Optimizing the conductivity of ceria-based oxides, or doped
ceria, is crucial to their use as electrolytes in future solid oxide fuel
cells.
Researchers from the NIST
Center for Nanoscale Science and
Technology and Arizona State University
have successfully used kinetic lattice Monte Carlo
simulations to predict the optimum dopant concentration for maximizing conductivity
for gadolinium doped ceria, and for double-doped (praseodymium and gadolinium)
ceria, at temperatures (773 K to 1,073 K) that are practical for fuel cell
operation.
Compared with the electrolytes that are commonly used in solid
oxide fuel cells, doped ceria has higher conductivity and therefore higher
efficiency. It also operates at lower temperatures, which may reduce the
overall material costs for the fuel cells.
The researchers used their previously published Monte Carlo model to calculate activation energies using
density functional theory that includes electron interactions (DFT + U) in
order to study time-dependent vacancy diffusion.
Their results showed that ionic conductivity is maximized
between 0.2 mole fraction and 0.25 mole fraction for gadolinium and decreases
slightly for higher concentrations. For the same doping concentrations,
double-doped ceria had higher ionic conductivity than single-doped, with
gadolinium-rich double-doped ceria having the highest conductivity.
The models explain the performance difference between double
and single doping by showing that in the double-doped ceria, vacancy diffusion
follows low energy migration paths.
The researchers’ calculations agree with available experimental
data, indicating that their model can be used to predict the behavior of other
lanthanide co-dopants in ceria-based oxides.
