Solid oxide fuel cell cathode materials

Abstract

A cathode in a solid oxide fuel cell containing AgPrCoO.sub.3. The operating temperature range of the cathode is from about 400° C. to about 850° C.

Claims

1. A composite cathode in a solid oxide fuel cell comprising: AgPrCoO.sub.3; and Gd.sub.0.1Ce.sub.0.9O.sub.2, wherein the operating temperature range of the cathode is from about 400° C. to about 800° C., wherein the weight ratio of AgPrCoO.sub.3 to Gd.sub.0.1Ce.sub.0.9O.sub.2 is 60:40.

2. The composite cathode of claim 1, wherein the solid oxide fuel cell utilizes a doped ceria barrier layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

(2) FIG. 1a depicts the thermogravimetric analysis (TGA) results of the state-of-the-art conventional SOFC cathode material Sm.sub.0.5Sr.sub.0.5. FIG. 1a depicts CoO.sub.3—Gd.sub.0.1Ce.sub.0.9O.sub.2 and Ag.sub.0.1. FIG. 1b shows Pr.sub.0.9CoO.sub.3—Gd.sub.0.1Ce.sub.0.9O.sub.2.

(3) FIG. 2a depicts the current voltage and current power density curves of a fuel cell with Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC cathode tested in hydrogen at 650° C. FIG. 2b is an enlarged section of FIG. 2a near 0.8 V.

(4) FIG. 3 depicts fuel cell performance at 0.8V and 650° C. for Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC cathode directly applied on YSZ electrolyte and SSC-GDC cathode with a GDC barrier layer. Cathode feed air contained 1.6% CO.sub.2.

DETAILED DESCRIPTION

(5) Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

(6) As briefly introduced above, the present embodiment provides a cathode material AgPrCoO.sub.3. The operating temperature range of the cathode is from about 400° C. to about 850° C. It is theorized that this new material when mixed with gadolinium doped ceria (GDC) exhibits superior mixed ionic and electronic conductivities, partially by overcoming stability issues of other cathode materials. It is also theorized that AgPrCoO.sub.3 (APC) show excellent long-term stability in CO.sub.2 containing environments. In one embodiment, use of AgPrCoO.sub.3 as a cathode material eliminates the use of barrier layers such as gadolinium doped ceria which has the ability to significantly reduce the material and fabrication costs of SOFCs.

(7) In one embodiment, AgPrCoO.sub.3 is made from Ag doping PrCoO.sub.3. This produces Ag doping levels of Ag.sub.xPr.sub.1-xxCoO.sub.3, x=0.05-0.15. In one non-limiting embodiment, the doping of PrCoO.sub.3 can be done by first dissolving metal nitrate hydrates with stoichiometric ratio in deionized water. Citric acid (CA) was added as a chelating agent with a CA-to-nitrate-ion molar ratio of around 1:2. Appropriate amount of ammonia water was then added to adjust the PH to ˜6. The resulting clear solution was heated at 90° C. for a prolonged period until a clear gel was formed. The gel was placed in an oven overnight at 150° C. to form a foam. The foam was then grinded and calcined at 800° C. for around 5 hours.

(8) For cathode ink preparation, Ag doped PrCoO.sub.3 were mixed with GDC powder in a weight ratio of 60:40. The composite cathode powder was further mixed with ink vehicle (Fuel cell materials) in a weight ratio of 60:40. The mixture was milled in a high energy ball mill at 350 rpm for 1 hour to form the cathode ink. The cathode ink was applied onto fuel cells with a cathode area of 12.25 cm.sup.2.

(9) Sample Preparation:

(10) Two types of baseline cells with yttria-stabilized zirconia (YSZ) electrolyte were produced: Type-1: NiO+YSZ anode/YSZ electrolyte/GDC barrier layer/APC-GDC cathode Type-2: NiO+YSZ anode/YSZ electrolyte/APC-GDC cathode

(11) Type-1 cells had a GDC hairier layer and Type-2 cells didn't contain a GDC barrier layer between cathode and electrolyte layers.

(12) The cathode was sintered at 900° C. or 950° C. for 2 hours, at a 2° C./min ramp rate. All fuel cells were held at 800° C. overnight in hydrogen before electrochemical testing. The fuel cell performance was evaluated between 500 to 750° C., and the impedance curves were taken at 650° C. under open circuit condition.

(13) Type-1 Cells Evaluation

(14) Table 1 below shows a summary of fuel cell performance with different cathode materials at 0.8V and 650° C. or 700° C. Based on Type-1 fuel cells, the Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC cathode showed the highest performance, which was higher than that of conventional Sr.sub.0.5Sm.sub.0.5CoO.sub.3 (SSC)-GDC and La.sub.0.6Sr.sub.0.4Co.sub.0.2Fe.sub.0.8O.sub.3 (LSCF)-GDC cathodes.

(15) TABLE-US-00001 TABLE 1 650° C. and 0.8V 700° C. and 0.8V Material Composition (mW/cm.sup.2) (mW/cm.sup.2) SSC-GDC 380 540 LSCF-GDC 377 537 SrCo.sub.0.8Ta.sub.0.1Nb.sub.0.1O.sub.3 268 398 (SCTN) PrBa.sub.0.5Sr.sub.0.5Co.sub.1.5 372 509 Fe.sub.0.5O.sub.5 + .sub.δ (PBSCF) PrCoO.sub.3-GDC 308 469 Ag.sub.0.05Pr.sub.0.95CoO.sub.3-GDC 355 355 Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC 437 636 Ag.sub.0.15Pr.sub.0.85CoO.sub.3-GDC 405 600

(16) The performance stability of Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC cathode in CO.sub.2 environment was evaluated using thermogravimetric analysis (TGA).

(17) The TGA program was as follows:

(18) (1) 25 to 600° C., 50° C./min (CO.sub.2)

(19) (2) 600 to 650° C., 10° C./min (CO.sub.2)

(20) (3) 650° C., 60 min (CO.sub.2)

(21) (4) 650° C., 120 min (Air)

(22) (5) 650° C., 120 min (Argon)

(23) (6) 650° C., 30 min (Air)

(24) FIG. 1a depicts the TGA results of (a) SSC-GDC cathode materials. FIG. 1b depicts the TGA results of Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC cathode materials. As shown in FIG. 1a and FIG. 1b, at 650° C., the SSC-GDC readily absorbed CO.sub.2, and started gaining weight due to SrCO.sub.3 formation, while the Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC showed no weight change. After switching to pure air, the SSC-GDC gradually lost weight due to the decomposition of SrCO.sub.3 while Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC experienced no weight change in the same period of time. After holding in air for 2 hours, a quick switch from air to Ar was carried out. Due to the formation of SrCO.sub.3, the SSC-GDC cathode experienced a slow weight loss comparing to a sharp change for Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC. This indicated the Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC has maintained high oxygen reduction reaction activity/performance even after a 100% CO.sub.2 treatment. The SSC-GDC cathode, however, significantly reduced the performance.

(25) The migration of Sr to the surface of the cathode was found to be an intrinsic property of the Sr containing cathode materials. The Sr readily reacted with YSZ electrolyte and formed a SrZrO.sub.3 insulator. To avoid the adversary reaction, a common practice is to apply a ceria-based barrier layer at the cathode-electrolyte interface. However, a CeZrO.sub.x solid solution layer with much lower conductivity might form after high temperature treatment. The CeZrO.sub.x solid solution layer could grow in thickness under SOFC operation condition. Besides, it is extremely hard to make a fully dense GDC layer on top of the YSZ electrolyte. With a porous GDC barrier layer, SrZrO.sub.3 layer was still found on the YSZ side of the GDC barrier layer and its thickness increased over time under electrical load.

(26) Type-2 Cells Evaluation

(27) Table 2 below summarizes the fuel cell performance with different cathode materials directly applied on YSZ electrolyte (Type-2 cell). The SSC+GDC cathode was directly sintered onto the YSZ at 950° C., while both the Ag.sub.0.05Pr.sub.0.95CoO.sub.3-GDC and Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC were sintered onto YSZ at 900° C.

(28) TABLE-US-00002 TABLE 2 650° C. and 0.8V 700° C. and 0.8V Material Composition (mW/cm.sup.2) (mW/cm.sup.2) SSC-GDC 22 56 Ag.sub.0.05Pr.sub.0.95CoO.sub.3-GDC 395 532 Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC 380 554

(29) The SSC-GDC cathode showed only 22 mW/cm.sup.2 power density at 0.8V and 650° C. due to the formation of SrZrO.sub.3 layer, while both the Ag.sub.0.05Pr.sub.0.95CoO.sub.3-GDC and Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC demonstrated a high performance of over 380 mW/cm.sup.2.

(30) The stability of the Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC cathode directly applied on YSZ electrolyte was evaluated in a 645.5 hours fuel cell test. The I-V curve at 650° C. and different fuel cell operation times of 195 hours, 261 hours and 605 hours are shown in FIG. 2a. FIG. 2b shows an enlarged section of I-V curve at 650° C. and different fuel cell operation times of 195 hours, 261 hours. The 605 h I-V curve was recorded after a 144 h test in 1.6% CO.sub.2 containing air and a 130 h accelerated test with a high current density of 1368 mA/cm.sup.2. A stable performance of 380 mW/cm.sup.2 at 650° C. and 0.8V was maintained after the long-term test as shown in FIG. 2a and FIG. 2b.

(31) During the long-term test, the cathode feed gas was switched from pure air to air containing 1.6% CO.sub.2. FIG. 3 shows that the Ag.sub.0.1Pr.sub.0.9CoO.sub.3-GDC cathode reached a steady stage during the 70 hours test in 1.6% CO.sub.2 mixed air, while the SSC-GDC cathode showed a high degradation rate of 34.6%/kh under the same test condition.

(32) In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiment of the present invention.

(33) Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.