High temperature kinetics of metal-oxygen reactions by solid oxide electrolyte cells: an application to nickel

Chronopotentiometric curves, generated by galvanostatic single steps, applied to a solid electrolyte cell, were analyzed on the basis of a dimensionless equation derived on the assumption that a scale of oxide grows at 1 of the electrode-electrolyte interfaces.  This process is rate limiting for developing the charge transfer-diffusion overvoltage; Wagner's theory on tarnishing, under inhibiting elec. field conditions, was assumed for treating the kinetics of the growing scale.  The kinetic model developed was tested for the reaction of solid Ni (pos. electrode) at 1268 K with O partial pressures ranging from 3.9 × 10-11 atm [equil. pressure of Ni(s)-NiO(s) system] to 1.7 × 10-10 atm.  Both the rate and the rate const. of the scale formation go through the max. but at different times, while the rate const. dependence of the O partial pressure agrees with the theory.  The max. rate const. was 9.8 × 108 mol. cm-1 s -1.  The calcd. scale thicknesses were between 6 and 24 nm.