst that allows the reaction to proceed efficiently may be supported on the semiconductor photoelectrode. The co-catalyst includes one or more substances selected from noble metals such as Pt and Pd, noble metal oxides such as RuO ₂ and IrO ₂ , and oxides such as titanium oxide, bismuth oxide, and tin oxide, or a composite thereof. Chemical substances. Supporting a cocatalyst on a semiconductor photoelectrode is particularly preferred for reactions involving halogen ions.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited at all by these Examples.

Example 1
First, 20 mL of a 0.5 mol / L Na ₂ WO ₄ aqueous solution was passed through a cation exchange resin in a burette, and while adding ion-exchanged water until this dripping solution became neutral, polyethylene glycol (average molecular weight 300) was 12.5 mL. Then, the solution was proton-exchanged into an aqueous solution of H ₂ WO ₄ by dropping it into a mixed solution of 20 mL of ethanol. Next, about 100 mL of this H ₂ WO ₄ aqueous solution was concentrated to about 13 mL at 75 ° C. to obtain a WO ₃ precursor aqueous solution. Then, the precursor aqueous solution is spin-coated on the surface of the F-SnO ₂ (FTO) film, which is a conductive substrate, and then air-fired at 550 ° C. This is repeated 12 times, and the n-type semiconductor is WO ₃ An anode electrode was produced. Next, this anode electrode is installed in the anode chamber of a two-chamber electrolytic cell using a cation exchange membrane as a diaphragm, and a cathode electrode made of Pt is installed in the cathode chamber, and these electrodes are electrically connected via a DC power source. Connected.

Then, 29 mL of a 1 mol / L H ₂ SO ₄ aqueous solution was put in each of the anode chamber and the cathode chamber, and an electric quantity of 7.2 C was passed at a constant current of 1 mA under irradiation with simulated sunlight. A color experiment confirmed that 38 μmol of persulfate ion (Faraday efficiency 100%) was generated in the electrolyte in the anode chamber. Next, the electrolytic solution containing 13 μmol of persulfate ions accumulated in the anode chamber was transferred to another container having high airtightness. Then, 128 μmol of Ag ⁺ catalyst was added to this electrolytic solution and stirred at 60 ° C. for 2 hours, and it was confirmed by gas chromatography that 6.5 μmol (yield 100%) of oxygen was generated. In this way, oxygen could be produced at a location away from the electrolytic cell. This oxygen can be collected and sold.

(Comparative Example 1)
Although it differs from Example 1 that 1 mol / L HClO ₄ aqueous solution was put into the electrolytic cell and an electric quantity of 7.2 C was flowed under simulated sunlight irradiation, it was the same as Example 1 except these. Electrolysis was performed. As a result, no peroxide was generated in the anode chamber. It was confirmed by the change over time using an oxygen sensor that 17.5 μmol (yield 94%) of oxygen was generated in the anode chamber. Although it has been reported that ClO ₄ ⁻ is oxidized to ClO ₄ radicals on the semiconductor photoelectrode, the ClO ₄ radicals are unstable and thus immediately decompose to oxygen (see Non-Patent Document 2). In this case, oxygen is directly generated on the semiconductor photoelectrode, so that production and collection of oxygen cannot be performed outside the electrolytic cell.

(Example 2)
First, the same anode electrode, cathode electrode, and bipolar electrical connection members as in Example 1 were installed in a two-chamber electrolytic cell using an anion exchange membrane as a diaphragm. Next, an aqueous solution in which both Ce (ClO ₄ ) ₃ and HClO ₄ were dissolved at 1 mol / L was placed in the anode chamber, and 29 mL of a 1 mol / L HClO ₄ aqueous solution was placed in the cathode chamber. And the electric quantity of 2.9C was sent with the constant current of 0.2mA under pseudo-sunlight irradiation. As a result, it was confirmed by a color experiment that 13 μmol of Ce ⁴⁺ (Faraday efficiency 43%) was generated in the anode chamber.

(Example 3)
First, the same anode electrode, cathode electrode, and bipolar electrical connection members as in Example 1 were installed in a two-chamber electrolytic cell using a cation exchange membrane as a diaphragm. Next, 29 mL of 0.2 mol / L NaIO ₃ aqueous solution was put into both the anode chamber and the cathode chamber, and an electric quantity of 7.2 C was applied at a constant current of 1 mA under simulated sunlight irradiation. A color experiment confirmed that 19 μmol of IO ₄ ⁻ (Faraday efficiency 50%) was produced in the electrolyte in the anode chamber.

Example 4
First, the same anode electrode, cathode electrode, and bipolar electrical connection members as in Example 1 were installed in a two-chamber electrolytic cell using a cation exchange membrane as a diaphragm. Next, 35 mL of 5.0 mol / L NaCl aqueous solution is placed in both the anode chamber and the cathode chamber, and an electric charge of 2.0 C is applied at a constant current of 1.0 mA under irradiation with ultraviolet and visible light using a xenon lamp. Washed away. A color experiment confirmed that 5.2 μmol of ClO ⁻ (Faraday efficiency 50%) was formed in the electrolyte in the anode chamber.

(Example 5)
First, the same anode electrode, cathode electrode, and bipolar electrical connection members as in Example 1 were installed in a two-chamber electrolytic cell using a cation exchange membrane as a diaphragm. Next, 35 mL each of 5.0 mol / L NaBr aqueous solution is placed in both the anode chamber and the cathode chamber, and an electric quantity of 2.0 C is applied at a constant current of 1.0 mA under irradiation with ultraviolet and visible light using a xenon lamp. Washed away. The anode chamber of the electrolytic solution of 7.8μmol BrO ^- (Faraday efficiency 75%) had to be generated and confirmed by coloration experiments.

  The present invention can be applied to a technique for producing and collecting high-purity oxygen as necessary. It can also be applied to the decomposition of organic pollutants using oxidation products.

DESCRIPTION OF SYMBOLS 10 Light energy utilization apparatus 12 Electrolyzer 14 Container 16 Diaphragm 18 Anode chamber 20 Cathode chamber 22 Anode electrode 24 Conductive substrate 26 N-type semiconductor 28 Cathode electrode

Claims (8)

The n-type semiconductor on the surface of the anode electrode provided in the anode chamber of the electrolytic cell is irradiated with light, and is generated from the oxide by an oxidation-reduction reaction in which the standard oxidation-reduction potential is positive from +1.23 V (RHE). An accumulation step of accumulating oxidation products in the anode chamber;
A reaction step of carrying out a chemical reaction involving reduction of the oxidation product after transferring the oxidation product accumulated in the anode chamber to the outside of the electrolytic cell;
A method of using light energy.   The method of using light energy according to claim 1, wherein the reaction step includes a step of transferring an oxidation product accumulated in the anode chamber to a container different from the electrolytic cell.   The method of using light energy according to claim 1, wherein the reaction step includes a process of decomposing an oxidation product to generate oxygen gas and collecting the generated oxygen gas. The oxide is sulfate ion and the oxidation product is persulfate ion, the oxide is Ce ³⁺ and the oxidation product is Ce ⁴⁺ , the oxide is IO ₃ ⁻ and the oxidation product is IO ₄ ^-, wherein the oxide is Cl ^- said oxidation product is ClO ^- or the oxidizable compound is Br, ^- light energy according to any one of claims 1 to 3 is ^- said oxidation product is BrO How to use   The method of using light energy according to any one of claims 1 to 4, wherein the n-type semiconductor includes one or more elements selected from Ti, W, V, Bi, Fe, Nb, lanthanoid, and Ta.   6. In parallel with the oxidation-reduction reaction in the anode chamber, the reduction product is generated from the substance to be reduced or the hydrogen gas is generated from water in the cathode chamber of the electrolytic cell. How to use light energy. A light energy utilization apparatus for use in the light energy utilization method according to any one of claims 2 to 6,
Having an electrolytic cell and a container different from the electrolytic cell,
The electrolytic cell has an anode chamber and a cathode chamber separated by a diaphragm, an anode electrode provided in the anode chamber and having an n-type semiconductor on a surface thereof, and provided in the cathode chamber and electrically connected to the anode electrode. A light energy utilization device comprising a cathode electrode.   The light energy utilization apparatus according to claim 7, further comprising a transfer member that transfers the oxidation product accumulated in the anode chamber to the container.

Images