Abstract:
A method for producing a fuel cell, including applying a porous, non-densified electrolyte layer over an anode substrate, sintering the electrolyte layer to the anode substrate, and applying a porous catalytic anode layer onto the electrolyte and spaced from the anode layer to define a fuel cell to produce a non-gastight fuel cell. Typically, the fuel cell is annealed and substantially porous.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of co-pending U.S. patent application Ser. No. 10/961,680, filed Oct. 8, 2004. 
     
    
     TECHNICAL FIELD  
       [0002]     The present novel technology relates generally to the field of electrochemical power generation, and, more specifically, to the solid oxide fuel cell systems.  
       BACKGROUND  
       [0003]     A fuel cell is a power generating electrochemical device that reacts chemical fuel with an oxidant to produce an electrical potential. A conventional fuel cell consists of two electrodes positioned around an electrolyte that serves to physically separate the chemical reactants (i.e., the fuel and the oxidant) from one another. The fuel may be hydrogen or a hydrocarbon (such as methane or propane) and the oxidant is typically oxygen. In the conventional fuel cell, oxygen flows over one electrode and hydrogen over the other. The reaction of hydrogen and oxygen generates electricity, water and heat. In the conventional fuel cell, hydrogen fuel is fed to the anode and oxygen is fed to the cathode. At the anode, atomic hydrogen is ionized to produce protons and electrons. The protons are conducted through the electrolyte, which is typically an ionic conductor and an electrical insulator (i.e., the electrolyte is characterized by a very high resistance to the flow of electrons.) The electrons therefore must travel around the electrolyte to the cathode and can thus be directed through a load to produce useful work. At the cathode, protons that have migrated through the electrolyte are combined with oxygen and electrons to balance the charges and produce water.  
         [0004]     Since fuel cells operate on the principles of electrochemistry rather than thermal combustion to produce power, fuel cells enjoy higher operating temperatures and greater energy conversion efficiencies. Further, fuel cell systems produce substantially less and much cleaner emissions than do known fuel combustion engines. However, although cleaner and more efficient than combustion, fuel cell technology is much newer and less commonplace than combustion technology, and is accordingly more expensive to support. While fuel cells are attractive for a myriad of reasons, including low pollution, high efficiency, low noise and increased power density, the first hurdle to be overcome in the expansion of fuel cell technology is the development of more cost competitive (i.e., cheaper) fuel cell hardware and support systems that can compete with conventional combustion-based power-generating engines on the basis of cost, weight and volume.  
         [0005]     Another kind of fuel cell design, suggested decades ago by Van Gool but only recently given any real attention, is the single chamber fuel cell design. The single chamber solid oxide fuel cell (SC-SOFC) utilizes surface migration of fuel and oxygen over the electrolyte to accommodate a mixture of reactants (i.e., a single mixture of fuel and oxidant.) While such a mixture of reactants experiences a thermodynamic driving force urging reaction, reactants may be chosen with sufficiently high activation barriers, slow reaction kinetics at room temperature, or the like, that the reaction effectively does not begin until the reactants are fed to the fuel cell electrodes. Another advantage of the SC-SOFC design is that use of mixed reactants obviates the requirement of bulky and heavy manifolding and gastight sealing for the separate supply of fuel and oxidants. Thus, the system may be simplified and lightened at the same time.  
         [0006]     Densified porous, gas permeable materials have been used for the electrolytes, as they effectively increase the surface area, and thus the available migration paths, over which migration takes place. This allows for increased migration of the fuel and oxidant species. However, SC-SOFC designs are still limited by the diffusion rate of the reactant gasses in the mixture.  
         [0007]     Further, by selectively choosing the electrode materials, a reduction reaction can be promoted at the cathode and an oxidation reaction at the anode, whilst the degree of parasitic reaction in the reactant mixture is negligible. The known SC-SOFC designs generally suffer the same disadvantages of the conventional fuel cells, and further are characterized by lower fuel efficiency and open cell voltages (usually due to parasitic fuel-oxidant reactions). With conventional electrode materials, the efficiency of mixed reactant fuel cells tend to be inferior to that of a conventional system in which the fuel and oxidant are maintained in separate feeds. However, other performance measures such as cost and power density may be significantly enhanced. A concern with mixed reactant fuel cells is that certain reactant mixtures have an inherent risk of uncontrolled catastrophic reaction (i.e., explosion.) However, this risk may be minimized with proper handling of the mixture, since the reactants do not necessarily combine simply because the reaction product is thermodynamically more stable.  
         [0008]     Another limitation of known fuel cells is that electrochemical reaction only occurs at an interface between three phases. In other words, electrochemical reaction is limited to sites on the catalyst where reactant and electrolyte meet together. This latter problem is not only a limitation in mixed reactant fuel cells, but is also a disadvantage of conventional fuel cells.  
         [0009]     Thus, there exists a need for a SC-SOFC that enjoys increased fuel efficiency and higher voltage outputs than may be currently achieved. The present novel technology addresses this need.  
       SUMMARY  
       [0010]     One aspect of the present novel technology relates to a SC-SOFC design that incorporates a porous, non-densified yttria-stabilized zirconia (YSZ) electrolyte material and porous NiO-YSZ and (La 0.8 Sr 0.2 )(Fe 0.8 Co 0.2 )O 3  catalytic electrodes along with an optimizes linear flow rate of the fuel-oxidant gas mixture over the electrolyte to maximize the diffusion rate of the fuel and oxidant through the cell to increase the SC-SOFC&#39;s fuel efficiency and open cell voltage output.  
         [0011]     One object of the present novel technology is to provide an improved fuel cell system. Related objects and advantages of the present novel technology will be apparent from the following description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1A  is a perspective view of a first embodiment single chamber solid oxide fuel cell device of the present novel technology.  
         [0013]      FIG. 1B  is a perspective view of an alternate embodiment single chamber solid oxide fuel cell device of the present novel technology.  
         [0014]      FIG. 2  is a 10,000× magnification SEM photomicrograph of a fracture surface of a porous YSZ electrolyte of the present novel technology.  
         [0015]      FIG. 3A  graphically displays I-V discharge profile and power density curves of the fuel cell of  FIG. 1B  at 556 degrees Celsius.  
         [0016]      FIG. 3B  graphically displays I-V discharge profile and power density curves of the fuel cell of  FIG. 1B  at 606 degrees Celsius for relatively low fuel gas flow rates.  
         [0017]      FIG. 4  is a schematic illustration of the surface migration at the electrodes and electrolyte of the fuel cell of  FIG. 1A .  
         [0018]      FIG. 5A  graphically displays of the impedance spectra of the fuel cell of  FIG. 1B  at 556 degrees Celsius.  
         [0019]      FIG. 5B  graphically displays of the impedance spectra of the fuel cell of  FIG. 1B  at 606 degrees Celsius for relatively low fuel gas flow rates.  
         [0020]      FIG. 6  graphically displays the area of specific resistance (ASR) of the fuel cell of  FIG. 1B  at 556 and 606 degrees Celsius.  
         [0021]      FIG. 7A  graphically displays I-V discharge profile and power density curves of the fuel cell of  FIG. 1B  at 606 degrees Celsius for relatively high fuel gas flow rates.  
         [0022]      FIG. 7B  graphically displays of the impedance spectra of the fuel cell of  FIG. 1B  at 606 degrees Celsius for relatively high fuel gas flow rates.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     For the purposes of promoting an understanding of the principles of the novel technology and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, with such alterations and further modifications in the illustrated device and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates.  
         [0024]     The present novel technology comprises multi-layer electrolyte/electrode compositions that are used as electrochemical power generation media (i.e., fuel cells.)  
         [0025]     One embodiment of this novel technology is shown in  FIG. 1A  as fuel cell system  10 . Fuel cell system  10  includes a fuel cell device  12 , which comprises an electrolyte substrate layer  14 , an anode layer  16  and a cathode layer  18 . A fluidic fuel/oxidant mixture  20  is flowed over the fuel cell device  12  at a predetermined flow rate. Oxygen  24  diffuses through the cathode  18  and migrates along the electrolyte  14  towards the anode  16 . Hydrocarbon fuel  21  is broken down into hydrogen  22  and carbonaceous molecules  23  at the anode  16 , and the hydrogen  22  migrates from the anode  16  along the electrolyte  14  towards the cathode  18 . The hydrogen  22  and carbonaceous molecules  23  are oxidized at the electrolyte  14  or near the anode  16 . Since the anode  16  has a higher catalytic activity for the oxidation of fuel  21  than the cathode  18 , an oxygen activity difference exists between the two electrodes  16 ,  18  and an electromotive force (EMF) is generated. Since the oxygen activity difference occurs locally in the region of the electrodes  16 ,  18 , the oxygen potential may be sustained without the requirement of separation of the fuel  21  and oxidant  24  fluids. Thus, the requirement of gas- or fluid-tightness of the fuel cell system  10  is avoided. In an alternative embodiment (shown as  FIG. 1B ), the electrodes  16 ,  18  are positioned on opposite sides of the electrolyte layer  14 .  
         [0026]     The electrolyte layer  14  is preferably characterized as electrically conducting primarily via an ionic mechanism. In other words, the electrolyte  14  is primarily an ionic conductor. The electrolyte layer  14  is also preferably porous. Commonly used materials for the electrolyte layer  14  include zirconia (ZrO 2 ), doped zirconia, yttria (Y 2 O 3 ) stabilized zirconia (YSZ), ceria (CeO 2 ), doped ceria, lanthanum-strontium galleate ((La, Sr)GaO 3 ) derivatives, and the like. YSZ is commercially available, and is typically found with yttria concentrations of about 8 atomic or molecular percent (the balance being zirconia.) Preferable dopants for ceria include Gd, Sm, and the like. Dopants for zirconia include yttrium, yttria, and the like. Alternately, the electrolyte layer  14  may conduct via other mechanisms. For example, the electrolyte layer  14  could conduct electricity primarily via a protonic mechanism. Examples of protonically conducting electrolyte  14  materials include Ba(Y)CeO 3  and Sr(Y)CeO 3 .  
         [0027]     The anode layer  16  is preferably porous as well. Commonly used materials for the anode  16  layer include platinum (Pt), palladium (Pd), cobalt (Co), nickel (Ni) (either individually, in combination, as oxides, or as combinations of oxides), metal oxide-YSZ compounds (MO-YSZ), or the like. One preferred range of anode compositions includes at least about 30 volume percent nickel or nickel oxide with the balance being zirconia, ceria, YSZ or the like (translating roughly to at least about 80 weight percent Ni or NiO.) This limitation arises from the connectivity requirement of nickel atoms arising from percolation of the nickel. If coated so as to yield better (electrical) connectivity, the minimum compositional requirement of the nickel/nickel oxide drops to about five percent.  
         [0028]     The cathode layer  18  is also preferably porous. Commonly used cathode layer  18  materials include compounds formed of oxides of elements from the lanthanide series (the lanthanides) and the transition metals. One preferred cathode layer  18  composition is an oxide of the form ABO 3 , wherein A is a lanthanide, such as La, and B is a transition metal such as Sr, Ca, Mg or Ba. Another preferred compositional range is La 1-x Sr x Q y Z 1-y O 3 , wherein Q is Mn, Co, or Mg, and Z is Cu or Fe, 0≦x≦0.8, and 0≦y≦1.  
         [0029]     In one embodiment, the electrolyte  14  is preferably formed onto an already fabricated anode layer  16 , since the anode layer  16  is typically thicker and more structurally sound. Typical anode thickness is about 1 mm or less. The electrolyte layer  14  may be formed by any convenient process, such as by screen-printing precursor ink onto the anode substrate  16 , and subsequently sintered to produce a porous electrolyte layer  14 . Preferably, the cathode layer  18  is of comparable thickness and is likewise positioned on the electrolyte  14 , such as by screen-printing. The cathode layer  18  is then preferably annealed. More preferably, the anode layer  16  and cathode layer  18  are porous. The anode  16  and cathode  18  may be separated by any convenient distances (for example 0.5, 1, 2 or more millimeters). In the case of the electrolyte layer  14  being positioned between the anode  16  and cathode  18  layers, the separation distance between the anode  16  and cathode  18  layers will be equal to the thickness of the electrolyte layer, which may be quite small if the electrode layer  14  is formed as a thin film.  
         [0030]     Once produced, the fuel cell device  12  may be incorporated into a fuel cell system  10  including a heat source (such as a tube furnace or the like) to maintain the fuel cell  12  above a predetermined minimum temperature. A fuel/oxidant gas mixture  20  is flowed over the fuel cell device  12 . The fuel portion  22  of the mixture  20  may be a hydrocarbon, such as methane, butane, propane, or the like, or may alternately be any appropriate combustible fluid. The oxidant portion  24  of the mixture  20  is typically air, oxygen, or an oxygen-rich gaseous composition, although other oxidizing fluids may be substituted. When the mixture  20  is flowing over the heated fuel cell device  12 , power may be extracted from the fuel cell system  10  (such as through connected current collectors (for example, Pt, Pd, Au or Ag mesh) attached to the area of the electrode  16 ,  18 .)  
         [0031]     Preferably, gas flow controllers (not shown) or the like are used to maintain the gas mixture  20  flow at a predetermined optimal rate, such as between about 300 and about 900 cubic centimeters per minute, to yield an optimum linear velocity over the fuel cell device  12 . (As used herein, linear velocity is defined as ‘gas flow rate/gas flow cross sectional area’.) The gas flow rate is a function of the geometry of the heat source (such as the diameter of a tube furnace, if a tube furnace is the heat source.) Typically, the gas velocity is maintained between from about 40 to about 120 centimeters per second. The temperature of the fuel cell device  12  is preferably maintained above about 300 degrees Celsius, more preferably above about 550 degrees Celsius, and even more preferably above about 600 degrees Celsius (as measured without gas flowing over the cell  12 , or as a set temperature, T s ; gas flowing over the fuel cell  12  has a cooling effect, which is generally more than offset by internal heat production when the cell  12  is operating.)  
         [0032]     In one typical fuel cell system  10  produced and operated as described above, the cell  12  temperatures were measured using a thermocouple directly placed on the cell  12 . Impedance spectroscopy techniques were utilized to investigate the cell  12  performance using a Solartron 1470 Battery Tester and 1255B Impedance Gain Phase Analyzer with a 4-probe configuration. The impedance spectra were obtained using a 1 mA load. The cell  12  measurements were conducted over 36 hours and showed reproducible results. The microstructure of the cell  12  was characterized by scanning electron microscopy (Hitachi S4700) and is illustrated in  FIG. 2 .  FIG. 2  shows fracture surface SEM images of the porous electrolyte  14 . The thickness of the electrolyte was about 18 μm. Well-connected open channels were observed in the electrolyte  14 , which allow gas permeation and diffusion therethrough.  
         [0033]      FIGS. 3A and 3B  show the discharge profile of the cell  12  with different linear velocities of gas flow at 556 and 606 degrees Celsius (set temperatures), respectively. The porous electrolyte  14  provided a sufficient barrier to separate the oxygen activities at the electrodes  16 ,  18  by optimizing gas flow. Open circuit voltages (OCV) of the porous electrolyte fuel cell  12  were measured in the range of 0.68 to 0.78 volts and were dependant upon the linear velocity of the gas mixture  20 . Comparably, the OCV for a densified 2 μm thick YSZ electrolyte cell was measured at about 0.8 V when run under identical same conditions.  
         [0034]     As shown in  FIG. 4 , H 2  or/and CO can diffuse from anode  16  to cathode  18  through the porous electrolyte  14  and as a result, oxygen  24  can be consumed at the cathode  18  and, accordingly, lower the oxygen partial pressure. Lowered oxygen partial pressure yields a decrease in the available OCV. This is borne out by an observed increase of the OCV as the linear velocity of the flowing gas mixture  20  is increased. The increase of linear velocity improves interfacial gas diffusion between the gas phase  20  and an electrode  16 ,  18  and results in increased catalytic activity at the electrode  16 ,  18 , thereby increasing the OCV.  
         [0035]     The cell  12  temperatures showed a strong dependence on the linear velocity of the flowing gas mixture  20 . While the cell  12  temperature in air  24  (without fuel  22 , and thus without the generation of electrochemical energy) decreased with increasing gas flow linear velocity (due to cooling from gas flow), the cell  12  temperature in air-fuel mixture  20  increased due to an increase of catalytic activity in the anode  16 . Note that this is one of advantages of SC-SOFC and certainly contributes to the performance of the system  10  at relatively low operating temperature.  
         [0036]     A maximum power density of about 0.66 watts per square centimeter was obtained from the system  10  at a temperature of 744 degrees Celsius (set temperature of 606 degrees Celsius) with a measured current density of 1.5 amps per square centimeter and a cell  12  voltage of 0.44 volts at 120 centimeters per second gas flow linear velocity. The performance of the cell  12  is dependent both upon the linear velocity of the gas mixture  20  over the cell  12  and the set temperature. Due to cooling from the gas flow, a degradation of cell performance with decreasing cell temperature occurs at excessively high gas flow linear velocities.  
         [0037]     Impedance spectra for the porous electrolyte SC-SOFC system  10  are shown in  FIGS. 5A and 5B . Relatively large electrode overpotentials exist (as compared to high frequency electrolyte resistances) due to gas diffusion and charge transfer effects. The overpotential resistances and the electrolyte resistances decrease as linear velocity of the flowing gas mixture  20  increases. As shown in  FIGS. 5A and 5B , an increase of cell  12  temperature is considered the main reason for the high efficiency of the cell  12  performance. The effect of gas mixture  20  linear velocity was also observed, i.e., higher linear velocities yielded improved cell efficiency for a given cell temperature. An increase of linear velocity typically results in an improvement of gas diffusion in the vicinity of electrodes and, therefore, overpotential resistances are reduced and catalytic activity of the anode  16  is increased, resulting in an increased cell temperature.  
         [0038]      FIG. 6  shows the area specific resistances (ASR) for the electrolyte  14  and overpotential as a function of cell temperature. As can be seen, the overpotential ASR showed a dependence on linear velocity with lower ASR being obtained for higher linear velocity, which resulted in better performance of the cell  12 . It is also seen that the ASRs of the porous electrolyte  14  were relatively low. It is inferred that the cell  12  is characterized by increased ionic conductivity in porous electrolyte  14  due to the surface migration effect.  
         [0039]     The electrolyte  14  resistance had little effect on the performance of the cell, with the performance being limited by the electrode overpotential. Thus, a SC-SOFC may be produced having a porous electrolyte  14 , which opens the opportunities to design both thermally and mechanically more robust cell designs operated on hydrocarbon fuels.  
         [0040]      FIG. 7A  shows the I-V discharge profile of the anode  16  supported porous YSZ electrolyte cell  12  in the air-fuel (air/methane) mixture  20  at 606° C. furnace temperature at higher gas flow rates. The anode  16  and the cathode  18  are exposed to the same gas mixture  20 . An open circuit voltage was measured as high as 0.75 V and the maximum power density of 0.66 W/cm 2  was measured at 900 cubic centimeters per minute gas flow rate.  FIG. 7B  shows the impedance spectra corresponding to the results shown in  FIG. 7A  for higher gas flow rates.  
       EXAMPLES  
     Example 1  
       [0041]     A fuel cell  12  was prepared by screen printing an electrolyte layer  14  having a thickness of 18 μm and having a composition of YSZ (approximately 16 mole percent yttria and the balance substantially zirconia) onto a 0.7 mm thick NiO-YSZ (80 weight percent NiO and 20 weight percent YSZ) substrate and sintered at 1400 degrees Celsius for one hour. A cathode layer  18  of La 0.8 Sr 0.2 Co 0.2 Fe 0.8 O 3  (LSCF) was screen printed onto the now-porous electrolyte  14  and annealed at 1000° C. for one hour. The fuel cell device  12  was then heated to 556 degrees Celsius and a fuel/oxidant mixture  20  (17 volume percent methane and 83 volume percent air) was flowed thereover. Pt and Au mesh were used as current collectors with the size adjusted to the area of the cathodes, which was 0.18 cm 2 . Gas flow controllers maintained the gas flow between 300˜900 cm 3  min −1 , which gave a linear velocity (gas flow rate/gas flow cross section area where cell was placed) over the fuel cell device  12  of from 40-120 centimeters per second (cm s −1 .)  
       Example 2  
       [0042]     A fuel cell  12  was prepared by screen printing an electrolyte layer  14  having a thickness of 18 μm and having a composition of YSZ (approximately 16 mole percent yttria and the balance substantially zirconia) onto a 0.7 mm thick NiO-YSZ (80 weight percent NiO and 20 weight percent YSZ) substrate and sintered at 1400 degrees Celsius for one hour. A cathode layer  18  of La 0.8 Sr 0.2 Co 0.2 Fe 0.8 O 3  (LSCF) was screen printed onto the now-porous electrolyte  14  and annealed at 1000° C. for one hour. The fuel cell device  12  was then heated to 606 degrees Celsius and a fuel/oxidant mixture  20  (17 volume percent methane and 83 volume percent air) was flowed thereover. Pt and Au mesh were used as current collectors with the size adjusted to the area of the cathodes, which was 0.18 cm 2 . Gas flow controllers maintained the gas flow between 300-900 cm 3  min −1 , which gave a linear velocity (gas flow rate/gas flow cross section area where cell was placed) over the fuel cell device  12  of from 40˜120 centimeters per second (cm s −1 .) A maximum power density of about 0.66 W cm −2  was obtained at a cell temperature of 744° C. (set temperature=606° C.) with a current density of 1.5 A cm −2  and cell voltage of 0.44 V at 120 cm s −1  linear velocity.  
       Example 3  
       [0043]     A fuel cell  12  having a 20 μm thick Gd-doped CeO 2  electrolyte layer  14  with a 0.8 mm thick Ni—ZrO 2  (35 volume percent Ni with the balance substantially zirconia) anode  16  and a La 0.8 Sr 0.2 Cu 0.8 Mg 0.2 O 3  cathode layer  18 . The fuel cell was annealed at 1100° C. until porous. The fuel cell device  12  was then heated to at least about 300 degrees Celsius and a fuel/oxidant mixture  20  (25 volume percent butane and 75 volume percent air) was flowed thereover. Pt and Ag mesh were used as current collectors with the size adjusted to the area of the cathodes, which was about 0.25 cm 2 . The gas mixture  20  was maintained at a velocity of about 80 centimeters per second (cm s −1 .)  
       Example 4  
       [0044]     A fuel cell  12  having a 15 μm thick porous ionically conducting doped CeO 2  electrolyte layer  14  with a 0.9 mm thick porous CoO 2  anode  16  and an acceptor doped LaMnO 3  cathode layer  18 . The fuel cell was annealed at 1100° C. and a porous electrolyte  14  microstructure was obtained. The fuel cell device  12  was then heated to at least about 700 degrees Celsius and a fuel/oxidant mixture  20  (15 volume percent propane and 85 volume percent oxygen rich nitrogen) was flowed thereover. Pd and Au mesh were used as current collectors with the size adjusted to the area of the cathode, which was about 0.20 cm 2 . The gas mixture  20  was maintained at a velocity of between about 50 and about 100 centimeters per second (cm s −1 .)  
         [0045]     While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.