We demonstrate the applicability of LAMOX oxide ion conductor as the electrolyte of single-chamber SOFC, using two compositions La0.9Dy0.1Mo2O9 (LDM) and La0.9Dy0.1Mo1W1O9 (LDMW55). The peak power of the SOFC based on La0.9Dy0.1Mo2O9 is measured 220 mW cm-2 at 700 C in flowing methane/air mixture with CH4:O2=2:1. Despite its high ion conductivity, the applicability of LAMOX electrolyte has been questioned for a long time, because it is susceptible to hydrogen reduction, reacts with the typical cathode compositions, expands more than other typical electrolytes at 500 – 800 C.

Single chamber SOFC based on LDMW electrolyte and its on-site anode initialization

Dah-Shyang Tsai,* Jen-Chieh Lo, Yu-Chen ChenDepartment of Chemical Engineering, National Taiwan University of Science and Technology

43, Keelung Road, Section 4, Taipei 10607 TaiwanE-mail address: dstsai@mail.ntust.edu.tw

ABSTRACT

IntroductionLAMOX stands for a group of oxygen ion conductors based on its parent crystal La2Mo2O9. Featured with high oxygen ion conductivity in the temperature range of SOFC interest, the crystal structure of -phase La2Mo2O9 is intrinsically defective. Partially filled oxygen sites, O(2) and O(3), are approximately 2/3 and 1/3 filled. Most of O(3) sites surround Mo6+, which is the key element for LAMOX. Meanwhile, the high valence Mo is also the reason for hydrogen reduction.

Figure 2 Comparison of La1.8Dy0.2Mo2O9 and La1.8Dy0.2Mo1.6W0.4O9 microstructures, that were first reduced in 3% H2 (97% Ar) at 600 C for 48 h then annealed in air at 1000 C to restore its microstructure. Before hydrogen reduction, the oxide surfaces were creamy white. Reduction at 600 C turned the surfaces into dark grey. Annealing in air can restore the color of oxide surface, but left with microscopic damages on the surface of La1.8Dy0.2Mo2O9, Figure 2(a). For La1.8Dy0.2Mo1.6W0.4O9, the microscopic damages are not found, Figure 2(b), showing the W substitution alleviates Mo reduction.

Sr is a common element in typical cathode composition, but LAMOX reacts with Sr and form SrMoO4 as low as 600 C. A ceria interlayer may act as diffusion barrier to block interdiffusion and solid-state reaction.

Figure 3 Reaction between La0.6Sr0.4Co0.8Fe0.2O3 (LSCF6482) and La0.9Dy0.1Mo01.6W0.4O9 (LDMW82) at 800 C for 6 h, (a) no reaction with a SDC interlayer in between, (b) a product layer between LSCF6482 and LDMW82. Note the grain coarsening of LSCF6482 and new phase of LDMW82 are accompanied with the product layer formation.

Figure 4 Solid state reaction between 50% LDMW92 and 50% LSCF6482 powder compact, soaked at 800 C for 6 h. XRD patterns of (a) 50% LDMW92 and 50% LSCF6482 mixture, (b) LDMW82, (c) LSCF6482. Note that SrMoO4 and Mo deficient LAMOX phase in the pattern (a).

Figure 5 Thermal expansion of LDM and LDMW55 measured with dilatometry. High thermal expansion between 480 and 800 C is difficult to match with. The TEC value of GDC is 12 ppm K-1, that of NiO 14 ppm K-1, that of Ni metal 17 ppm K-1. Although NiO+GDC and Ni+GDC are thermochemically matched with LAMOX, the matching of thermal expansion is a headache.

Figure 6 The experimental setup for measuring single chamber SOFC performance. The cell is housed in a quartz glass tube. The flowing CH4/Air is preheated before itmeets the cell, which is hanged on two gold wires connecting to the current collectors.

Attempts to build the dual chamber SOFC have failed, mainly because of the difficulty in finding suitable gas sealants for LDMW electrolyte.

However, we did succeed in fabricating the single chamber SOFC using the anode support design based on LAMOX electrolytes.

Figure 7 Configuration of the anode-supported cell. The matching cathode LSCF6482 is interfaced with the i-GDC protected LAMOX (LDM or LDMW55). NiO+GDC (4:6 wt ratio) mechanically supports the cell. The electrolyte thickness 60 m.

Figure 8 Plateform for SOFC measurement. Gas cylinders and mass flow controllers are placed at the right-hand side. The vacuum pump and the hydrogen detector are placed at the left-hand side. In between is the furnace and the quartz glass tube for housing the coin cell.

Initialization of single chamber SOFC is critical to its measurement. We adopt the on-site initialization to reduce the anode with a minimum influence on the cathode. Condition for initialization: CH4/air (CH4:O2=2:1) flow rate 350 sccm at 650 C

Figure 9 (a) Variation of cell voltage with time, (b) Incubation time versus the spacing between Ni+GDC and anode.

Figure 10 I-V and I-P characteristics of the cell Ni+GDC/LDM/i-GDC/LSCF6482 in flowing CH4/air (CH4:O2=2:1). The flow rate is 350 sccm. Two temperature readings are given for each measurement, the one being parenthesized is the cell temperature; the other is the furnace temperature.

Figure 11 I-V and I-P characteristics of the cell Ni+GDC/LDMW55/i-GDC/LSCF6482 in flowing CH4/air (CH4:O2=2:1). The flow rate is 350 sccm. Two temperature readings are given for each measurement, the one being parenthesized is the cell temperature; the other is the furnace temperature.

ConclusionsWe have shown the LAMOX ion conductor can be the electrolyte of SOFC. Similar to the ion conductor family of ceria, its OCV value is less than 1.0 when the anode-supported single chamber SOFC based on LDMW is immersed in the CH4/air mixture flow with CH4:O2=2:1.

Figure 1 -La2Mo2O9 crystallizes in SnWO4 structure with the unoccupied oxygen sites O(2) and O(3) to facilitate oxygen ion transport. Comparison of cation coordination environment indicates the 1/3 filled O(3) sites are closelyassociated with Mo6+ ion. (J. Mater. Chem. 11, 2001, 119)