Reduction kinetics of nickel-based supporting anode composite substrates under operating conditions of intermediate-temperature solid oxide fuel cells

Nickel oxide reduction kinetics and formation of the electric potential of anode-supported solid oxide fuel cell (SOFC) were studied using in situ Raman spectroscopy (RS) and conventional electrochemical techniques. It was shown that the time dependence of the electric potential and the intensity of the Raman spectra during anode substrate reduction can be conditionally divided into two stages. At the initial stage, in the first seconds after hydrogen supply, the open circuit voltage (OCV) quickly rises and stabilizes at a value of about 0.85 V. This stage is not accompanied by noticeable changes in RS. Then, there is a more gradual rise, requiring tens of minutes, to the OCV equilibrium value. The beginning of the second stage of the increase in the OCV coincides with a sharp change in the intensity of the specific spectral line in the Raman spectrum. An increase of the OCV in the first seconds after hydrogen supply is explained by an increase in the hydrogen concentration at the outer boundary of the SOFC anode. The sharp change in the intensity of the Raman spectrum, in turn, can be explained by the reduction of NiO to the metallic state in the near electrolyte region. In this work, a detailed reduction model of the NiO-YSZ (yttria-stabilized zirconia) cermet composite supporting substrate of an anode-supported SOFC was constructed. The model was developed on the assumption that the rate of the nickel oxide reduction reaction at each point of the sample depends only on the partial pressure of hydrogen, the fraction of oxidized nickel, and temperature. In the case of repeated reduction and at high temperatures, it can be assumed that the reaction has first-order kinetics, i.e., reaction rate is proportional to the fraction of oxidized nickel. At primary reduction and temperatures of 400–600 °C, the reaction is described by Avrami-Erofeev kinetics.