Physicochemical compatibility of SrCeO₃ with potential SOFC cathodes

Abstract

The chemical and physical compatibility of SrCeO₃ is investigated with respect to LaMO₃ (M=Mn, Fe, Co) and La_2−xSr_xNiO₄ (x=0, 0.8), via the reaction of fine-grained powder compacts and solid-state diffusion couples. Compositions were chosen so as to give predictive insight into possible candidate materials for all-oxide electrochemical devices. Results show the primary reaction in these systems to be the dissolution of SrO from SrCeO₃ into the LaMO₃/La_2−xSr_xNiO₄, and corresponding formation of La-doped CeO₂. Reaction kinetics are observed to be relatively fast, with element profiles suggesting the diffusion of Sr²⁺ in ceria to be surprisingly rapid. It is demonstrated that perovskite starting materials represent poor candidates for use with SrCeO₃, reacting completely to form Ruddlesden-Popper/K₂NiF₄ type oxides. Reaction with La₂NiO₄ is less pronounced, and formation of secondary phases suppressed for the composition La_1.2Sr_0.8NiO₄. It is thus concluded that Ruddlesden-Popper type oxides represent good candidate materials for use with a SrCeO₃-based electrolytes when doped with appropriate levels of Sr.

Introduction

Proton conducting oxides have attracted considerable recent attention owing to their potential for use in a wide range of electrochemical devices, particularly the technologies which underpin the hydrogen economy; hydrogen separators, steam electrolysers and fuel cells [1], [2], [3], [4], [5], [6]. For these applications protonic systems are particularly attractive as they avoid the problems associated with the fuel–product mixing that occurs in oxide ion conducting systems.

Current state of the art in ceramic proton conductors are the barium and strontium cerate families [7], [8]. These exhibit a distorted perovskite structure [9], and high levels of proton conductivity are achievable via the replacement of Ce⁴⁺ with an acceptor dopant such as Y³⁺, Nd³⁺ Yb³⁺ or Gd³⁺ [10], [11], [12], [13], [14]. The commercial application of these materials has been hindered however, by the instability of both Sr and Ba families with respect to their alkaline earth carbonates [15], [16], [17], and by the technical difficulty of creating fully dense ceramics with good mechanical properties [18]. A further challenge exists in the pairing of these materials with appropriate electrodes. Typically a porous electrode of a noble metal has been used [5], [10]. However, these are expensive and so there is some interest in the development of less costly solutions. Ideally a less expensive oxide ceramic would be used which exhibits combined fast protonic–electronic conductivity, but to date there are no oxides which fulfil these requirements. Possible alternatives exist in the use of a Ni-electrolyte cermet [19] under reducing conditions (i.e. fuel cell anode) and a mixed oxide ion/electronic conductor for oxidising conditions (i.e. fuel cell cathode) [10], [13], [20]. The use of a mixed electronic/oxide ion conductor greatly broadens the choice of potential electrodes, with the additional benefit that a good understanding of the synthesis, processing and chemistry already exists for many of the candidate materials.

With this in mind, we have investigated the chemical compatibility of the fast protonic conductor SrCeO₃ with a number of well-known mixed oxide-ion/electronic conducting oxides. The systems chosen (LaCoO₃, LaMnO₃, LaFeO₃ and La_2−xSr_xNiO₄) are representative of current materials and provide a solid basis for prediction of other likely candidates.