Functional properties of SOFC anode materials based on LaCrO₃, La(Ti,Mn)O₃ and Sr(Nb,Mn)O₃ perovskites: A comparative analysis

Abstract

The electrochemical, transport and thermomechanical properties of perovskite-type (La_1 − xSr_x)_1 − yMn_0.5Ti_0.5O_3 − δ (x = 0.15–0.75; y = 0–0.05), (La_0.75 − xSr_0.25 + x)_0.95Mn_0.5Cr_0.5 − xTi_xO_3 − δ (x = 0–0.5), (La_0.75Sr_0.25)_0.95Cr_1 − xFe_xO_3 − δ (x = 0.3–0.4), SrNb_1 − xMn_xO_3 − δ (x = 0.5–0.8) and (La_0.9Sr_0.1)_0.95Cr_0.85Mg_0.1Ni_0.05O_3 − δ have been appraised in light of their applicability for solid oxide fuel cell (SOFC) anodes. The electrical conductivity, measured in the oxygen partial pressure range of 10^− 20 to 0.5 atm at 940–1270 K, increases with manganese and strontium additions which lead, however, to higher reducibility. In addition to the thermodynamic stability limitations under the SOFC anodic conditions, the latter factor raises the importance of chemically induced expansion, as for Fe-substituted (La,Sr)CrO_3 − δ. The reduction of Ni-doped chromite results in the formation of nanosized metallic particles dispersed on the perovskite surface, and has no significant effect on the transport properties governed by the perovskite phase. The maximum electrochemical performance was observed for porous La_0.5Sr_0.5Mn_0.5Ti_0.5O_3 − δ, (La_0.9Sr_0.1)_0.95Cr_0.85Mg_0.1Ni_0.05O_3 − δ and (La_0.75Sr_0.25)_0.95Cr_0.7Fe_0.3O_3 − δ electrodes in the electrochemical cells with lanthanum gallate-based solid electrolyte and Ce_0.8Gd_0.2O_2 − δ interlayers.

Introduction

Utilization of hydrocarbon-containing fuels in solid oxide fuel cells (SOFCs) makes it necessary to develop novel high-performance anode materials with an improved durability. In general, these materials should satisfy severe requirements, including a high electrochemical and catalytic activity, chemical and microstructural stability under the SOFC operation and fabrication conditions, high electronic and ionic conductivity, compatibility with solid electrolytes, and low cost [1], [2], [3]. The conventional Ni-containing cermets exhibit important disadvantages, primarily degradation in hydrocarbon- or sulfur-containing atmospheres and sintering during operation [1], [2], [3], [4]. As substituting Ni with other metals does not allow to avoid these problems and/or leads to high costs [2], [3], [4], numerous studies were focused on the search of alternative oxide materials, in particular with ABO₃ perovskite structures (e.g. [2], [3], [5], [6], [7], [8], [9], [10], [11], [12], [13]). Quite promising results were reported for chromite- and titanate-based perovskite compositions, since their tolerance to A- and B-site substitution enables the optimization of anode functional properties [5], [6], [7], [8], [9], [10], [11], [12], [13]. Extensive doping is necessary for all candidate parent compounds known up to now. For example, chromites are thermodynamically stable under the SOFC anodic conditions and possess moderate thermal and chemical expansion, but suffer from the electronic conductivity drop in reducing atmospheres and a poor electrochemical activity [5], [6], [7], [8], [9], [12]. Moreover, the presence of chromium is associated with the formation of Cr^6 + under ambient conditions and volatilization of Cr-containing species [12], and should be minimized. An enhanced electrocatalytic activity and relatively high electronic conduction are characteristic of (La,Sr)(Cr,Mn)O_3 − δ and other Mn-containing perovskites where the B-site substituents may shift the onset of phase decomposition and/or conductivity drop to lower oxygen chemical potentials [6], [11], [14]. Another doping strategy relates to dissolving reducible components (e.g., Ni or Ru) in the perovskite structure and subsequent segregation of metallic particles at low p(O₂) [9], [12]; this improves catalytic behavior and transport, but may induce time degradation as for the conventional cermets. Perovskite-like titanates and their derivatives exhibit a high n-type electronic conductivity in reducing conditions; however, their functional properties are strongly susceptible to oxidation, whilst the redox kinetics is often too slow [10], [11], [13].

The present work is centered on the comparative assessment of several groups of the perovskite-based anode materials, including (La_1 − xSr_x)_1 − yMn_0.5Ti_0.5O_3 − δ (x = 0.15–0.75; y = 0–0.05), (La_0.75 − xSr_0.25 + x)_0.95Mn_0.5Cr_0.5 − xTi_xO_3 − δ (x = 0–0.5), (La_0.75Sr_0.25)_0.95Cr_1 − xFe_xO_3 − δ (x = 0.3–0.4), SrNb_1 − xMn_xO_3 − δ (x = 0.5–0.8) and (La_0.9Sr_0.1)_0.95Cr_0.85 Mg_0.1Ni_0.05O_3 − δ. The materials of the three first systems were characterized elsewhere [7], [15], [16], [17]; in this work their functional properties are compared to one another in order to select promising directions of the oxide electrode developments. In the case of (La_0.9Sr_0.1)_0.95Cr_0.85 Mg_0.1Ni_0.05O_3 − δ, the A-site cation deficiency and reduction of Ni under anodic conditions are expected to promote the formation of highly dispersed metallic particles stabilized on the perovskite oxide surface. For other perovskites the target state is single-phase, although the significant concentrations of manganese or iron may lead to partial decomposition in reducing atmospheres. In addition to the phase stability and anode performance, attention was focused on thermomechanical behavior and electronic transport as functions of the oxygen partial pressure and cation composition.