How To Get Mechanistic Information from Partial Pressure-Dependent Current–Voltage Measurements of Oxygen Exchange on Mixed Conducting Electrodes

The
oxygen incorporation and evolution reaction on mixed conducting
electrodes of solid oxide fuel or electrolysis cells involves gas
molecules as well as ionic and electronic point defects in the electrode.
The defect concentrations depend on the gas phase and can be modified
by the overpotential. These interrelationships make a mechanistic
analysis of partial pressure-dependent current–voltage experiments
challenging. In this contribution it is described how to exploit this
complex situation to unravel the kinetic roles of surface adsorbates
and electrode point defects. Essential is a counterbalancing of oxygen
partial pressure and dc electrode polarization such that the point
defect concentrations in the electrode remain constant despite varying
the oxygen partial pressure. It is exemplarily shown for La0.6Sr0.4FeO3−δ (LSF) thin film electrodes
on yttria-stabilized zirconia how mechanistically relevant reaction
orders can be obtained from current–voltage curves, measured
in a three-electrode setup. This analysis strongly suggests electron
holes as the limiting defect species for the oxygen evolution on LSF
and reveals the dependence of the oxygen incorporation rate on the
oxygen vacancy concentration. A virtual independence of the reaction
rate from the oxygen partial pressure was empirically found for moderate
oxygen pressures. This effect, however, arises from a counterbalancing
of defect and adsorbate concentration changes.