Catalyst material for a fuel cell or an electrolyser and associated production method

Abstract

The invention relates to a multi-component catalyst material for use in a fuel cell or electrolysis system, in particular in a regenerative fuel cell or reversible electrolyser. According to the invention, the catalyst material comprises a doped manganese oxide, a NiFe intercalation compound and a conductive carrier material, wherein the doped manganese oxide and the NiFe intercalation compound are supported on the carrier material.

Claims

1. A catalyst material for a fuel cell or an electrolyser, comprising a multi-component system including a manganese oxide doped with a dopant M, a NiFe intercalation compound, and a conductive carbon-containing carrier material on which the doped manganese oxide and the NiFe intercalation compound are directly or indirectly arranged.

2. The catalyst material according to claim 1, wherein the doped manganese oxide is a cryptomelane-type manganese dioxide according to the formula M:α-MnO.sub.2.

3. The catalyst material according to claim 1, wherein the dopant M is selected from the group of iron, nickel, copper, silver and/or cobalt.

4. The catalyst material according to claim 1, wherein the dopant M is contained in the manganese oxide in a proportion in the range from 0.1 to 3.5 wt %, in particular in the range from 0.2 to 3.35 wt %.

5. The catalyst material according to claim 1, wherein the NiFe intercalation compound is a NiFe LDH, which in particular has 1.5 to 5 times the proportion by weight of nickel in relation to iron.

6. The catalyst material according to claim 1, wherein the NiFe intercalation compound is a NiFe intercalation compound modified by anion exchange.

7. The catalyst material according to claim 1, wherein the conductive carbon-containing carrier material is selected from carbon black, graphene, nanostructured carbon, carbon nanotubes and/or a carbonised carrier particle.

8. The catalyst material according to claim 1, wherein the conductive carbon-containing carrier material is modified by oxygen, nitrogen and/or phosphorus.

9. The catalyst material according to claim 1, wherein the NiFe intercalation compound is grown directly on the conductive carbon-containing carrier material.

10. An electrode, in particular an oxygen electrode, for a fuel cell or an electrolyser, including a catalyst material according to claim 1.

11. A fuel cell, an electrolyser, a regenerative fuel cell or a reversible electrolyser including an ion exchange membrane which is coated at least on one side with a catalyst material according to claim 1.

12. A method for producing a catalyst material according to claim 1, comprising the following steps in the specified order: preparing a mixture of a doped manganese oxide, a NiFe intercalation compound and a conductive carbon-containing carrier material by means of blending, preparing a catalyst dispersion from the mixture by adding water, an organic solvent and optionally a binder, and applying the catalyst dispersion to a substrate.

13. The method according to claim 12, wherein there are 70 to 80 parts by weight of water, 15 to 25 parts by weight of the organic solvent and 2 to 5 parts by weight of the binder in relation to the sum of the components water, organic solvent and binder, or there are 0.5 to 5 parts by weight of water and 95 to 98.5 parts by weight of the organic solvent in relation to the sum of the components water and organic solvent, and the solids and the binder are present in a ratio from 5:3 to 7:3 in the mixture of water and the organic solvent.

14. The method according to claim 12, wherein the binder is a perfluorinated sulfonic acid polymer and/or a quaternary ammonium compound, in particular a doped polysulfone.

15. The method according to claim 12, wherein the substrate is a gas diffusion layer or a membrane, in particular an ion exchange membrane.

Description

(1) The invention is explained below in exemplary embodiments with reference to the associated drawings. Therein:

(2) FIG. 1 shows (a) XRD diffractograms of alpha-manganese (IV) oxide doped with cobalt, nickel, copper or silver and of non-doped alpha-manganese (IV) oxide, and (b) XRD diffractograms of a carbon carrier material, of NiFe LDH, of Cu:α-MnO.sub.2 and of a catalyst material according to the invention, comprising all of said components;

(3) FIG. 2 shows a graphical comparison of electrochemical RDE measurements of alpha-manganese (IV) oxide doped with cobalt, nickel, copper or silver;

(4) FIG. 3 shows a graphical representation of cyclic voltammograms of a catalyst material Cu:α-MnO.sub.2/carbon/NiFe LDH according to the invention compared with the individual components Cu:α-MnO.sub.2/carbon and NiFe LDH and a precious metal-based reference catalyst Pt/C-Ir/C, a) being ORR activity and b) being OER activity;

(5) FIG. 4 shows a graphical representation of cyclic voltammograms of the materials from FIG. 3 after electrochemical loading, a) being ORR activity, b) being OER activity;

(6) FIG. 5 shows a graphical representation of a galvanostatic stability test of Cu:α-MnO.sub.2/carbon/NiFe LDH on a rotating disk electrode;

(7) FIG. 6 shows a schematic sectional view of a unitised regenerative fuel cell (URFC) in a preferred embodiment of the invention;

(8) FIG. 7 shows a) ORR activities and b) OER activities of a modified catalyst material Cu:α-MnO.sub.2/mod. carbon/NiFe LDH (Cl.sup.−/ClO.sub.4.sup.−) according to the invention compared with the unmodified material Cu:α-MnO.sub.2/carbon/NiFe LDH;

(9) FIG. 8 shows a comparison of the ORR and OER activities and the total activity of the modified catalyst material Cu:α-MnO.sub.2/mod. carbon/NiFe LDH (Cl.sup.−/ClO.sub.4.sup.−) and the unmodified material Cu:α-MnO.sub.2/carbon/NiFe LDH compared with OER/OPR catalysts of the prior art; and

(10) FIG. 9 shows long-term stability of the a) modified catalyst material Cu:α-MnO.sub.2/mod. carbon/NiFe LDH (Cl.sup.−/ClO.sub.4.sup.−) b) compared with the unmodified material Cu:α-MnO.sub.2/carbon/NiFe LDH.

(11) FIG. 1 shows a) XRD diffractograms of undoped cryptomelane manganese oxide (α-MnO.sub.2) and of doped manganese oxide of the cryptomelane type (M:α-MnO.sub.2, M=Cu, Ni, Ag, Co). The crystal structure of the resulting manganese oxide is not affected by the metal doping. It can thus be concluded that the amount of dopants can be dramatically increased by the synthesis used. Part b) of FIG. 1 shows XRD diffractograms of the individual components of the three-component system and of a complete three-component system according to the invention.

(12) FIG. 2 shows results of electrochemical RDE measurements in oxygen-saturated 0.1-M KOH at 5 mV s.sup.−1, 1600 rpm and a catalyst load of 203.72 μg cm.sup.−2. The figure shows the ORR activities of various doped manganese oxides of the cryptomelane type (M:α-MnO.sub.2, M=Cu, Ni, Ag, Co). As shown in FIG. 2, Cu:α-MnO.sub.2 has the most positive potential at −3 mA cm.sup.−2 and the highest diffusion limit currents. Therefore, Cu:α-MnO.sub.2 can be regarded as the most active material among the materials investigated here and is used for further measurement of the catalyst according to the invention.

(13) FIG. 3 shows two diagrams which represent a) ORR activities and b) OER activities of NiFe-LDH, Pt/C-Ir/C, Cu:α-MnO.sub.2/carbon and physically mixed Cu:α-MnO.sub.2/carbon/NiFe LDH. It can be seen from the graphs that the transition metal oxide-based system according to the invention surpasses the commercial catalyst used as a reference with respect to ORR activity and OER activity. The ORR activity of the manganese oxide is only slightly influenced by the addition of NiFe LDH, while the OER activity is dramatically increased.

(14) FIG. 4 shows a) ORR and b) OER activities of Cu:α-MnO.sub.2/carbon, commercial Pt/C-Ir/C and Cu:α-MnO.sub.2/carbon/NiFe LDH after electrochemical loading. The stability of the transition metal catalyst surpasses the stability of the commercial precious metal reference catalyst. In addition, a simple comparison shows that NiFe LDH considerably improves the stability of the three-component catalyst according to the invention. While the activity of NiFe LDH-free Cu:α-MnO.sub.2/carbon decreases dramatically, the activity of the NiFe LDH-containing system is less strongly influenced. Therefore, a shielding effect of NiFe LDH is assumed.

(15) FIG. 5 shows a graphical representation of galvanostatic stability measurements on a rotating disk electrode in an oxygen-saturated 0.1-M KOH solution of the Cu:α-MnO.sub.2/carbon/NiFe LDH, a commercial reference catalyst and a transition metal-based comparison system (data of NiFe LDH/PANI from reference). The current was kept constant at −3 mA cm.sup.−2 for the oxygen reduction reaction and constant at 4 mA cm.sup.−2 for the oxygen evolution reaction. In contrast to the transition metal-based comparison system and the precious metal-based reference catalyst, the total activity remains stable over more than 18 alternating OER and ORR potential cycles.

(16) A schematic sectional view of an individual cell of a PEM fuel cell, labelled as a whole by 100, is shown in FIG. 6 to explain the structure and its operating principle.

(17) The fuel cell 100 comprises, as a core component, a membrane electrode assembly 6, which in the present case preferably has an ion exchange membrane 1, and in each case one electrode 2, specifically an anode and a cathode, adjoining one of the two flat sides of the membrane 1. The ion exchange membrane 1 can be a polymer electrolyte membrane, preferably an anion exchange membrane, for example Tokuyama A201, Selemion, Fumasep, which (selectively) permits ion diffusion from the anode chamber into the cathode chamber or vice versa. The electrodes 2 comprise a catalytic material that can be supported on an electrically conductive material, for example a carbon-based material.

(18) The electrodes 2 are each adjoined by a gas diffusion layer 3, the task of which is substantially to uniformly distribute the supplied operating gases over the main surfaces of the electrodes 2, or of the membrane 1.

(19) A bipolar plate 5 is arranged on the outer sides of each gas diffusion layer 3. Bipolar plates have the task of electrically interconnecting the individual membrane electrode assemblies 6 of the individual cells in the stack, cooling the fuel cell stack and supplying the operating gases to the electrodes 2. For the latter purpose, the bipolar plates 5 (also called flow field plates) have flow fields 4. The flow fields 4 comprise, for example, a plurality of flow channels which are arranged parallel to one another and which are incorporated in the form of grooves or slots in the plates 5. Usually, each bipolar plate has, on its one side, an anode flow field facing the anode 2 and, on its other side, a cathode flow field facing the cathode. In the present case, only one flow field 4 is depicted for each of the two bipolar plates 5 shown.

(20) During operation as a fuel cell, a fuel, in particular hydrogen (H.sub.2), is supplied to the anode flow field, while an operating medium, in particular air, containing oxygen (O2) is supplied to the cathode flow field.

(21) During operation as a reversible fuel cell, the function of the flow fields changes; the anode field is changed to the cathode field and vice versa. Therefore, it is more expedient to refer to the electrodes as the oxygen electrode and the hydrogen electrode. In this case, the reactions involving oxygen, i.e. the oxygen reduction and the oxygen evolution reaction, take place at the oxygen electrode. Conversely, the hydrogen evolution reaction and the hydrogen oxidation reaction take place at the hydrogen electrode.

(22) Since the reactions taking place at the hydrogen electrode are less complex and easier to catalyse, the catalyst material according to the invention is preferably arranged on the oxygen electrode.

(23) The reactions taking place at the electrodes are not shown in FIG. 1. The electrodes catalysed at the surface of the respective electrodes release ions, which then migrate through the membrane 1. In the case of a fuel cell reaction, this means that a catalytic reduction of oxygen O.sub.2 into hydroxide ions OH.sup.− takes place at the oxygen electrode by receiving electrons. The hydroxide ions migrate through the anion-conducting membrane 1 and reach the hydrogen electrode. The hydrogen H.sub.2 supplied reacts with the hydroxide ions OH.sup.− to form water H.sub.2O, with the hydrogen being oxidised and thus donating electrons. The electrons of the hydrogen electrode reaction are supplied to the oxygen electrodes via an external circuit (not shown here). It can be seen from FIG. 1 that the product water of the fuel cell 100 arises on the oxygen electrode side of the membrane 5.

(24) The catalytically coated substrate, which acts as an electrode, comprises a substrate and a catalyst layer applied thereto according to the present invention. The catalyst layer can function as an oxygen electrode in a fuel cell. The substrate may be a membrane or a gas diffusion layer. In the embodiment shown here, the substrate is a membrane. The membrane can in principle be any ion exchange membrane used in fuel cell technology. These include, for example, polymer electrolyte membranes which have an electrolytic conductivity based on their wetting with water (e.g. Nafion®), or which owe their conductivity to an acid bonded to the polymer material or, in the case of anion exchange membrane, to its quaternary amine, for example polysulfone doped with trimethylammonium, so that quaternary benzyltrimethylammonium groups are produced.

(25) The catalyst layer is composed at least of a catalyst material which is arranged on a carrier material. It may further comprise a solvent, an electrolyte and/or an electron conductor.

(26) The carrier material functions as an electron conductor. In general, the electron conductor is in the form of electrically conductive carbon particles. All carbon materials which are known in the field of fuel or electrolysis cells and have a high degree of electrical conductivity and a large surface area can be used as electrically conductive carbon particles. The surface is, for example, 50 to 200 m.sup.2/g. Preferably, carbon blacks, graphite or activated carbons are used. Carbon blacks of high conductivity, so-called conductive carbon blacks, are very particularly preferred. In addition, carbon can also be used in other modifications, for example in granular form or as so-called nanotubes.

(27) The preferred embodiment of the catalyst material according to the invention provides a novel catalyst system which can be used as a bifunctional oxygen electrode in unitised regenerative fuel cells (URFC) and metal-air batteries. The catalyst system comprises a physical mixture of a separately synthesised NiFe layer double hydroxide (NiFe LDH), M-doped alpha-MnO.sub.2 (M:α-MnO.sub.2) and an additional carbon carrier.

(28) In particular, M:α-MnO.sub.2 (M=nickel, copper, silver or cobalt) is the component that acts catalytically for ORR. The proportion (weight basis) of the dopant in M:α-MnO2 is preferably 0.2 wt %, up to 2.2 wt %. α-MnO.sub.2 crystallises in the cryptomelane structure, which is not affected by the dopants, as shown in FIG. 1. The carrier material used is preferably commercially available carbon black, for example Vulcan XC-72R@.

(29) The component which is active for oxidising water is an intercalation compound, preferably NiFe LDH. The molar ratio between nickel and iron is between 5:1 to 3:1. In order to obtain the electrocatalyst which is capable of catalysing said reactions, the various components are physically mixed using sonication. To this end, a suspension is produced which contains both electrocatalysts, the carrier material, high-purity water (obtained, for example, by means of Mili-Q), isopropanol and the binder. In the case of slow application to a substrate, the ratio (weight basis) between Mili-Q, isopropanol and the binder is preferably 76.8 wt %: 20.0 wt %: 3.2 wt %. The total concentration of the solids may be in the range of 3.5 mg L.sup.−1 to 5 mg L.sup.−1. In the case of rapid application, water and isopropanol are preferably present at 2 wt % and 98 wt % respectively. Solids and binders are then preferably present in the solvent mixture in a ratio of 7:3. The total mass of the solid can be in a range from 15000 mg L.sup.−1 to 22000 mg L.sup.−1. The binder used is a perfluorinated sulfonic acid polymer such as Nafion® or polysulfone containing quaternary benzyltrimethylammonium groups.

(30) α-MnO.sub.2 alone is not a sufficiently active and stable catalyst for the ORR. The application of α-MnO.sub.2 to carbon, in particular Vulcan XC-72R and the doping of α-MnO.sub.2, in particular with copper, provide a material which is a competitive ORR catalyst. NiFe LDH is a very active and stable OER catalyst. Other materials based on transition metals capable of catalysing OER with the same activity as NiFe LDH are nickel- or vanadium-doped cobalt ferrites (Co.sub.x(Ni/V).sub.3Fe.sub.2O.sub.4). The present electrocatalyst provides several advantages over other bifunctional systems. It is a very cost-efficient system, compared to electrocatalysts based on precious metals such as Pt/C-Ir/C, which achieve similar OER and ORR activities. Compared to other ORR catalysts based on transition metals and carbon materials, the synthesis of the ORR-active component is simple, fast, energy-efficient and easily scalable. The greatest advantage of the three-component system presented here is the extraordinary stability. Whereas the activity of systems such as NiFe LDH/PANI or Pt/C-Ir/C decreases significantly during sustained electrochemical loading, the activity of the catalyst material according to the invention remains intact despite the carbon component (FIG. 4).

(31) The mixture of M:α-MnO.sub.2, NiFe LDH and carrier material in the preferred composition provides an inexpensive and competitive electrocatalyst which is simultaneously active for OER and ORR. While the electrochemical splitting of water is catalysed by NiFe LDH, the oxygen reduction is catalysed by the carbon-supported M:α-MnO.sub.2. The above-mentioned components are respectively active only for OER or ORR, but not for both reactions.

(32) In the following, examples are provided to illustrate the catalyst material according to the invention.

EXAMPLE 1: PRODUCTION OF M:α-MNO.SUB.2.: M=CO, CU, AG, NI, COCU

(33) α-MnO.sub.2 was synthesised using a top-down approach described by Ding et. al. (Y. Ding, X. Shen, S. Gomez, R. Kumar, V. M. B. Crisostomo, S. L. Suib and M. Aindow, Chem. Mater., 2005, 17, 5382-5389). To determine the optimum dopant for α-MnO.sub.2, small amounts of cobalt, copper, silver, nickel and a mixture of cobalt and copper were added to the above synthesis and then tested with respect to ORR activity. The proportion of the dopant obtained was between 0.2 wt % (silver) and 2.25 wt % (copper plus cobalt). The crystal structure of the catalyst is not affected by the various dopants (FIG. 1a).

EXAMPLE 2: PRODUCTION OF NIFE LDH

(34) The second component, NiFe LDH, was produced using a microwave-assisted synthesis, a mixture of Fe(NO).sub.3×9 H.sub.2O and Ni(OAc).sub.2×4 H.sub.2O being dissolved in DMF/H.sub.2O and converted according to a procedure by Dresp et. al (S. Dresp, F. Luo, R. Schmack, S. Kühl, M. Gliech and P. Strasser, Energy Environ. Sc., 2016, 9, 2020-2024). The XRD diffractogram is shown in FIG. 1b.

EXAMPLE 3: ORR ACTIVITY OF M:α-MNO.SUB.2.: M=CO, CU, AG, NI

(35) To determine the ORR and OER activities of the undoped and doped manganese oxides from Example 1, a three-electrode RDE setup was used with a platinum gauze as a counter-electrode, a reversible hydrogen electrode (RHE) as a reference electrode and a glass-like carbon disk coated with the catalyst as a working electrode. As shown in FIG. 2, the copper-doped manganese oxide (Cu:α-MnO.sub.2) is the most active catalyst material with respect to ORR in this series.

EXAMPLE 4: ORR AND OER ACTIVITY OF CU:α-MNO.SUB.2./CARBON/NIFE LDH

(36) A three-component catalyst was produced using the Cu:α-MnO.sub.2 of example 1 and the NiFe LDH of Example 2 on an unmodified carbon carrier material (Vulcan XC-72R) in a ratio of 1:1:1 to 3:1:3. To determine the ORR and OER activities of the catalyst, the three-electrode RDE setup was used as in Example 3. For comparison, the corresponding activities of the individual components Cu:α-MnO.sub.2 and NiFe LDH and of a commercial Pt/C-Ir/C catalyst from the prior art were measured. FIGS. 3a and 3b show the electrochemical activities with respect to ORR and OER respectively. The potential difference between −3 mA cm.sup.−2 (ORR) and 10 mA cm.sup.−2 results in a total excess potential of 0.7086 V±0.0011 V. FIG. 4 shows the ORR and OER activities of these materials after electrochemical loading.

EXAMPLE 5: LONG-TERM STABILITY OF CU:α-MNO.SUB.2./CARBON/NIFE LDH

(37) The most important achievement of the catalyst material according to the invention is its long-term stability, which was likewise evaluated in a three-electrode RDE measurement setup for the catalyst according to Example 4. The galvanostatic measurement was carried out at a constant current density of 4 mA cm.sup.−2 for OER and −3 mA cm.sup.−2 for ORR. Current density relevant between ORR and ORR was changed hourly. The results of the stability testing are shown in FIG. 5.

EXAMPLE 6: PRODUCTION OF CHLORIDE- AND PERCHLORATE-MODIFIED NIFE LDH-(CL.SUP.−./CLO.SUB.4..SUP.−.)

(38) Chloride- and perchlorate-modified NiFe LDH was produced as described in Song et al. (Song, F., et al., Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis. Nature Communications 2014, 5 (1)).

(39) For the chloride exchange, 100 mg of the NiFe intercalation compound were dispersed in 100 ml of a saturated NaCl solution, which is mixed with 100 ml of a 0.01-M hydrochloric acid solution (HCl). The resulting reaction solution was stirred at 600 rpm for 24 hours at room temperature (RT). The product (chloride-NiFe intercalation compound; NiFe LDH-Cl.sup.−) was separated from the dispersion by centrifugation, washed with water and finally freeze-dried.

(40) The perchlorate exchange was carried out with the NiFe intercalation compound resulting from the chloride exchange. For this purpose, 100 mg of the starting material were dispersed in 100 ml of a saturated NaClO4 solution and 100 ml of a 0.01-M HCl solution were added. The reaction solution was stirred at 600 rpm for 24 hours at RT. The product (perchlorate-NiFe intercalation compound; NiFe LDH-ClO.sub.4.sup.−) was separated from the dispersion by centrifugation, washed with water and finally freeze-dried.

EXAMPLE 7: PRODUCTION OF N- AND N/P-MODIFIED CARBON CARRIER MATERIALS

(41) The modifications of the carbon-containing carrier materials were carried out on the basis of the above-mentioned procedures from the literature using Vulcan XC-72R and multi-walled carbon nanotubes (MWCNTs).

(42) The corresponding carbon-containing carrier material (MWCNT) was washed in a first step by means of concentrated hydrochloric acid. The second step was an oxygen modification of the materials. For this purpose, 100 ml of nitric acid (HNO.sub.3) were used per gram of carbon-containing carrier material. The resulting reaction solution was refluxed for 2 to 6 hours at 90° C. with stirring. The product was separated from the reaction solution by centrifugation, washed with ultrapure water until the supernatant had a neutral pH and finally dried by lyophilisation.

(43) The oxygen-modified carbon-containing carrier materials (OC) were subsequently modified by nitrogen and/or phosphorus. In order to carry out pure nitrogen modification, the OC were reacted with NH.sub.3 in an ammonolysis reaction at 400 to 800° C. for 2 to 5 hours. Nitrogen and phosphorus were incorporated by thermal treatment of the OC at 500 to 800° C. for 2 to 5 hours in the presence of a nitrogen- and phosphorus-containing precursor.

EXAMPLE 8: PRODUCTION OF NIFE LDH

(44) NiFe LDH, which is applied during synthesis to the carbon-containing carrier materials described in Example 7, was produced as described in Dresp et al. (S. Dresp, F. Luo, R. Schmack, S. Kühl, M. Gliech and P. Strasser, Energy Environ. c., 2016, 9, 2020-2024). For this purpose, a carbon-containing carrier material or the carbon-containing carrier material from Example 7 was dispersed in DMF, and then the metal precursors from example 2, Fe(NO).sub.3×9 H.sub.2O und Ni(OAc).sub.2×4 H.sub.2O, were added. The subsequent synthesis was carried out as described in Example 2. The resulting catalyst material can be further modified by the chloride and perchlorate anion exchange specified in Example 6.

EXAMPLE 9: ORR ACTIVITY AND OER ACTIVITY OF CU:α-MNO.SUB.2./MOD. CARBON/NIFE LDH-(CL.SUP.−./CLO.SUB.4..SUP.−.)

(45) Using a modified NiFe intercalation compound from Example 6 or Example 8, the oxygen, nitrogen, nitrogen- or phosphorus-modified carbon carrier material from Example 7 and the Cu:α-MnO.sub.2 from example 3, a modified three-component catalyst Cu:α-MnO.sub.2/mod. carbon/NiFe LDH-(Cl.sup.−/ClO.sub.4.sup.−) according to the invention was produced and, as described in Example 4, the ORR and OER activities were measured. FIGS. 7a and 7b show the electrochemical activities of the modified catalyst (modified material) compared with the unmodified catalyst Cu:α-MnO.sub.2/carbon/NiFe LDH (primary material) with respect to ORR or OER. It can be seen that the electrocatalytic stability of the catalyst material is further improved by the modifications.

(46) FIG. 8 shows the electrocatalytic ORR and OER activities (black and white bars) as well as the total activity (hatched bars) of the modified catalyst Cu:α-MnO.sub.2/mod. carbon/NiFe LDH-(Cl.sup.−/ClO.sub.4.sup.−) (modified material) and of the unmodified catalyst Cu:α-MnO.sub.2/carbon/NiFe LDH (primary material) compared with other bifunctional ORR/OER catalyst systems known from the literature. The characterisation was carried out in each case on the basis of the excess potential for providing −3 mA cm.sup.−2 (ORR) and 10 mA cm.sup.−2 (OER).

EXAMPLE 10: ORR ACTIVITY AND OER ACTIVITY OF CU:α-MNO.SUB.2./MOD. CARBON/NIFE LDH-(CL.SUP.−./CLO.SUB.4..SUP.−.)

(47) The electrocatalytic long-term stability of the modified catalyst Cu:α-MnO.sub.2/mod. carbon/NiFe LDH-(Cl.sup.−/ClO.sub.4.sup.−) from Example 9 was measured as described in Example 5. The result is shown in FIG. 9a, and FIG. 9b illustrates the result of the same measurement compared with the unmodified catalyst.

LIST OF REFERENCE NUMERALS

(48) 100 Combined fuel cell 1 Membrane 2 Electrode 3 Gas diffusion layer—GDL 4 Flow field 5 Bipolar plate—BPP 6 Membrane electrode assembly—MEA 10 Operation flow direction of fuel cell 11 Operation flow direction of reversible fuel cell