SOFC, half-cell reactions

In the cathode of an SOFC, oxygen, supplied typically as air to one side of the oxygen permeable membrane, is reduced by the cathode to oxygen ions via the following overall half-cell reaction (see Figure 8.7) ... 

An estimate shows that in the temperature range typical of low-T fuel cell operation (300-360 K), the second term on the right side of (1.76) is much smaller than the first one. Physically, the entropy change A5o due to the creation of new molecules (water) is much greater than the entropy change due to species heating. This is also true for the half-cell reactions in SOFCs. In the following we thus will take AS = ASo = const. 

Although SOFCs that rely on reforming of hydrocarbons to produce synthesis gas are a viable technology, it would be much simpler and more efficient if the reforming step could be avoided and the fuels used directly. Thus, one would prefer to have an anode half-cell reaction that involves the direct oxidation of a hydrocarbon fuel, such as the following ... 

Unlike the PEM, the ionic conduction occurs for the oxygen ion instead of the hydrogen ion. SOFCs are made of ceramic materials like zirconium (Z = 40) stabilized by yttrium (Z = 39). High-temperature oxygen conductivity is achieved by creating oxygen vacancies in the lattice structure of the electrolyte material. The half-cell reactions in this case are... 

Recently, Savoie etal. [24] reported an extensive investigation on catalysis in SC-SOFCs with Ni-YSZ (yttria-stabilized zirconia) anodes. Their investigations were performed on three different half-cells exposed at various temperatures to a methane-air gas mixture. The detected outlet gases contained CO and H2, but also CO2, suggesting that the complete oxidation reaction also occurred. Furthermore, at a fixed gas flow rate, the outlet gas composition was found to depend on both Rmix and the thickness of the anode. At 600 °C and Rmix = 1-2, 33% of methane was catalytically converted on a thin anode (0.05 mm), and more than double on a thick anode (1.52 mm). An increase in temperature led to an increase in methane conversion. At 800 °C and R ix = 1-2, the yield of H2 was 14 and 38% for thin and thick anodes, respectively. The production of syngas was significantly reduced at lower temperatures. From this study, it is obvious that the optimization of SC-SOFC systems requires extensive catalytic studies on the electrode materials. 

The power generation of SC-SOFC is dependent on the resistance of the materials. The electrolyte itself, the chemical reactions, and the overpotential contribute to the impedance, which is measured with a load of half the short circuit current applied to the cell. Figure 3 shows the impedance spectra of a particular cell, fitted to an equivalent resistor/capacitor (RC) circuit. Usually, R1 is considered to be the electrolyte resistance with R2 and R3 as the overpotential of the electrodes. The inductance of the cables and the relaxation frequency of R2 and C2 tended to introduce error into the measurement of Rl. Therefore, R1 is usually measured together with R2 as R1 + R2 [31], Some cells may be significantly affected by the electrolyte resistance, which depends on thickness. 

The second most common electrolytes, found mostly in solid oxide fuel cells (SOFCs), are permeable to oxide ions and lead to the half-reactions in Table 1-4. 

In a previous study at Haldor Topsoe A/S [24], it was calculated that approximately half of the waste heat from the oxidation reaction, however, is used to drive the internal reforming reaction, where methane in the anode feed is reformed with water to generate hydrogen. This reaction not only reduces the size of the in/ out exchanger E3 and reduces the parasitic loss for air compression, but also upgrades waste heat to chemical energy. This is a major reason for the higher electrical efficiency of a stationary SOFC CHP system compared to a stationary low temperature CHP fuel cell such as PEMFC. 

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