JP5994729B2 - A fuel cell catalyst electrode layer, a membrane electrode assembly, a fuel cell, and a method for producing a fuel cell catalyst electrode layer. - Google Patents

Description

  The present invention relates to a catalyst electrode layer for a fuel cell, a membrane electrode assembly, and a fuel cell.

  The polymer electrolyte fuel cell includes a membrane electrode assembly in which an anode and a cathode as catalyst electrode layers are formed on a polymer electrolyte membrane having proton conductivity. The catalyst electrode layer generally includes catalyst particles including a catalyst metal and a polymer electrolyte having proton conductivity. Conventionally, various studies have been made to improve the performance of such fuel cells.

  Since the polymer electrolyte generally has good proton conductivity in an appropriate humidity environment (appropriate wet state), good output characteristics cannot be obtained in an environment with low humidity (so-called dry state). On the other hand, in Patent Document 1, a specific catalyst electrode layer is formed on a polymer electrolyte membrane having a specific physical property in a polymer having a specific structure in order to obtain good output characteristics even in a low humidity state. Membrane electrode assemblies having the following have been proposed. In Patent Document 2, the ionomer (polymer electrolyte) covering the catalyst of the catalyst electrode layer has a thickness of 13 nm in order to promote the power generation reaction by sufficiently increasing the moisture concentration in the ionomer (increasing the wet state). The membrane electrode assembly specified below has been proposed.

JP 2008-166049 A JP 2008-192490 A

  Here, the above prior art is made for the purpose of improving the steady output characteristics, and the output response characteristics (hereinafter referred to as “the output power characteristics”) when changing from the low output state to the high output state (for example, the maximum output state). No consideration has been given to "transient output characteristics". However, for example, when a transient change between a low output state and a high output state is repeated as in a vehicle equipped with a fuel cell, the transient output characteristic of the fuel cell is important. Therefore, a technique for improving the transient output performance of the fuel cell has been desired.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms.

(1) According to one aspect of the present invention, a catalyst electrode layer for a fuel cell is provided. The catalyst electrode layer is formed of a catalyst electrode material containing carbon carrying a catalyst and a polymer electrolyte; the thickness of the polymer electrolyte covering at least a part of the carbon carrying the catalyst is , 2.4 nm to 2.9 nm in a state of 40% relative humidity, and 3.5 nm to 4.2 nm in a state of 200% relative humidity. According to the catalyst electrode layer for a fuel cell of this embodiment, the relative humidity of the catalyst electrode layer is 40% by producing a fuel cell using the membrane electrode assembly produced using at least one of the catalyst electrode layers. The transient output performance can be improved when the low output state corresponding to the dry state is changed to the high output state corresponding to the wet state where the relative humidity is 200%.

(2) In the catalyst electrode layer for a fuel cell of the above aspect, the thickness of the polymer electrolyte is 2.6 nm to 2.8 nm in the 40% relative humidity state, and 200% The relative humidity is preferably 3.8 nm to 4.0 nm. According to the fuel cell catalyst electrode layer of this aspect, the transient output when the low output state corresponding to the dry state where the relative humidity is 40% is changed to the high output state corresponding to the wet state where the relative humidity is 200%. The performance can be further improved.

(3) According to another aspect of the present invention, a method for producing a catalyst electrode layer for a fuel cell is provided. This method includes (a) preparing a catalyst electrode material containing a catalyst-supported carbon and a polymer electrolyte; and (b) applying the catalyst electrode material on a substrate sheet to produce the fuel cell. Forming a catalyst electrode layer. In the step (a), the equivalent mass of the ion exchange group of the polymer electrolyte and the ratio of the mass of the polymer electrolyte to the mass of the carbon are adjusted to adjust the fuel cell catalyst formed. Controlling a thickness of the polymer electrolyte in a first humidity state and a thickness of the polymer electrolyte in a second humidity state higher than the first humidity state in an electrode layer . According to the method of manufacturing the fuel cell catalyst electrode layer of this embodiment, the fuel cell catalyst in which the thickness of the polymer electrolyte in the first wet state and the thickness of the polymer electrolyte in the second wet state are controlled An electrode layer can be formed.

  The present invention can be realized in various forms other than those described above. For example, the present invention can be realized in the form of a membrane electrode assembly or a fuel cell.

It is a cross-sectional schematic diagram showing the schematic structure of the fuel cell as one Embodiment of this invention. It is a schematic diagram which expands and shows the structure of the cathode formed on an electrolyte membrane. It is a graph which shows the example of alternating current impedance measurement. It is a graph which shows the example of the IV characteristic in a low oxygen concentration environment. It is a table | surface which shows the parameter and evaluation result of each sample. It is the graph which showed the relationship between the ionomer thickness in 40% RH of each sample shown in the table | surface of FIG. 5, and a transient output. 6 is a graph showing the relationship between ionomer thickness and transient output at 200% RH for each sample shown in the table of FIG. 5. 6 is a graph showing the relationship between ionomer thickness and proton resistance at 40% RH for each sample shown in the table of FIG. 5. 6 is a graph showing the relationship between ionomer thickness and gas diffusion resistance at 200% RH for each sample shown in the table of FIG. 5.

  FIG. 1 is a schematic cross-sectional view showing a schematic configuration of a fuel cell as an embodiment of the present invention. The fuel cell of the present embodiment is a polymer electrolyte fuel cell that generates power upon receiving a supply of a reaction gas (a fuel gas containing hydrogen and an oxidizing gas containing oxygen). The fuel cell has a stack structure in which a plurality of single cells 10 are stacked, and FIG. 1 shows the structure of the single cells 10.

  The single cell 10 includes an MEA (Membrane Electrode Assembly) 27, gas diffusion layers 23 and 24, and gas separators 25 and 26. The MEA 27 includes an electrolyte membrane 20 and an anode 21 and a cathode 22 that are catalyst electrode layers formed on each surface of the electrolyte membrane 20. The MEA 27 is sandwiched between the gas diffusion layers 23 and 24, and the sandwich structure composed of the MEA 27 and the gas diffusion layers 23 and 24 is further sandwiched between the gas separators 25 and 26 from both sides.

  The electrolyte membrane 20 is a proton conductive ion exchange membrane formed of a polymer electrolyte material, for example, a fluororesin, and exhibits good electrical conductivity in a wet state.

  The cathode 22 and the anode 21 include carbon particles supporting a catalytic metal that proceeds with an electrochemical reaction, and a polymer electrolyte (referred to as “ionomer”) having proton conductivity. As the catalyst metal, for example, platinum or a platinum alloy made of platinum and another metal such as ruthenium can be used. As the polymer electrolyte, the same polymer electrolyte as the material forming the electrolyte membrane 20 can be used. The polymer electrolyte included in the catalyst electrode layer may be the same type of polymer as the polymer electrolyte constituting the electrolyte membrane 20 or a different type of polymer.

  The gas diffusion layers 23 and 24 are made of a member having gas permeability and electronic conductivity, and are formed of, for example, a metal member such as foam metal or metal mesh, or a carbon member such as carbon cloth or carbon paper. can do.

  The gas separators 25 and 26 are formed of a gas-impermeable conductive member, for example, a carbon-made member such as dense carbon that has been made to be gas-impermeable by compressing carbon, or a metal member such as press-molded stainless steel. ing. In the gas separators 25 and 26, flow channel grooves 28 and 29 through which a reaction gas flows are formed on the surface facing the catalyst electrode layer. A porous body for forming an in-cell gas flow path may be disposed between the gas separators 25 and 26 and the gas diffusion layers 23 and 24. In this case, the flow path grooves 28 and 29 are provided. May be omitted.

  An inter-cell refrigerant flow path is further formed inside the fuel cell (not shown). Such a refrigerant flow path may be formed, for example, between all the stacked single cells, or may be formed every time a predetermined number of single cells are stacked.

  Further, the fuel cell is formed with a plurality of flow paths that penetrate the fuel cell in the stacking direction. Specifically, a gas manifold for supplying / discharging the reaction gas to / from each cell and a refrigerant manifold for supplying / discharging the refrigerant to / from the refrigerant flow path described above are formed. Further, in the fuel cell, a seal member (not shown) is arranged at a predetermined position between the outer periphery of the MEA 27 or the gas separators 25 and 26. Such a seal member prevents fluid leakage and prevents short circuit between the gas separators 25 and 26.

  FIG. 2 is an enlarged schematic view showing the configuration of the cathode 22 formed on the electrolyte membrane 20. The cathode 22 includes carbon particles (catalyst-supporting carbon 30) that support the catalyst metal 31, and a polymer electrolyte (ionomer) 32 that covers at least a part of the surface of the catalyst-supporting carbon 30. Although illustration is omitted, the anode 23 is configured similarly to the cathode 22.

  The ionomer 32 has a thickness Ti in the range of t1 to t2 in the first humidity state (40% RH), and in the range of t3 (> t2) to t4 in the second humidity state (200% RH). It is formed to become. These four values t1 to t4 may be, for example, t1 = 2.4 nm, t2 = 2.9 nm, t3 = 3.5 nm, and t4 = 4.2 nm. “% RH” is a unit of relative humidity, 40% RH indicates that the relative humidity is 40%, and 200% RH indicates that the relative humidity is 200%. The thickness of the ionomer can be controlled by the ratio (I / C) of the ionomer mass (or weight) I to the carbon mass (or weight) C and the equivalent mass (EW) indicating the amount of ion-exchange groups. .

  Here, the steady low output state of the fuel cell corresponds to a low humidity state (so-called dry state), and the steady high output state corresponds to a high humidity state (so-called wet state). For this reason, when the low output state is changed to the high output state, the environment changes from the low humidity state (dry state) to the high humidity state (wet state). The transient output characteristics when changing from the low output state to the high output state are estimated to be affected by the performance to recover from the dry state to the wet state and the performance to suppress the over-wet state beyond the wet state. The

  In the MEA 27 of the embodiment, as described above, the thickness Ti of the ionomer 32 of the cathode 22 and the anode 23 is defined in each of the first humidity state (low humidity state) and the second humidity state (high humidity state). It is controlled to be within the specified range. Thereby, in the fuel cell with the single cell 10 using the MEA 27, the transient output characteristic changing from the low output state to the high output state (maximum output state) has the same performance as the steady high output state (maximum output state). Can be improved.

A. Sample preparation:
FIG. 5 is a table showing parameters and evaluation results of 14 types of samples prepared for confirming the effects of the embodiment.

(1) Sample 1
As shown below, a catalyst was produced, a catalyst ink was produced using the catalyst, a catalyst electrode layer was produced using the catalyst ink, and a membrane electrode assembly (MEA) was produced using the catalyst electrode layer.

<Production of catalyst>
The catalyst support carbon has a surface area of 850 m ² / g, a primary particle size of 12 nm, a bulk density of 0.02 g / mL, a crystal diameter (La) of 20 nm, an iodine adsorption amount of 870 mg / g, and a DBP oil supply amount of 280 mL. / G of acetylene black carbon was used. Acetylene black carbon (5.0 g) was added to 1.2 L of pure water and dispersed. A hexahydroxo platinum nitric acid solution containing 5.0 g of platinum was dropped into the obtained dispersion and stirred sufficiently. About 100 mL of 0.1N ammonia was added to the dispersion after stirring to adjust the pH to about 10, and a hydroxide was formed and precipitated on carbon. Further, ethanol was added and reduced at 90 ° C. The dispersion after the reduction was filtered, and the obtained powder was vacuum-dried at 100 ° C. for 10 hours. This produced a catalyst powder. The platinum loading density of the obtained catalyst powder was determined by analyzing the waste liquid after filtration and was 50 wt%. Moreover, the crystallite diameter of platinum calculated | required by XRD analysis was about 2 nm.

<Preparation of catalyst ink>
10 mL of ultrapure water was added to 1 g of the obtained catalyst and stirred, and then 5 mL of ethanol was added, and the mixture was suspended with sufficient stirring using a stirring bar. An ionomer solution having an EW of 910 as an ionic conductor is slowly added to the obtained suspension until the ratio (I / C) of the solid part mass of the ionomer solution to the mass of carbon in the catalyst becomes 0.92. The solution was dropped and dispersed with an ultrasonic disperser for 30 minutes to form a uniform slurry, thereby preparing a catalyst ink as a catalyst electrode material.

<Preparation of catalyst electrode layer>
The obtained catalyst ink was uniformly coated on the substrate sheet so that the weight per unit area of platinum in the catalyst was 0.2 mg / cm ² . The base material sheet coated with the catalyst ink was dried at 80 ° C. for 3 hours to prepare a catalyst electrode layer. As the base sheet, a PTFE (polytetrafluoroethylene) sheet, for example, Teflon (registered trademark) is used.

<Preparation of membrane electrode assembly>
Nafion (registered trademark) 117 manufactured by DuPont was used as the solid polymer electrolyte membrane, and the obtained catalyst electrode layer was used as both anode and cathode electrodes. With the solid polymer electrolyte membrane sandwiched between the anode and the cathode, hot pressing was performed at 170 ° C. for 300 seconds to produce a membrane electrode assembly (MEA).

(2) Samples 2 to 14
Samples 2 to 14 were prepared in the same manner as Sample 1. However, the EW of the ionomer solution when preparing the catalyst ink and the I / C for defining the dropping amount of the ionomer solution were changed to the values shown in FIG.

B. Evaluation The thickness of the ionomer in the catalyst electrode layer, gas diffusion layer resistance, proton resistance (H ⁺ resistance), and transient output were determined for each of the prepared samples by the following methods.

<Ionomer thickness>
About each sample, the cross section of produced MEA was cut out using the focused ion beam (FIB) apparatus FB2200 made from Hitachi, respectively. The cross section of the cut-out MEA was observed using a transmission electron microscope (TEM) H-9500 made by Hitachi, and the thickness of the cathode ionomer was measured at 100 points to calculate the average ionomer thickness T1 (see FIG. 2).

  Next, an ionomer layer not containing a catalyst was prepared for each sample. Specifically, first, in each sample, an ionomer solution containing no catalyst was prepared in the same manner as that for preparing the catalyst ink. The obtained ionomer solution was applied onto a PTFE sheet using a squeegee. The PTFE sheet coated with the ionomer solution was dried at 80 ° C. for 3 hours, and then hot pressed at 170 ° C. for 300 seconds to produce an ionomer layer. And the cross section of the ionomer layer corresponding to each sample cut out with the FIB apparatus was observed with TEM, and the thickness (hereinafter also referred to as “ionomer layer reference thickness”) T2 of the ionomer layer corresponding to each sample was obtained.

Next, for the ionomer layer corresponding to each sample, the thickness of the ionomer layer (hereinafter also referred to as “ionomer layer 40% RH thickness”) T3 was obtained under the conditions of 80 ° C. and 40% RH in a thermostatic bath. In addition, the thickness of the ionomer layer (hereinafter also referred to as “ionomer layer 200% RH thickness”) T4 was determined under conditions of 80 ° C. and 200% RH in a thermostatic bath. For each sample, the average ionomer thickness T1, the ionomer layer reference thickness T2, the ionomer layer 40% RH thickness T3, and the ionomer layer 200% RH thickness T4 are used in the following formulas (1) and (2). The cathode ionomer thickness Ti (40% RH) at 40% RH and the cathode ionomer thickness Ti (200% RH) at 200% RH were determined.
Ti (40% RH) = T1 · (T3 / T2) (1)
Ti (200% RH) = T1 · (T4 / T2) (2)

<Proton resistance>
About each sample, the impedance measurement by the alternating current impedance method was performed in 90 degreeC and 40% RH.
FIG. 3 is a graph showing an example of AC impedance measurement. For each sample, proton resistance was determined from the obtained impedance measurement results as shown in FIG. That is, AC impedance was plotted on a complex plane, and the plot was approximated by two straight lines L1 and L2. And the distance on the real part axis | shaft Z 'between the intersection Cp of those straight lines and the point C0 where the 1st straight line L1 is tolerance | permissible is made into proton resistance.

<Gas diffusion resistance>
For each sample, the current versus voltage characteristics (IV characteristics) were measured at 60 ° C. and 200% RH, with the reaction gas supply environment being a low oxygen concentration environment (hydrogen concentration 100%, oxygen concentration several%).
FIG. 4 is a graph showing an example of IV characteristics in a low oxygen concentration environment. For each sample, the limiting current density was determined from the obtained IV characteristics. That is, on the IV characteristics, the limit current density was calculated by dividing the value of the current (limit current) in the portion where the current does not increase even when the voltage is decreased by the surface area of the catalyst electrode layer. The gas diffusion resistance (unit: sec / m) is obtained by calculating the time required for one molecule of oxygen to move 1 m based on the obtained limiting current density, temperature and humidity, and oxygen concentration. It was.

<Transient output>
For each sample, a low output (output voltage 0.9 V) was maintained for 30 minutes as a reaction gas supply environment in which 100% hydrogen was sufficiently supplied to the anode and air was sufficiently supplied to the cathode. 0.5V) was instructed (so-called step-like output change was instructed), and the transient output was calculated from the current value and voltage value 10 seconds later.

  FIG. 6 is a graph showing the relationship between ionomer thickness and transient output at 40% RH for each sample shown in the table of FIG. FIG. 7 is a graph showing the relationship between the ionomer thickness and the transient output at 200% RH for each sample shown in the table of FIG. As shown in FIGS. 6 and 7, the ionomer thickness at 40% RH (hereinafter also referred to as “ionomer thickness (@ 40% RH)”) is in the range of 2.4 nm to 2.9 nm, and at 200% RH. Samples 1 to 7 having an ionomer thickness (hereinafter also referred to as “ionomer thickness (@ 200% RH)”) of 3.5 nm to 4.2 nm had a transient output of 99 kW to 102 kW. In contrast, in other samples 8 to 14 in which either the ionomer thickness (@ 40% RH) or the ionomer thickness (@ 200% RH) is not within the above range, the transient output is as low as 96 kW to 86 kW. . Since the measurement error of the transient output is ± 1 kW or less, the sample 1 to 7 having a transient output higher than the boundary value and the samples 8 to 14 having a transient output lower than the boundary value are compared with 97 kW as a boundary value. Therefore, it was judged that the transient output performance was excellent.

  FIG. 8 is a graph showing the relationship between the ionomer thickness and proton resistance at 40% RH for each sample shown in the table of FIG. As shown in FIG. 8, it can be seen that the proton resistance increases as the ionomer thickness (@ 40% RH) decreases. In general, when the proton resistance is high, the transient output increase rate tends to be low. Therefore, when the ionomer thickness (@ 40% RH) decreases, it is assumed that the transient output performance tends to decrease overall.

  FIG. 9 is a graph showing the relationship between ionomer thickness and gas diffusion resistance at 200% RH for each sample shown in the table of FIG. As shown in FIG. 9, it can be seen that the gas diffusion resistance increases as the ionomer thickness (@ 200% RH) increases. Generally, when the gas diffusion resistance is high, the transient output increase rate tends to be low. Therefore, when the ionomer thickness (@ 200% RH) increases, it is estimated that the transient output performance tends to decrease overall.

  As described above, both the ionomer thickness (40% RH) and the ionomer thickness (200% RH) are controlled within the specified ranges so that appropriate proton resistance and gas diffusion resistance are obtained. It can be considered that transient output performance can be obtained. Specifically, as can be understood from the results of samples 1 to 7, the ionomer thickness (40% RH) is in the range of 2.4 nm to 2.9 nm, and the ionomer thickness (200% RH) is 3.5 nm to It is preferable to be controlled within the range of 4.2 nm. More preferably, the ionomer thickness (40% RH) is controlled in the range of 2.6 nm to 2.8 nm, and the ionomer thickness (200% RH) is controlled in the range of 3.8 nm to 4.0 nm. Further, it is most preferable that the ionomer thickness (40% RH) is controlled to 2.8 nm and the ionomer thickness (200% RH) is controlled to 4.0 nm.

  In the embodiments and examples described above, the case where the ionomer thicknesses of both the cathode and the anode are controlled has been described as an example, but only one of them may be used.

  The present invention is not limited to the above-described embodiments, examples, and modifications, and can be realized with various configurations without departing from the spirit thereof. For example, the technical features in the embodiments, examples, and modifications corresponding to the technical features in each embodiment described in the summary section of the invention are to solve some or all of the above-described problems, or In order to achieve part or all of the above effects, replacement or combination can be performed as appropriate. Further, if the technical feature is not described as essential in the present specification, it can be deleted as appropriate.

DESCRIPTION OF SYMBOLS 10 ... Single cell 20 ... Electrolyte membrane 21 ... Anode 22 ... Cathode 23, 24 ... Gas diffusion layer 25, 26 ... Gas separator 27 ... MEA
28, 29 ... channel groove 30 ... catalyst-supporting carbon 31 ... catalyst 32 ... polymer electrolyte

Claims (5)

A catalyst electrode layer for a fuel cell,
It is made of a catalyst electrode material containing carbon carrying a catalyst and a polymer electrolyte,
The thickness of the polymer electrolyte covering at least a part of the carbon on which the catalyst is supported is 2.4 nm to 2.9 nm in a state of 40% relative humidity, and a state of 200% relative humidity. A catalyst electrode layer for a fuel cell having a thickness of 3.5 nm to 4.2 nm. The catalyst electrode layer for a fuel cell according to claim 1,
The thickness of the polymer electrolyte is 2.6 nm to 2.8 nm in the 40% relative humidity state, and 3.8 nm to 4.0 nm in the 200% relative humidity state. Catalyst electrode layer for fuel cells. A membrane electrode assembly comprising an electrolyte membrane and a pair of catalyst electrode layers provided on the electrolyte membrane,
The membrane electrode assembly according to claim 1, wherein at least one of the pair of catalyst electrode layers is the catalyst electrode layer according to claim 1 or 2. A membrane cell assembly in which a pair of catalyst electrode layers are formed on an electrolyte membrane, and a fuel cell in which a reaction gas flow path is formed on each of the catalyst electrode layers,
The fuel cell according to claim 3, wherein the membrane electrode assembly is the membrane electrode assembly according to claim 3. A method for producing a catalyst electrode layer for a fuel cell, comprising:
(A) preparing a catalyst electrode material comprising a catalyst-supported carbon and a polymer electrolyte;
(B) applying the catalyst electrode material on a base sheet to form the fuel cell catalyst electrode layer;
With
The step (a)
Equivalent mass of the ion exchange groups of the polymer electrolyte, and the ratio of the mass of the carbon and the mass of the polymer electrolyte, by adjusting, in the catalyst electrode layer is formed the fuel cell, said catalyst The thickness of the polymer electrolyte in the first humidity state of the polymer electrolyte covering at least a part of the supported carbon , and the second humidity state higher than the first humidity state A method comprising the step of controlling the thickness of the polymer electrolyte.

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