Anodization thick

The influence of porosity on the electrochemical activity has not been studied much for electrolyte-supported cells because anode pastes for electrolyte-supported cells are made for screen printing, and thus contain significant amounts of organics, which almost guarantees sufficient porosity. In addition, since the anode thickness for electrolyte-supported cells is only on the order of 50 pm, the concentration polarization itself becomes much less of an issue. In fact, Jiang et al. [44] showed that anode overpotential for cermet anodes prepared with extra graphite pore formers... 

FIGURE 2.21 Change of anode overpotential versus anode thickness for anodes made with and without 20 wt% graphite as pore former in electrolyte-supported cells. (From Jiang, S.P. et al., Solid State Ionics, 132 1-14, 2000. Copyright by Elsevier, reproduced with permission.)... 

By substituting Equation (A3.10) into (A3.9), and integrating through by the anode thickness (/ ). the following equation is obtained ... 

Figure 10.53 shows the measured surface shape of the warped cell. The cathode side is a convex shape and the top of the cell is warped by more than 2 mm from the comer position in a perpendicular direction to the plane for the 10 mm x 10 mm sample. From the simulation, it is found that the magnitude of the warp depends on the relative thickness between the anode substrate and electrolyte, as shown in Figure 10.54. The warp increases with the increase in the ratio of the anode thickness to electrolyte thickness. The calculated value of the warp is very close to the measured one indicating that this simulation is reliable in predicting the warp with various combinations of the thickness between the anode and electrolyte.

Monoliths that were anodized extensively (72) had an anodization thickness of up to 25 pm with a BET surface area of 40 m /g, which is sufficient for many applications. However, because this layer contained only mesopores (pore diameters up to 20 nm) and no macropores, internal diffusion limitations can easily be a problem. An extensive report on the anodization of aluminum monoliths, with the aim of using the anodization layer as catalyst support, was provided by Burgos et al. (73). 

EW Date Anode Thickness Anode Life Current Density... 

Figure 7.2 Current density for different anode thicknesses for cell operating at 0.7 V. The inlet fuel consists of 40%CH4 and 60%H2O at 800°C. Air enters the cathode side at 650°C and the air number X,=l (Eq.7.11)...

Figure 7.6 Species profiles within the fuel channel and anode. The inlet fuel consists of 14% CH4, 63%H2, 2% H2O, 20% CO and traces of CO2, which is 60% pre-reformed fuel resulting from an initial composition of 60% CH4 and 40% H2O. The drop down panels shows the species profiles across the anode thickness at various axial positions.

Based on a co-flow configuration, the effect of various parameters on cell performance has been studied systematically. The study covers the effect of (a) air flow rate, (b) anode thickness, (c) steam to carbon ratio, (d) specific area available for surface reactions, and (e) extend of pre-reforming on cell efficiency and power density. Though the model predicts many variables such as conversion, selectivity, temperature and species distribution, overpotential losses and polarization resistances, they are not discussed in detail here. In all cases calculations are carried for adiabatic as well as isothermal operation, fii calculations modeling adiabatic operation the outer interconnect walls are assumed to be adiabatic. All calculations modeling isothermal operation are carried out for a constant temperature of 800°C. Furthermore, in all cases the cell is assumed to operate at a constant voltage of 0.7 V. 

Though SOFC can be either of anode, electrolyte or cathode supported, in the case of cells running on hydrocarbon fuels, anode supported cells may be preferable to the others for the reasons of internal reforming. However, the optimal anode thickness required to support the cell mechanically and to achieve the desired level of internal reforming and optimal cell performance is rather a difficult task. 

Figure 7.12 Effect of anode thickness on efficiency and power density for a cell operating under adiabatic conditions. The inlet fuel consists of 40% vol. CH4 and 60% vol. H2O entering at 800°C. Cathode inlet is assumed to be air at 650 C.

Figure 7.13 presents the efficiency and power density as a function of anode thickness for the case of a cell operating isothermally and the same inlet fuel composition as in the adiabatic case. In this case both efficiency and power density increases with increasing anode thickness. Furthermore, isothermal operation results in better performance than adiabatic operation. A maximum efficiency of 59% is achievable with isothermal operation, while the maximum possible in the case of adiabatic operation is 45%. 

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