Types of Fuel Cells - Technology Status

Zirconia, stabilized with 8-9 % yttria (yttria stabilized zirconia - YSZ) is the most commonly used electrolyte for SOFCs because it exhibits predominant ionic conductivity (O2- transport number close to unity) over a wide range of oxygen partial pressures (1 to 10 20 atmospheres). YSZ provides sufficient conductivity at 

Planar SOFCs are composed of flat, ultra-thin ceramic plates, which allow them to operate at 800°C or even less, and enable less exotic construction materials. P-SOFCs can be either electrode- or electrolyte- supported. Electrolyte-supported cells use YSZ membranes of about 100 pm thickness, the ohmic contribution of which is still high for operation below 900°C. In electrode-supported cells, the supporting component can either be the anode or the cathode. In these designs, the electrolyte is typically between 5-30 pm, while the electrode thickness can be between 250 pm - 2 mm. In the cathode-supported design, the YSZ electrolyte and the LSM coefficients of thermal expansion are well matched, placing no restrictions on electrolyte thickness. In anode-supported cells, the thermal expansion coefficient of Ni-YSZ cermets is greater than that of the YSZ 

Ni-state-of-the-art anodes contain Cr to eliminate the problem of sintering. However, Ni-Cr anodes are susceptible to creep, while Cr can be lithiated by the electrolyte and consumes carbonate, leading to efforts to decrease Cr. State-of-the-art cathodes are made of lithiated-NiO. Dissolution of the cathode is probably the primary life-limiting constraint of MCFCs, particularly under pressurised operation. The present bipolar plate consists of the separator, the current collectors, and the seal. The bipolar plates are usually fabricated from thin sheets of a stainless steel alloy coated on one side by a Ni layer, which is stable in the reducing environment of the anode. On the cathode side, contact electrical resistance increases as an oxide layer builds up (US DOE, 2002 Larminie et al., 2003 Yuh et al., 2002). 

High operating temperatures are needed to achieve sufficient electrolyte conductivity. Most MCFC stacks operate at 650°C, as a compromise between high performance and stack life, because, above 650°C there are increased electrolyte losses due to evaporation and increased material corrosion. The voltage of MCFCs varies with the composition of the reactant gases. Increasing the reactant gas utilisation generally decreases cell performance. A compromise leads to typical utilisations of 75 to 85% of the fuel. The electrochemical reaction at the cathode involves the consumption of two moles C02 per mole 02, and this ratio provides the optimum cathode performance (US DOE, 2002 Larminie et al., 2003 Yuh et al., 2002). 

Endurance is a critical issue in the commercialisation of MCFCs. Adequate cell performance must maintain an average potential degradation no greater than 15 mV/a over a cell stack lifetime of 5 years, while state-of-the-art MCFCs exhibit an average degradation of 40 mV/a. At full load, MCFC system can achieve efficiencies up to 55%, which drops at partial loads. Typical MCFCs operate in the range 100-200 mA/cm2, at 750-900 mV/cell, achieving power densities even above 150 mW/cm2 (US DOE, 2002 Larminie et al., 2003 Yuh et al., 2002). 

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