Application of Various Reduction Techniques for the Improvement of Microstructure and Strength of YSZ–NiO(Ni) Solid Oxide Fuel Cell Anodes

Recently, solid oxide fuel cells (SOFCs) as devices for the production of green energy are being intensively developed. An anode substrate used in such devices is predominantly fabricated by conventional powder sintering or tape casting of yttria-stabilized zirconia/nickel oxide (YSZ–NiO) ceramics. The as-sintered YSZ–NiO anode ceramics should be preconditioned via reduction of NiO to Ni in a hydrogen-containing atmosphere. This also ensures an appropriate level of electrical conductivity of SOFC anodes. This work is aimed at studying both the effects of preconditioning in a high-temperature (600 °C) reduction atmosphere and cyclic treatment in reduction/oxidation atmospheres (redox cycling) on strength of YSZ–NiO(Ni) cermets. The specimens of size 0.5 × 5 × 25 mm3 were cut of YSZ–NiO ceramic plates. The preconditioning modes were as follows: (i) one-time reduction in Ar–5 vol% H2 mixture for 4 h at 600 °C under a pressure of 0.15 MPa; (ii) redox treatment for three cycles in Ar–5 vol% H2 mixture/air at 600 °C with intermediate degassing. A part of the specimens were then aged for 100 h at 600 °C in Ar–5 vol% H2 mixture under a pressure of 0.15 MPa. A series of as-sintered ceramic specimens and cermet specimens after treatments were tested under three-point bending. Scanning electron microscopy was used to analyze the peculiarities of changes in microstructure and fracture surface morphology of aged materials. It was revealed that the one-time reduction in Ar–5 vol% H2 mixture followed by exposure for 100 h under such conditions did not cause a discernible change in mechanical behavior of the material. In particular, flexural strength of the cermet lowered by 12–15% as compared to that of as-sintered ceramics. In contrast, specimens after redox treatment followed by aging for 100 h exhibited a significant drop in flexural strength (by 39–47%). Features of microstructure degradation were substantiated based on a developed pore coalescence model of damage accumulation.