Enhanced Low-Temperature Proton Conduction in Sr_0.02La_0.98NbO_4−δ by Scheelite Phase Retention

Abstract

The high-temperature scheelite phase of Sr0.02La0.98NbO4 materials has been retained to room temperature via vanadium substitution for the Sr0.02La0.98Nb1−x V x O4−δ compositional range of 0.25 ≤ x ≤ 0.325. Such structural stabilization avoids the characteristic break in the thermal expansion coefficient (TEC) that may otherwise be deleterious for the application of LaNbO4-based materials as proton-conducting electrolytes. The transport properties of composition Sr0.02La0.98Nb0.7V0.3O4−δ have been extensively characterized. The composition offers pure proton conduction with higher conductivity than the base composition Sr0.02La0.98NbO4−δ in the low-temperature range under wet oxidizing conditions. A wide ionic domain is observed, which increases as the temperature decreases. Suggested operational limits have been documented. Under dry oxidizing conditions, vanadium substitution is shown to suppress p-type conductivity, in comparison to the base composition Sr0.02La0.98NbO4−δ. In contrast, under wet conditions, Sr0.02La0.98Nb0.7V0.3O4−δ is observed to be a pure proton conductor in oxygen and argon atmospheres at temperatures lower than ∼700 °C. The total ionic conductivity equals that of the proton conductivity, within experimental error, suggesting negligible oxide-ion conduction in this material under these conditions.

Influence of Sb-substitution on ionic transport in lanthanum orthoniobates

“…In comparison to other isovalent substituents, such as Ta and V, 24,26 , the total conductivity of Sb-substituted lanthanum orthoniobate without acceptor doping increases with increasing Sb content. For the Ca-doped compounds the total conductivity is generally higher in the high temperature range, with an observable decrease of conductivity at lower temperatures in LCNSO30.…”

“…lanthanide vanadates, niobates, arsenates or antimonates) possess interesting functional properties, such as ferroelasticity [12][13][14] , photoluminescence [15][16][17][18][19][20] or ionic conductivity [21][22][23] . Out of this group lanthanum orthoniobate is considered to be among the best materials combining appreciable proton conductivity and chemical stability 22,[24][25][26][27] , reaching conductivity close to 10 -3 S/cm at 900 °C under wet conditions 22 . Doping and substitutions in lanthanum niobate, both alio-and isovalent, have a substantial influence on these materials properties and much effort to understand the influence of dopants on structural [24][25][26][28][29][30][31][32][33][34] and electrical [22][23][24][25][26][27][35][36][37][38][39][40][41][42][43] properties have been made.…”

“…Doping and substitutions in lanthanum niobate, both alio-and isovalent, have a substantial influence on these materials properties and much effort to understand the influence of dopants on structural [24][25][26][28][29][30][31][32][33][34] and electrical [22][23][24][25][26][27][35][36][37][38][39][40][41][42][43] properties have been made. Lanthanum orthoniobate undergoes a structural phase transition at about 500 °C, which is accompanied by a decrease in the activation energy of the conductivity 37,39,43 and nearly a twofold change of the thermal expansion coefficient (TEC) [24][25][26]29 . Changes in TEC may cause challenges for the materials' applicability in e.g.…”

“…fuel cells, yielding thermal incompatibility between the electrolyte and electrodes which may lead to strain, cracking and even failure of the electrochemical device. This challenge can be tackled by substitution of niobium with different isovalent elements, such as Ta 25,26,29 , V 24,29 or Sb 29 . Substitution of Nb by Ta increases the phase transition temperature, whereas substituents such as Sb and V give the opposite effect.…”

“…Substituting Nb with an element of different ionic radius affects bond lengths and therefore isovalent substituents may alter the electrical conductivity. It has been shown that in lanthanum niobate substituted by Ta the conductivity is decreased (when compared to lanthanum niobate without isovalent substituents) in a wide temperature range 26 , whereas in V-substituted lanthanum niobate the conductivity is increased in the low temperature range and decreased at high temperatures 24 .…”