Fast Oxygen Separation Through SO 2 - and CO 2 ‑Stable Dual-Phase Membrane Based on NiFe 2 O 4 -Ce 0.8 Tb 0.2 O 2‑δ María Balaguer, Julio García-Fayos, Cecilia Solís, and Jose ́ M. Serra* Instituto de Tecnología Química (Universidad Polite ́ cnica de Valencia - Consejo Superior de Investigaciones Científicas), Av. Naranjos s/n, E-46022 Valencia (SPAIN) * S Supporting Information ABSTRACT: Composite membranes with enhanced oxygen permeability and unprecedented stability in oxyf uel-like gas environments are reported. Specifically, 60 vol% NiFe 2 O 4 - 40 vol% Ce 0.8 Tb 0.2 O 2‑δ (NFO-CTO) composite has been successfully obtained by one-pot fabrication method showing both spinel and fluorite pure phases. Narrow grain size distribution centered around 1 μm and homogeneous distribution of grains is attained, as well as percolative pathways from side to side of the dual-phase membranes. The composite resisted a stability test in wet SO 2 and CO 2 containing gas at 800 °C for 170 h, which represents a step forward toward its use in oxyf uel power plants. The conductivity of both phases is investigated as a function of temperature and oxygen partial pressure (pO 2 ). Oxygen separation in this kind of NFO-doped-ceria composite membranes occurs via the separate ambipolar transport through the two distinct percolating networks. Oxygen permeation flux values of 0.17 mL·min -1 ·cm -2 and 0.20 mL·min -1 ·cm -2 are achieved at 1000 °C when argon and pure CO 2 are used as sweep gas, respectively, through a 0.68 mm- thick membrane. Experiments at 900 °C showed that the material is stable and effective in pure CO 2 atmospheres and the oxygen permeation is even improved after 76 h on CO 2 stream. KEYWORDS: composite membrane, oxygen transport membrane, doped ceria, spinel composite, sulfur stable, terbium oxide ■ INTRODUCTION Oxyfuel technology in different energy-demanding processes enables to reach important energy savings and facilitates the integration of CO 2 capture strategies that minimize greenhouse emissions. Oxyfuel technology consists of fuel combustion by using O 2 (instead of air) in a CO 2 sweeping stream. The absence of N 2 allows (i) more efficient combustion processes, (ii) minimizing NO x formation, and (iii) the direct CO 2 sequestration process. 1 However, oxyfuel overall efficiency is penalized by the high energetic and economic costs associated with oxygen production using state-of-the-art cryogenic distillation units. The current alternatives for cryogenic air separation are modules based on oxygen transport membranes (OTMs), which may be thermally integrated in the furnace. 2 OTMs are made of mixed ionic electronic conductor (MIEC) materials that allow 100% oxygen selectivity. 3 Up to date, materials with perovskite structure are the MIEC materials showing the highest electrical conductivities but permeabilities are often limited by the ionic conductivity, thus jeopardizing the ambipolar conductivity. 3 Doping strategies to increase the oxygen ion conductivity normally affect the crystal and thermo- mechanical stability. This increases the vulnerability of the membrane when exposed to large oxygen concentration gradients and atmospheres containing CO 2 , SO 2 , and H 2 O, that is, the operation conditions found in the oxyfuel process and catalytic membrane reactors in different intensified industrial processes. 4-6 Improved stability against oxygen partial pressure gradients and carbonation is expected when avoiding the presence of alkaline-earth elements in the oxide lattice and doping by only transition metals and lanthanides instead. The difficulty of joining all the desired characteristics in a single material pointed out to achieve them separately. Dual phase composite membranes try to combine the best characteristics of different compounds to achieve large oxygen permeability and relatively good chemical and mechanical stability at elevated temperatures. Thus, composite materials should consist of an electron conducting material, which allows the percolation of electrons and an ionic conductor that transports the oxygen ions through the membrane. Due to the existence of two phases, the influence of the grain boundary between them plays an important role, since it can either promote or block the transport of ionic and electronic species across the membrane. Furthermore, large catalytic activity toward surface oxygen exchange is required. The first reported dual phase membranes were ceramic-metal (cermet) composites consisting of a continuous oxygen ion- conducting oxide phase and a continuous electron-conducting Received: October 23, 2013 Revised: November 25, 2013 Published: November 26, 2013 Article pubs.acs.org/cm © 2013 American Chemical Society 4986 dx.doi.org/10.1021/cm4034963 | Chem. Mater. 2013, 25, 4986-4993