JP2007080742A - Carbon fiber sheet for solid polymer electrolyte fuel cell and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carbon fiber sheet suitable for a gas diffusion layer of a solid polymer electrolyte fuel cell, and a manufacturing method for the carbon fiber sheet. <P>SOLUTION: The carbon fiber sheet has a thickness of 100 to 350 μm, a basic weight of 50 to 100 g/m<SP>2</SP>, a specific resistance value of 0.2 to 10 mΩ cm, a carbon content rate of 94% of mass or more, an average pore size of 7 to 15 μm, and a gas permeability of 3,000 to 28,500 ml/cm<SP>2</SP>min. For this carbon fiber sheet, the X-ray crystal size of a carbon is preferably 2.1 to 4.8 nm. This carbon fiber sheet contains a polyacrylonitrile-based oxidation fiber, a binder fiber with a residual carbon content rate of 0.5 to 25% of mass, and a resin with a residual carbon content rate of 0.5 to 25% of mass. The carbon fiber sheet can be manufactured in such a method that an oxidation fiber sheet with a total content rate of the binder fiber and resin of 5 to 25% of mass is carbonized at a temperature of 1,400 to 2,300°C under the inert atmosphere. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a carbon fiber sheet suitable for an electrode for a gas diffusion layer of a solid polymer electrolyte fuel cell and a method for producing the same.

  Sheets using carbon fibers (carbon fiber sheets) have been developed and applied as electrodes for gas diffusion layers of solid polymer electrolyte fuel cells.

  In general, a carbon fiber sheet is obtained by mixing carbon fiber short fibers with a binder resin and / or binder fibers to obtain a carbon fiber paper sheet, and a thermosetting resin having a high residual carbon ratio at the time of carbonization is added to the paper sheet. It is made to contain 30 to 80%, and is obtained by thermosetting and molding to obtain an intermediate base material, and firing this intermediate base material at a high temperature of 1000 ° C. or higher in an inert gas atmosphere (for example, Patent Document 1). 2).

  In Patent Document 1, the carbon fiber papermaking sheet is used for an electrode material of a fuel cell or the like by carbonizing an intermediate base material obtained by containing 40 to 80% by mass of a foam-containing thermosetting resin. A technique for producing a porous carbon material is disclosed.

  In Patent Document 2, a carbon fiber papermaking sheet is impregnated with a mixture of a thermosetting resin and carbon powder to obtain an intermediate base material (of a thermosetting resin with respect to 100 parts by mass of carbon fibers in the intermediate base material). Content 30-250 parts by mass, carbon powder content 10-160 parts by mass), by carbonizing this intermediate substrate, for gas diffuser (pore diameter 25-55 μm) of a solid polymer electrolyte fuel cell A technique for manufacturing a porous carbon plate is disclosed.

  In a fuel cell, hydrogen, which is a reaction gas, and oxygen (or air) are reacted. This reaction produces water. The fuel cell extracts this reaction energy as electric energy. The electrode of the fuel cell is required to have conductivity in order to take out the electric energy, and to have a gas diffusion layer in order to allow the reaction gas to pass through the electrode and supply the reaction gas to the reaction part having the catalyst. As the gas diffusion layer, a material having as good a gas permeability as possible is required, and its improvement is being promoted.

  As one method for improving gas permeability, there is a method of enlarging the pore diameter through which the gas formed on the electrode passes. However, if the pore diameter is too large or the distribution is non-uniform, the amount of diffusion of the reaction gas that permeates through the gas diffusion layer and diffuses into the reaction section becomes non-uniform, resulting in a contact efficiency between the catalyst and the reaction gas. There is a problem that the battery performance is lowered.

As described above, none of the conventional manufacturing methods has obtained a carbon material for a gas diffusion layer having smaller pores and excellent gas permeability.
JP-A-10-167855 (paragraph numbers [0001], [0010] to [0020]) JP 2004-311431 A (paragraph numbers [0001], [0041] to [0051])

While the present inventor has made various studies in order to solve the above problems, the present inventors have learned the following and have completed the present invention.
1. By using polyacrylonitrile (PAN) -based oxidized fiber as a raw material,
(1) Oxidized fibers have many oxygen-containing surface functional groups on the fiber surface, high hydrophilicity, and good water dispersibility, so that a paper having good fiber dispersibility can be obtained.
(2) The fiber diameter after firing becomes thinner, contributing to the formation of uniform voids (pores) between the fibers.
2. After making the oxidized fiber using fibers having a binder effect and a low residual carbon ratio, a resin having a binder effect and a low residual carbon ratio is contained in an amount of 5 to 25% by mass in the total amount of the fiber and the resin. After obtaining an impregnated sheet and heat-compressing the resin impregnated sheet to obtain an oxidized fiber sheet, by carbonizing the oxidized fiber sheet, most of the resin and fibers disappear and the interfibers are lost. It remains only at the intersection, and as a result, uniform voids (pores) can be generated, and the resulting carbon fiber sheet is suitable as an electrode for a gas diffusion layer of a solid polymer electrolyte fuel cell.

  Accordingly, an object of the present invention is to provide a carbon fiber sheet for a solid polymer electrolyte fuel cell and a method for producing the same, which have solved the above problems.

  The present invention for achieving the above object is described below.

[1] Thickness of 100 to 350 μm, basis weight of 50 to 100 g / m ² , specific resistance value of 0.2 to 10 mΩ · cm, carbon content of 94% by mass or more, average pore diameter of 7 to 15 μm, gas permeation A carbon fiber sheet for a solid polymer electrolyte fuel cell having a property of 3000 to 28500 ml / cm ² · min.

  [2] The carbon fiber sheet for a solid polymer electrolyte fuel cell according to [1], wherein the carbon X-ray crystal size is 2.1 to 4.8 nm.

  [3] A polyacrylonitrile-based oxidized fiber and a binder fiber having a residual carbon ratio of 0.5 to 25% by mass are mixed to obtain an oxidized fiber paper sheet, and then the resulting oxidized fiber paper sheet has a residual carbon ratio of 0.5. After impregnating ˜25 mass% resin with 5-25 mass% in the total amount of the binder fiber and resin to obtain a resin-impregnated sheet, the resin-impregnated sheet is subjected to heat compression treatment to obtain an oxidized fiber sheet, Thereafter, the oxidized fiber sheet is carbonized at 1400 to 2300 ° C. in an inert atmosphere, and a method for producing a carbon fiber sheet for a solid polymer electrolyte fuel cell.

  The carbon fiber sheet for a solid polymer electrolyte fuel cell of the present invention has a small average pore size and good gas permeability, and is therefore suitable for a gas diffusion layer of a solid polymer electrolyte fuel cell.

  According to the method for producing a carbon fiber sheet for a solid polymer electrolyte fuel cell of the present invention, a PAN-based oxidized fiber is used as a raw material, and a fiber or a resin having a binder effect and having a low residual carbon ratio at the time of papermaking or after papermaking. Since a predetermined amount is contained to obtain an oxidized fiber sheet and the sheet is carbonized, the carbon fiber sheet having the above physical properties can be easily obtained.

  Hereinafter, the present invention will be described in detail.

The carbon fiber sheet for a solid polymer electrolyte fuel cell of the present invention has a thickness of 100 to 350 μm, a basis weight of 50 to 100 g / m ² , a specific resistance value of 0.2 to 10 mΩ · cm, and a carbon content of 94 mass. % Or more, the average pore diameter is 7 to 15 μm, and the gas permeability is 3000 to 28500 ml / cm ² · min.

  In general, the larger the average pore diameter of the carbon fiber sheet, the better the gas permeability. However, if the average pore diameter (A) is too large, the contact efficiency of the reaction gas to the base material (fiber surface) decreases, and the battery performance decreases. Therefore, the average pore diameter (A) is preferably 15 μm or less. When the average pore diameter (A) is less than 7 μm, the pores are blocked due to condensation of the generated water, resulting in a decrease in gas permeability and a decrease in battery performance.

  If the gas permeability (B) is outside the range indicated by the above formula, the battery performance is degraded.

  When the thickness of the carbon fiber sheet is less than 0.10 mm, the sheet strength decreases. When the thickness of the carbon fiber sheet exceeds 0.35 mm, the conductivity in the thickness direction decreases. Increased surface fluff.

When the basis weight of the carbon fiber sheet is less than 30 g / m ² , the sheet strength decreases. When the basis weight of the carbon fiber sheet exceeds 100 g / m ² , it becomes difficult to produce a thin layer sheet having the above thickness.

The bulk density of the carbon fiber sheet is preferably 0.25 to 0.45 g / m ³ . When the bulk density of the carbon fiber sheet is less than 0.25 g / m ³ , the sheet strength decreases. Conductivity decreases. When the bulk density of the carbon fiber sheet exceeds 0.45 g / m ³ , the sheet strength decreases. Increased surface fluff.

  The specific resistance value of the carbon fiber sheet is preferably low, but it is difficult to produce a carbon fiber sheet having a resistivity of less than 0.2 mΩ · cm. When the specific resistance value of the carbon fiber sheet exceeds 10 mΩ · cm, the battery performance decreases.

  When the carbon content of the carbon fiber sheet is less than 94% by mass, the electrical conductivity decreases. Fiber degradation is likely to occur during long-term battery operation.

  The strength of the carbon fiber sheet is preferably 6 to 40 N / cm. When the strength of the carbon fiber sheet is less than 6 N / cm, the handleability of the sheet is lowered. A carbon fiber sheet having a strength exceeding 40 N / cm is difficult to produce.

  The carbon X-ray crystal size of the carbon fiber sheet is preferably 2.1 nm to 4.8 nm. When the carbon X-ray crystal size of the carbon fiber sheet is less than 2.1 nm, the conductivity is reduced. When the carbon X-ray crystal size of the carbon fiber sheet exceeds 4.8 nm, fine powder tends to be generated.

  The carbon fiber sheet for a solid polymer electrolyte fuel cell of the present invention is not particularly limited in its production method as long as its physical properties are within the above range. For example, the PAN-based oxidized fiber and the carbon fiber sheet remaining during carbonization are not limited. Oxidized fiber paper sheet is obtained by mixing with binder fiber having a low charcoal ratio, and this oxidized fiber paper sheet is impregnated with a resin solution having a low residual carbon ratio during carbonization to obtain a resin-impregnated sheet. It can be manufactured by compression molding to obtain an oxidized fiber sheet and carbonizing the oxidized fiber sheet.

  Hereinafter, an example of the manufacturing method of the carbon fiber sheet for solid polymer electrolyte fuel cells of the present invention will be described in detail.

[Oxidized fiber of raw material]
A PAN-based oxidized fiber is used as a kind of oxidized fiber as a raw material for producing a carbon fiber sheet. By using this PAN-based oxidized fiber, a high-strength material can be obtained.

  The PAN-based oxidized fiber of the raw material fiber is made infusible and flame-retardant by causing a cyclization reaction, for example, by treating commercially available PAN-based fiber in air at a temperature of 200 to 300 ° C., increasing the amount of oxygen bonds. What is obtained by the flameproofing process to be used can be used.

  The PAN-based fiber is prepared by spinning, washing, drying, or drawing a spinning solution containing a homopolymer of acrylonitrile or a copolymer obtained by polymerizing a monomer containing 95% by mass or more of acrylonitrile in a wet or dry-wet spinning method. It can also be obtained by performing such processing. As the monomer to be copolymerized, methyl acrylate, itaconic acid, methyl methacrylate, acrylic acid and the like are preferable.

  The average cotton length (cut length) of the PAN-based oxidized fiber is preferably 3 to 15 mm. When the average cotton length is less than 3 mm, the oxidized fiber is difficult to be entangled with each other, so that the strength of the obtained carbon fiber sheet is lowered. When average cotton length exceeds 15 mm, the uniform dispersibility of a fiber will fall and the intensity | strength of the oxidized fiber sheet and carbon fiber sheet obtained in connection with it will fall.

  The fineness of the PAN-based oxidized fiber is preferably 0.5 to 1.5 dtex, more preferably 0.9 to 1.3 dtex. When the fineness is less than 0.5 dtex, the openability of the oxidized fibers is poor, and it is difficult to uniformly disperse the oxidized fibers at the time of blending. When the fineness exceeds 1.5 dtex, a high-strength carbon fiber sheet cannot be obtained.

  The dry strength of the PAN-based oxidized fiber is preferably 5 mN / dtex or more. The higher the dry strength, the higher the strength of the obtained oxidized fiber sheet and carbon fiber sheet. When the dry strength is less than 5 mN / dtex, fiber breakage occurs frequently when the sheet is produced, and papermaking is difficult.

[Binder fiber]
When producing an oxidized fiber papermaking sheet, PAN-based oxidized fibers and binder fibers are mixed. The preferable binder fiber content in the oxidized fiber papermaking sheet is 3 to 20% by mass. As the binder fiber, one having a residual carbon ratio of 0.5 to 25% by mass in the subsequent carbonization treatment is used. Specifically, polyester (PET), nylon, rayon, vinylon, olefin fiber, etc. are used.

[Oxidized fiber paper sheet]
The oxidized fiber papermaking sheet obtained by the above mixed papermaking has a thickness of 0.10 to 0.35 mm. When the thickness of the oxidized fiber papermaking sheet is less than 0.10 mm, the sheet strength decreases. The strength of the carbon fiber sheet obtained from the oxidized fiber papermaking sheet also decreases. When the thickness of the oxidized fiber papermaking sheet exceeds 0.35 mm, the conductivity in the thickness direction of the obtained carbon fiber sheet is lowered.

Basis weight of the oxidized fiber paper sheet is 50 to 250 g / m ^2. When the basis weight of the oxidized fiber papermaking sheet is less than 50 g / m ² , the sheet strength decreases. When the basis weight of the oxidized fiber paper sheet exceeds 250 g / m ² , it is difficult to produce a thin layer of oxidized fiber paper sheet and carbon fiber sheet having the above thickness.

(Resin for impregnation)
As the resin for impregnating the oxidized fiber papermaking sheet, a resin having a residual carbon ratio of 0.5 to 25% by mass is used similarly to the binder fiber. When this impregnating resin is used, the oxidized fiber papermaking sheet can be continuously dipped using polyvinyl alcohol (PVA), carboxymethyl cellulose, methyl methacrylate resin or the like as an aqueous solution or aqueous dispersion.

  The total content of the binder fiber in the oxidized fiber papermaking sheet and the resin for impregnation into the oxidized fiber papermaking sheet is 5 to 25% by mass. When the total content of the binder fiber and the resin for impregnation is less than 5% by mass, the sheet strength is lowered, and problems occur in the compression process and carbonization process in the subsequent process using this sheet. Manufacturing is difficult. When the total content of the binder fiber and the resin for impregnation exceeds 25% by mass, the sheet strength is reduced during the carbonization treatment. The obtained carbon fiber sheet has an average pore diameter (A) larger than 15 μm. The variation in the pore diameter increases, and the average pore diameter (A) and gas permeability (B) deviate from the range of the above formula.

[Compression heat treatment]
The resin-treated oxidized fiber papermaking sheet (resin-impregnated sheet) is subjected to compression heat treatment to obtain an oxidized fiber sheet as an intermediate raw material for carbonization treatment. Depending on the thermal properties of the resin impregnating the binder fiber and the oxidized fiber paper sheet to be mixed at the time of oxidized fiber papermaking, the optimum conditions for the compression heat treatment are slightly different, but the following conditions are preferable.

  The treatment temperature during the compression heat treatment is 100 to 350 ° C, preferably 110 to 250 ° C. When processing temperature is less than 100 degreeC, effects, such as a shaping property improvement, strength improvement, and thinning, about the obtained oxidized fiber sheet are not acquired. When the treatment temperature exceeds 350 ° C., fiber properties such as formability and strength are deteriorated with respect to the obtained oxidized fiber sheet. When the treatment temperature exceeds 350 ° C., there is a risk of causing problems such as heat storage or ignition during the compression heat treatment.

  The treatment pressure during the compression heat treatment is 0.5 to 50 MPa, preferably 2 to 30 MPa. When the treatment pressure is less than 0.5 MPa, effects such as improvement in formability, strength improvement, and thinning of the obtained oxidized fiber sheet cannot be obtained. When the treatment pressure exceeds 50 MPa, fiber properties such as formability and strength are deteriorated with respect to the obtained oxidized fiber sheet. Although the compression heat treatment atmosphere is not particularly limited, an atmosphere containing 20% by volume or less of oxygen is preferable.

[Carbonization treatment]
The carbon fiber sheet for a solid polymer electrolyte fuel cell of the present invention can be obtained by firing and carbonizing the oxidized fiber sheet subjected to the compression heat treatment in an inert atmosphere.

  The carbonization treatment is performed at a maximum temperature of 1400 to 2300 ° C. in an inert atmosphere such as nitrogen, helium, or argon. In addition, the temperature increase rate in the case of carbonization under temperature increase is preferably 200 ° C./min or less. When the maximum temperature during the carbonization treatment is less than 1400 ° C., effects such as improvement in characteristics inherent to carbon fibers, that is, improvement in heat resistance, strength, and electrical conductivity are not exhibited. When the maximum temperature during the carbonization treatment exceeds 2300 ° C., the fiber strength is deteriorated, and fine powder is frequently generated along with the deterioration. The carbonization treatment time at the maximum temperature is preferably 0.5 to 20 minutes.

  During the carbonization treatment, 50 to 60% of the oxidized fibers in the oxidized fiber sheet remain in the form of fibers and remain in terms of mass. On the other hand, the binder fiber is carbonized while melting, and 0.5 to 25% remains in terms of mass. The melted binder fiber exhibits an effect (binder effect) for connecting carbon fibers generated by carbonization. Due to this binder effect, the shapeability and strength of the carbon fiber sheet are improved, and a thinner carbon fiber sheet is obtained.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. Each physical property was measured by the following method.

[Total content of binder fiber and resin]
From the input mass (blending amount) of oxidized fiber and binder fiber in the mixing process during papermaking, the resin impregnation amount (blending amount) during resin treatment, the following formula:
[(Binder fiber content) + (Resin content)] × 100 / [(Oxidized fiber content) + (Binder fiber content) + (Resin content)]
Was used to calculate the total content (% by mass) of the binder fiber and the resin.

[Fiber characteristics: fineness, average fiber length (cut length)]
Measured based on JIS L 1015.

[Strength of oxidized fiber sheet and carbon fiber sheet]
The breaking strength when pulling at a gripping interval of 100 mm and pulling speed of 30 mm / min was defined as strong (N / cm).

[Sheet thickness]
The thickness when a 1.2 N load (61.9 kPa) was applied in the thickness direction with a circular pressure plate having a diameter of 5 mm was measured.

[Sheet weight]
It calculated from the mass value after drying a felt of 200 mm × 250 mm at 120 ° C. for 1 hour.

[Sheet bulk density]
It was calculated from the felt basis weight and the felt thickness.

[Remaining charcoal rate]
The residual carbon ratio (mass%) was calculated from the mass change when the temperature was raised from room temperature to 900 ° C. by thermal mass spectrometry (TGA), with a nitrogen flow rate of 100 ml / min, a temperature increase rate of 10 ° C./min.

[Average pore diameter]
A volume-based median pore diameter was determined by mercury porosimetry using a mercury porosimeter (PoreMaster-60 manufactured by Quantachrome), and this was defined as the average pore diameter (μm).

[Gas permeability]
In accordance with JIS P 8117, an aeration flow rate (ml / cm ² · min) per minute at 1 cm ² under a pressure of 100 mmH ₂ O was measured by the Gurley method.

[X-ray crystal size]
From the result of wide-angle X-ray measurement at a peak near 2Θ = 26 °, Scherrer's formula Lc (nm) = kλ / β · COSΘ
k: Device constant (0.9 in this measurement)
λ: X-ray wavelength (0.154 nm)
β: half width of peak near 2Θ = 26 ° Θ: peak position (°)
It calculated using.

[Carbon content]
The carbon content (% by mass) of the carbon fiber sheet was measured with a CHN coder (Carbo Elba, EA1108, CHNS-0).

[Specific resistance value]
An electric resistance value R (Ω) in the thickness direction when a load of 1 MPa is applied in the thickness direction by sandwiching a carbon fiber sheet with two 50 mm square (10 mm thick) gold-plated electrodes so that the electrodes are in contact with the entire surface. , And the following specific resistance value (Ωcm) = R × (S / L)
S: Contact area 5 × 5 = 25 cm ²
L: Sheet thickness during measurement (load 1 MPa)
Determined by

[Battery characteristics]
The carbon fiber sheet was cut into a 50 cm square, and 0.2 mg / cm ² of catalyst (Pt-Rt) was supported on the carbon fiber sheet. A carbon fiber sheet carrying the catalyst was bonded to both surfaces of the polymer electrolyte membrane (Nafion 117) to form a cell. Hydrogen and air were supplied to the cell, the cell voltage (V) at a temperature of 80 ° C. and a current density of 1.6 A / cm ² was measured, and this measured value was displayed as initial performance (battery characteristics).

[Examples 1 to 4, Comparative Examples 1 to 6]
Under the conditions shown in Tables 1 and 2, PAN-based oxidized fibers and binder fibers were mixed to obtain oxidized fiber paper sheets having a basis weight and thickness shown in Tables 1 and 2. The residual carbon ratio of the binder fiber was 1.2% by mass for PET and 8.5% by mass for cellulose.

  The obtained oxidized fiber papermaking sheet was immersed in an aqueous resin solution (resin concentration 5 mass%) under the conditions shown in Tables 1-2, dried at 120 ° C., and shown in Tables 1-2 for 100 parts by mass of oxidized fibers. The compounding amount was added and pressure treatment was performed at the temperatures and pressures shown in Tables 1 and 2 to obtain oxidized fiber sheets having the basis weight and thickness shown in Tables 1 and 2. The residual carbon ratio of the impregnating resin was 12.5% by mass for PVA and 21.4% by mass for the phenol resin.

  The obtained oxidized fiber sheet was heated at a rate of temperature increase of 100 ° C./min and in a nitrogen stream, and then carbonized at the post-temperature increase temperature shown in Tables 1 and 2 (maximum temperature) and the holding time at the same temperature. A carbon fiber sheet having physical properties shown in ˜2 was obtained.

  As shown in Tables 1 and 2, carbon fiber sheets with good physical properties were obtained in Examples 1 to 4.

  However, in Comparative Example 1, since the fineness of the oxidized fiber was small, the gas permeability was lower than the lower limit value obtained from the average pore diameter, the battery characteristics were low, and a carbon fiber sheet with good physical properties could not be obtained. In Comparative Example 2, since the fineness of the oxidized fiber was large, the gas permeability was higher than the upper limit value determined from the average pore diameter, the battery characteristics were low, and a carbon fiber sheet with good physical properties could not be obtained.

  In Comparative Example 3, since the maximum temperature at the time of carbonization treatment is low, a carbon fiber sheet with low carbon content, small X-ray crystal size, high specific resistance, low battery characteristics, and good physical properties can be obtained. There wasn't. In Comparative Example 4, the amount of the resin impregnated into the oxidized fiber papermaking sheet is large, and since the total content of the binder fiber and the resin in the oxidized fiber sheet is as large as 41% by mass, the average pore diameter is small and the gas permeability is low. Was lower than the lower limit obtained from the average pore diameter, the battery characteristics were low, and a carbon fiber sheet having good physical properties could not be obtained.

  In Comparative Example 5, since the blending amount of the binder fiber and the blending amount of the resin were small, the strength of the oxidized fiber sheet was lowered, the sheet was cut during the carbonization treatment, and the carbon fiber sheet was not obtained. In Comparative Example 6, the amount of the resin impregnated into the oxidized fiber papermaking sheet is large, and since the total content of the binder fiber and the resin in the oxidized fiber sheet is as large as 31% by mass, the average pore diameter is large and the gas permeability is high. Is higher than the upper limit value obtained from the average pore diameter, the battery characteristics are low, and a carbon fiber sheet having good physical properties could not be obtained.

  In Tables 1 and 2, portions indicated by x deviate from the configuration of the present invention.

Claims (3)

Thickness is 100 to 350 μm, basis weight is 50 to 100 g / m ² , specific resistance is 0.2 to 10 mΩ · cm, carbon content is 94% by mass or more, average pore diameter is 7 to 15 μm, and gas permeability is 3000. A carbon fiber sheet for a solid polymer electrolyte fuel cell, which is ˜28500 ml / cm ² · min. The carbon fiber sheet for a solid polymer electrolyte fuel cell according to claim 1, wherein the X-ray crystal size of carbon is 2.1 to 4.8 nm. A polyacrylonitrile-based oxidized fiber and a binder fiber having a residual carbon ratio of 0.5 to 25% by mass are mixed to obtain an oxidized fiber paper sheet, and then the residual carbon ratio of 0.5 to 25 mass is obtained in the obtained oxidized fiber paper sheet. % Resin is impregnated in a total amount of the binder fiber and the resin to obtain a resin-impregnated sheet, and then the resin-impregnated sheet is subjected to heat compression treatment to obtain an oxidized fiber sheet. A method for producing a carbon fiber sheet for a solid polymer electrolyte fuel cell, comprising carbonizing the oxidized fiber sheet at 1400 to 2300 ° C in an active atmosphere.