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, Mo|Ni(s)-NiO(s)|CSZ|Fe(s)-Fe1-δO(s)|Mo, (CSZ = CaO-contg. ZrO2) was studied with respect to the sintering time of the active powders.  Pellets from short (7 h) and long (14 h) sintering times were prepd. and assembled to give the above cells.  Each of them was thermally cycled and only the cells contg. Ni(s)-NiO(s) electrodes prepd. with a long sintering time gave emf. vs. T curves which were independent of cycle.  These values agreed closely with the literature.  For the cell reaction, NiO(s) + (1 - δ)Fe(s) = Ni(s) + Fe1-δO(s), ΔG = -(27.85 ± 0.06) - (0.02157 ± 0.00004)T kJ mol-1 at 977-1350 K.  To check the electrochem. reversibility, cyclic voltammetry was also used.  On the basis of these results and of SEM anal. 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.