JP2006102568A - Electrode catalyst, its production method and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode catalyst with excellent joint property, high catalytic activity and a low cost in which eluting of a metal ion from a catalytic particle is less and radical decomposition of peroxides can be suppressed, its production method and a fuel cell using this. <P>SOLUTION: The electrode catalyst is provided with an alloy containing Pt and metal M (provided that M is at least one selected from transition metal element, group III element and rare earth element); and phosphate and/or fluoride of the metal M fixed to the alloy surface. In this case, the content of phosphorus contained in the surface is preferably 0.05 at% or higher. Further, the content of fluorine contained in the surface is preferably 0.15 at% or higher. Further, the fuel cell uses the electrode catalyst. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrode catalyst, a method for producing the same, and a fuel cell, and more specifically, various fuel cells, a water electrolysis device, a hydrohalic acid electrolysis device, a salt electrolysis device, an oxygen and / or hydrogen concentrator, and a humidity sensor. The present invention relates to an electrode catalyst used in various electrochemical devices such as a gas sensor, a manufacturing method thereof, and a fuel cell using the same.

  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 and a solid polymer electrolyte.

  Conventionally, Pt has mainly been used as an electrode catalyst for a polymer electrolyte fuel cell. However, in order to obtain higher catalytic activity, attempts have been made to alloy various Pt by adding various metal elements. . In general, such an alloy catalyst is produced by first supporting Pt fine particles on the surface of a carrier, and then adding a metal salt thereto and heat-treating it. However, the electrode catalyst obtained by such a method usually contains a non-alloyed metal or a compound such as an oxide or hydroxide thereof.

  The inclusion of non-alloyed metal or oxides or hydroxides thereof in the catalyst particles is a cause of hindering adhesion when the catalyst layer and the electrolyte membrane are joined. In addition, since the catalyst layer is exposed to a strong acidic atmosphere in the operating environment of the fuel cell, the metal, which is not alloyed during use, elutes as ions, causing a decrease in the ionic conductivity of the electrolyte. . Further, in the electrode, peroxide is generated as a side reaction of the electrode reaction, but certain metal ions serve as a catalyst for radical decomposition of the peroxide and promote deterioration of the electrolyte.

  In order to solve this problem, various proposals have heretofore been made. For example, Patent Document 1 discloses a method for producing a platinum alloy catalyst in which a metal salt is added to a platinum catalyst, heat treated, acid treated with 3-9 N hydrochloric acid, and unalloyed metal is dissolved and extracted. . This document describes that a stable potential can be maintained for a long time by dissolving and removing unalloyed metal.

  In Patent Document 2, cobalt nitrate is added to platinum-supported carbon and heat-treated at 900 ° C., and this is dispersed in 105 wt% phosphoric acid to elute a part of cobalt. There is disclosed a method for producing a catalyst which is brought into contact with carbon monoxide and further heat-treated at 350 ° C. in a nitrogen atmosphere. In this document, when platinum-supported carbon alloyed by adding a cobalt salt is subjected to phosphoric acid treatment, the surface is made of only platinum, and the phosphoric acid-treated catalyst and carbon monoxide are brought into contact with each other. It is described that when heat treatment, carbon is fixed to vacancies of base metal (cobalt) generated on the platinum surface by phosphoric acid treatment, and lattice strain generated by alloying is stably maintained.

  Furthermore, Patent Document 3 discloses a fuel cell electrode catalyst body obtained by adding iron nitrate to platinum-supporting carbon, heat-treating it at 900 ° C., and acid-treating it with a 1M aqueous sulfuric acid solution. In this document, when the number of atoms of the metal constituting the metal oxide existing on the surface of the catalyst particles without being alloyed with platinum is less than 5% of the number of atoms of platinum, the surface area that becomes the reaction field of the alloy increases. The point that the utilization factor of the alloy is improved and the elution of the metal from the metal oxide is suppressed even when exposed to a strong acidic atmosphere are described.

JP-A-6-246160 JP 2001-52718 A JP 2003-92114 A

When the various methods described above are used, unalloyed metals and the like can be removed from the alloyed catalyst particles. Moreover, this can suppress the adhesion fall at the time of producing MEA and the elution of a metal ion to some extent.
However, it is difficult to completely suppress radical decomposition of peroxide by simply removing unalloyed metal from the alloyed catalyst. Therefore, there are cases where a fuel cell having practically sufficient durability cannot be obtained with an alloy catalyst according to a conventional method.

The problem to be solved by the present invention is to provide an electrode catalyst having excellent bonding properties, high catalytic activity and low cost, a method for producing the electrode catalyst, and a fuel cell using the electrode catalyst.
Further, another problem to be solved by the present invention is that not only the elution of metal ions from the catalyst particles is small, but also the radical decomposition of peroxide can be suppressed, thereby suppressing the deterioration of the electrolyte. An electrode catalyst, a method for producing the same, and a fuel cell using the same.

In order to solve the above problems, an electrode catalyst according to the present invention includes an alloy containing Pt and a metal M (where M is at least one selected from a transition metal element, a group III element, and a rare earth element); The gist of the invention is that it comprises the metal M phosphate and / or fluoride fixed on the alloy surface.
The method for producing an electrode catalyst according to the present invention includes an alloying step for obtaining an alloy containing Pt and a metal M (where M is at least one selected from a transition metal element, a group III element, and a rare earth element). And a phosphating / fluoriding step for bringing the metal M or its alloy not alloyed with Pt into contact with a solution containing phosphate ions and / or fluoride ions. This is the gist.
The gist of the fuel cell according to the present invention is that the electrode catalyst according to the present invention is used.

  The electrode catalyst according to the present invention is excellent in adhesion and catalytic activity because Pt and metal M are alloyed. In addition, when alloying Pt and metal M and then treating the catalyst particles with a solution containing phosphate ions or fluoride ions, if the solution composition is optimized, excess metal M that has not been alloyed or At the same time as the compound is removed, a portion of the unalloyed metal M is fixed to the catalyst particle surface as a phosphate or fluoride. The phosphate or fluoride of the metal M has an action of directly decomposing the peroxide into water and oxygen without radical decomposition. Therefore, the fuel cell using the electrode catalyst according to the present invention exhibits higher durability than the fuel cell using the electrode catalyst obtained by the conventional method.

  Hereinafter, an embodiment of the present invention will be described in detail. The electrode catalyst according to the present invention includes an alloy containing Pt and a metal M, and a phosphate and / or fluoride of the metal M.

A part of the metal M contained in the electrode catalyst is alloyed with Pt, and a part is fixed to the alloy surface as a phosphate or fluoride.
An electrode catalyst containing a metal M that is not alloyed with Pt and not fixed as a phosphate or a fluoride (hereinafter referred to as “excess metal M”) is applied to, for example, a polymer electrolyte fuel cell. If used, the metal M will elute as ions during use, leading to a decrease in the ionic conductivity of the electrolyte membrane or radical decomposition of the peroxide. Therefore, the smaller the content of the surplus metal M, the better. Specifically, the amount of surplus metal M contained in the alloy is preferably 0.1 at% or less, and more preferably 0.05 at% or less.

In addition, the metal M includes transition metal elements ( ₂₁ Sc to ₂₉ Cu, ₃₉ Y to ₄₇ Ag, ₇₂ Hf to ₇₉ Au), group III elements (B, Al, Ga, In, Tl), and rare earth elements ( ₅₇ At least one selected from La to ₇₁ Lu) is preferred. In the alloy, only one of these may be included, or two or more thereof may be included.
Among these, the metal M is preferably at least one selected from Fe, Ni, Co, Cr, Mn, Ti, Ag, Ce, La, Y, and Al. These metals M not only improve the catalytic activity when alloyed with Pt, but these phosphates and fluorides are both sparingly soluble and radically decompose the peroxide. There is an action (ion decomposition action) which makes it decompose directly into water and oxygen.

  As the content of the phosphate fixed on the alloy surface increases, an ionic decomposition reaction of peroxide tends to occur on the alloy surface. In order to obtain an electrode catalyst having a high peroxide decomposition action, the phosphorus content is preferably 0.05 at% or more, more preferably 0.1 at% or more.

  Similarly, as the content of fluoride fixed on the alloy surface increases, the ionic decomposition reaction of peroxide tends to occur on the alloy surface. In order to obtain an electrode catalyst having a high peroxide decomposition action, the fluorine content is preferably 0.15 at% or more, and more preferably 0.30 at% or more.

  Furthermore, both phosphate and fluoride may be fixed on the alloy surface. In this case, in order to obtain an electrode catalyst having a high peroxide decomposing action, it is sufficient that at least one content of phosphorus or fluorine satisfies the above-described conditions.

  The electrode catalyst according to the present invention may be used as it is in a powder state, or may be used by being supported on the surface of an appropriate carrier. The electrode catalyst according to the present invention can also be used in the form of a thin film. Furthermore, the electrode catalyst according to the present invention is particularly suitable as an electrode catalyst for a polymer electrolyte fuel cell, but an electrode catalyst for other types of fuel cells (for example, phosphoric acid fuel cells) or an electrolysis device. It can also be used as an electrode catalyst for various electrochemical devices such as sensors.

  Next, the method for producing the electrode catalyst according to the present invention will be described. The method for producing an electrode catalyst according to the first embodiment of the present invention is a method for producing an electrode catalyst in which an alloy is supported on a carrier, and includes a Pt supporting step, an alloying step, a phosphating / fluorination step. Process.

The Pt carrying step is a step of carrying Pt fine particles on the carrier surface. Specifically, the carrier (Pt-supported carbon) on which the Pt fine particles are supported is
(1) Disperse platinum salt and carrier in water,
(2) If necessary, add a reducing agent to evaporate to dryness.
(3) It can be obtained by heat treatment in a reducing atmosphere.

As the platinum salt, hexahydroxyplatinic acid (H ₂ Pt (OH) ₆ ), platinum sulfite complex, or the like is preferably used. In addition, it is preferable to use a carbon material such as carbon, activated carbon, fullerene, carbon nanophone, or carbon nanotube as the carrier. The amount of Pt supported on the support surface is selected in accordance with the material of the electrode catalyst and the catalyst support, the use of the MEA, the use conditions, and the like. The amount of Pt supported can be arbitrarily adjusted by changing the ratio of the platinum salt dispersed in water and the carrier.
The reducing agent is for reducing platinum ions, and generally an alcohol or an aqueous hydrazine solution is used.
Furthermore, the heat treatment conditions may be any conditions that reduce the platinum salt adsorbed on the support surface. Generally, reduction treatment is performed at 200 ° C. for about 2 hours in a hydrogen atmosphere.

The alloying step is a step of obtaining an alloy of Pt and metal M by further carrying metal M on the surface of the carrier carrying Pt. Specifically, the alloy of Pt and metal M is
(1) Disperse Pt-supported carbon and metal M salt in water;
(2) Ammonia water is added to adjust the pH to 10, and after the metal M hydroxide is precipitated on the surface of the support, the support is filtered,
(3) It is obtained by heat-treating the obtained carrier under an inert atmosphere.

The salt of the metal M may be water-soluble. Examples of the water-soluble metal M salt include nitrate, sulfate, and chloride. The amount of the metal M salt dissolved in the solution is selected in accordance with the composition of the target alloy.
Aqueous ammonia is added to remove unnecessary anions. Ammonia water was added to a solution prepared by dispersing a salt of the metal M, the solution alkaline ^- If (OH high state ionic), the metal M is precipitated as a hydroxide sparingly soluble, it is supported on the surface of the Pt-supported carbon The Since unnecessary anions (for example, nitrate ions) remain in the solution as they are, unnecessary anions can be removed by filtering the solution thereafter. In order to precipitate the hydroxide of metal M, use of another alkaline solution (for example, sodium hydroxide, potassium hydroxide, etc.) is preferable because unnecessary cations may remain on the surface of Pt-supported carbon. Absent.
Further, the optimum heat treatment conditions are selected according to the composition of the target alloy and the type of the metal M. For example, when iron nitrate is used as the metal M salt, heat treatment is preferably performed at 900 ° C. for about 2 hours in an Ar atmosphere.
When a salt of the metal M is supported on the surface of the Pt-supported carbon and heat-treated under predetermined conditions, a part of the metal M is alloyed with Pt, and the other part is an unalloyed metal M or The hydroxide or a compound such as an oxide (excess metal M) is fixed to the alloy surface.

  The phosphating / fluorination step is a step of bringing a carrier carrying an alloy of Pt and metal M into contact with a solution containing phosphate ions and / or fluoride ions.

Specific examples of the solution containing phosphate ions (PO ₄ ³⁻ ) include phosphoric acid (H ₃ PO ₄ ) aqueous solution, ammonium phosphate ((NH ₄ ) ₃ PO ₄ aqueous solution), and sodium phosphate (Na ₃ PO ₄ ) aqueous solution, potassium phosphate (K ₃ PO ₄ ) aqueous solution and the like.
Specific examples of the solution containing fluoride ions (F ⁻ ) include hydrofluoric acid (HF), ammonium fluoride (NH ₄ F) aqueous solution, sodium fluoride (NaF) aqueous solution, potassium fluoride ( KF) and aqueous solution.
The concentration of phosphate ions or fluoride ions in the solution is preferably from 0.5M to 1.0M. When the concentration of phosphate ions or fluoride ions is less than 0.5M, the elution of the metal M into the solution is difficult to occur, and as a result, the phosphate or fluoride of the metal M is hardly precipitated. On the other hand, when the concentration of phosphate ions or fluoride ions exceeds 1.0 M, the phosphate or fluoride of metal M is difficult to deposit on the surface of the carrier.

In this case, a smaller amount of an oxidizing agent (for example, hydrogen peroxide) may be added to the solution. When an appropriate amount of oxidizer is added to the solution, the reaction of surplus metal M fixed on the alloy surface is ionized (oxidation reaction of formula (1) or (3) described later) is shortened, so the processing time is shortened. can do.
Further, when an oxidizing agent is added to the solution, the surplus metal M is oxidized to a high valence ion, and a phosphate or fluoride having a high valence is generated. High valence phosphates or fluorides are less soluble than low valence phosphates or fluorides. For example, when the metal M is Fe, the solubility of Fe (II) iron phosphate (Fe ₃ (PO ₄ ) ₂ ) and Fe (III) iron phosphate (FePO ₄ ) is overwhelming. It is known that the latter is small. Therefore, it is possible to suppress a decrease in battery performance and a decrease in durability due to elution of metal M ions.

“Contacting” is usually performed by dispersing a carrier carrying an alloy in a solution containing phosphate ions or fluoride ions. The temperature at the time of contact is preferably room temperature or higher. The contact time is preferably 1.5 hours or longer.
Further, the carrier may be brought into contact with only one of the solution containing phosphate ions or the solution containing fluoride ions, or may be brought into contact with the other after contacting with either one. Further, the mixed solution containing both phosphate ions and fluoride ions may be brought into contact with the carrier. When the support is sequentially brought into contact with the solution containing phosphate ions and the solution containing fluoride ions, or the support is brought into contact with the mixed solution, an electrode catalyst in which both phosphate and fluoride are fixed is obtained. It is done.

  The precipitation of phosphate or fluoride on the surface of Pt-supported carbon is caused by the excess metal M supported on the surface of Pt eluting into the solution as one end ion, and then phosphate ions or fluoride ions in the solution. It is thought that it reacts with and precipitates on the alloy surface. When a small amount of ethanol is further added to the treated solution, such a reaction is likely to occur, and phosphate or fluoride is more likely to precipitate.

Next, the operation of the manufacturing method according to the present embodiment will be described. When a salt of metal M is supported on Pt-supported carbon and heat-treated at a predetermined temperature, a part of metal M is alloyed with Pt, and the other part becomes surplus metal M and is fixed on the alloy surface. The Next, when this carrier is brought into contact with a solution containing phosphate ions, the surplus metal M fixed on the alloy surface is eluted into the solution as ions of the metal M, and then the phosphate in the solution. It is thought that it reacts with ions and becomes phosphate and is fixed on the alloy surface. When the metal M is divalent in an aqueous solution, this reaction can be expressed by the following formulas (1) and (2).
M → M ²⁺ + 2e ⁻ (1)
3M ²⁺ + 2PO ₄ ^3- → M ₃ (PO ₄ ) ₂ (2)

Similarly, when the support obtained in the alloying step is brought into contact with a solution containing fluoride ions, the surplus metal M fixed on the alloy surface is once released into the solution as ions of the metal M. Then, it is considered that it reacts with fluoride ions in the solution to become fluoride and is fixed to the alloy surface. When the metal M is divalent in an aqueous solution, this reaction can be expressed by the following formulas (3) and (4).
M → M ²⁺ + 2e ⁻ (3)
M ²⁺ + 2F ⁻ → MF ₂ (4)

  Since the electrode catalyst obtained in this way is alloyed with Pt and metal M, it exhibits a higher catalytic activity than a catalyst consisting only of Pt. Although there are various reasons why the catalytic activity is improved by alloying Pt, the electronic state on the surface of Pt is changed by alloying, and the binding force with oxygen atoms is slightly weakened (that is, it is generated by electrode reaction). This is thought to be because water is easily detached from the catalyst surface and the active sites are always exposed.

  Further, if the surplus metal M remains in the electrode catalyst containing an alloy of Pt and the metal M, adhesion may be hindered when the MEA is manufactured. However, in the electrode catalyst according to the present invention, after the heat treatment for alloying is performed, the support is treated with a solution containing phosphate ions or fluoride ions. Few. Therefore, adhesion is not hindered when the MEA is manufactured.

In addition, when an alloy of Pt and metal M is supported on the surface of a support made of a carbon material and excess metal M is removed by acid treatment, the surface of the support is oxidized when treated with an acid having strong oxidizing power such as nitric acid. Generates a hydrophilic group such as a carboxyl group. When such an electrode catalyst is used in the catalyst layer of a polymer electrolyte fuel cell, so-called flooding, in which the three-phase interface in the catalyst layer is blocked by water, is likely to occur.
On the other hand, since the electrode catalyst according to the present embodiment is treated with a solution containing phosphate ions or fluoride ions, there is little possibility that a hydrophilic group is generated on the surface of the carrier. Therefore, even if this is used for the catalyst layer of a polymer electrolyte fuel cell, there is little possibility of flooding.

In addition, when the surplus metal M is exposed to an acidic atmosphere, it is ionized relatively easily as ions. When such an electrode catalyst is used for the catalyst layer of a polymer electrolyte fuel cell, the inside of the catalyst layer is in an acidic atmosphere, so that ions of metal M are eluted from the electrode catalyst. When ions of the metal M diffuse into the electrolyte membrane, the ions exchange with acid groups of the electrolyte membrane, which causes a decrease in the ionic conductivity of the electrolyte membrane. Further, some ions of the metal M have a function of radically decomposing a peroxide generated by a side reaction of the electrode reaction. Peroxide radicals generated by this decomposition cause degradation of even perfluoro-based electrolytes that are said to have excellent oxidation resistance, and cause fluoride ions to elute into the water discharged from the fuel cell. It becomes.
On the other hand, the electrode catalyst according to the present invention fixes the excess metal M as a hardly soluble phosphate or fluoride by treating with a solution containing phosphate ions or fluoride ions. It is possible to suppress the decrease in the ionic conductivity of the electrolyte membrane and the generation of peroxide radicals due to the elution of the ions.

  Furthermore, the phosphate and fluoride of metal M both have the effect of ion decomposition without radical decomposition of the peroxide. Therefore, if this is applied to a polymer electrolyte fuel cell, the peroxide can be decomposed before radicalization, thereby improving durability. Moreover, since the catalyst having such a peroxide decomposing action can be fixed at the same time as removal of the non-alloyed metal M or the like, compared to the case where the peroxide decomposing catalyst is fixed in a separate process, Manufacturing costs can be reduced.

  Next, the manufacturing method of the electrode catalyst which concerns on the 2nd Embodiment of this invention is demonstrated. The manufacturing method according to the present embodiment is a method of manufacturing an electrode catalyst in which an alloy is supported on a support, and includes a Pt supporting process, an alloying process, an acid treatment process, and a phosphating / fluoriding process. I have. Among these, the Pt supporting process, the alloying process, and the phosphating / fluoriding process are the same as those in the first embodiment, and thus the description thereof is omitted.

The acid treatment step is a step in which the support obtained in the alloying step is brought into contact with an acid to remove at least a part of the metal M or its compound that is not alloyed with Pt.
When the amount of surplus metal M is relatively small, in the phosphating / fluorination process described above, these are almost completely removed from the alloy surface and a part thereof is fixed as phosphate or fluoride. Can do. However, when the amount of surplus metal M is relatively large, these may remain only in the phosphating / fluorination step.
In addition, certain types of metal M ions serve as catalysts for promoting the reaction of breaking peroxides into radicals (Fenton reaction). When other metal ions coexist with these ions, the metal ions function as promoters. However, the Fenton reaction may be accelerated.
Therefore, in such a case, before the phosphating / fluorination step, the support is subjected to an acid treatment to remove excess surplus metal M and / or other metal or compound that functions as a promoter. Is preferred.

Specifically, hydrochloric acid, nitric acid, sulfuric acid or the like is preferably used as the acid. These are selected optimally according to the purpose of acid treatment.
For example, it is known that Fe ions serve as a catalyst for promoting the Fenton reaction. On the other hand, Pb ions serve as promoters for Fe ions, and when Fe ions and Pb ions coexist, the Fenton reaction is accelerated. This point has been found for the first time by the present inventors. Therefore, when Fe is used as the metal M to be alloyed with Pt, nitric acid is preferably used in the acid treatment step. In the case where an alloy is supported on a carrier, acid treatment with nitric acid has the disadvantage of hydrophilizing the carrier, but Pb oxides and salts are soluble in nitric acid. Therefore, when the carrier carrying the alloy is treated with nitric acid, the deterioration of the electrolyte due to the elution of Pb ions can be suppressed.

The acid concentration of the treatment solution is preferably 0.5 M or more and 1.0 M or less. If the acid concentration is less than 0.5M, it is difficult to remove excess excess metal M and / or other metals or compounds thereof in a relatively short time. On the other hand, if the acid concentration exceeds 1.0M, the acid becomes too strong, and it becomes difficult to form a phosphate or fluoride of metal M on the alloy surface.
For the same reason, the treatment temperature is preferably room temperature or more, and the treatment time is preferably within 2 hours.

In this case, a smaller amount of an oxidizing agent (for example, hydrogen peroxide) may be added to the solution. When an appropriate amount of oxidant is added to the solution, the reaction of ionizing the metal M fixed on the alloy surface or its hydroxide or oxide is promoted, so that the treatment time can be shortened.
Further, when an oxidizing agent is added to the solution, the metal M is oxidized to a high valence ion, a high valence salt is generated, and a part thereof is fixed to the alloy surface. When a carrier containing such a salt is brought into contact with a solution containing phosphate ions or fluoride ions, a high valence phosphate or fluoride can be formed on the surface of the alloy.

When the carrier carrying the alloy is treated with the above-described acid and then treated with a solution containing phosphate ions and / or fluoride ions, the excess metal M or the salt of the metal M generated by the acid treatment is converted into the metal M It can be fixed as phosphate and / or fluoride.
Since the electrode catalyst obtained by such a method contains an alloy of Pt and metal M, it exhibits high catalytic activity. Moreover, since the surplus metal M is hardly contained, adhesiveness is not inhibited when producing MEA. Furthermore, the elution of metal ions that serve as a catalyst or co-catalyst for accelerating the Fenton reaction is small, and the fixed phosphate and fluoride have an ion decomposing action of peroxide. When used in a fuel cell, it is possible to suppress deterioration in battery performance and durability.

  Next, the manufacturing method of the electrode catalyst which concerns on the 3rd Embodiment of this invention is demonstrated. The method for producing an electrode catalyst according to the present embodiment includes a Pt supporting process, an alloying process, a catalyst layer forming process, and an electrolysis process. Among these, the Pt carrying step and the alloying step are the same as those in the first embodiment, and thus the description thereof is omitted.

The catalyst layer forming step is a step of forming a catalyst layer using the carrier obtained in the alloying step.
For example, when the electrode catalyst according to the present invention is used in a polymer electrolyte fuel cell, a catalyst ink is prepared by adding an electrolyte in the catalyst layer to the support obtained in the alloying step, and this is prepared with a suitable substrate (for example, A polytetrafluoroethylene sheet) to form a catalyst layer.
The electrode of the polymer electrolyte fuel cell may be composed of only the catalyst layer, but may have a two-layer structure of the catalyst layer and the diffusion layer. In this case, the diffusion layer may be formed by directly applying the catalyst ink on the surface of the electrolyte membrane or the diffusion layer.

In a polymer electrolyte fuel cell, the electrolyte membrane generally includes a perfluorinated electrolyte excellent in oxidation resistance (for example, Nafion (registered trademark) manufactured by DuPont, Aciplex (registered trademark) manufactured by Asahi Kasei Corporation), Asahi Glass Flemion (registered trademark) manufactured by Co., Ltd.) is used, but a partial fluorine-based electrolyte containing both a C—F bond and a C—H bond in the polymer chain, or a C—H bond in the polymer chain The use of hydrocarbon-based electrolytes that contain only hydrogen is also being studied. For the electrolyte in the catalyst layer, an electrolyte made of the same material as the electrolyte membrane is usually used, but a different material may be used.
Further, the ratio of the electrode catalyst and the electrolyte in the catalyst layer contained in the catalyst ink is not particularly limited, and an optimal ratio is selected according to the types and applications of the electrode catalyst and the electrolyte in the catalyst layer.

The electrolysis step is a step of electrolysis using a catalyst layer as an anode in an electrolytic solution containing phosphate ions or fluoride ions.
Specifically, examples of the electrolytic solution containing phosphate ion (PO ₄ ³⁻ ) include an aqueous solution of phosphoric acid (H ₃ PO ₄ ), an aqueous solution of ammonium phosphate ((NH ₄ ) ₃ PO ₄ ), sodium phosphate (Na ₃ PO ₄ ) aqueous solution, potassium phosphate (K ₃ PO ₄ ) aqueous solution, and the like.
Specific examples of the electrolyte solution containing fluoride ions (F ⁻ ) include hydrofluoric acid, ammonium fluoride (NH ₄ F) aqueous solution, sodium fluoride (NaF) aqueous solution, and potassium fluoride (KF). There are aqueous solutions.
The concentration of phosphate ions or fluoride ions in the electrolytic solution is not particularly limited, and can be arbitrarily selected according to the type of metal M, characteristics required for the electrode catalyst, and the like.

In general, a polymer electrolyte fuel cell is formed by laminating both surfaces of an MEA with a separator having a gas flow path to form a single cell, and laminating a plurality of such single cells. When anodic decomposition of the catalyst layer, soaking in the electrolyte is
(1) Only the catalyst layer formed on the substrate surface,
(2) MEA (ie, an assembly of electrolyte membrane + catalyst layer (+ diffusion layer)),
(3) a module in which several single cells are stacked;
(4) A battery cell stack in which multiple layers of single cells are laminated,
Either may be sufficient.
However, when electrolysis is performed in the state of a module or battery cell stack, it is necessary to pay attention to ion contamination due to corrosion of metal materials such as piping.

The counter electrode may be (1) insoluble carbon, (2) noble metal such as Pt, Au, or (3) the catalyst layer described above (including the MEA, module, and battery cell stack including the catalyst layer) (Hereinafter referred to as “catalyst layer etc.”) may be used.
When insoluble carbon or noble metal is used as the counter electrode, the energization method is preferably an anodic decomposition method for a certain period of time.
On the other hand, when a catalyst layer or the like is used as the counter electrode, the energization method is preferably PR energization in which the direction of current is alternately switched or AC energization. When PR energization or AC energization is performed, two (or two sets) of catalyst layers can be processed at a time, which is efficient.
It is preferable to use a tool that does not elute ions when energized, or a tool that does not degrade the battery performance even if ions are eluted. Specifically, as the material of the energizing jig, noble metals such as carbon, Pt, and Au, Ti, and the like are preferable.

  Further, the catalyst layer may be electrolyzed using only one of a solution containing phosphate ions or an electrolyte solution containing fluoride ions, or after electrolysis using either one, the other May be used for electrolysis. Furthermore, electrolysis may be performed using a mixed electrolyte containing both phosphate ions and fluoride ions. When the catalyst layer is sequentially electrolyzed using both an electrolyte solution containing phosphate ions and an electrolyte solution containing fluoride ions, or when the catalyst layer is electrolyzed using a mixed electrolyte solution, the phosphate and fluoride An electrocatalyst in which both are fixed is obtained.

When a catalyst layer is formed using a support on which an alloy containing surplus metal M is supported and electrolyzed in an electrolytic solution containing phosphate ions and / or fluoride ions as an anode, the above-mentioned (1 ) And (2) and / or the above-described reactions (3) and (4) occur, and the surplus metal M on the alloy surface becomes phosphate and / or fluoride.
This method is a method of positively forming a poorly soluble phosphate or fluoride as compared with the methods according to the first and second embodiments described above, and it is necessary to add an oxidizing agent to the solution. There is also an advantage that processing can be performed in a short time.

The various methods described above can also be applied to the case where the alloy powder is used as it is without being supported on the carrier, and also when the thin film electrode catalyst is formed.
For example, when Pt fine particles are used without being supported on a carrier, the above-described Pt supporting step is omitted, and in the alloying step, Pt fine particles and metal M are alloyed using Pt fine particles instead of Pt-supported carbon. This may be used for the next step.
Further, for example, when using a thin film electrode catalyst, instead of the above-described Pt supporting step, a Pt thin film is formed on the surface of an appropriate base material (for example, a diffusion layer made of carbon paper, carbon cloth, etc.), and Instead of the alloying step described above, a salt of metal M may be applied and dispersed on the surface of the Pt thin film, alloyed by heat treatment, and used for the next step.

[1. Production of electrode catalyst]
(Comparative Example 1)
5 g of 20 wt% Pt / C was dispersed in an aqueous iron nitrate solution containing 0.5 g of Fe ions. Ammonia water was added thereto to adjust the pH to 10, and the mixture was stirred at room temperature for 3 hours to carry Fe on the Pt / C surface. The solution was then filtered and the support was vacuum dried. Further, the support was heat-treated at 900 ° C. for 2 hours in an Ar atmosphere to obtain a PtFe alloy catalyst.

(Comparative Example 2)
0.5 g of the electrode catalyst obtained in Comparative Example 1 was dispersed in 0.2 liter of 1M nitric acid aqueous solution and stirred at room temperature for 2 hours (solution (B)). Subsequently, this solution (B) was filtered and then washed with water until pH 7 was obtained, whereby an electrode catalyst was obtained.

Example 1
0.5 g of the electrode catalyst obtained in Comparative Example 1 was dispersed in 0.2 liter of 1M phosphoric acid aqueous solution and stirred at room temperature for 2 hours (solution (C)). Next, this solution (C) was filtered and then washed with water until pH 7 was obtained to obtain an electrode catalyst.

(Example 2)
0.5 g of the electrode catalyst obtained in Comparative Example 2 was dispersed in 0.2 liter of 1M phosphoric acid aqueous solution and stirred at room temperature for 2 hours (solution (D)). Subsequently, this solution (D) was filtered and then washed with water until pH 7 was obtained to obtain an electrode catalyst.

  About the filtrate (solution collect | recovered after filtration) of the solution (B)-solution (D) obtained by the comparative example 2 and Example 1, 2, Fe concentration was measured by atomic absorption analysis and Fe eluted by acid treatment The amount of was determined. Table 1 shows the results. Even when phosphoric acid was used, the amount of Fe elution was almost the same as when nitric acid was used, and it can be seen that almost unalloyed Fe was pickled and removed.

Moreover, the composition ratio of the elements on the electrode catalyst surfaces obtained in Comparative Examples 1 and 2 and Example 1 was measured by X-ray photoelectron spectroscopy. Table 2 shows the results.
Compared with Comparative Example 1 where pickling was not performed, Comparative Example 2 pickled with nitric acid and Example 1 pickled with phosphoric acid had a small Fe composition ratio and a large Pt composition ratio. This indicates that unalloyed Fe is eluted and the Pt surface appears. In Example 1, a small amount of P was detected. This is presumably because the catalyst contains hardly soluble iron phosphate precipitated during the acid treatment.

[2. Evaluation of battery characteristics]
(Example 3)
An electrocatalyst (Example 1) washed only with an aqueous phosphoric acid solution was mixed with a Nafion (registered trademark) solution (manufactured by DuPont) to prepare a catalyst ink. This catalyst ink was applied to one surface of a Nafion (registered trademark) film so that the amount of Pt was 0.5 mg per 1 cm ^{2 of} electrode area, thereby forming an air electrode catalyst layer.
Further, platinum-supported carbon (30 wt% Pt / C) not subjected to alloying treatment was mixed with Nafion (registered trademark) solution (manufactured by DuPont) to prepare a catalyst ink. This catalyst ink was applied to the other surface of the Nafion (registered trademark) film so that the amount of Pt was 0.15 mg per 1 cm ^{2 of} electrode area, thereby forming a catalyst layer of the fuel electrode.
The obtained electrolyte membrane-catalyst layer assembly (MEA) was hot-pressed and then set in a 13 cm ² cell together with a carbon cloth serving as a diffusion layer to obtain a polymer electrolyte fuel cell.

(Comparative Example 3)
A polymer electrolyte fuel cell was produced according to the same procedure as in Example 3, except that an air electrode side catalyst layer was produced using an electrode catalyst (Comparative Example 2) washed only with an aqueous nitric acid solution.

The battery characteristics of the two types of cells obtained in Example 3 and Comparative Example 3 were evaluated. The fuel electrode has humid hydrogen with a dew point of 80 ° C. at a rate of 0.5 L / min, and the air electrode has humid air with a dew point of 80 ° C. at a rate of 0.2 MPa and a rate of 1.0 L / mim. The polymer electrolyte fuel cell was operated at an operating temperature of 80 ° C.
FIG. 1 shows the battery voltage at a current density of 1.2 A / cm ² . FIG. 1 shows that the battery voltage of Example 3 is 12% higher than that of Comparative Example 3. This is because the oxidizing power of phosphoric acid is weaker than that of nitric acid, the battery cell of Example 3 is compared with the battery cell of Comparative Example 3, the generation of hydrophilic groups accompanying the oxidation of the carbon surface in the catalyst, and This is considered to be because the hydrophilization of the catalyst layer due to this was suppressed, and the flooding in the high current region was thereby alleviated.

[3. Measurement of fluorine ion elution rate during exposure test]
(Comparative Examples 4, 5 and Example 4)
MEAs using the electrode catalysts obtained in Comparative Example 1, Comparative Example 2 and Example 1 in the catalyst layer on the air electrode side (Comparative Example 4, Comparative Example 5 and Example 4 respectively) And an exposure test was conducted.
The MEA was produced by the following method. That is, each of the electrode catalysts obtained in Comparative Examples 1 and 2 and Example 1 was mixed with a Nafion (registered trademark) solution (manufactured by DuPont) to prepare a catalyst ink. This catalyst ink was applied to one surface of a Nafion (registered trademark) film so that the amount of Pt per 1 cm ^{2 of} electrode area was 0.5 mg, thereby forming an air electrode catalyst layer. The obtained MEA was hot-pressed and then set in a 13 cm ² cell to produce a cell in which a catalyst layer was formed only on the air electrode.

  In the exposure test, the cell temperature was 80 ° C., humidified hydrogen with a dew point of 45 ° C. was applied to the fuel electrode at a rate of 0.25 L / min under 0.1 MPa, and humidified hydrogen with a dew point of 55 ° C. was applied to the air electrode. Each of them was supplied at a rate of 0.5 L / min under 0.1 MPa, and the MEA was exposed to the supply gas for 24 hours. The concentration of fluorine ions contained in the recovered water was measured using ion chromatography, and the fluorine ion elution rate per unit electrode area from the electrolyte membrane was determined.

  FIG. 2 shows the result. When an MEA exposure test is performed, peroxide is generated in the air electrode side catalyst layer due to the reaction between hydrogen permeated from the fuel electrode side and oxygen on the air electrode side, and this peroxide undergoes radical decomposition. As a result, peroxide radicals are generated. When the fluorine-based electrolyte membrane is attacked by peroxide radicals, the electrolyte membrane is decomposed and fluorine ions are generated. That is, the faster the fluorine ion elution rate, the faster the decomposition rate of the electrolyte membrane.

From FIG. 2, the MEA using the electrode catalyst of Comparative Example 2 (Comparative Example 5) has a higher elution rate of fluorine ions than the MEA using the electrode catalyst of Comparative Example 1 (Comparative Example 4). Is 56%, 60% on the air electrode side, and 58% overall. This is considered to be because the decomposition of the film due to the Fenton reaction was suppressed by preliminarily dissolving and removing excess iron that was not alloyed with platinum iron by nitric acid treatment.
2, the MEA using the electrode catalyst of Example 1 (Example 4) has a fluorine ion elution rate of 88% on the fuel electrode side and on the air electrode side as compared with the MEA of Comparative Example 4. It can be seen that it has decreased by 82% and 85% overall. This is because, in addition to the removal of excess iron by the phosphoric acid treatment, part of the iron becomes iron phosphate and precipitates on the catalyst surface, and this iron phosphate promotes the ionic decomposition of hydrogen peroxide. This is probably because the generation of peroxide radicals was suppressed.

  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 electrode catalyst according to the present invention is a polymer electrolyte fuel cell, phosphoric acid fuel cell, water electrolysis device, hydrohalic acid electrolysis device, salt electrolysis device, oxygen and / or hydrogen concentrator, humidity sensor, gas sensor, etc. It can be used as an electrode catalyst used in various electrochemical devices.
In addition, the 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 battery voltage in the current density of 1.2 A / cm < ² > measured about the polymer electrolyte fuel cell obtained in Example 3 and Comparative Example 3. FIG. It is a figure which shows the elution rate of the fluorine ion in the exposure test measured about MEA obtained in Example 4 and Comparative Examples 4 and 5. FIG.

Claims (7)

An alloy containing Pt and a metal M (where M is at least one selected from a transition metal element, a group III element, and a rare earth element);
An electrode catalyst comprising the metal M phosphate and / or fluoride fixed on the alloy surface.   2. The electrode catalyst according to claim 1, wherein when at least the phosphate of the metal M is fixed on the alloy surface, the content of phosphorus contained in the alloy surface is 0.05 at% or more.   3. The electrode catalyst according to claim 1, wherein when at least the fluoride of the metal M is fixed on the alloy surface, the content of fluorine contained in the alloy surface is 0.15 at% or more.   The electrode catalyst according to any one of claims 1 to 3, wherein the metal M is at least one selected from Fe, Ni, Co, Cr, Mn, Ti, Ag, Ce, La, Y, and Al. . An alloying step of obtaining an alloy containing Pt and a metal M (where M is at least one selected from a transition metal element, a group III element, and a rare earth element);
An electrocatalyst comprising a phosphating / fluorination step of bringing the metal M or a compound thereof not alloyed with the Pt into contact with a solution containing phosphate ions and / or fluoride ions. Production method.   6. The acid treatment step according to claim 5, further comprising contacting the alloy with an acid before contacting with the solution to remove at least a part of the metal M or a compound thereof not alloyed with the Pt. A method for producing an electrode catalyst. A fuel cell using the electrode catalyst according to any one of claims 1 to 4.

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