Some Reasons of the Degradation of a Fine-Grained YSZ–NiO Anode Material During Intense Reduction and Reoxidation

This work is aimed at studying the high-temperature reduction of nickel oxide and reoxidation of metallic nickel in a fine-grained YSZ–NiO anode material. The YSZ–NiO anode substrates were manufactured by tape casting. As-sintered material (of mode 1) was undergone to two treatment mode: one-time reduction in pure hydrogen for 4 h at 600 °C under a pressure of 0.15 MPa (mode 2); reduction/oxidation (redox) for five cycles in hydrogen/air atmospheres at 600 °C (mode 3). As-sintered material (mode 1) showed the strength under three-point bending about 209 MPa that is high enough for anode materials. Significant changes in the microstructure of the material have been detected after reduction in hydrogen (mode 2). It consisted of agglomerates of particles (average size about 5 µm) with a predominance of the zirconium phase as well as regions of small particles (1 µm) with uniformly distributed both the nickel and zirconium phases. The intergranular fracture micromechanism prevailed in the specimens tested. The material exhibited strength of about 40 MPa that did not satisfy the requirements for anode materials. Despite too “rigid” mode of redox cycling in pure hydrogen and air at 600 °C (mode 3), according to the level of strength (85 MPa), the cermet is not inferior to those obtained using other techniques. On the specimen fracture surface, dense outer layers and a porous core have been revealed. An increased amount of nickel in the surface layer is probably a consequence of the redistribution due to the diffusion of nickel from the subsurface region towards the surface. The redox treatment led to the degradation of the specimen core by forming large pores, their coalescence into cracks, and loss of material integrity. The intergranular fracture micromechanism in the specimen core was noted, in contrast to the mixed one revealed in the surface layer. The obtained value of electrical conductivity (7.4·105 S/m) and a certain positive effect of this treatment on strength indicate the potential of the tape cast material in the case of the optimization of its redox treatment mode, in particular, by reduction in Ar–5 vol% H2 gas mixture instead of pure hydrogen.