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Solid Oxide Fuel Cells and Electrolysis Membranes
  Solid Oxide Fuels Cells and Electrolysis Membranes
 
 
Solid Oxide Fuel Cells (SOFC), also called "high-temperature" fuel cells, operate at temperatures from 600°C to 1000°C (1100°F – 1800°F). A diagram of the SOFC and its principle of operation are shown in Figure 1. The key to the operation is a thin, dense layer of Y-doped zirconia (YSZ), which separates the air-side from the fuel-side of the cell, and performs as the electrolyte. At high temperature, oxygen ions (O-2) can hop through the zircconia membrane, by this mechanism oxygen is transported from the air side of the fuel cell, through the YSZ electrolyte, to the fuel side where it reacts with H2 to make H2O. Because SOFCs operate at high temperature they can also use hydrocarbon based fuels; such as Jet fuel (JP-8), diesel and natural gas; which are converted to CO and then react with oxygen to form CO2. A X-section of a state-of-the-art (SOA) fuel cell, called anode supported, is also shown in Figure 1. The anode side of the cell is thick, on the order of 700 – 1000 microns (1.0 mm), in order to support the thin YSZ electrolyte which is only 10 microns. NASA applications for SOFCs include; Auxiliary Power Units (APUs) for commercial airlines, power sources for high altitude UAVs including reversible fuel cells (RFC) for water electrolysis; and CO2 electrolysis for Mars missions.

Principle of SOFC Operation
 
Figure 1. Principle of SOFC Operation (High temperature - 600-1000°C)
 

A recent Boeing study sponsored by NASA-GRC, determined that a commercial aircraft APU requires a specific power density of 1.0 kW/kg. NASA-GRC has developed a novel cell design and a novel ceramic fabrication technique that has a predicted specific power density to meet the 1.0 kW/kg target. In a state-of-the-art (SOA) fuel cell, called anode supported, the anode side of the cell is thick, on the order of 700 – 1000 microns (1.0 mm), in order to support the thin YSZ electrolyte which is only 10 microns. The cell is then connected to a metal interconnect, which has gas channels for both air and H2, which is about 3,000 microns or 3.0 mm thick. The GRC design, called a BSC for "bi-electrode supported cell", uses a thin ceramic interconnect rather than a metal interconnect; the "all ceramic" fabrication makes it ideally suited to operate at high temperatures, in the 800 to 900°C range, which allows the BSC to take advantage of the higher power density and efficiency.

In fabrication of the BSC, NASA adapted a technique called freeze-tape casting that has been tailored specifically for SOFC materials, which creates a graded pore structure, used for the electrolyte support and the electrode gas channels. Symmetrical cells are fabricated by taking two green parts cut from the green freeze-cast tape, depositing a thin electrolyte layer between the tapes, and laminating them together with the small pores facing each other, forming the YSZ tri-layer as shown in Figure 2. The natural channels created by the freeze casting form the gas channels seen in Figures 3 and 4. The fuel electrode materials are then infiltrated into the channels and heat treated, followed by infiltration of the oxygen electrode materials and heat treatment. The stack is fabricated by coating the top and bottom of the "green" or unfired tri-layer cell, with a thin layer of LaCaCrO3 (LCC) electronic conductor, producing a repeat unit, followed by applying the thin YSZ edge seals shown in grey in Figure 3. Multiple repeat units can be laminated in the green state and "fired" to produce a "unitized" BSC Stack. Figure 4 shows a cross section of three repeat units.
A thermoelectric energy conversion device
 
Figure 2. BSC cell fabrication by applying a thin electrolyte layer between the freeze-cast electrode tapes, with the smallest pores back to back, and laminating them together.
Figure 3. (right) BSC Stack: Blue layers are the electronically conductive LaCrO3 interconnect, the black layers are the YSZ electrolyte, the gray areas at the edges are the hermetic YSZ edge seals. The stack is assembled then sintered in the furnace.
 

A Single Repeat Unit
 
Figure 4. Single Repeat Unit shown at left with the YSZ scaffold supports and gas channels, and the LCC layer at the top and bottom of the slide; Multiple (3) Repeat unit shown on the right, the electronically conductive LaCrO3 interconnect layers are black.
 

The BSC technology has been rigorously tested in both fuel cell mode, for power production, and in electrolysis mode, for the production of H2 from steam. At present single cell and 3-cell stack development is in progress.

Special Processing and Equipment
Joining of the tape casting process and freeze casting process has resulted in a new freeze-tape casting process that has been developed as a direct means of forming and controlling complex pore structures in large area green tapes through commercially viable routes. The freeze-tape casting process not only allows engineering of pore structures through the cross section, but also allows for long range alignment of acicular pores from the surface that is not readily seen in freeze casting through the use of small dies. A standard tape caster is utilized that has been modified with a thermally isolated freezing bed to allow for uni-directional solidification of the tape after casting. As with traditional tape cast, the slip contains a significant quantity of organic binders that make the tape strong and flexible after solvent evaporation for handling and cutting. The freezing of the tape, typically solidified in several minutes, eliminates particles settling out of the suspension as opposed to the traditional process of allowing the solvent to slowly evaporate, which can ultimately result in compositional and physical changes in the uniformity of the product. Immediately following the casting process, “green” tapes are place in a freeze-dryer, where the ice crystals, which have formed the gas channels, are carefully removed.

The relationships between solids loading, freezing rates, and density have been examined as part of the development of the NASA GRC fuel cell design. The freeze-tape casting process, aside from minor variables, is a materials independent process allowing the fabrication of many ceramic materials, nano-powder ceramics, and tape thicknesses exceeding the outer bounds of traditional tape casting. The freeze-casting lab and the freeze drying equipment are shown below.

SOFC Fabricating Capabilities
 
Capability Established at GRC for Fabricating SOFC Cells: (top left) Freeze-Tape caster, (top center) Screen printing unit for depositing thin electrolyte and interconnect layers, (top right) Variety of sintering/heat treatment furnaces with and without controlled environments, (lower left and right) Cell performance measurement.
 

Publications
T.L. Cable, J.A. Setlock, S.C. Farmer and A.E. Eckel; "Regenerative Performance of the NASA Symmetrical Solid Oxide Fuel Cell Design", International Journal of Applied Ceramic Technology, 2009

T.L. Cable, S.A. Sofie, "A Symmetrical, Planar SOFC Design for NASA’s High Specific Power Density Requirements", Journal of Power Sources, 174 (2007) 221

S.C. Farmer, T.L. Cable, J.A. Setlock, 'Water Electrolysis and Regenerative Tests Conducted on NASA Glenn Solid Oxide Fuel Cell Demonstrated High Efficiency', NASA/TM-2007-214479, 2006 Research and Technology Report

S.C. Farmer, T.L. Cable, J.A. Setlock, "Proof of Concept (Design, Fabrication and Testing) of a Novel High-Power-Density Solid Oxide Fuel Cell Established", NASA/TM-2006-214016, 2005 Research and Technology Report

S.C. Farmer, S.W. Sofie, T.L. Cable, S.M. Salamone, 'Process Developed for Fabricating Engineered Pore Structures for High-Fuel-Utilization Solid Oxide Fuel Cells", NASA/TM-2005-213419, 2004 Research and Technology Report

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Last Updated: February 2, 2010