JP2013258129A - Solid fuel cell electrolyte and solid fuel battery cell using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel cell electrolyte that allows fuel cells to be activated under low-humidification conditions and exhibits high proton conductivity and high gas barrier properties.SOLUTION: A solid fuel cell electrolyte 1 contains graphene oxide, where the graphene oxide content is no less than 20 mass%, is preferably no less than 50 mass%, and more preferably no less than 70 mass%, has no upper limit and may be 100 mass%. The solid fuel cell electrolyte 1 further contains a sulfo group, where the sulfo group content is no more than 0.5 mmol/g and is preferably no more than 0.3 mmol/g.

Description

  The present invention relates to an electrolyte for a solid fuel cell and a cell for a solid fuel cell using the same.

  In a fuel cell, hydrogen gas supplied to the fuel electrode is divided into hydrogen ions and electrons at the catalyst electrode, hydrogen ions (protons) move through the electrolyte, electrons move to the air electrode through an external circuit, and oxygen and Water reacts to produce water, and electrons that move to the external circuit at this time are taken out as energy. Various electrolytes are being studied to improve the output and stability of fuel cells.

  For example, in order to obtain high proton conductivity and stability, a technique using a fluorine-based polymer having a sulfonic acid group as an electrolyte is known. In this case, since water molecules are necessary for proton conduction, Control of the amount is necessary, and further, a humidifying device is required for the fuel cell device, and there is a problem that the device becomes large.

  Moreover, in patent document 1, the graphene oxide (GO) which introduce | transduced the sulfo group and the sulfo group containing fluororesin are used as electrolyte for manufacturability and contact reduction. However, in this case as well, water molecules are necessary for proton conduction, which causes the same problem.

  On the other hand, in Patent Documents 2 and 3, carbon nanotubes (CNT), carbon nanofibers, and fullerenes are used as electrolytes in order to drive without humidification. However, materials such as CNT have a problem in gas barrier properties due to their molecular structure.

  In Non-Patent Document 1, a mixture of graphene oxide and polyethylene oxide is used as an electrolyte in order to improve high proton conductivity and mechanical properties. However, in this case, if the driving of the fuel cell is continued, there is a problem that the polyethylene oxide is dissolved by water generated from the fuel cell reaction, and the mechanical strength of the electrolyte is lowered.

  Further, in Non-Patent Document 2, in order to obtain high proton conductivity, graphene oxide and a sulfonic acid group-containing fluorine-based polymer are used and driven under humidified conditions. However, also in this case, water molecules are required for proton conduction, which causes the same problem as described above.

JP 2011-98843 A JP 2003-86022 A JP 2008-166147 A

A poly (ethylene oxide) / graphene oxide electrolyte membrane for low temperature polymer fuel cells (2011). Journal of Power Sources196 (2011) 8377-8382 A Graphite Oxide Paper polymer Electrolyte for Direct Methanol Fuel Cells (2011) SAGE-Hindawi Access to Research International Journal of Electorochemistry Vol.2011

  It is an object of the present invention to provide a fuel cell electrolyte that can drive a fuel cell with low humidification and that exhibits high proton conductivity and high gas barrier properties.

  The present inventor has intensively studied to solve the above problems. As a result, the inventors have succeeded in solving the above problems by using an electrolyte containing a specific amount of graphene oxide and having a sulfo group amount of a specific amount or less.

That is, the present invention provides the following.
[1] An electrolyte for a solid fuel cell, wherein the electrolyte contains 20% by mass or more of graphene oxide and has a sulfo group amount of 0.5 mmol / g or less.
[2] The graphene oxide has a ratio a / b of the total number of carbon atoms (a) to which the oxygen functional groups of carboxyl group, carbonyl group, hydroxyl group, and epoxy group are bonded, and the total number of carbon atoms (b) is 15 It is% or more, The electrolyte for solid fuel cells as described in [1] characterized by the above-mentioned.
[3] The electrolyte for a solid fuel cell according to [1] or [2], wherein the graphene oxide has an equivalent circle diameter in a plane direction of 3 μm or less.
[4] The graphene oxide according to any one of [1] to [3], wherein a ratio of carbon atoms to which an epoxy group is bonded is 10% or more in the total number of carbon atoms (b). The electrolyte for a solid fuel cell as described.
[5] The proton conductivity in an atmosphere of a temperature of 20 ° C. and a relative humidity of 20% is 3.0 × 10 ⁻⁸ S / cm or more, and any one of [1] to [4] Electrolyte for solid fuel cells.
[6] It is arranged in a solid fuel cell, and the short circuit current density when the solid fuel cell is operated at a relative humidity of 20% in the solid fuel cell is 4 mA / cm ² or more. The solid fuel cell electrolyte according to [5].
[7] It is disposed in the solid fuel battery cell, and an electromotive force when the solid fuel battery cell is operated at a relative humidity of 20% in the solid fuel battery cell is 0.3 V or more. The solid fuel cell electrolyte according to [5] or [6].
[8] A solid fuel battery cell in which the electrolyte for a solid fuel battery according to any one of [1] to [7] is laminated.
[9] The solid fuel battery cell according to [8], wherein the electrolyte and the electrode are integrated.
[10] The solid fuel battery cell according to [9], which is operated at a relative humidity of 30% or less.

The figure which shows an example of sectional drawing of a fuel cell. The figure which shows the AFM image (photograph) of the graphene oxide shown in the Example, and the film thickness measurement result of the graphene oxide by the tap mode of AFM. The figure which shows the structure of the fuel cell used in the Example. The figure which shows the structure of the gas-barrier property test measured in the Example.

The electrolyte for a solid fuel cell according to the present invention is an electrolyte containing 20% by mass or more of graphene oxide and having a sulfo group amount of 0.5 mmol / g or less.
Details will be described below.

<About graphene oxide>
In the present invention, graphene oxide refers to graphene modified with an oxygen functional group such as a carboxyl group, a carbonyl group, a hydroxyl group, and an epoxy group.
Proton conductivity can be obtained by binding a proton to the above-mentioned oxygen functional group in graphene oxide and acting as a hopping function.

  The graphene oxide used in the present invention does not have a substituent other than the oxygen functional group described above. In particular, it does not contain a sulfo group or the like. This is because the sulfo group is a hydrophilic group, and when used as an electrolyte, it tends to contain water generated by power generation, and the shape stability of the electrolyte may deteriorate, and the protons of the electrolyte will be separated at low humidity. This is because the proton conductivity may be reduced. In addition, since the inside of the fuel cell is brought into a strongly acidic atmosphere due to power generation due to the sulfo group, when the metal is used for the fuel cell assembly member, it is eluted into the water generated by the reaction due to oxidative deterioration of the member and the fuel cell. This is because acid water may be discharged outside.

The graphene oxide used in the present invention is not particularly limited, but the total number of carbon atoms (a) to which the oxygen functional groups of carboxyl group, carbonyl group, hydroxyl group and epoxy group are bonded and the total number of carbon atoms (b) of graphene oxide. The ratio a / b (%) is preferably 15% or more, more preferably 25% or more, and particularly preferably 30% or more. By being in the above range, the oxygen functional group may increase, and the electrolyte hopping function, that is, proton conductivity may be improved. Furthermore, it is preferable that the above-described oxygen functional group is bonded to a specific amount or more, so that π conjugation is cut and the electrical insulating property of graphene oxide may be increased. The upper limit of a / b is not particularly limited, and the larger the amount of oxygen functional groups that can be modified into graphene (graphene oxidation), the more preferable for improving proton conductivity and electrical insulation. In addition, the said ratio a / b (%) should just be satisfy | filled at least before use of electrolyte.
In addition, as a method for adjusting the amount of oxygen functional group, graphene oxide is photoreduced by irradiating light with a xenon lamp in an atmosphere of oxygen or hydrazine, a method of reducing with hydrazine vapor, a method of thermal reduction, etc. Is mentioned.

  The number of carboxyl groups, carbonyl groups, hydroxyl groups, and epoxy groups contained in the graphene oxide used in the present invention is not particularly limited, but the carbon to which the epoxy group is bonded with respect to the total number of carbon atoms of the entire graphene oxide. The ratio of the total number of atoms is preferably 10% or more from the viewpoint of improving proton conductivity of the electrolyte at low humidity. The condition that the ratio of the total number of carbon atoms to which the epoxy group is bonded is 10% or more as long as it is satisfied at least before use of the electrolyte.

  The reason why the proton conductivity of the electrolyte at low humidity is improved is not clear, but is presumed to be due to the following mechanism. In graphene oxide, a large number of oxygen functional groups are present in the thin film (flake) plane direction and edge of the graphene oxide, so that a continuous hydrogen-bonded phase is formed. This continuous hydrogen-bonded phase enables protons to move through the hydrogen bonds while reorganizing continuously so that the hydrogen-bonded molecules can easily pass protons even in a low-humidity environment with few water molecules. (Grotus mechanism), it is estimated that the proton conductivity of the electrolyte at low humidity is improved.

Graphene oxide has a thin film (flakes) structure. The equivalent-circle diameter in the plane direction of each flake of the graphene oxide used in the present invention is preferably 3 μm or less, more preferably 2.5 μm or less, and particularly preferably 2 μm or less. Moreover, it is preferable that it is 0.01 micrometer or more.
It is preferable that the equivalent circle diameter of the graphene oxide is in an appropriate range because the vertical proton conductivity of the stacked graphene oxide in the electrolyte is improved. In addition, by setting the thickness to 0.01 μm or more, high vertical gas barrier properties of graphene oxide can be obtained.
In the present invention, the equivalent-circle diameter in the plane direction of graphene oxide refers to a circle having the same area as the area in the plane direction in all graphene oxide pieces observed in the 10.0 × 10.0 μm visual field range of the AFM image. This means the diameter of this circle.

The method for obtaining graphene oxide used in the present invention is not particularly limited, and for example, a Hummers method, a Brodie method, a Staudenmaier method, or the like can be used. The Hummers method oxidizes using H ₂ SO ₄ , NaNO ₃ and KMnO ₄ (W. S Hummers, Jr. et al., J. Am. Chem. Soc., 1958, 80, 1339.) The Brodie method oxidizes using fuming nitric acid and KClO ₃ (BC Brodie, Philos. Trans. R. Soc., London, 1859 149, 249., BC Brodie, Ann. Chim. Phys., 1860, 59, 466.). The Staudenmaier method oxidizes using H ₂ SO ₄ , HNO ₃ and KClO ₃ (L. Staudenmaier, Ber. Dtsch. Chem. Ges., 1898, 31, 1481.).
For graphite oxide obtained by the Hummers method, etc., the interlayer is expanded by adding oxygen functional groups and the property of having hydrophilicity is used, so that ultrasonic waves are irradiated in water, or centrifugation and redispersion are performed. The graphene oxide can be obtained by allowing water molecules to permeate between the layers by repeating the process to separate the layer structure of the graphite oxide.

  The method of controlling the equivalent circle diameter in the surface direction of the graphene oxide flakes used in the present invention is not particularly limited. For example, the equivalent circle diameter can be reduced by increasing the oxidation time of the graphite oxidation process, or after the oxidation process. The equivalent circle diameter can be reduced by increasing the ultrasonic irradiation intensity of graphite, and the equivalent circle diameter can be reduced by increasing the ultrasonic irradiation time.

<About electrolyte>
The electrolyte of the present invention contains 20% by mass or more of graphene oxide, preferably 50% by mass or more, and more preferably 70% by mass or more. Moreover, there is no upper limit and 100% may be sufficient.
When the graphene oxide is in the above range, the proton conductivity, electrical insulation, and gas barrier properties of the electrolyte are improved.

In the present invention, components that may be contained in the electrolyte other than graphene oxide are not particularly limited as long as the effects of the present invention are not impaired, but metal oxides such as Fe and Ti, Ce (NO ₃ ) _And compounds having an oxo acid such as ₃ and aromatic compounds. By having these, graphene oxide and other components in the electrolyte can be easily stacked, and gas barrier properties and a continuous proton conduction path can be easily formed, which may be preferable. In addition, from the viewpoint of film formability, heat resistance, mechanical properties, and the like, it is also possible to use aromatic compounds such as polyimide and polystyrene used for plastic materials.
The proportion of components that may be contained in the electrolyte other than graphene oxide is not particularly limited as long as it does not impair the curing of the electrolyte of the present invention, but when it contains a metal oxide, 0.5 mass in the electrolyte. % Or more, preferably 1% by mass or more. Further, it is preferable that the electrolyte contains 30% by mass or less, more preferably 25% by mass or less, and particularly preferably 20% by mass or less. By including a metal oxide in an appropriate ratio, the power generation performance of the fuel cell is improved, and film formability tends to be obtained.

  The amount of sulfo group contained in the electrolyte of the present invention is 0.5 mmol / g or less. Preferably, it is 0.3 mmol / g or less, and it is more preferable that no sulfo group is contained. If there are more sulfo groups than the above, the sulfo group, which is a hydrophilic group, tends to contain water generated by power generation, so the shape stability of the electrolyte may deteriorate, and further, the protons of the electrolyte will be separated at low humidity. This is because the proton conductivity may be reduced. In addition, since the inside of the fuel cell is brought into a strongly acidic atmosphere due to power generation due to the sulfo group, when the metal is used for the fuel cell assembly member, it is eluted into the water generated by the reaction due to oxidative deterioration of the member and the fuel cell. This is because acid water may be discharged outside.

In the electrolyte of the present invention, the graphene oxide has excellent gas barrier properties because the thin films (flakes) as described above are stacked and has a dense structure. In the fuel cell electrolyte, this gas barrier property is one of the important functions.
For example, when the same carbon compounds and oxides such as fullerene and carbon nanotube, which have a three-dimensional structure, are used in a membrane electrolyte, voids are generated in the electrolyte, making it difficult to stack. May occur.

The electrolyte of the present invention preferably has a proton conductivity of 3.0 × 10 ⁻⁸ S / cm or more, and 1.0 × 10 ⁻⁷ S / cm or more in an atmosphere at a temperature of 20 ° C. and a relative humidity of 20%. It is more preferable that it is 1.0 × 10 ⁻⁶ S / cm or more. By having proton conductivity higher than the above, an electrolyte and a fuel cell excellent in power generation efficiency can be obtained.

In addition, it is preferable that the short-circuit current density when the fuel cell is operated at a relative humidity of 20% and a temperature of 20 ° C. is 4 mA / cm ² or more. Furthermore, the electromotive force when operated at a relative humidity of 20% in the fuel cell is preferably 0.3 V or more. Moreover, it is preferable that the output density when operated at a relative humidity of 20% and a temperature of 20 ° C. in the fuel cell is 0.4 V and 0.1 mW / cm ² or more. Since the electrolyte of the present invention has these performances, an electrolyte and a fuel cell excellent in power generation efficiency can be obtained.

The method for producing the electrolyte of the present invention is not particularly limited, and it is possible to use a known film formation method such as a filtration film formation method, a spin coating method, a drop casting method, an electrophoretic method, or a bar coating method.
The method of mixing graphene oxide and other components is not particularly limited as long as it does not impair the effects of the present invention, but a method of compounding graphene oxide and other components, or dissolving graphene oxide and other components simultaneously in a solution A method of dispersing, a method of adding other components to the graphene oxide dispersion and performing a redispersion treatment as necessary, a method of mixing the graphene oxide dispersion and the other component dispersion, and the like can be used.
A solution in which graphene oxide and / or other components are dissolved or dispersed is not particularly limited, but water, a polar solvent, alcohol, and the like are preferably used from the viewpoint of ease of dissolution and dispersion of graphene oxide.

<About solid fuel cells>
The solid fuel battery cell of the present invention has at least an electrolyte and an electrode. Moreover, you may have a catalyst, a separator, the extraction line of an electric current, etc. as needed.

An example of the configuration of the solid fuel battery cell of the present invention is shown in FIG.
FIG. 1 is a diagram schematically illustrating a solid fuel battery cell. As shown in the figure, an electrolyte 1, a catalyst 2, an electrode (a fuel electrode (negative electrode) 3, an air electrode (positive electrode) 4), and separators 5 and 5 'are provided. The electrolyte 1 is located at the center of the solid fuel battery cell, and the catalyst layer 2 is provided so as to contact the electrolyte 1. The catalyst layer 2 may be provided on both surfaces of the electrolyte 1 or only on one side. The fuel electrode 3 and the air electrode 4 are provided adjacent to the catalyst layer 2, and separators 5 and 5 ′ are disposed on the outer sides thereof. The separators 5 and 5 'may be formed with a flow path for feeding the fuel gas and the oxidant.
A membrane electrode assembly (hereinafter referred to as MEA) is composed of an electrolyte, an air electrode side catalyst disposed on one surface of the electrolyte, and a fuel electrode side catalyst disposed on the other surface of the electrolyte. A joined body called “sometimes called”.

For example, hydrogen gas is supplied as a fuel gas from the gas flow path of the separator 5 on the fuel electrode 3 side. On the other hand, for example, a gas containing oxygen is supplied as an oxidant gas from the gas flow path of the separator 5 ′ on the air electrode 4 side. An electromotive force can be generated between the fuel electrode and the air electrode by causing an electrode reaction between hydrogen and oxygen gas of the fuel gas in the presence of the catalyst. FIG. 1 shows a solid polymer fuel cell having a solid fuel cell structure, but a plurality of cells can be stacked via separators 5 and 5 'to form a fuel cell.

  Further, in the solid fuel battery cell of the present invention, the electrolyte and the electrode can be integrated by reducing the electrolyte surface and imparting electrical conductivity only to the surface (FIG. 3). By integrating the electrolyte and the electrode, cell thinning, gas barrier properties, cost reduction, and easy manufacture of the fuel cell can be performed.

  As the material of the fuel electrode, known materials can be used as the material of the fuel electrode of the solid fuel cell. For example, a mixture of a metal catalyst and a ceramic powder material made of an oxide ion conductor can be used. . As the metal catalyst used at this time, a material that is stable in a reducing atmosphere, such as nickel, iron, cobalt, or a noble metal (platinum, ruthenium, palladium, etc.) and has hydrogen oxidation activity can be used. In addition, as the oxide ion conductor, one having a fluorite structure or a perovskite structure can be preferably used. Examples of those having a fluorite structure include ceria-based oxides doped with samarium, gadolinium, and the like, and zirconia-based oxides containing scandium and yttrium. In addition, examples of those having a perovskite structure include lanthanum galide oxides doped with strontium and magnesium. Among the above materials, the fuel electrode 4 is preferably formed of a mixture of an oxide ion conductor and nickel. The mixed form of the ceramic material made of the oxide ion conductor and nickel may be a physical mixed form or a form of powder modification to nickel. Moreover, the ceramic material mentioned above can be used individually by 1 type or in mixture of 2 or more types. The fuel electrode 4 can also be configured using a metal catalyst alone.

As the material of the air electrode, known materials can be used as the material of the air electrode of the solid fuel cell. For example, a metal oxide made of Co, Fe, Ni, Cr or Mn having a perovskite structure or the like can be used. Can be used. Specifically, (Sm, Sr) CoO ₃ , (La, Sr) MnO ₃ , (La, Sr) CoO ₃ , (La, Sr) (Fe, Co) O ₃ , (La, Sr) (Fe, Co , Ni) O _{3 and the} like, and (La, Sr) MnO ₃ is preferable. The above-mentioned materials can be used alone or in combination of two or more.

  The air electrode and the fuel electrode are, for example, screen printing method, doctor blade method, spray coating method, spin coating method, dip coating method, electrophoretic electrodeposition method, roll coating method, gravure roll coating method, dispenser coating method, CVD. , EVD, sputtering, transfer, and other general printing methods.

  As the catalyst, known materials for solid fuel cell catalysts can be used. In addition to the platinum group elements of platinum, palladium, ruthenium, iridium, rhodium, and osmium, iron, lead, copper, chromium, cobalt, A metal such as nickel, manganese, vanadium, molybdenum, gallium, or aluminum, or an alloy thereof, an oxide, a double oxide, a carbide, or the like can be used.

  As a separator, a well-known thing can be used as a material of the separator of a solid fuel cell, and it is preferable that it is an electroconductive and impermeable material.

  The solid fuel battery cell of the present invention is preferably operated at a relative humidity of 30% or less, more preferably operated at a relative humidity of 20% or less, further preferably operated at a relative humidity of 10% or less, and a relative humidity of 0. It is particularly preferred to operate at% (no humidification). In this case, it is not necessary to provide a humidifier in the solid fuel battery cell, and the entire fuel cell module can be downsized.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limited to the following aspects.

[Composition analysis of graphene oxide]
The ratio of the substituents of graphene oxide was determined by XPS (Sigma Probe Model Thermo Fisher Scientific).
Monochromatic light AlKα was used as the X-ray source, and the analysis area was 0.4 mm diameter, the extraction angle was 65 °, 10 ⁻⁹ mbar, and the detection part Ag3d was 0.5 eV half-width. In addition, the C1s spectrum obtained from XPS measurement was peak-separated for a single bond of carbon and an oxygen functional group (hydroxyl group, epoxy group, carbonyl group, carboxyl group, and sulfo group. The assignment of the peak was a single bond of carbon 285 eV. The hydroxyl group was 286.6 eV, the epoxy group was 287.1 eV, the carbonyl group was 287.9 eV, and the carboxyl group was 289.1 eV, and for the sulfo group, the assigned peak was 169 eV in the S2p spectrum of sulfur.

[Preparation of graphene oxide solution (A)]
2.0 g of sodium nitrate was mixed with 2.0 g of graphite powder having a purity of 98% (manufactured by Wako). In an ice bath, 92 mL of concentrated sulfuric acid was added, and the mixture was stirred for 30 minutes so that the whole became uniform. To this solution, 12 g of potassium permanganate was added with stirring at a charging rate of 1 g every 5 minutes, and the mixture was stirred for 60 minutes in a 35 ° C. water bath. Thereafter, 92 mL of pure water was added, transferred to a 95 ° C. oil bath, and stirred for 40 minutes. Furthermore, 200 mL of pure water and 2 mL of 30% hydrogen peroxide were added to this solution and left at room temperature for 15 minutes. The color of the solution changed from brown to light yellow. After completion of the oxidation treatment, each was washed three times with 50 mL of 5% hydrochloric acid and 50 mL of pure water, and vacuum-dried at 70 ° C. for 1 week to obtain graphite oxide (a).

  60 mg of the dried graphite oxide (a) was collected and subjected to ultrasonic irradiation in 50 mL of pure water for 2 to 4 hours to peel off the carbon layer of graphite oxide. After ultrasonic irradiation, centrifugation was performed at 10,000 rpm for 30 minutes, and the supernatant was separated to obtain a graphene oxide solution (A).

A graphene oxide solution (A) was dropped onto a mica substrate and dried in a vacuum for 5 hours or longer to obtain an AFM image. An AFM image of the graphene oxide solution (A) is shown in FIG.
The film thickness was 1.27 nm from the measurement by AFM tap mode (Nanoscope 5 manufactured by Digital Instruments). Further, the equivalent circle diameters of all graphene oxide pieces observed in the visual field range of 10.0 × 10.0 μm of the AFM image were obtained. For the equivalent circle diameter, a circle having the same area as the surface direction of one graphene oxide piece was estimated and used as the diameter of this circle. The observed circle equivalent diameters of the graphene oxide pieces were all 3 μm or less.

[Example 1]
The graphene oxide solution (A) described above was poured into a Teflon (registered trademark) container and dried at 50 ° C. for 24 hours. After drying, the film formed on the bottom surface of the container was peeled off from the container to obtain an electrolyte 1. The dimension of the obtained electrolyte 1 was 3 × 3 cm, and the film thickness was 20 μm as observed with a field emission scanning electron microscope (FE-SEM, measurement mode: secondary electron).
Moreover, the ratio of the substituent which the graphene oxide contained in a graphene oxide solution (A) has was calculated | required by XPS.
In the graphene oxide solution (A), peaks attributed to the respective oxygen functional groups were confirmed. Moreover, the peak about a sulfo group was not seen. The ratio (%) of the total number of carbon atoms to which each oxygen functional group is bonded to the total number of carbon atoms of graphite oxide is 8.5% for hydroxyl groups, 19.3% for epoxy groups, 8.1% for carbonyl groups, and 7.8% for carboxyl groups. The ratio a / b (%) of the total number of carbon atoms (a) to which the oxygen functional group is bonded and the total number of carbon atoms (b) of the graphite oxide was 32.0%.

[Example 2]
The electrolyte 1 prepared in a 50 mL quartz beaker was charged, and the electrolyte 1 was photoreduced by irradiating light with a 500 W xenon lamp in an oxygen atmosphere for 10 minutes to obtain an electrolyte 2. The dimension of the obtained electrolyte 2 was 3 × 3 cm.
Electrolyte 2 was evaluated by XPS. The composition of the graphene oxide of electrolyte 2 is 22.1% hydroxyl group, 1.9% epoxy group, 1.9% carbonyl group, 11.7% carboxyl group, and the total number of carbon atoms (a) to which the oxygen functional group is bonded and the electrolyte The ratio a / b (%) of the number of carbon atoms (b) of 2 was 28.1%.

[Example 3]
To 60 mg of the above graphene oxide solution (A), add 5 mL of Ce (NO ₃ ) ₃ aqueous solution adjusted to 1 mM (Ce (NO ₃ ) ₃ is equivalent to 1.6 mg) and apply ultrasonic irradiation for 30 minutes. Implemented to obtain a dispersion. This dispersion was poured into a Teflon (registered trademark) container and dried at 50 ° C. for 24 hours. After drying, the film formed on the bottom surface of the container was peeled off from the container to obtain an electrolyte 3. The dimension of the electrolyte 3 was 3 × 3 cm. The film thickness was 23 μm as observed by FE-SEM.
The atomic% of each oxygen functional group of the graphene oxide used for the electrolyte 3 is the same as that of the electrolyte 1.

[Example 4]
To the above graphene oxide solution (A), Fe ₂ O ₃ powder corresponding to 10 wt% of graphene oxide as a solid content was added, and ultrasonic irradiation was performed for 30 minutes to obtain a dispersion. This dispersion was poured into a Teflon (registered trademark) container and dried at 50 ° C. for 24 hours. After drying, the film formed on the bottom surface of the container was peeled from the container to obtain an electrolyte 4. The dimension of the electrolyte 4 was 3 × 3 cm. The film thickness was 26 μm as observed by FE-SEM.
The atomic% of each oxygen functional group of the graphene oxide used for the electrolyte 4 is the same as that of the electrolyte 1.

[Example 5]
To the graphene oxide solution (A) described above, TiO ₂ powder corresponding to 10 wt% of graphene oxide as a solid content was added, and ultrasonic irradiation was performed for 30 minutes to obtain a dispersion. This dispersion was poured into a Teflon (registered trademark) container and dried at 50 ° C. for 24 hours. After drying, the film formed on the bottom surface of the container was peeled off from the container to obtain an electrolyte 5. The dimension of the electrolyte 5 was 3 × 3 cm. The film thickness was 23 μm as observed by FE-SEM. The atomic% of each oxygen functional group of the graphene oxide used for the electrolyte 5 is the same as that of the electrolyte 1.

[Example 6]
The graphene oxide solution (A) described above was poured into a Teflon (registered trademark) container and dried at 70 ° C. for 24 hours. After drying, the film formed on the bottom surface of the container was peeled off from the container to obtain an electrolyte 6. The dimension of the obtained electrolyte 6 was 3 × 3 cm. The film thickness was 20 μm as observed by FE-SEM. The atomic% of each oxygen functional group of the graphene oxide used for the electrolyte 6 is the same as that of the electrolyte 1.

[Comparative Example 1]
To the above graphene oxide solution (A), sodium polyacrylate (PAANa) powder equivalent to 90 wt% of graphene oxide in solid content was added and stirred sufficiently with a magnetic stirrer to dissolve PAANa. After confirmation, ultrasonic irradiation was performed for 30 minutes to obtain a dispersion. This dispersion was poured into a Teflon (registered trademark) container and dried at 50 ° C. for 24 hours. After drying, the film formed on the bottom surface of the container was peeled off from the container to obtain an electrolyte 7. The dimension of the electrolyte 7 was 3 × 3 cm. The film thickness was 25 μm as observed by FE-SEM.
The atomic% of each oxygen functional group of the graphene oxide used for the electrolyte 7 is the same as that of the electrolyte 1.

[Preparation of graphene oxide solution (B)]
A graphene oxide having an equivalent circle diameter different from that of the graphene oxide solution (A) described above was produced. The manufacturing method is as follows.
2.0 g of sodium nitrate was mixed with 2.0 g of graphite powder having a purity of 98% (manufactured by Wako). In an ice bath, 92 mL of concentrated sulfuric acid was added, and the mixture was stirred for 30 minutes so that the whole became uniform. To this solution, 12 g of potassium permanganate was added with stirring at a charging rate of 1 g every 5 minutes, and the mixture was stirred for 60 minutes in a 35 ° C. water bath. Thereafter, 92 mL of pure water was added, transferred to a 95 ° C. oil bath, and stirred for 40 minutes. Furthermore, 200 mL of pure water and 2 mL of 30% hydrogen peroxide were added to this solution and left at room temperature for 60 minutes. The color of the solution changed from brown to light yellow. After completion of the oxidation treatment, each was washed 3 times with 50 mL of 5% hydrochloric acid and 50 mL of pure water, and vacuum-dried at 70 ° C. for 1 week to obtain graphite oxide (b).

  After drying the graphite oxide (b) produced by the above method, 60 mg was taken and subjected to ultrasonic irradiation in 50 mL of pure water for 2-4 hours to peel off the carbon layer of graphite oxide. After ultrasonic irradiation, after ultrasonic irradiation, centrifugation was performed at 10,000 rpm for 30 minutes, and the supernatant was separated to obtain a graphene oxide solution (B). The circle equivalent diameters of the graphene oxide pieces observed on the mica substrate in the AFM tap mode were all 1.5 μm or less.

[Example 7]
The graphene oxide solution (B) described above was poured into a Teflon (registered trademark) container and dried at 50 ° C. for 24 hours. After drying, the film formed on the bottom surface of the container was peeled from the container to obtain an electrolyte 8. The dimension of the obtained electrolyte 8 was 3 × 3 cm, and the film thickness was 20 μm as observed by FE-SEM. When the ratio of the substituents of the graphene oxide contained in the graphene oxide solution (B) was determined by XPS, the ratio of the total number of carbon atoms to which each oxygen functional group was bonded to the total number of carbon atoms of the entire graphite oxide (%) Is 3.6% of hydroxyl groups, 33.9% of epoxy groups, 4.2% of carbonyl groups, 8.9% of carboxyl groups, the total number of carbon atoms to which oxygen functional groups are bonded (a) and the total number of carbon atoms in graphite oxide The ratio a / b (%) of (b) was 29.0%. There was no peak for the sulfo group.

[Preparation of graphene oxide solution (C)]
A graphene oxide having an equivalent circle diameter different from that of the graphene oxide solution (A) described above was produced. The manufacturing method is as follows.
Concentrated sulfuric acid (400 mL) was added to 98 g of graphite powder (manufactured by Wako Co., Ltd.) (3.0 g), and the whole was stirred for 30 minutes so as to be uniform. To this solution, 30 g of potassium permanganate was added with stirring at a charging rate of 1 g every 5 minutes, and stirred at 30 ° C. for one week. Thereafter, 400 mL of pure water was added, 2 mL of 30% hydrogen peroxide was added, and the mixture was left at room temperature for 60 minutes. The color of the solution changed from brown to light yellow. After completion of the oxidation treatment, each was washed 3 times with 50 mL of 5% hydrochloric acid and 50 mL of pure water, and vacuum dried at 70 ° C. for 1 week to obtain graphite oxide (c).

  After drying the graphite oxide (c) produced by the above method, 60 mg was taken and subjected to ultrasonic irradiation in 50 mL of pure water for 20 hours to peel off the carbon layer of graphite oxide. After ultrasonic irradiation, centrifugation was performed at 10,000 rpm for 30 minutes, and the supernatant was separated to obtain a graphene oxide solution (C). The circle-equivalent diameters of the graphene oxide pieces observed on the mica substrate in the AFM tap mode were all 0.5 μm or less.

[Example 8]
The graphene oxide solution (C) described above was poured into a Teflon (registered trademark) container and dried at 50 ° C. for 24 hours. After drying, the film formed on the bottom surface of the container was peeled off from the container to obtain an electrolyte 9. The dimension of the obtained electrolyte 9 was 3 × 3 cm, and the film thickness was 20 μm as observed by FE-SEM. When the ratio of the substituents of the graphene oxide contained in the graphene oxide solution (C) was determined by XPS, the ratio of the total number of carbon atoms to which each oxygen functional group was bonded to the total number of carbon atoms of the entire graphite oxide (%) Is a hydroxyl group of 3.0%, an epoxy group of 28.1%, a carbonyl group of 5.5%, a carboxyl group of 17.5%, the total number of carbon atoms to which the oxygen functional group is bonded (a) and the total number of carbon atoms in graphite oxide The ratio a / b (%) of (b) was 32.1%. There was no peak for the sulfo group.

[Preparation of graphene oxide solution (D)]
Graphene oxide was produced in the same manner as in Non-Patent Document 2, which is a prior art document.
The manufacturing method is as follows.
Concentrated sulfuric acid (92 mL) was added to 0.4 g of 98% pure graphite powder (manufactured by Wako) in an ice bath, and the mixture was stirred for 10 minutes so as to be uniform. To this solution, 12 g of potassium permanganate was added with stirring at a rate of 1 g every 5 minutes, and the mixture was stirred in a 37 ° C. water bath for 30 minutes. Thereafter, 12 mL of pure water was added, transferred to a 95 ° C. oil bath, and stirred for 15 minutes. Further, 280 mL of pure water and 20 mL of 30% hydrogen peroxide were added to this solution and left at room temperature for 5 minutes. The color of the solution changed from brown to light yellow. After completion of the oxidation treatment, it was washed with 50 mL of 5% hydrochloric acid and 50 mL of pure water, filtered, and vacuum dried at 70 ° C. for 1 week to obtain graphite oxide (d).

  After drying the graphite oxide (d) produced by the above method, 60 mg was taken and irradiated with ultrasonic waves in 50 mL of pure water for 2-4 hours to peel off the carbon layer of graphite oxide. After ultrasonic irradiation, centrifugation was performed at 10,000 rpm for 30 minutes, and the supernatant was separated to obtain a graphene oxide solution (D). The circle equivalent diameters of the graphene oxide pieces observed on the mica substrate in the AFM tap mode were all 1.5 μm or less.

[Comparative Example 2]
The graphene oxide solution (D) described above was poured into a Teflon (registered trademark) container and dried at 50 ° C. for 24 hours. After drying, the film formed on the bottom surface of the container was peeled from the container to obtain an electrolyte 10. The dimension of the obtained electrolyte 10 was 3 × 3 cm, and the film thickness was 20 μm as observed by FE-SEM. When the ratio of the substituents of the graphene oxide contained in the graphene oxide solution (D) was determined by XPS, the ratio (%) of the total number of carbon atoms to which each oxygen functional group was bonded to the total number of carbon atoms of the entire graphite oxide Is 2.7% hydroxyl group, 42.9% epoxy group, 3.3% carbonyl group, 10.5% carboxyl group, the total number of carbon atoms (a) to which the oxygen functional group is bonded and the total number of carbon atoms in graphite oxide The ratio a / b (%) of (b) was 33.5%. A peak derived from the sulfo group was also observed at 2.2%, indicating the presence of 0.7 mmol / g of the sulfo group.

[Measurement of proton conductivity of electrolyte]
The proton conductivity of the electrolyte 1 was measured by the AC four-terminal method under conditions where the room temperature was 20 ° C. and the relative humidity was changed to 20-100%. The proton conductivity of graphene oxide electrolyte improved with increasing humidity. The proton conductivity of graphene oxide electrolyte is 1.0 × 10 ⁻⁴ S / cm at relative humidity of 70% or higher, 1.0 × 10 ⁻⁶ S / cm at 40%, and low humidity conditions of 20-30%. 1.0 × 10 ⁻⁷ S / cm.

[Fuel cell test]
Platinum was sputtered on both surfaces of the electrolytes 1 to 10 as a catalyst in a circular shape having a diameter of 1 cm. Sputtering conditions were performed at 5 mA for 60 seconds for electrolytes 3, 4, 5, and 6 using a K575XD type apparatus manufactured by Emitech Co., and for 30 seconds at 5 mA for electrolytes 1, 2, 7 and 10. did.
A gold mesh was joined to both sides of each platinum-sputtered electrolyte as a current take-out line and a separator, and fixed with bolts to form a fuel cell (FIG. 3).
One side of the fuel cell using the platinum-sputtered electrolytes 1 to 10 is 50 mL / min of hydrogen at a relative humidity of 20% and a temperature of 16 ° C. on one side, and a relative humidity of 20% and a temperature of 16 ° C. on the other side. Oxygen was supplied at 50 mL / min, and electromotive force, short-circuit current value, and output density were measured from both ends of the gold net using IviumStat / CompactStat (manufactured by Ibium Technology).

[Comparative Example 3]
One side of the fuel cell using the platinum-sputtered electrolyte 6 is 50 mL / min of hydrogen at a relative humidity of 70% and a temperature of 16 ° C., and oxygen at a relative humidity of 70% and a temperature of 16 ° C. on the other side. 50 mL / min was supplied, and the electromotive force, short-circuit current value, output density, and AC resistance at 100 Hz were measured from both ends of the gold net using the above-described apparatus.

Conventionally, proton conductivity (1.3 × 10 ⁻⁸ S / cm, T. Okayasu) of perfluorosulfonic acid resin (for example, trade name Nafion®, manufactured by DuPont) used in polymer electrolyte fuel cells , et al., Polym. Adv. Technol., 2011, 22, 1229.), graphene oxide (Example 1) was confirmed to exhibit proton conductivity one order of magnitude higher under low humidification conditions.
Further, as shown in Examples 1 to 6 in Table 1, it was confirmed that the fuel cell generates power even at a low relative humidity, and high proton conductivity, current density, and output density were obtained. In addition, as shown in Examples 3 to 5 in Table 1, it was also confirmed that the power generation performance of the fuel cell was improved by adding an inorganic oxide. On the other hand, in Comparative Example 1 using an electrolyte in which the content of graphene oxide is less than 20% by mass, although the electromotive force was confirmed even under a relative humidity of 20%, the short-circuit current density and the output density were extremely low.
Further, in Comparative Example 3 humidified to a relative humidity of 70%, no current could be taken out at a voltage of 0.4V.

  Table 2 compares the methods for preparing graphene oxide solutions of Examples 1, 7, 8 and Comparative Example 2 and the graphene oxides having different equivalent circle diameters. The smaller the equivalent circle diameter of graphene oxide, the higher current density and output density were obtained at low relative humidity. On the other hand, in Comparative Example 2 (graphene oxide of Non-Patent Document 2) containing 0.7 mmol / g of sulfo group, compared with Example 7 in which the equivalent circle diameter of graphene oxide is the same, electromotive force and current The density and power density were small.

[Gas barrier test]
The gas barrier property of the graphene oxide film was evaluated by using a gas chromatograph cell and how much hydrogen permeated the graphene oxide film as shown in FIG. The gas chromatography used for the evaluation was Shimadzu GC-8A (TCD).
The sampling volume was 0.1 mL, the cell volume was 6 mL, the cell constant was 60, and the hydrogen flow rate was about 20 ml / min. The graphene oxide film used for the evaluation was prepared from the same graphene oxide dispersion as in Example 7. By laminating 1 to 3 films by the drop cast method, graphene oxide films having different film thicknesses of 24, 42 and 67 μm were obtained.

  As for the hydrogen gas barrier property of the graphene oxide film, only 1 μmol or less of hydrogen was detected even after 10 hours in the graphene oxide films of all film thicknesses of 24, 42 and 67 μm, indicating high gas barrier performance. On the other hand, in the absence of the graphene oxide film, 200 μmol of hydrogen corresponding to the hydrogen gas flow rate was detected.

DESCRIPTION OF SYMBOLS 1 ... Electrolyte, 2 ... Catalyst, 3 ... Fuel electrode, 4 ... Air electrode, 5 ... Separator

Claims (10)

An electrolyte for a solid fuel cell, wherein the electrolyte contains 20% by mass or more of graphene oxide and has a sulfo group amount of 0.5 mmol / g or less. In the graphene oxide, the ratio a / b of the total number of carbon atoms (a) to the total number of carbon atoms (b) to which the oxygen functional groups of carboxyl group, carbonyl group, hydroxyl group, and epoxy group are bonded is 15% or more. The electrolyte for a solid fuel cell according to claim 1, wherein the electrolyte is provided. 3. The solid fuel cell electrolyte according to claim 1, wherein the graphene oxide has a circle-equivalent diameter in a plane direction of 3 μm or less. 4. The solid according to claim 1, wherein the graphene oxide has a carbon atom ratio of 10% or more in the total number of carbon atoms (b). Fuel cell electrolyte. ^5. The solid fuel according to claim 1, wherein the proton conductivity in an atmosphere of a temperature of 20 ° C. and a relative humidity of 20% is 3.0 × 10 ⁻⁸ S / cm or more. Battery electrolyte. The short-circuit current density is 4 mA / cm ² or more when the solid fuel cell is disposed in a solid fuel cell and operated at a relative humidity of 20% in the solid fuel cell. 5. The electrolyte for a solid fuel cell according to 5. The electromotive force when the solid fuel cell is disposed in the solid fuel cell and the solid fuel cell is operated at a relative humidity of 20% in the solid fuel cell is 0.3 V or more. 7. The electrolyte for a solid fuel cell according to 5 or 6. The solid fuel battery cell which laminated | stacked the electrolyte for solid fuel batteries of any one of Claims 1 thru | or 7. The solid fuel battery cell according to claim 8, wherein the electrolyte and the electrode are integrated. The solid fuel battery cell according to claim 9, which is operated at a relative humidity of 30% or less.