Effect of tri- and tetravalent metal doping on the electrochemical properties of lanthanum tungstate proton conductors

Abstract

Rare-earth tungstates (La(28-y)W(4+y)O(54+δ)□(2-δ)) have attracted attention recently because of their relatively high proton-electron conductivity and high stability in a CO2 environment. Since doping on the tungsten-site may increase the conductivity, a new series of compounds with composition La(5.5)W(1-x)M(x)O(11.25-δ) (M = Al, Ti and Zr; x = 0, 0.05 and 0.10) have been investigated. The crystal structure of these materials has been studied using X-ray and time-of-flight neutron powder diffraction by Rietveld analysis. The concentration of oxygen vacancies for hydration in the structure has been indirectly determined by thermogravimetric analysis, and the total conductivity in several pO2, pH2O and pD2O atmospheres has been studied by impedance spectroscopy. An increase in the conductivity is observed, ranging from 4.1 mS cm(-1) for the undoped sample to 9.2 mS cm(-1) for La(5.5)W(0.9)Ti(0.1)O(11.25-δ), in wet N2 at 800 °C.

Metal-Doping of La5.4MoO11.1 Proton Conductors: Impact on the Structure and Electrical Properties

“…Samples with composition La 5.4 Mo 0.9 A 0.1 O 12−δ (A = Ti, Zr, and Nb) and La 5.3 B 0.1 MoO 11.05 (B = Ca, Sr, and Ba) were prepared by the freeze-drying precursor method, following the procedure described in a previous work for related materials . The starting reagents were La 2 O 3 (99.99%, Aldrich), MoO 3 (99.5%, Aldrich), CaCO 3 (99%, Alfa Aesar), SrCO 3 (99.99%, Alfa Aesar), BaCO 3 (99%, Alfa Aesar), Ti­[OCH­(CH 3 ) 2 ] 4 (99%, Aldrich), ZrO­(NO 3 ) 2 ·6H 2 O (99% Aldrich), and Nb­(HC 2 O 4 ) 5 ·H 2 C 2 O 4 (97% ABCR).…”

“…In recent years, lanthanum tungstates have been doped with molybdenum with the goal of increasing the electronic conductivity due to the higher reducibility of molybdenum compared to tungsten. − Mo-doping improves the hydrogen fluxes compared to the undoped compound, where the best results were obtained by Chen et al for hollow-fiber membranes of La 5.6 W 0.6 Mo 0.4 O 11.4 . Nb-doping also leads to an enhancement of the overall conductivity and hydrogen permeation. , Other substitutions, such as Ti 4+ , Zr 4+ , Al 3+ , and alkaline-earth metals, have been successfully performed, improving the electrical properties of these materials. , …”

“…13,14 Other substitutions, such as Ti 4+ , Zr 4+ , Al 3+ , and alkaline-earth metals, have been successfully performed, improving the electrical properties of these materials. 15,16 On the other hand, La 5.5 W 1−x Mo x O 11.25 compounds crystallize with a cubic structure for x ≤ 0.4, rhombohedral for 0.4 ≤ x < 0.8, and they are mixed compounds for x > 0.8. 17 Molybdates with smaller rare-earth cations, Ln 6−x MoO 12−δ , present different symmetries depending on the ionic radii of Ln: cubic fluorite-type for Ln = Sm 3+ , Gd 3+ , and Dy 3+ (s.g. Fm3̅ m) and a cubic bixbyite-type for Ln = Ho 3+ , Er 3+ , Tm 3+ , Yb 3+ , and Lu 3+ (s.g. Ia3̅ ).…”

Synergic Effect of Metal and Fluorine Doping on the Structural and Electrical Properties of La5.4MoO11.1-Based Materials

“…Materials with the composition La 5.4 Mo 1−x A x O 12−δ (x = 0, 0.1; A = Ti 4+ , Zr 4+ , Nb 5+ ) were prepared by the freeze-drying precursor method, following the synthetic procedure detailed in a previous work for similar materials. 25 The starting reagents were: La 2 O 3 (99.99%, Aldrich), MoO 3 (99.5%, Aldrich), Ti[OCH(CH 3 ) 2 ] 4 (99%, Aldrich), ZrO(NO 3 ) was added as a complexing agent in the molar ratio 1:1 with respect to the total metal content. The different cation solutions were mixed in stoichiometric amounts under stirring, giving transparent solutions with a concentration of approximately 0.1 mol L −1 and pH 7.…”

“…Materials with the composition La 5.4 Mo 1– x A x O 12−δ ( x = 0, 0.1; A = Ti 4+ , Zr 4+ , Nb 5+ ) were prepared by the freeze-drying precursor method, following the synthetic procedure detailed in a previous work for similar materials . The starting reagents were: La 2 O 3 (99.99%, Aldrich), MoO 3 (99.5%, Aldrich), Ti­[OCH­(CH 3 ) 2 ] 4 (99%, Aldrich), ZrO­(NO 3 ) 2 ·6H 2 O (99%, Aldrich), and Nb­(HC 2 O 4 ) 5 ·H 2 C 2 O 4 (97%, ABCR).…”