Kinetics of the electrode reaction at the CO-CO 2, porous Pt/stabilized zirconia interface

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To elucidate the reaction kinetics of CO gas at the electrodes of zirconia sensors and solid oxide fuel cells, measurements were made on the electrochemical impedance spectra and steady-state polarization current at the porous Pt/stabilized zirconia (SZ) electrodes in the COCO 2 atmosphere at 600–1000°C. The impedance arcs were depressed semicircles, the centers of which were 45° below the abscissa. The eletrode impedance was expressed by a parallel circuit of resistance, R E , and Warburg impedance, Z E =A(1−i)ω − 1 2 . Here, A is related to the diffusion constant, D, of the diffusion process in the electrode reaction by A=kD − 1 2 , where k is constant. The P O 2 and temperature dependences of D agreed with those of the electronic conductivity of SZ. It was concluded that Z E is determined by the oxygen chemical diffusion in SZ in the relaxation at the closely contacted interface of the Pt particles/SZ. From the steady-state current-potential relationships, the rate equation in the CO 2 rich atmosphere was determined as i=k OP 1 2 CO a 1 2 0−k 0P 1 2 CO2 . It was shown that the rate determining reaction is CO ad(Pt)+O ad(SZ)→CO 2ad(Pt) at the triple phase boundary. Possible mechanism in the CO rich atmosphere was also discussed.