Cathode-supported cells

Self-supported SOFC can be classified into anode-supported and cathode-supported fuel cells. The SOFC assembly for laboratory testing has a shape of button with 1 - 2 cm in diameter and less than 500 im in thickness. The majority of these button cells are anode-supported cells due to the easy of their fabrication as compared with that of the cathode-supported cell. These self-supported fuel cell usually possess thin (5-20 p,m) electrolyte and can operate at reduced temperatures (< 800 °C). The low temperature operation is the key to decrease... 

In order to deduce the local fuel utilization of the various sectors, a model of the single cell voltage sensitivity to FU has been developed. Each term of the polarization model is discussed and then some hypothesis are assumed for the particular case study of a tubular cathode supported cell (used in the SOGC CHP-100 plant). 

The tubular cell used in the SOFC CHP-100 is a cathode-supported cell then, the main effect of diffusion is mainly addressed to the cathode layer. In terms of physieal measurable parameters, the cathode limiting current density can be evaluated in the form ... 

At 1000°C the ohmie area speeifie resistance of tubular cathode-supported cells is aroimd 0.40 Q cm for an HPD5 cell is around 0.25 Q cm for an HPDIO cell is aroimd 0.15 D cm for a Delta 9 cell is around 0.18 D cm [32]. In Figure 26 the ionic resistance of 8YSZ is drawn as a function of temperature considering the main models used in literature [22,30,31] and a comparison with experimental values [33]. 

Mechanical strength If anode-supported cell configuration If cathode-supported cell configuration... 

The planer type SOFCs with ASC and MSC are selected for operation at temperature range of 400 to 850 °C. ESC and Cathode Supported Cell (CSC) are also concerned in our scope of research for consideration of their special characteristics and fimctions as well as niche benefits, such as the anti-redox capability of ECS and cost reduction of CSC. 

These features have been found to be highly correlated with the fabrication method/sequence as well as materials selected. For example, anode support cells have stable anodes but there remain several points to be optimized for a cathode-complex-layer structure. On contrary, cathode-support cells have the stable performance for cathodes, but anodes may have some changed in microstructure because of nickel sintering [63]. 

Other two components are then applied to this support in different ways as thin layers (tenths of a millimeter). Accordingly, one has anode-supported, electrolyte-supported, and cathode-supported cells. Sometimes, though, an independent metal or ceramic substrate is used, to which the three functional layers are then applied (Figure 8.1). In his review. Tucker (2010) dicusses progress in metal-supported SOFCs and the advantages of this cell variety. 

Planar SOFCs are generally manufactured in three different configurations depending on the structure-supported cell element and operating temperature range as shown in Figure 9.19. These configurations are referred to as (i) electrolyte-supported cell with thick electrolyte layer, (ii) anode-supported cell with thick anode layer, and (iii) cathode-supported cell with thick cathode layer. 

Different configurations of planar SOFC designs, (a) Electrolyte-supported cell, (b) Anode-supported cell, (c) Cathode-supported cell. 

Electrolyte-Supported Cell Anode-Supported Cell Cathode-Supported Cell... 

Immediately after the discovery of LSGM by Ishihara [7,8], it became clear that interdiffusion associated with LSGM is significant between LSGM and cathode electrode [26] this makes it difficult to prepare cathode-supported cells in which the cathode-electrolyte interfaces are exposed to high temperatures. Currently, (Sm,Sr)Co03 is widely utilized as a cathode material on the basis of Ishihara s results [27]. 

If the reaction kinetics of the electrode is assumed to be very rapid, mass transfer and ohmic resistance are the dominant resistances. Assuming a reaction zone that coincides with the electrode-electrolyte interface, the diffusion fluxes in stationary operation can be expressed simply in terms of bulk gas partial pressures and gas-phase diffusivities. This is illustrated schematically in Figure 11.8, which compares anode- and cathode-supported cell designs for the simple case of a H2/O2 fuel cell. The decrease in concentration polarisation at the cathode, rjcc- is obvious in the case of an anode-supported cell, while the model shows that concentration polarisation at the anode, tiac is relatively insensitive to anode thickness. The advantage of the mass transfer-based approach is that analytical expressions are obtained for the polarisation behaviour. These are rather simple if activation overpotential is excluded but may still become elaborate in the case of an internally reforming anode where a number of reactions (discussed in Section 11.3) may occur simultaneously within the pores of the anode. 

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