Patent Application
20080280189
This invention relates to the fields of electrochemical reactors and thermal spray deposition of materials. One embodiment of the invention provides methods for fabricating anodes suitable for use in solid oxide fuel cells.
Fuel cells convert chemical energy of suitable fuels into electrical energy without combustion and with little or no emission of pollutants. Fuel cells may be made on a wide variety of scales. Fuel cells can be used to generate electrical power in any of a wide variety of applications including powering vehicles, auxiliary power units (APUs) and cogeneration of power and heat for residential and business uses.
Solid Oxide Fuel Cells (SOFCs) are solid-state fuel cells that typically operate at high temperatures. SOFCs can be highly efficient. One application of SOFCs is in stationary power generation, including both large-scale central power generation, and distributed generation in individual homes and businesses. High operation temperatures produce fast reaction kinetics and high ionic conductivity, and therefore high efficiency, but also create technological problems related to materials design and cell processing.
Hydrogen can be used as a fuel by solid oxide fuel cells. Using hydrogen as a fuel has the benefits of no local emissions, relatively low degradation rates and fast electrochemical kinetics. However, hydrogen must be generated, compressed, and transported, all of which require energy. Thus hydrogen fuel can be more expensive than other fuels.
SOFCs can be made to consume carbon-containing fuels, such as coal gas, methanol, natural gas, gasoline, diesel fuel, and bio-fuels and can use carbon monoxide as a fuel, in addition to hydrocarbons and hydrogen. Hydrocarbon fuels, such as methane, are typically converted through a process known as steam reforming to CO and H2, which are then consumed electrochemically within the fuel cell. The reforming reaction can be performed outside of the fuel cell in a reformer. Reforming fuel outside of the fuel cell increases the overall cost and complexity of the system. In a high temperature SOFC system, fuel can be reformed within the fuel cell. A reforming catalyst, commonly nickel, may be provided in the SOFC, typically in the SOFC anode to assist the reforming reactions. This procedure is known as internal reforming. Internal reforming processes are described in J. Larminie, A. Dicks, Fuel Cell Systems Explained, Wiley, Chichester, 2000, pp. 190-197, for example.
Internal reforming eliminates the requirement for an external reformer and therefore simplifies the balance of plant system and reduces costs. In addition to reduced costs, internal reforming is endothermic for some fuels, such as methane, and can therefore assist in thermal management of the cell.
Internal reforming is limited in practice by technological issues. One issue is that internal reforming can result in carbon deposition on fuel cell anodes. Carbon deposition reduces the anode performance by blocking the reaction sites, and consequently, reduces the efficiency of the fuel cell. Also, some reforming processes require very high temperatures. For example, the equilibrium conversion of methane for a CH4/H2O ratio of one at 1 bar is only 37% at 600° C., 68% at 700° C., and 87% at 800° C. If reforming is to be performed internally in an SOFC, the high temperature requirement for equilibrium conversion limits the choice of materials that can be used to construct the fuel cell. 700° C. is at the working limit for many common metals. Another issue is that internal reforming processes can give rise to significant thermal gradients.
Direct oxidation of hydrocarbon (HC) fuels may alleviate some disadvantages of internal reforming. Fuel cells that directly oxidize hydrocarbons are described in R. J. Gorte, H. Kim, J. M. Vohs, Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbon, Journal of Power Sources 106 (2002), 10-15. However, when HC fuel is directly utilized on conventional nickel-based fuel cell anodes, carbon deposited on the anode material due to a secondary cracking reaction blocks the reactants from reaching the reaction sites over time, and dramatically reduces the fuel cell performance and stability. Previous studies show that nickel can be utilized in direct oxidation of methane at temperatures between about 500° C. and 700° C. without significant carbon formation. It is unlikely that this could be achieved with higher hydrocarbons since the temperature window for pyrolysis will be lower and carbon formation more severe.
Some studies have suggested the use of copper as an alternative to nickel as the electronic conductor in SOFC anodes. Copper has high electrical conductivity and relatively low catalytic activity for hydrocarbon cracking. However, copper also has a low catalytic activity for hydrogen or hydrocarbon electrochemical oxidation. To improve cell performance, copper-containing fuel cell anodes have been made with ceria and samaria doped ceria in place of yttria stabilized zirconia (YSZ). Carbon deposition was not observed using this anode design. Ceria provides improved catalytic activity and mixed ionic-electronic conductivity, which increases reaction surface area in comparison to YSZ. However, these anodes are manufactured in a multi-step wet ceramic technique that is even more undesirably complicated and expensive than the multi-step techniques used to make nickel-YSZ anodes.
A variety of processing techniques have been suggested for the manufacturing of SOFC components. In high performance SOFCs, it is desirable to provide a thin electrolyte, typically on the order of about 5 mm to 10 mm thick. A thin electrolyte tends to reduce ohmic losses. In anode-supported planar SOFCs, the cathode layer is usually also fairly thin (20-40 mm), while a thicker anode (0.5-3 mm) is used as the mechanical support layer of the cell. Making an SOFC having thin electrode and electrolyte layers comprising ceramic materials having high melting temperatures typically requires a complex multi-step process.
SOFC processing typically includes a combination of wet powder compaction steps such as tape casting or extrusion, followed by deposition by a chemical or physical process such as spray pyrolysis, screen printing, or electrochemical vapor deposition, and then densification at elevated temperatures. The nature of the multi-step wet ceramic manufacturing procedures makes control over the electrode microstructure and material composition difficult. Processing of copper-based SOFC anodes is even more challenging, because copper oxides cannot be sintered together with the YSZ or ceria based electrolyte due to the large differences in melting temperatures between the copper and the ceramic material. R. J. Gorte, H. Kim, J. M. Vohs, Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbon, Journal of Power Sources 106 (2002), 10-15 describe making copper-based SOFC anodes by impregnating a copper salt into a pre-sintered porous YSZ matrix. This method is also used for processing of Cu—Co based anodes.
The complex multi-step processing procedures are time consuming and involve significant capital costs, particularly when scaled up for mass production.
The inventors have recognized a need for cost-efficient methods for making electrodes, such as anodes for solid oxide fuel cells, and for improved electrode structures, particularly, improved structures for anodes for solid oxide fuel cells.
The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools, and methods which are meant to be exemplary and illustrative, not limiting in scope.
One aspect of the invention provides a method for making an electrodes. The method comprises thermal spraying onto a substrate a mixture comprising a copper-containing material and a second material having a melting temperature greater than a melting temperature of the copper-containing material to provide a coating on the substrate.
Another aspect of the invention provides methods for making electrodes. In some embodiments, the electrodes have application as anodes in solid oxide fuel cells. The method comprises providing a mixture comprising a first powder and a second powder and, thermal spraying the mixture onto a substrate. The first powder comprises a copper-containing material and the second powder is a powder comprising a second material having a melting temperature that is greater than a melting temperature of the copper-containing material.
Another aspect of the invention provides methods for forming porous copper-containing coatings on substrates. The methods comprise providing a mixture of a first powder comprising the copper in an oxidized state with a second powder comprising a ceramic material, plasma spraying the mixture onto a substrate and subsequently reducing the copper to metallic copper in situ.
Another aspect of the invention provides an anode for a fuel cell comprising a plurality of layers. The layers each comprise a mixture of a crystalline copper metal phase and a crystalline ceramic phase. The layers have differing compositions.
Further aspects of the invention and features of embodiments of the invention are set out below or will become apparent by reference to the drawings and by study of the following detailed descriptions.
Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.
Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
One aspect of this invention provides methods for making electrode structures which involve thermal spray deposition of a copper-containing material together with a ceramic material. The thermal spray deposition may comprise plasma spraying. Plasma spraying has the advantage of short processing time, material composition flexibility, and a wide range of controllable spraying parameters that can be used to adjust the properties of deposited coatings. Spraying and feedstock parameters may be controlled during spraying to optimize the characteristics of the deposited materials.
In block 23 method 20 provides a mixture 48 of a copper-containing material and a ceramic.
In block 24 the mixture of a copper-containing material and a ceramic are applied to the substrate by thermal spraying. The thermal spraying could comprise high velocity oxy-fuel (HVOF) spraying or plasma spraying, for example. In a preferred embodiment, the thermal spraying comprises plasma spraying. The plasma spraying may be performed, for example, using an axial injection plasma spraying system 42. In the embodiment illustrated in
Copper-containing material 49A may comprise, for example:
Powdered ceramic material 49B may comprise, for example:
Mixture 48 may optionally comprise a material that functions as a pore former. Some examples of pore formers are:
The particles of mixture 48 may optionally be fed into the plasma as a suspension in a suitable liquid. The liquid may be water, ethanol, mixtures of those, or other suitable liquids. The concentration of solids in the suspension may be 1-10 weight percent of solid in liquid in some embodiments. Other concentrations may also be used.
Where copper-containing material 49A comprises a copper oxide and it is desired that the structure being made comprises copper metal then the copper oxide may be reduced in situ after the plasma co-deposition has been performed. In
The methods described herein may be applied, for example, to make
In some embodiments, an electrode structure is formed in a series of layers each having differing properties. In such embodiments, the composition of the electrode varies with depth. For example, in some embodiments, an SOFC anode has higher ceramic content near its interface with the electrolyte, and higher metal content near the surface for better current collection. In some embodiments, the metal content exceeds 40% or 50% near the surface of the anode. In some embodiments, the properties of the deposited material are caused to vary with position. Improved ability to control and vary the microstructure and material composition across the electrode may lead to better performance and reduced thermal stresses resulting from thermal expansion coefficient (CTE) mismatch, and thus increase cell efficiency and durability.
Electrode structures according to some embodiments of the invention are characterized by one or more of the following features:
The substrate may be selected from a variety of suitable materials. For example, the substrate could comprise:
In an example embodiment a YSZ (Tosoh, 8 mol % Y2O3) substrate was made by ball-milling a mixture of 60 wt % YSZ powder, 12 wt % Ethyl Alcohol, 12 wt % Toluene, 5 wt % PVB, and 7 wt % Butyl benzyl phthalate for several hours. After ball-milling the mixture was tape cast. The tape was cut and sintered at 1400° C. to produce a dense electrolyte support.
In an example embodiment, a copper-SDC SOFC anode was made by co-depositing copper oxide and SDC (Ce0.8Sm0.2O1.9) on a one-inch circular YSZ substrate using an axial injection plasma torch. The resulting anode was subsequently reduced to Cu-SDC and then tested electrochemically in a double-anode symmetrical fuel cell.
Samaria doped ceria (Ce0.8Sm0.2O1.9) was synthesized by mixing cerium carbonate and samarium acetate (obtained from Inframat Advanced Materials, Connecticut, USA). The mixture was ball milled with 40 wt % ethanol for 48 hours. The ball milled mixture was then calcined at 1500° C. for 6 hrs.
Particle size analysis was conducted using a wet dispersion optical particle size analyzer (Malvern Mastersizer 2000™).
YSZ (yttria stabilized zirconia) substrates were prepared by pressing 4 g YSZ powder (available from Inframat Advanced Materials) into pellets with a 32 mm die. The pellets were sintered to substrates at 1400° C. for 4 hrs. The sintered YSZ substrates were sand blasted prior to spraying to create a coarse surface in order to allow better adhesion of the coating to the surface. After sand blasting, the surfaces were cleaned with acetone to remove any residue.
CuO and SDC powders were co-deposited to form a coating on the substrates. In one test, CuO powder (Inframat Advanced Materials, particle size d0.5=9 μm) and SDC powder (synthesized from pre-cursors and sieved to a particle size range of +32-75 μm) were mixed in a weight ratio of 1:1.
The dry mixed powders were plasma sprayed from a single hopper onto an electrolyte support utilizing a Mettech Axial III™ axial injection torch (available from Northwest Mettech Corp. of North Vancouver, Canada). The YSZ substrates were mounted onto a turntable to allow cooling of the substrate during the spraying by contact with the air during the turntable rotation. Table 1 shows the spraying and feedstock conditions for all coatings produced during this experiment.
Table 2 shows the spraying and feedstock parameters used for the plasma spraying. With the apparatus used in this experiment plasma gas flow rate, plasma gas composition, and gun current are independently controlled. Gun power is dependent on other settings. In each case the plasma gas was a mixture of 50% nitrogen and 50% argon.
The sprayed samples were cut and polished. The coating was imaged with a scanning electron microscope to study the porosity and uniformity of the microstructure.
In another experimental example embodiment, spray-dried SDC and CuO powders (available from Inframat Advanced Materials) were co-deposited by plasma spraying. Particles in a spray-dried powder tend to have spherical shapes that tend to reduce stratification of powders being fed together in a plasma spray system. The powder particles used in this experiment are agglomerates of nano-powder. SDC powder (Ce0.8Sm0.2O1.9) from Inframat Advanced Materials, particle size +45-75 μm, and CuO powder from Inframat Advanced Materials, particle size +45-75 μm were mechanically mixed in a weight ratio of 1.5 g SDC to 1 g of CuO.
The mixture was then plasma sprayed onto a YSZ substrate. Tables 3 and 4 show the plasma and feedstock conditions and spraying parameters that were utilized for the co-deposition of spray dried CuO and SDC.
Visual observation of the YSZ substrates revealed that the YSZ substrates tended to break during the spraying, presumably due to thermal shock. This problem was ameliorated by improving the cooling of the YSZ substrate during the spraying by improving the contact of the substrate holder with the cooling air. SEM imaging of the coating was performed to determine the porosity and uniformity of the microstructure. EDX imagining was performed to determine the relative amounts of CuO and SDC in the coating.
The relative amounts of Cu and SDC in the coatings of this Example #2 and of Example #3 below were calculated (Table 5). Both materials were present in all the coatings, but the relative amounts of each phase changed as a function of the spraying conditions. The relative deposition efficiency of CuO in the CuO-SDC coating was also calculated for the different spraying conditions. The initial volume of CuO in the CuO-SDC powder mixtures was 42.93%. The relative deposition efficiency was calculated as the ratio between the relative volume of CuO in the CuO-SDC coatings and the relative volume of CuO in the CuO-SDC powders. Table 5 also shows the calculated relative volume of Cu in the solid phase of Cu-SDC coatings after full reduction of the deposited CuO in the coatings to Cu.
The as-deposited coatings were then treated to reduce the CuO to copper.
CuO-SDC coatings were applied to substrates and then processed to reduce the CuO to copper. CuO and SDC powders were mechanically mixed with a weight ratio of 0.667. The powders were then sprayed on stainless steel coupons using the feedstock and spraying conditions in Table 3. Table 6 shows the spraying parameters utilized for the reduction studies of the coatings.
The coatings were reduced after deposition in dry hydrogen at 700° C. for 5 hours. X-ray diffraction and energy-dispersive X-ray analysis were conducted to determine the phases and elemental composition of the materials in the coating after the reduction.
Another test co-deposited CuO and SDC with spraying distances smaller than 150 mm. Particle sizes of both CuO and SDC were adjusted to improve the coating microstructures. The particle size of the SDC powder was decreased to allow better melting in lower plasma energy conditions, and thus to allow its deposition onto a YSZ substrate without breaking the substrate due to thermal shock. It was found that the CuO particles melt completely and form large continuous splats in even the lowest energy plasmas used for spraying. In some tests, smaller CuO particles (having diameters of approximately 25 μm) were used. The smaller particles allow more fine scale mixing of the CuO splats with the SDC in the coating, resulting in a better microstructure for use as an anode. In addition, the plasma gas flow rate was decreased to allow a higher residence time of the particles in the plasma. Higher residence time increases the particle temperature, and allows better melting in lower energy plasmas.
The conditions utilized in this test were found to produce porous well-mixed coatings. These conditions were used to deposit symmetrical concentric anodes on both sides of YSZ electrolyte substrates using a custom made mask. Tables 7 and 8 show, respectively, the spraying and feedstock conditions and the spraying parameters that were utilized for these tests.
The coating was reduced in H2 at 700° C. for 5 hours. SEM imaging of the coating was performed to determine the porosity and uniformity of the microstructure. Symmetrical cell testing was performed using an SOFC test station (AMEL, Italy) and an FRA and potentiostat (Solartron™ 1260 and 1470E, UK) after in-situ reduction of the anodes at 569° C. in hydrogen. Additional symmetrical cells and anode coatings were reduced in H2 at 700° C. for 5 hrs. EDX measurements were conducted on the reduced cells to confirm that a sufficient volume fraction of Cu was present in the coatings for full percolation of the Cu phase. The test station design includes a thermocouple that measures the temperature close to the cell. Table 9 shows the furnace temperature profile and atmospheres used in testing the symmetrical cells.
In Sample 16, the CuO particle size was decreased to reduce the size of the splats of the highly melted CuO particles and improve the extent of mixing with the SDC to improve the microstructure. SDC particle size was decreased to allow the coatings to be sprayed with a lower plasma power and to produce coatings on YSZ substrates without breaking them due to thermal shock. The plasma gas velocity was reduced to allow higher residence times of the particles in the flame and therefore better melting of the SDC particles. The decrease also reduces the particle velocity upon impacting the substrate, and thus can help to reduce the breaking of the SDC agglomerates upon impact, and thereby improve the microstructure by maintaining a more uniform particle size of the CuO and SDC in the final coating. The spraying distance was reduced to allow a more homogenous coating. Decreased spraying distance reduces the chances of re-solidification of the particles during flight before impacting the substrate.
Impedance spectroscopy was conducted at cell temperatures of 569° C., 620° C., 672° C., 723° C., and 772° C., using the testing conditions shown in Table 10. The measurements were repeated several times at each temperature.
Producing Cu-SDC anodes by plasma spraying allows a much faster method of producing direct oxidation SOFC anodes than is currently possible using wet ceramic techniques involving infiltration of a porous sintered pre-form. The technique developed allows CuO and SDC to be co-deposited by plasma spraying, despite the very large high melting temperature difference between the two materials. Control of the anode microstructure is possible during the deposition process by adjusting the spraying conditions and particle size distributions of the starting powders. CuO-SDC coatings with well mixed, porous microstructures demonstrate acceptable performance as anodes, even at fairly low temperatures and despite the low catalytic activity of copper. Further optimization of the microstructure of the coatings, together with the incorporation of additional materials with a higher catalytic activity, such as cobalt, can further improve the performance of the composite anode coatings for use in solid oxide fuel cells that can operate on multiple fuels.
While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.
This application claims the benefit under 35 U.S.C. §119 of U.S. application No. 60/730,380 filed on 27 Oct. 2005, which is hereby incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA2006/001770 | 10/27/2006 | WO | 00 | 4/28/2008 |
| Number | Date | Country | |
|---|---|---|---|
| 60730380 | Oct 2005 | US |
