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Y. Kharchenko et al.
be accidental interruptions in fuel supply; (4) performing shut down of the fuel cell
and its start-up without inlet of a neutral gas. The abovementioned reasons cause
deterioration of the structure and mechanical and physical properties of the YSZ–Ni
cermet anode [6]. This requires a significant effort of researchers to be applied in
order to improve SOFC technology.
Before considering the YSZ–Ni cermet material, one should study the reduction
and oxidation of pure nickel.
During the reduction of NiO in hydrogen, Ni clusters are nucleated. The cluster
growth rate is almost constant for the larger part of the reduction process. It is finally
lowered because of the formation of porous Ni layer which complicates the hydrogen
diffusion.
In the case of metallic nickel behavior in air, the oxidation forms on its top NiO
layer according to the reaction Ni + ½O 2 = NiO. This layer separates the oxygencontaining gas and the metallic Ni [7]. For the case when NiO layer thickness is less
than 0.1 μm, the oxidation process is performed according to the anodic reaction
(Ni = Ni
2+
+ 2e
− ) and the cathodic one (½O 2 + 2e
−
= O
2− ). For thicker layers,
this process is due to ion diffusion through the NiO scale.
At high temperatures, the Ni
2+ cation diffusion in NiO is much faster than the
O
2− anion diffusion. At these conditions, voids are formed inside the NiO layer
during its growth. Microcracking occurred in the bulk of the material promotes the
oxygen inward diffusion. The microstructure of the NiO layer may vary depending
on the oxidation temperature and the layer thickness.
When Ni particles are small, the difference in diffusion coefficients of Ni
2+ and
O
2− causes the growth of multiple internal nanopores in the NiO layer. At the temperatures in the range of 800–1000 °C, it is observed that the outward Ni
2+ cation diffusion is faster than the inward O
2− anion diffusion. This is a cause of NiO internal
porosity. It is known that smaller nanoparticles show single pores. In the case when
Ni particles are larger, the Ni self-diffusion is too slow and the multiple nanopores
cannot be condensed into a single pore [8]. So, the larger particles present multiple
pores.
It was found by some researchers that the oxidation rate decreases with decreasing
the oxygen partial pressure [9–12]. This may be used as the main principle of
microstructural modification during oxidation.
At low temperature (about 600 °C), diffusion of the Ni
2+ ions through boundaries
of the NiO grains occurs [13]. In this case, the oxidation rate depends on the grain
size in the NiO layer.
This work is aimed to perform a study on micro- and nanostructural changes
in nickel-containing cermets for SOFC anodes during high-temperature (600 °C)
reduction and reoxidation, highlighting both the positive and negative effects on
their physical and mechanical properties.
Y. Kharchenko et al.
be accidental interruptions in fuel supply; (4) performing shut down of the fuel cell
and its start-up without inlet of a neutral gas. The abovementioned reasons cause
deterioration of the structure and mechanical and physical properties of the YSZ–Ni
cermet anode [6]. This requires a significant effort of researchers to be applied in
order to improve SOFC technology.
Before considering the YSZ–Ni cermet material, one should study the reduction
and oxidation of pure nickel.
During the reduction of NiO in hydrogen, Ni clusters are nucleated. The cluster
growth rate is almost constant for the larger part of the reduction process. It is finally
lowered because of the formation of porous Ni layer which complicates the hydrogen
diffusion.
In the case of metallic nickel behavior in air, the oxidation forms on its top NiO
layer according to the reaction Ni + ½O 2 = NiO. This layer separates the oxygencontaining gas and the metallic Ni [7]. For the case when NiO layer thickness is less
than 0.1 μm, the oxidation process is performed according to the anodic reaction
(Ni = Ni
2+
+ 2e
− ) and the cathodic one (½O 2 + 2e
−
= O
2− ). For thicker layers,
this process is due to ion diffusion through the NiO scale.
At high temperatures, the Ni
2+ cation diffusion in NiO is much faster than the
O
2− anion diffusion. At these conditions, voids are formed inside the NiO layer
during its growth. Microcracking occurred in the bulk of the material promotes the
oxygen inward diffusion. The microstructure of the NiO layer may vary depending
on the oxidation temperature and the layer thickness.
When Ni particles are small, the difference in diffusion coefficients of Ni
2+ and
O
2− causes the growth of multiple internal nanopores in the NiO layer. At the temperatures in the range of 800–1000 °C, it is observed that the outward Ni
2+ cation diffusion is faster than the inward O
2− anion diffusion. This is a cause of NiO internal
porosity. It is known that smaller nanoparticles show single pores. In the case when
Ni particles are larger, the Ni self-diffusion is too slow and the multiple nanopores
cannot be condensed into a single pore [8]. So, the larger particles present multiple
pores.
It was found by some researchers that the oxidation rate decreases with decreasing
the oxygen partial pressure [9–12]. This may be used as the main principle of
microstructural modification during oxidation.
At low temperature (about 600 °C), diffusion of the Ni
2+ ions through boundaries
of the NiO grains occurs [13]. In this case, the oxidation rate depends on the grain
size in the NiO layer.
This work is aimed to perform a study on micro- and nanostructural changes
in nickel-containing cermets for SOFC anodes during high-temperature (600 °C)
reduction and reoxidation, highlighting both the positive and negative effects on
their physical and mechanical properties.