Patent Application
20110229794
1. Field of the Invention
The present invention relates to solid oxide fuel cells and, more specifically, to composite oxygen conducting cathodes for use in solid oxide fuel cell devices.
2. Technical Background
In recent years, solid oxide fuel cells (SOFC) have attracted interest for power generation, producing electricity from fuels including hydrogen, hydrocarbons and fossil fuels, and converting pollution-free the chemical energy of the fuel into electrical energy. A typical SOFC comprises a dense oxygen ion-conducting ceramic electrolyte layer sandwiched between porous air electrode (cathode) and porous fuel electrode (anode). In operation, electrical energy is produced by electrochemical combination of the fuel with the oxidant.
Yttria-stabilized zirconium oxide (YSZ) is currently the most commonly employed electrolyte material due to its mechanical, electrical, chemical and thermal properties. Cubic YSZ offers higher ionic conductivity and lower strain tolerance while 3YSZ offers higher strength at comparably lower (around a third) oxygen ion conductivity. At present, the anodes in most commercial and prototype solid oxide fuel cell devices are made of nickel-YSZ cermet, and the cathodes are typically made of lanthanum manganites, lanthanum ferrites or lanthanum cobaltites. In such fuel cells, the oxygen reacts with the electrons on the surface of the cathode to form oxygen ions that migrate through the electrolyte to the anode, where they react with fuel, such as hydrogen, to produce electrons and water. The electrons flow from the anode through an external circuit to the cathode, while providing usable power. The theoretical open circuit voltage of single cell devices composed of YSZ electrolyte with anode and cathode is usually not reached in experiments, due to ohmic resistance, restricted ion mobility and electrode polarization.
Oxygen incorporation at the cathode occurs through a number of different reaction steps such as diffusion through the cathode pore network, adsorption, dissociation, charge transfer and exchange with oxygen vacancies. All can contribute to the cathode resistance. For the different types of cathode materials typically used, the rate limiting steps for the oxygen incorporation can differ. For example, lanthanum strontium manganite (LSM) has low ionic conductivity; as a result, oxygen incorporation mainly occurs at the triple phase boundaries, the contact points between ion-conducting electrolyte, electron-conducting LSM and gas phase. Due to the limited number of triple phase boundary sites (even in a LSM/YSZ composite cathode), charge transfer at the triple phase boundary is usually rate-controlling at high temperature. Due to the limitation of the oxygen incorporation in LSM-based cathodes to the triple phase boundary, those cathodes are very vulnerable to all types of pollution, poisoning and reactions occurring at the triple phase boundary. Thus LSM-based cathodes typically suffer severe performance degradation under harsh processing or operating conditions. For example, during processing insulating phases such as pyrochlore can form by reaction between YSZ and LSM. Further, during firing or operation, impurities such as Si can segregate to the triple phase boundary and form blocking layers. Still further, when triple phases boundaries are flooded by borosilicates from glass seals or when chromites deposit by electrochemically induced decomposition of volatile CrO2(OH)2 or CrO3 gases, the oxygen incorporation rates during processing can be diminished to nearly a complete inhibition.
Accordingly, there is a need in the art for improved electrode materials that can exhibit improved performance and processing properties when used, for example, as cathodes in solid oxide fuel cell devices.
Embodiments of the present invention can provide composite electrode materials suitable for use as cathodes in solid oxide fuel cell devices. In one embodiment, the composite electrodes comprise a sintered mixture of lanthanum strontium ferrite component having the formula (LaxSry)1±δ(FeaMnbCoc)O3 and a stabilized zirconia; wherein 1.0≧x≧0.65; 0.35≧y≧0.0; x+y=1.0, δ=0-0.1, a+b+c=1 and a>0.6. Among several advantages, the composite electrodes can exhibit high chemical stability at temperatures up to at least 1250° C., reach high electrochemical performance, remain stable under polarization and preserve rather high performance during long-time cathode operation in the presence of conventional or currently known seal glasses or when also exposed to chromium sources. In use, the electrode materials of the present invention can therefore enable solid oxide fuel cell devices to operate at higher performance levels, such as for example increased power density.
In another embodiment, a method of making a porous composite electrode is provided. The method generally comprises depositing an unsintered mixture of a lanthanum strontium ferrite component having the formula (LaxSry)1±δ(FeaMnbCoc)O3 and a stabilized zirconia; wherein 1.0≧x≧0.65; 0.35≧y≧0.0; x+y=1.0, δ=0-0.1, a+b+c=1 and a>0.6. Once deposited, the deposited mixture can then be sintered or fired under conditions effective to convert the unsintered mixture of the lanthanum strontium ferrite component and yttria stabilized zirconia into a porous composite suitable for use as a cathode catalyst in the solid oxide fuel cell cathode. The cathode can be formed entirely of the described catalyst layer or, alternatively, can comprise the described catalyst layer and an additional current collector top layer. As will be appreciated by one of ordinary skill in the art, a variety of suitable current collectors are conventionally known and can, for example, be comprised of a variety of materials including porous zirconia-metal composites.
Additional embodiments of the invention will be set forth, in part, in the detailed description, and any claims which follow, and in part will be derived from the detailed description, or can be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as disclosed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate certain embodiments of the instant invention and together with the description, serve to explain, without limitation, the principles of the invention.
The following description of the invention is provided as an enabling teaching of the invention in its best, currently known embodiment. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various embodiments of the invention described herein, while still obtaining the beneficial results of embodiments of the present invention. It will also be apparent that some of the desired benefits of embodiments of the present invention can be obtained by selecting some of the features described herein without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof.
As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an “electrode” includes embodiments having two or more such electrodes unless the context clearly indicates otherwise.
Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.
As used herein, a “wt. %” or “weight percent” or “percent by weight” of a component, unless specifically stated to the contrary, is based on the total weight of the composition or article in which the component is included.
As briefly summarized above, embodiments of the present invention can provide oxygen conducting composite electrodes suitable for use in solid oxide fuel cell devices. The composite electrodes comprise a sintered mixture of lanthanum strontium ferrite and stabilized zirconia. Among several advantages exemplified in the examples which follow, the composite electrodes can exhibit relatively high chemical stabilities at temperatures up to at least 1250° C., can reach relatively high electrochemical performance, are relatively stable under polarization and preserve relatively high performance, such as power density, even during prolonged periods of operation and in presence of borosilicate and other glasses as well as chromium sources. Still further, the composite electrodes have a relatively broader processing window and, to that end, can be fired at temperatures up to at least 1250° C. for several hours without any substantial pyrochlore formation.
The composite electrodes are formed of a sintered mixture of lanthanum strontium ferrite and stabilized zirconia. The stabilized zirconia component of the mixture can comprise any desired amount of calcia, magnesia, yttria and other rare earth oxides, including for example a 3 mol % yttria stabilized zirconia, an 8 mol % yttria stabilized zirconia, or even a 10 mol % yttria stabilized zirconia. However, in one embodiment, a preferred yttria stabilized zirconia is the 3 mol % yttria stabilized zirconia also referred to herein as yttria (3 mol %) stabilized zirconia or 3YSZ.
The lanthanum strontium ferrite component, also referred to herein as LSF, can contain various small amount partial substitutions on the A-site others than Sr and La and can also contain partial substitutions on the perovskite B-site, such as for example Mn, Co and others. Accordingly, in one embodiment the LSF compound can be characterized by the formula (LaxSry)1±δ(FeaMnbCoc)O3; wherein 1.0≧x≧0.65; 0.35≧y≧0.0; x+y=1.0, δ=0-0.1, a+b+c=1 and a>0.6. In a preferred embodiment, the lanthanum strontium ferrite component can be characterized by the formula (LaxSry)1-δFeO3 and in an even further preferred embodiment as (La0.8Sr0.2)FeO3.
The components of the sintered mixture can be present in any desired weight ratio, however, in one embodiment it is preferred for the composite electrode to comprise from about 30 weight % to about 90 weight % of the lanthanum strontium ferrite and from about 70 weight % to about 10 weight % of yttria stabilized zirconia. In still a more preferred embodiment, the sintered composite electrode comprises about 40 weight % lanthanum strontium ferrite and about 60 weight % of the yttria stabilized zirconia.
To prepare the composite electrodes, an unsintered mixture of the lanthanum strontium ferrite component and the stabilized zirconia component can be deposited onto a substrate. In one embodiment, the composite electrode can be formed on and in direct contact with (i.e., in the absence of intervening layers) an electrolyte membrane or sheet, such as those commonly used in solid oxide fuel cell devices. For example, in one embodiment the substrate can be an electrolyte sheet comprised of a yttria stabilized zirconia. Further, the electrolyte sheet can have any desired thickness, including for example, a thickness that is less than or equal to 50 μm. In still another embodiment, it is desired that the electrolyte sheet be less than or equal to 40 μm, less than or equal to 30 μm, or even less than or equal to 20 μm. Once deposited, the mixture can then be sintered under conditions effective to form a sintered solid oxide fuel cell electrode on the substrate.
The unsintered mixture of lanthanum strontium ferrite and stabilized zirconia can be obtained by blending the desired relative amounts of the lanthanum strontium ferrite component and the stabilized zirconia component. As described above, these components can be blended together in any desired ratio, including for example about 30 weight % to about 90 weight % of the lanthanum strontium ferrite and from about 70 weight % to about 10 weight % of stabilized zirconia. The unsintered mixture can, for example, be deposited onto a substrate such as an electrolyte membrane, by a screen printing process. To that end, a printable ink composition can be obtained comprising the blended unsintered powder batch mixture dispersed in a liquid vehicle system which can further comprise one or more dispersants, binders, or organic solvents. The dispersed powders and the vehicle system can also be blended together in any desired ratio to reach the desired porosity in the resulting composite cathode material. For example, in one embodiment an exemplary ink composition can be obtained by providing an unsintered mixture of 40 volume % (La0.8Sr0.2) FeO3 and 60 volume % 3YSZ. The exemplary unsintered mixture can then be mixed with an organic liquid vehicle at a 10.5 vol % solids loading concentration. In another exemplary embodiment, a mixture of 40 volume % (La0.8Sr0.2) FeO3 and 60 volume % 3YSZ can be mixed and loaded at 15 vol % solids loading concentration into an organic vehicle. In still another embodiment, a mixture of 40 volume % (La0.8Sr0.2) FeO3 and 60 volume % 8YSZ can be mixed and loaded at 15 vol % solids loading concentration into an organic vehicle.
Once blended, an ink composition comprising the dispersed unsintered mixture can be deposited onto a substrate, such as for example, a ceramic electrolyte membrane suitable for use in a solid oxide fuel cell device. As mentioned above, in one embodiment the ink can be deposited using a screen printing process. If desired, the printing process can also be automated.
The deposited unsintered LSF/YSZ mixture can then be fired under conditions effective to convert the unsintered mixture into a sintered porous composite electrode comprising the selected lanthanum strontium ferrite component and the selected yttria stabilized zirconia component. It should be understood that the particular firing or sintering conditions to be used will depend, at least in part, on the particular composition of the unsintered mixture and the amount deposited onto the substrate. However, in one embodiment, suitable firing conditions can comprise heating the deposited mixture at a sintering temperature in the range of from 1000° C. to 1250° C. for approximately 2 hours.
In use, the composite electrodes are well suited for use as cathodes in solid oxide fuel cell devices and can exhibit several improved processing and performance characteristics. In one embodiment, the composite electrode materials of the can exhibit improved, i.e., reduced, levels of cathode area specific resistance when utilized as a cathode in a solid oxide fuel cell device. To that end, as referred to herein a cathode area specific resistance was determined by first measuring the total cathode area resistance for a cathode oxygen pump sample comprising two symmetrically identical cathodes positioned on either side of an electrolyte sheet and operated at 0.5V in air at 750° C. This total cathode pump resistance was then divided by two to determine the cathode area specific resistance for each of the two cathodes utilized in the oxygen pump sample.
For example, a cathode pump sample with two symmetric identical composite electrodes can exhibit a cathode resistance less than approximately 0.15 ohms cm2 when measured at 0.5V and at 750° C. (according to conventions established by those skilled in the art, the resistance of one individual cathode is considered as half of that value, 0.07 ohm cm2). Similarly, the inventive composite electrodes also exhibit an improved, i.e., increased, current density. For example, oxygen pump samples with a thin electrolyte and two of these composite electrodes can exhibit a current density of at least 1.0 A/cm2 when measured at 0.5 volts and 750° C. Still further, oxygen pump samples with a 20 micrometer thick thin electrolyte and two of these electrodes can even exhibit a current density of at least 1.3 A/cm2, or even at least 1.5 A/cm2 when measured at 0.5 volts and 750° C. Still further, the inventive composite electrodes show lower degradation of their performance during operation in cathode pump cells and in stacks. Lower degradation in a stack environment was simulated by cathode operation under polarization, in the presence of seal glass and when exposed to CrO3 vapor.
To further illustrate the principles of embodiments of the present invention, the following examples of inventive oxygen conducting composite electrode compositions are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the composite electrodes and methods claimed herein can be made and evaluated. They are intended to be purely exemplary of the invention and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperatures, etc.); however, some errors and deviations may have occurred. Unless indicated otherwise, parts are parts by weight, temperature is ° C. or is at ambient temperature, and pressure is at or near atmospheric.
In the following examples, the LSF/3YSZ composite cathodes evaluated had the stoichiometric formula (La0.8Sr0.2)FeO3 and 3YSZ. These cathodes were prepared by providing an unsintered mixture comprising 40 weight % of the LSF component and 60 weight % of the 3YSZ component. To provide a printable ink composition, the powders were mixed with a liquid vehicle system containing dispersants, binders, and an organic solvent. For higher porosity composite layers (type A), the 40 volume % (La0.8Sr0.2) FeO3 and 60 volume % 3YSZ powder mixture was loaded at 10.5 vol % concentration into the vehicle system. For lower porosity composites (type B), 40 volume % (La0.8Sr0.2) FeO3 and 60 volume % 3YSZ powder mixtures were loaded at 15 vol % concentrations into the vehicle system. For obtaining comparative LSF/8YSZ composites, 40 volume % (La0.8Sr0.2) FeO3 and 60 volume % 8YSZ were mixed and loaded at 15 vol % concentrations into the vehicle system. For obtaining comparative LSM/3YSZ composites, 40 volume % (La0.8Sr0.2)0.98 MnO3 and 60 volume % 3YSZ were mixed and loaded into the organic vehicle.
Once blended, the ink compositions were printed onto the YSZ electrolyte using a semi-automatic screen printer (de Haart). The substrate had a thickness of about 20 microns. The deposited layer of ink was approximately 4 microns thick. The 3YSZ ceramic substrate was mounted on the printer. A print of the planned design was placed on the cloth covered mounting platen. Aligning to the dried print on the mounting platen provided the desired registration of the print. Each substrate was then printed and dried for about 2 minutes at about 145° C., before printing the opposite side. It should be noted that the temperature of the drying oven also varied by about 10° C. as more substrates were being dried. The screens used for the printing were made of 250 & 200-mesh stainless steel wire bonded to a frame. The LSF/YSZ ink was then printed on a 1 cm×1.5 cm print area on both sides of the electrolyte with a 1 cm2 area of the print overlapping the print area on the opposite side of the substrate. This print design of the test samples provided an active cathode with an exact area of 1 cm2. After printing, the LSF/YSZ composite layers were dried and fired at 1250° C. To reach the firing temperature, temperatures were first slowly ramped to 1250° C., followed by a hold or soak period of 2 hours, after which the fired composition was slowly cooled to ambient conditions. After firing, a current collector was applied to the LSF/YSZ composite. In particular, an Ag/Pd-3YSZ ink was printed on top of the fired LSF/YSZ composite print, dried and subsequently fired at 850° C. for 2 hours. An SEM image of the LSF/3YSZ composite cathode (which is situated on 3YSZ electrolyte) is shown in
In the following examples, the LSF/8YSZ cathodes evaluated had the stoichiometric formula (La0.8Sr0.2)FeO3+8YSZ. These cathodes were prepared by providing an unsintered mixture comprising 40 weight % of the LSF component and 60 weight % of the 8YSZ component. The same process that was used to prepare inks and print the cathodes for LSF/3YSZ cathodes was also used for preparing the LSF/8YSZ cathodes. The resulting ink composition comprising the unsintered LSF/8YSZ mixture was deposited onto a yttria stabilize zirconia substrate using the screen printing process. The substrate had a thickness of about 20 microns. Once printed and dried, the LSF/8YSZ composite prints were slowly heated to 1150° C., hold at that temperature for 2 hours and then slowly cooled down. The fired LSF/8YSZ composite layer was approximately 4 microns thick.
In the following examples, the so-called LSM/3YSZ reference cathodes of good performance were based on (La0.8Sr0.2)0.97MnO3 and 3YSZ. They were prepared from an unsintered mixture comprising 40 weight % of the LSM component and 60 weight % of the 3YSZ component and contained some NiO/8YSZ. The same process that was used to prepare inks and print the cathodes for LSF/3YSZ cathodes was also used for preparing the LSM/3YSZ cathodes. The LSM/YSZ ink was deposited onto a yttria stabilize zirconia substrate using the screen printing process. The substrate had a thickness of about 20 microns. Once printed and dried, the LSM/3YSZ composite prints were slowly heated to 1250° C., hold at that temperature for 2 hours and then slowly cooled down. The fired LSM/3YSZ composite layer was approximately 4 microns thick.
Considerable pyrochlore formation occurs during high temperature processing or firing of lanthanum strontium ferrite with cubic yttria stabilized zirconia ceramics and even with single crystalline cubic 10 mol % yttria stabilized zirconia. For example, systematic decoration of the 8YSZ grain boundary lines in contact with LSF can occur at temperature of 1000° C. already after 100 hours. At 1250° C., substantial formation of large size plate-shaped grains of pyrochlore is observed in the contact plane of 8YSZ and LSF. However, during processing of the inventive composite LSF/3YSZ composite electrodes, it has been discovered that pyrochlore formation remains negligible after firing at temperatures of up to 1250° C. and for several hours. In fact, only very few isolated pyrochlore particles form, the density and size of which is similar to those found to be formed during the processing of 3YSZ/LSM composite cathodes containing excess Mn.
The electrochemical performance of the inventive composite electrodes when sampled in cathode/cathode oxygen pump single cells with 3YSZ electrolyte and Ag—Pd/3YSZ current collectors also shows significant improvements compared to corresponding LSM/3YSZ and LSF/8YSZ cathodes. Comparative data from this evaluation are illustrated in
Still further,
With reference to
During cathode operation, LSF/3YSZ cathodes can suffer under polarization and in presence of a chromium source the well-known Cr-poisoning from CrO3 or CrO2(OH)2 vapor that is electrochemically reduced at the cathode surface and forms chromium oxide Cr2O3 and spinel precipitates at the surface that inhibit the further oxygen incorporation into the cathode. To that end,
Lastly, it should also be understood that while the present invention has been described in detail with respect to certain illustrative and specific embodiments thereof, it should not be considered limited to such, as numerous modifications are possible without departing from the broad scope of the present invention as defined in the appended claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US08/09425 | 8/6/2008 | WO | 00 | 2/2/2010 |
| Number | Date | Country | |
|---|---|---|---|
| 60963932 | Aug 2007 | US |
