KINETICS OF THE ELECTRODE-REACTION AT THE CO-CO2, POROUS PT/STABILIZED ZIRCONIA INTERFACE

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-CO2 atmosphere at 600-1000-degrees-C. The impedance arcs were depressed semicircles, the centers of which were 45-degrees below the abscissa. The electrode impedance was expressed by a parallel circuit of resistance, R(E), and Warburg impedance, Z(E)=A(1-i)omega-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(O2) 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 CO2 rich atmosphere was determined as i=k0P(CO)1/2a0(1/2)-k0P(CO2)1/2 . It was shown that the rate determining reaction is CO(ad)(Pt) + O(ad)(SZ) --> CO2ad(Pt) at the triple phase boundary. Possible mechanism in the CO rich atmosphere was also discussed.