Investigation of O2 reduction on Pt/YSZ by means of thin film microelectrodes: The geometry dependence of the electrode impedance

The electrochemical oxygen exchange reaction on yttria stabilized zirconia (YSZ) was investigated by means of Pt microelectrodes. The circular-shaped electrodes were prepared by sputter deposition of platinum onto a heated YSZ substrate and photolithographic micro-structuring of the Pt film. Electrochemical characterization was carried out by impedance spectroscopy at temperatures ranging from 700 to 900 degrees C. Analysis of the results obtained on differently sized microelectrodes showed that the main part of the polarization resistance of O-2 reduction scales with the inverse length of the three phase boundary (TPB). Consequently, Pt (O-2)/YSZ represents a so-called surface path system with the TPB-zone as the active region for oxygen incorporation and the rate-limiting step being located close to this TPB. At 700 degrees C a TPB-related reciprocal electrode resistance (electrode conductivity) of 6.6 +/- 0.9 10(-7) Omega(-1) cm(-1) with an activation energy of 1.36 +/- 0.11 eV was determined. An additional, much smaller contribution to the polarization resistance scales with the inverse electrode area and is interpreted in terms of a limited oxygen reservoir at the Pt/YSZ interface. Several implications of temperature gradients resulting in microelectrode measurements (e.g. thermovoltage) are discussed. (C) 2010 Elsevier B.V. All rights reserved.