JP2013030286A - Catalyst ink for fell cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for improving a catalyst ink to improve durability of a fuel cell, and a technique for evaluating a catalyst ink capable of forming a fuel cell with high durability.SOLUTION: A catalyst ink for forming an electrode of a fuel cell includes: an aggregate of carbon carrying a catalyst; and ionomers. When the state of the aggregate and the ionomers is identified by a contrast modulated small angle neutron scattering method, the ionomers are substantially uniformly adsorbed on the surface of the aggregate.

Description

  The present invention relates to a catalyst ink used for producing an electrode of a fuel cell.

  Conventionally, in order to improve the power generation performance and durability of the fuel cell, the catalyst ink used for producing the catalyst layer of the fuel cell has been improved. For example, there is known a technique for producing a catalyst layer having excellent gas diffusibility by reducing the uneven distribution of the polymer electrolyte by setting the average inertia radius of the polymer electrolyte contained in the catalyst ink to 150 to 300 nm (patent) Reference 1).

JP 2005-10827 A

  However, there is still room for improvement in the technology for improving the power generation performance and durability of the fuel cell by improving the catalyst ink. For example, there is room for improvement in the technology for improving the catalyst ink to suppress the cracking of the catalyst layer and the technology for evaluating the catalyst ink.

  The present invention has been made to solve the above-described problems, and a first object of the present invention is to provide a technique for improving the durability of the fuel cell by improving the catalyst ink. A second object is to provide a technique for evaluating a catalyst ink capable of forming a highly durable fuel cell.

  In order to solve at least a part of the above problems, the present invention can be realized as the following aspects or application examples.

[Application Example 1]
A catalyst ink used to fabricate fuel cell electrodes,
A carbon agglomerate carrying a catalyst, and an ionomer,
A catalyst ink in which the state of the aggregate and the ionomer is specified by a contrast modulation small angle neutron scattering method, and the ionomer is adsorbed almost uniformly on the surface of the aggregate.

  According to this configuration, since the ionomer is adsorbed almost uniformly on the surface of the aggregate, the occurrence of cracks in the catalyst electrode layer is suppressed, so that the durability of the fuel cell can be improved.

[Application Example 2]
The catalyst ink according to application example 1,
When the scattering length density of the solvent contained in the catalyst ink is changed to prepare a plurality of samples, and the partial scattering function indicating the interaction between the aggregate and the ionomer is calculated from the scattering curve of each sample,
The partial scattering function is a catalyst ink that is a curve having a positive value and can be fitted with a high degree of fitness.

  According to this configuration, since the ionomer is adsorbed almost uniformly on the surface of the aggregate, the durability of the fuel cell can be improved.

[Application Example 3]
The catalyst ink according to Application Example 1 or Application Example 2,
The ionomer includes an adsorbed ionomer adsorbed on the aggregate and a non-adsorbed ionomer not adsorbed on the aggregate,
The non-adsorbed ionomer volume fraction, which is the ratio of the volume of the non-adsorbed ionomer to the volume of the catalyst ink, is φp_matrix,
When the total ionomer volume fraction, which is the ratio of the volume of the ionomer in the volume of the solution in which the aggregate contained in the catalyst ink is replaced with the same volume of solvent, is φp_prep,
φp_matrix / φp_prep ≦ 0.98
Catalyst ink.

  According to this configuration, since a part of the ionomer is adsorbed on the surface of the aggregate as an adsorbed ionomer, the durability of the fuel cell can be improved.

[Application Example 4]
The catalyst ink according to application example 3,
In the shell part of the composite having a shell-core structure in which the aggregate is a core and the adsorbed ionomer is a shell, an adsorbed ionomer volume fraction that is a ratio of the adsorbed ionomer volume to the entire shell volume is φp_shell. Then,
φp_shell / φp_matrix ≧ 2.0
Catalyst ink.

  According to this configuration, the adsorbed ionomer is gathered near the surface of the aggregate pair at a volume fraction more than twice that of the non-adsorbed ionomer, so that the durability of the fuel cell can be improved.

[Application Example 5]
The catalyst ink according to application example 4,
When the radius of the aggregate is Rc and the radius of the complex is Rp,
Rp−Rc ≧ 20 [Å]
Catalyst ink.

  According to this configuration, since the molecular layer of the adsorbed ionomer having a thickness of 20 [Å] or more is formed on the surface of the aggregate, the durability of the fuel cell can be improved.

  The present invention can be realized in various modes. For example, a membrane electrode assembly or a fuel cell including a catalyst electrode layer produced from the catalyst ink, a method for evaluating the catalyst ink, and production of the catalyst ink It is realizable with forms, such as a manufacturing method of a method and a fuel cell.

It is explanatory drawing for demonstrating schematic structure of the catalyst ink for fuel cell electrodes which concerns on 1st Example. It is explanatory drawing for demonstrating the scattering curve of a catalyst ink sample. It is explanatory drawing for demonstrating the relationship between the scattering intensity of a catalyst ink, and the partial scattering function of each component. It is explanatory drawing for demonstrating the scattering length density difference of each component contained in a catalyst ink. It is explanatory drawing for demonstrating the graph of the partial scattering function Scc (q) of catalytic carbon. It is explanatory drawing for demonstrating the graph of the partial scattering function Scp (q) showing the interaction of catalytic carbon and ionomer. It is explanatory drawing for demonstrating the graph of the partial scattering function Spp (q) of an ionomer. It is explanatory drawing for demonstrating the test result of a cycle durability test.

A. First embodiment:
FIG. 1 is an explanatory diagram for explaining a schematic configuration of the catalyst ink for a fuel cell electrode according to the first embodiment. The catalyst ink 100 for a fuel cell electrode according to the present embodiment is a precursor of an electrode (catalyst electrode layer) of a solid polymer fuel cell, and is applied to both sides of the solid polymer electrolyte membrane and dried to obtain an electrode (catalyst). Electrode layer). This catalyst ink 100 for a fuel cell electrode is for dispersing carbon particles (conductive carrier) 110 carrying a catalyst metal (here, platinum) 111, a polymer electrolyte (for example, a fluorine resin) 120, and these. A multi-component dispersion comprising: A method for producing this catalyst ink will be described later. Hereinafter, the fuel cell electrode catalyst ink 100 is also referred to as “catalyst ink 100”, the carbon particles 110 supporting the catalyst metal 111 are also referred to as “catalyst carbon 110”, and the polymer electrolyte 120 is also referred to as “ionomer 120”.

  As shown in FIG. 1, the catalyst carbon 110 is present as agglomerate particles 130 in which primary particles are aggregated in the catalyst ink 100. Hereinafter, the agglomerate particles 130 are also referred to as “aggregates 130”. In FIG. 1, for convenience of explanation, the aggregate 130 is shown as a perfect sphere, but the surface of the aggregate 130 has uneven shapes and a catalyst.

  In the catalyst ink 100 of this embodiment, a part of the ionomer 120 is adsorbed on the aggregate 130 to form an ionomer molecular layer on the surface of the aggregate 130. The ionomer molecular layer covers the surface of the aggregate 130 substantially uniformly. That is, in the catalyst ink 100 of the present embodiment, the ionomer 120 is adsorbed almost uniformly on the surface of the aggregate 130. As described above, a method for confirming whether or not the ionomer 120 is adsorbed almost uniformly on the surface of the aggregate 130 in the catalyst ink will be described later.

  Hereinafter, among the ionomers 120 included in the catalyst ink 100, the ionomer that is adsorbed on the aggregate 130 to form a molecular layer is also referred to as “adsorption ionomer 120 s”, and is released without being adsorbed on the aggregate 130. The ionomer is also specifically referred to as “non-adsorbing ionomer 120 m”. A complex having a shell-core structure in which the aggregate 130 is a core and the molecular layer formed by the adsorbed ionomer 120s is a shell is also referred to as a “composite 140”.

  Regarding the catalyst ink 100 of the present embodiment, it is preferable that the following formula (1) is satisfied.

ここで、Ｒcは、凝集体１３０の半径を表し、Ｒpは、複合体１４０の半径を表している（図１参照）。

Here, Rc represents the radius of the aggregate 130, and Rp represents the radius of the composite 140 (see FIG. 1).

  That is, it is preferable that the catalyst ink 100 of this embodiment includes the composite 140 in which the molecular layer of the adsorbed ionomer 120s is formed on the surface of the aggregate 130 with a thickness of 20 [Å] or more. A method for calculating the radius Rc of the aggregate 130 and the radius Rp of the composite 140 from the catalyst ink will be described later.

  Moreover, it is preferable that the catalyst ink 100 of the present embodiment is manufactured so that the following formulas (2) and (3) are satisfied.

ここで、
φp_shellは、複合体１４０のシェル部分において、シェル全体の体積に占める吸着アイオノマー１２０ｓの体積の割合である吸着アイオノマー体積分率を表す。
φp_matrixは、触媒インク１００の体積に占める、非吸着アイオノマー１２０ｍの体積の割合である非吸着アイオノマー体積分率を表す。
φp_prepは、触媒インク１００に含まれるアイオノマー１２０がすべて凝集体１３０に吸着していないと仮定したときの触媒インク１００の体積に占める、アイオノマー１２０の体積の割合である全アイオノマー体積分率を表す。

here,
φp_shell represents the adsorption ionomer volume fraction, which is the proportion of the volume of the adsorbed ionomer 120s in the total shell volume in the shell portion of the composite 140.
φp_matrix represents a non-adsorbed ionomer volume fraction, which is a ratio of the volume of the non-adsorbed ionomer 120 m to the volume of the catalyst ink 100.
φp_prep represents the total ionomer volume fraction, which is the ratio of the volume of the ionomer 120 to the volume of the catalyst ink 100 when it is assumed that all of the ionomer 120 included in the catalyst ink 100 is not adsorbed to the aggregate 130.

  The total ionomer volume fraction φPp_prep is equal to the ratio of the volume of the ionomer 120 to the volume of the solution in the solution in which the aggregate 130 included in the catalyst ink 100 is replaced with the same volume of solvent. The total ionomer volume fraction φp_prep can be calculated from the weight and specific gravity of the solvent and ionomer 120, respectively. A method for calculating the adsorption ionomer volume fraction φp_shell and the non-adsorption ionomer volume fraction φp_matrix will be described later.

  The expression (2) means that in the catalyst ink 100, the adsorbed ionomer 120s is gathered near the surface of the aggregate 130 at a volume fraction more than twice that of the non-adsorbed ionomer 120m. Further, in the formula (3), in the catalyst ink 100, when the volume fraction of the non-adsorbed ionomer 120m in the solvent is assumed that all the ionomers 120 are not adsorbed on the aggregate 130 but are dispersed in the solvent. This means that it is 98% or less of the volume fraction of the ionomer 120 in the solvent. This means that in the catalyst ink 100, a part of the ionomer 120 is adsorbed on the aggregate 130 as the adsorbed ionomer 120s.

  The preparation method and evaluation method of the catalyst ink 100 are shown below. First, catalytic carbon and ionomer (EW800) were added to the dispersion and stirred. The catalyst carbon had a catalyst (platinum) average particle diameter of 2 nm, and the platinum-supported weight ratio, which is the weight ratio between the carbon particles and the catalyst (platinum), was 30%. The ionomer (EW800) was adjusted so that the weight and weight ratio of the carbon particles contained in the catalyst carbon were 1: 1. The dispersion solution was adjusted so that the total weight of the catalyst carbon and ionomer was 10% of the total weight of the catalyst ink. After the above stirring, this solution was transferred to a magnetic (ZrO2) jar and highly dispersed by a planetary bead mill (150 rpm / 3 hr) to prepare a catalyst ink.

Here, a plurality of catalyst inks were prepared using a solvent in which light water (H ₂ O) and heavy water (D ₂ O) were mixed at various light water / heavy water ratios as a dispersion solvent. Specifically, catalyst inks were prepared using six types of dispersion solvents in which the molecular number ratio of D ₂ 0 was 0%, 10%, 20%, 60%, 70%, and 80%. Thereafter, the catalyst ink sample having a D ₂ 0 ratio of the solvent of 0% is “D0”, the catalyst ink sample having the D ₂ 0 ratio of the solvent of 10% is “D10”, and the catalyst ink having the D ₂ 0 ratio of the solvent of 20%. The sample is “D20”, the catalyst ink sample having a solvent D ₂ 0 ratio of 60% is “D60”, the catalyst ink sample having a solvent D ₂ 0 ratio of 70% is “D70”, and the solvent D ₂ 0 ratio is 80 % Catalyst ink sample is also referred to as “D80”. The above-mentioned heavy water (D ₂ O) is obtained by replacing light hydrogen (H) in light water (H ₂ O) with deuterium (D).

  Using these six types of catalyst ink samples, the structure (morphology / density / thickness) of the adsorbed ionomer 120s in the catalyst ink 100 was quantified by contrast modulation small angle neutron scattering (CV-SANS). Here, contrast-modulated small-angle neutron scattering (CV-SANS) refers to a plurality of samples included in a sample from small-angle neutron scattering (SANS) of a plurality of samples subjected to contrast modulation (CV). This is a technology for conducting structural analysis of substances.

  Here, the contrast variation (CV) is, as described above, changing the scattering length density of the solvent by changing the light water / heavy water ratio of the solvent, and a plurality of contained in the solution (catalyst ink 100). This is to change the scattering length density difference (contrast), which is the difference from each scattering length density of the substance (catalyst carbon 110, ionomer 120). Small-angle neutron scattering is a technique for clarifying structural characteristics of a sample (material) by irradiating the sample (material) with neutrons and observing the interference phenomenon of the scattered neutron wave. In this example, in the small-angle neutron scattering apparatus SANS-U of the University of Tokyo Institute of Physical Properties, contrast is obtained using a continuous neutron beam with a wavelength λ = 7.0 [Å] taken out from the 20 MW experimental reactor JRR-3. Modulated small angle neutron scattering (CV-SANS) was performed.

FIG. 2 is an explanatory diagram for explaining a scattering curve of a catalyst ink sample. The horizontal axis of FIG. 2 indicates the scattering vector magnitude q [Å ⁻¹ ], and the vertical axis indicates the scattering intensity I (q) [cm ⁻¹ ]. The size q of the scattering vector on the horizontal axis is given by the following equation (4), where 2θ is the scattering angle formed by the wave vector of the incident neutron and the wave vector of the scattered neutron, and λ is the wavelength of the neutron beam. Can be represented by

Here, since the wavelength λ of the neutron beam is constant, the magnitude q of the scattering vector depends on the scattering angle 2θ. FIG. 2 shows scattering curves for each of the six types of catalyst ink samples (D0, D10, D20, D60, D70, and D80) described above. From FIG. 2, in the catalyst ink sample having a low D ₂ 0 ratio of the solvent, the scattering length density difference (contrast) between the solvent and the ionomer 120 is small, so that the scattering from the catalytic carbon 110 is dominant. On the other hand, as the catalyst ink sample has a higher D ₂ 0 ratio of the solvent, the scattering length density difference between the solvent and the catalyst carbon 110 becomes smaller, so that the scattering from the catalyst carbon 110 is suppressed and the scattering from the ionomer 120 is dominant. It became a result. The reason for this will be described later with reference to FIG.

FIG. 3 is an explanatory diagram for explaining the relationship between the scattering intensity of the catalyst ink and the partial scattering function of each component. The scattering intensity I (q) of the catalyst ink 100 can be expressed as the sum of the partial scattering functions of the components included in the catalyst ink 100. Here, the scattering function S _cc (q) of the catalyst carbon 110 contained in the catalyst ink 100 and the partial scattering function S _pp (q) of the ionomer 120 are calculated from the scattering intensities I (q) of the six types of catalyst ink samples. The partial scattering function S _cp (q) representing the interaction between the catalytic carbon 110 and the ionomer 120 was calculated. The scattering intensity I (q) of the catalyst ink 100 is given by the following formula (5).

ここで、ρ_cは、触媒カーボン１１０の散乱長密度を表し、ρ_wは溶媒の散乱長密度を表し、ρ_pは、アイオノマー１２０の散乱長密度を表している。また、（ρ_c−ρ_w）は、触媒カーボン１１０と溶媒の散乱長密度差（コントラスト）を表し、（ρ_p−ρ_w）は、アイオノマー１２０と溶媒の散乱長密度差を表している。

Here, ρ _c represents the scattering length density of the catalytic carbon 110, ρ _w represents the scattering length density of the solvent, and ρ _p represents the scattering length density of the ionomer 120. Further, (ρ _c −ρ _w ) represents the scattering length density difference (contrast) between the catalytic carbon 110 and the solvent, and (ρ _p −ρ _w ) represents the scattering length density difference between the ionomer 120 and the solvent.

FIG. 4 is an explanatory diagram for explaining the scattering length density difference of each component contained in the catalyst ink. The horizontal axis of the table shown in the upper part of FIG. 4 shows the D ₂₀ ratio of the solvent contained in the catalyst ink, and the vertical axis shows the scattering length density of the substance contained in each catalyst ink. The scattering length density of heavy water is 6.38 × 10 ¹⁰ [cm ⁻² ], and the scattering length density of light water is −0.563 × 10 ¹⁰ [cm ⁻² ]. Therefore, the scattering length density of the solvent increases as the D ₂ 0 ratio increases. On the other hand, the scattering length density of the catalytic carbon and the scattering length density of the ionomer are both positive values and almost constant, and the scattering length density of the catalytic carbon is higher than the scattering length density of the ionomer.

When the scattering length density difference (ρ _c −ρ _w ) between the catalyst carbon and the solvent is Δρ _c , and the scattering length density difference (ρ _p −ρ _w ) between the ionomer and the solvent is Δρ _p , the solvent D ₂₀ ratio is low. It can be seen that the catalyst ink samples (D0, D10, D20) have a larger scattering length density difference Δρ _c between the catalytic carbon and the solvent than the scattering length density difference Δρ _p between the ionomer and the solvent. On the other hand, in the catalyst ink sample (D60, D70, D80) having a high D ₂ 0 ratio of the solvent, as the D ₂ 0 ratio increases, the scattering length density difference Δρ _c between the catalytic carbon and the solvent decreases, and the ionomer and the solvent scatter. It can be seen that the long density difference Δρ _p increases. As a result, in D0, D10, and D20 where the D ₂ 0 ratio of the solvent is low, the scattering length density difference Δρ _c between the catalytic carbon and the solvent is large, and the resulting scattering curve is the result of the scattering from the catalytic carbon 110 being dominant. (FIG. 2). On the other hand, D ₂ 0 ratio of the solvent is high D60, D70, the D80, as D ₂ 0 ratio of solvent becomes high catalyst ink, scattering curve scattering from the catalyst the carbon 110 is suppressed, dominated scattering from ionomer 120 Result (FIG. 2).

The scattering intensities I _n (q) (n = 1 to 6) of the six types of catalyst ink, the scattering length density difference Δρ _c between the catalyst carbon and the solvent, and the scattering length density difference Δρ between the ionomer and the solvent. _{From p} , the partial scattering function S _cc (q) of the catalytic carbon, the partial scattering function S _pp (q) of the ionomer, and the partial scattering function S _cp (q) representing the interaction between the catalytic carbon and the ionomer are obtained from p. Can be calculated. Equation (6) is a matrix representation of Equation (5) for the above six types of catalyst ink, and S _cc (q), S _cp (q), S _pp (q) are obtained by singular value decomposition. Can be determined.

FIG. 5 is an explanatory diagram for explaining a graph of the partial scattering function S _cc (q) of catalytic carbon. The horizontal axis in FIG. 5 indicates the magnitude q of the scattering vector, and the vertical axis indicates the partial scattering intensity. The dots in the figure indicate the partial scattering function S _cc (q) obtained from the equation (6). The solid line in the figure is a fitting curve according to the following equation (7). The radius Rc of the aggregate 130 can be calculated by fitting the partial scattering function S _cc (q) with the following equation (7).

式（７）では、触媒インク１００における凝集体１３０の半径Ｒｃは、ガウス分布（分散σ）しており、また、分布座標ｘでは、体積Ｖの凝集体１３０がｎ個存在しているものとしている。すなわち、下記の式（７）において、ｎは、散乱体（凝集体１３０）の数密度を表し、Ｖは、散乱体（凝集体１３０）一個の体積を表し、ｘは、ガウス分布における分布座標を表している。Ｇ（Ｒ，ｘ，σ）は、下記式（８）に示すように、確率密度関数を示している。また、Ｐ_sph（Ｒ，ｑ）は、式（９）（１０）に示すように、散乱体（凝集体１３０）が球体である場合の散乱関数である。

In Expression (7), it is assumed that the radius Rc of the aggregate 130 in the catalyst ink 100 has a Gaussian distribution (dispersion σ), and that there are n aggregates 130 of volume V at the distribution coordinate x. Yes. That is, in the following formula (7), n represents the number density of the scatterer (aggregate 130), V represents the volume of one scatterer (aggregate 130), and x is a distribution coordinate in the Gaussian distribution. Represents. G (R, x, σ) represents a probability density function as shown in the following formula (8). P _sph (R, q) is a scattering function when the scatterer (aggregate 130) is a sphere as shown in equations (9) and (10).

As a result of fitting the partial scattering function S _cc (q) by the above formula (7), the radius Rc of the aggregate 130 contained in the catalyst ink 100 was Rc = 570 [Å]. Further, the volume V and the number density n of the aggregate 130 are calculated from the equation (7).

FIG. 6 is an explanatory diagram for explaining a graph of the partial scattering function S _cp (q) representing the interaction between the catalytic carbon and the ionomer. The horizontal axis in FIG. 6 indicates the magnitude q of the scattering vector, and the vertical axis indicates the partial scattering intensity. The dots in the figure indicate the partial scattering function S _cp (q) obtained from the equation (6). The solid line in the figure is a fitting curve according to equation (11) described later.

In the catalyst ink 100 of this example, the partial scattering function S _cp (q) representing the interaction between the catalyst carbon and the ionomer has a positive value, and is represented by a graph having a sufficiently high fitness when fitted. ing. Therefore, an interaction occurs between the catalyst carbon 110 and the ionomer 120 in the catalyst ink 100, and the ionomer 120 is gathered around the catalyst carbon 110 and has the same shape as the catalyst carbon 110 as a whole. It is thought that it has. That is, it is considered that a complex 140 in which a part of the ionomer 120 (adsorption ionomer 120s) is adsorbed almost uniformly is formed on the surface of the aggregate 130. If the ionomer 120 is not adsorbed on the surface of the aggregate 130, the partial scattering function S _cp (q) is considered to be a negative value.

As shown in FIG. 6, the partial scattering function S _cp (q) representing the interaction between the catalytic carbon and the ionomer is fitted by the following equation (11). Further, as shown in FIG. By fitting the scattering function S _pp (q) by the following equation (14), the radius Rp of the complex 140, the adsorbed ionomer volume fraction φp_shell, and the nonadsorbed ionomer volume fraction φp_matrix can be calculated. .

ここで、ｎは、式（７）によって算出された散乱体の数密度を表し、Ｖｐは、複合体１４０の体積を表し、Ｖｃは、式（７）によって算出された凝集体１３０の体積を表している。また、Φ_sph（Ｒ_p，ｑ）は、複合体１４０が球体である場合の散乱関数を表し、下記の式（１２）で与えられる。

Here, n represents the number density of the scatterers calculated by Expression (7), Vp represents the volume of the composite 140, and Vc represents the volume of the aggregate 130 calculated by Expression (7). Represents. Φ _sph (R _p , q) represents a scattering function when the composite 140 is a sphere, and is given by the following equation (12).

式（１３）において、全アイオノマー体積分率φp_prepは、溶媒、アイオノマー１２０それぞれの重さと比重から算出することができる。全アイオノマー体積分率φp_prepは、φp_prep＝０．０２であった。また、凝集体１３０の体積分率φcは、触媒インク１００の体積に占める、凝集体１３０の比率であり、触媒インク１００および凝集体１３０のそれぞれの重さと比重から算出することができる。凝集体１３０の体積分率φcは、φc＝０．０１であった。 Φ _sph (R _c , q) represents a scattering function when the aggregate 130 is a sphere as shown in the equation (10). As for the volume fraction φp_matrix of the non-adsorbed ionomer, the following equation (13) is established between the adsorbed ionomer volume fraction φp_shell and the total ionomer volume fraction φp_prep. From equation (13), φp_matrix can be obtained from φp_prep and φp_shell.

In equation (13), the total ionomer volume fraction φp_prep can be calculated from the weight and specific gravity of the solvent and ionomer 120, respectively. The total ionomer volume fraction φp_prep was φp_prep = 0.02. Further, the volume fraction φc of the aggregate 130 is the ratio of the aggregate 130 to the volume of the catalyst ink 100, and can be calculated from the weight and specific gravity of the catalyst ink 100 and the aggregate 130, respectively. The volume fraction φc of the aggregate 130 was φc = 0.01.

  As a result of calculating these, the radius Rp of the complex 140 is Rp = 650 [Å], the adsorption ionomer volume fraction φp_shell is φp_shell = 0.35, and the volume fraction of the non-adsorption ionomer φp_matrix is φp_matrix = 0.018. It became.

FIG. 7 is an explanatory diagram for explaining a graph of the ionomer partial scattering function S _pp (q). The horizontal axis of FIG. 7 shows the magnitude q of the scattering vector, and the vertical axis shows the partial scattering intensity. The dots in the figure indicate the partial scattering function S _pp (q) obtained from the equation (6). The alternate long and short dash line in the figure shows the ionomer scattering function f (ionomer) obtained by matching the absolute value of the experimental result of the ionomer 120 solution with S _pp (q). The broken line in the figure shows the fitting curve (FIG. 6) of the partial scattering function S _cp (q) representing the interaction between the catalytic carbon and the ionomer. The solid line in the figure is a fitting curve according to the following equation (14).

Since the partial scattering function S _pp (q) does not coincide with the scattering function f (ionomer), the ionomer 120 in the catalyst ink 100 is not uniformly dispersed in the catalyst ink 100, and some of the ionomers 120 are not. It turns out that it has a specific structure. Specifically, in the region where the magnitude q of the scattering vector is small, the partial scattering function S _pp (q) is a fitting curve of the partial scattering function S _cp (q) representing the interaction between the catalytic carbon and the ionomer (FIG. 6). The ionomer 120 having the specific structure described above is an adsorbed ionomer 120s and is adsorbed on the aggregate 130. On the other hand, in the region where the magnitude q of the scattering vector is large, the partial scattering function S _pp (q) is almost the same as the scattering function f (ionomer), and therefore there is a non-adsorbing ionomer 120m free in the solvent. You can see that

  In the catalyst ink 100 of the present embodiment, when φp_shell and / φp_matrix shown in the above equation (2) are calculated from the adsorption ionomer volume fraction φp_shell calculated as described above and the non-adsorption ionomer volume fraction φp_matrix, φp_shell , /Φp_matrix≈19.4 (0.35 / 0.018). From this, it can be seen that in the catalyst ink 100, the adsorbed ionomer 120s is gathered near the surface of the aggregate 130 at a volume fraction of about 19 times or more of the non-adsorbed ionomer 120m. Therefore, in the catalyst ink 100 of this example, it can be said that the adsorbed ionomer 120s covers the aggregate 130 uniformly and densely. The volume fraction of the adsorbed ionomer 120s is not necessarily about 19 times or more than the volume fraction of the non-adsorbed ionomer 120m. If the volume fraction of the non-adsorbed ionomer 120m is two times or more, the adsorbed ionomer 120s. It can be said that the aggregate 130 is uniformly and densely coated.

  In addition, in the catalyst ink 100 of this example, φp_matrix and / φp_prep shown in Expression (3) are φp_matrix and /φp_prep≈0.9 (0.018 / 0.02). From this, it can be seen that in the catalyst ink 100, about 90% of the ionomer 120 exists as the non-adsorbed ionomer 120m, and about 10% exists as the adsorbed ionomer 120s. Therefore, it can be said that the catalyst ink 100 of this example covers the aggregate 130 with a sufficient amount of adsorbed ionomer 120s. The amount of the adsorbed ionomer 120s that covers the aggregate 130 is not necessarily 10% or more of the ionomer 120 in the catalyst ink 100. If the amount of the ionomer 120 is 2% or more of the ionomer 120, the aggregate 130 is sufficiently covered. can do.

  Further, in the catalyst ink 100 of this example, Rp−Rc represented by the formula (1) was Rp−Rc≈80 (650−570) [Å]. From this, it can be seen that the catalyst ink 100 contains the composite 140 in which the molecular layer of the adsorbed ionomer 120s is formed on the surface of the aggregate 130 with a thickness of about 80 [Å]. Therefore, it can be said that the catalyst ink 100 of this example covers the aggregate 130 with the molecular layer of the adsorbed ionomer 120s having a sufficient thickness. The molecular layer of the adsorbed ionomer 120s that covers the aggregate 130 does not necessarily have a thickness of 80 [Å] or more. If the thickness is 20 [Å] or more, the aggregate 130 can be sufficiently covered. it can.

In order to evaluate the cross leak resistance of the membrane / electrode assembly produced using the catalyst ink 100 of this example, a cycle durability test was conducted. The cycle endurance test was performed on two membrane electrode assemblies (Sample 1 and Sample 2). Sample 1 was formed by applying the catalyst ink 100 described above to an electrolyte membrane and drying. Sample 2 as a comparative example was formed by applying a catalyst ink having a negative partial scattering function S _cp (q) representing the interaction between catalytic carbon and ionomer to the electrolyte membrane and drying.

  For the two prepared samples, gas diffusion layers were arranged on both sides and sandwiched between fuel cells. With the hydrogen gas supplied to the anode of the fuel cell and the oxygen gas supplied to the cathode, the open circuit state and the state where the terminal voltage was 0.6 V were repeated for 30 seconds. The supply rate of hydrogen gas is 0.5 L / min, and the supply rate of oxygen gas is 1.0 L / min. The cell temperature is 80 ° C., and the relative humidity of the gas supplied to the anode and cathode is 50%. The anode and cathode gas outlet pressures are 200 kPa in absolute value for both electrodes.

  The fuel cell was made into the above state, and a cross leak test was performed every time a predetermined time passed. In the cross leak test, the pressure drop on the anode side after 10 minutes was measured with the anode side of the fuel battery cell sealed at 200 kPa and the cathode side opened to the atmosphere.

  FIG. 8 is an explanatory diagram for explaining the test results of the cycle durability test. The horizontal axis in FIG. 8 indicates the cycle durability endurance time (Hr), and the vertical axis indicates the cross leak amount (MPa). The cross leak amount is the above-described pressure drop amount on the anode side. FIG. 8 shows that sample 1 using the catalyst ink 100 of this example has higher durability than sample 2 as a comparative example. This is because the adsorption area of the ionomer with respect to the surface area of the catalytic carbon is larger than that of the catalytic electrode layer formed of the catalytic ink of the comparative example. It is thought that the crack which isolate | separates from the ionomer 120 does not generate | occur | produce easily, As a result, generation | occurrence | production of a cross leak is suppressed.

  According to the catalyst ink of the present embodiment described above, since a part of the ionomer 120 is adsorbed almost uniformly on the surface of the aggregate 130, the occurrence of cracks in the catalyst electrode layer can be suppressed. That is, since the ionomer 120 is adsorbed on the entire surface of the catalyst carbon 110, the catalyst electrode layer formed by the catalyst ink 100 can suppress the occurrence of cracks due to the separation of the catalyst carbon 110 and the ionomer 120. it can. Therefore, the durability of the fuel cell can be improved.

  Conventionally, there has been a problem that the power generation performance and durability of the fuel cell are reduced due to cracks in the catalyst electrode layer. Fuel cells generate hydroxyl radicals as a by-product in addition to water and electricity by an electrochemical reaction during power generation. The generated hydroxyl radical is invalidated on catalyst fine particles such as platinum in the electrode in a normal fuel cell. However, in the fuel cell in which the catalyst electrode layer is cracked, some hydroxyl radicals reach the electrolyte membrane without being invalidated and erode the electrolyte membrane. As a result, a hole is generated in the electrolyte membrane and cross leak occurs. Since the catalyst electrode layer formed of the catalyst ink 100 can suppress the occurrence of cracks as described above, the occurrence of cross leak can be suppressed.

  Conventionally, a technique for evaluating a catalyst ink capable of forming a highly durable and high performance fuel cell catalyst layer has been desired. When the present inventors specify the structure of the ionomer 120 in the catalyst ink 100 by contrast modulation small angle neutron scattering (CV-SANS), the adsorbed ionomer 120s is adsorbed almost uniformly on the surface of the aggregate 130. And the occurrence of cracks in the catalyst electrode layer was found to be suppressed. By applying this technology for evaluating catalyst ink by contrast-modulated small-angle neutron scattering to a fuel cell manufacturing method, a catalyst ink inspection method, and the like, a highly durable and high-performance fuel cell can be provided.

B. Variations:
The present invention is not limited to the above-described examples and embodiments, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible.

B-1. Modification 1:
The catalyst ink 100 of the present embodiment has been described on the assumption that carbon particles are used as the conductive carrier. However, in addition to the carbon particles, for example, carbon materials such as carbon black, carbon nanotubes, and carbon nanofibers are used. In addition, a carbon compound typified by silicon carbide or the like may be used as the conductive carrier. In addition to platinum, for example, platinum alloy, palladium, rhodium, gold, silver, osmium, iridium, or the like may be used as the catalyst metal.

B-2. Modification 2:
In this embodiment, the catalyst ink 100 has been described as satisfying the above formulas (1), (2), and (3). However, if the state of the aggregate 130 and the ionomer 120 is specified by the contrast-modulated small-angle neutron scattering, the catalyst ink 100 may temporarily have the above-described adsorption ionomer 120s adsorbed on the surface of the aggregate 130. Even if one or more of the formulas (1), (2), and (3) are not satisfied, the generation of cracks in the generated catalyst electrode layer can be suppressed. The catalyst ink satisfying one or more of the above formulas (1), (2), and (3) can further suppress the occurrence of cracks in the catalyst electrode layer.

B-3. Modification 3:
Even in a fuel cell in which the catalyst electrode layer generated by the catalyst ink 100 of this embodiment is used only for one electrode, durability and power generation performance can be improved. However, when the catalyst electrode layer generated by the catalyst ink 100 is used for both electrodes, durability and power generation performance can be further improved.

B-4. Modification 4:
In this embodiment, in order to perform the contrast variation SANS, six with varying D ₂ 0 ratio of solvent, using catalyst ink sample, D ₂ 0 ratios and a solvent, the catalyst ink sample The number can be set arbitrarily. In this embodiment, the small angle neutron scattering is performed in the small angle neutron scattering apparatus SANS-U of the University of Tokyo Institute of Physical Properties, but the small angle neutron scattering may be performed by an apparatus other than the above.

B-5. Modification 5:
In this embodiment, the catalyst ink 100 has been described as being used as an electrode of a solid polymer fuel cell. However, the catalyst ink 100 may be a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, or the like. The present invention can be applied to various fuel cells such as fuel cells.

DESCRIPTION OF SYMBOLS 100 ... Catalyst ink 110 ... Catalyst carbon 111 ... Catalyst metal 120 ... Ionomer 120m ... Non-adsorption ionomer 120s ... Adsorption ionomer 130 ... Aggregate 140 ... Composite

Claims (5)

A catalyst ink used to fabricate fuel cell electrodes,
A carbon agglomerate carrying a catalyst, and an ionomer,
A catalyst ink in which the state of the aggregate and the ionomer is specified by a contrast modulation small angle neutron scattering method, and the ionomer is adsorbed almost uniformly on the surface of the aggregate. The catalyst ink according to claim 1,
When the scattering length density of the solvent contained in the catalyst ink is changed to prepare a plurality of samples, and the partial scattering function indicating the interaction between the aggregate and the ionomer is calculated from the scattering curve of each sample,
The partial scattering function is a catalyst ink that is a curve having a positive value and can be fitted with a high degree of fitness. The catalyst ink according to claim 1 or 2,
The ionomer includes an adsorbed ionomer adsorbed on the aggregate and a non-adsorbed ionomer not adsorbed on the aggregate,
The non-adsorbed ionomer volume fraction, which is the ratio of the volume of the non-adsorbed ionomer to the volume of the catalyst ink, is φp_matrix,
When the total ionomer volume fraction, which is the ratio of the volume of the ionomer in the volume of the solution in which the aggregate contained in the catalyst ink is replaced with the same volume of solvent, is φp_prep,
φp_matrix / φp_prep ≦ 0.98
Catalyst ink. The catalyst ink according to claim 3,
In the shell part of the composite having a shell-core structure in which the aggregate is a core and the adsorbed ionomer is a shell, an adsorbed ionomer volume fraction that is a ratio of the adsorbed ionomer volume to the entire shell volume is φp_shell. Then,
φp_shell / φp_matrix ≧ 2.0
Catalyst ink. The catalyst ink according to claim 4,
When the radius of the aggregate is Rc and the radius of the complex is Rp,
Rp−Rc ≧ 20 [Å]
Catalyst ink.

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