The role of sintering time on reversibility of Ni(s)—NiO(s) electrodes for high temperature solid oxide electrolyte galvanic cells

The reversibility of solid electrolyte galvanic cells such as $${\text{Mo/Ni(s)}}--{\text{NiO(s)/CSZ/Fe(s)}}--{\text{Fe}}_{{\text{1}}--\delta } {\text{O(s)/Mo}}$$ has been studied with respect to the sintering time of the active powders. Pellets from short (7h) and long (14h) sintering times have been prepared and assembled to give the above cells. Each of them has been thermally cycled and only the cells containing Ni(s)-NiO(s) electrodes prepared with a long sintering time give emf versus T curves which are independent of cycle. These values are in close agreement with the literature. For the cell reaction $${\text{NiO(s)}} + (1 - \delta ){\text{Fe(s) = Ni(s)}} + {\text{Fe}}_{1 - \delta } {\text{O(s)}}$$ the free energy change $$\Delta G = - (27.85 \pm 0.06) - (0.02157 \pm 0.00004)T{\text{ kJ mol}}^{ - {\text{1}}} $$ has been found in the temperature range 977–1350 K. To check the electrochemical reversibility, cyclic voltammetry has also been used. On the basis of these results and of SEM analysis of the electrode pellets, a mechanism is proposed whereby only at long sintering time would a triple phase contact at the electrode/electrolyte interface be produced.