Composition for forming cathode catalyst layer of polymer electrolyte fuel cell, and cathode catalyst layer

Abstract

The present invention provides: a novel additive containing an ionic liquid that can be used for a composition for forming a cathode catalyst layer of a polymer electrolyte fuel cell; a novel composition for forming a cathode catalyst; and a novel cathode catalyst layer formed using the same.　The present invention relates to a composition for forming a cathode catalyst layer of a polymer electrolyte fuel cell, the composition containing an ionic liquid represented by formula (1). In the formula, X represents an atomic group containing a nitrogen atom or a phosphorus atom; Y represents a hydrogen atom, a carboxy group, or a sulfonic acid group; Z-represents a hydrogen sulfate anion, a trifluoromethanesulfonate anion, a bis(trifluoromethylsulfonyl)imide anion, a halogen anion, a tetrafluoroborate anion, a hexafluorophosphate anion, a bis(fluorosulfonyl)imide anion, or a nonafluorobutanesulfonate anion; and n represents an integer of 2-4.

Description

Composition for forming a cathode catalyst layer in a polymer electrolyte fuel cell and cathode catalyst layer

[0001] The present invention relates to a composition for forming a cathode catalyst layer in a polymer electrolyte fuel cell and to a cathode catalyst layer.

[0002] Fuel cells are environmentally friendly power generation devices that convert the chemical reaction between hydrogen and oxygen into electrical energy. In particular, polymer electrolyte fuel cells (PEFCs) are being actively developed for large commercial vehicles due to their excellent characteristics such as high power generation efficiency and low-temperature operation capability. The basic structure of a PEFC, the membrane electrode assembly (MEA), includes a cathode (air electrode; including a gas diffusion layer and a cathode catalyst layer), an anode (fuel electrode; including a gas diffusion layer and an anode catalyst layer), and a solid polymer electrolyte membrane. Of these, the oxygen reduction reaction (ORR) that proceeds in the cathode is slow and causes a decrease in MEA performance, so improvements in the performance of the cathode catalyst layer are being investigated.

[0003] Patent Document 1 describes a Pt / C catalyst used as a cathode catalyst, with an ionic liquid (IL) 1-methyl-2,3,4,6,7,8-hexahydro-1H-pyrimido[1,2-a]pyrimidine-9-ium 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonate ([MTBD][C) ４ F ９ SO ３ It has been reported that when the ionic liquid was brought into contact with the catalyst support at a weight ratio (IL / C) of 1.28, 2.56, or 3.84, the mass activity of the catalyst at 0.9 V increased by up to 27% compared to when the ionic liquid was not in contact, and that this result may be due to the hydrophobic coating that the ionic liquid imparts to the catalyst particles.

[0004] Patent Document 2 describes a cathode catalyst and an ionic liquid [MTBD][C] ４ F ９ SO ３It is stated that an MEA containing a composite cathode in which ] is mixed improves the performance of the MEA under both low and high humidity conditions, that under low humidity conditions this may be due to improved proton transport, and under high humidity conditions this may be due to rapid water removal and a decrease in gas diffusion resistance, and that the IL / C ratio is approximately 1:10.

[0005] Patent Document 3 describes a fuel cell membrane assembly having a gas diffusion catalyst electrode layer containing the ionic liquid diethylmethylammonium trifluoromethanesulfonate, diethylmethylammonium nonafluoromethanesulfonate, or diethylmethylammonium heptadecafluorooctanesulfonate, which has high power generation characteristics even in a low humidity environment and an IL / C of 0.40 to 0.60.

[0006] Japanese Patent Publication No. 2018-187618, Japanese Patent Publication No. 2020-109752, Japanese Patent Publication No. 2021-96940

[0007] The present inventors, while working to improve the performance of cathode catalyst layers by focusing on ionic liquids as additives to the cathode catalyst layer, found that the types (structures) of ionic liquids that have been conventionally studied are extremely limited, the amount of ionic liquid to the cathode catalyst support (IL / C) is relatively high, and the degree of freedom in selecting ionic liquids and the cost-effectiveness are insufficient. Therefore, the object of the present invention is to provide a novel additive containing an ionic liquid that can be used in a composition for forming a cathode catalyst layer of a polymer electrolyte fuel cell, as well as a novel composition for forming a cathode catalyst containing an ionic liquid and a novel cathode catalyst layer formed using the same. Another object of the present invention is to provide a composition for forming a cathode catalyst layer of a polymer electrolyte fuel cell and a cathode catalyst layer formed using the same that improve the mass activity of the catalyst even with a small amount of ionic liquid.

[0008] In order to solve the above problems, the present inventors have conducted intensive studies and, as a result, have found a novel composition for forming a cathode catalyst layer of a solid polymer fuel cell, an additive used in the composition, and a novel cathode catalyst layer formed using the composition, which contain an ionic liquid represented by the following formula (1). By containing the ionic liquid represented by the formula (1), it has been found that a composition for forming a cathode catalyst layer and a cathode catalyst layer with improved mass activity of the catalyst can be provided even when the addition amount of the ionic liquid is small. As a result of further research, the present invention has been completed.

[0009] That is, the present invention relates to the following. [1] A composition for forming a cathode catalyst layer of a solid polymer fuel cell, containing an ionic liquid represented by the following formula (1): In the formula, X is an atomic group containing a nitrogen or phosphorus atom, Y is hydrogen, a carboxy group or a sulfonic acid group, and Z － is a hydrogen sulfate anion, a trifluoromethanesulfonate anion, a bis(trifluoromethylsulfonyl)imide anion, a halogen anion, a tetrafluoroborate anion, a hexafluorophosphate anion, a bis(fluorosulfonyl)imide anion, or a nonafluorobutanesulfonate anion, and n is an integer of 2 to 4.

[0010] [2] The composition according to [1] above, wherein X is imidazolium, ammonium, piperidinium, phosphonium, pyrrolidinium, or pyridinium. [3] Z － is a hydrogen sulfate anion, a trifluoromethanesulfonate anion, or a bis(trifluoromethylsulfonyl)imide anion, and the composition according to [1] or [2] above.

[0011] [4] The ionic liquid represented by the formula (1) is 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate ([MSBIM][HSA]), 1-(4-sulfobutyl)-3-methylimidazolium trifluoromethanesulfonate ([MSBIM][TfO]), 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt ([MSBIM][Tf ２N]), 1-(2-carboxyethyl)-3-methylimidazolium bis(I) sulfate ([MCEIm][HSA]), 1-(2-carboxyethyl)-3-methylimidazolium trifluoromethanesulfonate ([MCEIm][TfO]), 1-(3-carboxypropyl)-3-methylimidazolium bis(I) sulfate ([MCPIm][HSA]), 1-(3-carboxypropyl)-3-methylimidazolium trifluoromethanesulfonate ([MCPIm][TfO]), 1-(3-carboxypropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([MCPIm][Tf ２ [N]), N,N-diethyl-3-sulfopropylammonium bis(trifluoromethylsulfonyl)imide salt [DESPA] [Tf ２ N], 1-(3-sulfopropyl)piperidinium bis(trifluoromethylsulfonyl)imide salt [SPPip] [Tf ２ N], 1-(4-sulfobutyl)pyrrolidinium bis(trifluoromethylsulfonyl)imide salt ([SBPyr][Tf ２ [N]), 1-(4-sulfobutyl)pyridinium bis(trifluoromethylsulfonyl)imide salt ([SBPy][Tf ２ [N]), tributyl(4-sulfobutyl)phosphonium bis(trifluoromethylsulfonyl)imide salt [TBSBP] [Tf ２ N], 1-(4-sulfobutyl)-3-methylimidazolium hydrochloride ([MSBIm][Cl]), 1-(4-sulfobutyl)-3-methylimidazolium nonafluorobutanesulfonic acid ([MSBIm][C]) ４ F ９ SO ３ ]), 1-butyl-3-methylimidazolium bis(TfO) ([BMIm][HSA]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm][TfO]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm][Tf ２ [N]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF ４]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF ６ ]), 1-(2-carboxyethyl)-3-methylimidazolium hydrochloride ([MCEIm][Cl]), or 1-(2-carboxyethyl)-3-methylimidazolium perfluorobutanesulfonate ([MCEIm][C ４ F ９ SO ３ The composition according to any one of [1] to [3] above, which is:

[0012] [5] The composition according to any one of [1] to [4], comprising a catalyst metal and a support for the catalyst metal, wherein the weight ratio of the ionic liquid to the support for the catalyst metal is greater than 0 and less than 0.35. [6] The composition according to any one of [1] to [5], comprising a catalyst metal and a support for the catalyst metal, wherein the weight ratio of the ionic liquid to the support for the catalyst metal is greater than 0 and 0.07 or less.

[0013] [7] Cathode catalyst layer of a polymer electrolyte fuel cell containing an ionic liquid represented by the following formula (1): In the formula, X is an atomic group containing nitrogen or phosphorus atoms, Y is hydrogen, a carboxyl group or a sulfonic acid group, and Z － n is a hydrogen sulfate anion, trifluoromethanesulfonate anion, bis(trifluoromethylsulfonyl)imide anion, halogen anion, tetrafluoroborate anion, hexafluorophosphate anion, bis(fluorosulfonyl)imide anion, or nonafluorobutanesulfonate anion, and n is an integer between 2 and 4.

[0014] [8] Additives for compositions for forming the cathode catalyst layer of a polymer electrolyte fuel cell, comprising an ionic liquid represented by the following formula (1): In the formula, X is an atomic group containing nitrogen or phosphorus atoms, Y is hydrogen, a carboxyl group or a sulfonic acid group, and Z －n is a hydrogen sulfate anion, trifluoromethanesulfonate anion, bis(trifluoromethylsulfonyl)imide anion, halogen anion, tetrafluoroborate anion, hexafluorophosphate anion, bis(fluorosulfonyl)imide anion, or nonafluorobutanesulfonate anion, and n is an integer between 2 and 4.

[0015] According to the present invention, by including an ionic liquid represented by formula (I) in the cathode catalyst layer forming composition, the dispersibility of the composition components can be improved, the viscosity of the composition can be reduced, and excellent coating properties can be provided. According to the present invention, by including an ionic liquid represented by formula (I) in the cathode catalyst layer forming composition, excellent coating properties can be provided even with a small amount of ionic liquid. According to the present invention, by including an ionic liquid represented by formula (I) in the cathode catalyst layer forming composition, cracking due to particle displacement during the drying process can be suppressed even with a small amount of ionic liquid. According to the present invention, the cathode catalyst layer formed using such a cathode catalyst layer forming composition is homogeneously formed, preventing, for example, blockage of reaction sites for oxidation-reduction reactions and oxygen gas flow paths due to cracks or irregularities on the film surface, improving oxygen mass transport, and improving the mass activity of the catalyst in a polymer electrolyte fuel cell.

[0016] This is a perspective view of a polymer electrolyte fuel cell. The viscosity change according to shear rate is plotted for a cathode catalyst layer formation composition (I / C = 0.7, IL / C = 0.03) containing ionic liquid [MSBIm] [HSA], prepared by stirring with a planetary ball mill. The viscosity change according to shear rate is plotted for a cathode catalyst layer formation composition (I / C = 0.7, IL / C = 0.03) containing ionic liquid [MCEIm] [TfO], prepared by stirring with a planetary ball mill. The viscosity change according to shear rate is plotted for a cathode catalyst layer formation composition (I / C = 0.7) without ionic liquid, prepared by stirring with a planetary ball mill (comparative example). These are secondary electron images of the cathode catalyst layer surface containing ionic liquid and the cathode catalyst layer surface without ionic liquid, taken using a Hitachi High-tech SEM (SU-9000). This graph plots the change in mass activity of a cathode catalyst layer containing ionic liquids [MSBIm] and [HSA], depending on the IL / C ratio.

[0017] The present invention will be described in detail below based on preferred embodiments of the present invention. The present invention relates to a composition for forming a cathode catalyst layer of a polymer electrolyte fuel cell, comprising an ionic liquid represented by the following formula (1): In the formula, X is an atomic group containing nitrogen or phosphorus atoms, Y is hydrogen, a carboxyl group or a sulfonic acid group, and Z － n is a hydrogen sulfate anion, trifluoromethanesulfonate anion, bis(trifluoromethylsulfonyl)imide anion, halogen anion, tetrafluoroborate anion, hexafluorophosphate anion, bis(fluorosulfonyl)imide anion, or nonafluorobutanesulfonate anion, and n is an integer between 2 and 4.

[0018] The polymer electrolyte fuel cell of the present invention includes one or more membrane electrode assemblies (MEAs) shown in Figure 1. The membrane electrode assemblies include an anode, a polymer electrolyte membrane, and a cathode. The anode includes a gas (hydrogen) diffusion layer and an anode catalyst layer, and the cathode includes a gas (oxygen) diffusion layer and a cathode catalyst layer. In the anode, the oxidation reaction of hydrogen, specifically H ２ →2H ＋ +2e － This process takes place, and in the cathode, an oxygen reduction reaction occurs, specifically, 1 / 2O ２ +2H ＋ +2e － →H ２ Process O is performed. The polymer electrolyte fuel cell can use a number of MEAs that are appropriately adjusted according to the scale in which it is used. In this specification, the catalyst film that becomes the cathode catalyst layer when applied to a polymer electrolyte fuel cell is referred to as the "cathode catalyst layer".

[0019] The composition for forming the cathode catalyst layer of a polymer electrolyte fuel cell in the present invention may include, in addition to the ionic liquid represented by the following formula (1), a desired catalyst metal, a support for the catalyst metal, a polymer ionomer, and / or a solvent, preferably all of these. In this specification, the composition for forming the cathode catalyst layer is also simply referred to as "the composition."

[0020] The catalyst metal is not particularly limited as long as it can promote the reduction reaction in the cathode of a polymer electrolyte fuel cell, but for example, platinum group elements and platinum group alloys can be used. From a kinetic standpoint of the reduction reaction, examples of catalyst metals include platinum (Pt), or alloys containing platinum (Pt) and one or more rare earth elements such as yttrium (Y) or cerium (Ce), transition metals such as cobalt (Co) or nickel (Ni), or tin (Sn), with platinum (Pt) being preferred.

[0021] The catalyst metal support is not particularly limited as long as it can promote the reduction reaction in the cathode of the polymer electrolyte fuel cell together with the catalyst metal, but for example, conductive fine particles can be used. From the viewpoint of surface area and conductivity, porous carbon and ceramic nanoparticles are examples of catalyst metal supports, and porous carbon is preferred. The content of the metal catalyst support in the cathode catalyst layer forming composition is typically 1 to 200 g / L, preferably 10 to 100 g / L, and more preferably 20 to 60 g / L.

[0022] The catalyst metal and the catalyst metal support may be prepared separately, with the catalyst metal being supported on the catalyst metal support, or commercially available catalyst metal supports with the catalyst metal already attached may be used. An example of a commercially available catalyst metal support with the catalyst metal already attached is TEC10E30E (manufactured by Tanaka Kikinzoku Co., Ltd., with a Pt load of 30%, carbon support).

[0023] As polymer ionomers, protons (H) that have moved from the solid polymer electrolyte membrane in polymer electrolyte fuel cells are used. ＋ The polymer is not particularly limited as long as it can promote the movement of ions in the cathode and accelerate the reduction reaction, but for example, an ionic polymer having proton conductivity can be used. Examples of polymer ionomers include fluororesins from the viewpoint of chemical stability and gas permeability, and Nafion (registered trademark), which is a copolymer of perfluorosulfonic acid and polytetrafluoroethylene, is preferred.

[0024] The content of polymer ionomer in the cathode catalyst layer formation composition is typically 1 to 200 g / L, preferably 10 to 100 g / L, and more preferably 20 to 60 g / L, from the viewpoint of preventing aggregation of the ionomer. When the cathode catalyst layer formation composition contains a catalyst metal and a support for the catalyst metal, the weight ratio of polymer ionomer to the support for the catalyst metal is such that protons (H ＋From the viewpoint of ensuring conduction pathways and oxygen diffusion pathways, the ratio is typically 0.1 to 2.0, preferably 0.3 to 1.5, and more preferably 0.6 to 1.3. In this specification, the weight ratio of the polymer ionomer (I) to the catalyst metal support (C) is also referred to as "I / C".

[0025] The solvent is not particularly limited as long as it can disperse the catalyst metal, the catalyst metal support, and / or the polymer ionomer with the ionic liquid, but examples include organic solvents and water. Examples of organic solvents include alcohols such as ethanol, 2-propanol, and 1-propanol, with ethanol being preferred. One or more solvents can be used.

[0026] The ionic liquid used in the present invention is represented by formula (1):

[0027] X is a cationic group of atoms containing nitrogen or phosphorus. Examples of X include imidazolium, ammonium, piperidinium, phosphonium, pyrrolidinium, and pyridinium, with imidazolium, ammonium, piperidinium, and phosphonium being preferred, and imidazolium being even more preferred. Y is hydrogen, a carboxyl group, or a sulfonic acid group, with carboxyl groups and sulfonic acid groups being preferred.

[0028] Z is a hydrogen sulfate anion, a trifluoromethanesulfonate anion, a bis(trifluoromethylsulfonyl)imide anion, a halogen anion, a tetrafluoroborate anion, a hexafluorophosphate anion, a bis(fluorosulfonyl)imide anion, or a nonafluorobutanesulfonate anion. Hydrogen sulfate anion, trifluoromethanesulfonate anion, and bis(trifluoromethylsulfonyl)imide anion are preferred as Z. n is an integer between 2 and 4.

[0029] Specific examples of ionic liquids represented by formula (1) include 1-(4-sulfobutyl)-3-methylimidazolium bisulfate ([MSBIm][HSA]), 1-(4-sulfobutyl)-3-methylimidazolium trifluoromethanesulfonate ([MSBIm][TfO]), and 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([MSBIm][TfO]). ２ N]), 1-(2-carboxyethyl)-3-methylimidazolium bis(I) sulfate ([MCEIm][HSA]), 1-(2-carboxyethyl)-3-methylimidazolium trifluoromethanesulfonate ([MCEIm][TfO]), 1-(3-carboxypropyl)-3-methylimidazolium bis(I) sulfate ([MCPIm][HSA]), 1-(3-carboxypropyl)-3-methylimidazolium trifluoromethanesulfonate ([MCPIm][TfO]), 1-(3-carboxypropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([MCPIm][Tf ２ [N]), N,N-diethyl-3-sulfopropylammonium bis(trifluoromethylsulfonyl)imide salt ([DESPA][Tf ２ [N]), 1-(3-sulfopropyl)piperidinium bis(trifluoromethylsulfonyl)imide salt ([SPPip][Tf ２ [N]), 1-(4-sulfobutyl)pyrrolidinium bis(trifluoromethylsulfonyl)imide salt ([SBPyr][Tf ２ [N]), 1-(4-sulfobutyl)pyridinium bis(trifluoromethylsulfonyl)imide salt ([SBPy][Tf ２ [N]), tributyl(4-sulfobutyl)phosphonium bis(trifluoromethylsulfonyl)imide salt ([TBSBP][Tf ２ N]), 1-(4-sulfobutyl)-3-methylimidazolium hydrochloride ([MSBIm][Cl]), 1-(4-sulfobutyl)-3-methylimidazolium nonafluorobutanesulfonic acid ([MSBIm][C]) ４ F ９ SO ３]), 1-butyl-3-methylimidazolium bis(TfO) ([BMIm][HSA]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm][TfO]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm][Tf ２ [N]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF ４ ]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF ６ ]), 1-(2-carboxyethyl)-3-methylimidazolium hydrochloride ([MCEIm][Cl]), or 1-(2-carboxyethyl)-3-methylimidazolium perfluorobutanesulfonate ([MCEIm][C ４ F ９ SO ３ Examples include: )

[0030] In one embodiment of the present invention, the ionic liquid represented by formula (1) is 1-(4-sulfobutyl)-3-methylimidazolium bisulfate ([MSBIm][HSA]), 1-(4-sulfobutyl)-3-methylimidazolium trifluoromethanesulfonate ([MSBIm][TfO]), and 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([MSBIm][TfO]). ２ N]), 1-(2-carboxyethyl)-3-methylimidazolium bis(sulfate) ([MCEIm][HSA]), 1-(2-carboxyethyl)-3-methylimidazolium trifluoromethanesulfonate ([MCEIm][TfO]), 1-(3-carboxypropyl)-3-methylimidazolium bis(sulfate) ([MCPIm][HSA]), 1-(3-carboxypropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([MCPIm][Tf ２ [N]), N,N-diethyl-3-sulfopropylammonium bis(trifluoromethylsulfonyl)imide salt [DESPA] [Tf ２N], 1-(3-sulfopropyl)piperidinium bis(trifluoromethylsulfonyl)imide salt [SPPip] [Tf ２ N], 1-(4-sulfobutyl)pyrrolidinium bis(trifluoromethylsulfonyl)imide salt ([SBPyr][Tf ２ [N]), 1-(4-sulfobutyl)pyridinium bis(trifluoromethylsulfonyl)imide salt ([SBPy][Tf ２ [N]), tributyl(4-sulfobutyl)phosphonium bis(trifluoromethylsulfonyl)imide salt [TBSBP] [Tf ２ N], 1-butyl-3-methylimidazolium bis(b)([BMIm][HSA]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate([BMIm][TfO]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([BMIm][TfO]) ２ [N]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF ４ ]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF ６ ]), 1-(2-carboxyethyl)-3-methylimidazolium hydrochloride ([MCEIm][Cl]), or 1-(2-carboxyethyl)-3-methylimidazolium perfluorobutanesulfonate ([MCEIm][C ４ F ９ SO ３ ]) is preferable.

[0031] In another embodiment of the present invention, the ionic liquid represented by formula (1) is 1-(4-sulfobutyl)-3-methylimidazolium bisulfate ([MSBIm][HSA]), 1-(4-sulfobutyl)-3-methylimidazolium trifluoromethanesulfonate ([MSBIm][TfO]), and 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([MSBIm][TfO]). ２N]), 1-(2-carboxyethyl)-3-methylimidazolium bis(I) sulfate ([MCEIm][HSA]), 1-(2-carboxyethyl)-3-methylimidazolium trifluoromethanesulfonate ([MCEIm][TfO]), 1-(3-carboxypropyl)-3-methylimidazolium bis(I) sulfate ([MCPIm][HSA]), 1-(3-carboxypropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([MCPIm][Tf ２ N) is preferred.

[0032] Ionic liquids generally possess properties such as high ionic conductivity, excellent electrochemical properties, low volatility, flame retardancy, and high thermal stability. The ionic liquid used in the present invention further has the property of improving the dispersibility of polymer ionomers that can result in high viscosity in the cathode catalyst layer forming composition, thereby improving the overall dispersibility of the components of the cathode catalyst layer forming composition and reducing the viscosity of the cathode catalyst layer forming composition. In addition, the ionic liquid used in the present invention can immediately reduce the viscosity of the cathode catalyst layer forming composition without requiring prolonged stirring of the composition.

[0033] The viscosity (at 25°C) of the cathode catalyst layer forming composition containing the ionic liquid of the present invention is, in one embodiment, measured using, for example, an Anton Paar viscometer MCR 102e, at a shear rate of 1 s. －１ The pressure is between 10 mPa·s and 100 mPa·s, and the shear rate is 100 s. －１ The viscosity is less than 10 mPa·s. In another embodiment, the viscosity (25°C) of the cathode catalyst layer forming composition containing the ionic liquid of the present invention is measured, for example, using a cone-plate viscometer RM100 CP2000 PLUS manufactured by Lamy Rheology, at a shear rate of 2000 s using spindle CP6010. －１ In measurements at a rotational speed of 350 rpm, the pressure is between 2.5 mPa·s and 3.3 mPa·s.

[0034] In one embodiment of the present invention, such a low viscosity of the cathode catalyst layer forming composition containing the ionic liquid of the present invention can be achieved without stirring or with a short stirring time, for example, even if the stirring time is 30 minutes or less. Examples of stirring means when preparing the cathode catalyst layer forming composition containing the ionic liquid of the present invention include ball mills, ultrasonic irradiation, and bead mills. From the viewpoint of processing scale, a ball mill is preferred as the stirring means, and from the viewpoint of fine grinding, ultrasonic irradiation is preferred.

[0035] A cathode catalyst layer-forming composition with reduced viscosity has excellent coating properties and provides a uniform surface when a film is formed. Unlike films made with a cathode catalyst layer-forming composition that have high viscosity, which can cause cracks and irregularities on the surface, creating accumulations of generated water and blocking the reaction active sites and oxygen gas channels in the oxidation-reduction reaction, this composition can smoothly promote the oxidation-reduction reaction and improve the mass activity of the catalyst.

[0036] When the cathode catalyst layer forming composition includes a catalyst metal and a support for the catalyst metal, the weight ratio of the ionic liquid to the support for the catalyst metal is typically greater than 0 and less than 0.35, preferably greater than 0 and less than 0.2, more preferably greater than 0 and less than 0.1, and even more preferably greater than 0 and 0.07 or less, from the viewpoint of preventing blockage of the reaction active sites of the reduction reaction. In this specification, the weight ratio of the ionic liquid (IL) to the support for the catalyst metal (C) is also referred to as "IL / C".

[0037] The present invention also relates to a cathode catalyst layer for a polymer electrolyte fuel cell, comprising an ionic liquid represented by the following formula (1): In the formula, X is an atomic group containing nitrogen or phosphorus atoms, Y is hydrogen, a carboxyl group or a sulfonic acid group, and Z － n is a hydrogen sulfate anion, trifluoromethanesulfonate anion, bis(trifluoromethylsulfonyl)imide anion, halogen anion, tetrafluoroborate anion, hexafluorophosphate anion, bis(fluorosulfonyl)imide anion, or nonafluorobutanesulfonate anion, and n is an integer between 2 and 4.

[0038] The cathode catalyst layer of a polymer electrolyte fuel cell can be formed by drying (removing the solvent from) the above-described cathode catalyst layer forming composition for polymer electrolyte fuel cells. The cathode catalyst layer of a polymer electrolyte fuel cell may contain a catalyst metal, a support for the catalyst metal, and / or a polymer ionomer, preferably all of these. The cathode catalyst layer of a polymer electrolyte fuel cell is substantially solvent-free.

[0039] The definition of the ionic liquid contained in the cathode catalyst layer, and the definitions of the catalyst metal, catalyst metal support, and / or polymer ionomer that the cathode catalyst layer may contain, can basically be applied to the definitions described in the cathode catalyst layer forming composition for polymer electrolyte fuel cells. Furthermore, the respective ratios of the ionic liquid, catalyst metal, catalyst metal support, and / or polymer ionomer in the cathode catalyst layer, e.g., IL / C and I / C, can basically be the values ​​described in the cathode catalyst layer forming composition for polymer electrolyte fuel cells. The amount of catalyst metal per unit area (basis weight) in the cathode catalyst layer should be 0.01 to 2.0 mg / cm² from the viewpoint of catalyst utilization. ２ The concentration is preferably 0.05 to 1.0 mg / cm³. ２ And more preferably 0.1 to 0.5 mg / cm² ２ That is the case.

[0040] The present invention also relates to an additive for a composition for forming a cathode catalyst layer of a polymer electrolyte fuel cell, comprising an ionic liquid represented by the following formula (1): In the formula, X is an atomic group containing nitrogen or phosphorus atoms, Y is hydrogen, a carboxyl group, or a sulfonic acid group, Z- is a bis(trifluoromethylsulfonyl)imide anion, a halogen anion, a tetrafluoroborate anion, a hexafluorophosphate anion, a bis(fluorosulfonyl)imide anion, or a nonafluorobutanesulfonate anion, and n is an integer from 2 to 4.

[0041] Next, the composition for forming a cathode catalyst layer in a polymer electrolyte fuel cell of the present invention and the cathode catalyst layer formed using the same will be described in more detail by the examples and comparative examples described below, but the present invention is not limited to these.

[0042] 1. Ionic liquid used (preparation of ionic liquid or commercially available product used) 50.0 g (0.609 mol) of 1-methylimidazole, 61.9 mL (0.609 mol) of 1,4-butanesultone, and 609 mL of acetonitrile were mixed and heated and stirred at 80°C for 20 hours. The resulting white solid was filtered and washed with acetonitrile to obtain 83.3 g (yield 63%) of 1-(4-sulfobutyl)-3-methylimidazole.

[0043] Preparation of 1-(4-sulfobutyl)-3-methylimidazolium trifluoromethanesulfonate ([MSBIm][TfO]): 28.4 g (0.130 mol) of 1-(4-sulfobutyl)-3-methylimidazole was dissolved in 130 mL of water, and 11.4 mL of trifluoromethanesulfonic acid was added. The mixture was heated and stirred at 80°C for 2 hours. The reaction mixture was filtered, the solvent was concentrated, and dried at 60°C until constant weight was obtained to yield 47.7 g of [MSBIm][TfO] (100% yield) (colorless liquid).

[0044] 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt ([MSBIm][Tf ２ Preparation of [MSBIm][Tf] 28.4 g (0.130 mol) of 1-(4-sulfobutyl)-3-methylimidazole was dissolved in 130 mL of water, and 36.6 g of bis(trifluoromethyl)sulfonylimide was added and heated and stirred at 80°C for 2 hours. The reaction mixture was filtered, the solvent was concentrated and dried at 60°C until constant weight was obtained [MSBIm][Tf] ２ 64.5 g of N was obtained (99% yield) (colorless liquid).

[0045] 53.7 g (0.654 mol) of 1-methylimidazole, 50.2 g (0.696 mol) of β-propiolactone, and 500 mL of water were mixed and heated and stirred at 120°C for 21 hours. After concentrating the water, the mixture was washed with acetone and purified using an activated alumina column (dichloromethane / methanol = 5:1) to obtain 80.7 g (80% yield) of 1-(2-carboxyethyl)-3-methylimidazole.

[0046] Preparation of 1-(2-carboxyethyl)-3-methylimidazolium bisulfate ([MCEIm][HSA]): 20 g (0.130 mol) of 1-(2-carboxyethyl)-3-methylimidazole was dissolved in 130 mL of water, and 13.4 g of concentrated sulfuric acid was added. The mixture was heated and stirred at 80°C for 2 hours. The reaction mixture was filtered, the solvent was concentrated, and the mixture was dried at 60°C until constant weight was obtained to yield 32.1 g of [MCEIm][HSA] (98% yield) (white crystals).

[0047] Preparation of 1-(2-carboxyethyl)-3-methylimidazolium trifluoromethanesulfonate ([MCEIm][TfO]): 20 g (0.130 mol) of 1-(2-carboxyethyl)-3-methylimidazole was dissolved in 130 mL of water, and 11.4 mL of trifluoromethanesulfonic acid was added. The mixture was heated and stirred at 80°C for 2 hours. The reaction mixture was filtered, the solvent was concentrated, and dried at 60°C until constant weight was obtained to yield 38.4 g of [MCEIm][TfO] (97% yield) (colorless liquid).

[0048] 51.1 g (0.623 mol) of 1-methylimidazole and 85.1 g (0.623 mol) of 4-methyl chlorobutyrate were mixed and heated and stirred at 70°C for 30 hours. After washing the reaction mixture with acetone, 57 mL of concentrated hydrochloric acid was added and heated and stirred at 110°C for 8 hours. The solvent was concentrated and the resulting crystals were washed with acetone to obtain 103.1 g (81% yield) of 1-(3-carboxypropyl)-3-methylimidazolium chloride salt.

[0049] Preparation of 1-(3-carboxypropyl)-3-methylimidazolium bisulfate ([MCPIm][HSA]): 20 g (0.098 mol) of 1-(3-carboxypropyl)-3-methylimidazolium chloride was dissolved in 100 mL of water, and 10.1 g of concentrated sulfuric acid was added. The mixture was heated and stirred at 80°C for 2 hours. The reaction mixture was filtered, the solvent was concentrated, and dried at 60°C until constant weight was obtained to yield 26.1 g of [MCPIm][HSA] (100% yield) (colorless liquid).

[0050] 1-(3-carboxypropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt ([MCPIm][Tf ２ Preparation of [MCPIm][Tf] 23.9 g (0.117 mol) of 1-(3-carboxypropyl)-3-methylimidazolium chloride salt was dissolved in 116 mL of water, and 32.8 g of bis(trifluoromethyl)sulfonyliimide was added and heated and stirred at 80°C for 2 hours. The aqueous layer was removed and the mixture was filtered and dried at 60°C until a constant weight was obtained. ２ 44.1 g of N was obtained (84% yield) (colorless liquid).

[0051] N,N-diethyl-3-sulfopropylammonium bis(trifluoromethylsulfonyl)imide salt [DESPA] [Tf ２ Preparation of [N]: 15.76 g of bis(trifluoromethyl)sulfonylimide and 23.15 g of 3-(diethylamino)propane-1-sulfonic acid were added and heated and stirred at 80°C for 4 hours. Vacuum dried at 80°C for 3 hours to obtain [DESPA][Tf ２ N was obtained.

[0052] 1-(3-sulfopropyl)piperidinium bis(trifluoromethylsulfonyl)imide salt [SPPip] [Tf ２ Preparation of [N]: 20.88 g of 1-pyridinepropanesulfonic acid was dissolved in 100 mL of water, and 28.29 g of bis(trifluoromethyl)sulfonyliimide was added and the mixture was heated and stirred at 80°C for 4 hours. The reaction mixture was filtered, the solvent was concentrated and vacuum-dried at 80°C for 4 hours to obtain [SPPip][Tf]. ２ N was obtained.

[0053] 13.2 g (0.185 mol) of pyrrolidine, 19.0 mL (0.187 mol) of 1,4-butanesultone, and 94 mL of toluene were mixed and heated and stirred at 80°C for 16 hours. The resulting yellow solid was filtered and reprecipitation with methanol / ethyl acetate to obtain 25.2 g (64% yield) of 1-pyrrolidinebutanesulfonic acid.

[0054] 1-(4-sulfobutyl)pyrrolidinium bis(trifluoromethylsulfonyl)imide salt [SBPyr][Tf ２ Preparation of [SBPyr][Tf]: 10.0 g of 1-pyrrolidinebutanesulfonic acid was dissolved in 50 mL of water, and 13.6 g of bis(trifluoromethyl)sulfonylimide was added and heated and stirred at 80°C for 2 hours. The reaction mixture was filtered, the solvent was concentrated and dried at 60°C until a constant weight was obtained. ２ 23.0 g of N was obtained (98% yield) (brown liquid).

[0055] 7.78 g (0.098 mol) of pyridine, 10.0 mL (0.187 mol) of 1,4-butanesultone, and 50 mL of acetonitrile were mixed and heated and stirred at 80°C for 25 hours. The resulting white solid was filtered and washed with acetonitrile to obtain 17.4 g (81% yield) of 1-pyridinebutanesulfonic acid.

[0056] 1-(4-sulfobutyl)pyridinium bis(trifluoromethylsulfonyl)imide salt [SBPy][Tf ２ Preparation of [SBPy][Tf]: 15.4 g of 1-pyridinebutanesulfonic acid was dissolved in 71 mL of water, and 20.0 g of bis(trifluoromethyl)sulfonylimide was added and heated and stirred at 80°C for 2 hours. The reaction mixture was filtered, the solvent was concentrated and dried at 80°C until a constant weight was obtained. ２ 35.2 g of N was obtained (yield 99%) (yellow liquid).

[0057] 14.6 mL (0.059 mol) of tributylphosphine and 20 mL of toluene were added to a glass container replaced with Ar and cooled in ice. After slowly adding 3.0 mL (0.0295 mol) of 1,4-butanethione thereto, it was heated at 130 °C for 36 hours. The white precipitate was collected by filtration, washed with toluene, and dried under reduced pressure to obtain 8.96 g (yield 90%) of 4-(tributylphosphino)butanesulfonic acid.

[0058] Preparation of tributyl(4-sulfobutyl)phosphonium bis(trifluoromethylsulfonyl)imide salt [TBSBP][Tf ２ N] 8.46 g of 4-(tributylphosphino)butanesulfonic acid was dissolved in 30 mL of water, 7.03 g of bis(trifluoromethyl)sulfonylimide was added, and it was heated and stirred at 80 °C for 2 hours. The reaction solution was filtered, the solvent was concentrated, and it was dried at 80 °C until a constant weight was obtained to obtain 15.3 g (yield 99%) of [TBSBP][Tf ２ N] (colorless liquid).

[0059] Preparation of 1-(2-carboxyethyl)-3-methylimidazolium hydrochloride ([MCEIm][Cl]) 23.1 g of 1-(2-carboxyethyl)-3-methylimidazole was dissolved in 150 mL of water, 15.6 g of 35% hydrochloric acid was added, and it was heated and stirred at 80 °C for 2 hours. The reaction solution was filtered, the solvent was concentrated, and it was dried at 60 °C until a constant weight was obtained to obtain 27.9 g (yield 97%) of [MCEIm][Cl] (white solid).

[0060] Preparation of 1-(2-carboxyethyl)-3-methylimidazolium perfluorobutanesulfonate ([MCEIm][C ４ F ９ SO ３ ) 12.7 g of 1-(2-carboxyethyl)-3-methylimidazole was dissolved in 82 mL of water, 24.7 g of perfluorobutanesulfonic acid was added, and it was heated and stirred at 80 °C for 2 hours. The reaction solution was filtered, the solvent was concentrated, and it was dried at 60 °C until a constant weight was obtained to obtain 36.4 g (yield 97%) of [MCEIm][C ４ F ９ SO ３ (colorless liquid).

[0061] Incidentally, the following ionic liquids were directly used as products manufactured by Tokyo Chemical Industry Co., Ltd. 1-(4-Sulfobutyl)-3-methylimidazolium hydrogen sulfate ([MSBIM][HSA]): Product number M3120 1-Butyl-3-methylimidazolium hydrogen sulfate ([BMIM][HSA]): Product number B5569 1-Butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][TfO]): Product number B2337 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt ([BMIM][Tf ２ N]): Product number B2477 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF ４ ): Product number B2195 1-Butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF ６ ): Product number B2320

[0062] 2. Evaluation of the viscosity reduction effect of ionic liquids on ionomer dispersions 2-1. Viscosity measurement of ionomer dispersions with added ionic liquids For approximately 30 mg of each of the ionic liquids prepared in 1 above and each commercially available ionic liquid, 5 mL of 5% Nafion dispersion DE521 CS type was mixed, stirred for 1 hour, and then the viscosity was measured using a cone-plate viscometer RM100 CP2000 PLUS manufactured by Rheology Co., Ltd. (Table 1).

[0063] 2-2. Viscosity measurement of ionomer dispersions without added ionic liquids An ionomer dispersion was prepared and its viscosity was measured in the same procedure as in 2-1 above, including 1 hour of stirring, except that a Nafion dispersion without ionic liquid was used. (Table 1).

[0064]

[0065] Table 1 shows the viscosity of mixtures of ionomer dispersions and ionic liquids. When each ionic liquid is added to the ionomer dispersion, the viscosity decreases compared to when the ionic liquid is not added. Specifically, the viscosity of the Nafion dispersion without added ionic liquid is 9.9 mPa·s, while the viscosity of the dispersion with added ionic liquid decreases significantly to 4.7 to 7.4 mPa·s. These viscosity reduction effects are expected to contribute to the uniformity and coating properties of the cathode catalyst layer forming composition.

[0066] 3. Preparation and viscosity evaluation of the composition for forming the cathode catalyst layer 3-1. Preparation and viscosity measurement of the composition for forming the cathode catalyst layer containing an ionic liquid 0.2 g of TEC10E30E (30% Pt supported, carbon support) manufactured by Tanaka Kikinzoku Co., Ltd. was mixed with 12 g of water and 8 g of ethanol and stirred in a planetary ball mill at 270 rpm for 30 minutes. 2.01 g of 5 wt% Nafion dispersion was added to this and stirred in a planetary ball mill at 270 rpm for 30 minutes (corresponding to I / C = 0.7). 1-(4-sulfobutyl)-3-methylimidazole bisulfate ([MSBIm][HSA]) manufactured by Tokyo Chemical Industry Co., Ltd. or [MSBIm][TfO], [MSBIm][Tf ２ Prepare the ionic liquid of [N] so that IL / C = 0.03, 0.07, 0.175, or 0.35, and also prepare [MCEIm][HSA], [MCEIm][TfO], [MCPIm][HSA] or [MCPIm][TfO] as prepared in 1 above. ２ The required amount of [N] was added so that IL / C = 0.03, and the mixture was stirred in a planetary ball mill at 270 rpm for 30 minutes. Finally, degassing treatment was performed as needed to prepare a cathode catalyst layer forming composition containing ionic liquid. The viscosity of the composition (IL / C = 0.03) with [MSBIm] [HSA] [MCEIm] [TfO] added and slightly stirred was measured using an Anton Paar viscometer MCR 102e (Figures 2-3).

[0067] 3-2. Preparation of a cathode catalyst layer-forming composition without ionic liquid and viscosity measurement. A cathode catalyst layer-forming composition was prepared using the same procedure as in 3-1 above, except that no ionic liquid was added, and the viscosity was measured using an Anton Paar MCR 102e viscometer (Figure 4).

[0068] Figures 2 to 4 show the results of comparing the change in viscosity of a cathode catalyst layer formation composition in response to shear rate when prepared using a planetary ball mill, depending on the presence or absence of an ionic liquid. In the case of the cathode catalyst layer formation composition without an ionic liquid, the viscosity is remarkably high immediately after mixing and shows irregular values ​​with respect to the shear rate, suggesting that the composition is non-uniform (Figure 4). On the other hand, in the case of the cathode catalyst layer formation composition containing an ionic liquid, the viscosity decreases rapidly immediately after mixing and decreases regularly in response to the shear rate, indicating that the composition is uniform (Figures 2 to 3). These properties are important from the viewpoint of film formation and reproducibility of the coating of the cathode catalyst layer formation composition.

[0069] 3-3. Preparation and viscosity measurement of the composition for forming the cathode catalyst layer The composition prepared in 1 above [MCEIm][Cl], [MCEIm][C ４ F ９ SO ３ ], and Tokyo Chemical Industries' [BMIm] [TfO], [BMIm] [Tf ２ N], [BMIm][BF ４ ], [BMIm][PF ６ Regarding the above, 0.2 g of TEC10E30E (30% Pt supported, carbon support) manufactured by Tanaka Kikinzoku Co., Ltd. was mixed with 12 g of water, 8 g of ethanol, 2.01 g of 5 wt% Nafion dispersion DE521 CS type, and the required amounts of each ionic liquid so that IL / C = 0.03, and the mixture was irradiated with ultrasound for 3 hours to prepare a composition for forming a cathode catalyst layer. The viscosity of the prepared cathode catalyst layer forming composition was measured using a cone-plate viscometer RM100 CP2000 PLUS manufactured by Lamy Rheology Co., Ltd. (Table 2).

[0070]

[0071] Table 2 shows the viscosity of the cathode catalyst layer forming compositions. When each ionic liquid is added, the viscosity decreases compared to when the ionic liquid is not added. Specifically, the viscosity of the cathode catalyst layer forming composition without the addition of an ionic liquid is 3.61 mPa·s, while the viscosity of the composition with the addition of an ionic liquid decreases to 2.7 to 3.3 mPa·s. These viscosity reduction effects are expected to contribute to the uniformity and coating properties of the cathode catalyst layer forming compositions.

[0072] 4. Catalyst Layer Manufacturing and Surface Observation 4-1. Manufacturing and Surface Observation of a Cathode Catalyst Layer Containing Ionic Liquid A catalyst layer was created by spray coating a Nafion film with a cathode catalyst layer formation composition containing the ionic liquid prepared in 3-1 above ([MCEIm][TfO] prepared so that IL / C = 0.03) using the pulsed swirl spray (PSS, Nordson Advanced Technologies, Inc.) method. A secondary electron image of the cathode catalyst layer surface was taken using a Hitachi High-tech SEM (SU-9000) (Figure 5). 4-2. Manufacturing and Surface Observation of a Cathode Catalyst Layer Without Ionic Liquid A cathode catalyst layer was manufactured using the same procedure as in 4-1 above, except that the cathode catalyst layer formation composition without the ionic liquid prepared in 3-2 above was used, and a secondary electron image of the cathode catalyst layer surface was taken (Figure 5).

[0073] Figure 5 shows a comparison of the cathode catalyst layer surface with and without ionic liquid. The cathode catalyst layer without ionic liquid exhibits cracks and significant surface irregularities. The resulting voids become accumulation sites for generated water when the fuel cell is in operation, potentially clogging catalytic active sites and oxygen gas flow paths. On the other hand, the cathode catalyst layer containing ionic liquid shows no surface cracks or irregularities. This leads to superior coating properties, forming a homogeneous catalyst surface, which is expected to contribute to an improvement in the catalyst's mass activity (MA).

[0074] 5. Performance Evaluation of Membrane Electrode Assembly (MEA) 5-1. Manufacturing and Performance Evaluation of MEA Containing Ionic Liquids NafionNRE-211 (Dupont, film thickness 25 μm) was used as the electrolyte membrane. 0.3 ± 0.03 mg of the cathode catalyst layer formation composition containing each ionic liquid prepared in 3-1 above, or a cathode catalyst layer formation composition without ionic liquid (comparative example), was applied to both sides of the electrolyte membrane. Ｐｔ cm －２ Electrode area 29.16cm ２ A catalyst-coated film (CCM) was fabricated by applying a pulsed swirl spray (PSS, Nordson Advanced Technologies, Inc.) to a size of (5.4 cm x 5.4 cm). The fabricated CCM was dried overnight in a 60°C constant temperature bath, and then heated at 140°C and 10 kgf cm². －２ After 3 min., it was hot-pressed and annealed. A MEA was fabricated by sandwiching both sides of the CCM with GDL (22BB, SGL Carbon Group) and a 150 μm gasket. Oxygen was introduced to the cathode and hydrogen to the anode with a back pressure of 50 kPaG, and the cell temperature was raised to 80°C. This was maintained at 0.6 V and held for approximately 10 hours until the current value stabilized. After that, the conditions were changed to 20% RH, 40% RH, 60% RH, and 80% RH, and the cell voltage at each current density was measured by introducing oxygen to the cathode and hydrogen to the anode with a back pressure of 50 kPaG.

[0075] Table 3 shows the catalytic activity (MA) results of MEA using cathode catalyst layer formation compositions containing each ionic liquid (all with IL / C = 0.03) or cathode catalyst layer formation compositions without ionic liquids. Also, [MSBIm][HSA], [MSBIm][TfO], [MSBIm][Tf ２ Table 3 shows the relationship between IL / C and catalytic activity (MA) of a MEA using a cathode catalyst layer formation composition containing an ionic liquid of [N] (IL / C = 0.03, 0.07, 0.175, or 0.35).

[0076] 5-2. Manufacturing and Performance Evaluation of MEA without Ionic Liquid The MEA was manufactured using the same procedure as in 5-1 above, except that the cathode catalyst layer without ionic liquid manufactured in 4-2 above was used, and its performance was evaluated (Tables 3-4).

[0077]

[0078]

[0079] Table 3 shows the performance of MEAs using cathode catalyst layers containing each ionic liquid. For all MEAs containing ionic liquids, an improvement in MA (Maximum Absorption) is observed compared to MEAs without ionic liquids. In particular, for MEAs containing [MSBIm] [HSA] and [MCEIm] [TfO], the MA at 80°C, 80% RH, and 0.85 V was 534 A / g and 531 A / g, respectively, which is approximately 1.6 times higher than the MA of the MEA without ionic liquids (326 A / g).

[0080] Table 4 shows the relationship between the weight ratio of the catalyst metal to the support of the ionic liquid (i.e., IL / C) and the MA. According to this, although the IL / C at which MA is maximized varies slightly depending on the ionic liquid, a significant improvement in catalytic activity is observed even with a low IL / C, in other words, with a small amount of ionic liquid. In particular, the ionic liquids [MSBIm][HSA] and [MSBIm][Tf ２ For [N], the best mass activity was obtained with an extremely small amount of ionic liquid at IL / C = 0.03. Furthermore, for [MSBIm][TfO], an MA of approximately 1.29 times higher was obtained with an extremely small amount of ionic liquid at IL / C = 0.07 compared to the case without ionic liquid. In Patent Document 1, a large amount of ionic liquid at IL / C = 1.28 to 3.84 was used, and the MA was improved by approximately 27% compared to the case without ionic liquid. However, the present invention can improve the MA with an even smaller amount of ionic liquid and is also cost-effective.

[0081] As described above, by including the ionic liquid represented by formula (I) of the present invention, the dispersibility of the components of the cathode catalyst layer forming composition can be improved, and the viscosity of the composition can be reduced. By using this cathode catalyst layer forming composition, a homogeneous cathode catalyst layer can be formed, which can improve the mass transport of oxygen in oxidation-reduction reactions, for example, and improve the mass activity of the catalyst.

Claims

1. A composition for forming a cathode catalyst layer in a polymer electrolyte fuel cell, comprising an ionic liquid represented by the following formula (1): In the formula, X is an atomic group containing nitrogen or phosphorus atoms, Y is hydrogen, a carboxyl group or a sulfonic acid group, and Z － n is a hydrogen sulfate anion, a trifluoromethanesulfonate anion, a bis(trifluoromethylsulfonyl)imide anion, a halogen anion, a tetrafluoroborate anion, a hexafluorophosphate anion, a bis(fluorosulfonyl)imide anion, or a nonafluorobutanesulfonate anion, and n is an integer between 2 and 4.

2. The composition according to claim 1, wherein X is imidazolium, ammonium, piperidinium, phosphonium, pyrrolidinium, or pyridinium.

3. Z － The composition according to claim 1, wherein the anion is a hydrogen sulfate anion, a trifluoromethanesulfonate anion, or a bis(trifluoromethylsulfonyl)imide anion.

4. The ionic liquid represented by the formula (1) is 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate ([MSBIM][HSA]), 1-(4-sulfobutyl)-3-methylimidazolium trifluoromethanesulfonate ([MSBIM][TfO]), 1-(4-sulfobutyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt ([MSBIM][Tf ２ N]), 1-(2-carboxyethyl)-3-methylimidazolium hydrogen sulfate ([MCEIM][HSA]), 1-(2-carboxyethyl)-3-methylimidazolium trifluoromethanesulfonate ([MCEIM][TfO]), 1-(3-carboxypropyl)-3-methylimidazolium hydrogen sulfate ([MCPI][HSA]), 1-(3-carboxypropyl)-3-methylimidazolium trifluoromethanesulfonate ([MCPI][TfO]), 1-(3-carboxypropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt ([MCPI][Tf ２ N), N,N-diethyl-3-sulfopropylammonium bis(trifluoromethylsulfonyl)imide salt [DESPA][Tf ２ N], 1-(3-sulfopropyl)piperidinium bis(trifluoromethylsulfonyl)imide salt [SPPip][Tf ２ N], 1-(4-sulfobutyl)pyrrolidinium bis(trifluoromethylsulfonyl)imide salt ([SB Pyr][Tf ２ N), 1-(4-sulfobutyl)pyridinium bis(trifluoromethylsulfonyl)imide salt ([SB Py][Tf ２ N), tributyl(4-sulfobutyl)phosphonium bis(trifluoromethylsulfonyl)imide salt [TBSBP][Tf ２ N), 1-(4-sulfobutyl)-3-methylimidazolium hydrochloride ([MSBIM][Cl]), 1-(4-sulfobutyl)-3-methylimidazolium nonafluorobutanesulfonate ([MSBIM][C ４ F ９ SO ３ ]), 1-butyl-3-methylimidazolium bis(TfO) ([BMIm][HSA]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm][TfO]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm][Tf ２ [N]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF ４ ]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF ６ ]), 1-(2-carboxyethyl)-3-methylimidazolium hydrochloride ([MCEIm][Cl]), or 1-(2-carboxyethyl)-3-methylimidazolium perfluorobutanesulfonate ([MCEIm][C ４ F ９ SO ３ The composition according to claim 1, which is ]).

5. The composition according to any one of claims 1 to 4, comprising a catalyst metal and a support for the catalyst metal, wherein the weight ratio of the ionic liquid to the support for the catalyst metal is greater than 0 and less than 0.

35.

6. The composition according to any one of claims 1 to 4, comprising a catalyst metal and a support for the catalyst metal, wherein the weight ratio of the ionic liquid to the support for the catalyst metal is greater than 0 and 0.07 or less.

7. Cathode catalyst layer of a polymer electrolyte fuel cell containing an ionic liquid represented by the following formula (1): In the formula, X is an atomic group containing nitrogen or phosphorus atoms, Y is hydrogen, a carboxyl group or a sulfonic acid group, and Z － n is a hydrogen sulfate anion, a trifluoromethanesulfonate anion, a bis(trifluoromethylsulfonyl)imide anion, a halogen anion, a tetrafluoroborate anion, a hexafluorophosphate anion, a bis(fluorosulfonyl)imide anion, or a nonafluorobutanesulfonate anion, and n is an integer between 2 and 4.

8. Additives for compositions for forming the cathode catalyst layer of polymer electrolyte fuel cells, comprising an ionic liquid represented by the following formula (1): In the formula, X is an atomic group containing nitrogen or phosphorus atoms, Y is hydrogen, a carboxyl group or a sulfonic acid group, and Z － n is an integer between 2 and 4.

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