Two types of O 2 ,Pt/YSZ electrode preparation (Pt/YSZ cermet and sputtered platinum film) have been characterized by SEM and by cyclic voltammetry and chronoamperometry at 450°C in 20 kPa oxygen. Cyclic voltammetry on the cermet and on the as-sputtered non-porous film electrode evidenced the characteristics of the PtO x /Pt couple. The corresponding redox reaction occurs at the metal/electrolyte interface and it manifests itself by an anodic wave and one of more cathodic peaks in the voltammogram. Heat treatment of the sputtered electrode at 700°C in oxygen atmosphere resulted in a porous structure by coalescence of the film. Cyclic voltammetry of the porous film electrode featured the characteristics of the O 2 /O 2) couple, i.e. the redox reaction of gaseous oxygen occurring at the tpb. Chronoamperometry at anodic potentials showed similar features for both electrode preparations: an initial inhibition, a current peak and a slow activation, the latter being related to the phenomenon of electrochemical promotion of catalysis.
Electrochemical impedance spectra at 450-600°C and P O2 ¼ 0:5 kPa of a rhodium catalyst interfaced with yttria-stabilizedzirconia (Rh/YSZ) were compared with a model based on the mechanism of electrochemical promotion. In the proposed equivalent electric circuit, existence of an ''effective'' double layer at the gas-exposed catalyst surface and its potential-controlled modification via diffusion of oxygen ions between the O 2) conducting solid electrolyte support (YSZ) and the catalyst are represented by two additional elements: adsorption capacitance and Warburg impedance. Under positive polarization, the adsorption capacitance increases dramatically indicating reinforcement of the ''effective'' double layer at the catalyst/gas interface, in agreement with the observation known from electrochemical promotion practice that positive polarization of a rhodium electrode leads to rhodium oxide reduction, hence, to dramatic increase in catalytic reaction rate.
The electrochemical activation of ethylene oxidation was studied over rhodium catalysts of different thickness (40, 100 and 160 nm) sputtered on top of a thin layer of TiO 2 deposited on YSZ. The strong relationship between catalytic activity and oxidation state of rhodium was confirmed. Under open-circuit operation the catalyst potential appears as a suitable indicator of the surface oxidation state of rhodium allowing a prediction of the catalytic behavior from solid electrolyte potentiometric measurements. Under closed-circuit conditions the catalyst potential was used as a tool to tune the catalytic activity of rhodium which showed increasing promotional efficiency with decreasing catalyst film thickness.
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