ES2343493A1 - Material for symmetrical electrode of solid oxides fuel batteries (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Mixture of materials (called composites) that can be used as symmetric electrodes, anode and cathode simultaneously, in solid oxide fuel cells (sofc). The idea is to use this new composite, which we can represent by m-ysz-ceo 2 , to replace the two materials that are used as standard in fuel cells: the ni-ysz cermet as anode and the la0.8sr0.2mno 3 - δ (lsm) as cathode. Where: - composite: combination of two or more materials to form a new material with special properties. - m = current collector, can be a noble metal such as platinum (pt), gold (au), silver (ag), etc ... Or else a chromite like la1-xsrxcro 3 - δ - ysz is stabilized zirconia. The fact of using only one material will make it possible to produce cheaper batteries, with less interaction between the different components, which can be prepared in a single thermal treatment and it would also be possible to avoid the problems of traditional batteries such as the formation of coal deposits ( when hydrocarbons are used as fuels) and poisoning by sulfur compounds (which usually accompany hydrocarbons). (Machine-translation by Google Translate, not legally binding)

Description

Material for symmetric electrode of batteries solid oxides fuel.

Technical sector

Materials science and technology.

Materials for fuel cells. Production of non-polluting electrical energy.

Introduction

A solid oxide fuel cell (SOFC) It is an energy conversion device that produces electricity directly by oxidation of a fuel and simultaneous reduction of an oxidant, both generally being in a gaseous state. Each battery consists of two electrodes, a separate anode and cathode for an electrolyte. The fuel is supplied to the anode, where the oxidation reaction occurs, and releases electrons to the circuit external. The oxidant is supplied to the cathode, where the electrons from the external circuit, and the reduction reaction occurs. The flow of electrons, from the anode to the cathode, produces current electric The electrolyte is an electronic insulator that allows the transport of oxide or proton ions -u other ionic species- between the two electrodes.

SOFC batteries are generally based on the ability of certain oxides to allow transport of oxide ions at moderately high temperatures (600-1000 ° C), achieving efficiencies of up to 85% (with cogeneration). In addition, high purity fuels are not required due to high operating temperatures and even mixtures can be used. High temperatures favor internal reforming to extract the hydrogen of some fuels, however, affect negatively to the durability of the equipment and limits the choice of materials such as stainless steel, which results in a product cost.

The electrolyte in this case is a non-porous solid oxide, generally ZrO2 stabilized with ytrium oxide (YSZ) or scandium oxide (SSZ). Normally the anode is a material composed of NiO and YSZ which, when reduced in situ, forms a Ni-YSZ cermet. The cathode is generally a manganite, for example LaMnO 3 doped with Mr.

State of the art

Electrolyte. Among the materials with potential properties to be used as electrolytes in SOFCs, highlight the oxides with fluorite type structure, among the The ZrO 2 and CeO 2 derivatives must be highlighted. He Zirconium oxide (ZrO2) without doping is not a good conductor ionic, however, the incorporation of ions such as Y3 +, Sc 3+ or Ca 2+, [1] makes it one of the best ionic conductors at high temperature. The so-called zircons stabilized are the most commonly used materials as electrolyte in the design of SOFC devices, (especially the composition Zr_ {0. 84} Y_ {0. 16} O_ {1. 92} abbreviated as YSZ), due to its greater stability at high temperature (800-1000ºC) and during high operating times. However, the fact of work at high temperatures remarkably expensive these devices, since it imposes quite restrictive conditions on the other components, preventing, for example, the use of steels as interconnecting materials.

At lower temperatures other ones can be used materials such as those derived from CeO2. [2] Similar to What happens in zircons is not an ionic conductor. Without However, the introduction of other species such as Gd3 +, Sm 3+, Y 3+, La 3+, Nd 3+ or Ca 2+, results in phases that have ionic conductivity values higher than Zircons at moderate temperatures (400-600ºC). He main drawback of these phases is the reduction of Ce (IV) to Ce (III) above 600 ° C and in conditions reducing, which brings with it a potential drop that affects The performance of the battery. The optimal compositions seem to be Ce_ {1-x} Ln_ {x} O_ {2-x / 2}, Ln = Sm 3+, Gd 3+ (x = 0.1-0.2), [2] generally the phase substituted with Gd is represented by CGO and that of Sm by CSO

Anode. In the anode oxidation occurs electrochemical of the fuel, which can be any species liable to be oxidized, although hydrogen is generally used or light hydrocarbons. The list of possible anode materials that meet all the requirements described above is not very broad [3], One of the most used strategies to obtain anode materials is to produce composites, that is, new materials that result from the combination of two or more materials in order to combine its properties.

The excellent catalytic properties of Pt do that could be considered as a component candidate of anode. However, its very high cost has resulted in the search of other materials. Thus, the cermets (ceramic composites and metal) of Ni-YSZ are the most used anodes in the SOFC technology, as they have high electronic conductivity (due to Ni), high ionic conductivity (due to YSZ support) and excellent catalytic activity for electrochemical oxidation of fuels Among the drawbacks, we must highlight the tendency to form carbon deposits when working with hydrocarbons, which can cause the battery to break after few hours of operation A possible solution to this problem is work at a lower temperature and with a higher degree of humidity, varying the fuel / water vapor ratio that reaches the anode. Moreover, these cermets are very sensitive to poisoning. by sulfur, which forces to work with high purity fuels making the whole process more expensive. Finally, we must add that these cermets tend to suffer from sintering problems Ni particles, an effect that is more serious when older are the Working temperatures and higher is the operating time. To these inconveniences, it should be added that Ni is generated from NiO, the latter being toxic and potentially carcinogenic [4] so that its use should be avoided.

One of the solutions adopted to overcome These problems have been the incorporation of alternative cermets [5] in which the role of Ni is carried out by Cu and CeO_ {2} within of a matrix of YSZ, although they are still in the phase of investigation. Another alternative to the use of cermets is the use of mixed oxides. Among the best performing materials are find (La, Sr) (Cr, Mn) O 3 [6] with results comparable to Ni-YSZ cermets in batteries fed with hydrogen. Other possible anode materials are the phases derived from SrTiO 3 substituted with different elements and of general formula (La, Sr) (Ti, M) O 3 (M = Ga, Mn, Se). [7] Recently, it has been shown that operating on batteries fueled with methane, they offer extraordinary performance and generate stable open circuit voltages and higher than 1.2 V, they do not favor the formation of carbon deposits and have a remarkable tolerance to fuels containing impurities of sulfur.

Cathode. In the cathode the reduction of the oxygen, a process that consists of several stages that occur in the breast of the material and its surface, and that depends fundamentally on the partial pressure of oxygen, temperature and characteristics of the electrode. High working temperatures of SOFC batteries (600-1000 ° C) cause candidates to operate as cathodes are compounds with electronic or mixed conductivity. The noble metals such as Pt or Pd, although they have adequate properties to be used as cathodes, they are too expensive for practical purposes. Currently, more cathode materials employees are lanthanum and strontium manganites (LSM), generally La_ {1-x} Sr_ {x} MnO_ {3- \ delta} (x = 0.2-0.5) with high conductivity values p-type electronics of the order of 200 Scm -1 to 1000 ° C [8]. However, there are some drawbacks arising from its use, so new materials such as ferrites have been sought with general formula La_ {1-x} Sr_ {x} FeO_ {3- \ delta} and cobalt-ferrites La_ {1-x} Sr_ {x} Fe_ {1-y} Co_ {y} O_ {3- \ delta}. [9] These materials are mixed conductors (ionic + electronic) and present a remarkable catalytic activity towards the reduction of oxygen, although the chemical compatibility of these materials with the YSZ is still questionable.

Currently a material has been tested that works as anode and cathode simultaneously, verifying said concept [10] and the first stack constructed of this type, with phases derived from the chromites (La, Sr) (Cr, Mn) O_ {3- δ}, -represented as (LSCM) - has allowed to generate up to 500 mW / cm2 working with hydrogen and 300 mW / cm2 when Use methane as fuel.

The LSCM although it works well presents the problem that the electrical conductivity under reducing conditions It is not very high and you have to use a current collector to replace this lack, for example in the study of the reference [10a] platinum was used.

We propose a new material that supplies LSCM deficiencies.

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References

[1] a) DW Strickler , WG Carlson , J. Am. Ceram. Soc . 1964 , 47, 122-127; b) SPS Badwal , Solid State Ionics 1992 , 52, 23-32; c) O. Yamamoto , Y. Arati , Y. Takeda , N. Imanishi , Y. Mizutani , M. Kawai and Y. Nakamura , Solid State Ionics 1995 , 79, 137-142; d) SPS Badwal , FT Ciacchi and D. Milosevic , Solid State Ionics 2000 , 136-137.91-99.

[2] a) BCH Steele , "High Conductivity Solid Ionic Conductors", Ed. T. Takahashi, World Scientific, Singapore , 1989 ; b) R. Gerhardt-Anderson and AS Nowick , Solid State Ionics 1981 , 5, 547-550; c) JA Kilner and RJ Brook , Solid State Ionics 1982 , 6 (3), 237-252.

[3] A. Atkinson , S. Barnett , RJ Gorte , JTS Irvine , AJ McEvoy , M. Mogensen , SC Singhal , J. Vohs, Nat. Mater. 2004 , 3.17-27.

[4] a) AD Ottolenghi , JK Haseman , WW Payne , HL Falk and HN MacFarland , J. Natl. Cancer Inst . 1974 , 54 (5), 1165-1172; b) PJ Haley , GM Shopp , JM Benson , YS Cheng , DE Bice , MI Luster , JK Dunnick and CH Hobbs , Fundam. Appl. Toxicol 1990 , 15, 476-487.

[5] RJ Gorte , H. Kim and JM Vohs , J. Power Sources 2002 , 106 (1-2), 10-15.

[6] a) J. Liu , BD Madsen , Z. Ji and SA Barnett , Electrochem. Solid-State Lett . 2002 , 5 (6), A122-A124; b) S. Tao and John TS Irvine , Nat. Mater . 2003 , 2, 320-323.

[7] a) JC Ruiz-Morales , J. Canales-Vázquez , CD Savaniu , D. Marrero-López , W. Zhou and JTS Irvine , Nature 2006 , 439, 568-571; b) J. Canales-Vázquez , JC Ruiz-Morales , JTS Irvine and W. Zhou , J. Electrochem. Soc . 2005 , 152, A1458-A1465; c) J. Canales-Vázquez , SW Tao and JTS Irvine , Solid State Ionics 2003 , 159, 159-165; d) A. Ovalle , JC Ruiz-Morales , J. Canales-Vázquez , D. Marrero-López and JTS Irvine , Solid State Ionics 2006 , 117 (19-25), 1997-2003; e) JC Ruiz-Morales , J. Canales-Vázquez , CD Savaniu , D. Marrero-López , P. Núflez , W. Zhou and JTS Irvine , Phys. Chem. Chem. Phys . 2007 , 9, 1821-1830.

[8] SC Singhal and K. Kendall , "High Temperature Solid Oxide Fuel Cells: Fundamental, Design and Applications", Elsevier , Oxford, 2004 .

[9] a) SP Simner , JF Bonnett , NL Canfield , KD Meinhardt , VL Sprenkle , and JW Stevenson , Electrochem. Solid-State Lett . 2002 , 5 (7), A173-A175; b) K. Huang , HY Lee and JB Goodenough , J. Electrochem. Soc . 1998 , 149 (9) 3220-3227; c) BCH Steele , Solid State Ionics 1996 , 86-88, 1223-1234; d) SP Jiang , Solid State Ionics 2002 , 146 (1-2), 1-22; e) L. Qiu , T. Ichikawa , A. Hirano , N. Imanishi and Y. Takeda , Solid State Ionics 2003 , 158 (1-2), 55-65; f) S. Carter , A. Selcuk , RJ Charter , J. Kajda , JA Kilner and BCH Steele , Solid State Ionics 1992 , 53-56, 597-605.

[10] a) JC Ruiz-Morales , J. Canales-Vázquez , J. Peña Martínez , D. Marrero-López and P. Núñez , Electrochimica Acta 2006 , 52 (1), 278-284; b) JC Ruiz-Morales , J. Canales-Vázquez , B. Ballesteros , J. Peña-Martínez , D. Marrero-López , JTS Irvine and P. Núñez , J. Eur. Ceram. Soc . 2006 , doi: 10.1016 / j.jeurcerarasoc.2007.02.117; c) JC Ruiz-Morales , H. Lincke , D. Marrero-López , J. Canales-Vázquez and P. Núñez , Bol. Soc. Esp. Ceram . V. 2007 (in press); d) D. Bastidas , S. Tao and JTS Irvine , J. Mater. Chem 2006 , 16, 1603-1605

Description of the invention

The invention consists in using a mixture of materials, a current collector (usually metallic) together with ceramic components, to produce a new material, such as symmetric electrode that works simultaneously as an anode and cathode of a fuel cell.

Ceramic materials consist primarily of electrolyte dust on the one hand, which ensures ionic conductivity, allowing the composite powder to be fixed relatively well to the dense pellet of the electrolyte, and that there are no significant differences in the coefficient of thermal expansion between the different materials that form the electrode, thus avoiding delaminations of them over time. It also benefits from the so-called extension of the Triple Phase Boundary (throughout the English Triple Phase Boundary ), along the entire electrode material, thus increasing the electrochemical active surface.

Another of the ceramic components is the dust of cerium oxide, CeO2, which although has no ionic conductivity, If you have electronics in reducing conditions.

The current collector is constituted, Generally, noble metals such as platinum (Pt), gold (Au) for temperatures 950-800ºC and silver (Ag) for lower temperatures Additionally another current collector that could be used would be the chromites used as interconnectors, but in this case it is not a metal but of Other ceramic material.

The powders of the two ceramic materials (electrolyte + CeO2) are weighed in the proper proportion, which It ranges around 50% of each compound. They are mixed in mortar Agate with acetone and let dry. Next a organic compound binder and fixative, called binder, at least in a 1: 1 ratio with respect to the previous mixture. This new viscous mixture, called slurry, is used to paint the electrodes on both sides of a dense electrolyte tablet and thus form a symmetrical stack. It is intended that the thickness of the electrode is of the order of 5-10 microns maximum. The cell with the painted electrodes is allowed to dry in an oven, between 50ºC and 80ºC. Finally the sample burns at high temperatures to fix the electrodes to the electrolyte, usually necessary temperatures above 1100ºC and up to a maximum of 1300 ° C, for 1-3 hours. Finally it would be paint a thin layer of the current collector. Subsequently burns the organic matter that contains the collector paint of current, in an oven at 800-950 ° C, for 30 min-2 hours Sometimes they can be impregnated previously electrodes with a diluted, alcoholic solution of the collector of current; which ensures greater electrochemical activity at Through all the material.

In the case of using chromites as a collector of current, what is done is that chromite dust is added in the first stage where electrolyte is mixed together with CeO 2. Again in a proportion of the order of 30-70% with respect to the mixture (electrolyte + CeO 2) and all the steps described above are repeated.

Embodiments of the invention Preparation of a battery with symmetric electrodes of Pt-YSZ-CeO_ {2}

1) Weigh 0.15 grams of YSZ and 0.15 grams of CeO 2. They are poured into an agate mortar and acetone is added so that it covers the powders, then they mix homogeneously the powders and, the acetone is allowed to evaporate.

2) The powders obtained in 1) are poured into a small glass canister, in which 0.2 grams of binder are poured, in our case DECOFLUX (WB41, Zschimmer and Schwartz). They mix to form a kind of homogeneous and viscous paint, known as slurry

3) Electrodes are painted using silkscreen symmetrical, with the slurry of point 2), on both sides of a dense YSZ tablet (1 mm thick and 15 mm wide). They are painted 10 mm diameter circles which gives rise to a geometric area 0.785 cm2. First a face is painted, dried in an oven to about 70 ° C and the other process is repeated with the other side.

4) Once dry, the cell is placed on top of a alumina substrate to avoid reactivities with the oven and it Sinter at 1200 ° C for 2 hours. With ramps of heating and cooling of 5º / min.

5) The upper part of both is painted electrodes with commercial platinum paint, to add the current collector It is introduced in an oven and burned at 950 ° C for 2 hours Again, the heating ramps and cooling are 5º / min. And with this the assembly of a battery.

Claims (7)

1. Symmetric electrode material, which acts as an anode and cathode simultaneously, for SOFC batteries, characterized by A, B and C, being:

Yet electrolyte comprising YSZ, either CGO or CSO or LSGM

B: CeO 2

C: a collector of current comprising, a noble metal or a chromite

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2. Symmetric electrode material, characterized according to claim 1, wherein:

A equals YSZ

B equals CeO 2

C equals Pt

3. Symmetric electrode material, characterized according to claim 1, wherein:

A equals CSO

B equals CeO 2

C equals Pt

4. Symmetric electrode material, characterized according to claim 1, wherein:

A equals YSZ

B equals CeO 2

C equals Au

5. Symmetric electrode material, characterized according to claim 1, wherein:

A equals CSO

B equals CeO 2

C equals Au

6. Symmetric electrode material, characterized according to claim 1, wherein:

A equals YSZ

B equals CeO 2

C equals Ag

7. Symmetric electrode material, characterized according to claim 1, wherein:

A equals CSO

B equals CeO 2

C equals Ag