JP2015140296A - Sheet-like graphene oxide and method for producing the same, and electrolyte film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide sheet-like graphene oxide useful for obtaining an electrolyte film which can perform fuel cell power generation at a high current density.SOLUTION: A sheet-like graphene oxide has a proton conductivity (S/cm) in a normal direction whose logarithm is -3.0 or more at a relative humidity of 95% at 25°C. The logarithm of the proton conductivity (S/cm) in the normal direction is preferably -2.5 or more at a relative humidity of 95% at 25°C. The sheet-like graphene oxide can be produced by removing a dispersion medium and a water-soluble polymer from a graphene oxide dispersion containing the water-soluble polymer. Preferable as the water-soluble polymer is cellulose acetate having a degree of acetyl substitution of 0.5 to 1.1.

Description

  The present invention relates to a sheet-like graphene oxide, a method for producing the same, an electrolyte membrane, and a fuel cell including the electrolyte membrane.

  Fuel cells are attracting attention as clean next-generation energy sources using hydrogen and oxygen as energy sources. Among them, a polymer electrolyte fuel cell used as an in-vehicle power source generally has a large number of unit cells, each having a structure in which a positive electrode (air electrode) and a negative electrode (fuel electrode) sandwich an electrolyte membrane, with separators interposed therebetween. The cell stack is the main body.

Conventionally, Nafion which is a fluorine-based ion exchange membrane is used as the electrolyte membrane. However, since Nafion has a high oxygen permeability of 5 × 10 ⁻⁵ cm ³ / (s · cm ² ), the film thickness has to be 100 μm or more in order to prevent the permeation of oxygen. Therefore, there is a problem that not only the material cost becomes high, but also the membrane resistance becomes high and the power generation efficiency is low.

  Recently, as an alternative technique, it has been proposed to use graphene oxide paper as an electrolyte membrane (see Non-Patent Document 1). Graphene oxide has the advantage that the film thickness can be reduced because it is relatively inexpensive as a material and has low oxygen permeability. However, the conventional graphene oxide paper cannot be said to have a sufficiently low film resistance, and high power generation efficiency cannot be obtained.

Journal of The Electrochemical Society, 160 (11), F1175-F1178 (2013)

An object of the present invention is to provide a sheet-like graphene oxide useful for obtaining an electrolyte membrane capable of performing fuel cell power generation at a high current density, and an industrially efficient production method thereof.
Another object of the present invention is to provide an electrolyte membrane composed of the above sheet-like graphene oxide and a fuel cell provided with the electrolyte membrane.

  As a result of intensive studies to achieve the above object, the present inventors have found that when a water-soluble polymer is used as a dispersant in preparing graphene oxide paper, the dispersibility of graphene oxide in the graphene oxide dispersion is improved. In addition, the proton conductivity in the thickness direction (normal direction) of the graphene oxide paper obtained by filtering and washing the dispersion is remarkably improved. According to such graphene oxide paper, the membrane resistance is greatly increased. It has been found that fuel cell power generation can be performed at a high current density. The present invention has been completed through further studies based on these findings.

  That is, the present invention provides a sheet-like graphene oxide having a logarithm of proton conductivity (S / cm) in the normal direction at a relative humidity of 95% and 25 ° C. of −3.0 or more.

  The present invention also provides the sheet-like graphene oxide having a logarithm of proton conductivity (S / cm) in the normal direction at 25% relative humidity of 95% and −2.5 or more.

  The present invention also provides an electrolyte membrane composed of the sheet-like graphene oxide.

  The present invention further provides a fuel cell comprising the electrolyte membrane.

  The present invention is also a method for producing the sheet-like graphene oxide, wherein the dispersion medium and the water-soluble polymer are removed from the graphene oxide dispersion containing the water-soluble polymer to obtain the sheet-like graphene oxide. A method for producing a sheet-like graphene oxide is provided.

  The present invention also provides a method for producing the sheet-like graphene oxide, wherein the dispersion medium and the water-soluble polymer are removed by filtering and washing the graphene oxide dispersion containing the water-soluble polymer.

  Moreover, this invention provides the manufacturing method of the said sheet-like graphene oxide whose water-soluble polymer is a cellulose acetate of acetyl total substitution degree 0.5-1.1.

According to the sheet-like graphene oxide of the present invention, the proton conductivity in the normal direction at a relative humidity of 95% and 25 ° C. is very high, so that the membrane resistance is small and fuel cell power generation can be performed at a high current density.
According to the method for producing a sheet-like graphene oxide of the present invention, the sheet-like graphene oxide having the excellent characteristics as described above can be produced industrially efficiently.

FIG. 1 is a schematic diagram of a method for measuring proton conductivity in the normal direction of sheet-like graphene oxide. FIG. 2 is a schematic view of a method for measuring oxygen permeability of sheet-like graphene oxide.

[Sheet graphene oxide]
The sheet-like graphene oxide of the present invention (hereinafter sometimes referred to as “graphene oxide paper”) has a proton conductivity (S / cm) in the normal direction (thickness direction) at a relative humidity of 95% and 25 ° C. A logarithm is −3.0 or more. The logarithm means a common logarithm. The film resistance of such a sheet-like graphene oxide (graphene oxide paper) at an assumed film thickness is lower than that of conventional graphene oxide paper and Nafion, and fuel cell power generation can be performed at a high current density.

  The logarithm of proton conductivity (S / cm) in the normal direction (thickness direction) at a relative humidity of 95% and 25 ° C. is preferably −2.8 or more, more preferably −2.5 or more. The larger the logarithm of the proton conductivity (S / cm) is, the better, but the upper limit is, for example, about -1.0.

  The sheet-like graphene oxide of the present invention may be a single layer or a multilayer.

[Production of sheet-like graphene oxide]
The sheet-like graphene oxide of the present invention can be obtained, for example, by removing the dispersion medium and the water-soluble polymer from the graphene oxide dispersion containing the water-soluble polymer. The method for removing the dispersion medium and the water-soluble polymer is not particularly limited. For example, the dispersion medium and the water-soluble polymer can be efficiently removed by filtering and washing the graphene oxide dispersion containing the water-soluble polymer. it can.

  The graphene oxide dispersion containing a water-soluble polymer contains a water-soluble polymer, graphene oxide, and a dispersion medium, and the graphene oxide is dispersed in the dispersion medium. As a dispersion medium, water; water-soluble such as methanol, ethanol, ethylene glycol, glycerin, 2-methoxyethanol, acetone, 2-pyrrolidone, 1-methyl-2-pyrrolidone, dimethyl sulfoxide, N, N-dimethylformamide, sulfolane An organic solvent; a mixed solution of water and the water-soluble organic solvent, or the like. A dispersion medium may be used individually by 1 type, and may be used in combination of 2 or more type. Especially, as a dispersion medium, water or the liquid mixture of water and a water-soluble organic solvent (these may be called "aqueous solvent") is preferable. The content of water in the aqueous solvent is, for example, 50% by weight or more, preferably 70% by weight or more, more preferably 90% by weight or more, and may be 100% by weight of water.

  The graphene oxide is not particularly limited and can be produced as a dispersion by a conventionally known method. For example, graphite is mixed with concentrated sulfuric acid and sodium nitrate, and the mixture is oxidized by adding potassium permanganate, then quenched with hydrogen peroxide and washed with distilled water to obtain graphite oxide. By dispersing the graphite oxide in the dispersion medium and irradiating ultrasonic waves, the graphite oxide is peeled off in the layer direction, and a dispersion of graphene oxide is obtained. This may be centrifuged, and the supernatant liquid may be recovered to form a graphene oxide dispersion.

  A graphene oxide dispersion containing the water-soluble polymer can be obtained by adding and mixing a water-soluble polymer to the graphene oxide dispersion obtained above. The water-soluble polymer will be described later.

  The concentration of graphene oxide in the graphene oxide dispersion containing the water-soluble polymer is, for example, 0.01 to 5% by weight, preferably 0.05 to 2% by weight. By setting the concentration of graphene oxide within the above range, the dispersibility and dispersion stability of graphene oxide can be further improved.

  The concentration of the water-soluble polymer in the graphene oxide dispersion containing the water-soluble polymer is, for example, 0.005 to 5% by weight, preferably 0.01 to 2% by weight. The amount of the water-soluble polymer in the graphene oxide dispersion containing the water-soluble polymer is, for example, 1 to 500 parts by weight with respect to 100 parts by weight of graphene oxide in the graphene oxide dispersion containing the water-soluble polymer. Preferably it is 5-300 weight part, More preferably, it is 20-200 weight part. By setting the concentration or amount of the water-soluble polymer within the above range, the dispersibility and dispersion stability of graphene oxide can be further improved.

  A general filtration method can be used for filtration of the graphene oxide dispersion containing a water-soluble polymer. The type of filtration may be any of natural filtration, vacuum filtration, pressure filtration, and centrifugal filtration. The filter medium may be, for example, a filter paper or a membrane filter, but a membrane filter is preferable. The pore size of the membrane filter is, for example, about 0.025 to 0.4 μm.

  For the washing after filtration, for example, the solvent exemplified as the dispersant (water, water-soluble organic solvent, mixed solution of water and water-soluble organic solvent) can be used. By this washing operation, the water-soluble polymer attached to the sheet-like graphene oxide can be removed. Sheet-like graphene oxide (graphene oxide paper) can be obtained by drying after washing.

  The thickness (film thickness) of the sheet-like graphene oxide (graphene oxide paper) is, for example, the graphene oxide concentration in the graphene oxide dispersion containing the water-soluble polymer, the graphene oxide dispersion containing the water-soluble polymer supplied to the filter medium It can be adjusted according to the amount of liquid.

[Water-soluble polymer]
The water-soluble polymer in the present invention is not particularly limited as long as it is a polymer compound that is soluble in water. For example, a water-soluble polymer such as a water-soluble cellulose polymer such as water-soluble cellulose acetate, carboxymethyl cellulose, and hydroxyethyl cellulose. Synthetic polymers; water-soluble natural polymers such as starch, mannan, pectin, alginic acid, dextran, pullulan, glue, gelatin, etc .; polyalkylene glycols and polyalkylene oxides (polyethylene glycol, polyethylene oxide, polypropylene glycol, polypropylene oxide, etc.), polyacrylic acid Examples thereof include water-soluble synthetic polymers such as sodium, polyacrylamide, polyvinyl alcohol, polyethyleneimine, and polyvinylpyrrolidone.

  Among these, from the viewpoint of the dispersibility and dispersion stability of graphene oxide, water-soluble cellulose-based polymers such as water-soluble cellulose acetate are preferable, and water-soluble cellulose acetate is particularly preferable. Hereinafter, the water-soluble cellulose acetate will be described.

[Water-soluble cellulose acetate]
(Total degree of acetyl substitution)
In the present specification, the water-soluble cellulose acetate has a total acetyl substitution degree of 0.5 to 1.1. When the total degree of acetyl substitution is within this range, the solubility in water is excellent, and when it is outside this range, the solubility in water tends to decrease. A preferred range of the total degree of acetyl substitution is 0.55 to 1.0, and a more preferred range is 0.6 to 0.95. The total degree of acetyl substitution can be measured by a known titration method in which cellulose acetate is dissolved in water and the degree of substitution of cellulose acetate is determined. The total degree of acetyl substitution can also be measured by NMR after propionylating the hydroxyl group of cellulose acetate (see the method described later), dissolving in deuterated chloroform.

The total degree of acetyl substitution is obtained by converting the degree of acetylation determined according to the method for measuring the degree of acetylation in ASTM: D-817-91 (test method for cellulose acetate, etc.) by the following formula. This is the most common method for determining the degree of substitution of cellulose acetate.
DS = 162.14 × AV × 0.01 / (60.052-42.037 × AV × 0.01)
DS: Degree of total acetyl substitution AV: Degree of acetylation (%)
First, 500 mg of dried cellulose acetate (sample) was accurately weighed and dissolved in 50 ml of a mixed solvent of ultrapure water and acetone (volume ratio 4: 1), then 50 ml of 0.2N sodium hydroxide aqueous solution was added, Saponify for 2 hours at 25 ° C. Next, 50 ml of 0.2N hydrochloric acid is added, and the amount of acetic acid released is titrated with a 0.2N sodium hydroxide aqueous solution (0.2N sodium hydroxide normal solution) using phenolphthalein as an indicator. In addition, a blank test (a test without using a sample) is performed by the same method. Then, AV (degree of acetylation) (%) is calculated according to the following formula.
AV (%) = (A−B) × F × 1.201 / sample weight (g)
A: Titration amount of 0.2N sodium hydroxide normal solution (ml)
B: Titration of 0.2N sodium hydroxide normal solution in blank test (ml)
F: Factor of 0.2N sodium hydroxide normal solution

(Composition distribution index (CDI))
In the present invention, the composition distribution (intermolecular substitution degree distribution) of the cellulose acetate is not particularly limited, and the composition distribution index (CDI) is, for example, 1.0 to 3.0. The composition distribution index (CDI) is preferably 1.0 to 2.0, more preferably 1.0 to 1.8, still more preferably 1.0 to 1.6, and particularly preferably 1.0 to 1.5. It is. When the composition distribution index (CDI) is 2.0 or less, the dispersibility and dispersion stability of graphene oxide in the graphene oxide dispersion are further improved.

  The lower limit of the composition distribution index (CDI) is 0, but this is achieved by a special synthetic technique such as acetylating only the 6th position of a glucose residue and not acetylating other positions with 100% selectivity. Such synthesis techniques are not known. In a situation where all of the hydroxyl groups of the glucose residue are acetylated and deacetylated with the same probability, the CDI is 1.0. However, in an actual cellulose reaction, a considerable amount is required to approach such an ideal state. It needs some ingenuity. The smaller the composition distribution index (CDI), the more uniform the composition distribution (intermolecular substitution degree distribution). When the composition distribution is uniform, water solubility can be secured in a range where the total degree of acetyl substitution is wider than usual.

  Here, the compositional distribution index (CDI) is the ratio of the measured value to the theoretical value of the half width of the composition distribution [(actual value of the half width of the composition distribution) / (theoretical value of the half width of the composition distribution)]. Defined. The composition distribution half-width is also referred to as “intermolecular substitution degree half-width” or simply “substitution degree distribution half-width”.

  In order to evaluate the uniformity of the total degree of acetyl substitution of cellulose acetate, the maximum peak half-value width (also referred to as “half-value width”) of the intermolecular substitution degree distribution curve of cellulose acetate can be used as an index. . The half width is the width of the chart at half the height of the peak of the chart, where the acetyl substitution degree is on the horizontal axis (x axis) and the abundance at this substitution degree is on the vertical axis (y axis). It is an index representing the standard of variation in distribution. The half value width of the substitution degree distribution can be determined by high performance liquid chromatography (HPLC) analysis. In addition, the method of converting the horizontal axis (elution time) of the elution curve of cellulose ester in HPLC into the substitution degree (0 to 3) is described in JP-A-2003-201301 (paragraphs 0037 to 0040).

ｍ：酢酸セルロース１分子中の水酸基とアセチル基の全数
ｐ：酢酸セルロース１分子中の水酸基がアセチル置換されている確率
ｑ＝１−ｐ
ＤＰｗ：重量平均重合度（ＧＰＣ−光散乱法による）
なお、重量平均重合度（ＤＰｗ）の測定法は後述する。 (Theoretical value of the half width of composition distribution)
The theoretical distribution of the half value width of composition distribution (substitution degree distribution half width) can be calculated stochastically. That is, the theoretical value of the composition distribution half width is obtained by the following equation (1).

m: Total number of hydroxyl groups and acetyl groups in one molecule of cellulose acetate p: Probability of hydroxyl groups in one molecule of cellulose acetate being acetyl substituted q = 1-p
DPw: weight average polymerization degree (by GPC-light scattering method)
In addition, the measuring method of a weight average polymerization degree (DPw) is mentioned later.

ＤＳ：アセチル総置換度
ＤＰｗ：重量平均重合度（ＧＰＣ−光散乱法による）
なお、重量平均重合度（ＤＰｗ）の測定法は後述する。 Formula (1) is the half width of the composition distribution that inevitably occurs when all hydroxyl groups of cellulose are acetylated and deacetylated with the same probability, and is derived according to the so-called binomial theorem. Furthermore, when the theoretical value of the composition distribution half width is represented by the substitution degree and the polymerization degree, it is expressed as follows. In the present invention, the following formula (2) is defined as a definition formula for obtaining a theoretical value of the composition distribution half width.

DS: Acetyl total substitution degree DPw: Weight average polymerization degree (by GPC-light scattering method)
In addition, the measuring method of a weight average polymerization degree (DPw) is mentioned later.

  By the way, in the formula (1) and the formula (2), the polymerization degree distribution should be taken into account more strictly. In this case, the “DPw” in the formula (1) and the formula (2) represents the polymerization degree. Substituting a distribution function, the entire equation should be integrated from 0 to infinity. However, as long as DPw is used, equations (1) and (2) give theoretical values with approximately sufficient accuracy. If DPn (number average degree of polymerization) is used, the influence of the degree of polymerization distribution cannot be ignored, so DPw should be used.

(Measured value of composition distribution half width)
In the present invention, the actual value of the composition distribution half width is the composition distribution half width obtained by HPLC analysis of cellulose acetate propionate obtained by propionylating all remaining hydroxyl groups (unsubstituted hydroxyl groups) of cellulose acetate (sample). It is.

  Generally, high performance liquid chromatography (HPLC) analysis can be performed on cellulose acetate having a total degree of acetyl substitution of 2 to 3 without pretreatment, whereby the half width of the composition distribution can be determined. For example, JP 2011-158664 A describes a composition distribution analysis method for cellulose acetate having a substitution degree of 2.27 to 2.56.

  On the other hand, in the present invention, the actual value of the composition distribution half-width (substitution degree distribution half-width) is obtained by derivatizing the residual hydroxyl group in cellulose acetate as a pretreatment before HPLC analysis, and then performing HPLC analysis. Ask. The purpose of this pretreatment is to convert the low-substituted cellulose acetate into a derivative that can be easily dissolved in an organic solvent to enable HPLC analysis. That is, the residual hydroxyl group in the molecule is completely propionylated, and the fully derivatized cellulose acetate propionate (CAP) is subjected to HPLC analysis to determine the composition distribution half width (actual value). Here, the derivatization is completely carried out, there should be no residual hydroxyl groups in the molecule, and only acetyl and propionyl groups must be present. That is, the sum of the degree of acetyl substitution (DSac) and the degree of propionyl substitution (DSpr) is 3. This is because the relational expression: DSac + DSpr = 3 is used to create a calibration curve for converting the horizontal axis (elution time) of the HPLC elution curve of CAP to the degree of acetyl substitution (0 to 3).

  Complete derivatization of cellulose acetate can be performed by reacting propionic anhydride with N, N-dimethylaminopyridine as a catalyst in a pyridine / N, N-dimethylacetamide mixed solvent. More specifically, a mixed solvent [pyridine / N, N-dimethylacetamide = 1/1 (v / v)] as a solvent is 20 parts by weight with respect to cellulose acetate (sample), and propionic anhydride as a propionylating agent. 6.0 to 7.5 equivalents based on the hydroxyl group of the cellulose acetate, and N, N-dimethylaminopyridine as a catalyst is used at 6.5 to 8.0 mol% with respect to the hydroxyl group of the cellulose acetate. Propionylation is carried out under conditions of reaction time of 1.5 to 3.0 hours. And after reaction, fully derivatized cellulose acetate propionate is obtained by making it precipitate using methanol as a precipitation solvent. More specifically, for example, at room temperature, 1 part by weight of the reaction mixture is added to 10 parts by weight of methanol to precipitate, and the resulting precipitate is washed 5 times with methanol and vacuum dried at 60 ° C. for 3 hours. Thus, fully derivatized cellulose acetate propionate (CAP) can be obtained. The polydispersity (Mw / Mn) and the weight average polymerization degree (DPw) described later were also measured by using cellulose acetate (sample) as a fully derivatized cellulose acetate propionate (CAP) by this method.

  In the above HPLC analysis, a plurality of cellulose acetate propionates having different degrees of acetyl substitution are used as standard samples, HPLC analysis is performed with a predetermined measuring apparatus and measurement conditions, and a calibration created using the analysis values of these standard samples. Obtain the composition distribution half-value width (actually measured value) of cellulose acetate (sample) from the curve [the curve showing the relationship between the elution time of cellulose acetate propionate and the degree of acetyl substitution (0 to 3), usually a cubic curve]. Can do. What is required by HPLC analysis is the relationship between the elution time and the distribution of acetyl substitution degree of cellulose acetate propionate. This is the relationship between the elution time of the substance in which all the remaining hydroxy groups in the sample molecule are converted to propionyloxy groups and the acetyl substitution degree distribution, so that the acetyl substitution degree distribution of the cellulose acetate of the present invention is obtained. And essentially the same.

The conditions for the above HPLC analysis are as follows.
Equipment: Agilent 1100 Series
Column: Waters Nova-Pak phenyl 60Å 4 μm (150 mm × 3.9 mmΦ) + guard column Column temperature: 30 ° C.
Detection: Varian 380-LC
Injection volume: 5.0 μL (sample concentration: 0.1% (wt / vol))
Eluent: Liquid A: MeOH / H ₂ O = 8/1 (v / v), Liquid B: CHCl ₃ / MeOH = 8/1 (v / v)
Gradient: A / B = 80/20 → 0/100 (28 min); Flow rate: 0.7 mL / min

  Substitution degree distribution curve obtained from the calibration curve [Substitution degree distribution curve of cellulose acetate propionate with the abundance of cellulose acetate propionate as the vertical axis and the acetyl substitution degree as the horizontal axis] ("Intermolecular substitution degree distribution curve )), The half value width of the substitution degree distribution is obtained as follows for the maximum peak (E) corresponding to the average substitution degree. A base (A) in contact with the base (A) on the low substitution degree side of the peak (E) and the base (B) on the high substitution degree side is drawn, and from this peak, the maximum peak (E) is drawn. Take a vertical line on the horizontal axis. An intersection (C) between the perpendicular and the base line (AB) is determined, and an intermediate point (D) between the maximum peak (E) and the intersection (C) is obtained. A straight line parallel to the base line (A-B) is drawn through the intermediate point (D), and two intersection points (A ′, B ′) with the intermolecular substitution degree distribution curve are obtained. A perpendicular line is drawn from the two intersections (A ′, B ′) to the horizontal axis, and the width between the two intersections on the horizontal axis is defined as the half-value width of the maximum peak (that is, the substitution value distribution half-value width).

The half-value width of the substitution degree distribution depends on the degree of acetylation of the hydroxyl chain of each glucose chain constituting the molecular chain of cellulose acetate propionate in the sample. (Retention time) is different. Therefore, ideally, the holding time width indicates the width of the composition distribution (in units of substitution degree). However, there are tubes (such as a guide column for protecting the column) that do not contribute to the distribution in HPLC. Therefore, depending on the configuration of the measurement apparatus, the width of the holding time that is not caused by the width of the composition distribution is often included as an error. As described above, this error is affected by the length and inner diameter of the column, the length from the column to the detector, the handling, and the like, and varies depending on the apparatus configuration. For this reason, the half value width of the substitution degree distribution of cellulose acetate propionate can be usually obtained as the correction value Z based on the correction formula represented by the following formula. By using such a correction formula, even if the measurement apparatus (and measurement conditions) are different, a more accurate substitution degree distribution half-value width (actual measurement value) can be obtained as the same (substantially the same) value.
Z = (X ² −Y ² ) ^1/2
[In the formula, X is the half-value width (uncorrected value) of the substitution degree distribution obtained with a predetermined measuring apparatus and measurement conditions. Y = (a−b) x / 3 + b (0 ≦ x ≦ 3). Here, a is an apparent substitution degree distribution half-value width of cellulose acetate having a total substitution degree of 3 obtained by the same measuring apparatus and measurement conditions as X (actually, there is no substitution degree distribution because the total substitution degree is 3, and b is It is an apparent substitution degree distribution half-value width of cellulose propionate having a total substitution degree of 3 determined by the same measuring apparatus and measurement conditions as those of X. x is the total acetyl substitution degree of the measurement sample (0 ≦ x ≦ 3)]

  The cellulose acetate (or cellulose propionate) having a total substitution degree of 3 is a cellulose ester in which all of the hydroxyl groups of cellulose are esterified, and (ideally) the half-width of the substitution degree distribution. (Ie, the substitution degree distribution half-width 0) cellulose ester.

  In the present invention, the measured value of the composition distribution half width (substitution degree distribution half width) of the cellulose acetate is preferably 0.12 to 0.34, more preferably 0.13 to 0.25.

  The substitution degree distribution theoretical formula described above is a probabilistic calculation value that assumes that all acetylation and deacetylation proceed independently and equally. That is, the calculated value according to the binomial distribution. Such an ideal situation is not realistic. The cellulose ester substitution degree distribution is probable unless the cellulose acetate hydrolysis reaction approaches an ideal random reaction and / or unless special measures are taken to create a fractional composition for post-treatment after the reaction. It is much wider than what is theoretically determined by the binomial distribution.

  As one of the special measures of the reaction, for example, it is conceivable to maintain the system under conditions where deacetylation and acetylation are in equilibrium. However, in this case, the decomposition of cellulose proceeds with an acid catalyst, which is not preferable. Another special contrivance for the reaction is to employ reaction conditions in which the deacetylation rate is slow for low substitution products. However, conventionally, such a specific method is not known. That is, there is no known special device for the reaction that controls the substitution degree distribution of the cellulose ester according to the binomial distribution according to the reaction probability theory. In addition, various circumstances such as non-uniformity in the acetylation process (cellulose acetylation process) and partial or temporary precipitation due to water added stepwise in the aging process (cellulose acetate hydrolysis process) It works in the direction to make the substitution degree distribution wider than the binomial distribution, avoiding all of these, and realizing the ideal condition is impossible in practice. This is similar to the fact that an ideal gas is an ideal product, and the behavior of an actual gas is more or less different.

  In the synthesis and post-treatment of conventional low-substituted cellulose acetate, little attention has been paid to the problem of such substitution degree distribution, and substitution degree distribution has not been measured, verified, or considered. For example, according to the literature (Journal of the Textile Society of Japan, 42, p25 (1986)), it is argued that the solubility of low-substituted cellulose acetate is determined by the distribution of acetyl groups to the glucose residues 2, 3, and 6. The composition distribution is not considered at all.

  According to the study by the present inventors, as described later, the substitution degree distribution of cellulose acetate can be surprisingly controlled by devising the post-treatment conditions after the cellulose acetate hydrolysis step. Literature (CiBment, L., and Rivibre, C., Bull. SOC. Chim., (5) 1, 1075 (1934), Sookne, AM, Rutherford, HA, Mark, H., and Harris, M. J. According to Research Natl. Bur. Standards, 29, 123 (1942), AJ Rosenthal, BB White Ind. Eng. Chem., 1952, 44 (11), pp 2693-2696.) Acetic acid with a degree of substitution of 2.3 In the precipitation fractionation of cellulose, it is said that fractionation depending on molecular weight and minute fractionation accompanying the degree of substitution (chemical composition) occur, and the degree of substitution (chemical composition) as found by the present inventors is remarkable. There is no report that fractionation is possible. Furthermore, it has not been verified that the substitution degree distribution (chemical composition) can be controlled by dissolution fractionation or precipitation fractionation for low-substituted cellulose acetate.

  Another device for narrowing the substitution degree distribution found by the present inventors is a hydrolysis reaction (aging reaction) of cellulose acetate at a high temperature of 90 ° C. or higher (or higher than 90 ° C.). Conventionally, although detailed analysis and consideration have not been made on the degree of polymerization of a product obtained by a high-temperature reaction, it has been considered that decomposition of cellulose is preferred in a high-temperature reaction at 90 ° C. or higher. This idea can be said to be a belief (stereotype) based solely on the consideration of viscosity. When the present inventors hydrolyze cellulose acetate to obtain low-substituted cellulose acetate, it is in a large amount of acetic acid at a high temperature of 90 ° C. or higher (or higher than 90 ° C.), preferably in the presence of a strong acid such as sulfuric acid. It was found that when the reaction was carried out with the above, the degree of polymerization was not lowered, but the viscosity was lowered with a decrease in CDI. That is, it has been clarified that the viscosity decrease due to the high temperature reaction is not due to a decrease in the degree of polymerization but is based on a decrease in structural viscosity due to a narrow substitution degree distribution. When cellulose acetate is hydrolyzed under the above conditions, not only forward reaction but also reverse reaction occurs, so the product (low-substituted cellulose acetate) has a very small CDI, and the solubility in water is significantly improved. On the other hand, when cellulose acetate is hydrolyzed under conditions where reverse reaction is unlikely to occur, the distribution of substitution degree becomes wide due to various factors, and cellulose acetate and acetyl substitution degree of less than 0.5 total acetyl substitution degree which is difficult to dissolve in water. The cellulose acetate content exceeding 1.1 increases, and the solubility in water as a whole decreases.

(Standard deviation of substitution degree at 2, 3, 6 position)
In the present invention, the degree of acetyl substitution at the 2, 3 and 6 positions of the glucose ring of cellulose acetate can be measured by the NMR method according to the method of Tezuka (Carbondr. Res. 273, 83 (1995)). That is, a free hydroxyl group of a cellulose acetate sample is propionylated with propionic anhydride in pyridine. The obtained sample is dissolved in deuterated chloroform, and a ¹³ C-NMR spectrum is measured. The carbon signal of the acetyl group appears in the order of 2, 3, 6 from the high magnetic field in the region of 169 ppm to 171 ppm, and the signal of the carbonyl carbon of the propionyl group appears in the same order in the region of 172 ppm to 174 ppm. From the abundance ratio of acetyl groups and propionyl groups at the corresponding positions, the degree of acetyl substitution at the 2, 3, and 6 positions of the glucose ring in the original cellulose diacetate can be determined. In addition, the sum of each acetyl substitution degree of the 2nd, 3rd, and 6th positions thus obtained is the total acetyl substitution degree, and the total acetyl substitution degree can also be obtained by this method. The total degree of acetyl substitution can be analyzed by ¹ H-NMR in addition to ¹³ C-NMR.

The standard deviation σ of the substitution degree at the 2nd, 3rd and 6th positions is defined by the following equation.

  In the present invention, the standard deviation of the degree of acetyl substitution at the 2, 3 and 6 positions of the glucose ring of cellulose acetate is preferably 0.08 or less (0 to 0.08). Cellulose acetate having a standard deviation of 0.08 or less is evenly substituted at the 2, 3, and 6 positions of the glucose ring, and has excellent solubility in water.

(Polydispersity (dispersity, Mw / Mn))
In the present invention, polydispersity (Mw / Mn) of molecular weight distribution (polymerization degree distribution) is determined by GPC-light scattering method using cellulose acetate propionate obtained by propionylating all remaining hydroxyl groups of cellulose acetate (sample). It is the value calculated | required by.

  The polydispersity (dispersity, Mw / Mn) of the cellulose acetate in the present invention is preferably in the range of 1.2 to 2.5. Cellulose acetate having polydispersity Mw / Mn in the above range has a uniform molecular size and is excellent in solubility in water.

The number average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity (Mw / Mn) of cellulose acetate can be determined by a known method using HPLC. In the present invention, the polydispersity (Mw / Mn) of cellulose acetate is determined by dissolving cellulose acetate (sample) in the same manner as in the case of obtaining the measured value of the half-value width of the composition distribution in order to make the measurement sample soluble in an organic solvent. ) Is completely derivatized cellulose acetate propionate (CAP), and then subjected to size exclusion chromatography analysis under the following conditions (GPC-light scattering method).
Equipment: GPC “SYSTEM-21H” manufactured by Shodex
Solvent: Acetone Column: 2 GMHxl (Tosoh), same guard column Flow rate: 0.8 ml / min
Temperature: 29 ° C
Sample concentration: 0.25% (wt / vol)
Injection volume: 100 μl
Detection: MALLS (multi-angle light scattering detector) (manufactured by Wyatt, “DAWN-EOS”)
MALLS correction standard substance: PMMA (molecular weight 27600)

(Weight average degree of polymerization (DPw))
In the present invention, the weight average degree of polymerization (DPw) is a value determined by GPC-light scattering method using cellulose acetate propionate obtained by propionylating all remaining hydroxyl groups of cellulose acetate (sample).

  The weight average polymerization degree (DPw) of the cellulose acetate in the present invention is preferably in the range of 50 to 800. If the weight average degree of polymerization (DPw) is too high, the filterability tends to deteriorate. The weight average degree of polymerization (DPw) is preferably 55 to 700, and more preferably 60 to 600.

  As with the polydispersity (Mw / Mn), the weight average degree of polymerization (DPw) is obtained by completely derivatizing cellulose acetate (sample) with a method similar to that for obtaining the measured half-value width of the composition distribution. It is calculated | required by performing size exclusion chromatography analysis after setting it as propionate (CAP) (GPC-light-scattering method).

  As described above, the molecular weight (polymerization degree) and polydispersity (Mw / Mn) of water-soluble cellulose acetate are measured by GPC-light scattering method (GPC-MALLS, GPC-LALLS, etc.). Note that detection of light scattering is generally difficult with an aqueous solvent. This is because aqueous solvents generally have a large amount of foreign matter and are easily contaminated by secondary contamination once purified. In addition, in the case of aqueous solvents, the spread of molecular chains may not be stable due to the influence of ionic dissociation groups present in a trace amount, and if water-soluble inorganic salt (for example, sodium chloride) is added to suppress this, it will dissolve. The state may become unstable, and an aggregate may be formed in an aqueous solution. One effective method for avoiding this problem is to derivatize water-soluble cellulose acetate so that it is dissolved in an organic solvent that is less contaminated and less susceptible to secondary contamination, and GPC-light scattering measurement is performed using the organic solvent. That is. Propionylation is effective for derivatization of water-soluble cellulose acetate for this purpose, and specific reaction conditions and post-treatment are as described in the explanation of the measured value of the half width of the composition distribution.

(6% viscosity)
The 6% viscosity of the cellulose acetate in the present invention is, for example, 5 to 500 mPa · s, preferably 6 to 300 mPa · s. If the 6% viscosity is too high, filterability may deteriorate.

The 6% viscosity of cellulose acetate can be measured by the following method.
Add 3.00 g of dry sample to a 50 ml volumetric flask and add distilled water to dissolve. The obtained 6 wt / vol% solution is transferred to a predetermined Ostwald viscometer mark and temperature-controlled at 25 ± 1 ° C. for about 15 minutes. Measure the flow-down time between the time marks and calculate the 6% viscosity by the following formula.
6% viscosity (mPa · s) = C × P × t
C: Sample solution constant P: Sample solution density (0.997 g / cm ³ )
t: The number of seconds that the sample solution flows down The sample solution constant is the same as described above using a standard solution for viscometer calibration [manufactured by Showa Oil Co., Ltd., trade name “JS-200” (based on JIS Z 8809)]. Measure the flow-down time and obtain it from the following formula.
Sample solution constant = {Standard solution absolute viscosity (mPa · s)} / {Standard solution density (g / cm ³ ) × Standard solution flow down seconds}

(Production of low-substituted cellulose acetate)
The cellulose acetate (low-substituted cellulose acetate) in the present invention is, for example, (A) a middle to high-substituted cellulose acetate hydrolysis step (aging step), (B) a precipitation step, and as necessary ( C) It can be manufactured by a washing and neutralization process.

[(A) Hydrolysis step (aging step)]
In this step, medium to high-substituted cellulose acetate (hereinafter sometimes referred to as “raw cellulose acetate”) is hydrolyzed. The medium to high-substituted cellulose acetate used as a raw material has a total acetyl substitution degree of, for example, 1.5 to 3, and preferably 2 to 3. As the raw material cellulose acetate, commercially available cellulose diacetate (acetyl total substitution degree: 2.27 to 2.56) and cellulose triacetate (acetyl total substitution degree: more than 2.56 to 3) can be used.

  The hydrolysis reaction can be performed by reacting raw material cellulose acetate with water in the presence of a catalyst (aging catalyst) in an organic solvent. Examples of the organic solvent include acetic acid, acetone, alcohol (such as methanol), and mixed solvents thereof. Among these, a solvent containing at least acetic acid is preferable. As the catalyst, a catalyst generally used as a deacetylation catalyst can be used. As the catalyst, sulfuric acid is particularly preferable.

  The amount of the organic solvent (for example, acetic acid) used is, for example, 0.5 to 50 parts by weight, preferably 1 to 20 parts by weight, and more preferably 3 to 10 parts by weight with respect to 1 part by weight of raw material cellulose acetate. .

  The amount of the catalyst (for example, sulfuric acid) used is, for example, 0.005 to 1 part by weight, preferably 0.01 to 0.5 part by weight, more preferably 0.02 to 1 part by weight of the raw material cellulose acetate. 0.3 parts by weight. If the amount of the catalyst is too small, the hydrolysis time becomes too long, which may cause a decrease in the molecular weight of cellulose acetate. On the other hand, if the amount of the catalyst is too large, the degree of change in the depolymerization rate with respect to the hydrolysis temperature increases, and even if the hydrolysis temperature is low to some extent, the depolymerization rate increases and it is difficult to obtain cellulose acetate having a somewhat high molecular weight. Become.

  The amount of water in the hydrolysis step is, for example, 0.5 to 20 parts by weight, preferably 1 to 10 parts by weight, and more preferably 2 to 7 parts by weight with respect to 1 part by weight of the raw material cellulose acetate. The amount of the water is, for example, 0.1 to 5 parts by weight, preferably 0.3 to 2 parts by weight, and more preferably 0.5 to 1 part per 1 part by weight of the organic solvent (for example, acetic acid). .5 parts by weight. Although all amounts of water may be present in the system at the start of the reaction, in order to prevent precipitation of cellulose acetate, a part of the water to be used is present in the system at the start of the reaction, and the remaining water is removed. It may be added to the system in 1 to several times.

  The reaction temperature in a hydrolysis process is 40-130 degreeC, for example, Preferably it is 50-120 degreeC, More preferably, it is 60-110 degreeC. In particular, when the reaction temperature is 90 ° C. or higher (or a temperature exceeding 90 ° C.), the equilibrium of the reaction tends to increase in the direction in which the rate of the reverse reaction (acetylation reaction) to the normal reaction (hydrolysis reaction) increases. As a result, the substitution degree distribution becomes narrow, and a low substitution degree cellulose acetate having an extremely small composition distribution index CDI can be obtained without any particular ingenuity in the post-treatment conditions. In this case, it is preferable to use a strong acid such as sulfuric acid as the catalyst, and it is preferable to use acetic acid in excess as the reaction solvent. Even when the reaction temperature is 90 ° C. or lower, as described later, in the precipitation step, precipitation is performed using a mixed solvent containing two or more solvents as the precipitation solvent, precipitation fractionation and / or dissolution. By performing the fractionation, a low-substituted cellulose acetate having a very small composition distribution index CDI can be obtained.

[(B) Precipitation step]
In this step, after completion of the hydrolysis reaction, the temperature of the reaction system is cooled to room temperature, and a precipitation solvent is added to precipitate low-substituted cellulose acetate. As the precipitation solvent, an organic solvent miscible with water or an organic solvent having high solubility in water can be used. Examples thereof include ketones such as acetone and methyl ethyl ketone; alcohols such as methanol, ethanol and isopropyl alcohol; esters such as ethyl acetate; nitrogen-containing compounds such as acetonitrile; ethers such as tetrahydrofuran; and mixed solvents thereof.

  When a mixed solvent containing two or more solvents is used as the precipitation solvent, the same effect as the precipitation fractionation described later can be obtained, the composition distribution (intermolecular substitution degree distribution) is narrow, and the low substitution with a small composition distribution index (CDI). Degree cellulose acetate can be obtained. Preferred examples of the mixed solvent include a mixed solvent of acetone and methanol, a mixed solvent of isopropyl alcohol and methanol, and the like.

  Further, by performing precipitation fractionation (fractional precipitation) and / or dissolution fractionation (fractional dissolution) on the low-substituted cellulose acetate obtained by precipitation, the composition distribution (intermolecular substitution degree distribution) is narrow, A low-substituted cellulose acetate having a very small composition distribution index CDI can be obtained.

  For precipitation fractionation, for example, low-substituted cellulose acetate (solid matter) obtained by precipitation is dissolved in water to obtain an aqueous solution having an appropriate concentration (for example, 2 to 10% by weight, preferably 3 to 8% by weight). Then, a poor solvent is added to this aqueous solution (or the aqueous solution is added to the poor solvent), and maintained at an appropriate temperature (for example, 30 ° C. or less, preferably 20 ° C. or less) to precipitate low-substituted cellulose acetate, This can be done by collecting the precipitate. Examples of the poor solvent include alcohols such as methanol and ketones such as acetone. The usage-amount of a poor solvent is 1-10 weight part with respect to 1 weight part of said aqueous solution, Preferably it is 2-7 weight part.

  For the dissolution fractionation, for example, the low-substituted cellulose acetate (solid matter) obtained by the precipitation or the low-substituted cellulose acetate (solid matter) obtained by the precipitation fractionation is mixed with water and an organic solvent (for example, acetone or the like). A mixed solvent of an alcohol such as ketone and ethanol) and stirring at an appropriate temperature (for example, 20 to 80 ° C., preferably 25 to 60 ° C.), followed by centrifugation to separate a concentrated phase and a diluted phase, A precipitation solvent (for example, a ketone such as acetone or an alcohol such as methanol) is added to the dilute phase, and the precipitate (solid matter) is recovered. The concentration of the organic solvent in the mixed solvent of water and organic solvent is, for example, 5 to 50% by weight, preferably 10 to 40% by weight.

[(C) Washing and neutralization step]
The precipitate (solid matter) obtained in the precipitation step (B) is preferably washed with an organic solvent (poor solvent) such as alcohol such as methanol and ketone such as acetone. Moreover, it is also preferable to wash and neutralize with an organic solvent containing a basic substance (for example, alcohol such as methanol, ketone such as acetone). The neutralization step may be provided immediately after the hydrolysis step. In that case, it is preferable to add a basic substance or an aqueous solution thereof to the hydrolysis reaction bath.

  Examples of the basic substance include alkali metal compounds (for example, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; alkali metal carbonates such as sodium carbonate and potassium carbonate; alkali metal carbonates such as sodium hydrogen carbonate) Hydrogen salts; alkali metal carboxylates such as sodium acetate and potassium acetate; sodium alkoxides such as sodium methoxide and sodium ethoxide), alkaline earth metal compounds (for example, alkaline earth such as magnesium hydroxide and calcium hydroxide) Alkaline earth metal carbonates such as metal hydroxide, magnesium carbonate and calcium carbonate; alkaline earth metal carboxylates such as magnesium acetate and calcium acetate; alkaline earth metal alkoxides such as magnesium ethoxide, etc.) can be used. . Among these, alkali metal compounds such as potassium acetate are particularly preferable.

  By washing and neutralization, impurities such as a catalyst (such as sulfuric acid) used in the hydrolysis step can be efficiently removed.

  The low-substituted cellulose acetate thus obtained can be adjusted to a specific particle size range by pulverization, sieving or granulation, if necessary.

[Electrolyte membrane]
The electrolyte membrane of the present invention is composed of the sheet-like graphene oxide of the present invention. This electrolyte membrane can be used as an electrolyte membrane for a fuel cell.

[Fuel cell]
The fuel cell of the present invention includes the electrolyte membrane of the present invention. The structure of the fuel cell is not particularly limited, and for example, a conventionally known structure can be used as a basic structure. For example, as the fuel cell of the present invention, a solid polymer electrolyte membrane of a so-called solid polymer fuel cell is replaced with the sheet-like graphene oxide, for example, a plus electrode (air electrode) and a minus electrode (fuel electrode). Examples thereof include a cell stack in which a large number of single cells having a structure sandwiching the electrolyte membrane of the present invention are stacked via separators.

  Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

Synthesis example 1
5.1 parts by weight of acetic acid and 2.0 parts by weight with respect to 1 part by weight of cellulose acetate (manufactured by Daicel, trade name “L-50”, acetyl group total substitution degree 2.43, 6% viscosity: 110 mPa · s) A part by weight of water was added and stirred at 40 ° C. for 5 hours to obtain a solution having a uniform appearance. 0.13 parts by weight of sulfuric acid was added to this solution, and the resulting solution was kept at 70 ° C. to carry out hydrolysis (partial deacetylation reaction; aging). In this aging process, water was added to the system twice in the middle. That is, 0.67 parts by weight of water was added 1 hour after the start of the reaction, and 1.67 parts by weight of water was added after another 2 hours, and the reaction was further continued for 6 hours. The total hydrolysis time is 9 hours. The first aging from the start of the reaction to the first water addition, the second aging from the first water addition to the second water addition, the reaction from the second water addition to the end of the reaction (ripening completed) This is called the third aging.
After carrying out the hydrolysis, the temperature of the system is cooled to room temperature (about 25 ° C.), and 15 parts by weight of acetone / methanol = 1/2 (weight ratio) mixed solvent (precipitating agent) is added to the reaction mixture to cause precipitation. Was generated.
The precipitate was recovered as a wet cake having a solid content of 15% by weight, and washed by adding 8 parts by weight of methanol and draining to a solid content of 15% by weight. This was repeated three times. The washed precipitate was further washed twice with 8 parts by weight of methanol containing 0.004% by weight of potassium acetate, neutralized and dried to obtain water-soluble cellulose acetate.

(Measurement of substitution degree (DS))
According to the method of Tezuka (Carbohydr. Res. 273, 83 (1995)), the unsubstituted hydroxyl group of the water-soluble cellulose acetate sample was propionylated. The total substitution degree of acetyl group of propionylated cellulose acetate was determined from 169 to 171 ppm acetylcarbonyl signal and 172 to 174 ppm propionylcarbonyl signal in ¹³ C-NMR according to the method of Tezuka (same). The total degree of acetyl group substitution of the water-soluble cellulose acetate thus determined was 0.87.

ＤＰｗ：重量平均重合度
ｐ：（未知試料のアセチルＤＳ）／３
ｑ：１−ｐ
このように求めた上記水溶性酢酸セルロースのＣＤＩは１．４であった。 (Measurement of composition distribution index (CDI))
The CDI of cellulose acetate was determined by conducting HPLC analysis under the following conditions after introduction into propionylated cellulose acetate.
Equipment: Agilent 1100 Series
Column: Waters Nova-Pak phenyl 60Å 4 μm (150 mm × 3.9 mmΦ) + guard column Column temperature: 30 ° C.
Detection: Varian 380-LC
Injection volume: 5.0 μL (sample concentration: 0.1% (wt / vol))
Eluent: Liquid A: MeOH / H ₂ O = 8/1 (v / v), Liquid B: CHCl ₃ / MeOH = 8/1 (v / v)
Gradient: A / B = 80/20 → 0/100 (28 min); Flow rate: 0.7 mL / min
First, a calibration curve of elution time versus DS was created by HPLC analysis of a sample with known DS in the range of 0 to 3 acetyl DS (acetyl group total substitution degree). Based on the calibration curve, the elution curve (time vs. detected intensity curve) of the unknown sample is converted to the DS vs. detected intensity curve (composition distribution curve), and the uncorrected half-value width X of this composition distribution curve is determined. The corrected half width Z of the distribution was determined.
Z = (X ² −Y ² ) ^1/2
Y is a device constant defined by the following equation.
Y = (a−b) x / 3 + b
a: X value of a sample with acetyl DS = 3 b: X value of a sample with acetyl DS = 0 x: Acetyl DS of unknown sample
From the corrected half width Z, the composition distribution index (CDI) was determined by the following formula.
CDI = Z / Z ₀
Here, Z ₀ is the composition distribution generated when acetylation and partial deacetylation in the preparation of all partially substituted cellulose acetates occur with equal probability for all hydroxyl groups (or acetyl groups) of all molecules. Yes, defined by

DPw: weight average polymerization degree p: (acetyl DS of unknown sample) / 3
q: 1-p
The CDI of the water-soluble cellulose acetate thus determined was 1.4.

(Measurement of weight average degree of polymerization (DPw), degree of dispersion (DPw / DPn))
The weight average polymerization degree and dispersion degree of cellulose acetate were determined by conducting GPC-light scattering measurement under the following conditions after being led to propionylated cellulose acetate.
Equipment: GPC “SYSTEM-21H” manufactured by Shodex
Solvent: Acetone Column: 2 GMHxl (Tosoh), same guard column Flow rate: 0.8 ml / min
Temperature: 29 ° C
Sample concentration: 0.25% (wt / vol)
Injection volume: 100 μl
Detection: MALLS (multi-angle light scattering detector) (manufactured by Wyatt, “DAWN-EOS”)
MALLS correction standard substance: PMMA (molecular weight 27600)
The water-soluble cellulose acetate thus obtained had a DPw of 180 and a DPw / DPn of 1.9.

Examples 1-4, Comparative Example 1
(Preparation of graphene oxide dispersion)
Graphene oxide was prepared by the method described in Journal of the Electrochemical Society, 160 (11), F1175-F1178 (2013). More specifically, first, 2.0 g of graphite powder (Wako Pure Chemical, special grade), 2.0 g of sodium nitrate (Wako Pure Chemical, special grade), and 92 ml of 98 wt% sulfuric acid (Wako Pure Chemical, special grade). Mixed at about 0 ° C. To this mixture, a total of 12 g of potassium permanganate (Wako Pure Chemicals, special grade) was added in small portions. The mixture was heated to 95 ° C. and stirred for 45 minutes to oxidize the graphite powder. The reaction mixture was diluted with distilled water, and 30 wt% aqueous hydrogen peroxide was added to 5 ml to neutralize the remaining potassium permanganate. The solid in the mixture was collected by centrifugation at 3,000 rpm, washed successively with 5 wt% aqueous hydrochloric acid (Wako Pure Chemicals, special grade) and distilled water, and after removing hydrochloric acid, it was dried at 70 ° C for 7 days. This solid was dispersed in distilled water and treated in an ultrasonic bath for 2 hours. This dispersion was centrifuged at 10,000 rpm, and the supernatant was used as a graphene oxide dispersion. The graphene oxide concentration of the graphene oxide dispersion was adjusted to 4 mg / ml.

(Preparation of graphene oxide paper)
A predetermined amount (described in Table 1) of the above water-soluble cellulose acetate was added to 1,000 parts by weight of a graphene oxide dispersion having a concentration of 4 mg / ml and stirred at room temperature (about 22 ° C.) for 24 hours (in Comparative Example 1) The following operation was carried out without adding water-soluble cellulose acetate). This dispersion was filtered under reduced pressure with a membrane filter (pore size: 0.025-0.4 μm), and sheet-like graphene oxide was obtained on the membrane filter. Subsequently, water-soluble cellulose acetate was removed from the sheet-like sample by adding a predetermined amount of a predetermined solvent to the sheet-like graphene oxide and filtering under reduced pressure on the same membrane filter (conditions are described in Table 1). The sheet sample on the membrane filter was dried under reduced pressure at 70 ° C. until a constant weight was obtained, thereby preparing graphene oxide paper. The thickness of the graphene oxide paper was adjusted to 55 ± 4 μm.

Evaluation test (1) Measurement of proton conductivity in normal direction The proton conductivity in the normal direction of graphene oxide paper obtained in Examples and Comparative Examples was measured by a two-terminal method (reference: “Sigma-Aldrich” FIG. 3b of “Introduction of Basics of Solid Oxide Fuel Cell and Its Material Evaluation Method” in “Basics of Materials Science (No. 2)”; Journal of The Electrochemical Society, 160 (11) F1175 (2013)). FIG. 1 shows a schematic diagram of a method for measuring proton conductivity in the normal direction of graphene oxide paper. In FIG. 1, 1 is a disk-shaped electrode, 2 is an electrolyte membrane (graphene oxide paper), D is a film thickness (cm), and an interelectrode distance. The resistance R (Ω) is obtained by the above method, and the proton conductivity σ _x (S / cm) of the electrolyte membrane (graphene oxide paper) can be obtained by the following formula. A (cm ² ) is the area of the disk-shaped electrode.
σ _x = D / (A · R)
The normal proton conductivity was measured under the conditions of a temperature of 25 ° C. and a relative humidity of 8%, 14%, 31%, 50%, and 95%. Table 2 shows the logarithm (logσ _x ) of the obtained proton conductivity σ _x (S / cm) in the normal direction.

(2) Measurement of oxygen permeability The oxygen permeability of the graphene oxide paper obtained in the examples and comparative examples was determined according to the "Proposal of targets / research and development issues and evaluation methods for solid polymer fuel cells" (Measurement according to JIS K7126-2 gas chromatographic method) in accordance with item III-1-2 (gas permeability measurement method), and using helium as the carrier gas. ). The measurement conditions are as follows.
[Gas permeation measurement (wet condition)]
Method: Isobaric method Test piece size (permeation surface diameter): 10 to 150 mmΦ
Number of tests: N = 3
Measurement temperature: 80 ° C
Relative humidity: 95%
Measurement gas: Oxygen Carrier gas: Helium FIG. 2 shows a schematic diagram of a method for measuring oxygen permeability of graphene oxide paper. In FIG. 2, 3 is a measuring gas (oxygen), 4 is a carrier gas (helium), 5 is a valve, 6 is a bubbling device, 7 is a flow meter, 8 is a permeation cell, 9 is a measuring tube, 10 is a gas chromatograph, 11 And 12 are constant temperature baths. The permeation cell 8 has a configuration of chamber / test piece (graphene oxide paper) / chamber. The gas that permeates the test piece in the permeation cell 8 is carried to the measuring tube by the carrier gas, and the entire capacity of the measuring tube 9 is introduced into the gas chromatograph 10 for quantitative analysis. And oxygen permeability [cm < ³ > / (s * cm < ² >)] is calculated | required by a following formula.
Oxygen permeability = oxygen permeation amount [cm ³ / s] / permeation cell effective area [cm ² ] In order to determine the oxygen permeation amount, the relationship between the oxygen amount and gas chromatographic detection sensitivity is separately investigated.
The results are shown in Table 2. The product name “Nafion 212” has an oxygen permeability (80 ° C., relative humidity 95%) of 5 × 10 ⁻⁵ cm ³ / (s · cm ² ) [Research Report of Kanagawa Industrial Technology Center, No. 15, pp64 (2009)].

(3) Trial calculation of proton transport resistance (Ω · cm ² ) at an assumed film thickness Proton transport resistance (Ω · cm ² ) at an assumed film thickness of graphene oxide paper obtained in Examples and Comparative Examples by the following method Was estimated. In the preparation of the membrane material, since the material cost of the membrane material increases when the film thickness is large, it is preferable that the film thickness is small from this viewpoint. On the other hand, when the film thickness is thin, desired performance (such as gas barrier properties) may not be achieved, but the other limiting factor is the handleability of the film material. That is, if the film material is too thin, it is too soft as a self-supporting film and is difficult to handle, or is easily broken during handling. From this viewpoint, it is preferable to secure a film thickness of 20 μm. In the trial calculation of proton transport resistance, although the material of the present invention has excellent oxygen barrier properties and can be made thin, the film thickness is assumed to be 20 μm in consideration of handleability. As for Nafion calculated as a comparison, it is considered that a minimum film thickness of 100 μm is required from the viewpoint of oxygen barrier properties, and therefore the assumed film thickness was set to 100 μm.
Proton transport resistance (Ω · cm ² ) was calculated by the following equation. Proton transport resistance [Ω · cm ² ] = assumed film thickness [cm] / proton conductivity [S / cm]
The results are shown in Table 3. Note that Nafion data is Nano Lett. Based on values described in 2010, 10, 3785-3790 (30 ° C., RH 90%).

1 Disc electrode 2 Electrolyte membrane (graphene oxide paper)
D Film thickness (distance between electrodes) (cm)
3 Measurement gas (oxygen)
4 Carrier gas (helium)
5 valve 6 bubbling device 7 flow meter 8 permeation cell 9 measuring tube 10 gas chromatograph 11 thermostat 12 thermostat

Claims (7)

  A sheet-like graphene oxide having a logarithm of proton conductivity (S / cm) in the normal direction at a relative humidity of 95% and 25 ° C. of −3.0 or more.   The sheet-like graphene oxide according to claim 1, wherein the logarithm of proton conductivity (S / cm) in the normal direction at a relative humidity of 95% and 25 ° C. is −2.5 or more.   The electrolyte membrane comprised with the sheet-like graphene oxide of Claim 1 or 2.   A fuel cell comprising the electrolyte membrane according to claim 3.   A method for producing the sheet-like graphene oxide according to claim 1 or 2, wherein the dispersion medium and the water-soluble polymer are removed from the graphene oxide dispersion containing the water-soluble polymer to obtain the sheet-like graphene oxide. A method for producing sheet-like graphene oxide.   The method for producing sheet-like graphene oxide according to claim 5, wherein the dispersion medium and the water-soluble polymer are removed by filtering and washing the graphene oxide dispersion containing the water-soluble polymer.   The method for producing sheet-like graphene oxide according to claim 5 or 6, wherein the water-soluble polymer is cellulose acetate having a total degree of acetyl substitution of 0.5 to 1.1.

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