Because of their high energy efficiency, solid oxide fuel cells (SOFCs) are expected to impact both terrestrial and aerospace industries. NASA has recently been investigating reversible SOFCs or regenerative fuel cells (RFCs) for their many applications, including UAVs and for water electrolysis for lunar missions.
RFCs can operate over temperatures from 600 to 1000 °C. Higher temperatures favor regenerative operation, requiring less energy to perform electrolysis, resulting in higher electrochemical efficiency. Most SOFC designs use a metal plate with gas channels to connect the ceramic cells in series. The metal/ceramic bond makes them difficult to seal, and the metal limits operation to lower temperatures. This prevents conventional SOFCs from meeting NASA’s challenging specific power density (kilowatts per kilogram) requirements.
BSC cross-flow stack. LaCrO3, lanthanum chromite.
To achieve a fourfold to fivefold increase in specific power density of 1.0 kW/kg,the NASA Glenn Research Center developed a novel cell design by adapting a special ceramic fabrication technique. The Glenn design, called a bielectrode supported cell (BSC), see the illustration, has low volume and low weight. The metal interconnects are removed, replaced by a thin ceramic interconnect, and gas flow channels are incorporated into the yttria-stabilized zirconia (YSZ) electrode supports. The use of all ceramic materials enables operation at high temperatures and makes hermetic ceramic-to-ceramic seals possible.
Multilayer, unitized BSC stack of two complete repeat units. The blue layers are the YSZ cells with the thin, lighter colored or white electrolyte in the center; the lanthanum chromite (LaCrO3) interconnect layers are black.
The BSC fuel cell design is particularly suited to regenerative (reversible) operation. The photomicrograph shows a “unitized” block of cells fabricated with a single firing step. The graph shows regenerative test results for a single BSC cell with excellent efficiency. The cell was operated from 0.4 to 1.0 V in fuel cell (FC) mode and then from 1.0 to 1.6 V in electrolysis (EL) mode. A smooth transition from FC mode on the right side (positive current density) to the EL mode on the left (negative current density) is evident. The cell was tested with three levels of H2O in the hydrogen (H2) feed gas: 11, 25, and 50 vol%. The voltage increases sharply and current is limited at 11 and 25 vol% H2O because the cell has consumed the available H2O and it is starved. The regenerative efficiency is the ratio of the power generated in fuel cell mode to the power required to perform electrolysis (PFC/PEL). The majority of the testing was performed at 850 °C to compare Glenn’s data with data in the literature at similar conditions. Recently published data report efficiencies from 83 to 85 percent (refs. 1 and 2), comparable to but lower than Glenn’s efficiencies of 89 to 93 percent.
BSC performance in EL mode. Voltage increases sharply and current density is limited at 11 and 25 vol% H2O, indicating H2O starvation.
Long description of figure 3.
RFCs are being considered for immediate applications to extend the missions of UAVs from 2 to 3 days to 90 days. Using a single tank of water, they would operate in EL mode using solar power during the day and operate in FC mode during the night.
Last updated: December 14, 2007
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