JP2009252587A - Manufacturing method of catalyst layer for fuel cell, and performance evaluation method of catalyst layer for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To define a high-output range of a catalyst layer for a fuel cell containing a catalyst constituent, an electrolyte material and a carbon carrier to prepare a high-performance catalyst layer for a fuel cell conforming to the range. <P>SOLUTION: This manufacturing method is used for manufacturing a catalyst layer for a fuel cell containing a catalyst constituent, an electrolyte material and a carbon carrier. The manufacturing method of the catalyst layer for a fuel cell is characterized in that the catalyst layer is prepared so that a ratio of a steam adsorption amount obtained from a steam adsorption isotherm to a nitrogen adsorption amount obtained from a nitrogen adsorption isotherm is set to 1.0-4.0. This evaluation method is used for evaluating performance of a catalyst layer for a fuel cell containing a catalyst constituent, an electrolyte material and a carbon carrier. The performance evaluation of the catalyst layer for a fuel cell is characterized by using, as an index, a ratio of a steam adsorption amount obtained from a steam adsorption isotherm to a nitrogen adsorption amount obtained from a nitrogen adsorption isotherm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a fuel cell catalyst layer and a performance evaluation method for a fuel cell catalyst layer. In particular, it relates to defining a range that maximizes the output performance of the fuel cell.

  The basic structure of a polymer electrolyte fuel cell is that a catalyst layer that serves as an anode and a cathode is disposed with a polymer electrolyte membrane having proton conductivity interposed therebetween, and a gas diffusion layer is disposed on the outer side with the catalyst layer interposed therebetween. A separator is arranged on the outer side to constitute a unit cell. Usually, a fuel cell is used that is stacked in accordance with the required output.

In order to take out the current from the fuel cell having such a 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 on both electrodes, and a reducing property such as hydrogen is supplied to the anode side. Gas is supplied to the catalyst layer through the gas diffusion layer. For example, when hydrogen gas and oxygen gas are used, H ₂ → 2H ⁺ + 2e ⁻ (E ₀ = 0V) occurring on the catalyst of the anode
And O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O that occurs on the cathode catalyst (E ₀ = 1.23 V)
The current is taken out using the energy difference of the chemical reaction.

  Therefore, a gas diffusion path through which oxygen gas or hydrogen gas can move from the gas flow path of the separator to the catalyst inside the catalyst layer, and a proton conduction path through which protons generated on the anode catalyst can be transmitted to the cathode catalyst through the proton conductive electrolyte membrane. In addition, the current cannot be efficiently extracted unless the electron transfer paths through which the electrons generated on the anode catalyst can be transmitted to the cathode catalyst through the gas diffusion layer, the separator, and the external circuit are not disconnected.

  Inside the catalyst layer, generally, pores formed in the gaps between the materials as gas diffusion paths, ionomers (electrolyte materials) as proton conduction paths, and conductive materials such as carbon materials and metal materials as electron conduction paths, A network is formed.

  In particular, an ion exchange resin typified by a perfluorosulfonic acid polymer is used as a polymer electrolyte material in the proton conduction path. These generally used ion exchange resins exhibit high proton conductivity for the first time in a wet environment, and the proton conductivity decreases in a dry environment. This is thought to be due to the interposition and accompanying of water molecules for proton transfer. Therefore, in order to operate the fuel cell efficiently, it is essential that the electrolyte material is always in a wet state, and it is necessary to always supply water vapor together with the gas supplied to both electrodes.

  In general, for the purpose of supplying water to the electrolyte material, a method of humidifying the gas supplied to the cell and operating the cell at a temperature near the dew point is employed. According to this method, the water vapor supplied into the cell partially aggregates to form droplets of aggregated water. On the other hand, when current is taken out by the cathode reaction described above, water is generated on the cathode catalyst. The generated water aggregates when the water vapor in the catalyst layer becomes supersaturated, and forms droplets of the aggregated water.

  The droplets formed by the water vapor supplied to humidify agglomerate in the gas diffusion layer or the catalyst layer or the water produced by the reaction agglomerate block the gas diffusion path. This phenomenon is called flooding, and is remarkable in the cathode in which a large amount of water is generated during a large current discharge, leading to insufficient supply of gas and causing an extreme voltage drop.

  For this reason, in order to operate the fuel cell stably, it is necessary to satisfy the conflicting requirements that the condensed water is quickly discharged out of the system while sufficiently humidifying the inside of the catalyst layer. For this, it is necessary to optimize the wet state in the MEA.

  For example, in Patent Document 1 below, by using a catalyst layer that reduces the amount of expensive noble metal used such as platinum used for the catalyst and efficiently transfers the gas, electrons, and protons inside the catalyst layer, In order to provide a low-cost fuel cell with excellent output characteristics, at least the catalyst layer of the cathode is composed of a mixture containing a catalyst component, an ionomer, and a carbon material, and the catalyst support carbon material has a temperature of 25 ° C. and a relative humidity. An invention is disclosed in which the water vapor adsorption amount at 90% is 50 ml / g or more.

  Patent Document 2 listed below is intended to provide an electrode structure that can maintain a moist environment suitable for an ionomer and extract the characteristics of the catalyst layer without disrupting the gas diffusion path inside the electrode. A gas diffusion layer for a fuel cell having a two-layer structure having a micropore layer mainly composed of carbon black on one side of a gas diffusion fiber layer mainly composed of a carbon-like carbon material, wherein the temperature of the carbon black of the micropore layer is 25 ° C. The invention discloses that the water vapor adsorption amount at a relative humidity of 90% is 100 ml / g or less.

  Patent Document 1 and Patent Document 2 focus on the water vapor adsorption amount as a parameter for knowing the wet state.

  However, in Patent Document 1, when the amount of ionomer contained in the electrode changes, it is impossible to evaluate the exact wet state of the electrode even if only the water vapor adsorption amount is defined. As the amount of ionomer in the electrode increases, the gas permeability decreases, so that the amount of gas adsorption on the catalyst particles coated with the ionomer decreases. Therefore, a phenomenon occurs in which the water vapor adsorption amount decreases.

  Patent Document 2 states that the wet state in the MEGA can be optimized by using a diffusion layer having a micropore layer having a water vapor adsorption amount of 100 ml / g or less. However, since the reaction related to power generation of the fuel cell occurs in the catalyst layer, and the catalyst layer is composed of Pt (alloy) -supported carbon and ion conductive ionomer, it is not possible to apply the conventional technology as it is to the catalyst layer. Of course not. When the catalyst species or ionomer species of the catalyst layer and the combination of the catalyst and ionomer are changed, the wet state in the catalyst layer is changed. Since the wet state is a phenomenon in the catalyst layer, the wet state cannot always be made optimal by the diffusion layer of 100 ml / g or less. That is, since the diffusion layer has a completely different function from the catalyst layer, the diffusion layer material cannot be diverted to the catalyst layer. When the material of the catalyst layer is changed, the wet state in the catalyst layer changes. Optimizing the water supply from the diffusion layer and the water discharge to the diffusion layer in order to keep the wet state optimal is considered as one means, but for this purpose, the water vapor adsorption amount of the catalyst layer and the water vapor adsorption of the diffusion layer are considered. It is necessary to quantify the balance with the quantity and specify the optimum range. However, the conventional technology tries to solve the problem only on the diffusion layer side, and it is impossible to keep the wet state in the catalyst layer optimal in any catalyst layer specification.

Japanese Patent Laying-Open No. 2005-332807 JP 2006-120335 A

  An object of the present invention is to define a high output range of a fuel cell catalyst layer containing a catalyst component, an ionomer, and a carbon support, and to prepare a high performance fuel cell catalyst layer suitable for the range.

  The inventor has found a new index representing a wet state when the amount of ionomer in the electrode catalyst layer is different, and has found a range in which the output performance of the fuel cell is maximized by the index, and has reached the present invention.

  That is, as in Patent Document 1, when the amount of ionomer contained in the electrode changes, it is impossible to evaluate the exact wet state of the electrode even if only the water vapor adsorption amount is defined. As the amount of ionomer in the electrode increases, the gas permeability decreases, so that the amount of gas adsorption on the catalyst particles coated with the ionomer decreases. Therefore, a phenomenon occurs in which the water vapor adsorption amount decreases. However, since the ionomer is hydrophilic, the actual electrode should become hydrophilic due to the increase in the amount of ionomer. Therefore, by measuring the amount of nitrogen adsorbed by nitrogen gas and dividing the amount of water vapor adsorption by the amount of nitrogen adsorption, the difference in gas permeability can be canceled, and the accurate wet state of the electrode can be evaluated.

  That is, first, the present invention is an invention of a method for producing a catalyst layer for a fuel cell comprising a catalyst component, an ionomer, and a carbon support. From the water vapor adsorption amount determined from the water vapor adsorption isotherm and the nitrogen adsorption isotherm. It is characterized in that it is prepared so that the ratio of the obtained nitrogen adsorption amount is 1.0 to 4.0.

  Specifically, by adjusting the ratio of the ionomer and the carbon support to 0.4 to 1.1, the ratio of the water vapor adsorption amount and the nitrogen adsorption amount can be adjusted to 1.0 to 4.0. When a fuel cell catalyst layer having a ratio between the water vapor adsorption amount and the nitrogen adsorption amount in the range of 1.0 to 4.0 is used, the maximum output can be obtained.

  2ndly, this invention is invention of the performance-evaluation method of the catalyst layer for fuel cells containing a catalyst component, an ionomer, and a carbon support, and is calculated | required from the water vapor adsorption amount and nitrogen adsorption isotherm calculated | required from a water vapor adsorption isotherm. The ratio of nitrogen adsorption is used as an index.

  Specifically, since the maximum output can be obtained when the ratio of the water vapor adsorption amount and the nitrogen adsorption amount is 1.0 to 4.0, the performance evaluation is determined to be good.

  According to the present invention, it is possible to easily determine the maximum output range of the fuel cell and to manufacture a high-performance fuel cell catalyst layer.

  The type of carbon support used in the fuel cell catalyst layer of the present invention is not particularly limited as long as it is a generally existing carbon material having electron conductivity. A material that causes the material constituting the carbon material to elute due to contact with the agglomerated water is not preferable, and a chemically stable carbon material is preferable.

  As a preferred carbon support, carbon black is the most common, but in addition, graphite, carbon fiber, activated carbon and the like, pulverized products thereof, carbon compounds such as carbon nanofiber and carbon nanotube, and the like can be used. Moreover, these two or more types can be mixed and used.

  The catalyst component used in the catalyst layer of the present invention and supported on the carbon support is not particularly limited. Specifically, examples of catalyst components include noble metals such as platinum, palladium, ruthenium, gold, rhodium, osmium and iridium, composites and alloys of noble metals obtained by combining two or more of these noble metals, noble metals and organic compounds, A complex with an inorganic compound, a transition metal, a complex of a transition metal with an organic compound or an inorganic compound, a metal oxide, and the like can be given. Moreover, what compounded these 2 or more types can also be used.

  A preferable content of the carbon support in the catalyst layer is not specified because it is affected by the type and content of the carbon support and the type and loading of the catalyst component. If it is the range of 5 mass% or more and 80 mass% or less, a fuel cell will function at least and the effect of this invention can be acquired. If a more preferable range is illustrated, it is 10 mass% or more and 60 mass% or less. If it is out of this range, the balance with other main components is deteriorated, and an efficient fuel cell cannot be obtained. For example, if it is less than 5% by mass, the amount of the catalyst component supported on the carbon support becomes too small. For example, if it exceeds 80% by mass, the amount of the electrolyte material becomes too small and the proton transmission path becomes poor, so that an efficient battery cannot be obtained.

  The polymer electrolyte membrane used in the fuel cell including the catalyst layer of the present invention and the ionomer used in the catalyst layer are polymers introduced with phosphoric acid groups, sulfonic acid groups, etc., such as perfluorosulfonic acid polymers and the like. Examples of the polymer include benzenesulfonic acid introduced, but the polymer is not limited to a polymer, and may be an inorganic or inorganic-organic hybrid electrolyte membrane. If a particularly preferable operating temperature range is exemplified, a range from room temperature to 150 ° C. is preferable.

  The method for producing the fuel cell catalyst layer of the present invention is not particularly limited. For example, an ink is prepared by adding a solution in which an electrolyte such as perfluorosulfonic acid polymer is dissolved or dispersed on a carbon support on which a catalyst is supported, and adding water or an organic solvent as necessary. This ink can be dried into a film and used as a catalyst layer.

  About the water vapor adsorption amount obtained from the water vapor adsorption isotherm as an index in the present invention and the nitrogen adsorption amount obtained from the nitrogen adsorption isotherm, about 50 mg of the electrode catalyst powder is weighed and vacuum degassed at 120 ° C. for 8 hours. The adsorption isotherm of water and nitrogen was measured for this powder using a constant volume method, and the amount of adsorption at a relative pressure of 0.5 was defined as the amount of water vapor and nitrogen adsorbed. The measurement temperature is nitrogen 77K and water vapor 323.15K.

Examples of the present invention will be described below.
[Preparation of catalyst powder and production of membrane electrode assembly (MEA)]
Carbon black of Ketjen Black EC (registered trademark, Ketjen Black International Co., Ltd.) is used as the catalyst-supporting carbon support, and the catalyst powder is obtained by supporting platinum particles on the carbon black at 60 wt%.

  The catalyst powder was mixed with a dispersion solvent composed of water and ethanol, and further mixed with Nafion (trade name, DuPont) which is an ionomer having ion conductivity. At this time, the ratio of the Nafion weight to the carbon black weight is 0.25, 0.375, 0.5, 0.75, 1.0, 1.1, 1.125, 1.15 and 1. 25.

In the mixed solution, catalyst particles are uniformly dispersed in a solvent by an ultrasonic homogenizer. The dispersion was applied onto a Teflon (trade name) sheet substrate. The coating amount was adjusted so that the Pt weight was 0.3 to 0.5 mg / cm ² . The coated sheet was dried in an environment of vacuum and 80 ° C. for 2 hours.

  The sheet was hot-pressed onto Nafion 112 (trade name), which is a polymer electrolyte membrane, and the substrate was peeled off to obtain a membrane electrode assembly (MEA).

  An electrode was prepared by joining a diffusion layer made of carbon cloth or carbon paper to the MEA, and a fuel cell was obtained by sandwiching the MEA with a carbon or metal separator.

  The electrode powder used for measuring the amount of water vapor adsorption and the amount of nitrogen adsorption was scraped from the coated sheet after drying.

[Battery performance evaluation]
A diffusion layer (GDL) composed of a carbon base material and a water repellent layer (carbon + PTFE) is disposed outside the MEA, and electricity is generated by flowing hydrogen on the anode side and air on the cathode side. The battery performance was evaluated based on the voltage value at each load current.

[Measurement of water vapor adsorption and nitrogen adsorption]
About 50 mg of electrode powder is weighed and vacuum degassed at 120 ° C. for 8 hours. The adsorption isotherm of water and nitrogen was measured for this powder using a constant volume method, and the amount of adsorption at a relative pressure of 0.5 was defined as the amount of water vapor and nitrogen adsorbed. The measurement temperature was nitrogen 77K and water vapor 323.15K.
Table 1 below shows a list of data on the amounts of water vapor and nitrogen adsorbed and the obtained voltage values.

図１に、従来の指標として用いられていた水蒸気吸着量とアイオノマー量との関係を示す。図１の結果より、アイオノマー量と湿潤状態との関係は水蒸気吸着量だけでは説明できないことがわかる。

FIG. 1 shows the relationship between the water vapor adsorption amount and the ionomer amount used as a conventional index. From the result of FIG. 1, it can be seen that the relationship between the ionomer amount and the wet state cannot be explained only by the water vapor adsorption amount.

  2 to 4 show the relationship between the water vapor adsorption amount / nitrogen element adsorption amount and the output at each current value of each electrode powder. From these figures, it can be determined that the water vapor adsorption amount / nitrogen adsorption amount is preferably in the range of 1.0 to 4.0 in order to obtain the maximum output.

  FIG. 5 shows the relationship between the ionomer amount / carbon amount and the water vapor adsorption amount / nitrogen adsorption amount. From the results of FIG. 5, it is considered that the hydrophilicity of the electrode increases as the ionomer amount of the electrode increases. It is preferable to use the water vapor adsorption amount / nitrogen adsorption amount as an index representing the wet state of the electrode powder, In order to obtain an output, in order to make the water vapor adsorption amount / nitrogen adsorption amount in the range of 1.0 to 4.0, it can be seen that the ionomer amount / carbon amount should be 0.4 to 1.1.

The reason why the water vapor adsorption amount / nitrogen adsorption amount is preferably in the range of 1.0 to 4.0 in order to obtain the maximum output is considered as follows.
(1) As a function of the electrode powder, proton conductivity is necessary, and accompanying water is necessary for proton conduction. When the water vapor adsorption amount / nitrogen adsorption amount is less than 1.0, the hydrophilicity of the electrode powder is insufficient, so that the accompanying water is insufficient and the proton conductivity is reduced, resulting in a decrease in performance.
(2) Gas diffusivity is necessary as another function of the electrode powder. If the water vapor adsorption amount / nitrogen adsorption amount exceeds 4.0, the retention of water produced by the reaction is too strong, and the amount of water that fills the pores existing in the electrode is retained, thus inhibiting gas diffusion. As a result, reaction inhibition is caused.

  As shown in FIG. 1, when the amount of ionomer in the electrode increases, the original gas permeability decreases, so that the correlation cannot be obtained only with the water vapor adsorption amount, which is a conventional index indicating a wet state. Therefore, it became possible to correctly represent the wet state by the water vapor adsorption amount / nitrogen adsorption amount, which is a new index.

  The maximum output range of the fuel cell can be easily determined and the production of a high-performance fuel cell catalyst layer can be achieved, thereby contributing to the practical application and popularization of fuel cells.

The relationship between the water vapor adsorption amount and the ionomer amount used as a conventional index is shown. The relationship between the water vapor adsorption amount / nitrogen element adsorption amount of each electrode powder and the output is shown. The relationship between the water vapor adsorption amount / nitrogen element adsorption amount of each electrode powder and the output is shown. The relationship between the water vapor adsorption amount / nitrogen element adsorption amount of each electrode powder and the output is shown. The relationship between the ionomer amount / carbon amount and the water vapor adsorption amount / nitrogen adsorption amount is shown.

Claims (4)

  In the method for producing a catalyst layer for a fuel cell including a catalyst component, an ionomer, and a carbon support, a ratio of a water vapor adsorption amount obtained from a water vapor adsorption isotherm to a nitrogen adsorption amount obtained from a nitrogen adsorption isotherm is 1.0 to 4. A method for producing a catalyst layer for a fuel cell, which is prepared so as to be zero.   The ratio of the ionomer and the carbon support is adjusted to 0.4 to 1.1 so that the ratio of the water vapor adsorption amount and the nitrogen adsorption amount is 1.0 to 4.0. Of manufacturing a fuel cell catalyst layer.   In a method for evaluating the performance of a catalyst layer for a fuel cell containing a catalyst component, an ionomer, and a carbon support, a ratio of a water vapor adsorption amount obtained from a water vapor adsorption isotherm and a nitrogen adsorption amount obtained from a nitrogen adsorption isotherm is used as an index. A method for evaluating the performance of a fuel cell catalyst layer.   4. The method for evaluating the performance of a fuel cell catalyst layer according to claim 3, wherein the performance evaluation is determined to be good when the ratio of the water vapor adsorption amount and the nitrogen adsorption amount is 1.0 to 4.0. 5.

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