Short-term strong cathodic polarization of Ni/YSZ and Pt/YSZ

The initial stages of YSZ reduction and formation of intermetallic phases at Ni-YSZ interfaces on strongly cathodically polarized electrodes were studied by a number of potential sweep and impedance techniques. The measurements were carried out in the potential range -1.0 V to -3.0 V vs E°(O2) in H2/H2O at 650 and 750 °C. Below -1.7 V the cathodic current increases almost exponentially, mainly due to electronic conductivity in the YSZ. Reduction and oxidation peaks develop below and above -1.9 V, respectively. The peaks reveal a simultaneous reduction of YSZ at the Ni-YSZ interface and reoxidation of a Ni-Zr phase. Charges calculated from the reoxidation peaks indicate a thickness of 200 nm for the layer formed by conditioning the electrode for 300 s at -2.6 V vs E°(O2). Impedance measurements show a suppressed arc decreasing with increasing polarization. Below -2.3 V vs E°(O2) a capacitive high frequency arc segment and an inductive low frequency loop develop. Both are ascribed to electronic conductivity. SEM/EDS microscopy on cross sections of samples cooled with and without polarization showed the formation of uniform and homogeneous intermetallic reaction layers which after reoxidation resulted in a two-phased nanostructures.