Study of Structural Changes in a Nickel Oxide Containing Anode Material During Reduction and Oxidation at 600 °C
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.
KeywordsSolid oxide fuel cell Anode material Nickel oxide Reduction Oxidation Redox cycling High temperature Structure Mechanical properties Degradation
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