Ceria electrolytes compositions

Xia C, Li Y, Tian Y, Liu Q, Wang Z, Jia L, Zhao Y and Li Y (2010), Intermediate temperature fuel cell with a doped ceria-carbonate composite electrolyte , / Power Sources, 195,3149-3154. 

Mizutania Y, Matsudaa H, Ishijia T, Furuya N, Takahashi K (2005) Improvement of electrochemical NOj sensor by use of carbon-fluorocaibon gas permeable electrode. Sens Actuators B 108 815-819 Mukundem E, Brosha E, Brown D, Garzon F (1999) Ceria-electrolyte-based mixed potential sensors for the detection of hydroceubons and ceubon monoxide. Electrochem Sohd State Lett 2 412-414 Neikayama S, Sadaoka Y (1994) Preparation of Na3ZrjSijPOjj-sodium aluminosilicate composite and its application as a solid-state electrochemiceil COj gas sensor. J Mater Chem 4(5) 663-668... 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally short-circuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must pos.scss good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a Ni-YSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic... 

Furthermore, the electrode material must be reasonably stable and not change its volume as a result of reduction or oxidation because such a volume change may cause the electrode to peel off the electrolyte. Also, the thermal expansion coefficient (TEC) of the electrode material must be close to the electrolyte material. Thus, in order to select a proper composition of the doped ceria for a specific electrode application, it is necessary to have knowledge of the ceria chemistry and its relation to the thermal, crystallograhical and electrical properties of the doped ceria. Therefore, the next sections briefly describe the ceria chemistry and the related properties. 

Figure 15.12. Electrode structure with two elements of composite structure. 1) The electrode adhesion on the interface towards ihe dense electrolyte is improved by a physical anchoring (YSZ-scalesK and 2> the electrode funciions are divided on two layers taking care of the electrochemical oxidation of hydrogen (ceria) and current collcclion respectively.

A particular emphasis has been placed on the detachment rate of colloids from the mineral surface. Colloid sorption is irreversible (or at least shows very slow desorption kinetics regardless of solution composition either in electrolyte solution or in the presence of a carrier colloid such as silica or humic acids (Table ID) moreover, desorption tests up to three months have not shown any colloid detachment. No marked influence of temperature on the release of retained ceria colloids has been observed between 20 and 90°C. 

Wang, B.H., Wang, J.D., Liu, R., Yie, Y.H., and Li, Z.J. 2007. Synthesis of ammonia from natural gas at atmospheric pressure with doped ceria -Ca/PO lj-KjPO composite electrolyte and its proton conductivity at intermediate temperature. Journal of Solid State Electrochemistry 11, 27-31. 

Di J, Chen M, Wang C, Zheng J, Fan L, Zhu B (2010), Samarium doped ceria-(Li/ Na)2COj composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell , 7 Power Sources, 195,4695 699. 

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