Influence of Isovalent ‘W’ Substitutions on the Structure and Electrical Properties of La2Mo2O9 Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells

Abstract

Lanthanum molybdenum oxide (La2Mo2O9, LAMOX)-based ion conductors have been used as potential electrolytes for solid oxide fuel cells. The parent compound La2Mo2O9 undergoes a structural phase transition from monoclinic (P21) to cubic (P213) at 580 °C, with an enhancement in oxide ion conductivity. The cubic phase is of interest because it is beneficial for oxide ion conduction. In search of alternative candidates with a similar structure that might have a stable cubic phase at lower temperatures, we have studied the variations of the crystal structure and ionic conductivity for 25, 50, 62.5 and 75 mol% W substitutions at the Mo site using high-temperature X-ray diffraction, dilatometry, and impedance spectroscopy. Highly dense ceramic samples have been synthesized by solid-state reaction in a two-step sintering process. Low-angle X-ray diffraction and Rietveld refinement confirm the stabilization of the cubic phase for all compounds in the entire temperature range considered. The substitutions of W at the Mo site produce a decrement in the lattice parameter. The thermal expansion coefficients in the high-temperature range of the W-substituted ceramics, as determined by dilatometry, are much higher than that of the unmodified sample. The impedance spectra have been modeled using a modified genetic algorithm within 300–600 °C. A distribution function of the relaxation times is obtained, and the contributions of ohmic drop, grains and grain boundaries to the conductivity have been identified. Overall, our investigation provides information about cationic substitution and insights into the understanding of oxide ion conductivity in LAMOX-based compounds for developing solid oxide fuel cells.

W-doped Lanthanum Molybdenum Oxide/Lithium-Sodium-Potassium Carbonate Composite Membranes for Carbon Dioxide Permeation

“…The Bragg reflections observed in the diffractogram shown in Figure 1 are consistent with the ICSD 172611 file, which has a lattice parameter of 7.142 Å, as determined using GSAS/EXPGUI software [ 29 ]. This agrees with the data reported for the cubic phase [ 8 , 16 ].…”

“…Moreover, La 2 Mo 2 O 9−δ is thermodynamically unstable at low oxygen partial pressures (p O2 ) due to the reducibility of Mo 6+ , resulting in the formation of phases with partial n-type conductivity and amorphization [ 4 , 5 ]. To address these issues, several studies have focused on cation substitutions (W [ 6 , 7 , 8 , 9 , 10 , 11 ], Nd [ 6 ], Ca [ 12 ], Ba [ 12 , 13 ], Sr [ 12 , 14 ], Y [ 15 ], Cr [ 7 , 14 ]) for La or Mo (LAMOX family). These substitutions aim to inhibit the phase transition, stabilize the β-phase, and improve the thermomechanical stability [ 16 ].…”

“…These substitutions aim to inhibit the phase transition, stabilize the β-phase, and improve the thermomechanical stability [ 16 ]. Substituting tungsten for molybdenum has been identified as an effective strategy to stabilize the cubic phase of La 2 Mo 2 O 9−δ at room temperature while preserving its ionic conductivity [ 8 ]. In addition to stabilizing the β-phase at room temperature, the La 2 Mo 2−y W y O 9−δ series also exhibits enhanced chemical stability under low p O2 , resulting in a reduction in the formation of phases with n-type conduction as the W content increases.…”

“…In addition to stabilizing the β-phase at room temperature, the La 2 Mo 2−y W y O 9−δ series also exhibits enhanced chemical stability under low p O2 , resulting in a reduction in the formation of phases with n-type conduction as the W content increases. Although substitutions at both the La and Mo sites lead to a decrease in the ionic conductivity due to long-range vacancy ordering, the substitution of Mo for W [ 8 ] results in a structurally and chemically stable W-LAMOX compound that is suitable for various applications.…”

Thermoanalytical and X-ray Diffraction Studies on the Phase Transition of the Calcium-Substituted La2Mo2O9 System

“…However, the molar ratio of initial metals remains the main factor that determines the formation of the La 2 Mo 2 O 9 composition. This is the reason why the partial substitution [ 20 , 21 , 22 ] of either lanthanum [ 23 , 24 , 25 ] or molybdenum [ 26 , 27 , 28 , 29 ] leads to the crystallization of side phases [ 30 , 31 ], which significantly affects the physical properties [ 32 , 33 ] of the corresponding compound. This effect is directly related to both the amount of the La 2 Mo 2 O 9 phase in the final ceramic mixture and the increased stabilization of the cubic phase at room temperature.…”