Protonic conduction in Ca2+-doped La2M2O7 (M = Ce, Zr) with its application to ammonia synthesis electrochemically

doi:10.1016/j.materresbull.2005.04.008

Materials Research Bulletin

Volume 40, Issue 8, 11 August 2005, Pages 1294-1302

https://doi.org/10.1016/j.materresbull.2005.04.008 Get rights and content

Abstract

Complex oxides La_1.95Ca_0.05M₂O_7−δ (M = Ce, Zr) were prepared by sol–gel method and characterized by thermal analysis (TG–DTA), X-ray diffraction (XRD). On the sintered complex oxides as solid electrolyte, the conductivity was measured in various atmospheres, and ammonia was synthesized from nitrogen and hydrogen at atmospheric pressure in the solid states proton conducting cell reactor by electrochemical methods. The rates of ammonia formation were up to 2.0 × 10⁻⁹ mol s⁻¹ cm⁻² for La_1.95Ca_0.05Zr₂O_7−δ and 1.3 × 10⁻⁹ mol s⁻¹ cm⁻² for La_1.95Ca_0.05Ce₂O_7−δ, respectively, at 520 °C.

Introduction

Oxides of composition A₂B₂O₇ have received great attention because of the presence in their structure of many nonoccupied positions, which makes them reasonable candidates as fast ionic conductors [1], [2]. Furthermore, their appreciable proton conduction at high temperature when exposed to wet hydrogen atmospheres is very promising for applications such as fuel cells and hydrogen sensors [3], [4], [5], [6].

For A₂B₂O₇ oxides, depending on the cation radius ratio (r³⁺/r⁴⁺), the metal ions will be ordered to different extents in the cation sub-lattice, yielding either the ordered pyrochlore structure (for example, La₂Zr₂O₇), space group Fd3m, or the disordered fluorite structure (such as La₂Ce₂O₇, space group Fm3m) [7]. However, in early literatures, the structure of La₂Ce₂O₇ was considered as pyrochlore-type wildly [8], and there was little report about the proton conduction in fluorite structure-type oxides. In this paper, the fluorite structure of La₂Ce₂O₇ and the proton conduction in fluorite structure-type oxides were confirmed, and the influence of the structural transformation from a pyrochlore structure to a defect fluorite structure on their chemical stabilities and electrical properties was studied.

Ammonia synthesis at atmospheric pressure through solid-state proton conductors as the electrolyte electrochemically has been investigated in our previous studies [9], [10] and by Marnellos and Stoukides [11], [12]. In the present study, fluorite structure-type oxide was used to synthesize ammonia for the first time, and evaluating electrical properties and protonic conductivity in different structure oxides through the rate of evolution of ammonia was attempted, from which we hoped to offer an alternative to evaluate the protonic conductivity for solid-state proton conductors.

Section snippets

Methods and experiments

The La_1.95Ca_0.05M₂O_7−δ (M = Ce, Zr) powders were prepared by sol–gel method. La₂O₃ (A.R.) and CaCO₃ (A.R.) were first converted to their nitrate in stoichiometric ratio. Calculated amount of ZrO(NO₃)₂·2H₂O (A.R.)/(NH₄)₂Ce(NO₃)₆ (A.R.) was added to the nitrate solution. Subsequently, solid citric acid was added to the solution, resulting in a mole ratio of citric acid:zirconium/cerium + lanthanum + calcium = 2:1. The solution was then slowly evaporated at 50–90 °C until a viscous liquid was obtained.

Thermal decomposition

The TG and DTA curves for La_1.95Ca_0.05M₂O_7−δ (M = Zr, Ce) precursors were presented in Fig. 2. Both the precursors decomposed to oxide via two distinct stages, the first stage was the decomposition of partial citrate and nitrate, and the second stage of decomposition with an exothermal peak centering at 500 °C (a)/400 °C (b) corresponding to the further decomposition of (La, Ce/Zr)₂O(CO₃)₂ to form oxide solid solution. The temperature ending the whole weight loss in Fig. 2b was lower than that in

Conclusion

Complex oxides La_1.95Ca_0.05M₂O_7−δ (M = Ce, Zr) were prepared by sol–gel method, single phase with well-defined pyrochlore-type structure was obtained at 1000 °C for La_1.95Ca_0.05Zr₂O_7−δ and fluorite-type structure for La_1.95Ca_0.05Ce₂O_7−δ at 800 °C. The relative densities of sintered plates were more than 90% when sintered at 1500 °C for 10 h.

La₂M₂O₇ (M = Ce, Zr)-based oxides under hydrogen-containing atmosphere at high temperature showed proton conduction regardless of their structures.

On the sintered

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Protonic conduction in Ca2+-doped La2M2O7 (M = Ce, Zr) with its application to ammonia synthesis electrochemically

Abstract

Introduction

Section snippets

Methods and experiments

Thermal decomposition

Conclusion

Solid State Ionics

Solid State Ionics

Solid State Ionics

Solid State Ionics

Solid State Ionics

Solid State Ionics

J. Alloys Compd.

Protonic conduction in Ca²⁺-doped La₂M₂O₇ (M = Ce, Zr) with its application to ammonia synthesis electrochemically