Gas diffusion electrode and fuel cell comprising such a gas diffusion electrode

Abstract

A gas diffusion electrode for a fuel cell which comprises a gas-permeable substrate that has functional groups is provided, said groups being capable of complexing cations, and catalytically active noble metal particles and/or atoms, said particles and/or atoms being bonded by the functional groups to a surface of a first flat side of the substrate and/or in a surface-proximal region of a first flat side of the substrate. The gas diffusion electrode according to the invention combines the functions of a gas diffusion layer and a catalytic layer in an integral component and is distinguished by a high long-term stability with respect to degradation phenomena of the catalyst.

Claims

1. A gas diffusion electrode for a fuel cell, comprising: a gas-permeable substrate which is formed from an organic polymer having an organic polymer backbone, wherein the polymer has functional groups covalently bonded to the organic polymer backbone and capable of complexing metal cations; and catalytically active noble metal particles and/or atoms that are bonded, by the functional groups, to a surface of a first flat side of the substrate and/or in a surface-proximal region of the first flat side of the substrate.

2. The gas diffusion electrode according to claim 1, wherein the functional groups comprise amidoxime groups, hydroxamic acid groups, amidrazone groups, or mixtures thereof.

3. The gas diffusion electrode according to claim 1, wherein the organic polymer is a nitrile-group-containing polymer whose nitrile groups are partially or entirely converted into amidoxime groups, hydroxamic acid groups, and/or amidrazone groups.

4. The gas diffusion electrode according to claim 1, wherein the organic polymer is a hydroxylamine-transformed copolymer of acrylonitrile and acrylic acid monomers, or a blend of hydroxylamine-transformed polyacrylonitrile and polyacrylic acid.

5. The gas diffusion electrode according to claim 1, wherein the gas-permeable substrate is formed from fibers of the organic polymer.

6. The gas diffusion electrode according to claim 1, wherein a concentration of the catalytically active noble metal particles and/or atoms decreases inward with a gradient from the direction of the first flat side of the substrate.

7. A method for producing a gas diffusion electrode for a fuel cell, comprising: providing a gas-permeable substrate which is formed from an organic polymer having an organic polymer backbone, wherein the polymer has functional groups covalently bonded to the organic polymer backbone and capable of complexing metal cations; charging the functional groups, which are present at a surface of a first flat side of the substrate and/or in a surface-proximal region of the first flat side of the substrate, with ions of a catalytically active noble metal under complexation thereof; and reducing the complexly bound noble metal ions to an oxidation state of zero.

8. A fuel cell, comprising: a polymer electrolyte membrane; gas diffusion electrodes arranged on both sides of the polymer electrolyte membrane, wherein each gas diffusion electrode comprises a gas-permeable substrate which is formed from an organic polymer having an organic polymer backbone, wherein the polymer has functional groups covalently bonded to the organic polymer backbone and capable of complexing metal cations, and further comprises catalytically active noble metal particles and/or atoms that are bonded, by the functional groups, to a surface of a first flat side of the substrate and/or in a surface-proximal region of the first flat side of the substrate, and wherein each of the first flat sides of the gas diffusion electrodes which have the catalytically active noble metal particles and/or atoms respectively contact the polymer electrolyte membrane; and bipolar plates adjoining the gas diffusion electrodes.

9. The fuel cell according to claim 8, wherein the polymer electrolyte membrane is a polyacrylonitrile-based membrane whose nitrile groups are at least partially converted into amidoxime groups, or which is impregnated with polyarylene ether sulfone.

10. The fuel cell according to claim 8, wherein no additional layer is present between the polymer electrolyte membrane and the gas diffusion electrodes adjoining thereto.

11. The gas diffusion electrode according to claim 3, wherein the organic polymer comprises polyacrylonitrile (PAN) transformed with hydroxylamine and/or hydrazine, or a copolymer or a blend thereof.

12. The gas diffusion electrode according to claim 5, wherein the fibers of the organic polymer are in the form of a fleece, a felt, or a fiber web.

13. The gas diffusion electrode according to claim 8, wherein no microporous layer and no catalytic layer is present between the polymer electrolyte membrane and the gas diffusion electrodes adjoining thereto.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The invention is explained below in exemplary embodiments in reference to the respective drawings. The following is shown:

(2) FIG. 1 a perspective section view of a fuel cell according to prior art, and the reactions taking place therein;

(3) FIG. 2 the schematic sequence of components of a fuel cell according to the prior art;

(4) FIG. 3 a schematic section view of a gas diffusion electrode according to an embodiment of the invention,

(5) FIG. 4 a schematic section view of a gas diffusion electrode according to a further embodiment of the invention,

(6) FIG. 5 the schematic sequence of the components of a fuel cell according to the invention, with gas diffusion electrodes according to FIG. 3, and

(7) FIG. 6 the schematic sequence of the components of a fuel cell according to the invention, with gas diffusion electrodes according to FIG. 4.

DETAILED DESCRIPTION

(8) FIG. 1 shows the structure and the occurring reactions of a fuel cell according to the prior art, designated as a whole with 10′.

(9) The fuel cell 10′ comprises a polymer electrolyte membrane 11 which exhibits an ionic conductivity, particularly for protons H.sup.+. The most common material for polymer electrolyte membranes in fuel cells today is a sulfonated polytetrafluoroethylene (PTFE) known under the trademark Nafion®. One of each electrode structure, namely an anode 12 and a cathode 13, respectively adjoin each of the two sides of the membrane 11. Each of the electrodes 12, 13 comprises a catalytic layer 14, a gas diffusion layer 16, and optionally a microporous layer 15 arranged between gas diffusion layer 16 and catalytic layer 14.

(10) The catalytic layers 14 typically have a supported catalytic material comprised of catalytic noble metal particles 141 which are present in fine distribution (see partial details in FIG. 1) on an electrically conductive substrate 142. The carrier material 142 of the particles is usually a carbon-based material such as activated carbon, carbon black, or the like. The catalyst material is held together by an ionically conductive polymeric binder 143 which is often selected from the same material as the polymer electrolyte membrane 11.

(11) The gas diffusion layers 16 serve for the uniform supply of the reaction gases to the catalytic layers 14, and for the removal of the reaction products. The gas diffusion layers according to the prior art are comprised of an electrically conductive and gas-permeable layer in the form of foams or fibers. Carbon-based materials, such as expanded graphites, carbon fibers, or carbonized PAN, or hydrophilized organic polymer materials, are generally used. The microporous layers 15 are likewise comprised of an electrically conductive, usually carbon-based material, often carbon paper. The microporous layers 15 should prevent the penetration of the catalytic particles from the catalytic layer 14 into the gas diffusion layer 16. Insofar as the components catalytic layer 14, microporous layer 15, and gas diffusion layer 16 are present in a material composite, this composite is also referred to as a gas diffusion electrode GDE. Alternatively, it is known to apply the catalytic layers 14 directly onto the polymer electrolyte membrane 11, which is then referred to as a catalytically coated membrane or CCM.

(12) A bipolar plate 17 adjoins each of the two sides of the gas diffusion layers 16. The bipolar plate has flow channels 171, 172 on each of its flat sides for supplying the reaction media and removing the fuel cell products. The anode-side flow channels 171 thereby serve to supply the fuel, mostly hydrogen H.sub.2, as well as to discharge unreacted fuel and the product water. The cathode-side flow channels 172 serve to supply the oxygen, usually in the form of air, and to discharge the cathode exhaust gas and the product water. Furthermore, the bipolar plates 17 have internal coolant channels 173 which serve to conduct a coolant and dissipate the heat of reaction. The bipolar plates 17 are likewise made of an electrically conductive material, usually a metallic material or a graphite material. The bipolar plates 17 are connected via an external circuit 18 into which an electrical load is integrated.

(13) The fuel cell 10′ shows the following mode of operation. Hydrogen H.sub.2 is supplied to anode 12 via the anode-side flow channels 171 of the bipolar plates 17. Air is supplied to cathode 13 via the cathode-side flow channels 172 of the bipolar plates 17. These operating gases diffuse through the respective gas diffusion layer 16 and the microporous layer 15 to the respective catalytic layer 14. At the anode-side catalytic layer 14, the hydrogen H.sub.2 at the catalytic noble metal particles 141 is oxidized to protons H.sup.+, wherein electrons e.sup.− are released which are discharged via the anode-side microporous layer 15 and gas diffusion layer 16, and the bipolar plate 17 adjoining thereto, via the circuit 18. The protons H.sup.+ generated in the anode reaction migrate via the ionically conductive polymer electrolyte membrane 11 to the cathode 13 of the fuel cell. Here, the reaction of the oxygen O.sub.2 of the supplied air with the protons takes place while taking up the electrons supplied via the circuit 18 to form water H.sub.2O. In sum, hydrogen and oxygen thus react to form water, wherein the electromotive force of this reaction serves to generate electrical energy to supply an electrical load, for example an electric motor for an electrically powered motor vehicle. As a rule, a fuel cell comprises a plurality of individual cells according to FIG. 1 in serial connection.

(14) FIG. 2 schematically shows the sequence of the components of a single fuel cell 10′ according to the prior art, using the same reference numerals as in FIG. 1. It can be seen that the structure is comparatively complex, owing to the multitude of components.

(15) FIG. 3 shows the structure of a gas diffusion electrode 20, designated as a whole with 20, for a fuel cell according to a first embodiment of the present invention.

(16) The gas diffusion electrode 20 comprises a gas-permeable substrate 221 in the form of a flat structure that has a first flat side 222 and a second flat side 223 arranged opposite said first flat side 222. The substrate 221 is formed from an organic polymer which has functional groups that are covalently bonded to the backbone structure. The functional groups are suitable to chemically complex metal cations in the sense of a coordinative chemical bond. For this, it is required that the functional group possesses at least one free electron pair, preferably at least two free electron pairs.

(17) Suitable functional groups of the polymer of the gas-permeable substrate 221 are selected from the group comprising primary amine groups (—NH.sub.2); secondary amine groups (—NH—); tertiary amine groups (═N—); nitrosyl groups (—N═O); nitrile or cyanide groups (—CN); isocyanide groups (—NC), cyanate groups (—CNO); isocyanate groups (—NCO), carbonyl groups (—C═O); carboxyl groups (—C(O)OH); carboxylic acid ester groups (—C(O)OR); amide groups (—C(O)—NH.sub.2); amidoxime groups (—C(NH.sub.2)═N—OH); hydroxamic acid groups (—CO—NHOH); amidrazone groups (—C(═NH)—NH—NH.sub.2 including the tautomers —C(NH.sub.2)═N—NH.sub.2 and —CH(NH.sub.2)—N═NH); and mixtures thereof. All of the preceding functional groups are characterized in that they have free electron pairs which are suitable for complex bonding, meaning that they are Lewis bases.

(18) Amidoxime groups, amidrazone groups, and hydroxamic acid groups are particularly advantageous in the context of the present invention. The amidoxime group has two nitrogen atoms; the amidrazone group even has three nitrogen atoms, each having a free electron pair which are suitable for complexing a metal ion. The hydroxamic acid group has two free electron pairs which are provided by the carbonyl function and the amine group for coordinative bonding. It is thus in each case an at least bidentate ligand.

(19) Modified polyacrylonitrile (PAN) represents a particularly advantageous polymer that may be used as the material for the substrate 221. Polyacrylonitrile is already, without subsequent reaction, furnished with nitrile groups (—CN) which as such are already capable of complexing metal cations. Moreover, nitrile groups may easily be chemically converted into other functional groups, in particular amidoxime groups, amidrazone groups, or hydroxamic acid groups. PAN is preferably used whose nitrile groups have, for example, been converted wholly or partly into hydroxime groups by reaction with hydroxylamine (H.sub.2N—OH) (see reaction equation below). Polyacrylonitrile is additionally characterized by a high chemical resistance to acids and many organic solvents, as well as a high hydrolysis resistance.

(20) ##STR00001##

(21) Amidoxime groups have a particularly high affinity to copper ions, but may also bind platinum and palladium ions in a very stable manner. The following series shows the affinity of amidoxime to various metal ions: Cu>Au>V>U>Fe>Pd>Pt Zn>Cd>Cr>Ni>Pb.

(22) In order to increase the ionic and electrical conductivity of the organic polymer of the gas-permeable substrate 221, it is preferably provided that the polymer additionally comprises acid groups. For this purpose, the organic polymer having the functional groups may be copolymerized with comonomers having corresponding acid groups. In particular, here a copolymer of acrylonitrile monomers and acrylic acid monomers whose nitrile groups are completely or partially converted into amidoxime groups may be used. Alternatively, a blend of polyacrylonitrile and polyacrylic acid may be used in which the nitrile groups of the polyacrylonitrile are converted into amidoxime groups before or after blending.

(23) The substrate 221 must have a sufficient gas permeability. For this purpose, the substrate is preferably formed from fibers of the organic polymer that are present in the form of a non-woven material, felt, or fiber web.

(24) According to FIG. 3, the gas diffusion electrode 20 according to the embodiment of the invention also has a catalytic segment 224 that is formed in the area of the first flat side 222 of the substrate 221, as well as in a region near the surface of the substrate. Within the catalytic segment 224, the gas diffusion electrode 20 has catalytic noble metal particles and/or noble metal atoms that have been formed by reducing the noble metal cations complexly bound by the functional groups of the organic polymer of the substrate 221. In particular, the noble metal particles or atoms are complexly bound by the functional groups of the substrate 221, preferably chelated by two or more functional groups. The catalytic noble metal particles or atoms are those which are suitable for catalyzing the fuel cell reactions, in particular those of the platinum group. The catalytic noble metal particles or atoms of the catalytic segment 224 particularly preferably comprise platinum and/or palladium. Furthermore, the gas diffusion electrode 20 according to the embodiment of the invention differs from the prior art in that the catalytic function is developed within the gas-permeable substrate 221, i.e., the gas diffusion layer, and is not present in the form of a coating on this. The gas diffusion electrode 20 according to the embodiment of the invention thus does not have the structure of a fabric or a composite of two layers, but rather is an integral component which combines the functions of the gas diffusion layer and the catalytic layer.

(25) FIG. 4 shows a gas diffusion electrode 20 according to a second embodiment of the invention. Unlike the electrode shown in FIG. 3, here the catalytic noble metal particles/atoms are not limited to a relatively sharply and closely defined segment of the substrate 221. Instead, the concentration of noble metal exhibits a gradient decreasing from the first flat side 222 toward the interior of the substrate.

(26) In a highly schematic representation, FIG. 5 shows the structure of a fuel cell 10 according to an embodiment of the invention with gas diffusion electrodes 20 according to FIG. 3. The fuel cell has a polymer electrolyte membrane 11 that is arranged between two gas diffusion electrodes 20 according to the invention. The gas diffusion electrodes 20 are arranged in such a way that each of their first flat sides 222, which are charged with the catalytic noble metal, respectively face toward the membrane 11 and contact it directly. One bipolar plate 17 each respectively adjoins each of the respective second flat sides 223 of the gas diffusion electrodes 20. It can be seen that the fuel cell 10 according to the embodiment of the invention has a simple structure made up of a few components. Thus, the gas diffusion electrode 20 replaces the catalytic layer 14, the microporous layer 15, and the gas diffusion layer 16 of conventional fuel cells (see FIG. 2).

(27) FIG. 6 shows a fuel cell 10 according to an embodiment of the invention, containing gas diffusion electrodes 20 according to FIG. 4.

(28) In the fuel cell 10 according to the embodiment of the invention, a polymer electrolyte membrane 11 is preferably used which is based on the same or a similar polymer base as the substrate of the gas diffusion electrode 20. Insofar as the GDE 20 according to the embodiment is based on modified polyacrylonitrile, a polyacrylonitrile impregnated with poly(arylene ether sulfone) is preferably used, or polyacrylonitrile whose nitrile groups are entirely or partially converted into amidoxime groups. In both instances, a correspondingly treated PAN nonwoven material may be used as a basis of the membrane 11.

(29) The mode of operation of the fuel cell 10 according to embodiments of the invention corresponds to what is described in connection with FIG. 1.

(30) The gas diffusion electrode 20 according to an embodiment of the invention may be manufactured as follows. A felt is used which is formed from fibers of a copolymer of acrylonitrile (AN) and acrylic acid (AA), thus poly(acrylonitrile-co-acrylic acid), or from a blend made up of polyacrylonitrile (PAN) and polyacrylic acid (PAA). For example, the felt is transformed with hydroxylamine, wherein the nitrile groups react to form amidoxime groups. After cleaning and drying of the felt treated in such a manner, the first flat side of the felt is charged with a suitable platinum salt solution (for example aqueous platinum (II) chloride solution). This may be done in that the noble metal salt solution is sprayed onto the first flat side 222, or in that the felt is immersed with its first flat side 222 in the noble metal solution, wherein only the first flat side 222 or the region near the surface thereof is dipped into the solution. Due to the high affinity of the amidoxime groups, a rapid complexing of the noble metal cations occurs here in the form of a chelation. Insofar as a structure with a concentration gradient of the catalytic noble metal according to FIG. 4 is to be produced, the immersion time in the noble metal salt solution may be selected to be correspondingly long so that a suction of the solution in the direction of the interior of the substrate 221 occurs as a result of capillary forces. The felt is subsequently rinsed and dried. The noble metal cations located on the substrate 221 are then converted into noble metal particles with an oxidation state of zero via application of a reducing agent, for example hydroquinone.

(31) The assembly of the fuel cell 10 then takes place in that the polymer electrolyte membrane 11 is arranged between two gas diffusion electrodes 20 produced in the manner described above, wherein each of the first flat sides charged with the catalytic noble metal are respectively arranged on the membrane. This structure of membrane 11 and gas diffusion electrodes 20 is alternately stacked with bipolar plates 17 to form a fuel cell stack, and is compacted via external tensioning means or devices.

Exemplary Embodiment

(32) For the following preparation, a protocol analogous to Duk Man Yu et al. was used (“Properties of sulfonated poly(arylene ether sulfone)/electrospun nonwoven polyacrylonitrile composite membrane fro [sic] proton exchange membrane fuel cells” J. Membrane Sci. 446 (2013), 212-219).

(33) 1. Amidoxime Functionalization of PAN Felt (PAN-Oxime)

(34) 32 g of hydroxylammonium chloride (ClH.sub.4NO) were dissolved with stirring in 800 ml of demineralized water at 80° C. (see Table 1). 18 felt pieces PAN (Heimbach Company, Type 876531 5/5 PAN (H) 500 g/m.sup.2 of felt) of size 2 cm×2 cm and with a thickness of 2.6 mm were then added into the solution. Sodium bicarbonate (NaHCO.sub.3) was subsequently added in portions until the CO.sub.2 development had concluded (approximately 1 h 10 min) and a complete conversion of hydroxylammonium chloride with sodium bicarbonate to form hydroxylamine was to be assumed:
NaHCO.sub.3+HONH.sub.3Cl.fwdarw.CO.sub.2+H.sub.2O+H.sub.2N-OH+NaCl

(35) The preparation was stirred for a further 3 h at 80° C., wherein after 1 h, 2 h, and 3 h, respectively, 6 felt pieces were removed from the reaction solution and transferred for washing into 250 ml of demineralized water at room temperature and with stirring. The felts floated at the top of the wash water. The washing process was repeated 3 times for 10 minutes each.

(36) The felts had a light yellow color after 1 h; a light yellow color after 2 h; and a honey yellow color after 3 h.

(37) TABLE-US-00001 TABLE 1 Conversion of PAN with the hydroxylammonium chloride solution ClH.sub.4NO Reaction Felt Area solution ClH.sub.4NO NaHCO.sub.3 time at 80° C. samples (cm.sup.2) (mol/l) (g) (g) (h) Color 019-024 24 0.58 32 51.6 1 light yellow 025-030 24 0.58 32 51.6 2 yellow 031-036 24 0.58 32 51.6 3 honey yellow

(38) 2. Complexing of Fe.sup.3+ with PAN-Oxime

(39) 200 ml of a 7.5.Math.10.sup.−4 mol/l FeCl.sub.3 solution which were already dark yellow/orange were used in each case. The PAN-oxime felts from Example 1 were then added and stirred for 20 min at room temperature. The felts discolored orange-light brown. The felt pieces were subsequently removed from the solutions and washed in demineralized water.

(40) TABLE-US-00002 TABLE 2 Pretreatment of PAN-oxime felts with FeCl.sub.3 Reaction time of FeCl.sub.3 solution conversion with Felt samples (mol/l) hydroxylamine (h) 019-021 7.5 .Math. 10−4 mol/l 1 025-027 7.5 .Math. 10−4 mol/l 2 031-033 7.5 .Math. 10−4 mol/l 3

(41) 3. Complexing of Pt.sup.2+ with PAN-Oxime

(42) The FeCl.sub.3-treated PAN felts from Example 2 were washed with concentrated hydrochloric acid (HCl), wherein they decolored. A saturated solution of PtCl.sub.2 in concentrated HCl (sediment) was prepared, and the saturated supernatant was decanted. 3 of the FeCl.sub.3-pretreated and HCl-washed felts, as well as 3 untreated with FeCl.sub.3, in each case were shaken with 10-15 ml of the saturated PtCl.sub.2 solution for 24 h at room temperature. After 24 h, the PtCl.sub.2 solution was decanted, and the felts were washed with dilute hydrochloric acid for 30 min and subsequently washed 2 times with demineralized water.

(43) 4. Reduction of the Pt.sup.2+ Complexed with PAN-Oxime, with Various Reducing Agents

(44) The felts from Example 3 were placed for 60 min in a solution of a reducing agent (sodium thiosulfate, sodium sulfite, or hydroquinone) according to Table 3 and shaken, and subsequently were washed with 10-15 ml of demineralized water. The preparations with Na.sub.2S.sub.2O.sub.3 5 H.sub.2O were cloudy. The felts were dried in air at room temperature. The felts treated with Na.sub.2S.sub.2O.sub.3 5 H.sub.2O showed a honey yellow to brownish coloration; the felts treated with Na.sub.2S.sub.2O.sub.3 showed a light yellow coloration; and the felts treated with hydroquinone showed a light yellow to pale brownish coloration. The colorations of the felts showed no dependency upon a pretreatment with FeCl.sub.3 taking place or not taking place, or upon the duration of the conversion with hydroxylamine.

(45) TABLE-US-00003 TABLE 3 Reducing the Pt.sup.2+ to Pt.sup.0 with different reducing agents Pretreatment Reaction time of Reducing agent FeCl.sub.3 conversion with Felt sample 3.44 .Math. 10.sup.−1 mol/l solution hydroxylamine (h) 019 Na.sub.2S.sub.2O.sub.3 yes 1 020 Na.sub.2S.sub.2O.sub.3•5H.sub.2O yes 1 021 hydroquinone yes 1 022 Na.sub.2S.sub.2O.sub.3 no 1 023 Na.sub.2S.sub.2O.sub.3•5H.sub.2O no 1 024 hydroquinone no 1 025 Na.sub.2S.sub.2O.sub.3 yes 2 026 Na.sub.2S.sub.2O.sub.3•5H.sub.2O yes 2 027 hydroquinone yes 2 028 Na.sub.2S.sub.2O.sub.3 no 2 029 Na.sub.2S.sub.2O.sub.3•5H.sub.2O no 2 030 hydroquinone no 2 031 Na.sub.2S.sub.2O.sub.3 yes 3 032 Na.sub.2S.sub.2O.sub.3•5H.sub.2O yes 3 033 hydroquinone yes 3 034 Na.sub.2S.sub.2O.sub.3 no 3 035 Na.sub.2S.sub.2O.sub.3•5H.sub.2O no 3 036 hydroquinone no 3

(46) In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled.