Ionic and Electronic Conductivities and Fuel Cell Performance of Oxygen Excess-Type Lanthanum Silicates

Abstract

Highly dense pellets of an oxygen excess-type lanthanum silicate ͑La 9.333+x Si 6 O 26+1.5x , x Ͼ ca. 0.3, OE-LSO͒ were successfully fabricated, and their electrical conducting properties were studied. The replacement of Si by Al enhanced its conductivity, and the slightly Al-doped OE-LSO specimen ͓La 9.62 ͑Si 5.79 Al 0.21 ͒O 26.33 ͔ had excellent features as a solid electrolyte; that is, it had high ionic conductivity and was highly stable under reducing as well as oxidizing conditions at 873-1073 K. In addition, the ionic transference number was higher than 0.99. In the fuel cell utilizing this electrolyte ͑0.72 mm thick͒, ͑La 0.6 Sr 0.4 ͒͑Co 0.2 Fe 0.8 ͒O 3−␦ cathode, and Ni-Ce 0.9 Gd 0.1 O 1.95−␦ anode, good performance with the maximum power density of ca. 0.25 W cm −2 was obtained at 1073 K. In addition, this electrolyte also had high compatibility with these conventional mixed conducting electrodes, according to an analysis near the electrode/electrolyte interfaces after the fuel cell test.Solid oxide fuel cells ͑SOFCs͒ attract much attention as commercial or household energy conversion devices capable of producing heat as well as electricity directly from natural gas. Because they exhibit high energy efficiency compared with present energy conversion systems, it is expected that developing practical fuel cells will give us a gift for the next generation, encouraging a sustainable use of resources and reducing carbon dioxide emission. In SOFCs, oxide ion or proton conducting oxide ceramics materials are employed as electrolytes. Well-known zirconia-based oxide ion conductors, such as yttria-stabilized zirconia ͑YSZ͒, have long been used as electrolyte materials for SOFCs, but are just now entering the long-awaited stage of practical application.Cost, however, presents a problem. One of the reasons for this is the necessity of using high cost heat-resistive interconnecting materials available for a high operating temperature of around 1273 K. Fuel cell reliability is also a problem due to poor mechanical strength of the electrolyte during operation at such a high temperature or during startup/shutdown cycles. Because of the high activation energy for the ionic conduction of YSZ ͑ca. 0.9 eV͒, its good ionic conductivity ͑ca. 10 −1 S cm −1 at 1273 K͒ falls to ca. 10 −2 S cm −1 at 973 K. If one could develop a novel solid electrolyte that conducts ions sufficiently ͑above 10 −1 S cm −1 ͒ in a reducing temperature region ͑773-1073 K͒, then less expensive stainless steel could be used for the surrounding materials of fuel cells. Hence, there is an escalating need for developing solid electrolytes for this temperature region to improve reliability as well as reduce cost.Oxide ion conduction through material based on lanthanum silicate ͑La 9.333+x Si 6 O 26+1.5x , LSO͒ with apatite-type structure, shown in Fig. 1, was originally discovered by Nakayama et al. 1,2 in 1995. This material is notable because of its low activation energy for conduction, while the reported ionic conductivity was about 10 −3 S cm −1 at 97...

Sinterability and conductivity of barium doped aluminium lanthanum oxyapatite La9.5Ba0.5Si5.5Al0.5O26.5 electrolyte of solid oxide fuel cells

“…Thus, one of the major challenges in the development of intermediate temperature SOFCs is to develop a solid oxide electrolyte material with a high conductivity to maintain the low electrolyte resistance during operation. Lanthanum silicate oxyapatite is emerging as new class of oxide ion conductors and a substantial level of oxygen ionic transport has been reported for apatite type phases A 10−y Si 6 O 26±ı , where A corresponds to rare-earth and alkaline-earth metal cations [1][2][3][4][5][6][7][8][9][10][11][12][13]. The apatite lattice consists of covalent SiO 4 tetrahedra and ionic-like La/O channels [4,7,14].…”

Electrochemical behavior of mixed conducting oxide cathode on oxygen excess-type solid electrolyte

“…Specimens were obtained from a mixture of raw powders of La 2 O 3 (99.99% purity, Kishida Chemical Co., Ltd.), SiO 2 (99% purity, Kishida Chemical Co., Ltd.), and Al(OH) 3 (99.5% purity, Kishida Chemical Co., Ltd.). Finally, powders were pressed into a disk at 57 MPa, followed by sintering at 1973 K for 10 h in air as described elsewhere [10]. After the thickness of the obtained disk (17 mm in diameter) was adjusted to 1.0 mm, LSCF and platinum pastes were applied on each surface of the disk (electrode surface area: 0.38 cm 2 ), and used as working and counter electrodes, respectively.…”

“…We have shown that the oxygen excess-type Al-doped LSO, La 10 (Si 5.8 Al 0.2 )O 26.9 on weighted basis (OE-ALSO) was the best composition as a solid electrolyte [9] because of its high electrical conductivity (6.3 Â 10 À2 S cm À1 at 1073 K [10]), high ionic transport number (>0.996 at 1073 K [10,11]), high chemical stability under presence of carbon dioxide as well as under the reducing conditions [12], and high ability to construct efficient electrode/electrolyte interfaces using MIEC electrode [10]. In addition, we have examined its potential as the solid oxide fuel cell (SOFC) electrolyte, and reported that a maximum power density of ca.…”

“…In addition, we have examined its potential as the solid oxide fuel cell (SOFC) electrolyte, and reported that a maximum power density of ca. 0.25 W cm À2 could be drawn from an SOFC unit cell based on OE-ALSO (0.72 mm thick) operating at 1073 K [10]. From the view point of the element strategy and green and sustainable development, LSO is also attractive because one of major constituent atoms of this electrolyte is environment friendly and common element, silicon.…”

Research progress in nondoped lanthanoid silicate oxyapatites as new oxygen-ion conductors

“…Mineshige et al measured the average transference number of oxygen ions in La 9.67 (SiO 4 ) 6 O 2.535 ceramics using an oxygen concentration cell over the p O2 range of 10 ¹2 10 ¹24 at 873 and 1073 K. 148) They concluded that the average transference number of oxygen ions was greater than 0.99 under the aforementioned conditions. However, measurement of the transference number at temperatures above 1173 K would be necessary to allow a comparison with the results reported in their previous study, 125) because hole conductivity may have influenced the results.…”