Support (La0.8Sr0.2)0.95MnO3−δ extrusion
Oxygen electrode (La0.8Sr0.2)0.95MnO3−δ/8YSZ painting/YSZ dip-coat
Electrolyte 8YSZ dip-coat/painting ink
Fuel electrode NiO-8YSZ painting ink
Fuel electrode current collector Ni mesh/Ag paste mesh wound/painting Seal/interconnect Cu-Ag braze or ceramic paste brazing or painting Table 1.3.5: Proposed materials and manufacturing methods for the tubular reversible SOFC. The inks used will be screen printing inks. 8YSZ is ZrO2 doped with 8 mol%
Y2O3, or Zr0.852Y0.148O2−δ.
1.4 Reversible SOFC microstructural characterisation
The SOFC has a microstructure on the order of 100 nm - 100 µm, due to the particle
sizes of the ceramic powders used to make it, and is therefore suited to examination with a scanning electron microscope (SEM), which can reveal the details of the microstructure. Fig. 1.4.1 shows a diagram of an SEM. It operates in high vacuum, using a beam of electrons incident on the sample, which allows for a higher resolution than an optical microscope, because the wavelength of the electrons is shorter than the wavelength of photons in visible light. The voltage of the electron beam may be adjusted in the kV range.
Figure 1.4.1: Schematic of a scanning electron microscope (SEM), not to scale [89].
Electrons and radiation are back-scattered from the sample, and detected. An image is pro- duced from the signal. Non- conductive samples to be ex- amined are often coated with gold in an evaporation or sput- tering process. This prevents the surface of the sample char- ging under the electron beam which distorts the image. How- ever, samples that have a mix- ture of a conductive and non- conductive phases, such as a reduced Ni-YSZ fuel electrode, may be imaged without a gold
coating at low accelerating voltages. This method was applied to a cross section of Ni-YSZ electrode, using a field emission-SEM and a secondary electron detector. Particles which are less conductive were observed to charge more and appear brighter. Therefore, four phases could be distinguished, as they back-scattered different numbers of electrons at the low accelerating voltage used: YSZ, non-percolating Ni, percolating Ni, and pores [90].
Software can be used to analyse SEM pictures. One study analysed the 2D micro- structure from a series of SEM images, then created a 3D model of the microstructure from the information. This was used to inform a computer model of SOFC operation - to relate microstructural features to performance of the cell [91, 92]. SEM images can also be analysed by software to determine levels of porosity, and pore size distributions, based on the contrast between pores and ceramic grains in an SEM image.
Energy-dispersive X-ray spectroscopy may be used to understand qualitatively and quantitatively, the distribution of elements through a sample. It is typically carried out at higher accelerating voltages in an SEM. The incident electrons strike the sample, exciting core electrons in surface atoms into higher energy levels. When these relax to lower energy levels, they emit X-rays which have energies that match the difference between the excited and the ground state energy levels. As each element has different gaps between the electron energy levels, the X-rays emitted are characteristic for that element. A detector measures these X-rays, and can resolve the position of the atoms of each element. If standard samples are employed, the system can be calibrated to measure quantitatively the amount of each element present on the surface in a specific area.
Therefore, by using EDX, phases in the sample can be distinguished. This is useful to detect the formation of undesirable phases, such as lanthanum/strontium zirconates in LSM-YSZ electrodes. It was used to determine where the elements in a Ag-CuO braze segregated to, and showed the formation of Cu-Cr compounds at the interface [39]. This is very useful, as even small amounts of a compound at an interface can have a dramatic effect on the ionic or electronic conductivity across the interface.
There are several advanced techniques for directly measuring the microstructure of the fuel cell, which can be either destructive, such as focused-ion beam (FIB)-SEM, or non-destructive, such as X-ray computed tomography. FIB-SEM was applied to a Ni-YSZ fuel electrode, by using an ion beam to ablate a series of 50 nm layers from the electrode, with SEM images being taken after each layer was removed. The images were analysed by a computer, and a very detailed 3D model of the electrode was obtained. From this, parameters such as the percolation of the Ni and YSZ phases, the TPB length, porosity, tortuosity, etc. could be determined [93]. This is one of the best ways to fully characterise the microstructure, because of the high resolution obtainable in 3 dimensions, down to 20 nm - 30 nm [94, 95]. However, it is quite time-consuming, and only examines a small area of the electrode.
A comparable non-destructive method is X-ray computed tomography, which is extens- ively used in medicine. X-rays are passed through the sample, and an image is recorded, it is rotated slightly, and then the process is repeated. The resolution is 50 nm or better. The method is more accurate at microstructure and pore characterisation than other methods such as mercury-intrusion porosimmetry (MIP), because it can see isolated pores that MIP cannot, and provides better estimates of other parameters [96].
The particle size of the ceramic powders used is also very important in determining the sintering behaviour, and the electrochemical performance of the cell. This can be measured by a particle size analyser, which uses a laser to measure the size of particles in suspension
1.5. REVERSIBLE SOFC PERFORMANCE 47