JP2009129646A - Polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polymer electrolyte fuel cell which is less in reduction of battery performance due to deterioration of a catalyst metal. <P>SOLUTION: The polymer electrolyte fuel cell contains one or more ions selected from the group consisting of trisbipyridyl iron ion ([Fe(dpy)<SB>3</SB>]<SP>2+</SP>or [Fe(dpy)<SB>3</SB>]<SP>3+</SP>) and trisphenanthroline iron ion ([Fe(phen)<SB>3</SB>]<SP>2+</SP>or [Fe(phen)<SB>3</SB>]<SP>3+</SP>) in an air electrode catalyst layer containing platinum as an electrode catalyst. The ion exchange ratio of these ions is preferable to be 10% or more. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polymer electrolyte fuel cell, and more particularly to a polymer electrolyte fuel cell suitable as an in-vehicle power source, a stationary small power generator, a cogeneration system, and the like.

  A solid polymer fuel cell has a membrane electrode assembly (MEA) in which electrodes are bonded to both surfaces of a solid polymer electrolyte membrane as a basic unit. In the polymer electrolyte fuel cell, the electrode generally has a two-layer structure of a diffusion layer and a catalyst layer. The diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and carbon paper, carbon cloth, or the like is used. The catalyst layer is a part that becomes a reaction field for electrode reaction, and generally comprises a composite of carbon carrying an electrode catalyst such as platinum and a solid polymer electrolyte.

The electrolyte membrane or the catalyst layer electrolyte that constitutes such an MEA includes a perfluorinated electrolyte excellent in oxidation resistance (an electrolyte that does not contain a C—H bond in the polymer chain. For example, Nafion (registered trademark, DuPont). ), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), etc.) are generally used.
In addition, perfluorinated electrolytes are excellent in oxidation resistance but are generally very expensive. Therefore, in order to reduce the cost of the solid polymer fuel cell, a hydrocarbon electrolyte (an electrolyte that includes a C—H bond and does not include a C—F bond in a polymer chain) or a partial fluorine electrolyte The use of (electrolytes containing both C—H bonds and C—F bonds in the polymer chain) has also been studied.

  However, in order to put the polymer electrolyte fuel cell into practical use as a vehicle-mounted power source, there remain problems to be solved. For example, in order to reduce the cost of a polymer electrolyte fuel cell, it is necessary to reduce the amount of expensive noble metal catalyst such as platinum, and for that purpose, it is necessary to uniformly disperse fine catalyst particles. . However, the noble metal catalyst has a problem that it easily aggregates on the surface of the support.

In order to solve this problem, various proposals have heretofore been made.
For example, Patent Document 1 discloses an electrode catalyst for an air electrode composed of composite fine particles of platinum and cobalt oxide. This document describes that platinum has a high affinity with cobalt oxide, so that elution of platinum into the electrolyte can be suppressed.

  Patent Document 2 discloses an electrode catalyst for a polymer electrolyte fuel cell obtained by annealing a platinum-based noble metal catalyst supported on a conductive carrier at 50 to 90 ° C. in air. This document describes that the durability of the catalyst is improved because a predetermined amount of oxygen is held and fixed on the surface of the platinum-based noble metal catalyst by the annealing treatment.

  Patent Document 3 discloses a cathode catalyst for a phosphoric acid fuel cell that is made of an alloy of platinum and cobalt and has a platinum ratio of 67% to 75% in atomic ratio. This document describes that the alloy phase is stabilized by alloying platinum and cobalt, so that the durability of the catalyst is improved.

Patent Document 4 discloses a catalyst for a phosphoric acid fuel cell made of a Pt—Ti—Co ternary alloy. This document describes that the use of Ti and Co as alloy components can improve both catalytic activity and stability.
Further, Patent Document 5 discloses a catalyst for a phosphoric acid fuel cell made of a Pt—Mo—Co ternary alloy. This document describes that when Mo and Co are used as alloy components, both catalytic activity and stability can be improved.

JP 2006-134613 A JP 2004-349113 A JP 2001-345107 A Japanese Patent Laid-Open No. 5-135773 JP 5-135772 A

In general, platinum group elements (Pt, Pd, Rh, Ru, Ir) used as a catalyst for the air electrode of a fuel cell elute from the catalyst layer during operation and precipitate inside the membrane, or reprecipitate in the catalyst layer. Thus, it is known that grain growth occurs and battery performance decreases. In addition, it is said that the Pt—Ru alloy used as a fuel electrode catalyst also deteriorates in battery performance due to elution or grain growth of Pt and Ru. It is known that the deterioration of these electrodes is promoted by fluctuations in the operating potential.
In order to prevent the deterioration of battery performance due to these, measures as described in Patent Documents 1 to 5 are considered, but it is not sufficient yet.

  The problem to be solved by the present invention is to provide a polymer electrolyte fuel cell in which the decrease in cell performance due to deterioration of the catalyst metal is small.

In order to solve the above problems, a polymer electrolyte fuel cell according to the present invention includes trisbipyridyl iron ions ([Fe (dpy ₃ )] ²⁺ , or [ Fe (dpy) ₃ ] ³⁺ ) and any one or more ions selected from the group consisting of trisphenanthroline iron ions ([Fe (phen) ₃ ] ²⁺ or [Fe (phen) ₃ ] ³⁺ ) It is made to include.

  When trisbipyridyl iron ions and / or trisphenanthroline iron ions are included in the air electrode catalyst layer, elution of platinum from the air electrode catalyst layer can be suppressed. This is presumably because the trisbipyridyl iron ion and / or trisphenanthroline iron ion inhibits the diffusion of platinum ions eluted from the electrode catalyst.

Hereinafter, an embodiment of the present invention will be described in detail.
A polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in which electrodes are bonded to both surfaces of a solid polymer electrolyte membrane. In addition, the polymer electrolyte fuel cell is usually formed by sandwiching both sides of such an MEA with a separator having a gas flow path and laminating a plurality of them.

In the present invention, the material of the solid polymer electrolyte membrane is not particularly limited, and various materials can be used.
That is, the material of the solid polymer electrolyte membrane is a hydrocarbon-based electrolyte containing a C—H bond in the polymer chain and not containing a C—F bond, and a fluorine-based material containing a C—F bond in the polymer chain. Any of electrolytes may be used. Further, the fluorine-based electrolyte may be a partial fluorine-based electrolyte containing both C—H bonds and C—F bonds in the polymer chain, or contains C—F bonds in the polymer chain, and A perfluorinated electrolyte containing no C—H bond may be used.
In addition to the fluorocarbon structure (—CF ₂ —, —CFCl—), the fluorine-based electrolyte includes a chlorocarbon structure (—CCl ₂ —) and other structures (eg, —O—, —S—, —C ( ═O) —, —N (R) —, etc. provided that “R” may be an alkyl group). The molecular structure of the polymer constituting the solid polymer electrolyte membrane is not particularly limited, and may be either linear or branched, or may have a cyclic structure.

  Also, the type of acid group provided in the solid polymer electrolyte is not particularly limited. Examples of the acid group include a sulfonic acid group, a carboxylic acid group, a phosphonic acid group, and a sulfonimide group. Only one of these acid groups may be contained in the solid polymer electrolyte, or two or more of them may be contained. Furthermore, these acid groups may be directly bonded to the linear solid polymer compound, or may be bonded to either the main chain or the side chain of the branched solid polymer compound.

Specifically, as the hydrocarbon electrolyte,
(1) Polyamide, polyacetal, polyethylene, polypropylene, acrylic resin, polyester, polysulfone, polyether, etc. in which an acid group such as a sulfonic acid group is introduced into any of the polymer chains, and derivatives thereof (aliphatic hydrocarbons) System electrolyte),
(2) Polystyrene having an acid group such as a sulfonic acid group introduced into any of the polymer chains, polyamide having an aromatic ring, polyamideimide, polyimide, polyester, polysulfone, polyetherimide, polyethersulfone, polycarbonate, and the like, and These derivatives (partial aromatic hydrocarbon electrolytes),
(3) Polyetheretherketone, polyetherketone, polysulfone, polyethersulfone, polyimide, polyetherimide, polyphenylene, polyphenylene ether, polycarbonate, in which an acid group such as a sulfonic acid group is introduced into any of the polymer chains, Polyamide, polyamideimide, polyester, polyphenylene sulfide, etc., and derivatives thereof (fully aromatic hydrocarbon electrolytes),
Etc. are mentioned as a suitable example.

As the partial fluorine-based electrolyte, specifically, a polystyrene-graft-ethylenetetrafluoroethylene copolymer (hereinafter referred to as “PS”) in which an acid group such as a sulfonic acid group is introduced into any of the polymer chains. -G-ETFE "), polystyrene-graft-polytetrafluoroethylene, and derivatives thereof.
Further, as the perfluorinated electrolyte, specifically, Nafion (registered trademark) manufactured by DuPont, Aciplex (registered trademark) manufactured by Asahi Kasei Co., Ltd., Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., etc. A suitable example is a derivative.

  Furthermore, in the present invention, the solid polymer electrolyte membrane constituting the MEA may be composed only of a solid polymer electrolyte, or a reinforcing material composed of a porous material, a long fiber material, a short fiber material, or the like. It may be a complex containing.

  Among these, fluorinated electrolytes, particularly perfluorinated electrolytes, have a C—F bond in the polymer chain and are excellent in oxidation resistance. Therefore, if the present invention is applied thereto, A polymer electrolyte fuel cell excellent in oxidation resistance and durability can be obtained.

  The electrode constituting the MEA usually has a two-layer structure of a catalyst layer and a diffusion layer, but may be constituted only by the catalyst layer. When the electrode has a two-layer structure of a catalyst layer and a diffusion layer, the electrode is joined to the electrolyte membrane via the catalyst layer.

  The catalyst layer is a part serving as a reaction field for the electrode reaction, and includes an electrode catalyst or a carrier carrying the electrode catalyst, and an electrolyte in the catalyst layer covering the periphery thereof. In general, an optimum electrode catalyst is used depending on the purpose of use of MEA, use conditions, and the like. In the case of a polymer electrolyte fuel cell, the electrode catalyst generally includes platinum, palladium, ruthenium, rhodium, iridium, etc., or an alloy containing one or more of these, or a platinum group element such as platinum and cobalt. Alloys with transition metal elements such as iron and nickel are used. As the amount of the electrode catalyst contained in the catalyst layer, an optimal amount is selected according to the use of MEA, use conditions, and the like.

  The catalyst carrier is for carrying a fine electrode catalyst and simultaneously transferring electrons in the catalyst layer. Generally, carbon, activated carbon, fullerene, carbon nanophone, carbon nanotube or the like is used as the catalyst carrier. As the amount of the electrode catalyst supported on the surface of the catalyst carrier, an optimum amount is selected according to the material of the electrode catalyst and the catalyst carrier, the use of the MEA, the use conditions, and the like.

  The electrolyte in the catalyst layer is for exchanging protons between the solid polymer electrolyte membrane and the electrode. For the electrolyte in the catalyst layer, the same material as that constituting the solid polymer electrolyte membrane is usually used, but a different material may be used. As the amount of the electrolyte in the catalyst layer, an optimal amount is selected according to the use of MEA, use conditions, and the like.

  The diffusion layer is for supplying and receiving a reaction gas to the catalyst layer at the same time as transferring electrons to and from the catalyst layer. Generally, carbon paper, carbon cloth, or the like is used for the diffusion layer. In order to increase water repellency, a surface of carbon paper or the like coated with a mixture of water repellent polymer powder such as polytetrafluoroethylene and carbon powder (water repellent layer) is used as a diffusion layer. Also good.

The polymer electrolyte fuel cell according to the present invention is characterized by further comprising the following configuration in addition to the configuration described above.
(A) The air electrode catalyst layer contains platinum as an electrode catalyst.
(B) In the air electrode catalyst layer, trisbipyridyl iron ion ([Fe (dpy) ₃ ] ²⁺ or [Fe (dpy) ₃ ] ³⁺ ) and trisphenanthroline iron ion ([Fe (phen) ₃ ] ²⁺ or any one or more ions selected from the group consisting of [Fe (phen) ₃ ] ³⁺ ).

In the present invention, the electrode catalyst contained in the air electrode catalyst layer may be composed of only platinum, or an alloy of platinum and a noble metal other than platinum, an alloy of platinum and a transition metal, or a noble metal other than platinum and platinum. And an alloy of a transition metal.
Trisbipyridyl iron ions and trisphenanthroline iron ions are considered to be present in the form of ion exchange with protons of acid groups in the electrolyte in the catalyst layer contained in the air electrode catalyst layer. The amount of these ions contained in the air electrode catalyst layer can be arbitrarily selected according to the purpose. In general, the greater the amount of these ions, the greater the effect of suppressing the elution of platinum.
In order to suppress the elution of platinum, the ion exchange rate by these ions is preferably 10% or more. The “ion exchange rate” refers to the ratio (%) of the number of acid groups ion-exchanged to the number of acid groups contained in the electrolyte in the catalyst layer.

Next, a method for producing a polymer electrolyte fuel cell according to the present invention will be described.
The catalyst layer containing trisbipyridyl iron ions and / or trisphenanthroline iron ions,
(1) A catalyst ink is prepared by dissolving or dispersing an electrode catalyst or an electrode supported on a carbon carrier and an electrolyte in the catalyst layer in an appropriate solvent,
(2) Apply the catalyst ink to a suitable substrate (for example, polytetrafluoroethylene sheet), remove the solvent to make a catalyst sheet,
(3) Immerse the catalyst sheet in a solution containing trisbipyridyl iron ions and / or trisphenanthroline iron ions, and ion-exchange all or part of the acid groups of the electrolyte in the catalyst layer,
(4) cleaning the catalyst sheet,
Can be obtained.
The ion source includes
(1) [Fe (dpy) ₃ ] SO ₄ , [Fe (dpy) ₃ ] Cl ₂ , [Fe (dpy) ₃ ] Br ₂ , [Fe (dpy) ₃ ] I ₂ , [Fe (dpy) ₃ ] (ClO ₄ ) ₃ ,
(2) [Fe (phen) ₃ ] SO ₄ ,
Etc. can be used.
MEA can be obtained by joining the obtained catalyst sheet for air electrode and the catalyst sheet for fuel electrode prepared by a usual method to both surfaces of the electrolyte membrane. Further, if both sides of the MEA are sandwiched by separators having gas flow paths and a required number of them are stacked, the polymer electrolyte fuel cell according to the present invention can be obtained.

Next, the operation of the polymer electrolyte fuel cell according to the present invention will be described.
When trisbipyridyl iron ions and / or trisphenanthroline iron ions are included in the air electrode catalyst layer, elution of platinum from the air electrode catalyst layer can be suppressed. This is presumably because the trisbipyridyl iron ion and / or trisphenanthroline iron ion inhibits the diffusion of platinum ions eluted from the electrode catalyst.

[1. experimental method]
[1.1. Introduction of ions into the air electrode catalyst layer]
A polytetrafluoroethylene sheet coated with a catalyst layer (Pt / carbon / Nafion mixing ratio: 0.45 / 0.55 / 0.41, Pt amount: 0.4 mg · cm ⁻² ) in 50 mL of ultrapure water It was immersed and placed in a desiccator that was evacuated.
A solution containing various ions was added to ultrapure water and allowed to stand at 60 ° C. for 7 hours. Since the affinity for the sulfonic acid group in the electrolyte in the catalyst layer is H ⁺ << “other ions”, the added ion is considered to replace H ⁺ of the sulfonic acid group. The ion species (reagents) added this time are Co ²⁺ (CoSO ₄ ), Ce ³⁺ (Ce ₂ (SO ₄ ) ₃ ), Fe ²⁺ (FeSO ₄ ), Fe ³⁺ (Fe (NO ₃ ) ₃ ). , [Fe (dpy) ₃ ] ²⁺ ([Fe (dpy) ₃ ] SO ₄ ), [Fe (phen) ₃ ] ²⁺ ([Fe (phen) ₃ ] SO ₄ ), VO ²⁺ (VOSO ₄ ) , Ba ²⁺ (Ba (NO ₃ ) ₂ ), and H ⁺ (H ₂ SO ₄ ). The number of moles of added ions was set to be equal to or greater than “the number of moles of sulfonic acid groups in the electrolyte in the catalyst layer / the valence of added cations”.

[1.2. Preparation of membrane electrode assembly]
The catalyst layer after the above treatment is applied to the cathode, and another catalyst layer (Pt / carbon / Nafion mixing ratio: 0.6 / 0.4 / 0.4, Pt amount: 0.2 mg · cm ^−2). These were joined to a perfluorinated electrolyte membrane (hot press conditions: 120 ° C., 15 min, 500 N · cm ⁻² ).

[1.3. An endurance test]
The MEA was assembled in a single cell, the cell temperature was 80 ° C., hydrogen was supplied to the anode, and nitrogen was supplied to the cathode. The supply conditions for both hydrogen and nitrogen were 0.1 L · min ⁻¹ , 0.1 MPa, and RH 100%.
After maintaining the cathode potential at 1.0 V for 30 s, holding at 0.6 V for 30 s was repeated. Cyclic voltammetry (0.09 V⇔0.5 V, 0.05 V · s ⁻¹ ) was performed every 50 minutes during the test to measure the effective platinum surface area (ECSA) of the cathode.

[2. result]
FIG. 1 shows [1.1. ] Shows the change of ECSA with respect to the durability test time of MEA produced by adding H ⁺ . FIG. 1 shows that the ECSA decreases with time. Here, assuming that the ECSA reduction rate is proportional to ECSA (S (t)) at that time, the following equation (1) is obtained.
dS (t) / dt = −kS (t) (1)
When the equation (1) is integrated, the following equation (2) is obtained.
S (t) = S ₀ exp (−kt) (2)
k can be considered to represent the likelihood of catalyst degradation. Hereinafter, this is referred to as a deterioration rate constant. A curve obtained as a result of the least square fitting using the equation (2) is a solid line in FIG.

Similar fitting was performed for other samples, and a degradation rate constant was calculated for each sample. Table 1 shows the results. From Table 1, it can be seen that the deterioration rate constant becomes the smallest when [Fe (dpy) ₃ ] ²⁺ or [Fe (phen) ₃ ] ²⁺ is added.

FIG. 2 shows the relationship between the ion exchange rate and the ECSA retention rate when the electrolyte in the air electrode catalyst layer is ion-exchanged with [Fe (dpy) ₃ ] ²⁺ ions. The “ECSA maintenance rate” refers to the ratio (%) of ECSA after the durability test (test time: 40 hours) with respect to ECSA before the durability test. FIG. 2 shows that the higher the ion exchange rate, the higher the ECSA maintenance rate. It can also be seen that when the ion exchange rate is 10% or more, the ECSA maintenance rate exceeds 40%.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The polymer electrolyte fuel cell according to the present invention can be applied to an in-vehicle power source, a stationary small power generator, a cogeneration system, and the like.

It is a figure which shows the relationship between the durability test time of a catalyst layer processed with H ^<+> , and an effective platinum surface area (ECSA). It is a figure which shows the relationship between the ion exchange rate when the electrolyte in an air electrode catalyst layer is ion-exchanged with [Fe (dpy) ₃ ] ²⁺ ion, and an ECSA maintenance factor.

Claims (2)

In an air electrode catalyst layer containing platinum as an electrode catalyst, trisbipyridyl iron ions ([Fe (dpy) ₃ ] ²⁺ or [Fe (dpy) ₃ ] ³⁺ ) and trisphenanthroline iron ions ([Fe ( phen) ₃ ] ²⁺ or [Fe (phen) ₃ ] ³⁺ ), a polymer electrolyte fuel cell containing any one or more ions selected from the group consisting of:   The solid polymer fuel cell according to claim 1, wherein an ion exchange rate by the ions is 10% or more.

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