09-20 solid oxide fuel cell

[Image above] A solid oxide fuel cell stack designed and assembled for the Bipropellant Enabled Electrical Power Supply, or BEEPS, effort by Air Force Research Laboratory collaborator OxEon Energy. Credit: Air Force Research Laboratory

By Laurel Sheppard

Deployed spacecraft are expected to remain in operation for years without in-person servicing. The GPS satellite USA-132, for instance, is still operating after it was launched in 1997.

However, mass and volume limitations make it challenging—and often impossible—to load a spacecraft with all the equipment needed for the mission. Dual function systems are thus required, as well as enough electrical power to run the payload and satellite functions.

Fuel cells are one option for powering spacecraft. Fuel cells convert hydrogen or other fuels into electricity. Compared to batteries, fuel cells typically are much more energy dense, which allows them to be more compact and lighter, plus produce electricity continuously for much longer on a single fill.

In recent years, solid oxide fuel cells (SOFCs) have been a focus for next-generation spacecraft. These fuel cells use a hard, nonporous ceramic compound as the electrolyte and operate at very high temperatures. The high-temperature operation provides several benefits, including

  • Removes the need for an expensive precious-metal catalyst; and
  • Allows SOFCs to reform fuels internally, which enables a variety of fuels to be used and reduces the cost associated with adding a reformer to the system.

High-temperature operation does lead to several challenges, however, including a slow startup and the need for significant thermal shielding to retain heat and protect personnel.

In a recent effort, researchers at the Air Force Research Laboratory (AFRL) are developing SOFCs that can convert chemical energy in a spacecraft’s bipropellant* into electricity.

This technology, dubbed Bipropellant Enabled Electrical Power Supply (BEEPS), “would allow spacecraft operators to tap stowed bipropellant to get a boost of electrical power or act as an auxiliary power supply if needed,” explains Thomas Peng, BEEPS program manager, in an AFRL press release.

*NOTE: Bipropellants are rocket propellants consisting of a separate fuel and oxidizer that come together only in a combustion chamber.

BEEPS is being developed through a three-year Seedlings for Disruptive Capabilities Program effort. Three AFRL technical directorates are collaborating: Space Vehicles, Aerospace Systems, and Materials and Manufacturing. Another collaborator is OxEon Energy, which is using the AFRL technology to create a single package of SOFC stacks.

Peng recently presented more details on BEEPS during an August 15 event sponsored by Parallax Advanced Research and The Ohio State University on the future of hydrogen fuel cells.

For proof of concept, Peng says they fabricated button cells consisting of a nickel oxide (NiO) anode, a lanthanum strontium manganite (LSM) cathode, and a scandia-doped zirconia electrolyte. A test station consisting of an electrochemical impedance spectroscopy analyzer, multigas unit, and potentiostat was used for electrochemical characterization, including open circuit voltage measurements and voltage-to-current curves.

Peng says they used aerosol jet printing to fabricate the anode interlayers on nickel oxide–yttria stabilized zirconia supports because it enables controlled deposition of active materials. After the anode/electrolyte ink was formulated (which contains a solvent, binders, or plasticizer and pore formers), it was jet printed by optimizing the following parameters.

  • Exhaust flow rate
  • Atomizer pressure
  • Substrate temperature
  • Raster speed
  • Number of layers

Printer layer properties were improved, including density, porosity, thickness, and tortuosity. A thermal processing step was used to burn out the additives.

Peng says the results confirmed the potential of running the NiO/scandia-doped zirconia/LSM SOFCs on bipropellants under test conditions of 800°C, ambient pressure, and flow rates of 150/150 standard cubic centimeters per minute.

However, with 4% H2/N2O, there was a sharp drop in voltage with increasing density. This result may be due to mass transport limits arising from a low concentration of H2.

The data also showed

  • NH3 is likely decomposing into N2 and H2, and N2O is decomposing into N2 and O2.
  • Ohmic loss due to electrolyte resistance is the major loss mechanism, although voltage vs. current density slopes are similar (except for the 4% H2/N2O pair).
  • By supplying a green bipropellant pair (ammonia and nitrous oxide) to a SOFC, electricity can generate electricity.
  • Bipropellant decomposition appears to be a major factor in the electric power an SOFC can output.

Peng says more research is needed to optimize

  • Electrochemistry of catalytic electrodes and solid-state electrolytes;
  • Fuel cell operation (feed pressures and rates, exhaust recirculation and release rate);
  • Bipropellant compatibility with fuel cells;
  • Cell design to produce useful voltages and currents;
  • Thermal control to achieve an operating temperature above 600°C; and
  • Fuel/oxidizer/exhaust management.

These many challenges must be addressed before SOFCs can be successfully integrated with a bipropellant thruster. However, Peng believes the AFRL technology can also be used to create solid oxide electrolysis cells that electrolyze water into hydrogen and oxygen.

“By pairing fuel cell and electrolysis technology,” Peng says in the AFRL press release, “we can set up a rechargeable energy storage system that can have more than 20 times the specific energy and more than 10 times the energy density of state-of-the-art rechargeable lithium-ion batteries.”

For further information on BEEPS, contact Peng at afrl.vss@us.af.mil.