Patent Application
20140287341
The present invention relates to a method of improving the performance of the fuel electrode in a solid oxide electrochemical cell. More specifically, the invention concerns a modified anode/electrolyte structure for a solid oxide electrochemical cell, and further the invention concerns a method for making said structure.
A solid oxide fuel cell (SOFC) is an electrochemical cell with an anode (fuel electrode) and a cathode separated by a dense oxide ion conductive electrolyte, said cell operating at high temperatures (800-1000° C.). The function of an anode in the solid oxide fuel cell is to react electrochemically with the fuel, which may be hydrogen and hydrocarbons, while the cathode reacts with air or oxygen to produce electric current. The anode of an SOFC comprises a catalytically active, conductive (for electrons and oxide ions) porous structure, which is deposited on the electrolyte.
The conventional SOFC anodes include a composite mixture of a metallic catalyst and a ceramic material, more specifically nickel and yttria-stabilized zirconium oxide (YSZ), respectively.
The anodes (fuel electrodes) must be capable of yielding a high performance in terms of high electrochemical activity and good redox stability to be employed in fuel cells such as SOFCs. Current state-of-the-art Ni—YSZ anodes provide a reasonable electrochemical activity at high operating temperatures, often above 800° C., but they are not redox stable. Any volume changes in Ni—YSZ anodes due to reduction and oxidation of Ni will result in inexpedient mechanical stresses in the anode material, which in turn will impair the overall performance of the fuel cell.
Over the recent years a number of efforts have been made to improve the functioning of SOFC anodes. For instance, the published US patent application No. 2009/0218311 describes the preparation of a catalyst with a layered structure in the electrode/electrolyte interface of a fuel cell. A plastic or glass substrate is used together with an electrolyte (such as YSZ), a catalyst layer (such as Ni or Pd) and a porous layer. However, the catalyst keeps its layered structure during the process, and thus it is not distributed.
US 2010/0075194 discloses a high performance, low cost cathode with low polarization resistance, which binds well to an electrolyte. This publication deals with an ion-conductive layer (doped cerium oxide) followed by a mixed ion-conductive and electron-conductive layer. Again, the catalyst remains in the layered structure and therefore does not become distributed.
US 2009/0148742 concerns high performance multilayer electrodes and i.a. mentions insertion of a cerium oxide based ion-conductive and electron-conductive layer in the interface between anode and electrolyte to improve the electrochemical performance of SOFC anodes.
In U.S. Pat. No. 6,420,064 it is described how a composite cathode containing mixed electron-conductive (Pd) and ion-conductive (YSZ) functional layers gets deposited on the electrolyte by e.g. screen printing. Then lanthanum cobaltite is printed over the functional layers, which are subsequently sintered in situ during operation of the SOFC.
US 2009/0011314 concerns an SOFC with reduced electrical resistance, which comprises an interfacial layer containing an ion-conductive material inserted between an electrode layer and an electrolyte layer. The ion-conductive material can i.a. be YSZ or GDC, preferably inserted by atomic layer deposition (ALD), and a catalytic metal, such as Pt, may be present.
Finally, US 2009/0061284 belonging to the present applicant describes that i.a. niobium-doped strontium titanate can be used as SOFC anode and impregnated with Ni and doped cerium oxide. The interface of the electrode/electrolyte was not modified in this instance, but the same niobium-doped strontium titanate as in the present invention was present.
The recent developments within high performance SOFC anodes have been focused on utilizing the redox stable electronically conductive perovskite oxides, such as niobium-doped strontium titanate (STN). While STN is stable under anode testing conditions and also compatible with the electrolyte, it lacks electrochemical catalytic activity for the hydrogen oxidation, and moreover, the ionic conductivity is insufficient for an efficient anode performance.
STN deposited on the electrolyte has a skeletal porous structure (termed “backbone” in the following), which is capable of holding the electrocatalyst. One of the recent trends within the development of anodes has been to incorporate a nanostructured electrocatalyst in the backbone by catalyst infiltration of one of the respective salts, such as nickel nitrate or nickel chloride. The electrocatalyst can be a metal, a ceramic material such as gadolinium-doped cerium oxide (CGO) or a mixture of both. In addition, CGO provides ionic conductivity in the backbone.
The present invention is based on the surprising finding that the performance of the STN backbone as an SOFC anode is dramatically improved, if thin metal layers (such as Ni, Pd and combinations thereof), ceramic layers (such as CGO, YSZ and combinations thereof) or both metal and ceramic layers are introduced in the interface of the backbone/electrode assembly (BEA), whereupon the finished assembly is heated to a high temperature, possibly to distribute the metal/ceramic functional interlayers in the backbone and into the BEA. Such distributed functional interlayers act as electrochemically active electrodes, and furthermore, infiltration of the electrocatalyst into the STN backbone improves the anode performance dramatically, as already mentioned.
More specifically, the present invention concerns a novel modified anode/electrolyte structure for a solid oxide electrochemical cell, said structure being an assembly comprising (a) an anode consisting of a backbone of electronically conductive perovskite oxides selected from the group of niobium-doped strontium titanate, vanadium-doped strontium titanate, tantalum-doped strontium titanate and mixtures thereof, (b) a scandia and yttria-stabilised zirconium oxide electrolyte and (c) a metallic and/or a ceramic electrocatalyst in the shape of interlayers incorporated in the interface between the anode and the electrolyte. This assembly is first subjected to sintering at a temperature of about 1200° C. in air, and then the sintered assembly is heated to about 1000° C. for up to 5 hours in H2/N2 in a separate furnace. These heat treatments result in the metallic and/or ceramic interlayers being distributed in the electrolyte/anode backbone junction.
Furthermore the invention concerns a process for the preparation of the inventive anode/electrolyte structure, said process comprising the steps of (a) depositing a ceramic interlayer onto one side of the electrolyte, (b) optionally applying a metallic interlayer thereon, (c) repeating steps (a) and (b), (d) applying a layer of the selected anode backbone onto the electrolyte with applied interlayers, (e) sintering the raw assembly by heating it to about 1200° C. in air, whereafter the sintered assembly is heated to about 1000° C. for up to 5 hrs in H2/N2, and (f) infiltrating the electrocatalyst precursor into the sintered assembly and further heat treating the sintered assembly at a temperature of about 350-650° C. in air to incorporate the electro-catalyst into the anode backbone.
The idea of depositing a metallic layer, a ceramic layer or a combination of both in the BEA interface to locate the catalyst in the required site for the electrochemical reaction of fuel constitutes a novel approach in designing SOFC anodes. It is also novel to utilize the well-known solution infiltration technique on a modified backbone, whereby the performance of the anode is surprisingly enhanced with the increase of loadings of the electrocatalyst.
Use of the conventional solution infiltration technique to incorporate an electrocatalyst in the STN backbone does not in itself ensure that the BEA interface is sufficiently covered or coated. Thus, no further decrease in the interfacial resistance is possible, even after increasing the loadings of the electrocatalyst. On the contrary, by the present invention the electrocatalysts are located either on the BEA interface or on sites which are favourable for the improved electrochemical reaction. This way the interfacial resistance is further decreased.
The metal-based functional layer (MFL) is preferably Pd, but other metals, such as Ni, Pt and Ru, are also conceivable. Furthermore, instead of single metals it is possible to use binary alloys of the above metals, such as Pd—Ni, or even ternary alloys, such as Pd—Ni—Ru. As to the ceramic-based functional layer (CFL), this is preferably gadolinium-doped cerium oxide (CGO), but it could also be e.g. samarium-doped cerium oxide.
Using the present invention it is possible to avoid the known blending of metal (such as Ni) and ceramics (such as YSZ) to form a composite anode. Further, the solution infiltration technique to incorporate an electrocatalyst in a perovskite-based anode is supplemented.
The present invention offers a number of advantages over the prior art technique, first of all lowering the interfacial resistance by several orders of magnitude compared with conventional anodes. The invention also provides a suitable way to lower the operating temperatures of solid oxide fuel cells (<600° C.). Furthermore, the process according to the invention, where thin metal or ceramic film layers are deposited on the electrolyte surface, makes it possible to increase the production speed considerably when making solid oxide fuel cells.
The invention will now be illustrated further by the following specific examples. Reference is also made to the accompanying
This example illustrates the method steps involved in the production of SOFC anodes according to the invention. The example is supported by
As electrolyte a tape of ScYSZ (scandia and yttria-stabilized zirconia) with a thickness of 120 μm was used. The backbone was niobium-doped strontium titanate (STN) placed upon the electrolyte to form a backbone/electrolyte assembly (BEA).
A functional layer was introduced in the BEA, i.e. between the backbone and the electrolyte. Said functional layer may be a metal-based functional layer (MFL), e.g. Pd in a layer thickness of 20-200 nm, or a ceramic-based functional layer (CFL), e.g. gadolinium-doped cerium oxide (CGO) in a layer thickness of 20-500 nm. The functional layer may also be a combination of a metal-based and a ceramic-based layer.
In practice, the functional layer is first applied to the electrolyte tape, which is done by sputtering (MFL) or spin coating (CFL). When a combined functional layer is used, the electrolyte tape is first spin coated with CGO and then sputtered with Pd. This is done on both sides of the electrolyte in case of symmetrical cells used for electrochemical electrode characterizations.
When the electrolyte has been provided with the intended functional layer(s), it is screen printed with STN ink, resulting in a layer, 18-20 μm thick, optionally on both sides of the electrolyte. The resulting “raw” assembly (
As the final process step, the electrocatalyst is infiltrated in the form of a precursor solution into the presintered backbone (
This example shows a few distinct Pd particles located in the interface of STN and ScYSZ electrolyte (
This example presents the performance results obtained with anodes, which have been prepared as described in Example 1, but without infiltration. As reference an anode without any functional layer was used.
The tested anodes are summarized in Table 1:
The results obtained (gas conditions: 97% H2, 3% H2O; temperature: 600° C.) are listed in Table 2 below.
The anode with no functional layer (anode Nos. 1) clearly shows the poorest performance, i.e. the highest interfacial resistance, of the anodes tested. The impedance spectra are shown in
In this example the performance results obtained with five anodes, which have been prepared as described in Example 1, i.e. including infiltration, are presented.
The tested anodes are summarized in Table 3 below.
The results obtained (gas conditions: 97% H2, 3% H2O; temperature: 600° C.) are illustrated in
Table 4 below is a summary of some of the favourable results obtained with anodes according to the invention compared to reference anodes with no functional layer. The first three anodes in the table are reference anodes, whereas the rest are anodes according to the invention.
The results shown in this example (
This example, shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2011 00810 | Oct 2011 | DK | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2012/070949 | 10/23/2012 | WO | 00 | 4/23/2014 |
