SC-SOFC

Interestingly, research has started on single chamber SOFC (SC-SOFC) concepts. However, the SC-SOFC exhibits inherently low power density and is therefore primarily of academic interest. It has the potential to relax cell component requirements and probably to ease manufacture. The principle of SC-SOFC is that it is fed by an air fuel mixture which flows onto the PEN contained in a single compartment, avoiding the use of gas separator plates and high temperature sealants. The fluid may flow simultaneously or sequentially along the electrodes. Both electrodes are either built onto the same side of the electrolyte some distance apart or on opposite sides. Low temperature operation would apparently suppress direct combustion of the air fuel mixture provided the electrode materials chosen are highly selective towards their respective catalytic reactions. SC-SOFC stacks may hold prospects in specific applications where the reaction products are the prime focus. 

Abstract Single-chamber solid oxide fuel cells (SC-SOFCs) immerse the entire cell in a mixture of fuel and oxidizer gases within a single chamber, which eliminates the need for high temperature sealant, simplifies construction, and increases reliability over traditional double-chamber cells. However, there are challenges, such as low fuel utilization and electrode catalytic selectivity, that need to be overcome. This brief review paper looks at recent improvements in materials, processing, and operation of SC-SOFCs, which are rapidly approaching the performances of the double-chamber fuel cells and may become attractive for specific fuel cell applications. 

Keywords solid oxide fuel cell, SOFC, hydrogen, hydrocarbon, single chamber, SC-SOFC... 

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 with no sealing required and direct use of hydrocarbon fuel [1, 2], 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... 

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 carbon deposition, which has been a significant problem in double-chamber SOFCs [5], However, the local mixture ratios were also dependent on catalytic activity and testing conditions and varied accordingly for the optimum [1],... 

An ideal SC-SOFC has the same OCV and I-V output as a double-chamber cell, given a uniform oxygen partial pressure. A difference in catalytic properties of the electrodes must be sufficient to cause a substantial difference in oxygen partial pressure between the electrodes. 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 [30], Advances in electrode catalyst materials are needed for SC-SOFC to have the same performance as conventional double-chamber SOFC with a significant reduction in complexity and cost. 

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. 

Figure 8. I-V discharge profile (solid symbols) and power density (open symbols) of the porous electrolyte SC-SOFC. Data collected at a set temperature of 606°C [4]...

In this case, compared with the direct electrochemical oxidation of the fuel [Eq. (2.2)], the Faraday efficiency is only 75% since the production of syngas (H, + CO) is not an electrochemical process, and electrons (six instead of eight) are generated only by the electrochemical oxidation of the syngas. In addition to this intrinsically reduced efficiency, current SC-SOFC systems show very low fuel utilization (1-8%) and thus efficiencies [4,18, 19]. While gas intermixing, small-scale electrodes, and high flow rates contribute to the low efficiency, non-ideally selective electrode materials and parasitic reactions are the primary reason. The further development of SC-SOFCs therefore requires active and selective materials for optimized performance. 

A fuel-to-oxygen ratio corresponding to the partial oxidation of the fuel (i.e., = 2 in the CH4-O2 system) was found appropriate for SC-SOFC operation [13, 15, 20, 21]. This optimized R ix value also depends on the electrode structures and thickness. Fuel-rich gas mixtures can cause carbon deposition whereas oxygen-rich gas mixtures can favor complete fuel oxidation and increase the risk of explosions. 

Despite the numerous investigations on understanding SC-SOFCs over the past 20 years, little attention has been paid to the crucial role of catalysis in such... 

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 electrode and electrolyte materials used in SC-SOFCs are similar to the ones being used in conventional SOFCs operated with separate fuel and air compartments. However, selectivity and catalytic activity requirements and overheating impose different constraints on the cell component materials under single-chamber operating conditions. 

LSM is the most commonly used cathode material in SC-SOFCs. However, the sintering process and the cell operating temperature strongly affect its catalytic activity and selectivity [23, 39, 40]. Under single-chamber operation, the cathode is required only to promote the oxygen reduction [Eq. (2.3)]. However, when sintered at low temperature, LSM shows catalytic activity towards hydrocarbon oxidation [23, 39, 40]. When sintered at higher temperatures, an increased density leads to negligible catalytic activity for methane conversion [23]. 

---