High power density solid oxide fuel cell having a graded anode

Details High power density solid oxide fuel cell having a graded anode High power density solid oxide fuel cell having a graded anode

1998-12-08

2001-05-08

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S047000, C502S101000

Reexamination Certificate

active

06228521

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved high power density solid oxide fuel cell which uses graded density for the anode, while still incorporating a relatively thick anode. More specifically, an anode of nickel and YSZ (zirconium oxide doped with yttrium oxide) is fabricated such that a major layer initially has about 80 percent by volume of NiO and a minor layer initially has about 60 percent by volume of NiO. The region containing about 80 percent by volume has a greater porosity which allows for easier transport of reactive gases. The invention permits the use of thicker and thus stronger anodes without sacrificing electrochemical performance. This invention makes it possible to achieve a high power density and at the same time have a thick electrode for durability and mechanical reliability greater than is observed in the fuel cells of the art.
2. Description of Related Art
A goal of current fuel cell research and technology is to provide a high power density and at the same time provide for improved durability and mechanical reliability.
Some patents of interest are as follows:
B. S. Baker in U.S. Pat. No. 4,329,403 discloses an electrolyte-electrode assembly for a fuel cell. The electrolyte component is adapted to exhibit a gradual transition in the coefficient of thermal expansion going from the anode of the inner electrolytic region and in going from the cathode region to the inner electrolyte region.
A. C. Khandkar in U.S. Pat. No. 5,171,645 discloses a graded electrolyte of zirconia and bismuth oxide. The strength and reduction resistance of zirconia with the high ionic conductivity of bismuth oxide form a superior oxygen ion-conducting electrolyte which is especially useful in fuel cells.
A. V. Virkar, et al. in U.S. Pat. No. 5,543,239 disclose an improved electrode design by incorporation of a porous layer of electrolyte material over the dense electrolyte creating an enhanced three phase (TPB) length. This design allows for fuel cells to have improved performance at lower operating temperatures.
H. L. Tuller, et al. in U.S. Pat. No. 5,509,189 disclose an electrochemical device which includes a solid electrolyte and solid electrode composed of materials having different chemical compositions and characterized by different electrical properties but having the same crystalline phase.
None of these U. S. patents teach or suggest the present invention.
All articles, patents, applications, references, standards and the like cited herein are incorporated by reference in their entirety.
It is desirable to have a high power density solid oxide fuel cell which has superior power densities and at the same time are more durable and mechanically reliable. The present invention provides such an improved fuel cell.
SUMMARY OF THE INVENTION
In one aspect, the present invention relates to an improved method to produce a high power density solid oxide fuel cell having a graded anode, which method comprises:
(a) obtaining a powder mixture of nickel oxide and YSZ, said YSZ having a composition of between about 5 and 15 mole percent of Y
2
O
3
doped ZrO
2
, wherein the ratio of nickel oxide to YSZ is between about 70 to 90 volume percent of the total volume of powder;
(b) creating a pellet of the mixture of step (a) by consolidation, including but not limited to die pressing at a pressure between about 20 and 300 MPa at ambient temperature;
(c) producing a powder mixture of nickel oxide and YSZ, said powder mixture having between about 50 and 65 volume percent of nickel oxide to the total mixture;
(d) producing a slurry of the powder mixture of step (c) with water, a C1 to C4 alcohol, a volatile chlorinated hydrocarbon, or mixtures thereof in a weight to volume ratio corresponding to between about 0.5 to 3 g of powder mixture to between about 30-50 ml of alcohol, optionally subjecting the obtained slurry to ultrasonic mixing;
(e) contacting the pellet of step (b) with the slurry of step (d) followed by evaporation of the alcohol wherein the coated pellet is isostatically pressed at between about 150 and 250 MPa and ambient temperature to produce a graded pellet;
(f) depositing a layer of YSZ on the surface of the graded pellet of step (e);
(g) sintering the pellet of step (f) in air at between about 1100 and 1600° C. for between about 0.5 and 3 hr producing densification of the pellet in three layers, said layers comprising YSZ electrolyte, NiO plus YSZ inner anode layer and NiO plus YSZ outer anode and support, wherein said YSZ electrolyte layer has porosity of less than 4% by volume, said NiO plus YSZ inner anode layer has a porosity of less than about 10% by volume, and said NiO plus YSZ outer anode layer and support has a porosity of less than about 10% by volume;
(h) obtaining a powdered LSM by calcining a three component mixture, e.g. MnO
2
, La
2
O
3,
and SrCO
3,
at between about 800 and 1200° C. for between about 6 to 10 hr;
(i) obtaining and calcining a powdered YSZ at between 1000 and 1300° C. for between about 0.5 and 5 hr;
(j) producing a powder mixture of 50 weight percent LSM and 50 weight percent YSZ which is contacted with an organic liquid having a boiling point of less than 200° C. to produce a paste;
(k) successively coating the pellet of step (g) with the paste of step (j) followed by heating at between about 350 and 500° C. for between about 0.5 and 5 hr to produce a layer of between about 40 and 100 micrometers in thickness;
(l) heating the coated pellet of step (k) at between about 1000 and 1400° C. for between about 0.5 hr and 5 hr to create the unreduced fuel cell;
(m) contacting the unreduced fuel cell of step (l) at between about 500° C. and 1000° C. with a mixture of water vapor and hydrogen in a ratio of between 1% and 10% volume percent water vapor for between about 0.5 and 5 hr; and
(n) producing a reduced fuel cell wherein the thicker anode of about 70 to 90 volume percent nickel oxide has an open porosity of between about 30 and 40 volume percent, and the thinner anode of about 50 and 65 volume percent of nickel oxide has a porosity of between about 20 to 29.9 volume percent.
The consolidation of the powder to create the pellet in step (b) may occur by die-pressing tape casting, slip-casting, electrophoretic deposition, injection-moldy and the like.
In another aspect, the present invention relates to the method wherein
in step (a), nickel oxide is present in about 80 volume percent;
in step (b), the pressure is between about 50 and 100 MPa;
in step (c), the nickel oxide is about 60 volume present;
in step (d), the alcohol is ethanol or methanol;
in step (e), the isostatic pressure is between about 175 and 225 MPa;
in step (g), the sintering is performed at between about 1200 and 1500° C. for about 1 hr;
in step (h), the temperature is between about 900 and 1100° C. for about 8 hrs;
in step (i), the temperature is maintained about 1200° C. for about 1 hr;
in step (k), the temperature is between about 400 and 450° C.;
in step (l), the temperature is between about 1100 and 1300° C. and the time is about 1 hr; and
in step (m), the temperature is between about 600 and 900° C.
In another aspect, the present invention relates to the method wherein
in step (a), nickel oxide is present in about 80 volume percent;
in step (b), the pressure is about 60 MPa;
in step (c), the nickel oxide is about 60 volume present;
in step (d), the alcohol is ethanol;
in step (e), the isostatic pressure is about 200 MPa;
in step (g), the sintering is performed at between about 1400° C. for about 1 hr;
in step (h), the temperature is about 1000° C. for about 8 hrs;
in step (i), the temperature is maintained about 1200° C. for about 1 hr;
in step (k), the temperature is about 400° C.;
in step (l), the temperature is about 1250° C. and the time is about 1 hr; and
in step (m), the temperature is between about 650 and 900° C.
In another embodiment of the present invention is an improved fuel cell wherein the anode when reduced in water vapor and hydrogen has a graded porosity, wherein one layer adjacent to the electrolyte

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