Electrolytes ceramic electrolyte

The molten salt, sodium aluminum chloride, fulfills two other tasks in the cell system. The ceramic electrolyte "-alumina is sensitive to high-current spots. The inner surface of the ceramic electrolyte tube is completely covered with molten salt, leading to uniform current distribution over the ceramic surface. This uniform current flow is one reason for the excellent cycle life of ZEBRA batteries. 

Sol-gel techniques have been widely used to prepare ceramic or glass materials with controlled microstructures. Applications of the sol-gel method in fabrication of high-temperature fuel cells are steadily reported. Modification of electrodes, electrolytes or electrolyte/electrode interface of the fuel cell has been also performed to produce components with improved microstructures. Recently, the sol-gel method has expanded into inorganic-organic hybrid membranes for low-temperature fuel cells. This paper presents an overview concerning current applications of sol-gel techniques in fabrication of fuel cell components. 

Ionic conductors, used in electrochemical cells and batteries (Chapter 6), have high point defect populations. Slabs of solid ceramic electrolytes in fuel cells, for instance, often operate under conditions in which one side of the electrolyte is held in oxidizing conditions and the other side in reducing conditions. A signihcant change in the point defect population over the ceramic can be anticipated in these conditions, which may cause the electrolyte to bow or fracture. 

The successful operation of SOFCs requires individual cell components that are thermally compatible so that stable interfaces are established at 1000°C (1832°F), i.e., thermal expansion coefficients for cell components must be closely matched to reduce stresses arising from differential thermal expansion between components. Fortunately, the electrolyte, interconnection, and cathode listed in Table 8-1 have reasonably close thermal expansion coefficients [i.e., 10 cm/cm°C from room temperature to 1000°C (1832°F)]. An anode made of 100 mol% nickel would have excellent electrical conductivity. However, the thermal expansion coefficient of 100 mol% nickel would be 50% greater than the ceramic electrolyte, or the cathode tube, which causes a thermal mismatch. This thermal mismatch has been resolved by mixing ceramic powders with Ni or NiO. The trade-off of the amount of Ni (to achieve high conductivity) and amount of ceramic (to better match the other component thermal coefficients of expansion) is Ni/YSZ 30/70, by volume (1). 

Having discussed the way in which blocking interfaces behave we must now consider how blocking metallic contacts can be made on a given material, e.g. a ceramic electrolyte. Frequently a relatively inert metal such as Pt or Au is evaporated onto a ceramic material which has been polished... 

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

SOE cells utilize solid ceramic electrolytes (e.g. yttria stabilized zirconia) that are good oxygen ion (0 ) conductors at very high temperatures in the range of 1000°C [8]. The operating temperature is decided by the ionic conductivity of the electrolyte. The feed gas, steam mixed with hydrogen, is passed through the cathode compartment. At the cathode side, the reaction is... 

Kharton VV, Marques FMB, Atkinson A (2004) Transport properties of solid oxide electrolyte ceramics a brief review. Solid State Ionics 174 135-149... 

Protonic ceramic fuel cell (PCFC)—The ceramic electrolyte can elec-trochemically oxidize fossil fuels, eliminating the need for fuel reformers. 

Unlike molten carbonates, solid oxides use a hard ceramic electrolyte instead of a liquid. That means the fuel cell can be cast into a variety of useful shapes, such as tubes. With higher temperatures, sofcs may be able to cogenerate steam at temperatures as high as i,ooo°f. The Siemens Westinghouse Power Corporation has built the first advanced hybrid system, which combines a gas turbine with a tubular sofc. As of 2003, the 220 kW hybrid system has operated in California for more than 2,000 hours with a respectable 53 percent efficiency, comparable to current combined cycle gas turbines. The ultimate goal is an efficiency of 70 percent or more. 

The Na/S and ZEBRA batteries, which incorporate ceramic electrolytes, will be discussed in detail below but first the elementary basic science is summarized. 

Fig. 4.31 shows Arrhenius plots for YSZ together with other candidate electrolyte ceramics illustrating how the combination of temperature and ceramic... 

Fig. 4.31 Ionic conductivities for candidate electrolyte ceramics. The arbitrary assumption that for a planar cell format a resistance of <15 gQm 2 is required places an upper limit on the permitted thickness of the electrolyte lower values of conductivity demand thinner membranes whilst higher values permit correspondingly thicker membranes. Electrolyte thicknesses greater than approximately 150/an are considered mechanically self-supporting. After B.C.H. Steele [11],...

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... 

Low rate solid-electrolyte-based cells For instance, Li/I2 cells used primarily in implantable medical devices are well established. Another example is the developmental all-solid-state Li metal/phosphorous oxynitride (PON)/intercalation cathode cells conceived for use in microelectronic circuits. The PON is a glassy ceramic electrolyte which is stable to over 5 V [25],... 

However, 1000 °C leads to a very rapid reaction if anode reform is attempted and in many cases the result is excessive thermal stress of the ceramic electrolyte, so that conventional reformers must be used. As a consequence there has come about a class of intermediate-temperature SOFCs based on alternative ceramic formulations, 500 °C operation having been achieved by a metal/ceramic fuel cell by the company Ceres (see Chapter 4) set up by Imperial College London. 

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