Diffusion‐Limited Charge Transport at Platinum Electrodes on Doped CeO2

We have measured the d‐c and a‐c properties of platinum paste electrodes on samples of the solid electrolyte ( CeO 2 ) 0.9 ( Gd 2 O 3 ) 0.1 in the temperature range 700°–900°C and the oxygen pressure range 1 ≥ P O 2 ≥ 10 − 4 atm . The d‐c measurements showed that the current through the system saturated at a value proportional to the oxygen pressure and that the cathode (the electrode where oxygen enters) limits the current. We explain the limitation by a gas diffusion mechanism, which adds a small term to the dynamic resistance of the sample. The dynamic resistance is mainly determined by the resistivity of the electrolyte together with the current constriction at the triple line gas‐electrolyte‐electrode. A‐C measurements gave impedance plots with the shape of a quarter circle, pointing to a diffusion process. We propose as model that part of the oxygen molecules arriving at the triple line is ionized directly and enters the electrolyte, and a second part diffuses as atoms along the interface electrolyte‐electrode before being ionized. This model explains the observed quarter circle as well as the impedances measured for ω = 0 and for ω=∞.