Mechanical Characterization of Solid Oxide Fuel Cells and Sealants

3
-
2
. The sequential tape casting simplified the production 
into two manufacturing technologies and only three sintering steps.
Figure 3-2: Schematic overview and comparison of the traditional and novel manufacturing 
route 
[22]. 
3.2.
Anode materials 
3.2.1. Working principle
The most common anode material is a porous Ni-YSZ cermet due to its good electro-catalytic 
properties along with the low cost. These anodes are typically prepared by mixing NiO with YSZ 
usually with an additional pore former [24]. In order to simplify the production procedure, the 
anode is normally produced in an oxidized state by warm pressing or tape casting [20]. Once it is 
exposed to hydrogen gas, the NiO is reduced to Ni. The Ni metal phase provides the electronic 
conductivity and catalytic activity [25]. To achieve a high efficiency and sufficient conductivity 

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Literature review 
9 
(e.g. 1000 S cm
-1
), a porous Ni cermet has to be created. An oxidized state with usually a 
porosity of around 13 vol% Ni leads to the necessary porosity of 30 vol% in the reduced state [6, 
26]. The gas diffuses through the pores and reacts chemically at the triple phase boundaries (TPB) 
of the electrochemically active anode layer which has usually the same composition as the anode 
substrate but a lower porosity, see 
Figure 
3-3
 [27]. 
Figure 3-3: Anode reaction process in a SOFC [27]. 
However, the good electronic and ionic conductivity also depend on the geometry of the anode, 
i.e. the required electronic conductivity can vary from 1 to 100 S cm
-1
and the ionic conductivity 
should be greater than 0.1 S cm
-1 
[28, 29]. The electrical conductivity of Ni-YSZ cermet strongly 
depends on its Ni content. To achieve a higher electrical conductivity, the Ni content needs to be 
optimized. The conductivity of the cermet as a function of Ni content is an S-shaped (
Figure
 3-4
) 
curve as predicted by percolation theory. The percolation threshold for conductivity is located at 
a Ni content of ~30 vol% [30, 31]. The porosity affects the performance, and the optimal 
porosity for the electrochemical operation was found to be around 40% [32]. 
Hydrocarbon fuels (e.g. CH
4
) are considered as the main choices to be used as SOFC fuels due to 
their world wide availability [33]. In fact, Ni is also a particularly good catalyst for decomposing 
CH
4
[34]
. 
Continuing efforts to improve performance and reliability of SOFCs require further development 
of materials [14], especially in terms of mechanical aspects in particular with respect to the 
substrate in the anode-supported cell design [35, 36]. The anode-supported Jülich SOFC design 

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Literature review 
10 
consists of a thin (10~30 µm) anode functional layer adjacent to the electrolyte and a thick anode 
substrate (500~1000 µm) [37]. To achieve a sufficient operational performance, the anode of a 
SOFC has to meet a number of stringent criteria, such as catalytic activity towards the 
electrochemical oxidation of the desired fuel, good electronic conductivity (continuous Ni-phase), 
small mismatch in the real expansion coefficient, high permeability for the fuel gas and reaction 
products (well-sized and connected porous structure) and robustness if used as a mechanical 
support [24, 36, 38].
Figure 3-4: Conductivity of Ni-YSZ cermet as a function of Ni content [30]. 
As outlined above, a higher Ni content leads to a good conductivity, but on expense of stability 
due to thermal expansion coefficient mismatch with YSZ cermet [39]. Ni has a higher thermal 
expansion coefficient than YSZ, which causes the a thermal expansion mismatch between anode 
and YSZ electrolyte (~10.5 · 10
-6
K
-1
) [40].