SISTEMA EXPORTACIÓN

Various microstructural factors, such as porosity, electrode thickness, particle size, and percolation have a large impact on the cell performance. These arise from the starting materials and fabrication methods. Therefore, an optimised microstructure is crucial to achieve a high performance cell.

The porosity of the electrodes is usually measured as the volume percentage of pores in the electrode, but the pore size distribution is also important. If the porosity is too low, and there is a high flow of gas through the electrode, e.g. at high currents, this can cause a diffusion polarisation resistance to be observed. This may also appear if there is a stagnant layer of gas over the electrode [29]. However, if the porosity is too high, electronic and ionic conductivity will limit cell performance. These effect is particularly important in tubular supports, which are many times thicker than the other active layers.

The effect of particle size distribution on porosity and performance is illustrated by a study on a Ni/CGO (cerium doped gadolinium oxide, an ion conductor) supported tubular cell. Two different sizes of NiO particles were used to make the anode support - 0.5µm and

2.5µm. Different microstructures were seen by SEM examination, and after reduction, the

electronic conductivity was similar in both supports. However, the cell made with 2.5µm

NiO had a maximum power density twice as high as the cell made with 0.5µm particles, and

impedance measurements showed that the difference was due to diffusion in the support. Although the coarser microstructure had a shorter TPB length, the larger pores and high porosity allowed greater diffusion of gases in the support, which improved the performance [30].

Other microstructural parameters of importance are the tortuosity and percolation of the electrodes. Tortuosity is the ratio of the path length of interconnected particles of the same type, e.g. LSM, though a electrode, to the thickness of the electrode. Percolation expresses the fraction of particles of one type that are connected to each other. A per- colation threshold for a material in a composite electrode is often seen. This is usually expressed in vol.% of a particular phase, above which value there is a sudden increase in

peroclation. This is about 30 vol% for NiO/YSZ composites [31]. These concepts are often used to predict the ohmic and polarisation resistances of composite electrodes in computer models by modelling inter-particle contact and calculating electronic and ionic pathways [32].

Sealing of the reversible SOFC is also necessary to prevent crossover leakage and direct combustion of fuel gases. Tubular SOFC are usually designed such that one end is closed, and the other is left open as a gas inlet. They are sealed to the gas inlet tube, which may also function as the current collector or interconnect between cells, connecting several cells in series. If this is the case, the seal must be electrically conductive. Glass-ceramics are used for non-conductive seals, and metal brazes for conductive seals.

Glass-ceramics are materials which are designed to soften but not fully melt at a tem- perature above the SOFC operation temperature, thus filling the joint. When they are cooled to the SOFC operating temperature, they pass through a glass transition temperat- ure, and crystallise, becoming rigid and making a seal. Various types of glass-ceramics have been tested, but the most effective have been silicate based glasses doped with alkaline- earth metals [33]. Another study reported the successful sealing of FeCrAlY steel (22 % Cr, 5% Al, 0.2 % Y, balance Fe), to a fuel cell with sodium aluminosilicate glasses. A MgO filler was also tested, which modified the thermal expansion coefficient of the glass-ceramic, matching it more closely with that of the steel [34].

Braze materials may also be used, if an electrically conductive seal is required. The silver-copper oxide system is the most common braze material used [35]. This consists of a mixture of copper and silver powders, sometimes mixed with an organic binder, that is applied to the fuel cell and gas delivery tube/interconnect. It may be sintered in a vacuum furnace or a reducing atmosphere, or also in air, in which case the copper oxidises. The CuO coats the surfaces of the interconnect and the ceramic fuel cell, allowing the silver to wet them and make a seal between the two components [36, 37]. Such seals have been tested in a fuel cell environment, i.e. with hydrogen on one side and oxygen on the other. Evidence of hydrogen and oxygen diffusion into the seal is seen, but this does not affect its mechanical integrity [38]. The seals have also been subjected to thermal cycling without failure [39], when the fuel cell has been brazed to FeCrAlY steel, which has a similar thermal expansion coefficient to YSZ. The Ag-CuO braze material is quite ductile, but has a higher thermal expansion coefficient than the steel. Furthermore, if braze is placed in a joint which is under mechanical load, it may be be squeezed out during brazing when it melts. These problems have been addressed with low CTE filler material, such as alumina, which reduces the CTE of the braze, and increases its viscosity [40].

A tubular support material should have high electronic conductivity, and sufficient porosity to allow diffusion of gas through it. Tubular supports are usually made from either LSM [18, 41]or Ni/YSZ [42, 43]. The advantage of using LSM is that the fuel electrode can be on the outside of the cell, so the hydrogen and steam may circulate in a reversible SOFC system between the cells by convection. If the electrode was on the inside of the cell, steam would build up as the cell was run in fuel cell mode, hydrogen would be depleted, and the performance of the cell would become limited by diffusion

1.3. REVERSIBLE SOFC GEOMETRY AND FABRICATION 39 losses. However, the disadvantage of using LSM as the support material is that the YSZ electrolyte must be coated on top of it, and sintered to a high temperature, which may cause lanthanum and strontium zirconate formation. If a NiO/YSZ support is used, the electrolyte may be sintered to a high temperature, and the LSM/YSZ electrode may be coated on at a lower temperature.

The gas delivery tube/interconnect connected to the SOFC is usually made of either a ceramic material, or a stainless steel with a similar thermal expansion coefficient to that of the SOFC materials. Stainless steels such as FeCrAlY or Crofer-22-APU, a special steel developed specifically for fuel cells, are often used. However, coatings have to be applied to these steels to prevent rapid oxidation under fuel cell operating conditions, or poisoning of the fuel cell electrodes with chromium [44].