Synthesis and structural characterization of Co-doped lanthanum strontium titanates

Abstract

Lanthanum strontium titanates based oxides, materials with perovskite-type crystal structure, are good candidates for symmetrical fuel cell electrodes. They can become good mixed conductors (electronic and ionic) and reach adequate electrocatalytic activity for both oxygen reduction and fuel oxidation reactions if the right doping or structural modification of the (La,Sr)TiO₃ compound is performed. However, the crystallographic information reported in the literature indicates a noticeable lack of reproducibility, since it is strongly dependent on synthesis conditions. In this work, La_0.4Sr_0.6Ti_1−yCo_yO_3+δ (0.0 ≤ y ≤ 0.5) synthesis by a low-temperature chemical route and its structural characterization by X-ray powder diffraction and transmission electron microscopy are reported. From Rietveld refinements results, charge compensation mechanisms are studied and their correlation with synthesis parameters is discussed. We show that the creation of Sr vacancies at the A-site of the perovskite-type structure is the main compensation mechanism and their concentration decreases with the incorporation of Co in the crystal structure.

Introduction

Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants. Hydrogen is normally used as the fuel and oxygen as the oxidant. However, solid-oxide fuel cells (SOFCs) are distinctive since they can operate with hydrocarbons as fuel, for instance methane, instead of hydrogen. Nevertheless, the requirement of high operating temperature (typically higher than 1073 K) implies great challenge for materials compatibility and stability leading to high manufacturing costs. The search for new cathode and anode materials is oriented to achieve high electronic and ionic conductivities (i.e. ‘mixed ionic-electronic conductors’, MIECs), high electrocatalytic activity for oxygen reduction or fuel oxidation reactions, chemical stability with electrolyte and interconnection materials, thermal expansion coefficients compatible with that of the electrolyte, etc. In the case of the anode, an additional requirement is high performance with other fuels besides hydrogen (such as methane, natural gas, biogas, etc.) [1], [2].

A new materials search branch has emerged recently from the Symmetrical Solid Oxide Fuel Cells (SSOFC) concept published by Ruiz-Morales et al. [3], [4] where the same material is used as anode and cathode simultaneously. This new approach could solve two of the main SOFCs problems, sulfur poisoning and carbon deposits when the cell is using hydrocarbon as fuel, by simply inverting the gas flux. Besides, this design should be less complicated than the conventional SOFCs due to the smaller number of components.

La_xSr_1−xTiO₃ (LST) has been widely reported as a possible SOFC anode material [2], [5], [6] and oxides based on this compound are also between the few reported as a possible electrode material for SSOFCs [7]. More generally, structural characterization of this compound has been reported in several articles with the common agreement that LST oxides have a cubic perovskite structure belonging to $Pm\overline{3}m$ space group for x ≤ 0.4. However, lattice parameter evolution with La content (x) has been a major source of disagreement between all these works. It has been reported a decrease [8], as well as a slight [9] and even a strong [10] increment in the lattice parameter with x. This lack of reproducibility on its structural characterization was not explained until the correlation between structural parameters, charge compensation mechanisms and synthesis parameters was reported by Hashimoto et al. [11].

Proposed charge compensation mechanisms for LST are: perovskite A-site vacancy creation (at low temperature synthesis), perovskite B-site transition metal oxidation state changes (at high temperature synthesis and reductive atmospheres) or oxygen non-stoichiometry. A-site vacancies and oxygen rich phases are related through a structural reordering leading to a Ruddlesden-Popper phase [12] but evidences of this change (SrO characteristic planes) are difficult to acquire through conventional X-ray powder diffraction techniques [8], [11]. Equation (1a) presents the A-site compensation mechanism while the 1b is the charge compensation one. However, the combination of 1a and 1b in Equation (2) reflects the relationship of A-site vacancies and transition metal oxidation state indicating that vacancies decrease when the amount of ${\text{B}}_{\text{B}}^{\prime }$ increases.

$${\text{La}}_2{\text{O}}_3+2{\text{Sr}}_{\text{Sr}}^{\text{x}}\leftrightarrow 2\text{SrO}+2{\text{La}}_{\text{Sr}}^{•}+{\text{V}}_{\text{Sr}}^{″}+{\text{O}}_{\text{O}}^{\text{x}}$$

$$2{\text{Ti}}_{\text{Ti}}^{\text{x}}+{\text{O}}_{\text{O}}^{\text{x}}\leftrightarrow 2{\text{Ti}}_{\text{Ti}}^{\prime }+V_{\text{O}}^{••}+\frac{1}{2}{\text{O}}_2\left(g\right)$$

$$2{\text{La}}_{\text{Sr}}^{•}+{\text{V}}_{\text{Sr}}^{″}+2{\text{Ti}}_{\text{Ti}}^{\text{x}}+2\text{SrO}\leftrightarrow 2{\text{Ti}}_{\text{Ti}}^{\prime }+{\text{V}}_{\text{O}}^{••}+{\text{La}}_2{\text{O}}_3+2{\text{Sr}}_{\text{Sr}}^{\text{x}}+\frac{1}{2}{\text{O}}_2\left(g\right)$$

On the other hand, through several experimental techniques a new explanation for La-doped Strontium Titanates structure has been reported by Canales-Vázquez et al. [13]. They discarded the A-site vacancies formation or Ti reduction mechanisms in favor of oxygen rich layered defects, based on the discovery of the new family of layered La-doped Strontium Titanates La₄Sr_n−4Ti_nO_3n+2. These ordered compounds were obtained via solid state reaction using high temperatures (up to 1873 K) and long time (up to 120 h) thermal treatments.

At SOFC cathode side, La_xSr_1−xCoO_3−δ is a well studied perovskite with excellent electrochemical properties that have turned it into one of the most successful SOFC cathodes until now [1]. Because SSOFC implicit design, its electrodes have to fulfill both SOFC anode and cathode requirements in the same material and, in this way, the substitution of Ti for Co in (La,Sr)TiO₃ is proposed in the present work. Oxygen ion migration in SrBO₃ compounds has been calculated by Li et al. [14] using density-functional theory in the search of the best element for SrTiO₃ B-site doping. According to this work, the lowest oxygen ion migration energy corresponded to Sc, while Co resulted the second best for a 3d transition metal doping choice.

Previous reports on (La,Sr)(Ti,Co)O₃ have been mostly related with their magnetic properties on epitaxially grown thin films [15], [16] although some works have recently appeared in the fuel cells area: a small cobalt substitution in LST in order to obtain a good SOFC anode material was proposed by Yoo et al. [17] and Li et al. [18], while a Ti-doped SrCoO₃ cathode was presented by Shen et al. [19].

A complete structural characterization of SOFC electrodes is required for a comprehensive understanding of their electrochemical properties. For example, electrode properties are influenced by all charge compensation mechanism cited before: perovskite A-site vacancies affects the chemical compatibility with electrolyte and, together with oxygen vacancies, also the ionic conductivity; structural distortions (e.g. oxygen octahedral tilting) along with transition metal valence state are key factors for electronic conductivity [20]. In this work the synthesis of a new possible electrode material for SSOFCs based on La_0.4Sr_0.6Ti_1−yCo_yO_3+δ (LSTC) with 0.0 ≤ y ≤ 0.5 oxides by a citrate chemical route and their structural characterization through synchrotron X-ray powder diffraction (XPD) is reported. Correlation between structural parameters, charge compensation mechanism and synthesis parameters is discussed.