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  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M8/12—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte
  • H01M8/124—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte characterised by the process of manufacturing or by the material of the electrolyte
  • H01M8/1246—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte characterised by the process of manufacturing or by the material of the electrolyte the electrolyte consisting of oxides
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  • C01B2203/0465—Composition of the impurity
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  • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
  • C01B2203/0465—Composition of the impurity
  • C01B2203/0495—Composition of the impurity the impurity being water
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Definitions

• the invention relates to a C0 2 tolerant, mixed conductive oxide, which is particularly suitable for use as a membrane for hydrogen separation or as a fuel cell electrode even at high temperatures.
• MPEC materials are known from many fields of materials engineering and are used, for example, in the manufacture of dense ceramic membranes.
• Such membranes can be used, for example, as a solid electrolyte in a Proton Conductive Solid Oxide Fuel Cell (PC-SOFC), as an electrode component in a proton-conducting solid oxide fuel cell irrespective of electrolyte type, in systems for selective gas separation, in particular hydrogen separation, or also in petrochemistry.
• PC-SOFC Proton Conductive Solid Oxide Fuel Cell
• the Pre-Combustion concept involves several stages of the process, in particular the partial oxidation of a fossil fuel (methane, natural gas or coal) to syngas (CO + H 2 ), the water gas shift reaction (WGS), which minimizes the amount of CO in a syngas and at the same time the H 2 content is increased, and the CO 2 / H 2 separation by an MPEC membrane.
• a fossil fuel methane, natural gas or coal
• WGS water gas shift reaction
• the dissociative adsorption of the water on the membrane comprises the following substeps: a) water molecules are split into hydroxyl groups (OH “ ) and protons (H + ), b) the hydroxyl groups (OH " ) absorb at oxygen ion vacancies (V "), which form a Since no free electrons are generated during this process, the membrane acting mainly as a proton conductor can advantageously be operated under moist conditions, which makes such a membrane suitable for use as a dense electrolyte, for example PC SOFC attractive.
• Ii 2 - »2H '+ 2e' (equation 2) generates free electrons in addition to the generation of protons. These protons are then incorporated into the oxide lattice by protonating the lattice oxide ions.
• the protons in the structure are also referred to as interstitial defects (H '), although their positions are not on proper interstitials, but in the electron cloud of the "guest" oxide.
• H ' interstitial defects
• both the protons and the electrons are charge carriers, thus contributing to the overall conductivity of the material.
• the total conductivity ⁇ ⁇ consists of the contributions of the proton conductivity ⁇ - ⁇ and the electronic conductivity a e together with:
• the total conductivity of the material as well as the ionic conductivity can be measured directly.
• the electronic conductivity can thus be calculated by subtracting the ionic conductivity from the total conductivity.
• transport number t This transport number can be determined for each carrier.
• the conductivity of protons is proportional to their charge z, their concentration c, their mobility ⁇ and their diffusion coefficient D:
• Equation 6 The permeation of hydrogen through a dense ceramic membrane is proportional to the "ambivalent" conductivity, whereby instead of the ionic conductivity ⁇ , the proton conductivity is now set, so that the flux density of the protons through a membrane can be represented as follows: Equation 6) Due to the partial pressure gradient across the MPEC membrane, the protons produced by the hydrogen oxidation reaction move through the membrane in the same direction as the protons. They combine at the side of the membrane, which has the lower partial pressure of hydrogen, to hydrogen, H 2 .
• the protons within the structures of the membrane move mainly by jumps between the stationary "guest” oxide ions (Grotthuss mechanism) .
• the best-investigated MPEC materials with a perovskite structure suitable for high-temperature use are also SrCe0 3 , BaCe0 3 , SrZr0 3 .
• Zerates and zirconates are currently state of the art as MPEC materials and show the highest proton conductivity in oxide systems.
• a well-studied class of mixed proton and electron conductors include the partially substituted perovskites, such as CaZr0 3 , SrCe0 3, and BaCe0 3 , in which the substitution of cerium or zirconium with trivalent cations causes oxygen vacancies and other charge defects, resulting in mixed conductivity Gas mixtures lead, which have oxygen, hydrogen and water vapor.
• such a material should also have high catalytic activity for the oxidation and evolution of hydrogen at the solid / gaseous interface.
• a material should also have high catalytic activity for the oxidation and evolution of hydrogen at the solid / gaseous interface.
• dense BaCeO, 8YO 2 0 3 ceramics to a high hydrogen permeation rate.
• High-temperature hydrogen separation membranes make it possible to implement pre-combustion strategies in power plants so that CO2 and hydrogen can be separated after the shift reaction, creating an exhaust flow of moist C0 2 that can be easily liquefied and stored.
• High-temperature membranes for the separation of hydrogen are usually based on two types of membranes, on the one hand the hydrogen-permeable metals, such as palladium alloys or Nb / Ta / V, or on the other hand, the mixed protons and electron-conducting oxides, which at the high Temperatures in a hydrogen atmosphere Sphere are stable.
• Typical operating conditions include temperatures between 400 and 900 ° C, pressures between 2 and 50 bar and a surrounding area, the very high levels of water and C0 2 as well as low concentrations of H 2 S (typically 5 to 200 ppm) can have.
• the aim of the material development was to generate defect centers for high oxygen ion and electron conductance by substitution of the A and / or B position in the Perowski structure ABO3 with cations of lower valence.
• Shimura [1] also investigated two materials with a smaller ionic radius doped on the A-site than La 3+ , namely (Lao, 93 ro, o6) 5.8WOi i, 9 and (La 0; 9Nd 0j yWO j.
• the object of the invention is to provide a material for a high-temperature membrane for the separation of hydrogen, which overcomes at least some of the disadvantages of the prior art. Furthermore, it is the object of the invention to provide a production method for such a material.
• MPEC mixed conductive
• An MPEC material has intrinsically ion-conducting, in particular oxygen ions or proton-conducting properties as well as electron-conducting properties, which typically move in the same order of magnitude or at most differ by about one order of magnitude.
• an oxidic material with improved mixed conductivity, improved chemical stability and improved sintering properties has been found, which can be used in particular as material for a hydrogen-separating membrane at higher temperatures.
• the lanthanide metal on the A-site is partially replaced by at least one other metal with a similar ionic radius and an oxidation state between +2 and +4.
• substitution elements besides the metals from the same period (La to Lu), Ca, Mg, Sr, Ba, Th, In or Pb are also particularly suitable.
• the tungsten metal cation on B-site is partially replaced by at least one other metal element with a similar ionic radius and an oxidation state between +4 and +6.
• the mixed conducting material has the following composition (in the anhydrous state)
• Ln element of the group (La, Pr, Nd and Sm)
• A at least one element from the group (La, Ce, Pr, Nd, Eu, Tb, Er, Yb, Ca, Mg, Sr, Ba, Th, In, Pb),
• B at least one element from the group (Mo, Re, U, Cr, Mn, Nb),
• Substitution at the A and / or B site may result in a stoichiometric deviation ⁇ of up to 0.3 for the oxygen. If there is more than one cation on the A or B site, the indices x and y are each the sum of corresponding substitution elements.
• All the materials according to the invention have a single-phase structure based on a fluorite structure.
• the production of the inventive materials can be carried out via different ways.
• 0 ⁇ x ⁇ 0.7 is allowed in the material according to the invention, which has proved to be advantageous in the range 0 ⁇ x ⁇ 0.5.
• a specific embodiment of the mixed proton and electron-conducting material with an A-site substitution has the general composition (Ln 5 A) 6 W z O] 2-5 , in which x - 1 is chosen.
• Ln lanthanum or neodymium.
• the invention is not limited to this.
• the surplus in the production should only ensure the single-phase nature of the material.
• the improvement of the proton conductivity in the case of the inventive materials is achieved in particular by the fact that, compared to Ln 6 W0 12 by the aforementioned substitutions on the A and / or B place a change in the structure, the ion arrangement or the number, or The arrangement of the oxygen vacancies is achieved.
• These changes usually affect the concentration and the stability as well as the mobility of the protons in the hydrogenated oxidic material under normal operating conditions.
• the improvement in the electronic conductivity is achieved on the one hand by the altered structure, but in particular by the fact that the substitution cations with different oxidation states are incorporated. Although these cations basically fit well into the given structure and can under controlled conditions partially change the oxidation state, but not until complete reduction to the metal. Despite this change in the oxidation state of the substituted cations, this does not lead to large structural changes within the oxidic material, ie no large symmetry or structural changes or high chemical expansion. By chemical expansion is meant here the effect that the change in the oxidation state of the various metallic cations can lead to an increased ionic radius which widens the crystal lattice. This effect is usually caused by a change in temperature or changes in the surrounding atmosphere.
• the oxidation or reduction of the cation underlying the change in the oxidation state must be reversible.
• the change in the oxidation state advantageously leads to a reduction in the band gap, ie the energy gap between valence band and conduction band of the material, and thus also to an increase in the electronic conductivity.
• the abovementioned material according to the invention has particular advantages when used as a crystalline and gas-tight, hydrogen-permeable membrane for the separation of hydrogen at relatively high temperatures, in particular in a power plant, or as an electrolyte in an SOFC fuel cell.
• FIG. 1 schematically illustrates the process of hydrogen separation in a water gas shift reactor.
• the CO content in a synthesis gas is minimized and at the same time the H 2 content is increased.
• molecular hydrogen is adsorbed on the hydrogen-rich side of the membrane in a dissociative manner and enters the oxide material of the MPEC membrane as a proton with electron donation.
• the protons combine again to form molecular hydrogen and are released into the gas phase.
• the manufacturing method used here is based on a modified citrate complex formation, to obtain stable tungsten and lanthanum-containing ions in the solution.
• the lanthanum oxides eg Nd 2 0 3 , purity 99.9%
• the lanthanum oxides are dissolved in concentrated, hot nitric acid (65% by volume.)
• the resulting nitrate is with Citric acid at a molar ratio of 1: 2 (cation charge to citric acid) complexed.
• Another solution is prepared for the B-site ions (purity> 99 5) using ammonium tungstate, ammonium heptamolybdate or uranyl nitrate and which are also complexed with citric acid (Fluka 99.5%) at the same molar ratio.
• the metal complexation is enhanced in both cases by a heat treatment at 120 ° C for 1 hour.
• Both solutions are then neutralized by the controlled addition of ammonium hydroxide (32% by weight) and mixed at room temperature (20-25 ° C).
• the resulting solution is gradually auilconzentriert by stepwise heating up to 150 ° C with stirring and then foamed, ie polymerized and then dried the resulting foam.
• the product thus produced is subsequently calcined in air to drive off carbon impurities and promote crystallization of the mixed oxide.
• the crystallized material is heated to 1 150 or 1350 ° C. It should also be noted that to achieve the phase stability during production, a 10% excess of tungsten was used, which is accordingly taken into account in the following by the parameter z.
• the ratio of A-site cations to B-site cations was always chosen to be 6: 1, 1. In this way, for example, the following materials were produced.
• Nd 5 SmW u Oi2- 8 Nd 5 euw u Oi 2-8, Nd 5 GdW Oi. 2 8 , Nd 5 TbW u Oi 2 -
• La 5 GdWo, 6Cro 50i2-5, La 5 GdWNbo, i0 12 -8, and La 5 GdWo, 6Nbo, 5 Oi2-8.
• the dimensions in the green state were 40 x 5 x 4 mm 3 .
• the rod-shaped samples were sintered in air for 4 hours either at 1150 ° C or at 1350 ° C.
• XRD measurements can be used to study the structure of materials. The measurements were carried out with an X-ray diffractometer system from PANalytical. The Xert Pro system in conjunction with the high-speed detector X "Celerator is equipped with a copper X-ray tube to produce monochromatic Cu Radiation operated. The XRD patterns were recorded in the 2Teta range between 20 ° to 90 ° and analyzed using the X'Pert Highscore Plus software (PANalytical).
• FIGS. 2a to 2i and FIGS. 3a to 3d show the results of the structural investigations.
• the investigated materials of the aforementioned group A and B show a fluorite structure ( Figures 2 and 3). This indicates the formation of a proton-conducting phase and the incorporation of the doping elements (A2 and B2) into the oxide lattice.
• a simultaneous incorporation of two or more A-place elements and / or two or more B-place elements in the generic connection A 6 BOi 2 is so far, except for the connection La 6 - x Zr x W0i 2 from [1] not reported in the literature.
• the determination of the electrical conductivity was carried out by default using the 4-tip method on the sintered rectangular samples.
• the measurements were carried out under different atmospheric conditions, such as under argon and hydrogen, each at 20 ° C and saturated with water.
• the constant current was provided by means of a programmable current source (Keithley 2601) while the voltage drop across the sample was detected using a multimeter (Keithley 3706). To eliminate thermal effects and to avoid non-ohmic responses, the voltage along with the current was measured forward and backward in both directions.
• FIGS. 4a to 4i and 5a to 5g show the improvement in the total conductivity of the materials according to the invention in a humid atmosphere of argon and of hydrogen in comparison with the undoped sample of NdOw ⁇ i O ⁇ .
• the electrical conductivities of the examined samples at 800 ° C in a humid and dry atmosphere are also listed in tabular form below. These samples were also sintered at 1350 ° C.
• Table 1 Combined ion and electron conductivity measurements at A-site
• Transmittance measurements were made on 15 mm diameter sample disks.
• the samples each consisted of a gas-tight 900 ⁇ thick material disc, which were sintered at 1550 ° C. Both sides of the disks were coated with the aid of screen printing with a 20 ⁇ m thick layer of Pt ink (Mateck, Germany), with the aim of improving surface hydrogen exchange. The seals were made with the help of golden O-rings.
• the total continuous gas flow of the gas mixture was 120 mL / min and that of the purge gas argon 180 mL / min.
• the hydrogen content in the purge gas on the permeate side of the membrane was analyzed using a gas chromatograph (Varian CP-4900 microGC with Molsieve5A, PoraPlot-Q glass capillary and CP-Sil modules). Together with the gas flow of the purge gas, the hydrogen flow rate can thus be determined assuming the ideal gas law.
• the permeability results showed a higher permeability of the substituted materials according to the invention Nd 5 EuW 1 (1 0 12 .g in comparison to Nd 6 W 1; iO i 2.
• a comparison of the investigations for the permeability to moist H 2 in a test membrane from an A Space-substituted material (Nd.sLaWyOi ⁇ and a substituted on the B-site material (Nd 6 Wo > 6 Re 0) 50i 2 .5) with a test membrane comprising NdöWi Oia-s is shown in the following table (from Figure 6a with 20% H 2 mixture and from 6b with 50% H 2 mixture).
• the substituted on the B-site material Nd 6 W 0j 6RE 0; 5 O 12- 5 currently indicates the best transmittance values for hydrogen (see Table 3), and is thus particularly advantageous for use as a mixed proton and electron conducting membrane in a Pre-combustion power plant suitable.
• it shows a high mixed conductivity and is characterized by a good chemical resistance to aggressive atmospheres as well as by a good longevity. Particularly noteworthy is that these properties are present even at moderate temperatures around 800 ° C in this material.
• the hydrogen flux through a membrane of Nd 6 Wo ) 6 Re 0> 50i2-8 (B-space substitution) is 3 to 4 times even at 800 ° C compared to a membrane of d 6 Wi ; i Oi2-8 (also substituted B-site).

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