Cathode for solid oxide fuel cells

Abstract

A solid oxide fuel cell comprising an anode layer, an electrolyte layer, and a two phased cathode layer. The two phased cathode layer comprises praseodymium and gadolinium-doped ceria. Additionally, the solid oxide fuel cell does not contain a barrier layer.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

This invention relates to cathodes for solid oxide fuel cells.

BACKGROUND OF THE INVENTION

It is well known that most cathode materials such as lanthanum strontium manganite, lanthanum strontium cobalt ferrite and samarium strontium cobalt react with the yttrium doped zirconia (YSZ) electrolyte and form an insulating phase, SrZrO₃, significantly reducing fuel cell performance and stability. A typical mitigation strategy is to apply a thin barrier layer such as gadolinium doped ceria (GDC) between the cathode and YSZ electrolyte. However, a low conductivity solid solution CeZrOx, forms at the interface between GDC and YSZ at temperatures higher than 1200° C. When the sintering temperature is lower than 1250° C., the GDC barrier layer is known to be too porous to achieve high performance. It is extremely difficult to obtain dense GDC barrier layer while avoiding the formation of CeZrOx.

There exists a need for an improved cathode for solid oxide fuel cells.

BRIEF SUMMARY OF THE DISCLOSURE

A solid oxide fuel cell comprising an anode layer, an electrolyte layer, and a two phased cathode layer. The two phased cathode layer comprises praseodymium and gadolinium-doped ceria. Additionally, the solid oxide fuel cell does not contain a barrier layer.

In an alternate embodiment, solid oxide fuel cell comprising an anode layer, an electrolyte layer, and a two phased cathode layer is taught. The two phased cathode layer comprising a first phase consisting essentially of: praseodymium; cobalt; nickel; oxygen; and silver, and a second phase comprising essentially of: gadolinium-doped ceria. In this embodiment, the solid oxide fuel cell does not contain a barrier layer and the bonding strength of the two phased cathode layer and the electrolyte layer is formed by sintering the solid oxide fuel cell at temperatures less than 950° 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. 1 depicts a conventional SOFC structure.

FIG. 2a depicts a SOFC with a barrier layer.

FIG. 2b depicts a SOFC without a barrier layer.

FIG. 2c depicts the associated structure layers of the SOFC with a barrier layer.

FIG. 2d depicts the associated structure layers of the SOFC without a barrier layer.

FIG. 3a depicts the XRD patterns of the PCNA-YSZ composite.

FIG. 3b depicts the XRD patterns of the PCNA-YSZ composite.

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.

The present embodiment describes a solid oxide fuel cell with an anode layer, an electrolyte layer, and a two phased cathode layer comprising praseodymium and gadolinium-doped ceria. It is envisioned that the solid oxide fuel cell does not contain a barrier layer.

In one embodiment, the two phased cathode layer can comprise a first phase and a second phase. The first phase can either be consisting essentially of or consisting of praseodymium, cobalt, nickel, oxygen, and silver. The second phase can be consisting essentially of or consisting of gadolinium-doped ceria.

In one embodiment, the praseodymium is silver-doped praseodymium cobaltite. In another embodiment, the praseodymium is nickel-doped praseodymium cobaltite.

In yet another embodiment, the bonding strength of the two phased cathode layer and the electrolyte layer is formed by sintering the solid oxide fuel cell at temperatures less than 950° C. It is envisioned that only one sintering step for the solid oxide fuel cell cathode is performed.

In yet another embodiment, a solid oxide fuel cell is described with an anode layer, an electrolyte layer, and a two phased cathode layer comprising a first phase consisting essentially of: praseodymium; cobalt; nickel; oxygen; and silver, and a second phase comprising essentially of: gadolinium-doped ceria. In this embodiment, the solid oxide fuel cell does not contain a barrier layer and the bonding strength of the two phased cathode layer and the electrolyte layer is formed by sintering the solid oxide fuel cell at temperatures less than 950° C.

The following examples of certain embodiments of the invention are given. Each example is provided by way of explanation of the invention, one of many embodiments of the invention, and the following examples should not be read to limit, or define, the scope of the invention.

The first phase of the cathode layer was first formed by dissolving metal nitrate hydrates with stoichiometric ratios in deionized water. Citric acid (CA) was added as a chelating agent with a CA-to-nitrate-ion molar ratio of 1:2. 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 ground and calcined at 800° C. for 5 h. The resultant product was PrCo_0.9Ni_0.1O_3-δAg_0.1 (PCNA).

The second phase of the cathode layer gadolinium-doped ceria (GDC) was mixed with PCNA in a weight ratio of 60% PCNA:40% GDC to form a cathode powder. The cathode mixture was then mixed with a terpineol-based ink vehicle obtained from Nexceris, LLC in a weight ratio of 65% cathode powder:35% ink vehicle and milled in a high-energy ball mill at 350 rpm for 1 h to form the cathode ink.

This cathode ink was then used to form a novel solid oxide fuel cell (SOFC) as compared to a conventional SOFC as depicted in FIG. 1.

In a conventional anode supported SOFC, the process will begin by casting, either by spray coating or any other conventionally known technique, an electrolyte layer on top of an anode which is then followed by sintering at temperatures greater than 1400° C. This will be subsequently followed by casting, either by spray coating or any other conventionally known technique, a barrier layer on top of the electrolyte which is then followed by sintering at temperatures greater than 1,000° C. This temperature is depicted in FIG. 1 as 1,250° C. A cathode layer is then spray coated on top of the electrolyte, followed by a second sintering at a temperature of 950° C.

Applicants novel process begins similarly to the conventional method by casting, either by spray coating or any other conventionally known technique, an electrolyte layer on top of an anode which is then followed by sintering at temperatures greater than 1400° C. Applicants process then differs by directly casting, either by spray coating or any other conventionally known technique, a two-phase cathode layer on top of the electrolyte. This is followed by sintering only once at a lower temperature of 900° C.

FIG. 2a depicts a SOFC with a barrier layer and FIG. 2c depicts the associated structure layers of the SOFC with a barrier layer. FIG. 2b depicts a SOFC without a barrier layer and FIG. 2d depicts the associated structure layers of the SOFC without a barrier layer.

The fuel cells were then tested between 500 and 750° C., and the impedance curves were taken at 650° C. under open circuit conditions.

As depicted below in Table 1 in which all the cells had a GDC barrier layer, PCNA-GDC provided enhanced performance. The Ag doping level was optimized to be approximately 10 mol % in the A site to provide the best fuel cell performance. At 650° C. and 0.8 V, the performance of the APC-GDC cathode layer reached 437 mW/cm², which is much higher than that of the undoped PrCoO3 (PC)-GDC cathode. Several state-of-the-art SOFC cathode materials were studied for comparison, including commercially available SSC-GDC and LSCF-GDC composites and in-house-prepared SrCo_0.8Ta_0.1Nb_0.1O₃ (SCTN) and PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF). It is theorized that the significant performance enhancement may come from the increased active sites introduced by the Ag and Co₃O₄ nanoparticles together with the superior electronic conductivity of PC even under low-oxygen partial pressures.

TABLE 1
	650° C. and	700° C. and
Material composition	0.8 V (mW/cm2)	0.8 V (mW/cm2)
SSC-GDC	380	540
LSCF-GDC	377	537
SrCo0.8Ta0.1Nb0.1O3 (SCTN)	268	398
PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF)	372	509
PrCoO3-GDC	308	469
Ag0.05Pr0.95CoO3-GDC	355	525
Ag0.1Pr0.9CoO3-GDC	437	636
Ag0.15Pr0.85CoO3-GDC	405	600
PrCo0.9Ag0.1O3-GDC	390	604

The refined Ag concentration is about 4.3 wt % in PCNA. Small nanoparticles (˜10 nm) were found on the PCNA substrate. FIG. 3a shows the XRD patterns of the PCNA-YSZ composite with a weight ratio of 50:50 and sintered between 850 and 1,100° C. A secondary phase with a Pr₂Zr₂O₇ structure (main peak around 29.08°) was found at sintering temperatures of 950° C. and above see FIG. 3b. Therefore, 900° C. was determined to be the optimal sintering temperature for both PCN-GDC and PCNA-GDC. At this temperature, both good cathode-electrolyte bonding strength and minimal phase interactions were achieved.

Test results of fuel cells without a cathode barrier are shown in Table 2 below. PCA-GDC, PCN-GDC, and PCNA-GDC showed much higher performance than LSM-YSZ cathode.

TABLE 2
	Power density at 0.8 V (mW/cm2)
Material compositions	600° C.	650° C.	700° C.
LSM-YSZ	70	136	235
PC-GDC	203	406	585
APC-GDC	226	397	554
PCA-GDC (two phase cathode)	248	424	610
PCN-GDC (two phase cathode)	261	445	650
PCNA-GDC (two phase cathode)	330	500	782

The novel PCNA-GDC cells showed lower polarization resistance (Rp) than in conventional SOFC cells. The PCA-GDC showed the smallest Rp because of the extruded nano Pr₅O₉ oxides on the surface. With more oxygen vacancies created in PCN, a low Rp of 0.15 Ωcm² at 650° C. was achieved for PCN-GDC. Further Ag doping of PCNA-GDC reduced the Rp to 0.12 Ωcm² at 650° C. PCA-GDC, PCN-GDC, and PCNA-GDC performed better than conventional SOFC cells. In the novel cells, Rp was reduced by more than 50% compared to that of the SSC-GDC in conventional SOFC cells

Table 3 depicts further results of the two phase cathode with different cathode materials.

TABLE 3
	Rp (Ω cm2)
Material composition	600° C.	650° C.	700° C.
APC-GDC	0.67	0.22
PCA-GDC (two phase cathode)	0.37	0.12	0.03
PCN-GDC (two phase cathode)	0.54	0.15	0.054
PCNA-GDC (two phase cathode)	0.41	0.12	0.04

A fuel cell with a PCN-GDC cathode and without a GDC barrier layer showed 445 mW/cm² at 650° C. With Ag added, the PCNA-GDC performance reached a record high performance of 500 mW/cm² at 650° C. and 330 mW/cm² at 600° C. Neither of the cathode materials showed degradation in the short-term fuel cell tests. After 140 h of operation, Ag nano particles could still be observed on PCNA surface, and although the SSC was used as the cathode contact layer, no SrZrO₃ insulator could be detected at the interface based on EDX mapping results. The formation of GdCeZrYO_x insulator was avoided by removing the GDC barrier layer, which further reduced the interfacial resistance. During the two phase cathode testing, impedance curves were recorded at 650° C. under open current voltage (OCV) conditions. The bulk conductivity of the two phase cathode was calculated assuming a YSZ thickness of 10 μm.

Table 4 depicts the bulk conductivity, which was improved by applying PCA-GDC, PCN-GDC, and PCNA-GDC cathode layers directly on the YSZ electrolyte

TABLE 4
                             Bulk (S/cm)
Material composition         conductivity
SSC-GDC                      0.0030
PCA-GDC (two phase cathode)  0.0082
PCN-GDC (two phase cathode)  0.0045
PCNA-GDC (two phase cathode) 0.0076
YSZ                          0.008

The conductivity of cells with PCA-GDC and PCNA-GDC cathode materials was very close to the theoretical YSZ conductivity value of 0.008 s/cm at 650° C. However, because of formation of the GdCeZrYO_x insulator between the YSZ and GDC electrolyte, the bulk conductivity of the baseline cell with the SSC-GDC cathode in a conventional SOFC was only 37.5% of the theoretical value for YSZ.

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 solid oxide fuel cell comprising: an anode layer;an electrolyte layer; anda two phased cathode layer comprising: praseodymium and gadolinium-doped ceria, andwherein the solid oxide fuel cell does not contain a barrier layer separating physical contact between the electrolyte layer and the two phased cathode layer, andwherein the two phased cathode layer comprises a first phase and a second phase and the first phase consists essentially of: praseodymium; cobalt nickel; oxygen; and silver.

2. The solid oxide fuel cell of claim 1, wherein the second phase consists essentially of: gadolinium-doped ceria.

3. The solid oxide fuel cell of claim 1, wherein the bonding strength of the two phased cathode layer and the electrolyte layer is formed by sintering the solid oxide fuel cell at temperatures less than 950° C.

4. The solid oxide fuel cell of claim 3, wherein the solid oxide fuel cell undergoes only one sintering phase.

5. The solid oxide fuel cell of claim 1, wherein the praseodymium is silver-doped praseodymium cobaltite.

6. The solid oxide fuel cell of claim 1, wherein the praseodymium is nickel-doped praseodymium cobaltite.

7. A solid oxide fuel cell comprising: an anode layer;an electrolyte layer; anda two phased cathode layer comprising a first phase consisting essentially of: praseodymium; cobalt; nickel; oxygen; and silver, and a second phase consisting essentially of: gadolinium-doped ceria, andwherein the solid oxide fuel cell does not contain a barrier layer separating physical contact between the electrolyte layer and the two phased cathode layer and the bonding strength of the two phased cathode layer and the electrolyte layer is formed by sintering the solid oxide fuel cell at temperatures less than 950° C.