The O_2 reduction on Pt electrode with an yttria stabilized
zirconia (YSZ) electrolyte is examined with potential step,
voltammetry and impedance measurements. Inductive hysteresis are
observed in all cases, indicating an activation-deactivation
process for the electrode reaction. The same is found when the
electrolyte is Gd doped ceria. The activation is generated by
current passage. The time constant for the hysteresis is large
considering the high operating temperatures, 800- 1000^oC. For the
activation process potential steps give two time constants 10^2s
and 10^3s for an anodic current, 10^3s and 10^5s for a cathodic
current. For the deactivation is the time constant about 10^4s.
The origin for the hysteresis is not clear, but expansion of the
three phase boundary (TPB) or change of the catalytic properties
due to surface segregation are suggested.The hysteresis phenomenon
is also treated by modelling. The phenomenological model proposed
can explain the principal behaviour of the inductive hysteresis.
The activation process has first order dependence of the current
density and the deactivation first order with respect to the
activation.AFM pictures of the electrode-electrolyte interface
show dynamics of the YSZ surface and formation of a bank of YSZ
along the TPB. These changes are induced by passage of current.
The origin of the dynamics behaviour may be a localised
temperature increase or it might be driven by segregation. The
dynamics of the YSZ surface seems to be irreversible to annealing
at 1000^oC.A separated part of the project was performed at
National Institute of Materials and Chemical Research, Tsukuba,
Japan. Here YSZ, Pr doped YSZ and Y doped SrCeO_3 were tested as
electrolytes in a one chamber fuel cell. Electrochemical
measurements and SIMS analysis of tracer exchange experiments
shows that the solubility of hydrogen in YSZ increases by doping
and might affect the performance of the one chamber fuel cell.
|Number of pages||118|
|Publication status||Published - 1998|