Article ID Journal Published Year Pages File Type
1293351 Journal of Power Sources 2011 9 Pages PDF
Abstract

The electrical conductivity, crystal structure and phase stability of La0.99Ca0.01Nb1−xTaxO4−δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, δ = 0.005), a potential candidate for proton conductor for solid oxide fuel cells (SOFCs), have been investigated using AC impedance technique and in situ X-ray powder diffraction. Partially substituting Nb with Ta elevates the phase transition temperature (from a monoclinic to a tetragonal structure) from ∼520 °C for x = 0 to above 800 °C for x = 0.4. AC conductivity of the La0.99Ca0.01Nb1−xTaxO4−δ both in dry and wet air decreased slightly with increasing Ta content above 750 °C, while below 500 °C, it decreased by nearly one order of magnitude for x = 0.4. It was also determined that the activation energy for the total conductivity increases with increasing Ta content from 0.50 eV (x = 0) to 0.58 eV (x = 0.3) for the tetragonal phase, while it decreases with increasing Ta content from 1.18 eV (x = 0) to 1.08 eV (x = 0.4) for the monoclinic phase. By removing the detrimental structural phase transition from the intermediate-temperature range, consequently avoiding the severe thermal expansion problem up to 800 °C, partial substitution of Nb with Ta brings this class of material closer to its application in electrode-supported thin-film intermediate-temperature SOFCs.

► We measured the electrical conductivity, crystal structure and phase stability of La0.99Ca0.01Nb1−xTaxO4−δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, δ = 0.005). ► Substituting Nb with Ta increased the monoclinic-to-tetragonal phase transition temperatures from ∼520 °C for x = 0 to near 800 °C for x = 0.4. ► The conductivity decreased slightly from 3.5 × 10−4 S cm−1 for x = 0 to 2.3 × 10−4 S cm−1 for x = 0.4 at 850 °C in humidified air. ► Substitution of Ta onto Nb slightly increases the activation energy of the tetragonal phase, and it decreases the activation energy for the monoclinic phase.

Related Topics
Physical Sciences and Engineering Chemistry Electrochemistry
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