High performance cathode with controlled operating temperature range

Information

  • Patent Application
  • 20080081243
  • Publication Number
    20080081243
  • Date Filed
    October 02, 2006
    18 years ago
  • Date Published
    April 03, 2008
    16 years ago
Abstract
In a solid oxide fuel cell having an anode, a cathode, and a dense electrolyte disposed between the anode and the cathode, the cathode having a ceramic-ionic conducting phase of a plurality of ionic conducting particles and a metallic phase of a plurality of metallic particles. The metallic phase includes a metal alloy having an oxide-to-metal transition temperature in the range of about 600° C. to about 800° C. With this cathode, solid oxide fuel cell operating temperatures as low as about 600° C. may be possible.
Description

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein:



FIG. 1 is a diagram showing a thermogravimetric (TG) analysis of palladium powder at various oxygen partial pressures;



FIG. 2 is a diagram showing the metal oxide-to-metal transition temperature as a function of Pd content of Pd/Ag alloys;



FIG. 3 is a diagram showing cell voltage versus time at 700° C. with 4A current steps at two (2) minute intervals;



FIG. 4 is a diagram showing cell voltage versus time at 650° C. with 4A current steps at two (2) minute intervals;



FIG. 5 is a diagram showing cell voltage versus time at 600° C. with 4A current steps at two (2) minute intervals;



FIG. 6 is a diagram showing current-voltage curves at temperatures in the range of about 600° C. to about 700° C.;



FIG. 7 is a diagram showing the steady state voltage and current for a solid oxide fuel cell having a Pd-YSZ cathode operating at a temperature of about 700° C.;



FIG. 8 is a diagram showing current-voltage curves for a cell at temperatures in the range of about 650° C. to about 800° C. with a Pg/Ag cathode (70:30% w/w in alloy powder);



FIG. 9 is a schematic representation of a cross-sectional view of a cathode in accordance with one embodiment of this invention; and



FIG. 10 is a scanning electron micrograph of a solid oxide fuel cell in accordance with one embodiment of this invention.





DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The invention disclosed herein is a solid oxide fuel cell and solid oxide fuel cell stack operating at a temperature in the range of about 600° C. to about 800° C.


The invention disclosed herein is a composite electrode for a solid oxide fuel cell, which electrode, by virtue of its composition, enables operation of the solid oxide fuel cell and solid oxide fuel cell stack at high performance levels at temperatures as low as about 600° C.


The solid oxide fuel cell, as shown in FIG. 10, comprises a dense electrolyte layer 112 sandwiched between an anode electrode layer 113 and a cathode electrode layer 114. The cathode layer 10, as depicted in FIG. 9, shown bonded to an electrolyte 11, comprises a ceramic-ionic conducting phase comprising a plurality of ionic conducting particles 16 and a metallic phase comprising a plurality of metallic particles 14. The ionic conducting particles combine to form ionic conducting paths (I) from the electrolyte 12 to the electrochemically active sites 18. The metallic phase forms electronic conducting paths (E) through the electrode 10 to a contact paste (not shown) and cathode electronic conducting strip (not shown). The electrochemically active area coincides with a three phase boundary 18 which extends along the common boundary of the gaseous pore phase, the ceramic phase 16 and the metallic phase 14.


The metallic phase, in accordance with one embodiment of this invention, comprises at least one of a metal and a metal alloy having an oxide-to-metal transition temperature in a range of about 600° C. to about 800° C. In accordance with one embodiment of this invention, the metal or metal alloy comprises a noble metal. In accordance with one preferred embodiment of this invention, the noble metal is selected from the group consisting of silver (Ag), gold (Au), iridium (Ir), osmium (Os), palladium (Pd), ruthenium (Ru), rhodium (Rh) and platinum (Pt). In accordance with one particularly preferred embodiment of this invention, the noble metal is palladium.


More particularly, the invention disclosed herein is a cathode electrode comprising an alloyed metal and YSZ or other ceramic oxide ionic conductor such as gadolinia-doped ceria as the key components whereby the alloyed metal has a lower oxide-to-metal transition temperature at 0.21 atm pO2 than the metal components individually, in the range of about 600° C. to about 800° C. This includes Pd/Ag alloys where the Pd content is greater than about 50% by weight of the alloy, Pd/Au alloys where the Pd content is greater than about 70% by weight of the alloy, Pd/Pt alloys where the Pd content is greater than about 70% by weight of the alloy, and Pd/Cr alloys or Pd/Nb alloys wherein the Pd content is greater than about 80% by weight of the alloy.


As previously indicated, the electrode of this invention is formed by mixing ceramic ionic conducting particles and metallic electrocatalyst particles into a composite electrode which is then applied to a dense electrolyte substrate. In accordance with one embodiment of this invention, the ionic conducting particles comprise ceramic particles which may be yttria stabilized zirconia and the metallic electrocatalyst particles are particles of a metal alloy comprising at least one noble metal. In accordance with one embodiment, the mixture comprises metallic particles ranging in size from about 1-2 microns in diameter and 8 mol % yttria stabilized zirconia (8YSZ) particles ranging in size from about 0.1 to about 0.3 microns in diameter. The preferred microstructure in accordance with one embodiment of this invention comprises about 1-10 vol % Pd and 40-80 vol % YSZ balanced with about 20-50 vol % porosity. It should be noted that all references herein to volume percentages of the electrocatalyst phase and the ionic conducting phase are of the volume of the entire solid phase.


The cathode layer in accordance with one embodiment of this invention has a thickness of less than about 10 microns, preferably less than about 5 microns. It is screen-printed and co-sintered with a screen-printed electrolyte layer (8YSZ), screen-printed anode functional layer and tape-casted anode substrate. A layer of ionic conducting ceramic materials (perovskites) is printed on the other surface after co-firing. The perovskite particles enter the cathode porous structure during the printing process. The layer is then fired in-situ at the operating temperature of the tri-layer, about 600° C. to about 850° C. This layer provides electrical contact from the cathode to the bipolar plates of the fuel cell stack (interconnects).


Due to its materials composition, the majority of the cathode materials (YSZ) is the same as the electrolyte materials. During the co-firing, sufficient sintering between the cathode and electrolyte occurs, creating a strong interface that is less prone to mechanical and thermal mechanical failure during stack assembly and operation.



FIG. 1 is a diagram showing a thermogravimetric (TG) analysis of palladium powder at various oxygen partial pressures. As shown therein, as the cathode is polarized, the oxygen partial pressure falls in the cathode in accordance with the Nernst equation. Within the solid oxide fuel cell operating temperature range of about 725° C. to about 850° C., approximately every 50 mV polarization produces an order of magnitude lower oxygen partial pressure (effectively) in the cathode, although the exact value varies with temperature. FIG. 1 shows a significant lowering of the oxide-to-metal transition temperature (dashed lines) as pO2 is lowered over 2 orders of magnitude (approximately equivalent to about 100 mV cathode polarization). Based upon these results, it is reasonable to expect similar performance in a solid oxide fuel cell stack operation and it establishes a lower temperature limit of about 700° C. to about 725° C. required for metallic palladium.


We have discovered that even lower operating temperatures, in the range of about 600° C. to about 650° C., may be achieved by alloying the palladium or choosing other precious metal alloys for use as an electrocatalyst in the cathode. FIG. 2 shows that by alloying palladium with silver (Ag), the oxide-to-metal transition temperature can be significantly lowered. For an alloy comprising 70 wt % Pd and 30 wt % Ag, the oxide-to-metal transition temperature is reduced from 800° C. for Pd metal alone to about 650° C. for the alloy.


A single cell test was conducted using a cathode contact of perovskites and 10% v/v silver addition for the dual purpose of a sinter aid for the perovskite powder (silver has a low melting point of 962° C.) and potentially to modify the oxide transition temperature of palladium in the cathode, allowing the cell to operate at lower temperatures. FIGS. 3-5 show cell voltage stepped by 4A every two minutes at temperatures of 700, 650 and 600° C., respectively. The large increases in cell voltage with current increase is due to increases in overpotential at the cathode, lowering the pO2 and causing an oxide-to-metal transition which decreases overvoltage, thereby increasing cell voltage. Over time, oxide reforms, again lowering cell voltage, and the cycle is repeated at the next current load step. FIG. 6 shows current-voltage data for the temperature range of about 600° C. to about 700° C. These tests, together with TG measurements demonstrating a lowering of oxide transition temperature for a Pd/Ag alloy cathode, suggest that any method of lowering the oxide-to-metal transition temperature while maintaining cathode microstructure will result in an improved low temperature cathode, which, in turn, should lead to lower degradation rates due to operation at lower temperatures.



FIG. 7 shows a cell with a Pd-YSZ cathode operating at 100 A (1.23 A/cm2) and 700° C. As can be seen, cell voltage increased from about 591 mV to about 605 mV in about two days, and one week later was running at 607 mV with no degradation. Unexpectedly, the degradation rate at 100 A and 700° C. is lower than at 40.5 A (0.5 A/cm2) and 750° C. (not shown) despite operating at about 2.5 times the current density. This suggests that oxide transition is a limiting factor in reducing cell operation of a Pd cathode and that the oxide-to-metal transition temperature of Pd can be reduced to 700° C. by imposing >100 mV polarization at the cathode. At such high current density, the cathode polarization exceeds this value and, thus, the cell is stable. At this temperature and 0.5 A/cm2, the cell degrades rapidly.


Cell tests have shown reversible degradation below 700° C. due to the cathode. In one test, a cell was cycled between 650° C. and 750° C. at 0.5 A/cm2 and showed very high degradation at 650° C., but on returning to 750° C., the cell voltage was within 1 mV of the voltage previously at 750° C., indicating reversible degradation at 650° C. This is due to the palladium-to-palladium oxide reversible transition and data from this test is summarized in Table 1.









TABLE 1







Steady State












Voltage
Shunt
Time Elapsed
%/1000 hr

















Start 650° C.
0.646
40.50
N/A
N/A



End
0.597
40.50
96
79.0



Start 750° C.
0.855
40.50
N/A
N/A



End
0.835
40.50
24
97.5



Start 650° C.
0.648
40.50
N/A
N/A



End
0.600
40.50
48
154.3 



Start 750° C.
0.836
40.50
N/A
N/A



End
0.832
40.50
24
19.9



Start 650° C.
0.647
40.50
N/A
N/A



End
0.586
40.50
48
196.4 



Start 750° C.
0.831
40.50
N/A
N/A



End
0.827
40.50
24
20.1



Start 650° C.
0.641
40.50
N/A
N/A



End
0.576
40.50
48
211.3 



Start 750° C.
0.911
40.50
N/A
N/A



End
0.906
40.50
72
 7.6







Note:



Time is calculated in 24 hr increments







FIG. 8 shows VII curves for a cell with a Pd—Ag cathode (70:30% w/w in alloy powder). The cell has a voltage of 801 mV at 700° C. and 0.74 A/cm2. The cell became unstable when run long term at low temperature (650° C.), after which time powder curves were repeated and cell voltage dropped to 715 mV at the same condition, indicating that the electrode is unstable. But, when the alloy is stable and its oxide transition temperature is as low as for Pd—Ag (70:30), i.e. 650° C., low temperature cell performance is possible.


While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.

Claims
  • 1. In a solid oxide fuel cell having an anode, a cathode, and a dense electrolyte disposed between said anode and said cathode, the improvement comprising: said cathode comprising a ceramic-ionic conducting phase comprising a plurality of ionic conducting particles and a metallic phase comprising a plurality of metallic particles, said metallic phase comprising a metal alloy having an oxide-to-metal transition temperature in a range of about 600° C. to about 800° C.
  • 2. A solid oxide fuel cell in accordance with claim 1, wherein said metallic phase comprises at least one noble metal.
  • 3. A solid oxide fuel cell in accordance with claim 2, wherein said at least one noble metal is selected from the group consisting of Ag, Au, Ir, Os, Pd, Ru, Rh and Pt.
  • 4. A solid oxide fuel cell in accordance with claim 3, wherein said at least one noble metal is Pd.
  • 5. A solid oxide fuel cell in accordance with claim 4, wherein said Pd is alloyed with a metal selected from the group consisting of Au, Ag, Pt, Cr, Nb and mixtures and alloys thereof.
  • 6. A solid oxide fuel cell in accordance with claim 1, wherein said ionic conducting particles and said metallic particles are interspersed and the mean size of the metallic particles is one of substantially equal to and greater than the mean size of said ionic conducting particles.
  • 7. A solid oxide fuel cell in accordance with claim 1, wherein said cathode has a thickness less than about 10 μm.
  • 8. A solid oxide fuel cell in accordance with claim 4, wherein said Pd comprises one of more than and substantially equal to about 50% by weight of said metallic phase.
  • 9. A solid oxide fuel cell in accordance with claim 1, wherein said metal alloy has an oxide-to-metal transition at a pO2 equal to about 0.21 atm. within an operating temperature range of said solid oxide fuel cell.
  • 10. A solid oxide fuel cell comprising: a dense electrolyte disposed between an anode electrode and a cathode electrode, said cathode electrode comprising an ionic conducting phase having a plurality of ionic conducting particles interspersed with a metallic phase having a plurality of metallic particles, said metallic particles comprising a metal alloy having an oxide-to-metal transition temperature in a range of about 600° C. to about 800° C.
  • 11. A solid oxide fuel cell in accordance with claim 10, wherein said metallic phase comprises at least one noble metal.
  • 12. A solid oxide fuel cell in accordance with claim 11, wherein said at least one noble metal is selected from the group consisting of Ag, Au, Ir, Os, Pd, Ru, Rh and Pt.
  • 13. A solid oxide fuel cell in accordance with claim 12, wherein said at least one noble metal is Pd.
  • 14. A solid oxide fuel cell in accordance with claim 13, wherein said Pd is alloyed with a metal selected from the group consisting of Au, Ag, Pt, Cr, Nb, and mixtures and alloys thereof.
  • 15. A solid oxide fuel cell in accordance with claim 10, wherein the mean size of said metallic particles is one of substantially equal to and greater than the mean size of said ionic conducting particles.
  • 16. A solid oxide fuel cell in accordance with claim 14, wherein said Pd comprises one of greater than and substantially equal to about 50% by weight of said metallic particles.
  • 17. An electrode forming part of a solid oxide fuel cell comprising a dense electrolyte layer, said electrode comprising a porous three-dimensional solid phase comprising: an electrocatalytic phase comprising a plurality of metallic electrocatalytic particles comprising a metal alloy having an oxide-to-metal transition temperature in a range of about 600° C. to about 800° C.; andan ionic conducting phase comprising a plurality of ionic conducting particles.
  • 18. An electrode in accordance with claim 17, wherein said electrode is a cathode.
  • 19. An electrode in accordance with claim 17, wherein said electrocatalyst particles comprise at least one noble metal.
  • 20. An electrode in accordance with claim 19, wherein said at least one noble metal is Pd.
  • 21. An electrode in accordance with claim 20, wherein said Pd is alloyed with a metal selected from the group consisting of Au, Ag, Pt, Cr, Nb, and mixtures and alloys thereof.
  • 22. An electrode in accordance with claim 17, wherein a mean size of said electrocatalytic particles is one of greater than and substantially equal to a mean size of said ionic conducting particles.