ISSN 1063-7834, Physics of the Solid State, 2017, Vol. 59, No. 4, pp. 694–702. © Pleiades Publishing, Ltd., 2017. Original Russian Text © E.Yu. Pikalova, D.A. Medvedev, A.F. Khasanov, 2017, published in Fizika Tverdogo Tela, 2017, Vol. 59, No. 4, pp. 679–687.
SEMICONDUCTORS
Structure, Stability, and Thermomechanical Properties of Ca-Substituted Pr2NiO4 + δ E. Yu. Pikalovaa, b, D. A. Medvedeva, b, *, and A. F. Khasanova, b a
Institute of High-Temperature Electrochemistry, Ural Branch, Russian Academy of Sciences, ul. Akademicheskaya 20, Yekaterinburg, 620137 Russia b Ural Federal University, ul. Mira 19, Yekaterinburg, 620002 Russia *e-mail:
[email protected] Received May 30, 2016; in final form, July 30, 2016
Abstract—Ca-substituted layered nickelates with a general Pr2 – xCaxNiO4 + δ composition (x = 0–0.7, Δx = 0.1) were prepared in the present work and their structural and physic-chemical properties were investigated in order to select the most optimal materials, which can be used as cathodes for solid oxide fuel cells. With an increase in Ca content in Pr2 – xCaxNiO4 + δ the following tendencies were observed: (i) a decrease in the concentration of nonstoichiometric oxygen (δ), (ii) a decrease in the unit cell parameters and volume, (iii) stabilization of the tetragonal structure, (iv) a decrease of the thermal expansion coefficients, and (v) enchancement of thermodynamic stability and compatibility with selected oxygen- and proton-conducting electrolytes. The Pr1.9Ca0.1NiO4 + δ material, having highest δ value, departs from the general “properties– composition” dependences ascertained. This indicates that oxygen non-stoichiometry has determining influence on the functional properties of layered nickelates. DOI: 10.1134/S1063783417040187
1. INTRODUCTION In recent years, layered nickelites of the Ruddlesden–Popper homological series Ln2NiO4 + δ (Ln = La, Pr, Nd) have been objects of intense attention due to the potential possibility of their application as an oxygen electrode in intermediate-temperature solidoxide fuel cells (SOFCs) and solid oxide electrolysis cell [1]. When operating temperatures decrease, the activation polarization of an oxygen electrode makes the main contribution to total polarization losses during operation of SOFCs. In the case of the electrode with a mixed oxygen-ionic and electronic conductivity, the polarization is related to the following stages [2, 3]: the adsorption of oxygen from the gaseous phase and the diffusion of atomic oxygen at the electrode surface, reduction of oxygen atoms with formation of O2– ions, the diffusion of oxygen ions through the particles of the electrode with mixed conductivity, and the transport of the ions through the electrolyte/mixed conductor interface. Thus, it is preferable to use oxygen electrodes made of materials with high mixed (oxygen-ionic and electronic) conductivity, a high diffusion coefficient, and a high surface exchange constant. Another important factor that determines the longterm stability is also the consistency of the thermal expansion coefficients of the electrode and electrolyte
materials, along with the absence of chemical interaction between them. At room temperature, the Ln2NiO4 + δ compounds crystallize in the K2NiF4 structural type with a space group Fmmm or Bmab [1, 2]. The orthorhombic distortions of the unit cell increase as the rare-earth element radius decreases; in this case, the content of the superstoichimetric oxygen (δ) increases, compensating stresses in the structure [4]. The superstoichiometric oxygen ions in these compounds possess high mobility and, as theoretical calculations show, the migration of these ions determines the oxygen transport in the nickelate layered structures [5]. The maximum oxygen diffusion coefficients are attained in Pr2NiO4 + δ (for example, 8 × 10–8 cm2 s–1 at 600°C [6]). According to the available data, the cathodes based on Pr2NiO4 + δ exhibit the lowest polarization resistances among layered nickelates [7–9]. In addition, there is indirect evidence of the existence mixed H+/O2–/e– transport in Pr2NiO4 + δ [10], which opens the prospect of using it in SOFCs based not only on oxygen-ionic [11], but also proton-conducting electrolytes [12]. However, the problem of the stability of electrodes based on layered praseodymium nickelate remains unsolved. This is known that Pr2NiO4 + δ decomposes at temperatures higher than 850°C with the formation of Pr-containing impurities [13]. It
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