SOFC Electrolyte Membrane

Water transport in the SOFC electrolyte membrane is primarily by diffusion mass transport... 

This presentation reports some studies on the materials and catalysis for solid oxide fuel cell (SOFC) in the author s laboratory and tries to offer some thoughts on related problems. The basic materials of SOFC are cathode, electrolyte, and anode materials, which are composed to form the membrane-electrode assembly, which then forms the unit cell for test. The cathode material is most important in the sense that most polarization is within the cathode layer. The electrolyte membrane should be as thin as possible and also posses as high an oxygen-ion conductivity as possible. The anode material should be able to deal with the carbon deposition problem especially when methane is used as the fuel. 

The principle of the fuel cell was first demonstrated by Grove in 1839 [W. R. Grove, Phil. Mag. 14 (1839) 137]. Today, different schemes exist for utilizing hydrogen in electrochemical cells. We explain the two most important, namely the Polymer Electrolyte Membrane Fuel Cell (PEMFC) and the Solid Oxide Fuel Cell (SOFC). 

Yasumoto K, Itoh H, and Yamamoto T. Anode supported interconnect for electrolyte membrane SOFC. Electrochem. Soc. Proc. 2003 2003(07) 832-840. 

Note PAFC phosphoric acid fuel cell PEMFC proton exchange membrane fuel cell/polymer electrolyte membrane fuel cell MBFC microbiological fuel cell DMFC direct methanol conversion fuel cell AFC alkaline fuel cell MCFC molten carbonate fuel cell SOFC solid oxide fuel cell ZAFC zinc air fuel cell. 

In the SOFC, the thickness of the electrolyte layer is a vital consideration, as is any additional biconductor interface 20-50 jtm is referred to as a thick electrolyte film, a design compromise between the minimisation of electrical losses and freedom from defects. Thin films may go down to nanometres and are more difficult to make without defects. Global Thermoelectric (Kuo etal., 1999) is in SOFC production at 6 xm and InDEC (Section 4.12) at 5 p,m. In the small-tube SOFC the membrane thickness is about half a millimetre. It is difficult to see the latter thickness coming down for improved efficiency to 6 xm, impossibly delicate tubing. 

Proton-conducting ceramic membranes have been studied as SOFC electrolytes for intermediate temperature, around 800°C, and can also be used as electrolytes in steam electrolysis. The families of SrCeOj and BaCeOj with dopants such as Y, Yb, and Nd on the Ce site show good selectivity for proton transport. The advantage of... 

Fuel cells are classified primarily according to the nature of the electrolyte. Moreover, the nature of the electrolyte governs the choices of the electrodes and the operation temperatures. Shown in table 10.1 are the fuel cell technologies currently under development. "" Technologies attracting attention toward development and commercialization include direct methanol (DMFC), polymer electrolyte membrane (PEMFC), solid-acid (SAFC), phosphoric acid (PAFC), alkaline (AFC), molten carbonate (MCFC), and solid-oxide (SOFC) fuel cells. This chapter is aimed at the solid-oxide fuel cells (SOFCs) and related electrolytes used for the fabrication of cells. 

The most important fuel cells that are in use nowadays are the polymer electrolyte membrane fuel ceU (PEMFC), the molten carbonate fuel cell (MCFC), and the solid oxide fuel cell (SOFC). In a PEMFC, the electrolyte is a polymer membrane that conducts protons, in an MCFC the electrolyte is a carbonate melt in which oxygen is conducted in the form of carbonate ions, CO , and in an SOFC the electrolyte is a solid oxide that conducts oxygen ions, While a PEMFC can be operated at low temperatures of about 80 °C, an MCFC works at intermediate temperatures of about 650 °C, and an SOFC needs relatively high temperatures of 800-1000 °C (see next sections). 

The last section (19.6) is focused on the commercial potential and perspectives of using metal ammines in connection with, for example, polymer electrolyte membrane (PEM) and solid oxide fuel cells (SOFCs) as well as selective catalytic reduction (SCR)-DeNO c (NO c removal) in the transport sector, and it includes comments on the global availability and low cost of the carrier salts. This section also provides the authors perspectives on future trends and challenges in metal ammine research, along with links to the interested reader for further information on key articles, companies and websites. 

Selection of an appropriate solute is important for the formulation of an effective electrolyte. Maximum conductivity, for example, seems to be associated with a size homogeneity between the substituting species and the majority cation in the cubic structure, as well as its concentration in solid solution. Figure 3 presents the effects on the ionic conductivity of stabilised zirconia at a fixed temperature, on variation of the cationic substituting species. It is evident that the optimised yttrium solid solution has a conductivity of about 0.015 S cm at 800°C, so that only a very thin electrolyte membrane can provide a technically acceptable current density at that temperature. The well-established Westinghouse SOFC system therefore operates closer to 1000°C to take advantage of the rapid increase of electrolyte conductivity with temperature (7) (see also Fig. 7). This dependance, particularly steep for YSZ, is presented for several solid ionic conducting materials in Fig. 4. 

Fig. 1 Operation principle of the various types of fuel cells PEMFC polymer electrolyte membrane fuel cell, AFC alkatine fuel cell, PAFC phosphoric add fuel cell, MCFC molten carbonate fuel cell, SOFC sohd oxide fuel cell...

The electrolyte membrane is an oxide ion conductive ceramic, whose thickness depends on the cell design. One may distinguish electrolyte-supported cell from electrode-supported cell (Fig. 15.6). In the first case, anode and cathode are deposited onto both faces of the electrolyte membrane. As a direct consequence, the membrane must be mechanically strong, and a minimal thickness of 100 pm is required. In the case of the electrode-supported cell, the anode is actually the mechanical support of the electrolyte first, and next the cathode on the top. Thus, the electrolyte thickness can be greatly reduced, down to 8 pm for classical SOFC devices. More recently, with the development of micro-SOFC, it can reach 100 nm to 1 pm. 

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