In this study, the steady state current-overpotential (I-η) characteristics of the oxygen reduction reaction on the system Pt|yttria-stabilized zirconia (YSZ) were investigated at temperatures between 600 and 720°C. The I-η curve of Pt thin-film model electrodes on YSZ (100) exhibited a quite unexpected behavior, with diffusion limitation at lower and an exponential I-η relationship at high cathodic polarization. This situation was interpreted in terms of two parallel electrochemical oxygen reduction pathways. The diffusion limited path – with an activation energy of the limiting current of 1.57 eV – is attributed to a classical surface path on Pt with diffusion through an impurity phase at the triple phase boundary (TPB) being rate limiting. Stoichiometry polarization is most likely responsible for the exponential part of the I-η curve. In the corresponding second pathway, the oxygen reduction takes place on the YSZ surface with a rate limiting electron supply via the YSZ electrolyte. This interpretation is further supported by good accordance of the obtained activation energy (3.65 eV) with the activation energy of electronic conductivity in YSZ. The data can be used to calculate current contributions in broad temperature and overpotential ranges.
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