Oxyfuelled Carbon Capture and Storage (CCS) is one means of achieving the necessary reductions in the emission of carbon dioxide from fossil fuel-fired power plants. In this approach, a hydrocarbon fuel is burned in pure oxygen, yielding only carbon dioxide and steam as combustion products. Since the steam may be easily condensed out of the exhaust stream, the CO₂ can be easily separated for subsequent sequestration in, for example, geological storage sites.

Clearly, one of the import requirements for implementing this is providing an economical and reliable source of pure oxygen. Mixed electronic – oxygen ionic conducting ceramic membranes can fulfil both of these criteria. However, single phase ceramics do not offer the required chemical and mechanical stability for this application, and so it is thought that composite materials could provide the necessary performance.

The oxygen exchange properties of the pure ionic conductor-mixed electronic conductor Ce_0.9Gd_0.1O_1.95-La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ system were investigated by the Isotope Exchange Diffusion Profiling method. The “macroscopic” (integrating over many grains of each phase) properties were measured with a Duoplasmatron source-based Atomika 6500 Secondary Ion Mass Spectrometer (SIMS), and compared with “microscopic” measurements from within single grains of each phase, obtained using a Gallium liquid metal ion source Focussed Ion Beam (FIB)-SIMS instrument.

Although the variation in the diffusion (D) and surface exchange (k) coefficients measured on the macro-scale appears relatively straightforward, the microscopic oxygen exchange behaviour is more interesting. The surface ¹⁸O isotopic fraction measured on grains of the CGO phase is higher than expected from literature values, suggesting the presence of LSCF enhances oxygen exchange in these materials. This will be discussed in terms of the interdiffusion of cations between the two phases on sintering, which has been indicated by both XRD and SEM-EDX.

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