CATALYST SYSTEM, ELECTRODE, AND FUEL CELL OR ELECTROLYZER

Abstract

A catalyst system comprises an electrically conductive carrier metal oxide and an electrically conductive, metal oxide catalyst material, wherein the carrier metal oxide and the catalyst material differ in their composition and wherein the catalyst material and the carrier metal oxide are each stabilized with fluorine. A near-surface pH value, designated pzzp value (pzzp=point of zero zeta potential), of the carrier metal oxide and of the catalyst material differ from one another, wherein the pzzp value of either the carrier metal oxide or the catalyst material is at most pH=5. The catalyst material and the carrier metal oxide form an at least two-phase disperse oxide composite. The catalysts system may be used in an electrode which may be used in a fuel cell or an electrolyzer.

Claims

1. A catalyst system comprising an electrically conductive carrier metal oxide having an electrical conductivity σ1 of at least 10 S/cm, wherein the carrier metal oxide has at least two first metallic elements selected from the group of non-precious metals and has a structure comprising oxide grains with a grain size of at least 30 nm, an electrically conductive, metal-oxide catalyst material having an electrical conductivity σ2 of at least 10 S/cm, wherein the catalyst material has at least one second metallic element from the group of non-precious metals, wherein the first metallic elements in the carrier metal oxide and the at least one second metallic element in the catalyst material are each present in a solid stoichiometric compound or solid homogeneous solution, wherein the carrier metal oxide and the catalyst material differ from one another in their composition and each are stabilized with fluorine, and wherein a near-surface pH value, designated point of zero zeta potential (pzzp) of the carrier metal oxide and of the catalyst material differ from one another, wherein the pzzp value of either the carrier metal oxide or the catalyst material is at most pH=5, and the catalyst material and the carrier metal oxide form an at least two-phase disperse oxide composite.

2. The catalyst system according to claim 1, wherein the first metallic elements are formed by at least two metals from the group consisting of tin, tantalum, niobium, titanium, hafnium and zirconium.

3. The catalyst system according to claim 2, wherein the first metallic elements are formed by tin and furthermore by at least one metal from the group consisting of tantalum, niobium, titanium, hafnium and zirconium.

4. The catalyst system according to claim 1, wherein the at least one second metallic element is formed by at least one metal from the group comprising tantalum, niobium, titanium, hafnium, zirconium, iron and tungsten.

5. The catalyst system according claim 1, wherein the catalyst material has a structure comprising oxide grains with a grain size in the range from 1 nm to 50 nm.

6. The catalyst system according to claim 1, wherein the carrier metal oxide has a first crystal lattice structure comprising first oxygen lattice sites and first metal lattice sites, wherein the carrier metal oxide on the first metal lattice sites is doped with at least one element from the group comprising titanium, zirconium, hafnium, vanadium, niobium, tantalum, aluminum, iron, tungsten, molybdenum, iridium, rhodium, ruthenium and platinum.

7. The catalyst system according to claim 1, wherein the carrier metal oxide has a first crystal lattice structure comprising first oxygen lattice sites and first metal lattice sites, wherein the carrier metal oxide on the first oxygen lattice sites is doped with at least one element from the group comprising nitrogen, carbon and boron.

8. The catalyst system according to claim 1, wherein the catalyst material has a second crystal lattice structure comprising second oxygen lattice sites and second metal lattice sites, wherein the catalyst material on the second metal lattice sites is doped with at least one element from the group comprising titanium, zirconium, hafnium, vanadium, niobium, tantalum, iron, tungsten, molybdenum, iridium, rhodium, ruthenium and platinum.

9. The catalyst system according to claim 1, wherein platinum is applied to a surface of the catalyst system in an amount of at most 0.1 mg/cm.sup.2.

10. An electrode comprising a catalyst system according to claim 1.

11. The electrode according to claim 10, which further comprises at least one ionomer and at least one binder.

12. The electrode according to claim 11, wherein the at least one binder comprises at least one fluorinated hydrocarbon or at least one polysaccharide.

13. The electrode according to claim 10, wherein the electrode has a coating thickness in the range of from 0.5 to 20 μm.

14. The electrode according to claim 10, wherein platinum is applied to a free surface of the electrode in an amount of at most 0.2 mg/cm.sup.2.

15. An oxygen-hydrogen fuel cell or electrolyzer, comprising at least one electrode according to claim 10 and at least one polymer electrolyte membrane.

16. The fuel cell or electrolyzer according to claim 15, wherein the polymer electrolyte membrane and the ionomer in the electrode are formed from identical materials.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIGS. 1 to 6 and Table 1 are intended to explain the catalysts system in an exemplary manner. In the figures:

[0032] FIG. 1 shows a bipolar plate having an electrode containing the catalyst system;

[0033] FIG. 2 schematically shows a fuel cell system comprising a plurality of fuel cells;

[0034] FIG. 3 shows a section through the arrangement according to FIG. 1;

[0035] FIG. 4 shows a section through two bipolar plates and a polymer electrolyte membrane according to FIG. 2 arranged there between;

[0036] FIG. 5 shows a phase diagram of Ta.sub.2O.sub.5—SnO.sub.2 above 1200° C.; and

[0037] FIG. 6 shows the calculated activities of Ta.sub.2O.sub.5 and SnO.sub.2 at 1500° C.

DETAILED DESCRIPTION

[0038] FIG. 1 shows an electrode 1 on a bipolar plate 2 which has a carrier plate 2a. The electrode 1 contains the catalyst system 9 (see FIG. 3) and forms a cathode. The electrode 1 has a coating thickness in the range of from 1 to 2 μm and, in addition to the catalyst system 9, also comprises an ionomer and a binding agent in the form of agar-agar. The bipolar plate 2 has an inflow area 3a with openings 4 and an outlet area 3b with further openings 4′ which are used to supply a fuel cell with process gases and to remove reaction products from the fuel cell. The bipolar plate 2 also has a gas distribution structure 5 on each side, which is provided for contact with a polymer electrolyte membrane 7 (see FIG. 2).

[0039] FIG. 2 schematically shows a fuel cell system 100 comprising a plurality of fuel cells 10. Each fuel cell 10 comprises a polymer electrolyte membrane 7 which is adjacent to both sides of bipolar plates 2, 2′. The same reference symbols as in FIG. 1 indicate identical elements.

[0040] FIG. 3 shows a section through the bipolar plate 2 according to FIG. 1. The same reference symbols as in FIG. 1 indicate identical elements. The carrier plate 2a, which is formed here from stainless steel, can be seen, which can be constructed in one part or in several parts. A gas diffusion coating 6 is arranged between the carrier plate 2a and the electrode 1 which contains the catalyst system 9. It can also be seen that a further anode-side coating 8 of the carrier plate 2a is provided. This is preferably a coating 8 which is designed according to DE102016202372 A1. A further gas diffusion coating 6′ is located between the coating 8 and the carrier plate 2a. The gas diffusion coatings 6, 6′ are designed to be electrically conductive, and in particular are made of a fiber mat made of carbon material.

[0041] FIG. 4 shows a section through two bipolar plates 2, 2′ and a polymer electrolyte membrane 7 according to FIG. 2 arranged therebetween, which together form a fuel cell 10. The same reference symbols as in FIGS. 1 and 3 indicate identical elements. It can be seen that the electrode 1 of the bipolar plate 2 as the cathode and the coating 8 of the bipolar plate 2′ as the anode are arranged adjacent to the polymer electrolyte membrane 7. The gas diffusion coatings 6, 6′ can also be seen.

[0042] In the following, a catalyst system 9 is presented using the example of the quasi-binary oxide phase diagram Ta.sub.2O.sub.5—SnO.sub.2.

[0043] FIG. 5 shows a calculated phase diagram for the catalyst system Ta.sub.2O.sub.5—SnO.sub.2 for temperatures above T=1200° C., which originates from the dissertation “The Impact of Metal Oxides on the Electrocatalytic Activity of Pt Catalysts” by A. Rabis, ETH Zurich 2015. The mutual solubilities at lower temperatures must be extrapolated and estimated. The phase diagram shows that tin oxide in tantalum oxide has an initial solubility of about 7 mol % at the temperature mentioned, while the initial solubility of tantalum oxide in tin oxide is 1.1 mol %. It can accordingly be assumed that the solubilities are lower at room temperature or the operating temperature of a fuel cell.

[0044] The activity profile of the two oxides at 1500° C. in the respective mixed phases is as shown in FIG. 6 (J. Am. Ceram. Soc., 95 [12], 4004-4007, (2012)). The stable thoreaulite phase SnTa.sub.2O.sub.7 is not included in this phase diagram according to FIG. 6. The tin is tetravalent in this compound. In the solid solution of tin oxide with tantalum oxide, the electrical conductivity of the tin oxide is drastically increased. With an addition of tantalum oxide up to a maximum α solubility of 1.1 mol % to tin oxide, electrical conductivities of 7×10.sup.2S/cm.sup.2 are achieved.

[0045] The increase in electrical conductivity increases steadily with the concentration of the solution up to the aforementioned phase boundary and then decreases again. When the solubility limit according to the phase diagram shown in FIG. 6 is exceeded, a two-phase region is formed from the SnO.sub.2—Ta.sub.2O.sub.5 phase and the thoreaulite SnTa.sub.2O.sub.7 in equilibrium. The composition of the heterogeneous structure can be calculated with given concentrations according to the lever law. If, for example, a total concentration of 10 mol % Ta.sub.2O.sub.5 in SnO.sub.2 is chosen, the result is a composition of the heterogeneous structure of 88% Sn.sub.0.99Ta.sub.0.01O.sub.2 and 2% SnTa.sub.2O.sub.7 as oxide composite.

[0046] The electrically highly conductive tin dioxide phase Sn.sub.0.99Ta.sub.0.01O.sub.2 forms the carrier metal oxide and the thoreaulite phase SnTa.sub.2O.sub.7 forms the catalyst material which is finely dispersed in the grain of the carrier metal oxide. The precipitation conditions are determined on the one hand by the grain size produced and on the other hand by the temperature-time diagram for setting the structure. By varying the composition, the proportions of the two phases of the oxide composite are changed.

[0047] However. the chemical activities of the first and second metallic elements in the oxides remain unchanged in the two-phase region, as do the respective basic electrical and chemical-physical properties. For catalysis, the triple phase boundary lengths) as well as the energetic surface states of the carrier metal oxide can be set via the quantity and size ratios. Since the two phases, i.e., the carrier metal oxide and the catalyst material, are present in crystallographic structures that differ from one another, they are inherently dissolved with one another, i.e., the catalyst material is present as inherently dissolved dispersoids in the carrier metal oxide.

[0048] With RDE investigations (RDE=ring disc electrode) it was found that both the tantalum-rich β phase and the thoreaulite phase SnTa.sub.2O.sub.7 have a comparatively good catalytic activity for oxygen reduction. This was verified with experiments in which the catalyst system was treated with a solution containing 2-[1-[difluoro[(trifluoroethenyl) oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2-tetrafluoroethanesulfonic acid as a polymer electrolyte material or ionomer, also known under the trade names Nation or Flemion, was applied to a carbon substrate (glassy carbon) to form an electrode. The onset voltages U were in the range of platinum. However, the specific currents i found were low: i<0.1 A/cm.sup.2 at U=0.65 V.

[0049] In the next step, the individual phases were eliminated from the two-substance mixture. As stated above, the carrier metal oxide used was SnO.sub.2 with about 1 mol % Ta.sub.2O.sub.5, wherein the mass fraction of this phase was in the range from 70 to 95% by weight.

[0050] Table 1 below shows the results of the catalyst systems. The results were determined by means of a single cell consisting of two end plates, two graphite plates, two bipolar plates 2, 2′ made of graphite, two gas diffusion coatings 6, 6′, the electrode 1 (cathode side), a standard Pt/C catalyst (anode side) and a polymer electrolyte membrane 7 made from Nation. The process gases, here air and hydrogen, were humidified differently on the cathode side and the anode side.

[0051] The electrode 1 had an electrode area of 30 mm×30 mm. The cell was operated at T=85° C. with p=2.5 bar. On the hydrogen side, λ=1.5 was set, and λ=2 on the air side. A reference humidification temperature TB was set at 80° C.

[0052] The prepared coating thicknesses of the electrode 1 were in the range of from 1 to 5 μm.

TABLE-US-00001 TABLE 1 Ratio of carrier (metal oxide)/ Grain size of the I (A/cm.sup.2) Onset catalyst Carrier (metal oxide)/ carrier metal & 0.7 V voltage material in wt. % catalyst material oxide in nm (T = 85° C.) in VNHE 80/20 C/Pt 2-4 1.1 0.95 50/7  SnO.sub.2—1%Ta.sub.2O.sub.5/ 100-125 0.65 1.05 Ta.sub.2O.sub.5-dot. 35/15 SnO.sub.2—1%Ta.sub.2O.sub.5/ 100-125 0.75 1.1 Ta.sub.2O.sub.5-dot. 35/15 SnO.sub.2—1%Ta.sub.2O.sub.5/ 100-125 1.4 1.1 Ta.sub.2O.sub.5-dop.-0.1%Pt 50/7  SnO.sub.2—2.5%Nb.sub.2O.sub.5/  80-100 0.5 1.15 Ta.sub.2O.sub.5-dot. 35/15 SnO.sub.2—2.5%Nb.sub.2O.sub.5/  80-100 0.8 1.2 Ta.sub.2O.sub.5-dop. 35/15 SnO.sub.2—2.5%Nb.sub.2O.sub.5/ 100-125 1.35 1.2 Ta.sub.2O.sub.5-dop.-0.1%Pt 30/10 SnO.sub.2—2.5%Nb.sub.2O.sub.5/  80-100 0.95 0.85 (Ti90Nb10)O.sub.2—0.1%Pt

[0053] According to Table 1, the current densities for catalyst systems are 5 to 8 times higher than in experiments in which individual oxidic phases were deposited on a carbon substrate. The results in Table 1 impressively show that it is possible to produce platinum-free and carbon-free electrodes with similarly good activities as in the conventional system of “platinum on carbon carriers”.

[0054] The triple phase boundary length (TPB), the nanodisperse precipitated electrocatalytically active thoreaulite phase (=catalyst material) as well as the size of the individual grains in the microstructure can be optimized via the precipitation conditions from the Sn—Ta—O system. In this way, the electrolytic activity of the catalyst system for oxygen reduction can also be optimized.

[0055] The conductivity of the tin oxide, in which the tantalum oxide is dissolved up to the maximum limit solubility (approx. 1.1 mol %), depends heavily on the sintering temperature. It is important to ensure that the oxygen partial pressure above the powder is always high enough that the fully oxidized compounds are established. Otherwise, post-oxidation during cell operation and loss of activity can be expected. It is currently unclear whether the thoreaulite phase or the tantalum-rich β phase actually occurs under the oxidative test conditions chosen. According to the test results, this is not decisive for the effectiveness of the catalyst system.

[0056] Furthermore, a sintering temperature must be set so high that later grain agglomeration is not to be expected and, on the other hand, the catalyst system is sufficiently stable even for use at lower temperatures. This risk would exist if the mutual solubilities in the α and β phases were to change significantly. This is why the temperature program was chosen in such a way that initially sintering was carried out at higher temperatures of up to T=900° C. and the grain was adapted as closely as possible to the conditions in cell operation in the cooling program. Accordingly, a holding phase at T=250° C. over a period of 60 minutes is preferably set in the cooling program.

[0057] Furthermore, a study was made of how further deposition of nanodisperse platinum particles affects the electrocatalytic effectiveness of the catalyst system. The platinum was deposited on the surface of the coating 6 by means of sputtering technology with an area coverage of <0.1 mg/cm′. The platinum cluster sizes were determined from different samples by means of TEM measurements and X-ray diffractometry.

[0058] When comparing the values determined by TEM measurement with those obtained by X-ray diffractometry, it has been shown that similar cluster sizes are obtained with both methods (TEM: 6-11 nm; XRD: 7 nm). Only statements about the tendencies in changes of cluster sizes should be made here. For this reason, the cluster sizes were determined by means of X-ray diffractometry, as this method is not only much easier to carry out, but also contains broader statistical information, since only a small section of the sample can be viewed with TEM measurements.

[0059] Overall, it can be stated that surprisingly high activities for oxygen reduction are found in the embodiments of the catalyst system both without platinum and with platinum. Using extremely loaded electrochemical investigations with CV measurements up to anodic potentials of 2000 mV NHE in sulfuric acid solution at pH=3 and T=85° C., it was also possible to demonstrate the high oxidation stability in 30-fold repeated cycles. It could even be shown that even up to 3000 mV NHE, especially in the phases rich in thoreaulite or β phase, the samples show very good resistance to passivation and dissolution.

[0060] Similar results were achieved with the same type of niobium-containing tin oxide composites. Niobium oxide has a slightly higher solubility in tin oxide than tantalum oxide. The limit solubility for niobium oxide is 2.5 at. %. With niobium oxide, stable stoichiometric phases SnNb.sub.20.sub.7 (“froodite”) similar to the thoreaulite phase are formed. The activities measured are lower than with the tantalum-based catalyst systems, which can be explained by, among other things, the different pzzp values. However, it should be noted at this point that the activities depend very heavily on the manufacturing conditions.

[0061] The use of the catalyst system for future fuel cells brings with it considerable advantages, both economically and in terms of long-term stability and increased catalytic activity.

[0062] Furthermore, catalyst systems based on titanium niobium oxide were investigated. To increase the electrical conductivity, these oxides were doped with iridium. Doping of 0.1 mol % in the catalyst system was sufficient to set electrical conductivities σ>5*10.sup.2 S/cm.sup.2.

[0063] The catalyst system based on Ti—Ta—O has also proven itself to be useful with the setting of the two-phase region on the tantalum oxide-rich β phase, which in the two-phase region is in equilibrium with the stoichiometric phase Ti.sub.3Ta.sub.2O.sub.11. Tantalum oxide has only a low solubility for titanium oxide in the β phase. In this phase, a pzzp value of pH=1 to 2 can be assumed, while the stoichiometric phase has a pzzp value above pH=4. In the context of the embodiments disclosed herein, a reverse setting was tested here, in which the active β phase functions as a carrier metal oxide and the stoichiometric phase is precipitated in nanodisperse form. In a further step, the surface of the coating 6—as described above—is covered with platinum metal islands.

[0064] The temperature treatment of the catalyst system has a great influence in several respects on the desired results with regard to the activity and electrical conductivity of the catalyst system. On the one hand, the density of the carrier metal oxide, for example the stoichiometric tin oxide, is set by means of the temperature treatment, taking into account the decomposition pressure of the compound at sintering temperatures above 950° C. On the other hand, the temperature treatment determines the precipitation conditions of the dispersoids, i.e., the catalyst material. For example, if the oxide is treated appropriately, pure Ta.sub.2O.sub.5 is precipitated at the grain boundaries of the tin oxide. It follows from this that the temperature treatment, as described above, must take place in such a way that the phases that are stable for fuel cell operation are established. For example, the SnO.sub.2—Ta.sub.2O.sub.5 carrier material is produced in such a way that the starting materials are intimately mixed in the desired ratio in a ball mill and tempered at a temperature in the range of 700-800° C. under oxygen for a period of t1=30 min. It is then cooled to a temperature of 250° C. and this temperature is maintained for a period of time t2=1 h. Finally, the catalyst system is cooled to room temperature.

LIST OF REFERENCE SYMBOLS

[0065] 1, 1′ Electrode (cathode side) [0066] 2, 2′ Bipolar plate [0067] 2a, 2a′ Carrier plate [0068] 3a Inflow area [0069] 3b Outlet area [0070] 4, 4′ Opening [0071] 5 Gas distribution structure [0072] 6, 6′ Gas diffusion coating [0073] 7 Polymer electrolyte membrane [0074] 8 Coating (anode side) [0075] 9 Catalyst system [0076] 10 Fuel cell [0077] 100 Fuel cell system