Solid oxide fuel cell cathode materials

Abstract

A cathode in a solid oxide fuel cell containing AgPrCoO3. The operating temperature range of the cathode is from about 400° C. to about 850° C.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

This invention relates to a solid oxide fuel cell cathode material.

BACKGROUND OF THE INVENTION

A solid oxide fuel cell (SOFC) is an electromechanical device that continuously converts chemical energy into electrical energy by exploiting the natural affinity of oxygen and hydrogen to react. By controlling the means by which such a reaction occurs and directing the reaction through a device it is possible to harvest the electrical energy given off by the reaction.

Generally, an SOFC stack repeat unit contains multiple layers such as a support substrate, an active anode layer, an electrolyte layer, a barrier layer, a cathode, an interconnect, an anode current collecting layer, a cathode current collecting layer, an anode seal, and a cathode seal.

There exists a need for new novel cathode components for SOFC's that would enable greater electrical output and lower material and fabrication costs.

BRIEF SUMMARY OF THE DISCLOSURE

A cathode in a solid oxide fuel cell containing AgPrCoO₃. The operating temperature range of the cathode is from about 400° C. to about 850° C.

A composite cathode in a solid oxide fuel cell. The composite cathode comprises AgPrCoO₃ and Gd_0.1Ce_0.9O₂. The operating temperature range of the cathode is from about 400° C. to about 800° C.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

FIG. 1a depicts the thermogravimetric analysis (TGA) results of the state-of-the-art conventional SOFC cathode material Sm_0.5Sr_0.5. FIG. 1a depicts CoO₃—Gd_0.1Ce_0.9O₂ and Ag_0.1. FIG. 1b shows Pr_0.9CoO₃—Gd_0.1Ce_0.9O₂.

FIG. 2a depicts the current voltage and current power density curves of a fuel cell with Ag_0.1Pr_0.9CoO₃-GDC cathode tested in hydrogen at 650° C. FIG. 2b is an enlarged section of FIG. 2a near 0.8 V.

FIG. 3 depicts fuel cell performance at 0.8V and 650° C. for Ag_0.1Pr_0.9CoO₃-GDC cathode directly applied on YSZ electrolyte and SSC-GDC cathode with a GDC barrier layer. Cathode feed air contained 1.6% CO₂.

DETAILED DESCRIPTION

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.

As briefly introduced above, the present embodiment provides a cathode material AgPrCoO₃. 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 AgPrCoO₃ (APC) show excellent long-term stability in CO₂ containing environments. In one embodiment, use of AgPrCoO₃ 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.

In one embodiment, AgPrCoO₃ is made from Ag doping PrCoO₃. This produces Ag doping levels of Ag_xPr_1-xxCoO₃, x=0.05-0.15. In one non-limiting embodiment, the doping of PrCoO₃ 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.

For cathode ink preparation, Ag doped PrCoO₃ 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 cm².

Sample Preparation:

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

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

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.

Type-1 Cells Evaluation

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_0.1Pr_0.9CoO₃-GDC cathode showed the highest performance, which was higher than that of conventional Sr_0.5Sm_0.5CoO₃ (SSC)-GDC and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF)-GDC cathodes.

TABLE 1
	650° C. and 0.8V	700° C. and 0.8V
Material Composition	(mW/cm2)	(mW/cm2)
SSC-GDC	380	540
LSCF-GDC	377	537
SrCo0.8Ta0.1Nb0.1O3	268	398
(SCTN)
PrBa0.5Sr0.5Co1.5	372	509
Fe0.5O5 +δ (PBSCF)
PrCoO3-GDC	308	469
Ag0.05Pr0.95CoO3-GDC	355	355
Ag0.1Pr0.9CoO3-GDC	437	636
Ag0.15Pr0.85CoO3-GDC	405	600

The performance stability of Ag_0.1Pr_0.9CoO₃-GDC cathode in CO₂ environment was evaluated using thermogravimetric analysis (TGA).

The TGA program was as follows:

(1) 25 to 600° C., 50° C./min (CO₂)

(2) 600 to 650° C., 10° C./min (CO₂)

(3) 650° C., 60 min (CO₂)

(4) 650° C., 120 min (Air)

(5) 650° C., 120 min (Argon)

(6) 650° C., 30 min (Air)

FIG. 1a depicts the TGA results of (a) SSC-GDC cathode materials. FIG. 1b depicts the TGA results of Ag_0.1Pr_0.9CoO₃-GDC cathode materials. As shown in FIG. 1a and FIG. 1b, at 650° C., the SSC-GDC readily absorbed CO₂, and started gaining weight due to SrCO₃ formation, while the Ag_0.1Pr_0.9CoO₃-GDC showed no weight change. After switching to pure air, the SSC-GDC gradually lost weight due to the decomposition of SrCO₃ while Ag_0.1Pr_0.9CoO₃-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 SrCO₃, the SSC-GDC cathode experienced a slow weight loss comparing to a sharp change for Ag_0.1Pr_0.9CoO₃-GDC. This indicated the Ag_0.1Pr_0.9CoO₃-GDC has maintained high oxygen reduction reaction activity/performance even after a 100% CO₂ treatment. The SSC-GDC cathode, however, significantly reduced the performance.

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 SrZrO₃ 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_x solid solution layer with much lower conductivity might form after high temperature treatment. The CeZrO_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, SrZrO₃ layer was still found on the YSZ side of the GDC barrier layer and its thickness increased over time under electrical load.

Type-2 Cells Evaluation

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_0.05Pr_0.95CoO₃-GDC and Ag_0.1Pr_0.9CoO₃-GDC were sintered onto YSZ at 900° C.

	TABLE 2
		650° C. and 0.8V	700° C. and 0.8V
	Material Composition	(mW/cm2)	(mW/cm2)
	SSC-GDC	22	56
	Ag0.05Pr0.95CoO3-GDC	395	532
	Ag0.1Pr0.9CoO3-GDC	380	554

The SSC-GDC cathode showed only 22 mW/cm² power density at 0.8V and 650° C. due to the formation of SrZrO₃ layer, while both the Ag_0.05Pr_0.95CoO₃-GDC and Ag_0.1Pr_0.9CoO₃-GDC demonstrated a high performance of over 380 mW/cm².

The stability of the Ag_0.1Pr_0.9CoO₃-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% CO₂ containing air and a 130 h accelerated test with a high current density of 1368 mA/cm². A stable performance of 380 mW/cm² at 650° C. and 0.8V was maintained after the long-term test as shown in FIG. 2a and FIG. 2b.

During the long-term test, the cathode feed gas was switched from pure air to air containing 1.6% CO₂. FIG. 3 shows that the Ag_0.1Pr_0.9CoO₃-GDC cathode reached a steady stage during the 70 hours test in 1.6% CO₂ mixed air, while the SSC-GDC cathode showed a high degradation rate of 34.6%/kh under the same test condition.

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.

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.

Claims

1. A composite cathode in a solid oxide fuel cell comprising: AgPrCoO3; andGd0.1Ce0.9O2,wherein the operating temperature range of the cathode is from about 400° C. to about 800° C.,wherein the weight ratio of AgPrCoO3 to Gd0.1Ce0.9O2 is 60:40.

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