Tailoring phase stability and electrical conductivity of Sr0.02La0.98Nb1–xTaxO4 for intermediate temperature fuel cell proton conducting electrolytes

Abstract

a b s t r a c t a r t i c l e i n f oO 4 (SLNT, with x = 0.1, 0.2, and 0.4) proton conducting oxides were synthesized by solid state reaction for application as electrolyte in solid oxide fuel cells operating below 600°C. Dense pellets were obtained after sintering at 1600°C for 5 h achieving a larger average grain size with increasing the tantalum content. Dilatometric measurements were used to obtain the SLNT expansion coefficient as a function of tantalum content (x), and it was found that the phase transition temperature increased with increasing the tantalum content, being T = 561, 634, and 802°C for x = 0.1, 0.2, and 0.4, respectively. The electrical conductivity of SLNT was measured by electrochemical impedance spectroscopy as a function of temperature and tantalum concentration under wet (p H2O of about 0.03 atm) Ar atmosphere. At each temperature, the conductivity decreased with increasing the tantalum content, at 600°C being 2.68 × 10 −4 , 3.14 × 10 −5 , and 5.41 × 10 −6 Scm −1 for the x = 0.1, 0.2, and 0.4 compositions, respectively. SLNT with x = 0.2 shows a good compromise between proton conductivity and the requirement of avoiding detrimental phase transitions for application as a thin-film electrolyte below 600°C.

Influence of Sb-substitution on ionic transport in lanthanum orthoniobates

“…This results in relatively high apparent activation energies if the data is interpreted according to Arrhenius behaviour (cf. Table 2 and literature data for undoped, Ca-doped 38,59 or Sr-doped Ta substituted lanthanum niobate 26 ). Interestingly, the case of Mg-doping 39 is an exception showing an activation energy of 0.74 eV.…”

“…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 .…”