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The phenomenon of electrochemical promotion of catalysis (EPOC) is most often fully reversible. Subsequent to long-lasting polarization, however, the new steady-state open-circuit catalytic activity after current interruption may remain significantly higher than that before polarization. This phenomenon, discovered in our laboratory in the late 1990s and called permanent electrochemical promotion of catalysis (P-EPOC), has been observed on both oxide (IrO 2 , RuO 2 ) and metal (Rh) catalysts. P-EPOC is out of the state-of-the-art model of reversible EPOC, which considers the gas-exposed catalyst surface as the unique location of charge storage via backspillover of electrochemically generated promoter species accompanied by their consumption in the catalytic reaction ('sacrificial' promoter). Double step chronoamperometric and linear sweep voltammetric characterization of Pt catalyst deposited on YSZ solid electrolyte revealed the existence of a somewhat delayed oxygen storage occurring at the vicinity of the catalyst/electrolyte interface during prolonged anodic polarization. It is proposed that oxygen stored at this location, hidden for the reactant, and then released during relaxation was at the origin of P-EPOC on the Pt/YSZ catalyst observed in catalytic combustion of ethylene with oxygen. The effect of this 'hidden' promoter on the catalytic reaction rate was found to be highly non-Faradaic.

The electrochemical behavior of Pt/YSZ electrodes in oxygen containing atmosphere at 450°C has been investigated by double-step chronoamperometry and programmed linear sweep cyclic voltammetry. The response of the O 2 (g),Pt/YSZ system in these experiments could be separated into a time dependent and a steady state contribution, the former being dominated by pseudocapacitive processes. It is proposed that Pt-O type species were stored via different processes at three different locations in the O 2 (g),Pt/YSZ system: (1) Build-up of a platinum oxide monolayer at the Pt/YSZ binary interface. (2) Formation of Pt-O species at the triple phase boundary and their spreading-out along the Pt/gas interface. (3) Growth of the platinum oxide layer from the binary Pt/YSZ interface toward the bulk of the platinum electrode.

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.