Study of Structural Changes in a Nickel Oxide Containing Anode Material During Reduction and Oxidation at 600 °C

Abstract

In this work, the substructure changes in the YSZ–NiO ceramic material for solid oxide fuel cell anodes, during its reduction and oxidation at 600 °C, have been studied. A series of the YSZ–NiO specimens were undergone to three treatment modes at 600 °C, namely: (1) one-time reduction in a hydrogenous atmosphere; (2) redox cycling (five cycles), each redox cycle comprises the stages of isothermal dwell in a hydrogenous atmosphere and in air; and (3) redox cycling (five cycles), with extra stages of degassing. Two extra modes were used to simulate the behavior of materials in a water vapor-containing atmosphere. Increased porosity, along with an increased amount of reduced Ni, has been revealed in specimens after mode 2 test. It was established that in case of such treatment, a reaction of oxygen with the remaining hydrogen on the stage of isothermal dwell in air at 600 °C takes place followed by a substantial increase of water vapor local pressure. Such high-pressure conditions occur in small pores causing nucleation of nanocracks on three-phase (“nickel phase-zirconium phase-pore”) boundaries. Such effect of water vapor is probably the main reason of structural degradation of the cermet. After mode 3 test of specimens, it was revealed that the stage of degassing between half-cycles of reduction and oxidation plays a substantial role in the formation of Ni-network. Contrary to mode 2, the following structural peculiarities were detected: (1) formation of a network of nanopores in the particle outer layer; (2) reduction of Ni-phase particle size by separating Ni clusters of the particle outer layer; (3) redistribution of fine Ni particles that allows the porosity to be partially decreased; and (4) formation of a network of reduced Ni particles that improves electric conductivity and structural strength of the cermet.