Electrode catalysts coated with phosphonium-type ionic liquids or phosphonium-type salts, and fuel cells using the same.

Abstract

Instead of the perfluorosulfonic acid-based cation exchange membrane, which is a coating material for the cathode electrode catalyst (catalyst Pt: 102, electrode carbon powder: 103), find an ionic liquid that has high conversion efficiency and high durability coating characteristics even for the FC stack of large vehicles. 【Solution means】Use an electrode catalyst coated with the ionic liquid or its salt 103 represented by the following chemical formula 1. (R1)3-P - -(CH2) n -Y X - (Chemical formula 1) Here, R1 is a saturated or unsaturated chain alkyl group having a linear or branched chain with 1 to 18 carbon atoms, a phenyl group, or a phenyl group having a linear or branched chain alkyl group with 1 to 18 carbon atoms as a substituent, and (CH2) n is a linear alkyl group of (0 ≦ n ≦ 8), Y is a functional group having an amine structure, and X - is an anion component.

Description

[Technical Field]

[0001] This invention relates to catalytic electrodes coated with a phosphonium-type ionic liquid or phosphonium-type salt, which are used in fuel cells and the like. [Background technology]

[0002] In recent years, with carbon dioxide being considered a contributing factor to global warming, fuel cells, which do not emit carbon dioxide during use, have been attracting increasing attention. Fuel cells have been put into practical use, including polymer electrolyte fuel cells for individual households and phosphoric acid fuel cells for factories and buildings, as well as fuel cell vehicles (FCVs). On the other hand, further development of FC stacks (fuel cell stacks) is necessary for their widespread adoption, and this is being actively researched around the world.

[0003] A fuel cell is a power generation device that generates electricity directly by chemically reacting hydrogen and oxygen. In a fuel cell, on the anode side, electrons are extracted from the supplied hydrogen by reacting with a catalyst to form protons (H + On the other hand, on the cathode side, protons that have passed through the electrolyte and supplied oxygen undergo an oxygen reduction reaction with electrons supplied from the cathode electrode and an electrode catalyst to produce water, and the water produced is discharged outside the system.

[0004] The electrolyte near the electrode catalyst of this cathode electrode is required to block the intrusion of hydrogen while allowing protons and oxygen to permeate and come into contact with the catalyst, and to rapidly discharge water produced by the oxidation-reduction reaction. Under these conditions, the cathode side typically uses an electrode catalyst in which the catalyst is supported, and its surface is generally coated with a perfluorosulfonic acid-based cation exchange film, trade name "Nafion" (registered trademark of The Chemours Company FC Limited Liability Company).

[0005] FCVs for small and medium-sized vehicles have been commercialized and reached the popularization stage. However, when considering the application of FCVs to large vehicles such as large buses and large trucks, the power consumption during operation is higher than that of small and medium-sized vehicles, so the hydrogen consumption inevitably increases. On the other hand, it is known that the perfluorosulfonic acid-based cation exchange membrane inhibits the oxygen reduction reaction due to poisoning of the catalyst surface derived from sulfonic acid groups. In addition, the water generated by the oxygen reduction reaction also increases, leading to a high-load state, which may damage the durability of the electrode or reduce the reaction efficiency with oxygen. In addition, the perfluorosulfonic acid-based electrolyte membrane has a high cost due to its complex synthetic raw materials and manufacturing process, and its heat resistance is also not sufficient.

[0006] By the way, in recent years, various types of ionic liquids have been developed as substances having the property of conducting electricity, and their use as materials for electronic devices has been studied. An ionic liquid is a liquid having ionic conductivity consisting only of ions in the atmosphere, and has characteristics such as a wide liquid temperature range, low volatility, non-flammability, and high electrochemical stability. For example, in Patent Document 1, a technique of using a phosphonium ionic liquid as an electrolyte of an electronic device is disclosed.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] Therefore, the problem to be solved by the present invention is to find a phosphonium-type ionic liquid or a phosphonium-type salt having a coating property with higher conversion efficiency and higher durability, in place of the perfluorosulfonic acid-based cation exchange membrane which is a coating material for the cathode electrode catalyst.

Means for Solving the Problems

[0009] The most important feature of the electrode catalyst of the present invention is that it is coated with a phosphonium-type ionic liquid or phosphonium-type salt represented by the following chemical formula 1. (R1)3-P + -(CH2) n -Y X - (chemical formula 1) In the above chemical formula 1, R1 is a saturated or unsaturated alkyl group having a straight or branched chain with 1 to 18 carbon atoms, a phenyl group, or a phenyl group having a straight or branched alkyl group having 1 to 18 carbon atoms as a substituent, (CH2) n is a linear alkyl group (0 ≤ n ≤ 8), Y is a functional group having an amine structure, and X- is an anionic component. [Effects of the Invention]

[0010] In this invention, while taking hydrophobicity into consideration, the activity of the oxygen reduction reaction on a platinum electrode catalyst is improved, and the H itself is also improved. + By focusing on a cationic structure with functional groups that improve conductivity, we were able to propose an ionic liquid and salt that, by imparting this structure to a phosphonium cation, forms a partially hydrophilic region within the hydrophobic matrix structure and exhibits oxygen reduction reaction activity through cocatalysis. This allows for the control of washout (dissolution of drain water) outside the fuel cell system. As a result, H near the catalyst, which is necessary for the oxygen reduction reaction, is present. + This will solve both the problem of transportation shortages and the challenge of increasing the thickness of the ionic liquid layer necessary for improved durability.

[0011] The electrode catalyst coated with the phosphonium-type ionic liquid or phosphonium-type salt of the present invention has the potential to be manufactured at a lower cost than conventional electrode catalysts coated with perfluorosulfonic acid-based cation exchange membranes, while exhibiting performance equivalent to or better than that in terms of hydrogen conversion efficiency and durability. In addition, since the FC cell can be made lighter, the FC stack can also be made lighter, or even if the same weight FC stack is installed, it can be composed of more FC cells, thus enabling the creation of a high-power FC stack. [Brief explanation of the drawing]

[0012] [Figure 1] This diagram schematically illustrates the electrode catalyst structure coated with the phosphonium-type ionic liquid or phosphonium-type salt of the present invention. [Figure 2] A schematic diagram illustrating a cathode electrode equipped with the electrode catalyst of the present invention. [Figure 3] An explanatory diagram showing an overview of an FC cell equipped with the electrodes of the present invention. [Figure 4] Bar graph 1 below comparing mass activity and surface area specific activity of Group 1 [Figure 5] Bar graph 2 below compares mass activity and surface area specific activity of the second group. [Figure 6] Bar graph 3 below compares the mass activity and surface area specific activity of the third group. [Figure 7] Bar graph 4 below compares mass activity and surface area specific activity of the 4th group. [Figure 8] Bar graph 5 below compares the mass activity and surface area specific activity of the 5th group. [Figure 9] Bar graph 6 below compares the mass activity and surface area specific activity of the 6th group. [Figure 10] Bar graph 7 below shows a comparison of mass activity and surface area specific activity for group 7. [Figure 11] Graph 1 below comparing the retention rate of Pt surface area after a 10,000-hour durability test for Group 2. [Figure 12] Graph 2 below comparing the retention rate of Pt surface area after a 10,000-hour durability test for the third group. [Figure 13]Comparison graph 3 of the Pt surface area retention rate after 10,000 hours of durability test for the following Group 4 [Figure 14] Comparison graph 4 of the Pt surface area retention rate after 10,000 hours of durability test for the following Group 5 [Figure 15] Comparison graph 5 of the Pt surface area retention rate after 10,000 hours of durability test for the following Group 6

Best Mode for Carrying Out the Invention

[0013] Hereinafter, the present invention will be described in detail with reference to the drawings. However, the electrode catalyst, electrode, and battery of the present invention are not limited to the forms shown in the drawings.

[0014] FIG. 1 is an explanatory diagram schematically showing an electrode catalyst structure coated with a phosphonium-type ionic liquid or phosphonium-type salt of the present invention. An electrode catalyst is generally a carbon fine powder or porous carbon material 103 serving as an electrode, on which platinum fine particles 102 serving as a catalyst are supported (sometimes referred to as "Pt / C catalyst"). The Pt / C catalyst is commercially available. Specifically, "TEC10V30E" manufactured by Tanaka Kikinzoku Kogyo K.K. (weight ratio of about 30 wt.%), commercially available fuel cell catalysts, and Pt-supported porous carbon catalysts (products such as "Vulcan" and "Ketjen" made of porous carbon materials or mesoporous carbon (MPC) with Pt supported thereon) prepared by thermal reduction method, hydrogen reduction method, alcohol reduction method, etc. can be mentioned.

[0015] In the present invention, the electrode catalyst is coated with a phosphonium-type ionic liquid or phosphonium-type salt represented below. (R1)3-P + -(CH2) n -Y X - (Chemical formula 1) However, in the above Chemical formula 1, R1 is a saturated or unsaturated chain alkyl group having a straight or branched chain with 1 to 18 carbon atoms, a phenyl group, or a phenyl group having a straight or branched chain alkyl group with 1 to 18 carbon atoms as a substituent, and (CH2) nis a linear alkyl group (0 ≤ n ≤ 8), Y is a functional group having an amine structure, and X- is an anionic component.

[0016] The difference between a phosphonium-type ionic liquid and a phosphonium-type salt as referred to in this invention is whether the melting point is 100°C or below (ionic liquid) or above 100°C (salt). In the electrode catalyst of this invention, whether the melting point of the compound of chemical formula 1 is 100°C or below or above 100°C does not affect the effect of this invention, so it can be used in either state.

[0017] In the selective functional group Y of chemical formula 1, the term "amine" refers to a general term for compounds in which a hydrogen atom of ammonia is replaced by a hydrocarbon group or an aromatic group, and includes aliphatic amines, aromatic amines, and heterocyclic amines. A functional group having an amine structure is a functional group having such an amine structure, and specifically, in addition to an amino group, examples of functional groups having an aliphatic amine include dimethylamino group, diethylamino group, dipropylamino group, and amidine group, examples of functional groups having an aromatic amine include aniline group and pyridine group, and examples of functional groups having a heterocyclic amine include benzotriazole group, triazine group, and purine bases such as adenine. Among these, examples from the present invention have shown that functional groups having a guanidine skeleton in their structure, such as purine bases such as guanine, are preferred.

[0018] In Chemical Formula 1, R1 is a saturated or unsaturated alkyl group having a straight or branched chain with 1 to 18 carbon atoms, a phenyl group, or a phenyl group having a straight or branched alkyl group having 1 to 18 carbon atoms as a substituent. In particular, examples of the present invention have shown that in all of the above cases, good properties are observed when the number of carbon atoms is between 3 and 9.

[0019] The anionic component (X) in chemical formula 1 -The anion can be any hydrophobic anion without particular restrictions, but specifically, examples include bis(trifluoromethanesulfonyl)imide anion, trifluate anion, bis(pentafluoroethyl)sulfonylimide anion, (fluorosulfonyl)-N-(fluoromethylsulfonyl)imide anion, nonafluorobutanesulfonic acid anion, (fluorosulfonyl)-N-(pentafluoroethylsulfonyl)imide anion, (trifluoromethylsulfonyl)-N-(nonafluorobutylsulfonyl)imide anion, bis(nonafluorobutylsulfonyl)imide, 2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide anion, or 2,2,2-(trifluoromethyl)sulfonyl-N-cyanamide anion. These anionic components can be used individually or in combination.

[0020] The raw materials for the aforementioned phosphonium-type ionic liquids or phosphonium-type salts are generally available for purchase on the market. If purchasing or contract synthesis of ionic liquids is difficult, it is possible to synthesize ionic liquids through a two-step process: first, synthesizing an intermediate phosphonium halide using trialkylphosphine as a starting material, and then performing anion exchange on the phosphonium halide.

[0021] First, the method for synthesizing the intermediate phosphonium halide necessary for the synthesis of ionic liquids is as follows. The reaction conditions for obtaining the intermediate phosphonium halide are as follows: Add 0.5 to 2 moles, preferably 0.9 to 1.2 moles, of an alkyl halide precursor to the trialkylphosphine, and react in a chlorine-free inert solvent such as toluene at 20 to 150°C, preferably 30 to 100°C, for 3 hours or more, preferably 5 to 12 hours.

[0022] The reaction atmosphere is not particularly limited as long as it is oxygen-free, but a nitrogen or argon atmosphere is preferred. Reacting trialkylphosphine with an alkyl halide precursor in an oxygen-containing atmosphere is undesirable because it generates trialkylphosphine oxide, in which oxygen is bonded to the trialkylphosphine, resulting in a decrease in yield. Trialkylphosphine oxide can be removed by washing with an organic solvent as appropriate, but as the total number of carbon atoms in the intermediate phosphonium halide increases, the intermediate phosphonium halide also tends to dissolve in the organic solvent, making removal difficult. Therefore, it is preferable to carry out the reaction under an inert atmosphere to prevent the generation of trialkylphosphine oxide.

[0023] As the metal salt of the anionic component used to introduce other anions from the intermediate phosphonium halide by anion exchange, alkali metal salts such as the Li salt of the aforementioned anionic component can be used. Using alkali metal salts is preferable because the alkali halide produced by the reaction with alkylphosphine can be easily removed by washing with water.

[0024] Ultrapure water or deionized water can be used for washing, and it is preferable to repeat the washing as needed until the impurity content decreases. Impurities to be removed by washing include unreacted raw materials and alkali halides. In addition, washing with an organic solvent can be performed as needed to remove unreacted raw materials and by-products. As for organic solvents that can be used for washing, non-polar solvents that do not contain chlorine, such as pentane, hexane, and heptane, are preferable because they can efficiently remove non-polar organic compounds such as impurities without dissolving the quaternary phosphonium salt.

[0025] The method for coating a Pt / C catalyst with the phosphonium-type ionic liquid is as follows. First, a solution is prepared by mixing the phosphonium-type ionic liquid or phosphonium-type salt with isopropyl alcohol at a concentration that provides sufficient fluidity (preferably 10 wt% to 60 wt% by weight ratio to isopropyl alcohol). The Pt / C catalyst is then mixed into this solution. At this time, it is desirable to perform the mixing in an inert gas environment, as mixing in the atmosphere may cause ignition. The solution containing the Pt / C catalyst and ionic liquid is ultrasonically stirred, and then the isopropyl alcohol is evaporated using an evaporator to obtain a Pt / C catalyst with the phosphonium-type ionic liquid or phosphonium-type salt coated on its surface. Figure 1 schematically shows the electrode catalyst coated with the phosphonium-type ionic liquid or phosphonium-type salt of the present invention obtained in the above step, in which minute (generally several nanometers in diameter) platinum particles 102 are in contact with porous carbon powder 103, and the phosphonium-type ionic liquid or phosphonium-type salt 101 is coated around this structure.

[0026] Furthermore, the phosphonium-type ionic liquid or phosphonium-type salt 101 is composed of a functional group 111 having an amine structure that acts as a functional group to improve oxygen reduction activity, a functional group 121 having a hydrophobic mother skeleton structure, and a phosphorus ion 131. An enlarged schematic diagram of the ionic liquid or salt located near the platinum nanoparticles is shown in 104.

[0027] Figure 2 schematically illustrates how the coated Pt / C catalyst obtained in Figure 1 is located at the cathode electrode. Figure 3 illustrates the structure of an FC cell (fuel cell) and the location of the cathode electrode shown in Figure 2 within the FC cell.

[0028] The coated catalyst electrode of the present invention operates as a fuel cell as follows: Oxygen diffuses from the gas diffusion layer 201 into the Pt / C catalyst portion (102, 103) coated with a phosphonium-type ionic liquid or phosphonium-type salt 101, and is decomposed into oxygen ions and electrons by the catalyst. The oxygen ions combine with protons (hydrogen ions) that move to the cathode side via the electrolyte membrane 304 and polymer electrolyte membrane 202 to produce water, and an electric current is generated by the movement of electrons and protons. Oxygen from the air is used, and air is introduced through the battery's air inlet 302. The produced water and excess air are discharged from the system through the outlet 303. Hydrogen is introduced through the hydrogen inlet 305 on the anode electrode side, and excess hydrogen is discharged from the system through the hydrogen outlet 311. [Examples]

[0029] [Manufacturing] As the Pt / C catalyst, a commercially available product with the trade name "TEC10V30E" (approximately 30 wt.% by weight, manufactured by Tanaka Kikinzoku Co., Ltd.) was used. Next, the target ionic liquid was mixed with isopropyl alcohol (weight ratio of isopropyl alcohol (wt%) 20.0 wt%). The mixture and the Pt / C catalyst were mixed in an inert gas and ultrasonically stirred. After that, the isopropyl alcohol was evaporated using an evaporator to obtain a Pt / C catalyst with the ionic liquid coated on its surface.

[0030] As the phosphonium-type ionic liquid or phosphonium-type salt of chemical formula 1 of this invention, those described in groups 1 to 7 below were used. The method for coating the Pt / C catalyst with the phosphonium-type ionic liquid is as described above. In any of the ionic liquids, the anion (X - ) used TFSI (bis(trifluoromethanesulfonyl)imide). In addition, as a comparative reference example with the prior art (a), a Pt / C catalyst coated with the perfluorosulfonic acid-based cation exchange membrane "Nafion" (registered trademark) (hereinafter referred to as "Nafion") was used.

[0031] [Group 1] (R1)3-P + -(CH2) n -Y X - In (Chemical Formula 1), an ionic liquid was used in which R1 was a phenyl group, n=1, and Y was the functional group described below. (i) Ionic liquid in which Y is an amino group (c) Ionic liquid in which Y is a dimethylamino group (e) Ionic liquid in which Y is a diethylamino group (O) Ionic liquid in which Y is a dipropylamino group (k) Ionic liquid in which Y is a pyridine group (Ki) Ionic liquid in which Y is a benzotriazole group (k) Ionic liquid in which Y is a triazine group (Ke) Ionic liquid in which Y is a purine base (C) Ionic liquid in which Y is an amidine group

[0032] [Group 2] (R1)3-P + -(CH2) n -Y X - An ionic liquid was used in which Y is an aniline group and R1 is the functional group described below, with n=1, in (Chemical Formula 1). (S) Ionic liquid in which R1 is a linear alkyl group of CH3 (c) Ionic liquid in which R1 is a linear alkyl group C3H7 (S) R1 is C6H 13 ionic liquids which are linear alkyl groups (Se) R1 is C9H 19 ionic liquids which are linear alkyl groups (So) R1 is C 18 H 37 ionic liquids which are linear alkyl groups

[0033] [Group 3] (R1)3-P + -(CH2) n -Y X - An ionic liquid was used in which Y is an aniline group and R1 is the functional group described below, with n=1, in (Chemical Formula 1). (T) Ionic liquid in which R1 is a branched alkyl group of 1-methylethyl (C3H7) (C) R1 is 1-methylpentylC6H 13 ) Branched-chain alkyl group ionic liquid (T) R1 is 1-methyloctyl(C9H 19 ) Branched-chain alkyl group ionic liquid (Te) R1 is a 1-methylheptadectyl group (C 18 H 37 ) Branched-chain alkyl group ionic liquid

[0034] [Group 4] (R1)3-P + -(CH2) n -Y X - An ionic liquid was used in which Y is an aniline group and R1 is the functional group described below, with n=1, in (Chemical Formula 1). (t) Ionic liquid in which R1 is a C2H4=CH group (Na) R1 is C5H 10 =CH group ionic liquid (ii) R1 is C8H 16 =CH group ionic liquid (Nu) R1 is C 17 H 34 =CH group ionic liquid

[0035] [Group 5] (R1)3-P + -(CH2) n -Y X - An ionic liquid was used in which Y is an aniline group and R1 is the functional group described below, with n=1, in (Chemical Formula 1). (Ne) Ionic liquid in which R1 is a phenyl (Ph-) group (no) Ionic liquid in which R1 is a C3H7-Ph- group (H) R1 is C6H 13 -Ph- group ionic liquid (Hi) R1 is C9H 19 -Ph- group ionic liquid (F) R1 is C18 H 37 -Ph- group ionic liquid

[0036] [6th group] (R1)3-P + -(CH2) n -Y X - An ionic liquid was used in which Y is an aniline group and R1 is the functional group described below, with n=1, in (Chemical Formula 1). (H) Ionic liquid in which R1 is a 1-methylethyl-Ph- group (e) Ionic liquid in which R1 is a 1-methylpenty-Ph- group (M) Ionic liquid in which R1 is a 1-methyloctyl-Ph- group (m) Ionic liquid in which R1 is a 1-methylheptadectyl-Ph- group

[0037] [Group 7] (R1)3-P + -(CH2) n -Y X - In (Chemical Formula 1), Y is an aniline group and R1 is a phenyl group, so (CH2) n An ionic liquid was used with functional groups having the following number of n. (M) Ionic liquid where n=0 (Me) Ionic liquid where n=1 (Mo) Ionic liquid with n=3 (Ya) Ionic liquid n=6 (Yu) Ionic liquid with n=8

[0038] 〔evaluation〕 The activity of Pt / C catalysts coated with each phosphonium-type ionic liquid or phosphonium-type salt was evaluated by measuring the mass activity and surface area specific activity of platinum. Mass activity and surface area specific activity were evaluated using the rotational electrode (RDE) method and the oxygen reduction reaction (ORR) activity evaluation method. Durability was evaluated by cyclic voltammetry after applying a predetermined load. These evaluation methods are standard evaluation and analysis protocols created in cooperation with the Fuel Cell Commercialization Promotion Council (FCCJ) and the New Energy and Industrial Technology Development Organization (NEDO), and details are described in the "NEDO PEFC Cell Evaluation and Analysis Protocol March 2022 Edition". The results of the evaluation of mass activity and surface area specific activity are shown in Figures 4 to 10. Durability was evaluated by assessing the electrochemical surface area (ECSA) of the catalyst after 10,000 cycle tests, and these results are shown in Figures 11 to 15. Higher mass activity and surface activity indicate better activity characteristics, and a higher retention rate after cycle testing also indicates better characteristics.

[0039] The graph in Figure 4 shows the results of an example comparing the efficiency of electrocatalysts using a Pt / C catalyst coated with a phosphonium-type ionic liquid in which R1 is a phenyl group, n is 1, and Y is a different functional group shown in [Group 1], and a Pt / C catalyst coated with Nafion, which is commonly used. In the figure, the vertical axis shows catalytic activity per unit surface area (Pt surface area specific activity, (B) in the figure) and activity per unit mass of Pt (mass activity, (A) in the figure).

[0040] Experiments conducted in this invention revealed that all examples yielded superior results in terms of mass activity and surface area specific activity compared to the reference example using Nafion. A detailed examination revealed that the ionic liquid using a triazine group for the Y functional group showed the best properties. Following that, the one using a purine group showed superior properties. From this, it is thought that a nitrogen-containing functional group, especially one with a cyclic structure, contributes to improving the activity of the oxygen reduction reaction on the platinum catalyst. While the functional groups using a linear or unsaturated nitrogen-containing carbon and hydrogen for Y did not show outstandingly good properties compared to the triazine group or purine group, in all examples, the oxygen reduction reaction (ORR) activity was improved, i.e., the efficiency as an electrode catalyst was improved, compared to the reference example using Nafion.

[0041] Figure 5 is a graph, similar to Figure 4, showing catalytic activity per unit surface area (Pt surface area specific activity, (B) in the figure) and activity per unit mass of Pt (mass activity, (A) in the figure) on the vertical axis. The comparison shows a Pt / C catalyst coated with the phosphonium-type ionic liquid shown in [Group 2], i.e., an ionic liquid in which Y is an aniline group, n is 1, and R1 is a linear alkyl group with a different number of carbon atoms, and a Pt / C catalyst coated with Nafion.

[0042] In the experiments conducted in this invention, all examples showed better mass activity and surface area specific activity compared to those using the reference example Nafion. Further investigation revealed that the activity tended to improve as the number of carbon atoms in the linear alkyl group R1 increased, but the properties decreased when the number of carbon atoms was increased to 18. In other words, linear alkyl groups with 3 to 9 carbon atoms showed particularly good properties.

[0043] Figure 6 is a graph showing catalytic activity per unit surface area (Pt surface area specific activity, (B) in the figure) and activity per unit mass of Pt (mass activity, (A) in the figure) on the vertical axis. The graph compares a Pt / C catalyst coated with the phosphonium-type ionic liquid shown in [Group 3], i.e., an ionic liquid in which Y is an aniline group, n is 1, and R1 is a branched alkyl group with a different number of carbon atoms, with a Pt / C catalyst coated with Nafion.

[0044] In the experiments conducted in this invention, similar to the results when using a linear alkyl group for R1, all examples showed good properties in terms of mass activity and surface area specific activity compared to the reference example using Nafion. Furthermore, our results showed particularly good properties with branched alkyl groups having 3 to 9 carbon atoms. Similar to the case of linear groups, it is thought that the oxygen reduction activity function works best at this carbon number range.

[0045] Similarly, Figure 7 is a graph showing the Pt surface area specific activity (B) and Pt mass activity (A) on the vertical axis. The phosphonium-type ionic liquids being studied are those shown in [Group 4], i.e., Pt / C catalysts coated with functional groups having unsaturated bonds with double bonds having different numbers of carbon atoms, where Y is an aniline group and n is 1, and R1 is a double bond with different numbers of carbon atoms, and Pt / C catalysts coated with Nafion.

[0046] In the experiments conducted in this invention, similar to the results obtained when using linear or branched alkyl groups for R1, all examples showed good properties in terms of both mass activity and surface area specific activity compared to the reference example using Nafion. Furthermore, our results showed particularly good properties with functional groups having unsaturated bonds with double bonds ranging from 3 to 9 carbon atoms. We believe that the same effect as in the cases of linear and branched groups is at work here.

[0047] Figure 8 is a graph showing Pt surface area specific activity (B) and Pt mass activity (A) on the vertical axis. It compares a Pt / C catalyst coated with the phosphonium-type ionic liquids shown in [Group 5], i.e., a Pt / C catalyst coated with a phenyl group or a phenyl group having a linear alkyl group with a different number of carbon atoms, where Y is an aniline group and n is 1, and R1 is a phenyl group, with a Pt / C catalyst coated with Nafion.

[0048] In experiments conducted in this invention, the ionic liquid using the simplest phenyl group for R1 showed the best properties. As the number of carbon atoms in the linear alkyl group attached to the phenyl group increased, the surface area specific activity and mass activity tended to decrease, but for 18 or more carbon atoms, better properties were confirmed than those using the reference example Nafion.

[0049] Figure 9 is a graph showing Pt surface area specific activity (B) and Pt mass activity (A) on the vertical axis. It compares a Pt / C catalyst coated with phenyl groups having branched alkyl groups with different numbers of carbon atoms, with Y being an aniline group and n being 1, and a Pt / C catalyst coated with Nafion.

[0050] In the experiments conducted in this invention, the ionic liquid using (1-methylethyl-Ph-) for R1 showed the best properties. As the number of carbon atoms in the branched alkyl group attached to the phenyl group increased, the surface area specific activity and mass activity tended to decrease, but for 18 or more carbon atoms, better properties were confirmed than those using the reference example Nafion. This trend was similar to that observed in the case of phenyl groups with linear alkyl groups attached, as obtained in [Group 5].

[0051] Figure 10 is a graph showing catalytic activity per unit surface area (Pt surface area specific activity, (B) in the figure) and activity per unit mass of Pt (mass activity, (A) in the figure) on the vertical axis. The graph compares Pt / C catalysts coated with phosphonium-type ionic liquids, as shown in [Group 7], i.e., ionic liquids in which Y is an aniline group and R1 is a phenyl group, with n varying from 0 to 8, with Pt / C catalysts coated with Nafion.

[0052] In the experiments conducted in this invention, the surface area specific activity and mass activity showed the best characteristics when n was 1, and tended to decrease when n was 0 or 3 or greater. However, for n ≤ 8, both characteristics were found to be better than those of the reference example using Nafion.

[0053] A Pt / C catalyst coated with a phosphonium-type ionic liquid in which R1 is a phenyl group, n is 1, and Y is a different functional group shown in [Group 1] was used, was compared in terms of durability with a conventionally used Pt / C catalyst coated with Nafion. After 10,000 loading cycles, the Pt surface area retention rate (%) was calculated by cyclic voltammetry. A higher Pt surface area retention rate (%), i.e., a smaller rate of change from the start of the test (i.e., a retention rate closer to 100%), indicates higher durability.

[0054] In all of the examples, no inferior characteristics were observed in terms of Pt surface area retention rate (%) compared to the example using Nafion, and excellent durability results were obtained.

[0055] Figure 11 shows an example of the results of a durability test. The vertical axis (C) shows the percentage of Pt surface area retention after 10,000 loading cycles. The comparison between the phosphonium-type ionic liquid shown in [Group 2], i.e., a Pt / C catalyst coated with a linear alkyl group of different carbon numbers, where Y is an aniline group and n is 1, and R1 is a linear alkyl group of different carbon numbers, and a Pt / C catalyst coated with Nafion, is shown as the target material.

[0056] In the experiments conducted in this invention, all examples showed a better retention rate (%) of Pt surface area compared to the example using Nafion. Further investigation revealed that increasing the number of carbon atoms in the linear alkyl group of R1 tended to improve the retention rate (%) of Pt surface area. This is thought to be because increasing the number of carbon atoms in the linear alkyl group improved hydrophobicity in particular while maintaining the reaction efficiency with oxygen, suppressing side reactions with water, and thereby improving the durability of the electrode.

[0057] Figure 12 also shows an example of the results of a durability test. The vertical axis (C) shows the result of calculating the retention rate (%) of the Pt surface area after 10,000 loading cycles. The comparison is shown between a Pt / C catalyst coated with a phosphonium-type ionic liquid, as shown in [Group 3], i.e., a Pt / C catalyst coated with a branched alkyl group of different carbon numbers, where Y is an aniline group and n is 1, and a Pt / C catalyst coated with Nafion.

[0058] In the experiments conducted in this invention, similar to the results when using a linear alkyl group for R1, all examples showed a good retention rate (%) of Pt surface area compared to the reference example using Nafion. Furthermore, our results showed that the properties improved as the number of carbon atoms increased. It is thought that the same effect as in the case of a linear group is at work.

[0059] Figure 13 also shows an example of the results of a similar durability test. The vertical axis (C) shows the result of calculating the retention rate (%) of the Pt surface area after 10,000 loading cycles. The comparison is shown between a Pt / C catalyst coated with a phosphonium-type ionic liquid as shown in [Group 4], i.e., a Pt / C catalyst coated with a functional group having an unsaturated bond with a double bond of a different number of carbon atoms, where Y is an aniline group and n is 1, and a Pt / C catalyst coated with Nafion.

[0060] In the experiments conducted in this invention, similar to the results when using an alkyl group for R1, a good retention rate (%) of the Pt surface area was obtained in all examples compared to the reference example using Nafion. In the series of experiments in Group 4, in particular, when R1 was set to (C 17 H 34 The best retention rate was observed with ionic liquids composed of CH groups.

[0061] Figure 14 also shows an example of the results of a durability test. The vertical axis (C) shows the result of calculating the retention rate (%) of the Pt surface area after 10,000 loading cycles. The comparison shows a Pt / C catalyst coated with the phosphonium-type ionic liquid shown in [Group 5], i.e., a Pt / C catalyst coated with a phenyl group or a phenyl group having a linear alkyl group with a different number of carbon atoms, where Y is an aniline group and n is 1, and a Pt / C catalyst coated with Nafion.

[0062] In the experiments conducted in this invention, in all examples using the phosphonium-type ionic liquid shown in [Group 5], a better retention rate (%) of the Pt surface area was obtained compared to the example using Nafion. In particular, R1 (C 18 H 37 Ionic liquids composed of -Ph- groups showed excellent retention. However, we believe that the side reactions with water are more suppressed in all cases, and that this effect leads to improved durability.

[0063] Figure 15 also shows an example of the results of a similar durability test. The vertical axis (C) shows the result of calculating the retention rate (%) of the Pt surface area after 10,000 loading cycles. The comparison is shown between a Pt / C catalyst coated with a phosphonium-type ionic liquid, as shown in [Group 6], where Y is an aniline group, n is 1, and R1 is a phenyl group having a branched alkyl group with a different number of carbon atoms, and a Pt / C catalyst coated with Nafion.

[0064] In the experiments conducted in this invention, all examples using the ionic liquids shown in Group 6 showed better retention rates (%) of Pt surface area compared to those using the reference example Nafion. In particular, the properties of phenyl groups with branched alkyl groups having a large number of molecules in R1 were excellent. We believe that, similar to phenyl groups with linear alkyl groups, the durability is improved because the side reactions with water are suppressed by the effect of the ionic liquid.

[0065] Incidentally, the target phosphonium-type ionic liquids are the ionic liquids shown in [Group 7], i.e., Y is an aniline group, (R 1 A comparison was also made between Pt / C catalysts coated with ionic liquids in which n was varied from 0 to 8 using phenyl groups, and Pt / C catalysts coated with Nafion. However, in all the examples using phosphonium-type ionic liquids shown in [Group 6], no inferior characteristics were shown in terms of Pt surface area retention rate (%) compared to the example using Nafion. [Industrial applicability]

[0066] The electrode catalyst coated with the phosphonium-type ionic liquid or phosphonium-type salt of the present invention exhibits higher oxygen reduction reaction activity and greater durability than conventional electrode catalysts coated with perfluorosulfonic acid-based cation exchange membranes. Therefore, electrodes using this catalyst hold great promise for industrial applications, particularly as materials for fuel cells used in large vehicles. [Explanation of symbols]

[0067] 101 Phosphonium-type ionic liquid or phosphonium-type salt 102 Platinum fine particles 103 Carbon fine powder or porous carbon material 104: Enlarged schematic diagram of the vicinity of the platinum nanoparticle catalyst 111: Functional groups that enhance the oxygen reduction activity of phosphonium-type ionic liquids or phosphonium-type salts 121: Hydrophobic region constituting a phosphonium-type ionic liquid or phosphonium-type salt 131: Phosphate ions constituting phosphonium-type ionic liquids or phosphonium-type salts 201 Gas diffusion layer 202 Polymer electrolyte membrane 203 Structure of the cathode electrode 301 Cathode electrode for fuel cell according to the present invention 302 Air (oxygen) inlet 303 Water and excess air (oxygen) outlet 304 Electrolyte membrane 305 Hydrogen Inlet 306 Anode side electrode of fuel cell 307 cases 308 Gas diffusion layer of cathode electrode 309 Gas diffusion layer of the anode electrode 310 Enlarged schematic diagram of the electrolyte membrane side of the cathode electrode (same as shown in Figure 2) 311 Excess hydrogen outlet (A) Mass activity (A / g) (B) Surface area specific activity (A / m2) (C) Percentage of Pt surface area retention after 10,000 durability tests

Claims

1. An electrode catalyst coated with a phosphonium-type ionic liquid or phosphonium-type salt represented by the following chemical formula 1. (R) １ ) ３ -P + - (CH ２ ) ｎ -Y X － (Chemical Formula 1) (In Chemical Formula 1, R １ is a saturated or unsaturated chain alkyl group having a straight-chain or branched-chain with 1 to 18 carbon atoms, a phenyl group or a phenyl group having a straight-chain or branched-chain alkyl group with 1 to 18 carbon atoms as a substituent, (CH ２ ) ｎ is a straight-chain alkyl group of (0 ≦ n ≦ 8), Y is a functional group having an amine structure, and X － is an anion component.)

2. The electrode catalyst according to claim 1, wherein Y in the chemical formula 1 is an amino group, a dimethylamino group, a diethylamino group, a dipropylamino group, an aniline group, a pyridine group, a benzotriazole group, a triazine group, an amidine group, or a purine base.

3. R of the above chemical formula 1 １ The electrode catalyst according to claim 1, wherein is a saturated alkyl group having a straight or branched chain with 1 to 18 carbon atoms.

4. The aforementioned R １ The electrode catalyst according to claim 3, wherein the number of carbon atoms is 3 to 9.

5. R of the above chemical formula 1 １ The electrode catalyst according to claim 1, wherein is an unsaturated alkyl group having a straight or branched chain with 1 to 18 carbon atoms.

6. The aforementioned R １ The electrode catalyst according to claim 5, wherein the number of carbon atoms is 3 to 9.

7. The aforementioned R １ The electrode catalyst according to claim 1, wherein is a phenyl group or a phenyl group having a linear or branched alkyl group having 1 to 18 carbon atoms as a substituent.

8. In the aforementioned chemical formula 1, R １ (CH ２ The electrode catalyst according to claim 1, wherein n of n is 1 and Y is an aminobenzyl group.

9. The anionic component (X) described in the above chemical formula 1 － The electrode catalyst according to claim 1, wherein the anion component is bis(trifluoromethanesulfonyl)imide anion, trifluate anion, bis(pentafluoroethyl)sulfonylimide anion, (fluorosulfonyl)-N-(fluoromethylsulfonyl)imide anion, nonafluorobutanesulfonic acid anion, (fluorosulfonyl)-N-(pentafluoroethylsulfonyl)imide anion, (trifluoromethylsulfonyl)-N-(nonafluorobutylsulfonyl)imide anion, bis(nonafluorobutylsulfonyl)imide, 2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide anion, or 2,2,2-(trifluoromethyl)sulfonyl-N-cyanamide anion, or consists of multiple components from the anion components listed above.

10. An electrode comprising an electrode catalyst as described in any of claims 1 to 9.

11. A fuel cell having the electrode described in claim 10.

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