Electrochemical reduction of NiO in a composite electrode

Electrochemical reduction of NiO in a composite electrode along with 8 mol% Y2O3 stabilized zirconia (8YSZ) is studied. Voltage sweeps are performed on cells with a counter electrode made from Pt or the composite of (La0.75Sr0.25)0.95MnO3 ± δ (LSM25) and 8YSZ, but the composite electrode gives a lower overpotential. Microstructures of NiO with different reduction degrees are shown. Electrochemical impedance spectroscopy is carried out during the reduction process. Electrochemical reduction of NiO may need an induction period. When NiO is reduced at a constant voltage the current initially increases, hereafter a decay is observed. The kinetics of electrochemical reduction of NiO can be described by the Avrami equation. In performed experiments the exponent of the Avrami equation is in the range of 0.5–0.7 and the overall rate constant varies from 1.19 to 7.73 × 10− 3 and increases with temperature and in particular with the applied voltage. The maximum reaction rate is obtained when 2–11% NiO is reduced. The increasing resistance in NiO reduction can be exclusively ascribed to one arc in impedance spectra.