(Invited) The Role of Solid-Gas Electrochemical Interfaces for Mixed Ionic Electronic Conducting Oxygen Transport Membranes

Oxygen transport membranes (OTMs) are attracting great interest for the separation of oxygen from air in an energy efficient way. In the last decade one major driver was the development of CO₂ mitigation scenarios utilizing carbon capture and storage (CCS) technology for large point sources. One very promising concept is the oxyfuel technology, which realizes the combustion of fossil fuels with oxygen-enriched recirculated flue gas thus requiring enormous quantities of oxygen. OTMs can deliver this oxygen efficiently if the required heat is delivered by the process itself as it is in the case of power, cement, glass, or steel plants. Another possible application is the operation of a membrane reactor, in which oxygen is directly consumed by a chemical reaction. In this context, the research focuses on the partial oxidation of methane or even the oxidative coupling of methane to higher hydrocarbons such as ethylene, propylene, aromatics etc.

Such OTMs consist of mixed ionic electronic conductors (MIEC), which can consist of a single phase MIEC material or a composite of two separate phases, each phase providing electronic or ionic conductivity, respectively. In general, the bulk transport is based on defects in the crystal lattice, i.e. oxygen vacancies and electron holes. This leads to a trade-off between permeability requiring a high defect concentration and stability, requiring a low defect concentration. Therefore, materials with limited permeability have to be used in many applications to provide a long term stable operation. In order to still reach sufficiently high permeation rates, thin supported membrane layers are developed in different shapes, mainly planar or as capillaries. In case that the membrane thickness is below a characteristic thickness, surface exchange kinetics become rate limiting because of the relatively fast diffusion through the thin bulk membrane. Consequently, the solid-gas electrochemical interfaces become more and more important, making catalytically active layers, such as porous electrodes, a key element in the future development of long term stable, high performance membranes.

In this presentation the development of thin (8µm – 400µm) supported membrane layers is described using high performance perovskites, i.e. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and La_0.6-xSr_0.4Co_0.2Fe_0.8O_3-δ (LSCF). Oxygen permeation measurements with varying conditions, i.e. temperature as well as feed gas composition and sweep gas flow rates, are used to identify limiting transport processes. The overall oxygen transport process of BSCF membranes is analyzed by combining different modelling approaches. While the gas diffusion in the gas phase and the porous support is modelled by cfd and binary friction model respectively, the transport through the bulk and the surface exchange is addressed in combination using a modified Wagner approach, accounting for the electrochemically active surface areas. According to this model, either gas diffusion in the porous support or the oxygen surface exchange is limiting the transport depending on experimental conditions.

Surface exchange limitations can be partly overcome using porous electrode layers made of the membrane material. Due to the high electrochemical activity of the perovskites used, an additional permeation enhancement using noble metal catalysts cannot easily be realized at high temperatures. In addition, dual phase membranes were investigated, which are much more prone to surface exchange limitations because of the limited length of the active triple phase boundaries. Electrodes made of a single phase MIEC material, i.e. LSCF, show evidence of these limitations even using 1 mm thick samples.