Patent Application
20150244001
The present invention relates to a doped metal oxide structure for use in electrochemical devices and to a method of manufacturing said doped metal oxide structure.
The performance and long term stability of solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs), oxygen membranes and electromotive force (EMF) devices may be influenced by the oxygen electrode used therein.
U.S. Pat. No. 6,017,647 relates to an oxygen electrode for a solid state electrochemical device comprising a porous structure having an interpenetrating network of an ionically conductive and an electronically conductive material. An electrocatalyst different from the electronically conductive material is dispersed within the pores of the porous structure by infiltration after the sintering step.
U.S. Pat. No. 6,319,626 relates to an yttria-stabilized zirconia (YSZ) cathode comprising a high performance electrocatalyst based on perovskites. The electrode is formed and sintered prior to impregnation.
WO 00/25898 discloses doping of cerium oxides for the production of oxygen and hydrogen. The oxides are obtained by precipitating the respective composition from a solution containing all desired cations, followed by sintering the dried aggregates.
WO 2007/027144 relates to a doped ceria ion conductor which enables higher ionic conductivity at lower temperatures.
However, the structures disclosed in the application mentioned above do not increase the performance, the long term stability, and the tolerance to high temperature treatment of the oxygen electrode.
Hence, an improved electrode and method for manufacturing said electrode would be advantageous, and in particular a more efficient and/or reliable electrode would be advantageous.
It is an object of the invention to provide an electrode with improved long term stability and a method of manufacturing said electrode.
It may be also seen as an object of the invention to provide a method of manufacturing an electrode and an electrode with improved heat treatment stability, i.e. high tolerance to heat treatments.
A further object of the present invention is to provide an alternative to the prior art.
In particular, it may be seen as an object of the present invention to provide a method of manufacturing an electrode and an electrode that solve the above mentioned problems of the prior art by elements infiltration into doped metal oxides.
Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a method of manufacturing an electrode, the method comprising: a) infiltrating of at least one element into a doped metal oxide; b) heating said doped metal oxide infiltrated with the at least one element.
Thereby, in a first aspect of the invention, the method of manufacturing an electrode provides an electrode with improved long term stability and improved heat treatment stability, i.e. high tolerance to heat treatments.
In order to improve performance and long term stability, electrodes need to exhibit low polarization losses. This can be achieved if the electrode material shows, e.g. fast oxygen redox kinetics, high electronic and ionic conductivity, chemical compatibility with other components, and stable microstructure at the operating temperature.
A method to produce composite electrodes exhibiting low polarization losses compared to conventional routes is the so-called infiltration or impregnation route. This may be applied to structures usually consisting of an oxide ion conducting porous backbone that is infiltrated with an electronic or mixed ionic and electronic conducting (MIEC) electrocatalyst.
The nanoparticulate grains of the electrocatalyst, obtained through the infiltration route, generally have a high surface area and therefore enhance the available area for oxygen surface exchange. However, it has been found that during long term operation and/or exposure to high temperatures, i.e.>600° C., the formed nanoparticles exhibit significant grain growth and subsequent loss of percolation thus increasing both the ohmic and polarization resistance.
In search of solutions, the inventors devise the invention by introducing an infiltration step of at least one element into a doped metal oxide preceding the heating step and the electrocatalyst infiltration step, thereby providing an infiltrated doped metal oxide structure.
Importantly the infiltration step of at least one element into a doped metal oxide occurs before the heating step and before the infiltration of the electrocatalyst. The electrode comprises a metal oxide referred to also as the backbone, ionic conducting backbone or simply the backbone material. The backbone material preparation lies within the person skilled in the art.
In some further embodiments the at least one element is a lanthanide, preferably a lanthanide soluble into ceria.
The at least one element may be selected from the group of Pr, Nd, Er, Tb, or a combination thereof.
In some other embodiments the at least one element is a transition metal.
In some further embodiments the at least one element is selected from the group of Cr, Mn, Fe, Co, Ni, V, Ru or a combination thereof.
In some even further embodiments the at least one element is or is selected from the group of W, Nb, Ta, Mo or a combination thereof.
Thus, in some embodiments the at least one element is praseodymium (Pr).
In some embodiments the at least one element are two elements.
Those two elements may be praseodymium and terbium.
The doped metal oxide may have the following general formula: M1−xAxO2−δ, wherein A is an alkaline earth or a rare earth and M is cerium or zirconium and wherein x is in the range between 0 and 1 and wherein δ is in the range between 0 and 1. The value of δ is generally determined by the oxygen activity around the material.
In some other embodiments the doped metal oxide has the following general formula: Bi2−2xA2xO3−δ, wherein A is an alkaline earth or a rare earth and wherein x is in the range between 0 and 1 and wherein δ is in the range between 0 and 1. In some further embodiment A is selected from the group of Mg, Ca and Sr. A may be also selected from the group of Sc, Y, Gd, La, Sm, Er, Dy.
The infiltration step may occur by contacting the backbone material with a solution. Contacting may be achieved by providing a solution containing ions of the element, to at least one surface of the backbone so that the backbone is in contact with the solution containing the ions of the element. In some embodiments, infiltration may be achieved by soaking the doped metal oxide in a solution containing the ions of the element. The solution may have different concentrations of elements salts, e.g. between 0.001M to 10 M.
The infiltration step allows for introduction and penetration of the salt containing the ions of the element into the doped metal oxide backbone.
In some further embodiments, the infiltration of the at least one element occurs from an aqueous solution.
In some other embodiments the aqueous solution comprises nitrates.
Thus, the electrode may comprise gadolinium doped cerium oxide (CGO) as a backbone material and may be infiltrated or impregnated with an element such as praseodymium ions from an aqueous solution of praseodymium salt, such as praseodymium nitrate, prior to the heating step.
In some embodiments, the heating is performed at a temperature, higher or equal to 500° C., thereby causing the formation of nanoparticulate precipitates at the surface of the backbone and subsequent full or partial sintering or reaction with the backbone structure.
The heating step is a thermal treatment process occurring in the presence of air or some other atmosphere like dry H2 and may be also referred to as sintering step, calcination step or simply as calcination. The heating is performed at high temperature, such as higher or equal to 500° C., preferably between 500 and 1500° C., even more preferably between 900 and 1250° C.
The high temperature treatment allows the ions of the element to diffuse into the CGO backbone. The resulting backbone has a co-doped subsurface exhibiting electronic conductivity. The remaining particles of oxide of the element in the surface may also be advantageous and exhibit enhanced activity at the surface. Subsurface is defined herein as within 0.1 nm from the external surface of the backbone grain.
In some further embodiments the method according to first aspect of the invention further comprises: a1) pre-heating at temperature lower or equal to 500° C., preferably at 350° C.
This pre-heating, pre-sintering or pre-calcination step allows for further infiltrations before the heating, sintering or calcination step.
In some further embodiments the method further comprises repeating step a) and a1) before step b).
If multiple infiltrations are desired, the infiltrated backbone is pre-calcined at temperature lower or equal to 500° C., preferably at 350° C., before each infiltration. This pre-calcination step at low temperature, such as at 350° C., follows each infiltration step and is performed before the sintering step. As a last step, the infiltrated backbone is heated at high temperature, such as 1000° C. In some further embodiments the method further comprises: c) infiltrating an electrocatalyst.
After the sintering step, an electrocatalyst is impregnated or infiltrated into the electrode. The infiltration of the electrocatalyst lies within the person skilled in the art.
Examples of electrocatalyst that can be infiltrated or impregnated are electronically conducting oxides e.g. Sr-doped LaMnO3 and MIEC electrocatalysts e.g. Sr-doped LaCoO3 and Sr/Fe-doped LaCoO3.
The above described object and several other objects are intended to be obtained in a second aspect of the invention by providing an electrode manufactured according to the method according to the first aspect of the invention.
The electrode may be an oxygen electrode.
In some other embodiments, the electrode is a fuel electrode.
In some embodiments, the electrode comprises grains of the doped metal oxide and the electrode has a structure characterized by a concentration gradient of the at least one element within the grains of the doped metal oxide. The content of the at least one element varies from the grain interior towards the surface in a manner characterized by a core-shell type structure, so that the at least one element is present at the edge of the grain but is substantially not present at the core of the grain. The structure of the electrode is thus realized by controlled interdiffusion of the at least one element from the surface of the grains of the doped metal oxide during heating, where the element has been introduced via infiltration.
Thus, a partial substitution with at least one element occurs to the doped metal oxide. The partial substitution is brought about in a process that involves an infiltration and a heat treatment. The heat treatment leads to a decomposition of the infiltrated compound and the at least one element diffuses into the doped metal oxide.
Concentration gradient or functional gradient is herein defined as a gradient of concentration, i.e. a change in concentration of the element as a function of distance within and through the grains of the doped metal oxide. The definition of the concentration gradient becomes more apparent from and elucidated with reference to the embodiments described in the description of the invention. Grains refer to the crystal grains of the doped metal oxide, e.g. crystal grains of CGO.
In some embodiments the grains are surrounded by a continuous layer, i.e. without interruption, of doped metal oxide that was previously infiltrated with the at least one element.
In some further embodiments the electrode further comprises particles of the at least one element and/or particles of an oxide of the at least one element onto the surface of the grains of the doped metal oxide.
For example, the electrode may have a structure characterized by the presence of a continuous layer of praseodymium doped gadolinia doped ceria (CGPO) surrounding gadolinia doped ceria (CGO) grains.
The electrode is characterized by the presence of a doped cerium oxide infiltrated by praseodymium ions, producing, following the sintering step, a thin layer, e.g. of a thickness between 0.1 and 200 nm, of praseodymium doped CGO (CGPO) onto grains of CGO.
Thus, the electrode may comprise gadolinia doped ceria (CGO) surrounded by a continuous layer of praseodymium doped gadolinia doped ceria (CGPO).
In some embodiments the electrode may further comprise praseodymium oxide particles onto the continuous layer of CGPO.
The presence of praseodymium oxide particles is further advantageous as it may enhance the performance of the electrode, e.g. the polarization resistance, Rp, may be between 30-80% lower than the Rp of an electrode with un-modified CGO at a T between 600° C. and 800° C.
In some embodiments the electrode may comprise praseodymium doped gadolinia doped ceria (CGPO) grains having a concentration gradient of praseodymium within said grains.
In general, the electrode obtained with the proposed new method possesses high performance, long term stability, and high tolerance to heat treatment. Due to the impregnation step prior to firing, electronic conductivity and catalytic activity specifically on the surface of the CGO backbone are enhanced, leading to lower electrode polarization resistance.
The electrode obtained with the above described method achieves the best results regarding the suppression of a degradation of electrochemical performance. By comparison, pre-mixing CGO and a Pr oxide via conventional routes to form the electrode comprising a Pr-doped CGO backbone shows lower performance and less advantageous results. Further, Pr-doped CGO (CGPO) has a very high thermal expansion coefficient causing difficulties in getting CGPO to adhere to other substrates that usually have a lower thermal expansion coefficient. Thermal expansion mismatch usually results in mechanical failures which increase Rp. Thus, conventional procedure leads to lower performance, i.e. higher Rp.
The first, second and other aspects and embodiments of the present invention may each be combined with any of the other aspects or embodiments. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
The method and the electrode according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.
a is a schematic drawing of the steps of the method according to one aspect of the invention.
b is a schematic cross section showing grains modified by the method according to one of the aspect of the invention.
a presents a Dark-field scanning transmission electron microscopy (STEM) image where the CGO grains according to some embodiments of the invention, with Pr and LSC impregnations are represented in the image as bright regions.
b shows X-ray Kα peak intensities for the elements La, Pr and Ce from a STEM-EDS (energy dispersive X-ray spectroscopy) line scan in the 10 points indicated in
a is a schematic drawing of the steps of the method according to one aspect of the invention.
A part 1 of an electrochemical device is shown comprising an electrolyte, such as a solid electrolyte 2, and doped metal oxide grains, such as CGO grains 3.
The grains 3 supported by the electrolyte 2 are soaked in a solution, such as an aqueous solution 4 containing praseodymium salt. The precalcination step where heat 28 is applied leads to the formation of particles 5 containing praseodymium.
The last step is the sintering step where heat 29 is applied to the part 1 leading the praseodymium ions to diffuse into the CGO grain forming a continuous layer 7 of praseodymium doped gadolinia doped ceria surrounding gadolinia doped ceria grains 3.
In some embodiments, following the sintering step, praseodymium oxide particles 6 may be present onto the continuous layer 7.
b is a magnified schematic cross section of two grains 3 of CGO surrounded by a continuous layer 7 of CGPO.
In
The electrocatalyst 8 may be an electronic or mixed ionic and electronic conducting (MIEC) electrocatalyst such as Sr doped La CoO3 (LSC) or Sr/Co doped La FeO3 (LSCF).
In
Points 14 connected by a dashed line represent Rp as a function of LSC firing temperature for electrodes comprising CGPO prepared by conventional routes, i.e. by solid state synthesis.
Points 13 connected by a dashed line represent Rp as a function of LSC firing temperature for electrodes comprising CGO with no further co-doping.
Points 12 connected by a dashed line represent Rp as a function of LSC firing temperature for electrodes according to some embodiments of the invention comprising CGO that has been infiltrated with Pr and has been fired at 1000° C.
It can be easily seen that the performance of the electrodes produced according to the method of the invention is improved with respect to an un-doped CGO, i.e. points 13, or CGPO produced via conventional routes, i.e. points 14. In particular it can be noticed that the Rp is 35% lower for the electrode according to the invention than the un-modified CGO at Tmax=600° C.
Dashed line 15 represents a possible sealing temperature to which the cell will be exposed to when part of an SOFC or SOEC stack.
Points 16 connected by a dashed line represent Rs as a function of LSC firing temperature for electrodes comprising CGPO prepared by conventional routes, i.e. by solid state synthesis.
Points 17 connected by a dashed line represent Rs as a function of LSC firing temperature for electrodes according to some embodiments of the invention comprising CGO that has been infiltrated with Pr and has been fired at 1000° C.
Points 18 connected by a dashed line represent Rs as a function of LSC firing temperature for electrodes comprising CGO with no further co-doping.
The cathode or oxygen electrode 27 according to the invention has a high electronic conductivity, a high ionic conductivity, and a fast oxygen surface exchange characteristics and thus minimize internal losses, i.e. resistance and provide a stable electrode suitable for long term operation.
In order to compare the performance of electrodes produced through the method of the invention and by conventional routes, such as solid state reactions, some samples were prepared, tested and reported below in example 1 and 2.
Ce0.9Gd0.09Pr0.0102−δ (CGPO-sss) was prepared via a conventional route, i.e. through a solid state reaction, by weighing precise amounts of CeO2 (99.9%, Johnson Matthey Germany), Gd203 (99.9%, Alfa Aesar Germany), and Pr6011 (99.9%, Alfa Aesar Germany) powders. The powders were mixed by ball milling with ethanol for 48 hours. The resulting mixture was dried and calcined at 1400° C. for 5 hours in air with heating and cooling rates of 2° C. min−1. The resulting powder was ball milled in ethanol for 45 hours to break the agglomerates and obtain a more uniform particle size distribution. A screen printing ink of CGPO is prepared by adding the corresponding powder into a mixture of terpineol (Aldrich), dispersant (Solsperse 3000, Lubrizol), dibutyl phthalate (Merck), and ethyl cellulose (Fluka) in 50:9:2:1:0.075 weight ratio, respectively. The CGPO-sss backbone was prepared by screen printing the CGPO ink onto both sides of a 5×5 cm2, dense 290 μm thick Ce0.9Gd0.101.95 (CGO) (KERAFOL) and sintering at 1150° C. for 2 hours in air.
In the infiltration route, an in-house CGO ink was screen printed onto both sides of a 5×5 cm2, dense 180 or 290 μm thick CGO (KERAFOL). The samples were then fired at 1150° C. or 1250° C. for 2 h in air. To introduce Pr ions into the backbone, the infiltration method was employed. In this study, two concentrations of Pr precursor solutions were investigated, i.e. 0.013 M and 1 M.
During the process of Pr infiltration, Pr nitrate is introduced into a CGO backbone. If multiple infiltrations are desired, the infiltrated backbone is precalcined at 350° C. before each infiltration. As a last step, the backbone is fired at 1000° C.
All the backbones are further infiltrated six times with LSC and fired at varying firing temperatures (Tmax). To investigate the performance with varying infiltrate firing temperature, the infiltrated cathodes were heated in-situ in an electrochemical setup at temperatures ranging from 300-900° C. A temperature program providing a desired temperature profile was used to ensure that an impedance spectrum from each measuring temperature is obtained as samples are heated and cooled progressively in the rig. Initially, the samples are heated to a maximum firing temperature, i.e. a Tmax up to 900° C., and then they are cooled to 300° C. where an impedance measurement is performed. The samples are then heated further, carrying out impedance measurements at each step. Impedance spectra from each measuring temperature are compared from their corresponding Tmax. In order to ensure sufficient current collection, Pt paste was painted onto both sides of the symmetric cells. It has previously been established that the Pt paste used for current collection showed inferior electrochemical performance compared to that of standard perovskite based SOFC cathodes. Electrochemical impedance spectroscopy (EIS) was carried out with a frequency range from 0.07 Hz to 100 kHz or with a frequency range from 0.06 Hz to 1 MHz under open circuit conditions and a 50 mV amplitude AC signal.
The EIS plane plots of symmetric cathodes composed of various backbones sintered at 1150° C. and infiltrated six times with LSC have been analysed. The trend at varying Tmax shows increasing polarization resistance, Rp. The Rp values plotted against LSC firing temperatures are summarized in
In the case of Rs, as shown in
Another batch of samples was tested. This time employing a CGO backbone sintered at 1250° C. with and without Pr. Further, the concentration of Pr (0.0132 M) was significantly lower than the first case. The CGO backbone with Pr was also preheated at 1000° C. prior to LSC infiltration like in the first case. Both samples were infiltrated six times with LSC and fired at varying temperatures. The Rp in both samples increases with increasing LSC firing temperature. However, it seen from the sample with Pr that the increase in Rp, at Tmax=900° C. was not as significant compared to the sample without Pr. This result was also seen in the abovementioned discussion. Regarding Rs the sample without Pr shows increasing Rs with increasing LSC firing temperature while the sample with Pr shows almost no change in Rs.
In general conventionally produced CGPO backbones infiltrated with LSC, e.g. as described in U.S. Pat. No. 5,670,270, results in electrodes having inferior performance when compared to the one produced according to the method of the invention. Differently from the conventional production of CGPO, the method of the invention introduces an infiltration step prior the sintering and the infiltration of the electrocatalyst, allowing for diffusion of an element, such as a lanthanide into the grains of doped metal oxides. Thus produces a structure as described in example 4.
The method in the present invention can also be applied to fabricate a fuel electrode, e.g. an anode in a solid oxide fuel cell. A CGO backbone can be fabricated as described in Examples 1 and 2. This backbone can consequently be infiltrated with a 1 molar WO3 precursor such as ammonium metatungstate. The infiltrated structure can be heated to 650° C. in air to decompose the precursor and form WO3. A final heating at temperatures≧1000° C. can be performed to allow W to diffuse into the CGO grains. This modified backbone can then be infiltrated with an electrocatalyst such as a Ni-CGO mixture.
A sample previously tested for electrochemical performance was prepared for transmission electron microscopy (TEM) by casting the sample in epoxy for stability and successively thinning to a thickness of ca. 100 nm with a focused ion beam (FIB) by using a Zeiss Crossbeam 1540xb. Transmission and dark field scanning transmission electron microscopy (STEM) was carried out by using a JEOL 3000F and equipped with a STEM unit and a high angle annular dark field (HAADF) detector. The microscope was operated at 300 kV and with a nominal probe size of 1 nm. For compositional analysis, energy dispersive spectroscopy (EDS) was carried out by using an Oxford Instruments EDS detector.
a presents a Dark-field STEM image where the CGO grains with Pr and LSC impregnations are represented in the image as bright regions. The dark regions in the image represent epoxy filling the sample pores. For the 10 points indicated in
b also presents a La signal at the CGO grain edge. In several scans a signal of La is observed at the CGO grain edges. Often the La signal is stronger closer to the pores than the Pr signal indicating that the LSC is situated at the CGO grain surfaces. For a few CGO grains, no Pr was observed at the edge.
Example 4 thus confirms the core-shell type structure CGPO-CGO produced as shown in
Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12189207.9 | Oct 2012 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/DK2013/050319 | 10/10/2013 | WO | 00 |
