Solid Oxide Fuel Cell With Single Material for Electrodes and Interconnect

The present invention is directed to a microchannel solid oxide fuel cell constructed of only two material components. More particularly, the invention is directed to a solid oxide fuel cell constructed of two material components which do not include nickel and have a relatively thin interconnect between the electrodes which also are quite thin to enhance electrical conductivity. One material, such as a Sr doped lanthanum chromite, can be used to construct both electrodes, electrical connector and fuel-oxygen separator between adjacent cells; and the second material is the electrolyte ZrO.sub.2 -Y.sub.2 O.sub.3.

Conventional designs developed for solid oxide fuel cells include tubular, planar, and monolithic designs. In the tubular design, the traditional four fuel cell materials (anode 4, electrolyte 5, cathode 3, and interconnect 6) are deposited sequentially on a porous support tube by the electrochemical vapor deposition (ECVD) process. In FIG. 1 (described below), a tubular design is shown. The interconnect structure extends from the cathode layer 3 on the inside of the tube to the exterior surface where it contacts the anode layer 4 of the next cell electrical series. The tubular single cells are stacked together in a square array in which two or more cells are connected in parallel (side by side) while several cells are connected in series to build up the voltage of the system to practical levels. While the tubular concept is quite far advanced and has experienced considerable success in performance both as individual cells and in complete power systems, the concept suffers from high manufacturing cost due to the complexity of the ECVD process. The concept has also suffered from uncertain reliability of individual cells in the system.

In a planar design, essentially the same four fuel cell materials have been used to build single cells and stacks in a flat geometry instead of the tubular geometry. FIG. 2 (described below) shows the principle of the planar design. Individual cells are fabricated by tape casting the electrolyte layer 5, sintering and flattening the electrolyte layer, then applying the anode 4 and cathode layers 3 to the sintered electrolyte. The interconnect material 6 is formed with grooves for air and fuel gas flow. Alternating layers of single cells and interconnect material are stacked to form the fuel cell assembly. The flatness of the electrolyte and the interconnect layers is critical to good performance of the planar concept because good electrical contact must be maintained over the entire surface area of each cell. In addition, it is important to maintain good gas seals at the edges to prevent cross-leakage of the fuel and air streams. These requirements of extreme flatness and good gas seals limit the performance of the planar concept.

The same four fuel cell materials have been used to build single cells and stacks in a monolithic design. In the monolithic design, thin composites of anode 4/electrolyte 5/cathode 3 (A/E/C) and anode 4/interconnect 6/cathode 3 (A/I/C) materials are made either by tape casting or hot roll calendering. FIG. 3 (described below) shows the configuration of the co-flow design of the monolithic concept. The A/E/C composites are corrugated to form the air and fuel flow passages, and the A/I/C composites connect the cells in electrical series. While the monolithic concept has the potential for the highest performance in terms of power density and efficiency of any of the solid oxide fuel cell concepts, it suffers from the difficulty of fabricating a continuous, bonded structure with four different materials.

It is, therefore, an object of the invention to provide an improved solid oxide fuel cell and method of manufacture thereof.

It is another object of the invention to provide a novel solid oxide fuel cell constructed using only two basic materials.

It is further object of the invention to provide an improved solid oxide fuel cell constructed only of an electrolyte and a strontium doped lanthanum chromite.

It is yet another object of the invention to provide an improved solid oxide fuel cell having its electrodes and interconnect layer constructed of one basic material.

It is still a further object of the invention to provide a novel solid oxide fuel cell having reduced size gas flow channels enabling manufacture of a compact fuel cell with shortened electrical conduction paths.

It is yet an additional object of the invention to provide an improved solid oxide fuel cell constructed of materials tolerant of sulfur-containing fuels.

The subject invention relates generally to a solid oxide fuel cell using a single material for the electrodes and electrical interconnect, such as a La.sub.0.9 Sr.sub.0.1 CrO.sub.3 which is stable under oxidizing and reducing environments. The use of a single material also advantageously provides a compatible material for the electrolyte and a favorable interface with the electrolyte and fuel or oxygen. Use of a single material also simplifies cell construction and problems of sintering shrinkage, thermal expansion and chemical incompatability are substantially minimized. Construction from a single material further eliminates problems which arise in conventional cells from use of sulfur-containing fuels or from chemical degradation of electrode materials upon cross leakage between fuel and air channels.

Other advantages and features of the invention are set forth in the following description and figures described herein below wherein like elements have like numerals throughout the several drawings.

FIG. 1 illustrates a prior art tubular solid oxide fuel cell;

FIG. 2 illustrates a prior art planar solid oxide fuel cell;

FIG. 3 illustrates a prior art monolithic solid oxide fuel cell;

FIG. 4 illustrates a solid oxide fuel cell constructed in accordance with the invention;

FIG. 5 illustrates the distribution of Nernst potential across the area of the two-component solid oxide fuel cell;

FIG. 6 illustrates the distribution of current density across the area of the two-component solid oxide fuel cell;

FIG. 7 illustrates the distribution of oxygen partial pressure across the area of the two-component solid oxide fuel cell;

FIG. 8 illustrates the distribution of fuel stream temperature across the area of the two-component solid oxide fuel cell; and

FIG. 9 illustrates the distribution of air stream temperature across the area of the two-component solid oxide fuel cell.

A solid oxide fuel cell constructed in accordance with one form of the invention is indicated generally at 10 in FIG. 4. Both anode 12 and cathode 14 are comprised of any of a variety of conventional electrical interconnect materials, such as La.sub.1-x Sr.sub.x CrO.sub.3 and La.sub.1-x Mg.sub.x MnO.sub.3. Such materials are stable in both oxidizing and reducing environments, and therefore, it could be used for both the fuel and air electrodes. While the conductivity of such materials is not as high as common anode materials (such as, nickel-zirconia cermet) or conventional cathode materials (such as La.sub.0.9 SR.sub.0.1 MnO.sub.3), the fuel cell 10 has thin electrodes 12, 14 in order to provide short conduction paths and low voltage losses with materials of mediocre electrical conductivity. Electrolyte 16 has generally the largest resistance in the fuel cell 10, but its resistance is reduced to practically useful levels by reducing the operating dimensions to minimum practical thicknesses (see FIG. 4).

Interconnect 18 is adjacent to the electrolyte 16 and defines part of a fuel channel 20 and preferably is porous. Interconnect 22, along with the interconnect 18, and 26 also defines the fuel channel 24 and can be either porous and/or dense to meet design objectives (e.g., matching coefficients of thermal expansion of adjacent components). Dense interconnect 26, along with another one of the interconnects 18, defines an air channel 28 and preferably is dense with low porosity.

These differences in density for the interconnects can be achieved by adjustment of the particle size distributions in selected regions of part or all of the above-described elements of the fuel cell 10. One can also use various sintering aids to densify the various interconnects 18, 22, 26 where necessary. Generally such interconnect materials do not easily sintar to low porosity. Thus, sinrating aids can be concentrated, for example, in the bipolar plate region to cause sinrating to low porosity in part or all of the interconnect 26.

The porosity of the various elements can further be controlled, such as in the electrode regions, by adjusting the particle size distributions in the materials used. In addition, the sinrating conditions can be modified to enhance sinterability of the interconnects 18, 22, 26. In conventional four material systems, it is necessary to minimize sintering temperature to minimize migration of elements such as Mn from the cathode 14. In the fuel cell 10, however, both the electrolyte 16 and the interconnects 18, 22, 26 are stable to quite high temperatures in both oxidizing and reducing environments. Consequently, substantial degrees of freedom are accorded by being able to adjust the sintering temperature and atmosphere to achieve the desired properties.

In the construction of the fuel cell 10 of the invention, the distance between the cells 10 is also reduced to minimize electrical resistance. The size of the air channels 28 is also reduced to minimize the cell-to-cell distance. Consequently, the pressure drop through the air channels 28 will be higher than conventional monolithic designs. Preferably, air channel 28 is larger than the fuel channel 24 because the air flow required for heat removal is higher than the fuel flow.

Variations in the fuel flow between channels will not typically be as demanding as with prior art fuel cell designs because the interconnect material is much more stable. If fuel 30 is depleted in one of the fuel channels 24, the various electrode materials will remain intact and continue to function as a conductor.

In the fuel cell 10, the lengths of the air channels 28 and the fuel channels 24 can also be adjusted as needed to charge or equalize the pressure drops in the two gas streams.

Therefore, in view of the structure, advantages attendant to the fuel cell 10 are, for example:

1. The two-material structure makes the construction process much simpler, in terms of ceramic formulation, shrinkage matching, coefficient of thermal expansion matching, and lay-up of stacks.

2. The interconnect material is stable in both air and fuel; therefore, cross leakage will have no effect on the stability of the electrodes. The stability of both the interconnect material and the electrolyte material will allow the fuel cell to operate for very long times with essentially no detrimental migration of elements from one layer into the other.

3. Since the interconnect material is stable in both oxidizing and reducing environments, it should be possible push the fuel utilization well beyond the usual 85% targeted in other solid oxide fuel cell concepts. The only concern will be efficiency of operation rather than possible oxidation of the anode material or reduction of the cathode material. Fuel flow maldistribution should have little effect on fuel cell performance or lifetime.

4. Because the two-component fuel cell has no nickel in the fuel electrode, sulfur is not expected to have a significant effect on the fuel electrode performance. The fuel cell can operate on high-sulfur fuels or even pure H.sub.2 S.

The following non-limiting Example describes one form of the invention.

The performance of the fuel cell 10 was determined using a conventional, well known calculational model "CROSSFLOW" (see Appendix A) a source code output, which was developed by the inventors to evaluate the concepts described herein. The CROSSFLOW model is designed to accept any desired geometric configuration, any desired materials resistivities, and any desired process parameters (such as gas inlet temperatures, compositions, and flows). The model calculates the performance of a single cell in a fuel cell stack based on the input data, and the calculated parameters are output in the form of a data summary sheet (see Table 1) and five plots (see FIGS. 5-9) of gas temperatures, Nernst potentials, local current densities, and oxygen pressure as a function of position in the single cell layer.

The results of analysis of the two-component fuel cell with the CROSSFLOW model are summarized in the output data sheet, Table 1. The case that was analyzed was a fuel cell module with overall dimensions of 10 cm on each side. The electrolyte and interconnect layers were assumed to be 25 microns thick, as were the electrode layers. The fuel and air channels were 200 microns and 300 microns square, respectively. These are fairly small channels compared with "conventional" fuel cell designs; however, the pressure drops are reasonable at 0.03 atm and 0.1 atm, respectively for the fuel and air streams. The power generated by this 10-cm cubic fuel cell is 1.7 kW at 0.7 V/cell, 87% fuel utilization, and the efficiency is 47%. While the power density (1.7 kW/L) is low in comparison to monolithic solid oxide fuel cell calculations (4.0 kW/L), it is adequate for many applications, including cogeneration, stationary power, and many portable-power applications.

The calculated Nernst potential distribution across the fuel cell area is shown in FIG. 5, and the resulting local current density distribution is shown in FIG. 6. The oxygen utilization is illustrated by the plot of oxygen pressure distribution shown in FIG. 7. The air and fuel temperatures are plotted in FIGS. 8 and 9, respectively, for the case in question. The fuel cell design was not optimized in this series of calculations, e.g., the layer thickness, channel sizes, cell heights, etc., were not studied as variables. Only one case was calculated, and the fuel and air flows were adjusted to give the desired fuel utilization and average air outlet temperature. The performance of the two-component fuel cell would be quite satisfactory for most power source applications. The important advantages of this concept are its simplicity and ease of fabrication, and the stability of the electrode material in oxidizing and reducing environments and, possibly, in sulfur-containing fuels.

While preferred embodiments have been described herein, it will be clear to those skilled in the art that various changes and modifications can be made without departing from the invention in its broader aspects as set forth in the claims provided herein.