Electronic and structural properties of La_0.4Sr_0.6Ti_1−yCo_yO_3±δ electrode materials for symmetric SOFC studied by hard X-ray absorption spectroscopy

Highlights

• XANES/EXAFS reveal invariance of the local structure and formal valence of Ti in LSTC.

• Dramatic changes at the Co-site are produced with increasing Co content.

• Stability of Ti⁴⁺ triggers the formation of A-site vacancies in LSTC materials.

• The reduction of A-site deficiency with increasing Co content in LSTC is explained.

• An improved electrochemical performance for higher Co content is predicted.

Abstract

We present combined Synchrotron X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) study of La_0.4Sr_0.6Ti_1−yCo_yO_3±δ (0 ≤ y ≤ 0.5), which are promising electrode materials for symmetric solid oxide fuel cells. The measurements were performed at room temperature at the Ti and the Co K-edges in order to determine the local structural and electronic changes around the two transition metals. We find that Ti remains in a higher formal valence (around 4+) independent of the Co concentration. In contrast to this, dramatic and systematic changes are observed for the Co as a function of y. We conclude that the stability of the Ti⁴⁺ triggers the A-site deficiency in our samples and predicts that oxygen vacancies are much more easily formed at large Co content, which in turn will greatly enhance the performance as electrode material.

Introduction

ABO₃ perovskites with Ti in the B site are promising electrode materials for a new subtype of Solid Oxide Fuel Cells (SOFC), the so called Symmetrical SOFC (SSOFC), reported for first time by Ruiz-Morales et al. [1], [2]. SSOFC are based on a symmetric design where the same compound is used as cell anode and cathode. Advantages over traditional SOFC relies on their simpler design, dealing with compatibility and inter-diffusion problems between cell components and even helping to manage the anode sulfur and carbon poisoning by simply reversing the gas flux, issues that strongly affects SOFC anodes long term efficiency.

Perovskites belonging to the La_xSr_1−xTiO₃ (LST) family are recognized for their good properties in a SOFC anode reductive environment [3]. Besides, the replacement of small amounts of Ti with other transition metals can enhance their ionic and electronic conductivity in oxidizing atmospheres, therefore, enabling the use of doped LST as possible SOFC cathodes [4], [5]. Attending the wide experience gained with the La_1−xSr_xCoO_3−δ (LSC) as mixed conductor for SOFC cathodes [6], [7] and the support of theoretical calculations [8], the substitution of Ti by Co atoms seems to be an adequate way to enhance LST mixed conductivity.

However, substitution of atoms with other valences in the ABO₃ structure might induce oxidation and reduction processes in the components or even creation of vacancies in the oxygen or the A-site sublattice, in order to compensate for charge and size changes [9]. Some authors reported that doping SrTiO₃ with 3+ ions on the perovskite A-site may increase the oxygen non-stoichiometry, creates A-site deficiency and/or changes the ratio of Ti⁴⁺ to Ti³⁺ thus enhancing the conductivity. The predominance of one of these compensating mechanisms depends on synthesis temperature and reducing conditions [3], [10]. The already mentioned replacement of Ti⁴⁺ on the perovskite B-site can also alter the structural properties of the titanates [11], [12]. Therefore, substitutions may not only affect the electrochemical behavior, but also the material structure itself. An important requirement in the optimization of doped titanates as SSOFC electrodes is to identify and understand the active charge compensation mechanisms and their influence over structural parameters and electronic configuration. For example: A-site deficiency is related to phase stability [10] and to mixed conductivity [13], O vacancies strongly influences ionic conductivity [6] and the transition metal (TM) oxidation state affects the electronic conductivity [11].

In a previous work we investigated the series La_xSr_1−xTi_1−yCo_yO_3±δ (LSTC) with x = 0.4 and y varying between 0.0 and 0.5 [14]. A structural characterization using high resolution synchrotron X-ray Powder Diffractometry (XPD) and Transmission Electron Microscopy (TEM) methods showed that the creation of Sr vacancies at the A-site is the main compensation mechanism for charge and size changes, and that the A-site vacancies concentration decreases with the incorporation of Co in the crystal structure. The existence of Sr segregation in samples with A-site deficiency was supported by the presence of an amorphous background in the XPD data and local crystallinity loss observed by TEM. The physical reason for the creation of A-site vacancies, however, remained to be clarified. Simultaneously, the characterization of La_0.5Sr_0.5Ti_0.5Co_0.5O_3−δ through neutron diffraction measurements was reported [15], indicating that this compound is fully stoichiometric at room temperature and presented a different space group than our samples (i.e., for y = 0.5 and x = 0.5, this work reported a crystal structure corresponding to the orthorhombic Pbnm space group, while for a similar composition, y = 0.5 and x = 0.4, our best fit of XPD data was obtained assuming the R-3c rhombohedral space group).

We concluded from our previous studies that the presence of Co, which should modify its oxidation state more easily than Ti, might preclude the formation of A-site vacancies, even at low synthesis temperature. However, the lack of published data on the electronic properties of the B-site transition metals in this series made it impossible to validate that assumption. Moreover, the knowledge of the electronic configuration of the B-sites is crucial in order to understand the electrochemical properties of SSOFC electrodes.

In this work, we experimentally investigate the electronic configuration, oxidation state and nearest neighbors coordination behavior of cobalt and titanium in the same samples and tested our previous hypothesis using synchrotron X-ray Absorption techniques. These results will help to establish and characterize the LSTC charge compensation mechanisms and understand the physical origin of the electrochemical properties of these materials as SOFC electrode.