JP2012128946A - Method for manufacturing electrode for fuel battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an electrode for a fuel battery having a large three-phase interface area and a high catalyst particle surface utilization rate.SOLUTION: The method for manufacturing an electrode for a fuel battery comprises: the step of preparing an electrolyte precursor solution; the step S12 of applying the electrolyte precursor to a catalyst porous structure including a porous body S11 made up of carbon particles and catalyst particles; and the step S13 of forming an electrolytic layer in the porous structure by polymerizing the electrolyte precursor applied to the catalyst porous structure. Thus, the electrolyte precursor in a low-molecular condition is distributed everywhere even close to the catalyst particles in a pore structure into which no polymer electrolyte can be introduced. The electrolyte precursor thereafter goes through a polycondensation reaction, whereby the rise in the molecular weight thereof is facilitated. Further, the electrolytic layer making a path for proton transport can be formed to be distributed close to the catalyst particles at high density. Therefore, the area of a three-phase interface becomes larger, which increases the catalyst particle surface utilization rate.

Description

  The present invention relates to a method for manufacturing a fuel cell electrode, and more particularly to a method for manufacturing a polymer electrolyte fuel cell electrode.

  A fuel cell generates electric power by electrochemically reacting a fuel capable of generating protons such as hydrogen and an oxidant containing oxygen such as air.

  In the cathode of the fuel cell, a catalytic reaction in which water is generated on the surface of the catalyst particles is generated by gaseous oxygen, liquid protons, and electrons from conductive fine powder that is solid.

  The reaction center where the catalytic reaction occurs is generally called a three-phase interface, and the area of the three-phase interface is an effective area of catalyst particles in contact with protons efficiently. The battery performance is improved.

  In general, a catalyst layer of a fuel cell electrode is produced by stirring and mixing a polymer electrolyte and conductive fine powder carrying catalyst particles (Patent Document 1).

  However, usually, in the catalyst layer formed by stirring and mixing the conductive fine powder carrying the catalyst particles and the polymer electrolyte material, the catalyst surface is buried and the area of the three-phase interface becomes small. Had.

Therefore, a method has been proposed in which a catalyst layer is formed by forming a porous catalyst electrode layer in order to bring the catalyst particles to the outermost surface, and then applying a polymer electrolyte dispersion on the catalyst electrode layer. (Patent Documents 2 and 3). As an electrolyte membrane for fuel cells and an electrolyte layer in an electrode, a perfluorosulfonic acid polymer electrolyte represented by Nafion ^(R) (a product name manufactured by DuPont ⁾ is generally used.

  These polymer electrolyte materials have a large particle diameter in the dispersion solvent and do not fill even the small voids of the porous electrode layer. For this reason, the electrolyte material that supplies protons to the vicinity of the catalyst fine particles does not reach and the area of the three-phase interface is reduced, that is, the problem of low utilization of the catalyst particle surface has not been solved.

  Thus, there is a problem that the proton concentration near the catalyst particles is low, so that the area of the three-phase interface is small, and as a result, the effective area of the catalyst particles is small.

JP 2006-075709 JP 2008-104424 A Japanese Patent No. 3686364 JP 2007-123259 A JP 2005-026005

  As described above, in the conventional configuration, the mixing of the polymer electrolyte and the catalyst particles is insufficient, or the polymer electrolyte is not dispersed in the catalyst porous structure, so that protons are sufficiently present in the vicinity of the catalyst particles. Not supplied. Therefore, the problem is that the utilization factor of the catalyst surface is lowered.

  The present invention solves the above-described conventional problems, and an object of the present invention is to provide an electrode manufacturing method for a fuel cell electrode for increasing the utilization ratio of the catalyst particle surface by increasing the area of the three-phase interface. .

  In order to solve the above-mentioned conventional problems, a method for producing an electrode for a fuel cell according to the present invention comprises a step of forming a catalyst porous structure in which catalyst particles are made of fine carbon powder, a solvent, and at least an electrolyte material. A step of preparing a precursor, a step of applying the electrolyte precursor to the catalyst porous structure, and a step of forming a catalyst electrolyte composite precursor by applying the electrolyte precursor to the catalyst porous structure. The catalyst electrolyte composite precursor is subjected to a step of forming an electrolyte layer in the catalyst electrolyte composite precursor by performing a reduced pressure drying process and a heat drying process.

  With this configuration, electrolyte precursors in a low-molecular state are arranged in the vicinity up to the vicinity of the catalyst particles, and the molecular weight of the electrolyte precursor is increased in situ via a polycondensation reaction, so that an electrolyte layer that becomes a proton transport path is formed. High density and high dispersion can be formed up to the vicinity of the catalyst particles.

  According to the fuel cell electrode and the method of manufacturing the same of the present invention, the area of the three-phase interface can be increased to improve the utilization rate of the catalyst particle surface.

Process drawing shown in the manufacturing method of the electrode for fuel cells in Embodiment 1 of this invention Process drawing which showed the formation method of the electrolyte layer in Embodiment 1 of this invention The graph which shows the cyclic voltammogram measured in the catalytic reaction area evaluation of the electrode for fuel cells of Example 1 of this invention

  Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1)
In the present embodiment, a method for producing a fuel cell electrode composed of a porous body made of fine carbon powder and a catalyst porous structure composed of catalyst particles and an electrolyte layer will be described. FIG. 1 shows a process chart relating to a method for producing a fuel cell electrode of the present invention.

  First, the catalyst porous structure (S12) in the present embodiment is produced by supporting catalyst particles on the porous body (S11) made of carbon fine powder. The electrolyte layer (S25) in this embodiment has a low molecular electrolyte material (S23) having a polymerizable functional group and an ionic functional group (especially a sulfonic acid group) in the molecule, and a polymerizable functional group but an ionic functional group. A solution in which an organic solvent is mixed with a low-molecular spacer material that does not have is used as the electrolyte precursor (S24).

  Next, the electrolyte precursor (S24) is impregnated and applied to a porous body (S11) made of fine carbon powder and a catalyst porous structure (S12) composed of catalyst particles. Thereafter, low molecular volatile components such as a solvent are removed by a drying step so that the electrolyte layer (S25) is uniformly dispersed and applied to the catalyst porous structure (S12) via a polymerization reaction. An electrode (S14) can be produced.

  In addition, the order of the second step <electrolyte precursor impregnation coating (S12 → S13)> and the third step <formation of electrolyte layer by reduced pressure drying and heat drying (S13 → S14)> in the first embodiment is switched. The fuel cell electrode produced in this manner does not exhibit the effects of the present invention. This is because the polymerized electrolyte layer particles cannot be uniformly introduced into the catalyst porous structure.

  In addition, even if the polymer electrolyte is impregnated and applied to the catalyst porous structure (S12) in the present embodiment, the polymer mass is larger than the pore size of the porous material, so that the polymer is uniformly dispersed and applied. I can't.

  Next, a method for forming the electrolyte layer (S25) from the electrolyte raw material (S21) via the electrolyte precursor (S23) will be described. A process diagram related to the method for forming an electrolyte layer of the present invention is shown in FIG.

  In this embodiment, the electrolyte layer (S25) is formed by first diluting the electrolyte raw material (S21) with an organic solvent and then oxidizing it with an oxidizing agent, so that a polymerizable functional group and an ionic functional group are present in the molecule. Into a low molecular electrolyte material (S23) having Subsequently, the electrolyte material (S23) is mixed with a low molecular spacer material having a polymerizable functional group but not having an ionic functional group to obtain an electrolyte precursor (S24). And the electrolyte layer (S25) is formed via the polymerization reaction by drying the electrolyte precursor (S24).

  Note that the organic solvent used for mixing and diluting the electrolyte raw material in the present embodiment is preferably a low polarity solvent (S21 → S22).

  In the electrolyte precursor (S24) in the present embodiment, it is desirable that the low molecular electrolyte material (S23) has a sulfonic acid group as an ionic functional group.

  In addition, in the electrolyte precursor (S24) in this implementation, in order to impart insolubility to water, water insolubility and polymerization of the low molecular electrolyte material (S23) having solubility and polymerizability in water in the molecule. It takes the form of a low molecular weight organic compound solution mixed with a low molecular spacer material having a property.

  An electrolyte layer (S25) is formed from the electrolyte precursor (S24) via a polymerization reaction, and these are insoluble in water.

The electrolyte layer (S25) having the above-described structure needs to have an EW value comparable to or lower than that of the conventional material Nafion ^(R) .

  For the electrolyte layer (S25) having the above structure, in order for the EW value to be 1000 or less and insoluble in water, the molar ratio 1: n in the mixed solution of the low molecular electrolyte material (S23) and the low molecular spacer material is: Usually, n is preferably in the range of 0.25 to 5. In particular, it is more preferably within the range of 0.5 or more and 3 or less.

  In addition, it is more preferable that the polymerization reaction through which the electrolyte layer is formed particularly through a condensation reaction by reduced pressure or heat drying.

  Note that the catalyst porous structure has pores having a size of several nm at the minimum, and an electrolyte layer can be formed up to these pores through the steps in the present embodiment.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

Formation of Electrolyte Layer According to the method described in Embodiment 1, the electrolyte layer is formed by first treating the diluted solution of the electrolyte raw material with an oxidizing agent to convert it into an electrolyte material having an ionic functional group. Thereafter, a low molecular weight material insoluble in water is added as a spacer molecule and mixed to obtain an electrolyte precursor. Finally, a volatile component such as a solvent is removed by drying to obtain an electrolyte layer insoluble in water via a copolymerization reaction.

Specifically, the procedure is as follows. A 10 wt% solution is prepared by diluting 15 mmol of trialkoxysilane compound ((MeO) ₃ Si— (CH ₂ ) ₃ —SH) having a thiol group in the molecule with t-BuOH. 30% hydrogen peroxide solution was added to the thiol compound solution, and the mixture was stirred and mixed at room temperature in a nitrogen atmosphere for 15 hours. Thereafter, 15 mmol of (MeO) ₃ Si-Me was added, stirred for 15 minutes, and ultrapure water was further added and mixed to prepare an electrolyte precursor as a colorless transparent uniform solution. Silane compounds ((RO) ₃ Si— (CH ₂ ) ₃ —SO ₃ H, (RO = HO, MeO)) and (MeO) in which thiol groups in the molecule are oxidized and converted into sulfonic acid groups by this process A homogeneous solution with a ₃ : 1 Si-Me molar ratio is obtained.

In addition, the following method is also considered as a preparation method of said electrolyte precursor. For example, a solution in which a trialkoxysilane compound ((MeO) ₃ Si- (CH ₂ ) ₃ -SH) having a thiol group and (MeO) ₃ Si-Me are mixed in a desired molar ratio using t-BuOH as a solvent in advance. Add 30% hydrogen peroxide solution. A thiol group can be converted into a sulfonic acid group by an oxidation reaction between the solution and hydrogen peroxide solution.

  Next, after the above solution, which is an electrolyte precursor, is developed on a container, the polymerization reaction proceeds by gradually distilling off volatile components such as a solvent under reduced pressure. As a result, the film-like substance is insoluble in water. An electrolyte layer was obtained. The above material appears to have a siloxane (Si-O-Si) skeleton.

  In order to confirm the insolubility of the electrolyte layer obtained as a film-like substance in water, the film-like substance was immersed in water and stirred for one day. The supernatant liquid was taken and water was distilled off under reduced pressure, but no polysiloxane film-like substance was confirmed.

In addition, solid-state NMR measurement was performed on the synthesized film-like substance, which was measured in ¹³ C-DDMAS-NMR (single pulse & 1H decouple) and ²⁹ Si-CPMAS-NMR (1H → 13C cross polarization & 1H decouple). The chemical shift value of the signal peak was in good agreement with the theoretical value expected from its molecular structure, and it was found that the synthesized film-like substance was a copolymer having the target molecular structure.

In addition, various molar ratios of (RO) ₃ Si— (CH ₂ ) ₃ —SO ₃ H and (MeO) ₃ Si—Me 1: n (n = 0) by using the above-described method for preparing an electrolyte precursor , 0.5,1,2,3) can be prepared. Each electrolyte precursor was developed in a petri dish and then subjected to a polymerization reaction by distilling off the solvent under reduced pressure to obtain an electrolyte layer of a film-like substance.

  It was found that the electrolyte layers with n = 1, 2, 3, 4, 5 were insoluble in water. On the other hand, when n = 0, 0.5, it easily dissolves in water and is unsuitable for use as an electrolyte layer in a fuel cell catalyst electrode.

  In addition, when the solubility of the above electrolyte layer (n = 1, 2, 3) in an organic solvent was examined, the above electrolyte layer was immersed in acetone, ethyl alcohol, a chlorine-containing solvent, and dimethylacetamide, and stirred overnight. did. However, it did not dissolve at all and resulted in only a precipitate.

In addition, an electrolyte precursor prepared by mixing (RO) ₃ Si— (CH ₂ ) ₃ —SO ₃ H and (MeO) ₃ Si—C ₆ H ₁₃ having a C6 alkyl chain at a molar ratio of 1: n The electrolyte layer was obtained by drying and polymerizing. The electrolyte layer with n = 0.50,0.75,1,2,3 was immersed and stirred in acetone, ethyl alcohol, a chlorine-containing solvent, and dimethylacetamide for a whole day and night.

An electrolyte precursor prepared by mixing (RO) ₃ Si— (CH ₂ ) ₃ —SO ₃ H and (MeO) ₃ Si—C ₁₀ H ₂₁ having a C10 alkyl chain at a molar ratio of 1: n The electrolyte layer was obtained by drying and polymerizing. The electrolyte layer with n = 0.50,0.75,1,2,3 was immersed and stirred in acetone, ethyl alcohol, a chlorine-containing solvent, and dimethylacetamide for a whole day and night.

  Solvents capable of preparing the above electrolyte precursor include, except for t-BuOH, lower alcohols such as acetone and ethanol, dimethylacetamide, and the like.

Preparation of catalyst porous structure 4.0 g of acetylene black having a diameter of about 50 nm (Electrochemical Industry), 2.0 g of polyacrylonitrile (Sigma Aldrich) and dimethylacetamide (Wako Pure Chemical Industries) were mixed by a ball mill. 1.69 g of this mixed dispersion was dropped on carbon paper having an area of 19.6 cm 2, and the solvent was evaporated in a vacuum container at room temperature. Next, the carbon paper was heat-treated at 120 ° C. for 2 hours using a constant temperature vacuum dryer. Finally, the carbon paper was transferred into an infrared image furnace under an argon atmosphere, heated from room temperature at 20 ° C. per second, and heat-treated at an ultimate temperature of 800 ° C. for 30 minutes. As described above, carbon paper having a layer in which carbon fine powder was bound with a carbon thin film was obtained.

  Platinum-containing polyamic acid solution prepared by mixing 0.95 g of chloroplatinic acid (IV) hexahydrate (Wako Pure Chemical), 7.85 g of polyamic acid solution, and 17.5 g of dimethylacetamide (special grade of Wako Pure Chemical) Was dropped onto the carbon paper obtained above, and the solvent was removed in vacuo. Next, the carbon paper was dried at 200 ° C. for 2 hours using a constant temperature vacuum dryer. Finally, heating was performed in an infrared image furnace under an argon atmosphere at a temperature rising rate of 10 ° C. per second and an ultimate temperature of 800 ° C. for 30 minutes. As described above, a catalyst porous structure having a structure in which platinum nanoparticles were highly dispersed and fixed with respect to a porous body made of fine carbon powder was produced.

  The polyamic acid solution was prepared by polymerizing 5.00 g of 4,4 ′ diaminodiphenyl ether (Tokyo Kasei) and 5.45 g of pyromellitic anhydride (Tokyo Kasei) using 120 g of the solvent dimethylacetamide. It is.

Example 1 Production of Fuel Cell Electrodes A to G First, a method for producing a fuel cell electrode using the electrolyte precursor obtained by the method described in the formation of the electrolyte layer will be described. First, as shown in Table 1, seven types of electrolyte precursors were prepared. These seven types of electrolyte precursors are divided into _three types: electrolyte material (RO) ₃ Si— (CH ₂ ) ₃ —SO ₃ H and low molecular spacer material (MeO) ₃ Si—R (R: linear alkyl group). Each is contained in a predetermined molar ratio. The electrolyte materials listed in Table 1 and the low molecular spacer materials and their mixing ratios are suitable compositions having current-voltage characteristics as catalyst electrodes within the range of water-insoluble materials in the formation of the electrolyte layer. The choice is not limited to this. Each component of (RO) ₃ Si— (CH ₂ ) ₃ —SO ₃ H / (MeO) ₃ Si—R (R: linear alkyl group) contained in this electrolyte precursor is solvated in a low molecular state. Has been.

Next, the electrolyte precursor is dropped into the porous porous body made of carbon fine powder obtained by the method described in the preparation of the porous catalytic structure, and the electrolyte precursor is dropped for 1 hour.・ Stand still. Thereafter, removal of volatile components under reduced pressure and vacuum drying under heating were performed to produce fuel cell electrodes A to G including an electrolyte layer.

  In general, the electrolyte layer was formed under conditions of vacuum drying at 80 ° C. for 2 hours following the polysiloxane synthesis reaction example, but this is not restrictive.

  (Table 1) Preparation conditions and evaluation results of fuel cell electrodes in Example 1 and Comparative Example 1 of the present invention

(Comparative Example 1) Production of Comparative Electrode a A comparative electrode a was produced using an ethanol dispersion of a perfluorosulfonic acid electrolyte Nafion ^{(R) having} a EW value of 1100, which is a commercially available polymer electrolyte. The production procedure is as follows. The catalyst porous structure obtained by the method described in the preparation of the catalyst porous structure, in which platinum nanoparticles are supported on the porous body made of carbon fine powder, was allowed to stand on a petri dish. An ethanol dispersion of perfluorosulfonic acid electrolyte Nafion ^(R) was dropped into the catalyst porous structure, and immersed and left for 1 hour. Thereafter, removal of volatile components under reduced pressure and vacuum drying under heating produced a reference electrode a including a Nafion ^(R) electrolyte layer.

(Comparative example 2) Manufacture of comparative electrode b Moreover, preparation of the comparative electrode b was tried using the electrolyte layer superposed | polymerized through the drying process from the said electrolyte precursor. Specifically, first, the EW value obtained by mixing the electrolyte material (RO) ₃ Si— (CH ₂ ) ₃ —SO ₃ H and the low molecular spacer material (MeO) ₃ Si—Me at a molar ratio of 1: 3. 380 electrolyte precursors were prepared. The above electrolyte precursor was developed on a petri dish made of Teflon (registered trademark), and then a solid powder electrolyte layer was synthesized through removal of volatile components under reduced pressure and vacuum drying under heating. An attempt was made to impregnate and apply this solid powder electrolyte layer to the porous catalyst structure, but the electrolyte layer was insoluble in various solvents, and a dispersion solution could not be prepared. For this reason, the solid lump electrolyte layer once synthesized from the electrolyte precursor via the polymerization reaction cannot be impregnated and applied to the catalyst porous structure after being re-dispersed and the reference electrode b cannot be produced. It was. As described above, even if the same electrolyte precursor as that constituting the fuel cell electrode B is used, (FIG. 1) the second step in the process diagram of the production of the fuel cell electrode <the electrolyte precursor impregnation coating If the order of (S12 → S13)> and the third step <formation of electrolyte layer by reduced pressure drying and heat drying (S13 → S14)> is reversed, the effect of the present invention is not achieved.

(Comparative Example 3) Production of Comparative Electrode c An attempt was made to produce the comparative electrode c using an electrolyte precursor containing no low molecular spacer material. Specifically, an electrolyte precursor composed only of an electrolyte material (RO) ₃ Si— (CH ₂ ) ₃ —SO ₃ H was prepared, and a comparative electrode c was produced. The other production conditions are the same as in Example 1. Next, when the reference electrode c in which the electrolyte layer was formed in the catalyst porous structure was immersed in hot water at 60 ° C. for 2 hours, the formed electrolyte layer was dissolved in water and removed from the catalyst porous structure. It was. Therefore, it does not have an appropriate current-voltage characteristic and cannot be used as a fuel cell electrode.

Evaluation of catalytic reaction area of fuel cell electrode Each electrode produced by the above method was incorporated as a cathode electrode in a fuel cell, and the catalytic reaction area was evaluated by a cyclic voltammetry method. A carbon paste electrode carrying Pt 2.0 mg / cm ² was used as the anode electrode. Cyclic voltammetry measurement was performed while supplying hydrogen gas (65 ° C., 100% RH) to the anode electrode and nitrogen gas (65 ° C., 100% RH) to the cathode electrode. At this time, the sweep speed was set to 10 mV / sec, and the sweep potential width was set from the lower limit: natural potential to the upper limit: 1.0 V. The natural potential is a potential between electrodes in an open circuit state under the gas conditions at both electrodes as described above. (FIG. 3) shows a cyclic voltammogram obtained by measurement of the fuel cell electrode G (solid line) and the comparative electrode a (broken line). The amount of charge involved in proton desorption on platinum was estimated from the voltammogram obtained for each electrode, and the catalytic reaction area per unit platinum amount was estimated from the amount of charge. For example, the charge amount at the comparison electrode a is estimated from the area of the hatched area surrounded by the broken cycle upper curve and the solid level line. Table 1 summarizes the results of the catalytic reaction area and the natural potential for each electrode.

For the reference electrode a coated with a perfluorosulfonic acid polymer electrolyte dispersion often used in conventional materials, the catalytic reaction area per unit platinum amount is 23 m ² / g, and the natural potential of the cathode electrode is The result stayed high at around 100mV (vs. SHE). Here, the natural potential is an equilibrium potential between the two electrodes regarding proton desorption on the platinum surface. In other words, in this case, which is a positive value, it is considered that protons cannot reach the platinum surface sufficiently at the cathode electrode.

In contrast, in the electrode G in which an electrolyte precursor in which an electrolyte material and a low molecular spacer material are dispersed in a low molecular weight state is applied and dry polymerized to form an electrolyte layer, the reaction area dramatically increases. A result of 52 m ² / g was given. In addition, the natural potential of the electrode G was significantly reduced to about 20 mV (vs. SHE).

In addition, as shown in Table 1, the electrodes A to F other than the electrode G also had an increase in catalytic reaction area and a decrease in natural potential compared to the comparative electrode a using the polymer electrolyte material. .
The factors for the two effects of expansion of the reaction area and reduction of the natural potential are considered as follows.

  In general, dispersions of polymer electrolytes have a large polymer size, so it is extremely difficult to uniformly and sufficiently dispose the electrolyte in a catalyst structure having a pore structure of a small size (several to several tens of nanometers). It is. On the other hand, by using an electrolyte precursor in a low molecular weight state, the electrolyte precursor can be easily introduced into the pore structure on which the catalyst particles are supported, and the applied polymerization reaction can be performed in situ. By immobilizing via, a sufficient electrolyte layer is formed in the vicinity of the catalyst particles. As a result, a high proton concentration state in the vicinity of platinum was created, which led to an increase in the three-phase interface area and a decrease in the natural potential.

  The method for producing a fuel cell electrode according to the present invention has a large three-phase interface area and high power generation characteristics, and is useful for a fuel cell electrode and a fuel cell using the same. In addition, it is effective for high-density immobilization of electrolytes on electrode particles or catalyst particles finely dispersed in a porous structure, and can be widely applied to uses such as inexpensive electrochemical electrodes.

Claims (4)

Forming a catalyst porous structure in which the catalyst particles are made of fine carbon powder;
Preparing an electrolyte precursor with a solvent and at least an electrolyte material;
Applying the electrolyte precursor to the catalyst porous structure;
The step of forming a catalyst electrolyte composite precursor by applying the electrolyte precursor to the catalyst porous structure, and the catalyst electrolyte composite by performing reduced-pressure drying treatment and heat drying treatment on the catalyst electrolyte composite precursor. A method for producing an electrode for a fuel cell, comprising a step of forming an electrolyte layer in an electrolyte composite precursor. The low-molecular electrolyte material having an ionic functional group in a molecule, a low-molecular spacer material having no ionic functional group, and a solvent are used as the electrolyte precursor in a mixture. A method for producing an electrode for a fuel cell according to 1. The electrolyte material is (RO) ₃ Si-CH ₂ CH ₂ CH ₂ SO ₃ H (R = H, Me, Et), and the low molecular spacer material is (RO) _m SiR ′ _n (m + n = 4 , m ≧ 2) (R = H, Me, Et. R ′ = C _k H _{2k + 1} (k = 1 to 10)) . The method for producing a fuel cell electrode according to claim 1, wherein the solvent contained in the electrolyte precursor is a low polarity solvent.

Images