JP2012252865A - Electrode material and fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel cell electrode material capable of maintaining output performance of a fuel cell at high level for a long time.SOLUTION: There are provided an electrode material and a fuel cell including an electrode manufactured using the electrode material. The electrode material comprises a catalyst particle in which a plurality of catalyst metal particles are supported on a conductive carbon fine particle and a part or the whole of the supported catalyst metal particles is covered with an oxide of transition metal. In the electrode material, a ratio of numbers of atoms between the number A of atoms of the catalyst metal and the number B of atoms of the transition metal satisfies 0<B/A≤3.5, each of the catalyst metal particles includes one or more components selected from among Pt, Ru, Rh, Pd, Co, Ni and Au, and the oxide of the transition metal includes one or more oxides selected from among Si, Zr, Ti, Mg, Al and In.

Description

  The present invention relates to a fuel cell electrode material and a fuel cell comprising an electrode using the electrode material.

  In a general polymer electrolyte fuel cell, a catalyst layer that serves as an anode and a cathode is disposed with a proton conductive electrolyte membrane interposed therebetween, a gas diffusion layer is disposed on the outer side, and a separator is disposed on the outer side. The arranged basic structure is used as a unit cell. Normally, the unit cells are stacked to form a battery according to the required output.

  In order to extract the current from the fuel cell having the basic structure, an oxidizing gas such as oxygen or air is supplied to the cathode side from the gas flow path of the separator disposed at both the anode and cathode, and hydrogen or the like is used to the anode side. Reducing gas is supplied to the catalyst layer through the gas diffusion layer. Hereinafter, the catalyst layer is also referred to as an electrode. Moreover, the material which comprises an electrode is called electrode material.

For example, when hydrogen gas and oxygen gas are used, H ₂ → 2H ⁺ + 2e ⁻ (E0 = 0 V) that occurs on the catalyst of the anode
Chemical reaction (oxidation reaction) and O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O occurring on the cathode catalyst (E0 = 1.23 V)
Power is generated using the energy difference (potential difference) of the chemical reaction (reduction reaction). As a catalyst for the reaction field, generally, a catalyst metal supported on conductive carbon is used. Under the operating conditions of the fuel cell, the catalyst metal is exposed to an environment in which elution and aggregation of the catalyst metal can occur, such as high temperature, low pH, and high potential in the cathode, and an oxidizing atmosphere. For this reason, elution and aggregation of the catalyst metal occur due to long-time operation, the surface area of the catalyst metal involved in the power generation reaction is reduced, and the performance of the fuel cell is reduced.

  In Patent Document 1, for the purpose of solving the problem that the performance of a fuel cell deteriorates due to the elution of catalyst metal particles, the catalyst metal is eluted by coating the catalyst metal particles with a porous inorganic (transition metal) material. The electrode material which can prevent is disclosed.

JP 2008-004541 A

  However, the electrode material disclosed in Patent Document 1 in which the catalyst metal particles are covered with a porous inorganic material suppresses the elution of the catalyst particles and greatly improves the durability of the fuel cell. Since the material is thickly coated in the range of 2 to 20 nm, the supply of the gas serving as the fuel onto the catalyst metal is hindered. As a result, there arises a problem that the overvoltage is increased and the power generation performance of the fuel cell is lowered.

  An object of the present invention is to provide an electrode material for a fuel cell capable of maintaining high output performance of the fuel cell for a long time and a fuel cell including an electrode using the electrode material.

  In the electrode material in which the catalytic metal particles supported on the conductive carbon fine particles are coated with the oxide of the transition metal, the present inventors have a ratio B / A of the atomic number A of the catalytic metal and the atomic number B of the transition metal. The oxygen reduction activity before and after the durability evaluation and the change in the electrochemical surface area (ECSA) of the catalytic metal were investigated. ECSA corresponds to the catalytic metal surface area that can participate in the power generation of the fuel cell, and can be calculated from the area of the hydrogen desorption wave in the cyclic voltammogram as the electrochemically effective catalytic metal surface area. Therefore, when the catalytic metal is eluted or agglomerated to reduce the surface area of the catalytic metal, the ECSA measurement value is reduced.

  As a result of the investigation, it has been found that there is a region with a ratio B / A in which the oxygen reduction activity of the electrode material continues to be kept high for a long time, elution of the catalyst metal is suppressed, and ECSA is unlikely to decrease.

The gist of the invention is as follows.
(1) A catalyst in an electrode material comprising a plurality of catalyst metal particles supported on conductive carbon fine particles, the supported catalyst metal particles being partly or entirely covered with an oxide of a transition metal. An electrode material, wherein the atomic number ratio of the number A of metal atoms to the number B of transition metal atoms is 0 <B / A ≦ 3.5.
(2) The electrode material according to (1), wherein the catalytic metal particles include one or more components of Pt, Ru, Rh, Pd, Co, Ni, and Au.
(3) The oxide of the transition metal includes one or more oxides of oxides of Si, Zr, Ti, Mg, Al, and In. (1) or (2) The electrode material as described.
(4) A fuel cell comprising an electrode using the electrode material according to any one of (1) to (3).

  With the electrode material of the present invention, it is possible to provide an electrode material for a fuel cell that can maintain high activity for a long time without impairing the output performance of the fuel cell.

It is a figure which shows the relationship between ECSA before and after durability evaluation, and transition metal atom number B / catalyst metal atom number A in an electrode material. Oxygen reduction current at 0.5 V (vs. standard calomel electrode (SCE)) is defined as oxygen reduction activity (ORR activity), ORR activity before and after durability evaluation, number of transition metal atoms in electrode material B / number of catalyst metal atoms It is a figure which shows the relationship with A. FIG. It is an electron microscope observation image figure in case the oxide of a transition metal covers the whole catalyst metal particle surface. It is an electron microscope observation image figure in case the oxide film of a transition metal covers a part of catalyst metal particle surface. It is an electron microscope observation image figure when the thing in which the oxide of a transition metal covers the whole catalyst metal particle surface and the thing in which the oxide film of a transition metal covers a part of catalyst metal particle surface are mixed.

[First Embodiment]
In the first embodiment, a plurality of catalyst metal particles are supported on conductive carbon fine particles, and the supported catalyst metal particles are composed of catalyst particles partially or entirely covered with an oxide of a transition metal, The electrode material is characterized in that the atomic number ratio of the number A of catalyst metal atoms to the number B of transition metal atoms in the electrode material is 0 <B / A ≦ 3.5.

(Thickness of the transition metal oxide film covering the catalytic metal particles)
When electrochemically evaluating the oxygen reduction activity and durability of the catalyst constituting the electrode of the fuel cell, the inventors have determined the oxygen reduction current and ECSA at 0.5 V (vs. SCE) before and after the durability evaluation test. The change was investigated. The oxygen reduction current at 0.5 V (vs. standard calomel electrode (SCE)) is also referred to as oxygen reduction activity (ORR activity). More specifically, an electrode material comprising catalyst particles in which a plurality of catalyst metal particles are supported on conductive carbon fine particles, and the supported catalyst metal particles are partially or entirely covered with an oxide of a transition metal. The effect of the ratio of the number of transition metal atoms B / the number of catalyst metal atoms A in the electrode material on the changes in ECSA and ORR activity before and after durability evaluation was evaluated.

  The catalyst metal is Pt, the transition metal is Si, and B / A is changed from 0 to 3.7 to prepare an electrode material. The electrode material is subjected to ECSA and an oxygen reduction current at 0.5 V (vs. SCE). For clarity, electrochemical measurements were performed using disk electrodes. These electrode materials were subjected to a durability test, and the electrochemical measurements were performed before and after the durability test. The initial value is before the durability test and the elapsed value is after the durability test. Specific details of the electrode material preparation conditions, electrochemical measurement conditions, and durability test conditions are the same as in Example 1 described later.

  In FIG. 1, the horizontal axis is B / A, the vertical axis is ECSA, and the initial value (●) and the elapsed value (Δ) are plotted. From FIG. 1, it was confirmed that the ECSA initial value of the electrode material of the present invention coated with an oxide coating of a transition metal was less lowered than the ECSA initial value of the uncoated electrode material. In addition, while the elapsed value of the uncoated (B / A = 0) catalyst is reduced by about 60% with respect to the initial value, the initial value of the electrode material of the present invention coated with the transition metal oxide film is It was confirmed that even if the value of B / A was reduced to 0.01, it decreased only by about 20%.

  In FIG. 2, the initial value (●) and elapsed value (Δ) are plotted with the oxygen reduction current at 0.5 V (vs SCE) as the oxygen reduction activity (ORR activity) on the vertical axis and the horizontal axis as B / A. . According to FIG. 2, the initial value of the oxygen reduction activity of the electrode material of the present invention coated with the oxide film of transition metal is less decreased than the initial value of the electrode material without coating (B / A = 0). confirmed. In addition, while the elapsed value of the uncoated catalyst is reduced by about 50% with respect to the initial value, the initial value of the electrode material of the present invention coated with the transition metal oxide film has a B / A value of 0. It was confirmed that even if it was reduced to 0.01, it decreased only by about 20%.

Catalyst metal and atomic number of the transition metal in the electrode material, the electrode material is analyzed by ICP-MS method, since the mass a and the mass b of the transition metal of the catalyst metal can be measured, the catalytic metal atomic weight M _a and a transition metal By dividing by the atomic weight M _b of
A = a / M _a
B = b / M _b
Can be calculated as

In FIG. 1, the range in which the initial value of ECSA (m ² / g) is 50 to 40 and the elapsed value is 38 to 35 is 0 <B / A ≦ 3.5. In FIG. 2, the range in which the oxygen reduction current at 0.5 V (vs SCE) is 0.003 or more is 0 <B / A ≦ 3.5.

(Catalyst metal and transition metal)
The catalyst metal can be used as long as it has a catalytic action for oxygen reduction reaction or a catalytic action for hydrogen oxidation reaction. Any known catalyst may be used as long as it contains one or more of Pt, Ru, Rh, Pd, Co, Ni, and Au.

  The transition metal oxide may be one containing at least one of Si, Zr, Ti, Mg, Al, and In oxides.

  The situation in which the catalytic metal particles supported on the conductive carbon fine particles are covered with the transition metal oxide can be confirmed by an electron microscope. There are three types of forms in which the transition metal oxide film covers the catalyst metal particles. FIG. 3 shows an electron microscope observation image when the transition metal oxide covers the entire surface of the catalyst metal particles. The electron microscope observation image figure when an oxide film covers a part of catalyst metal particle surface is shown in FIG. 4, and the transition metal oxide film covers all of the catalyst metal particle surface and the transition metal oxide film. FIG. 5 shows an electron microscope observation image diagram in the case where a material covering a part of the catalyst metal particle surface is mixed.

(Conductive carbon material)
The conductive carbon fine particle is not particularly limited as long as it is a carbon material having an electron conductivity that is generally present, but the carbon material is configured by performing a chemical reaction other than the originally required reaction or contacting with water. A material from which the substance is eluted is not preferable, and a chemically stable carbon material is preferable. An example of a preferable conductive carbon fine particle material is carbon black.

[Second Embodiment]
The second embodiment is a polymer electrolyte fuel cell comprising an electrode using the electrode material described in the first embodiment. An electrode using the electrode material described in the first embodiment is provided as a catalyst layer serving as an anode or a cathode with a proton conductive electrolyte membrane interposed therebetween, and a gas diffusion layer is further disposed on the outer side of the electrode layer. A separator is placed outside.

[Example 1]
(Production of electrode material)
A commercially available carbon black (BET surface area = 1270 m ² / g) is used as the conductive carbon fine particles, and a specific example in which Pt is supported as a catalytic metal on the carbon black will be described. However, the present invention is not limited thereto. Absent. First, carbon black, chloroplatinic acid, water, and ethanol were dispersed in a mixed solution in a predetermined ratio, and then degassed. Next, aqueous ammonia was slowly added dropwise as a precipitant (reducing agent) and stirred for 1 hour. Cleaning and filtration were performed using the ammonia water. Thereafter, the Pt particles were supported on the carbon black in a highly dispersed manner by calcining in He gas at a temperature of 623 K for 3 hours. Thereby, carbon black carrying Pt particles could be obtained.

  Next, an example in which carbon black supporting Pt particles is coated with an oxide of Si will be described, but the present invention is not limited to this.

  First, carbon black carrying Pt particles obtained in the above process was dispersed in a dehydrated ethanol solution. Next, 3-aminopropyltriethoxysilane was added dropwise and stirred for 30 minutes. Thereafter, tetraethoxysilane was added dropwise and stirred for 60 minutes. Thereafter, the mixture was stirred while slowly dropping water over 24 hours. Thereafter, washing with alcohol and filtration were performed, followed by vacuum drying. Thereafter, heat treatment was performed at a temperature of 573 K in an Ar atmosphere to obtain a desired electrode material. The obtained electrode material was subjected to quantitative analysis of elements by ICP-MS.

(Electrode material using Pt and Si)
Clarify oxygen reduction current at ECSA and 0.5 V (vs. SCE) for electrode materials in which Pt is the catalyst metal, Si is the transition metal, and B / A is changed from 3.7 to 0.0. Therefore, electrochemical measurement was performed using a disk electrode.

  The electrochemical measurement was performed by the following procedure using a rotating ring disk evaluation apparatus (RRDE-1) for measuring the thickness of the sun (RRDE-1) and a potentiostat SI1287 manufactured by Solartron.

  A slurry obtained by dispersing 3 mg of the produced electrode material in 500 mg of ethanol containing 60 mg of a 5 mass% Nafion (registered trademark) solution on a commercially available disk electrode (disk diameter 6 mm) was dried and used as a test electrode. The amount of slurry applied was adjusted to 0.02 mg. A 0.1N sulfuric acid aqueous solution was used as the electrolyte, and a cell configuration was used in which SCE was used as the reference electrode and a Pt plate was used as the counter electrode. The temperature of the electrolyte was 293 K, and oxygen gas was bubbled to saturate oxygen. The electrode of the electrode rotates at 2400 rpm, and the potential is swept from 0.7 V (vs. SCE) to 0.1 V (vs. SCE) at a rate of 10 mV / sec. The oxygen reduction current value flowing through the electrode was measured, and the oxygen reduction current value at a working electrode potential of 0.5 V (vs. SCE) was recorded as the initial value of the oxygen reduction activity (ORR activity). Next, the electrolytic solution is replaced with a nitrogen-saturated electrolytic solution, the temperature is set to 293 K, and a cyclic scan is performed at a potential scanning speed of 10 mV / sec in the range of −0.3 V (vs. SCE) to 1.0 V (vs. SCE). Click voltammogram was performed, ECSA was calculated from the electric quantity of hydrogen desorption wave, and used as the initial value of ESCA.

  Next, as a durability evaluation test, a rectangular wave potential cycle was applied for 20000 cycles, with 0.6 V (vs. SCE) holding for 6 seconds and 1.0 V (vs. SCE) holding for 6 seconds as one cycle. .

  When the durability evaluation test was completed, cyclic voltammogram was performed at a potential scanning speed of 10 mV / sec in the range of −0.3 V (vs. SCE) to 1.0 V (vs. SCE), ECSA was calculated, and ESCA The elapsed value of Thereafter, the electrolyte was replaced, oxygen gas was bubbled, and the temperature was 293 K in a state where oxygen was saturated. The electrode of the electrode rotates at 2400 rpm, and the potential is swept from 0.7 V (vs. SCE) to 0.1 V (vs. SCE) at a rate of 10 mV / sec. The relationship with the oxygen reduction current flowing through the electrode was measured, the oxygen reduction current value at a working electrode potential of 0.5 V (vs. SCE) was recorded, and this was taken as the elapsed value of oxygen reduction activity (ORR activity).

Table 1 shows the initial value and elapsed value of ESCA, and the initial value and elapsed value of oxygen reduction activity (ORR activity). The initial value and elapsed value of ECSA are both 38 m ² / g or more, and the initial value and elapsed value of oxygen reduction current (ORR activity) at a working electrode potential of 0.5 V (vs. SCE) are both 0.003 mA / In the case of cm ² or more, it is evaluated as ○. Either the initial value of ESCA or the elapsed value is less than 38 m ² / g, and the initial value or elapsed value of the oxygen reduction current (ORR activity) at a working electrode potential of 0.5 V (vs. SCE) is 0.003 mA. When it is less than / cm ² , it is evaluated as x.

  When 0 <B / A ≦ 3.5, the electrochemical measurement result can be evaluated as ◯. However, when B / A = 0 or 3.7, the electrochemical measurement result is evaluated as x.

Example 2 (electrode material using Pt and Zr, Ti, Mg, Al, In)
Pt was used as the catalyst metal, and Zr, Ti, Mg, Al, and In were used as the transition metal. The tetraethoxysilane in Example 1 was changed to tetraethoxytitanium (for Ti), tetraethoxyzirconium (for Zr), aluminum triethoxide (for Al), indium triethoxide (for In), magnesium diethoxide (for Mg). ) Respectively. About each, B / A was changed into two types, 2.0 and 3.7. The electrode material manufacturing conditions other than those described above are the same as in Example 1. In order to clarify the oxygen reduction current at ECSA and 0.5 V (vs SCE), electrochemical measurement was performed using a disk electrode. The electrochemical measurement, durability evaluation test conditions, and evaluation determination conditions were the same as those in Example 1 above. These results are shown in Table 2 below.

  When 0 <B / A ≦ 3.5, the electrochemical measurement result can be evaluated as ◯. However, in the case of 3.7, the electrochemical measurement result is evaluated as x.

[Example 3] (Ru, Rh, Pd, Co, Ni, Au, and electrode material using Si)
Examples in which Ru, Rh, Pd, Co, Ni, and Au are used as the catalyst metal and Si is used as the transition metal are shown in Table 3. The same conditions as in Example 1 are adopted except that the type of catalyst metal is changed. In Table 3, “Pt—Ru”, “Pt—Rh”, “Pt—Au”, “Pd—Ni”, and “Pd—Co” mean alloys of the described transition metals.

  In either case, 0 <B / A ≦ 3.5, and the electrochemical measurement result can be evaluated as ◯.

[Example 4] (Evaluation as a fuel cell)
A fuel cell was constructed and evaluated using the electrode materials described in Examples 1 to 13 and Comparative Examples 1 to 7 in [Example 1].

  Add 5% Nafion solution (manufactured by Aldrich) in an argon stream so that the mass of Nafion solids is 3 times the mass of the catalyst, and after gently stirring, pulverize the catalyst with ultrasonic waves to remove the platinum catalyst and Nafion. Butyl acetate was added with stirring so that the combined solid content concentration was 2% by mass to prepare each catalyst layer slurry.

  The catalyst layer slurry was applied to each side of a Teflon (registered trademark) sheet by a spray method, dried in an argon stream at 80 ° C. for 10 minutes, and then dried in an argon stream at 120 ° C. for 1 hour. Solid polymer fuel cell electrodes containing the electrode materials described in Examples 1 to 13 and Comparative Examples 1 to 7 in the catalyst layer were obtained.

For each electrode, conditions such as spraying were set so that the amount of platinum used was 0.10 mg / cm ² . The amount of platinum used was determined by measuring the dry mass of a Teflon (registered trademark) sheet before and after spray coating and calculating the difference.

Two 2.5 cm square pieces are cut from the obtained polymer electrolyte fuel cell electrode, and the electrolyte membrane (Nafion 112) is sandwiched between two electrodes of the same type so that the catalyst layer is in contact with the electrolyte membrane. And hot pressing at 130 ° C. and 90 kg / cm ² for 10 minutes. After cooling to room temperature, only the Teflon (registered trademark) sheet was carefully peeled off to fix the anode and cathode catalyst layers to the Nafion membrane. Further, two commercially available carbon cloths (EC-CC1-060 manufactured by ElectroChem) were cut into 2.5 cm square sizes, and the anode and cathode fixed on the Nafion membrane were sandwiched at 130 ° C. and 50 kg / cm ² . Hot pressing was performed for 10 minutes to prepare 20 types of membrane / electrode assemblies (Membrane Electrode Assembly, MEA).

  The initial current density was measured by incorporating each manufactured MEA into a fuel cell measuring apparatus. Current density measurement measures the current density that flows when the cell terminal voltage is 0.8V by gradually changing the voltage between the cell terminals from the open circuit voltage (usually about 0.9 to 1.0V) to 0.2V. did. The current density after the endurance test was the same as the measurement of the initial current density after the endurance test.

  As an endurance test, a cycle of holding the cell terminal voltage at 0.9 V for 15 seconds and holding the cell terminal voltage at 1.3 V for 15 seconds was performed 4000 times.

  The gas is supplied to the cathode with air and pure hydrogen to the anode so that the utilization rates are 50% and 80%, respectively, and the pressure of each gas is adjusted to 0.1 MPa by a back pressure valve provided downstream of the cell. did. The cell temperature was set to 70 ° C., and the supplied air and pure hydrogen were bubbled in distilled water kept at 50 ° C. and humidified.

Table 4 shows the initial current density of each MEA and the current density after the durability test. When both the initial current density and the value of the current density after the durability test were 130 (mA / cm ² ) or more, they were evaluated as “good”, and those less than this were evaluated as “poor”.

  MEAs using the catalysts of Examples 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 of the present invention exhibit excellent initial battery performance, High battery performance is maintained after the durability test.

  It can be seen that the MEA using the catalyst of Comparative Example 2 has excellent initial battery performance, but has low battery performance after the durability test and is inferior in durability. In addition, the MEA using the catalysts of Comparative Examples 1, 3, 4, 5, 6, and 7 has a low deterioration rate indicating a difference between the initial battery performance and the battery performance after the durability test, and is excellent in durability. The initial battery performance is inferior to the MEA using the catalysts of Examples 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and 13 of the present invention.

1: catalytic metal particles 2: oxide film of transition metal 3: conductive carbon particles

Claims (4)

  A plurality of catalyst metal particles are supported on conductive carbon fine particles, and the supported catalyst metal particles are composed of catalyst particles partially or wholly covered with an oxide of a transition metal, and the catalyst metal atoms in the electrode material An electrode material wherein the ratio of the number of atoms A to the number of transition metal atoms B is 0 <B / A ≦ 3.5.   2. The electrode material according to claim 1, wherein the catalytic metal particles comprise one or more components of Pt, Ru, Rh, Pd, Co, Ni, and Au.   3. The electrode material according to claim 1, wherein the transition metal oxide includes one or more oxides of Si, Zr, Ti, Mg, Al, and In.   A fuel cell comprising an electrode using the electrode material according to any one of claims 1 to 3.

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