  The efficacy of the electrical conductivity relaxation (ECR) technique for
investigating the oxygen transport properties of mixed conducting oxides has
been evaluated. Fifteen mol% samarium doped ceria (SDC15), for which
approximate values of the two principal transport properties, bulk oxygen
diffusivity, $D_{Chem}$, and surface reaction rate constant, $k_S$, can be
found in the literature, was chosen as the benchmark material against which to
validate the methodology. Measurements were carried out at temperatures between
750 $^\circ$C and 850 $^\circ$C and over a wide range of oxygen partial
pressures. An unexpectedly high p-type electronic transference number enabled
ECR measurements under oxidizing conditions. A systematic data analysis
procedure was developed to permit reliable extraction of the kinetic parameters
even in the general case of simultaneous bulk and surface limitation. The
$D_{Chem}$ from this study showed excellent qualitative and quantitative
agreement with expected values, falling in the range from $\sim 2\times10^{-5}$
to $2\times10^{-4}$ cm$^2$/s. The surface reaction constant under H$_2$/H$_2$O
mixtures also showed good agreement with literature results. Remarkably, this
value increased by a factor of 40 under mixtures of CO/CO$_2$ or O$_2$/Ar. This
observation suggests kinetic advantages for production of CO rather than H$_2$
in a two-step solar-driven thermochemical process based on samarium doped
ceria.
