CCS and Power Systems

Advanced Energy Systems - Solid Oxide Fuel Cells


Solid Oxide Fuel Cell Research and Development


Performer: Argonne National Laboratory

Project No: FWP-49071


Accomplishments


  • Determined trends for strontium surface segregation in lanthanum strontium manganite (La1-xSrxMnO3; LSM), lanthanum strontium cobaltite (La1-xSrxCoO3; LSC), and lanthanum strontium cobalt ferrite (La1-xSrxCo1-yFeO3; LSCF) as a function of temperature and oxygen partial pressure. In doing so, equilibrium, high-temperature results were compared with room temperature non-equilibrium measurements.

  • Demonstrated the utility of resonant X-ray scattering techniques and inelastic X-ray measurements for probing the chemical state of materials under conditions approximating those of operating SOFCs.

  • Completed X-ray scattering and spectroscopy experiments comparing LSM (011) on yttria-stabilized zirconia (YSZ) (111) that were electrochemically conditioned in situ and ex situ. Ex situ conditioned samples were found to be more electrochemically stable when subject to the same applied potential for further experiments.

  • Demonstrated that surface strontium (Sr) segregation on LSM (011) on YSZ (111) is dependent on the distance from the contact wires under cathodic potential. Strontium was found to segregate in areas of the LSM thin film that is less active under potential.

  • Found that LSC (011) without a gadolinium-doped ceria (GDC) buffer layer reacts with YSZ (111) forming a resistive layer. LSC (001) on a GDC (001) buffer film was found to decompose if the GDC film is less than 60 nanometers thick.

  • Discovered that the LSCF (001) surface has a different crystal structure than bulk LSCF, and determined the sensitivity of the surface reconstruction to temperature and oxygen partial pressure (pO2).

  • Developed a detailed model of the surface polarization and position-dependent current for thin-film cathodes with wire electrodes. This included measuring and modeling the lateral potential drops from an LSCF on YSZ thin film when using a series of wires as current collector.

  • Found that, upon high temperature annealing, Sr segregates to the surface, new phases form, and there are chemical shifts on the surfaces of LSCF films grown on YSZ (111) or GDC/YSZ (001) with a strong dependence on the distance from the electrode contact.

  • Demonstrated that a thin LSM overlayer increases stability of LSCF films compared to bare LSCF films without the overlayer under similar annealing conditions.

  • Determined the lattice parameter and electrical conductance of thin LSCF films as functions of temperature, pO2, and electrochemical potential.

  • The effects of long-term applied electrochemical potential on the surface structure and chemical state of pulse laser deposited thin-film cathodes were measured.

  • A symmetric SOFC cell for ultra-small angle X-ray scattering studies was designed and constructed. The cell was used to determine the baseline porosity of sintered cathodes in preparation for atomic layer deposition infiltration studies.

  • Determined oxygen exchange rates in LSCF films based on conductivity and lattice parameter relaxation rates when cathodic/anodic potentials are applied.

  • Observed that Co and Sr segregate to the surface of LSCF film while Fe and La desegregate at operating temperature at open circuit potential (OCP).

  • Observed that only Co segregates and desegregates under electrochemical biases; Co remains segregated under OCP and anodic electrochemical biases while it desegregates under cathodic bias (decreased oxygen vacancies).

  • In situ ultra-small angle X-ray scattering measurements show that the average size of the porous cathodes increases rapidly above 900 °C and, albeit slowly, increases even at 800 °C implicating a slow degradation of active surface area during extended operations.


Project Details