Synthesis of pure and Sr-doped LaGaO₃, LaFeO₃ and LaCoO₃ and Sr,Mg-doped LaGaO₃ for ITSOFC application using different wet chemical routes

Abstract

Pure and Sr-doped LaGaO₃, LaFeO₃ and LaCoO₃ and Sr,Mg-doped LaGaO₃ were synthesized by various wet chemical routes, namely combustion, co-precipitation and citrate-gel methods. The effect of the various process parameters on the phase purity, particle size and surface area and morphology of the synthesized powders were determined by XRD, simultaneous TG-DTA, laser light scattering, BET and scanning electron microscopy. The stability of the synthesized pure phases in oxidizing and reducing atmosphere was also studied by thermogravimetry. It was observed that pure and Sr-doped single perovskite phases of lanthanum ferrite, cobaltite and gallate and Sr,Mg-doped lanthanum gallate could be synthesized by combustion and citrate-gel methods under suitable process conditions. Synthesis using the co-precipitation method yielded incomplete reaction irrespective of the calcination temperature adopted. The citrate-gel method yielded better powder properties in terms of particle size and morphology and surface area compared to combustion synthesis. It was found that pure and Sr-doped lanthanum ferrite, lanthanum cobaltite, lanthanum gallate and Sr,Mg-doped lanthanum gallate were stable in the oxidizing atmosphere. In the reducing atmosphere, pure and Sr-doped lanthanum ferrite and Sr,Mg-doped lanthanum gallate was found to be stable at least during the timeframe of the thermogravimetric experiment whereas pure and Sr-doped lanthanum cobaltite was partially reduced in hydrogen atmosphere.

Introduction

The present state of the art solid oxide fuel cells (SOFC) are based on (Y₂O₃)ZrO₂ solid electrolyte and operating in the temperature range of 800–1000 °C [1]. High temperature of operation of the SOFC is dictated by the ionic conductivity requirement of the solid electrolyte (>0.1 S cm⁻¹). However, the higher temperature of operation of these solid oxide fuel cells results in several material problems, including requirement of high temperature sealing and therefore a high cost. From the materials and cost point of view, it is advantageous to operate the fuel cells at intermediate temperatures of 600–800 °C, which retains the advantages of internal reforming and co-generation but imposes less stringent requirements on the materials. The main limitation of operating the solid oxide fuel cell at lower temperature is to find alternate oxygen ion conducting electrolyte materials, which have comparable ionic conductivities at these lower temperatures. In addition to the conductivity requirements of the solid electrolyte, thermal and chemical compatibility with the anode and cathode as well as stability under both oxidizing and reducing conditions are the other essential requirements.

Since the discovery of high and exclusive oxygen ion conductivity in doped cubic zirconia by Kiukkola and Wagner [2] in 1957, several other materials with high oxygen ion conductivity has been reported in the literature [1], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Oxygen ion conductivity has not been restricted to the cubic fluorite structure, but has been found in perovskite and pyrochlore structures [1], [3], [4], [5], [6]. One of the promising electrolyte candidate for a low or intermediate temperature SOFC (ITSOFC) is La(Sr)Ga(Mg)O_3−δ known as LSGM which has a perovskite structure and ionic conductivity several times higher than (Y₂O₃)ZrO₂ in the temperature range of 600–800 °C [10], [11], [12], [13], [14]. Although La(Sr)Ga(Mg)O_3−δ has a thermal expansion coefficient similar to (Y₂O₃)ZrO₂ and therefore the same cathode and anode materials as that used in the conventional cell can in principle be used, the chemical compatibility between these materials are not well established [3], [15]. It has been reported that ZrO₂ reacts with lanthanum oxide and forms several stable interoxide compounds and therefore zirconia-based material is not suitable for use as anode when LSGM is used as an electrolyte [15]. Further, Sr-doped lanthanum manganite, which is used as a cathode in the conventional SOFC offers very high interfacial resistance at the cathode–electrolyte interface [16], [17], [18], [19]. Lanthanum cobaltite and lanthanum ferrite are being considered as potential cathode materials for SOFC applications [1], [9], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. Lanthanum cobaltite has a higher electronic conductivity but lower thermal expansion compatibility [23], [24], [25], [26], [27], [28], [31], whereas lanthanum ferrite (LSF) has higher thermal expansion compatibility but lower electronic conductivity [28], [29], [30], [31]. It has been reported that mixed La(Sr)Fe(Co)O_3−δ (LSCF) has the optimum combination of electronic conductivity and thermal expansion compatibility with LSGM [31], [32], [33], [34], [35], [36].

Knowledge of the preparation of pure single-phase ceramic powders with controlled particle size and morphology and surface area is essential for their subsequent processing. Most common ceramic processing techniques such as slip and tape casting, extrusion, screen printing, calendaring and electrophoretic deposition require fine particle size and high surface area to form stable aqueous and non-aqueous suspensions with high solid loading. The conventional solid-state reaction method is in general employed to prepare mixed oxide compounds because of its lower manufacturing cost and simplicity. Several investigators [14], [18], [31], [37], [38], [39], [40], [41], [42], [43] have employed a solid-state reaction method to synthesize perovskite oxides pertinent to IT-SOFC. The main drawback of the solid state route is the long diffusion distances and slow reaction kinetics hindering the formation of pure single phases and lack of control of particle size and surface area. In general, solid-state synthesis requires a temperature higher than 1500 °C and time period in excess of 48 h.

In order to overcome the disadvantages of solid-state synthesis, a number of chemical synthesis techniques such as wet combustion synthesis [44], [45], [46], [47], [48], [49], [50], sol–gel [12], [51], citrate-gel/Pechini [34], [52], [53], [54], [55], [56], [57] and co-precipitation [18], [58], [59] methods have been reported in the literature for the preparation of these oxides. These methods provide mixing of the elements at an atomic scale accelerating pure phase formation and since these methods employ lower calcination temperatures, more control over the particle size and morphology and surface area can be exercised. In the combustion process, glycine or urea is used as a fuel for combustion as well as to form complexes with metal ions to increase solubility and prevent selective precipitation (during water removal). The resultant oxide ash after combustion is generally composed of very fine particles of the desired stoichiometry linked together in a networked structure. This process produces oxide powders of good compositional homogeneity in a short time, but forms agglomerates readily.

Co-precipitation method involves the precipitation of oxalates from a solution of metal nitrates at pH < 1 [60], and its subsequent thermal dissociation to form stoichiometric oxide compounds. In another variant of the co-precipitation method, the respective stoichiometric elemental nitrates are taken into solution and co-precipitated as hydroxides using ammonia as the precipitating agent at an alkaline pH range. Several parameters such as pH, mixing rates, temperature and concentration have to be controlled to produce the desired single phase. However, different rates of precipitation of each individual compound may lead to inhomogeneity and agglomerates are generally formed during the calcination treatment.

The Pechini or citrate gel process involves two basic chemical reactions: (i) complexation or chelation between metal ions and citric acid, and (ii) polyesterification of complexes with ethylene glycol. The complexation and poly-esterification reactions preserve the homogeneity of the metal salt solution in a gel. This polymerization reaction forms three-dimensional structures and minimizes segregation. The biggest advantage of this method is the high purity and excellent control over the composition of the resulting powders. In all these methods, a subsequent calcination step is necessary to completely drive out the organic products and achieve the desired single phase. Although some of these methods have been reported in the literature for the synthesis of pure and Sr,Mg-doped lanthanum gallate, lanthanum ferrite and lanthanum cobaltite, no systematic and comparative study highlighting the effect of various process parameters on the particle properties are available. Further, very little information is available on the long-term stability of these compounds in reducing and oxidizing atmospheres.

In the present work, pure single phase lanthanum cobaltite (LaCoO₃ i.e. LCO), lanthanum ferrite (LaFeO₃, i.e. LFO), lanthanum gallate (LaGaO₃ i.e. LGO), Sr-doped lanthanum cobaltite (La_0.90Sr_0.10CoO_2.95 i.e. LSC), lanthanum ferrite (La_0.90Sr_0.10FeO_2.95 i.e. LSF), and Sr- and Mg-doped lanthanum gallate (La_0.90Sr_0.10Ga_0.90Mg_0.10O_2.9 i.e. LSGM) powders were attempted to be prepared by combustion, co-precipitation and citrate-gel methods and their characteristics studied. The stability of these pure phases in reducing, oxidizing and ambient atmospheres was also investigated.