A single-chamber solid-oxide fuel cell (SC-SOFC), which operates using a mix­ture of fuel and oxidant gases, provides several advantages over the conventional double-chamber SOFC, such as simplified cell structure (no sealing required) and direct use of hydrocarbon fuel [15,16]. Figure 11.5 shows a schematic diagram of SC-SOFC operation. The oxygen activity at the electrodes of the SC-SOFC is not fixed and one electrode (anode) has a higher electrocatalytic activity for the oxidation of the fuel than the other (cathode). Oxidation reactions of a hydrocarbon fuel can be represented with a simplified multistep, quasi-general mechanism as follows:

On the other hand, the cathode has a higher electrocatalytic activity for the reduction of oxygen according to the reaction:

1/2O2 + 2e- ^ O2- (11.4)

These reactions lead to a low oxygen partial pressure at the anode locally, while the oxygen partial pressure at the cathode remains relatively high. As a

Furnace thermocouple

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FIGURE 11.5 Schematic diagram of a single-chamber solid-oxide fuel cell operating with a mixture of fuel and air.

result, an electromotive force (emf) between two electrodes is generated with a mixed fuel and air mixture. Due to the presence of oxygen at the anode, SC-SOFC is not affected by the problems associated with carbon deposition, which is a significant drawback for double-chamber SOFCs when Ni-cermet is used as anode material.

The fuel/air mixtures for SC-SOFC were generally chosen to be richer than the upper explosion limits, yet they were fuel-lean enough to prevent the carbon deposition, which has been a significant problem in double-chamber SOFCs [17]. However, variations in the ratios of the local fuel-air mixture were also dependent on catalytic activity and test conditions that affect the performance of the fuel cell [15]. An ideal SC-SOFC has the same open circuit voltage (OCV) and I-V output as a double-chamber cell, given a uniform oxygen partial pressure. The difference in catalytic properties of the electrodes must be sufficient to cause a significant difference in oxygen partial pressure between the anode and the cathode. For the ideal SC-SOFC, one electrode would be reversible toward oxygen adsorption and inert to fuel, while the other electrode would be reversible toward fuel adsorption and completely inert to oxygen [18]. If the electrode materials are not sufficiently selective, a parasitic reaction creates mixed poten­tials at the electrodes, which reduces the efficiency of the cell. Compared to traditional double-chamber fuel cells, parasitic reactions in a single-chamber fuel cell have historically reduced the OCV by about half. This is analogous to a leak that allows the fuel to seep into the oxidizer side of a double-chamber fuel cell [19]. Advances in electrode catalyst materials for SC-SOFC may lead to a similar performance as a conventional double-chamber SOFC with a substantial reduction in complexity and cost of the fuel cell.

Significant improvement in the performance of single-chamber solid-oxide fuel cells has been achieved in recent years [15, 20-22]. Since SC-SOFC does not require high-temperature sealing materials to prevent the mixing of fuel gas and oxygen at operation temperatures, it offers a robust and more reliable alter­native to double-chamber SOFC for special applications. As further advances are made toward controlling the catalytic activity of electrode materials, electrolyte resistance particularly at lower operating temperatures, optimizing of the gas flow rate and the cell configuration, SC-SOFCs may find widespread implementation as compact power sources in the future.

Several recent studies on the development of SC-SOFCs have been conducted in our laboratory to improve their performance and understand complex electrode reactions [20,23-26]. Initial experiments were carried out using fuel cells pre­pared by deposition of YSZ thin-film electrolytes (1-2 pm thickness) on the NiO-YSZ anode as a substrate with (La, Sr)(Co, Fe)O3 (LSCF) as the cathode (Figure 11.6). A power density of 0.12 W cm-2 was obtained at an OCV of >0.8 V using a methane-air gas mixture as a fuel [23].

In another study, the effect of mixed gas flow rates on the performance of SC-SOFCs has been investigated using a cell that consists of a 18-pm thickYSZ porous electrolyte on a NiO-YSZ anode substrate with a (La, Sr)(Co, Fe)O3 cathode. Higher gas flow rates led to an increase of cell temperature due to

increasing methane reaction rate, which resulted in improved cell performance. Figure 11.7 shows that optimization of gas flow rate (linear velocity) lead to a decrease of the operating temperature effectively and increased cell performance as well as fuel efficiency. At a cell temperature of 744°C (furnace temperature: 606°C), an OCV of ~0.78 V and a maximum power density of ~660 mW cm-2 (0.44 V) were obtained. The results indicated that a porous ion-conducting mem­brane provides sufficient separation of oxygen activity at the electrodes by selec­tion of an optimum operation temperature and a gas flow rate. Thus, it appears that SC-SOFCs with porous electrolyte provide opportunities to design thermally and mechanically more robust stacks by utilizing hydrocarbon fuels. It also allows fabrication of the cells at lower temperatures using conventional processing tech­niques such as screen printing, since densification of the electrolyte at high sintering temperatures is not required.