RU2561711C2 - Method of catalytic electrode manufacturing based on heteropoly compounds for hydrogen and methanol fuel elements - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention relates to the electric engineering, namely to the method of manufacturing of the catalytic electrode of the membrane-electrode block, mainly for hydrogen and methanol fuel elements. The method of manufacturing of the fuel element catalytic electrode includes manufacturing of the composite catalyst based on the heteropoly compounds and active catalytic layer on its basis with addition of the water-repellent agent. The catalytic electrode is porous nanostructured composite layer 5-15 mcm comprising: catalyst, i.e. composite out of proton-conducting heteropoly compound in form of caesium salt of phosphotungstic acid and electric conducting addition out of the carbon material or alloyed stannic oxide, on which particles of the catalytic metal of the platinum group are applied with average size 3 nm, and 5-20% of hydrophobisator, preferably PTFE. At that content of the precious metals in the composite catalyst is from 5 to 30 wt %, of the electric conducting components - from 2 to 30 wt %.

EFFECT: increased electrochemical reactivity of the catalytic electrode and its corrosion resistance are the technical result of the invention.

5 cl, 3 dwg, 2 ex

Description

The invention relates to the field of fuel cells, in particular hydrogen and methanol low-temperature polymer electrolyte fuel cells. The technical result of the invention is to increase the discharge characteristics of the fuel cell.

The proposed method for the manufacture of a catalytic electrode of a fuel cell includes the manufacture of a composite catalyst and an active catalytic layer based on it heteropoly compounds with the addition of electrically conductive and hydrophobizing additives, preferably polytetrafluoroethylene. According to the invention, the anode electrode of the membrane-electrode block of the fuel cell contains, as the active catalytic component, a catalyst - noble metal particles deposited on a carrier - a composite of a proton-conducting heteropoly compound, cesium salt of phosphoric tungsten acid and an electrically conductive additive - carbon material or doped tin dioxide. Moreover, in the composite carrier, the content of electrically conductive components is from 2 to 30% of the mass, and the noble metal content in the composite catalyst is from 5 to 30% of the mass.

In the manufacture of a catalytic electrode based on heteropoly compounds, a catalytic electrode is used, which is a porous composite layer 5-15 μm thick, consisting of the following components: a composite carrier on which platinum group catalytic metal nanoparticles with an average size of 3 nm are chemically deposited.

The suspension of the active composite mass is prepared by dispersing the composite catalyst and the hydrophobizing additive in a mixture of water, isopropyl alcohol and glycerol in a ratio of 0.5: 0.2: 0.3, respectively, and applying the suspension (by any means: brush, airborne or ultrasonic spraying, screen printing) on a gas diffusion layer followed by heat treatment at 120 ° C.

The proposed method of preparation allows you to create the optimal porous structure of the catalytic electrode of hydrogen and methanol fuel cells with increased electrochemical activity.

As is known, the role of the electrodes of membrane-electrode blocks and a fuel cell consists in the catalytic oxidation of hydrogen or methanol at the anode and the reduction of oxygen at the cathode, as well as in the supply / removal of reagents, the reaction product at the cathode, electrons and protons. The supply (removal) of reagents, reaction product, electrons and protons should be carried out to (from) the surface of the catalyst. Nanosized particles of a noble metal introduced into the composition of the catalytic active layer of the electrode are used as a catalyst. As a rule, to introduce the active layer into the active layer, platinum particles are not used directly, but are additionally applied to the surface of the particles of a conductive carrier. Particles of the carrier coated with platinum particles are introduced into the composition of the catalytic active layer of the electrode. For the fuel cell to work effectively, it is necessary to ensure the optimal transport balance of the flows of electrons, protons, gas reagents and water throughout the thickness of the active layer of electrodes.

It is known that the most effective electrocatalysts for the cathode and anode of low-temperature fuel cells are catalytic systems based on platinum group metals and especially platinum. In the early years of the development of low-temperature FC technology, large quantities of platinum in the form of black were used in the active electrode layer with a load of 4 to 30 mg Pt / cm ² [Wilson MS, Valerio JA, Gottesfeld S. Low platinum loading electrodes for polymer electrolyte fuel cells fabricated using thermoplastic ionomers. Electrochim Acta 1995; 40: 355-63]. A significant step forward was the transition from platinum mobile to catalytic structures using nanoscale platinum. To stabilize active metal nanoclusters, electrically conductive carriers with a high surface area are used. Known technical solution [Raistrick ID. Electrode assembly for use in a solid polymer electrolyte fuel cell. United States patent US 4876115. 1989 Oct 24], in which the electrode is nanosized particles of platinum deposited on carbon black, while the catalytic layer of the electrode is a composite consisting of a catalyst and an ion-conductive additive - ionomer. This structure of the electrode can significantly reduce the loading of platinum to 0.3-0.4 MGPt / cm ² while maintaining the power characteristics of the fuel cell.

This method of forming catalytic electrodes based on platinum catalysts on carbon carriers allows the manufacture of highly active membrane-electrode blocks with a reduced platinum content for pure hydrogen fuel cells. However, their serious drawback is the low efficiency in technical hydrogen fuel cells, even with a trace amount of CO impurity, or in alcohol solutions. This is primarily due to the poisoning of platinum catalysts with impurities of carbon monoxide in hydrogen, as well as the products of electrooxidation of alcohols (CO, formaldehyde, etc.) [Ota K, Nakagawa Y, Takahashi M. Reaction products of anodic oxidation of methanol in sulfuric acid solution. // J Electroanal Chem. 1984. Vol. 179. P. 179-86], [Lamy C, Lima A, LeRhun V, Delime F, Coutanceau C, Lüger JM. Recent advances in the development of direct alcohol fuel cells (DAFC). / J Power Sources. 2002. Vol. 105. P. 283], [Wu J, Yuan X.Z., Wang H., Blanco M. .., et al. Durability of PEM fuel cells. Presented at: Hydrogen and Fuel Cells 2007 International Conference and Trade Show; 2007 Apr 29-May 3; Vancouver, Canada].

To overcome this problem, scientific and technical solutions based on the use of heteropolyacids or their salts as components of membrane-electrode blocks are known. A number of authors have proposed a technical solution for the use of heteropoly compounds as additives to increase the proton conductivity of the components of catalytic electrodes [McGrath; James E .. Hickner; Michael Sulfonated polymer composition for forming fuel cell electrodes. US 7544764 2009 Jun 9], [Yasuo Miyake, Gohei Yoshida, Kazutaka Ikeda. Electrode for fuel cell and fuel cell therewith US 2004/0115516 A1 2003 Nov 26]. Such additives, in addition to ensuring transport balance in the catalytic layer, can also have a promoting effect in the oxidation of CO and organic fuels, as a result of which the use of heteropoly compounds in the composition of the electrodes leads to an increase in their tolerance to catalytic poisons. A number of acids used for this purpose is quite wide, while the content of conductive additives based on heteropoly acids in the electrode can be from 0.01 to 60% of the mass. The disadvantage of these technical solutions is the low degree of contact of the additives of heteropoly compounds with the active catalytic metal and a weakly expressed synergistic effect.

Known technical solution for the use of salts of heteropoly acids as components of catalysts for the oxidation of hydrogen with impurities CO [Hitoshi Nakajima, Itaru Homma. Catalyst for fuel cell and electrode using the same. United States patent US 01413342003 Nov 12]. The authors proposed catalysts based on salts of heteropoly acids with cations of alkali and alkaline earth metals, some of which are replaced by cations of noble metals Ru, Rh, Pd, Ag, Ir, Pt or Au, or a number of transition metals Ti, V, Cr, Mn, Fe , Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ta, or W. To form an electrode, catalysts based on heteropoly acid salts were deposited on a carbon substrate, or the catalytic layer was formed by mixing the catalyst with a carbon conductive material. The resulting electrodes were highly resistant to carbon monoxide poisoning.

The closest analogue of the invention, selected as a prototype [In-Hyuk Son, Sang-Il Han .. Electrode for fuel cell, and membrane-electrode assembly and fuel cell system including the same. US 7910263 2011 Mar 22], is a catalytic electrode containing a carbon substrate on which there is a catalytic layer comprising an active catalyst and a heteropoly acid deposited on an inert carrier - silicon oxide or aluminum. Moreover, the heteropoly acid has one of the following anion compositions: [PMo ₁₂ O ₄₀ ] ^3- , [PW ₁₂ O ₄₀ ] ^3- , [GeMo ₁₂ O ₄₀ ] ^4- , [GeW ₁₂ O ₄₀ ] ^4- , [P ₂ W ₁₈ O ₆₂ ] ^6- , [SiW ₁₂ O ₄₀ ] ^4- , [PMo ₁₁ O ₃₉ ] ^7- , [P ₂ Mo ₅ O ₂₃ ] ^6- , [H ₂ W ₁₂ O ₄₀ ] ^6- , [PW ₁₁ O ₃₉ ] ^7- . The content of heteropoly acid in relation to the carrier is from 0.01 to 10% of the mass. The addition of heteropoly compounds deposited on an oxide carrier is 0.1-5% of the mass. in relation to the mass of the catalytic layer. The technical effect of using the present invention is to increase the resistance of such catalytic electrodes to poisoning by catalytic poisons, in particular CO. The disadvantages of this technical solution is the relatively low stability of these catalytic electrodes in a fuel cell.

First of all, this can be explained by both the insufficient ionic conductivity of the used heteropoly compounds and the ineffectively organized structure of the active layer. The technical solution described is selected for the prototype.

It is preferable to use catalysts based on noble metals supported on a composite carrier, which would ensure the optimal transport balance of the flows of electrons, protons, gas reagents and water in the entire volume of the active layer of electrodes and at the same time have an active promoting effect on the catalyzed process.

The technical task of the proposed method is to obtain highly active catalytic electrodes for the anode of hydrogen and methanol fuel cells.

The task involves the development of a method for the manufacture of a catalytic electrode based on heteropoly compounds for hydrogen and methanol fuel cells, which combines: a method for producing an effective composite catalyst based on heteropoly compounds and an active catalytic layer based on it with the addition of water-repellent additives in order to most effectively use the noble metal in catalytic electrode.

For more efficient use of the active area of the electrode, it is necessary to organize three-phase boundaries in its volume, however, all known technical solutions for the construction of bulk electrodes for fuel cells using non-carbon catalyst supports use the same designs as for catalytic electrodes based on carbon nanostructures. However, due to the lower conductivity of composite systems based on doped oxides and heteropoly compounds, a thinner layer of a nanostructured electrocatalytic material is effectively working. In addition, the composition of the active layer must also be individually adapted for a particular catalytic system.

Therefore, the problem is solved due to the fact that in order to more efficiently use the noble catalytic metal in the proposed electrode design, a thinner layer of electrocatalytic material with an average thickness of 5-15 μm and the formation of the porous structure of the electrode due to hydrophobic additives is used. In order to increase the activity of the electrode in the reactions of electrooxidation of industrial hydrogen and methanol, a highly active catalyst is used, which has increased resistance to poisoning by catalytic poisons.

As a catalytic base, a composite carrier based on the proton-conducting cesium salt of phosphoric tungsten acid Cs _3-x H _x PW ₁₂ O ₄₀ · nH ₂ O and 2-30% of the mass is used. conductive antimony doped tin dioxide or carbon material with platinum nanoparticles with an average size of 3 nm deposited on the surface of the composite. The formation of the active composite mass is carried out by dispersing a catalyst based on heteropoly compounds and 5-20% of a hydrophobizing agent, preferably polytetrafluoroethylene, in a mixture of water, isopropyl alcohol and glycerol in a ratio of 0.4: 0.2: 0.4, respectively, and applying a suspension to the required substrate, followed by heat treatment at 120 ° C. This method of manufacturing the electrode allows you to create the optimal porous structure with increased electrochemical activity.

The implementation of the method is illustrated by the following examples.

Example 1

A hetero-based catalytic electrode for a hydrogen fuel cell.

Catalyst Preparation Method

An industrial phosphoric tungsten heteropoly acid (FVC) is purified by ether extraction from a saturated aqueous solution. For this, a few drops of fuming nitric acid are added to a saturated aqueous solution of FVC, after which they are mixed with an excess of diethyl ether. The ether layer containing PVA is separated using a separatory funnel, and after washing with a 0.1 M hydrochloric acid solution and distilled water, the ether is distilled off on a rotary evaporator.

The synthesis of cesium salt of phosphoric tungsten acid is carried out by neutralizing a solution of cesium carbonate with a solution of heteropoly acid. The concentration of acid and cesium carbonate are 0.03 and 0.04 mol / L. Reagents are taken in a stoichiometric ratio to obtain the following composition Cs _2.3 H _0.7 PW ₁₂ O ₄₀ . The precipitate obtained is washed with distilled water (at least 6 times) and centrifuged at 9000 rpm. The precipitates are dried in vacuo to constant weight at 50 ° C.

The synthesis of antimony doped tin dioxide Sn _0.95 Sb _0.05 O ₂ (Sb / Sn = 0.05) is carried out by the reverse micelle method. Metal salts SnCl ₄ · 5H ₂ O, SbCl _{3 are} dissolved in cyclohexane containing a surfactant (cetyltrimethylammonium bromide (CTAB) (5 μmol / L), then NaOH is added to pH = 13 and after thorough mixing the mixture is left for a day to form oxide particles. The resulting materials are annealed in air for 1 hour at a temperature of 500 ° C.

To obtain a catalyst, the platinum precursor H ₂ PtCl _{6 is} mixed in an aqueous solution with a heteropoly acid salt and an electrically conductive additive (doped tin oxide Sn _0.95 Sb _0.05 O ₂ ) in an ultrasonic bath at room temperature for 5 hours. The resulting suspension is centrifuged, decanted. The precipitate obtained is dried at 60 ° C and then reduced in a hydrogen atmosphere for 1 hour at a temperature of 300 ° C. The oxide content is 30% of the mass. in relation to hetero-compound. The platinum content in the catalyst is 10% of the mass. in relation to the composite medium.

The formation of the active composite mass is carried out by dispersing the catalyst Pt-Cs _2.3 H _0.7 PW ₁₂ O ₄₀ -Sn _0.95 Sb _0.05 O ₂ and 10% of the mass. a polytetrafluoroethylene hydrophobizing agent in a mixture of water, isopropyl alcohol and glycerol in a ratio of 0.5: 0.2: 0.3, respectively, in an ultrasonic bath for 1 hour at a temperature of 45 ° C. The resulting suspension is applied with a brush to heated to 70 ° C carbon hydrophobized paper (Toray TGPH-190, 20% PTFE). Losses should not exceed 10%.

Properties of the resulting materials.

сингония кубическая. Средний размер частиц, оцененный из диффрактограмм с использованием формулы Шерера по рефлексу [211]: составил 7 нм, а согласно сканирующей микроскопии полученное гетерополисоединение представляет собой сферические образования диаметром 50-150 нм, имеющие сложную внутреннюю структуру. Они состоят из плотноупакованных сфер меньшего диаметра (5-20 нм). Удельная поверхность цезиевой соли - 140 м²/г.According to the XRD results, the unit cell parameters of the cesium salt of phosphoric tungsten acid $$\alpha =11.82\pm \mathrm{0,03}\AA ;$$

cubic syngony. The average particle size, estimated from diffraction patterns using the Scherrer formula for reflex [211]: was 7 nm, and according to scanning microscopy, the resulting heteropoly compound is a spherical formation with a diameter of 50-150 nm, having a complex internal structure. They consist of densely packed spheres of smaller diameter (5-20 nm). The specific surface area of cesium salt is 140 m ² / g.

The figure 1 shows the images obtained by scanning electron microscopy for the cesium salt of phosphoric tungsten acid Cs _2.3 H _0.7 PW ₁₂ O ₄₀ (a) and doped tin oxide Sn _0.95 Sb _0.05 O ₂ (b).

The obtained oxide component of the carrier has a rutile-like structure (P4 / mnm), has a spherical shape of particles with an average diameter of about 20 nm and a narrow particle size distribution. The electronic conductivity of the carrier is 3 (Ohm · cm) ^-1 . The specific surface area of the oxide is about 100 m ² / g.

The platinum content in the catalyst is about 10 wt.%, The average particle diameter of platinum is 3 nm. The specific active surface of the catalyst is 59 m ² / g (Pt). The thickness of the electrode is 12 μm.

The activity of the catalytic electrode with the OIE composition of a hydrogen fuel cell when using hydrogen with CO impurity (100 ppm) was about 300 mW / cm ² at a voltage of 0.5 V. Experimental conditions: gas flow - 100 ml / min, gas humidity 50% HR . The platinum loading on the OIE anode was 0.3 mg / cm ² . At the cathode, a commercial 20% Pt / C catalyst (E-TEK) was used. In all series of experiments, the currents were recorded after reaching the stationary mode. The figure 2 shows the corresponding current-voltage and power characteristics of the membrane-electrode block with a catalyst Pt / Cs _2.3 H _0.7 PW ₁₂ O ₄₀ -Sn _0.95 Sb _0.05 O ₂ on the anode of a hydrogen fuel cell, the fuel used is hydrogen with an admixture of 100 ppm CO.

Example 2

A hetero-based catalytic electrode for a methanol fuel cell.

The electrode preparation process is similar to that shown in example 1 and differs in that the content of the polytetrafluoroethylene hydrophobizing agent is 14%.

Properties of the resulting materials.

The thickness of the electrode is 15 μm. The activity of the catalytic electrode with the OIE composition of the methanol fuel cell when using 0.5 M methanol solution as fuel was about 30 mW / cm ² at a voltage of 0.5 V. Temperature 35 ° C. The platinum loading on the OIE anode was 0.6 mg / cm ² . A commercial 20% Pt / C (E-TEK) catalyst was used at the cathode. In all series of experiments, the currents were recorded after reaching the stationary mode. Figure 3 shows the corresponding current-voltage and power characteristics of a membrane-electrode block with a Pt / Cs _2.3 H _0.7 PW ₁₂ O ₄₀ -Sn _0.95 Sb _0.05 O ₂ catalyst on the anode of a methanol fuel cell, the fuel used is a 0.5 M methanol solution. The platinum content on the electrodes is 0.8 mg / cm ² .

Claims (5)

1. A method of manufacturing a catalytic electrode based on heteropoly compounds for hydrogen and methanol fuel cells, characterized in that the active layer uses a catalyst based on a noble metal, preferably platinum, supported on a carrier - a composite consisting of proton conducting heteropoly compounds of cesium salt of phosphoric-tungsten acid Cs _3-x H _x PW ₁₂ O ₄₀ · nH ₂ O, and an electrically conductive additive of carbon nanomaterials or doped tin dioxide, as well as a hydrophobic additive, pre respectfully polytetrafluoroethylene. 2. A method of manufacturing a catalytic electrode based on heteropoly compounds according to claim 1, characterized in that in the composite carrier the content of electrically conductive components is from 2 to 30 wt.% 3. A method of manufacturing a catalytic electrode based on heteropoly compounds according to any one of claims 1 and 2, characterized in that the noble metal content in the composite catalyst is from 5 to 30 wt.% 4. A method of manufacturing a catalytic electrode based on heteropoly compounds according to any one of claims 1 to 3, characterized in that a catalytic electrode is used, which is a porous composite layer 5-15 microns thick, consisting of the following components: a composite carrier on which nanoparticles are chemically deposited a catalytic metal of a platinum group with an average size of 3 nm, as well as 5-20% water repellent, preferably polytetrafluoroethylene. 5. A method of manufacturing a catalytic electrode based on heteropoly compounds for hydrogen and methanol fuel cells according to any one of claims 1 to 4, characterized in that the suspension of the active composite mass is prepared by dispersing the composite catalyst and the hydrophobizing additive in a mixture of water, isopropyl alcohol and glycerol in the ratio of 0.5: 0.2: 0.3, respectively, and the application of the suspension on the desired substrate, followed by heat treatment at 120 ° C.

Images