JP2022122209A - diffusion layer - Google Patents

Abstract

To provide a low-cost diffusion layer excellent in corrosion resistance and electroconductivity.SOLUTION: The diffusion layer comprises a porous substrate made from an electroconductive material, a first coating film made from titanium phosphide formed on one face of the substrate, and a second coating film made from noble metal, a noble metal alloy including two or more species of noble metal, or an electroconductive noble metal oxide, formed on the other face of the substrate. The titanium phosphide preferably is of a composition represented by MxTiyP (provided that M is one or more metal elements selected from the group consisting of Ni, Fe, Mn, Cr, and Co, 0.6≤x≤1.4, and 0.6≤y≤1.4). The thickness of the first coating film preferably is not smaller than 30 nm and not larger than 500 nm, and the thickness of the second coating film preferably is not smaller than 30 nm and not larger than 500 nm.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a diffusion layer, and more particularly to a diffusion layer used for a gas diffusion layer of a polymer electrolyte fuel cell (PEFC), a gas diffusion layer of a polymer electrolyte membrane (PEM) water electrolysis device, and the like.

A polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in which electrodes (catalyst layers) containing a catalyst are bonded to both sides of an electrolyte membrane. A gas diffusion layer (GDL) is arranged on both sides of the MEA, and a separator (also called current collector) having a gas flow path is arranged on the outside thereof. A polymer electrolyte fuel cell usually has a structure (fuel cell stack) in which a plurality of single cells each including such an MEA, a gas diffusion layer, and a separator are stacked. When a fuel gas and an oxidant gas are supplied to the anode and cathode of such a fuel cell, respectively, water is produced at the cathode and electric power can be extracted.

On the other hand, a PEM water electrolyzer has almost the same structure as a polymer electrolyte fuel cell, but causes a reaction opposite to that of the polymer electrolyte fuel cell. That is, when water is supplied to the oxygen electrode and electric power is supplied between the electrodes, electrolysis of water proceeds and hydrogen and oxygen can be extracted.

In addition, in the PEM water electrolysis device, the gas diffusion layer is also called a porous transport layer (PTL).
In the present invention, the term "diffusion layer" refers to a generic term for these, that is, a generic term for members inserted between the MEA and the separator regardless of the application of the MEA.

In polymer electrolyte fuel cells and PEM water electrolyzers, polyperfluorocarbon sulfonic acid membranes are usually used as electrolyte membranes. Therefore, the diffusion layer is exposed to a strongly acidic atmosphere during use. If the surface of the diffusion layer is oxidized during use and a high-resistance layer is formed on the contact surface with the electrode (catalyst layer) or separator, electrode reaction or electrolytic reaction is inhibited.

In order to solve this problem, various proposals have been conventionally made.
For example, Non-Patent Document 1 discloses an anode gas diffusion layer for a PEM water electrolysis cell made of Pt-coated porous titanium.
This document describes that since the potential on the anode side is high, it is necessary to coat the surface of the anode gas diffusion layer made of titanium with Pt in order to improve durability.

As described in Non-Patent Document 1, coating the surface of porous titanium with Pt improves the corrosion resistance of the anode gas diffusion layer. However, the method described in Non-Patent Document 1 uses an expensive noble metal (Pt), resulting in high cost.

In addition, plating has been conventionally used to coat the anode gas diffusion layer with Pt. However, the plating method
(a) Because it is a wet process, it requires a multi-step process including pretreatment (for example, roughening the surface) to facilitate plating and post-treatment such as cleaning.
(b) a wasteful and costly process due to low utilization of expensive precious metal raw materials;
(c) In order to completely cover the entire surface of the substrate made of porous Ti with the Pt coating, the thickness of the Pt coating must be about 1 μm.
There were problems such as
Furthermore, there is no example in the past that has proposed a diffusion layer that can be manufactured by significantly reducing the amount of expensive materials such as noble metals and without using high-cost processes.

Feature SPECIAL REPORTS, Toshiba Review, Vol.73, No.3 (May 2018) 9-12, "Precious Metal Saving Electrodes for PEM Water Electrolysis"

The problem to be solved by the present invention is to provide a diffusion layer which is excellent in corrosion resistance and electrical conductivity and which is inexpensive.

In order to solve the above problems, the diffusion layer according to the present invention comprises:
a porous substrate made of a conductive material;
a first coating made of titanium phosphide formed on one surface of the substrate;
A second coating formed on the other surface of the base material and made of a noble metal, a noble metal alloy containing two or more kinds of noble metal elements, or a noble metal oxide having conductivity.

The separator-side surface of the substrate is exposed to a strongly acidic atmosphere, but does not come into direct contact with the MEA, so it does not require as high an oxidation resistance as the MEA-side surface. Also, titanium phosphide is inferior in oxidation resistance to noble metals, but has sufficient corrosion resistance and electrical conductivity to protect the surface on the separator side. Furthermore, a coating made of titanium phosphide can be deposited by a relatively low-cost sputtering method.
Therefore, if a first coating made of titanium phosphide is formed on the separator-side surface of the base material, and a second coating made of a noble metal, a noble metal alloy, or a noble metal oxide is formed on the MEA-side surface, corrosion resistance and electrical conductivity can be improved. An excellent diffusion layer can be obtained at low cost.

FIG. 2 is a graph showing the relationship between current density and cell voltage of PEM water electrolysis cells obtained in Example 1 and Comparative Examples 1 to 3;

An embodiment of the present invention will be described in detail below.
[1. diffusion layer]
The diffusion layer according to the present invention is
a porous substrate made of a conductive material;
a first coating made of titanium phosphide formed on one surface of the substrate;
A second coating made of a noble metal, a noble metal alloy containing two or more kinds of noble metal elements, or a noble metal oxide having conductivity is formed on the other surface of the base material.

[1.1. Base material]
The base material is a porous member made of a conductive material. As noted above, the diffusion layer is interposed between the MEA and the separator. Therefore, the base material
(a) conductivity to the extent that electrons can be transferred between the MEA and the separator;
(b) It must have porosity to the extent that raw materials and fuel can be supplied from the separator to the MEA and reaction products can be discharged from the MEA to the separator.

The material of the base material is not particularly limited as long as it exhibits such functions. Examples of materials for the base material include:
(a) metal meshes, foam metals or sintered metals made of various metals such as titanium or titanium alloys, stainless steel, aluminum or aluminum alloys;
(b) porous carbon sheets such as carbon paper and carbon non-woven fabric;
and so on.

Among these, titanium or a titanium alloy forms a passivation film mainly composed of TiO ₂ on the surface under oxidizing conditions. Therefore, even if a part of the surface of the substrate is exposed, there is an advantage that titanium ions and the like are less likely to be eluted.
Stainless steel has the advantage of being inexpensive and having excellent workability.
Aluminum, aluminum alloys, and polymeric materials are advantageous in that they are inexpensive, lightweight, and have excellent workability.

[1.2. First coating]
[1.2.1. material]
The first coating consists of titanium phosphide. In the present invention, "titanium phosphide" means a compound containing Ti, metal elements M other than Ti, and P.

The titanium phosphide is particularly preferably a compound having a composition represented by the following formula (1).
_{MxTiyP (} 1)
however,
M is any one or more metal elements selected from the group consisting of Ni, Fe, Mn, Cr, and Co;
0.6≦x≦1.4, 0.6≦y≦1.4.

The metal element M is preferably Ni, Fe, Mn, Cr, or Co. All of these metal elements M have the effect of improving the corrosion resistance and electrical conductivity of titanium phosphide. The titanium phosphide may contain any one of these metal elements M, or may contain two or more of them.

In formula (1), x represents the ratio of the number of atoms of the metal element M to the number of P atoms. If x is too small, Ti or TiP may form and the electrical conductivity and/or corrosion resistance may decrease. Therefore, x is preferably 0.6 or more. x is preferably 0.7 or more, more preferably 0.8 or more.
On the other hand, if x becomes too large, MP may be generated and the electrical conductivity and/or corrosion resistance may decrease. Therefore, x is preferably 1.4 or less. x is preferably 1.3 or less, more preferably 1.2 or less.

In formula (1), y represents the ratio of the number of Ti atoms to the number of P atoms. If y is too small, contact resistance increases. Therefore, y is preferably 0.6 or more. y is preferably 0.7 or more, more preferably 0.8 or more.
On the other hand, if y becomes too large, the contact resistance will rather increase. Therefore, y is preferably 1.4 or less. y is preferably 1.3 or less, more preferably 1.2 or less.

Any titanium phosphide having a predetermined composition has high corrosion resistance in a fuel cell environment or a water electrolyzer environment and also has high electrical conductivity, and is therefore suitable for the first coating. The first coating may contain any one of these titanium phosphides, or may contain two or more.
Preferably, the titanium phosphide constituting the first coating contains, as the metal element M, at least one element selected from the group consisting of Ni, Fe, and Mn. All of these titanium phosphides have both high durability and high electrical conductivity.

The first coating preferably consists essentially of titanium phosphide, but may contain other phases as long as they do not impede high corrosion resistance and high electrical conductivity.
Other phases include, for example,
(a) unavoidable impurities;
(b) highly corrosion resistant materials other than titanium phosphide;
and so on.

[1.2.2. thickness]
The thickness of the first coating is not particularly limited, and an optimum thickness can be selected according to the purpose. In general, if the thickness of the first coating is too thin, sufficient corrosion resistance cannot be obtained. Therefore, the thickness of the first coating is preferably 30 nm or more. The thickness of the first coating is preferably 40 nm or more, more preferably 50 nm or more.
On the other hand, if the thickness of the first coating is too thick, the adhesion to the substrate may be reduced, and peeling or cracking may occur. Therefore, the thickness of the first coating is preferably 500 μm or less. The thickness of the first coating is preferably 300 nm or less, more preferably 200 nm or less.

[1.2.3. Formation position of the first coating]
A first coating is formed on one surface of the substrate. In the present invention, "one side" refers to the separator-side side of the polymer electrolyte fuel cell or the PEM water electrolysis device. The first coating may be formed on the entire one surface of the base material, or may be formed on a part thereof.

The separator is usually provided with flow paths for the flow of fuel for power generation, oxidant, raw materials for electrolysis, or reaction products. Generally, the diffusion layer does not contact the separator on all of one surface, but contacts only the protruding portion for forming the flow path. Even if a high-resistance layer is formed on the non-contact surface of the base material with respect to the separator, it does not significantly hinder the transfer of electrons. Therefore, the first coating may be formed only on the contact surface with the separator among the one surfaces.

In order to suppress an increase in contact resistance due to the formation of the high-resistance layer, the first coating preferably covers 90% or more of the contact surface with the separator in one surface of the substrate. The first coating preferably covers 95% or more, more preferably 99% or more of the contact surface with the separator. The first coating preferably completely covers the contact surface with the separator.

[1.3. Second Coating]
[1.3.1. material]
The second coating consists of a noble metal, a noble metal alloy containing two or more noble metal elements, or a noble metal oxide having electrical conductivity. The second coating is formed on the MEA-side surface of the substrate surface. The reason why a noble metal, a noble metal alloy, or a noble metal oxide is used as the material of the second coating is that the second coating is required to have higher corrosion resistance than the first coating.

Specifically, the material for the second coating is
(a) noble metals consisting of Au, Ag, Pt, Pd, Rh, Ir, Ru, or Os;
(b) precious metal alloys containing two or more precious metal elements such as Pt--Pd alloys, Pt--Rh alloys, Pt--Ir alloys, Pd--Ir alloys, Pt--Au alloys;
(c) conductive noble metal oxides such as platinum oxide, rhodium oxide, iridium oxide and palladium oxide;
The second coating may consist of any one of these, or may consist of a mixture of two or more.

[1.3.2. thickness]
The thickness of the second coating is not particularly limited, and an optimum thickness can be selected according to the purpose. In general, if the thickness of the second coating is too thin, sufficient corrosion resistance cannot be obtained. Therefore, the thickness of the second coating is preferably 30 nm or more. The thickness of the second coating is preferably 40 nm or more, more preferably 50 nm or more.
On the other hand, if the thickness of the second coating is too thick, the adhesion to the substrate may be reduced, and peeling or cracking may occur. Moreover, if the thickness of the second coating is too thick, the cost will increase. Therefore, the thickness of the second coating is preferably 500 μm or less. The thickness of the second coating is preferably 300 nm or less, more preferably 200 nm or less.

[1.3.3. Formation position of the second coating]
A second coating is formed on the other side of the substrate. In the present invention, "the other side" refers to the MEA side of the polymer electrolyte fuel cell or the PEM water electrolysis device. The diffusion layer usually has a size equal to or larger than that of the catalyst layer of the MEA. Also, normally, no gap is intentionally formed between the MEA and the diffusion layer. Therefore, the second coating is preferably formed on 90% or more of the other surface of the substrate. The second coating preferably covers the entire surface of the other surface.

In order to suppress an increase in contact resistance due to the formation of the high resistance layer, the second coating preferably covers 90% or more of the contact surface with the MEA on the other surface of the substrate. The second coating preferably covers 95% or more, more preferably 99% or more of the contact surface with the MEA. The second coating preferably completely covers the contact surface with the MEA.

[1.4. Application]
The diffusion layer according to the present invention is
(a) Anode-side gas diffusion layer or cathode-side gas diffusion layer for polymer electrolyte fuel cells (b) Oxygen electrode-side gas diffusion layer or hydrogen electrode-side gas diffusion layer for PEM water electrolysis equipment can be done.
The diffusion layer according to the present invention is particularly suitable as a gas diffusion layer on the oxygen electrode side of a PEM water electrolysis device. Since the oxygen electrode side gas diffusion layer is exposed to a high potential, the durability of the PEM water electrolysis device can be improved by applying the diffusion layer according to the present invention to the oxygen electrode side gas diffusion layer.

[2. Diffusion layer manufacturing method]
A method for manufacturing a diffusion layer according to the present invention includes:
A first step of forming a first coating made of titanium phosphide on one surface of a base material using a dry coating method (A);
A second coating of a noble metal, a noble metal alloy containing two or more noble metal elements, or a conductive noble metal oxide is formed on the other surface of the substrate using the dry film forming method (B). It has a process.

[2.1. First step]
First, a first coating made of titanium phosphide is formed on one surface of the substrate using the dry film forming method (A) (first step). In the present invention, the type of dry film formation method (A) is not particularly limited as long as it is pinhole-free, thin, and uniform in thickness. The dry film forming method (A) includes, for example, a sputtering method, a PVD method, an ion plating method, and a vapor deposition method.
In particular, the sputtering method is suitable as a method for forming the first coating because it can form a thin and uniform first coating at low cost and can easily form a large-area film.

[2.2. Second step]
Next, a second coating made of a noble metal, a noble metal alloy containing two or more noble metal elements, or a conductive noble metal oxide is formed on the other surface of the substrate using the dry film forming method (B). (Second step). In the present invention, the type of dry film forming method (B) is not particularly limited as long as it is pinhole-free, thin, and uniform in thickness. Examples of the dry film forming method (B) include a sputtering method, a PVD method, an ion plating method, and a vapor deposition method. The dry film formation method (B) may be the same as the dry film formation method (A), or may be different.
In particular, the sputtering method is suitable as a method for forming the second coating because it can form a thin and uniform second coating at low cost and can easily form a large-area film.

[2.3. Third step]
In the method for manufacturing a diffusion layer according to the present invention, after the first step and before the second step,
The method may further include a third step of heat-treating the first coating at a temperature of 500° C. or higher and 800° C. or lower in an inert atmosphere or a reducing atmosphere to improve the crystallinity of titanium phosphide.

When the first film is formed using the dry film forming method (A), the crystallinity of the first film may be lowered and the electrical conductivity may be lowered. On the other hand, if heat treatment is performed after forming the first coating, the crystallinity of the first coating may be improved. Further, the heat treatment may improve the crystallinity of the titanium phosphide, further improving the electrical conductivity of the first coating. Therefore, heat treatment of the first coating may be performed as necessary.

As for the heat treatment temperature, it is preferable to select an optimum condition according to the purpose. Generally, if the heat treatment temperature is too low, the heat treatment cannot be completed within a practical time. In addition, it may not be possible to improve the crystallinity of the titanium phosphide. Therefore, the heat treatment temperature is preferably 500° C. or higher. The heat treatment temperature is preferably 600° C. or higher, more preferably 650° C. or higher.
On the other hand, if the heat treatment temperature is too high, the substrate may be damaged. Therefore, the heat treatment temperature is preferably 800° C. or lower. The heat treatment temperature is preferably 780° C. or lower, more preferably 740° C. or lower.

The optimum heat treatment time is selected according to the heat treatment temperature. In general, the higher the heat treatment temperature, the shorter the time it takes to complete the crystallinity improvement treatment of titanium phosphide. A suitable heat treatment time depends on the heat treatment temperature, but is usually about 2 hours to 10 hours.

[3. action]
Diffusion layers used in polymer electrolyte fuel cells and PEM water electrolysis devices are required to have high corrosion resistance because they are exposed to a strongly acidic atmosphere during use. Conventionally, in order to solve this problem, a Pt coating has been formed on both surfaces of the diffusion layer substrate using a plating method. However, in order to form a pinhole-free film using the plating method, the thickness of the Pt film had to be about 1 μm. Therefore, conventional diffusion layers are expensive.

On the other hand, the separator-side surface of the substrate is exposed to a strongly acidic atmosphere, but does not come into direct contact with the MEA, so it does not require as high oxidation resistance as the MEA-side surface. Also, titanium phosphide is inferior in oxidation resistance to noble metals, but has sufficient corrosion resistance and electrical conductivity to protect the surface on the separator side. Furthermore, a coating made of titanium phosphide can be deposited by a relatively low-cost sputtering method.
Therefore, if a first coating made of titanium phosphide is formed on the separator-side surface of the base material, and a second coating made of a noble metal, a noble metal alloy, or a noble metal oxide is formed on the MEA-side surface, corrosion resistance and electrical conductivity can be improved. An excellent diffusion layer can be obtained at low cost.

The diffusion layer according to the present invention provides a low-cost fuel cell system and water electrolysis system because it achieves high corrosion resistance and high conductivity despite the fact that the amount of expensive precious metals used is less than conventional ones. can do.
Moreover, since the film is formed by a low-cost dry film-forming method (for example, a sputtering method) instead of a high-cost plating method, not only the material cost but also the process cost are low.

(Example 1, Comparative Examples 1 to 3)
[1. Preparation of sample]
[1.1. Fabrication of Oxygen Electrode Side Diffusion Layer]
[1.1.1. Example 1]
A Ti mesh was used as the base material. NiTiP and Pt were used as sputtering targets for the first coating and the second coating, respectively. A sputtering target folder was filled with targets, and a NiTiP coating was formed on one surface (separator-side surface) of the Ti mesh by a sputtering method, and a Pt coating was formed on the other surface (MEA-side surface). The thicknesses of the NiTiP coating and the Pt coating were each set to 100 nm.

[1.1.2. Comparative Example 1]
A Pt coating was formed on both sides of the Ti mesh using a plating method. The thickness of the Pt coating was 1 μm.

[1.1.3. Comparative Example 2]
A NiTiP coating was formed on both sides of the Ti mesh using a sputtering method. The conditions for forming the NiTiP coating were the same as in Example 1.

[1.1.4. Comparative Example 3]
An uncoated Ti mesh was used as it was as the oxygen electrode side diffusion layer.

[1.2. Preparation of water electrolysis cell]
An oxygen electrode catalyst sheet was prepared by adding an ionomer to IrO ₂ of the oxygen electrode catalyst. Further, an ionomer was added to platinum/carbon (Pt/C) of the hydrogen electrode catalyst to produce a hydrogen electrode catalyst sheet.
Next, the oxygen electrode catalyst sheet and the hydrogen electrode catalyst sheet were transferred to both sides of the electrolyte membrane by a thermal transfer method to produce a membrane electrode assembly (MEA). Furthermore, the oxygen electrode-side diffusion layer and the oxygen electrode-side channel block (separator) were arranged on one surface of the MEA, and the hydrogen electrode-side diffusion layer and the hydrogen electrode-side channel block (separator) were arranged on the other surface. .

[1.1. ] was used. In Example 1, the diffusion layer was placed so that the Pt coating was in contact with the MEA. A Pt-plated Ti member was used for the oxygen electrode side channel block. Further, a carbon member was used for each of the hydrogen electrode-side diffusion layer and the hydrogen electrode-side channel block.
After assembling the PEM water electrolysis cell, the hermeticity was checked by a differential pressure test. After that, it was subjected to a water electrolysis evaluation test.

[2. Test method]
Water electrolysis was performed while the temperature of the PEM water electrolysis cell was kept constant at 80°C. Furthermore, changes in cell voltage were measured when the current density was increased to 3.0 A/cm ² .

[3. result]
FIG. 1 shows the relationship between the current density and the cell voltage of the PEM water electrolysis cells obtained in Example 1 and Comparative Examples 1-3. From FIG. 1, the following can be understood.
(1) Example 1 showed a low voltage almost equivalent to Comparative Example 1 using a conventional Pt plating diffusion layer. In the case of Example 1, the voltage at a current density of 3.0 A/cm ² was about 1.7 V, indicating excellent cell characteristics. It was confirmed that even if the current density increased, the electrical conductivity and corrosion resistance were maintained without corrosion and increased resistance.

(2) By changing the method of forming the Pt film from the plating method to the sputtering method, the film thickness of the Pt film could be reduced to 1/10. Furthermore, by coating only one side with Pt, the amount of Pt used could be halved. As a result, the total amount of Pt used could be reduced to 1/20. The covered area by the sputtering method was about a fraction of that by the plating method. That is, in Example 1, the same water electrolysis performance could be exhibited with a Pt usage amount that was about two orders of magnitude less than in Comparative Example 1.

(3) When a Ti mesh diffusion layer with NiTiP sputtered on both sides was used (Comparative Example 2), the voltage increased to about 2.0 V at a current density of 3.0 A/cm ² . In the uncoated Ti mesh diffusion layer (Comparative Example 3), the cell voltage increased sharply as the current density increased. It is believed that corrosion formed an oxide layer with high resistance on the surface, increasing the resistance.

(Examples 2 to 9)
[1. Preparation of sample]
Instead of the NiTiP target,
(a) CrTiP target (Example 2),
(b) CoTiP target (Example 3),
(c) MnTiP target (Example 4),
(d) FeTiP target (Example 5),
(e) Ni _x Ti _y P target (x=0.72, y=0.74) (Example 6),
(f) Ni _x Ti _y P target (x=1.31, y=1.33) (Example 7),
(g) Ni _x Ti _y P target (x=0.45, y=0.54) (Example 8) or (h) Ni _x Ti _y P target (x=1.67, y=1.56 ) (Example 9)
An oxygen electrode-side diffusion layer was produced in the same manner as in Example 1, except that .
Thereafter, in the same manner as in Example 1, a PEM water electrolysis cell was produced.

[2. Test method]
Water electrolysis was performed while the temperature of the PEM water electrolysis cell was kept constant at 80°C. Furthermore, the cell voltage was measured when the current density was increased to 3.0 A/cm ² .

[3. result]
Table 1 shows the results. Table 1 also shows the results of Example 1 and Comparative Examples 1-3. Table 1 shows the following.
(1) Even when Cr, Co, Mn, or Fe is used as the metal element M, the voltage at a current density of 3.0 A/cm ² is about 1.7 V, showing excellent cell characteristics. rice field.
(2) When the first coating consists of Ni _x Ti _y P, x is 0.6 or more and 1.6 or less and y is 0.6 or more and 1.6 or less (Example 6, In 7), the voltage at a current density of 3.0 A/cm ² was about 1.7 V, showing excellent cell characteristics.

(3) When the first coating consists of NiTiP, when x is less than 0.6 and y is less than 0.6 (Example 8), compared to Examples 1, 6, and 7, increased cell voltage. However, it was found that the increase in resistance due to corrosion was suppressed compared to the uncoated Ti mesh (Comparative Example 3).
(4) Similarly, when the first coating consists of Ni _x Ti _y P and x is greater than 1.4 and y is greater than 1.4 (Example 9), Compared to 1, 6 and 7, the cell voltage increased. However, it was found that the increase in resistance due to corrosion was suppressed compared to the uncoated Ti mesh (Comparative Example 3).

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The diffusion layer according to the present invention can be used as a gas diffusion layer for a polymer electrolyte fuel cell, a gas diffusion layer for a polymer electrolyte membrane (PEM) water electrolysis device, and the like.

Claims (10)

a porous substrate made of a conductive material;
a first coating made of titanium phosphide formed on one surface of the substrate;
A diffusion layer comprising a second coating made of a noble metal, a noble metal alloy containing two or more kinds of noble metal elements, or a noble metal oxide having conductivity, formed on the other surface of the base material. 2. The diffusion layer according to claim 1, wherein said titanium phosphide has a composition represented by the following formula (1).
_{MxTiyP (} 1)
however,
M is any one or more metal elements selected from the group consisting of Ni, Fe, Mn, Cr, and Co;
0.6≦x≦1.4, 0.6≦y≦1.4. 3. The diffusion layer according to claim 1, wherein the first coating has a thickness of 30 nm or more and 500 nm or less. 4. The diffusion layer according to any one of claims 1 to 3, wherein the first coating covers all or part of one surface of the base material. 5. The diffusion layer according to any one of claims 1 to 4, wherein the second coating has a thickness of 30 nm or more and 500 nm or less. 6. The diffusion layer according to any one of claims 1 to 5, wherein the second coating covers 90% or more of the other surface of the base material. 7. The diffusion layer according to any one of claims 1 to 6, wherein the substrate is made of titanium or a titanium alloy. The diffusion according to any one of claims 1 to 7, which is used as a gas diffusion layer of a polymer electrolyte fuel cell (PEFC) or a gas diffusion layer of a polymer electrolyte membrane (PEM) water electrolysis cell. layer. 9. The diffusion layer according to claim 8, wherein the first coating covers 90% or more of the contact surface with the separator in the one surface of the base material. 10. The diffusion layer according to claim 8, wherein the second coating covers 90% or more of the contact surface with the membrane electrode assembly in the other surface of the base material.

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