Short-Term Strong Cathodic Polarization of Ni/YSZ and Pt/YSZ

Karin Vels Hansen,Kosova Kreka,Torben Jacobsen,Vignesh Balasubramanian

doi:10.1149/1945-7111/abf87e

Karin Vels Hansen, Kosova Kreka + Show 2 more

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https://doi.org/10.1149/1945-7111/abf87e

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Abstract

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.

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Three cycles were recorded at each sweep rate, but—except for Fig. 3-only the second sweep is shown in the figures
The first time the electrode is polarized below −2.1 V vs E°(O2) reduction of YSZ in contact with Ni is initiated within 5 min and a Ni-Zr metallic layer with inclusions of yttria is formed at the interface

Summary

Introduction

Users may download and print one copy of any publication from the public portal for the purpose of private study or research. In the reoxidation process following the pulse, a nanoporous Ni-zirconia structure is formed.[8] In SOEC’s the Ni cathode can at high current densities be polarized to potentials where a partial reduction of YSZ occurs. The importance of this for the cell degradation is not known at present, but it emphasizes that the Ni-YSZ interface is a dynamic structure that may change with varying current load and polarization. In very recent SOEC degradation studies with rather large active electrode areas, inductive low frequency loops were observed and attributed to an adsorption process that allows H2O reduction on Ni sites polluted with Si impurities from the sealing,[12] electronic conduction through the YSZ electrolyte[13] or reduction of YSZ.[14]

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Discussion

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