Work Significance
Hydrogen (H2) separation ceramic-based membranes must exhibit both ionic and electronic conductivity in order to achieve H2 permeation flux suitable for practical applications. Also, they must be dense in order to achieve infinite H2 selectivity. At high temperatures (>500oC), ceramic oxide materials with perovskite structures (A2+B4+O3) have the required ionic-electronic conductivity when the B-sites are doped with trivalent cations (M). This doping process creates oxygen-ion vacancies (Vo..) within the lattice as means of charge
compensation. The general formula of a doped perovskite material is given as ABxM1-xO3-δ, where x and δ represent cations molar ratio and oxygen-ion vacancy respectively. The mixed conductivity of doped perovskite ceramic oxides have been receiving considerable attention in the development of solid oxide fuel cells (SOFCs), gas sensors, and H2 permeation membranes. The doped perovskite structure for the H2 separation membranes have several characteristics including, infinite H2 selectivity, low H2 chemical potential gradient, lower operating temperature range of 500-1000oC, inexpensive compared to Pd membranes, simple and flexible compared to pressure swing adsorption process, and does not require external power.A typical H2 permeation process, through the doped perovskite ceramic oxides, involves three consecutive steps: (i) a gas-solid interfacial reaction for the adsorption of H2 at the membrane surface, where it dissociates into protons and electrons, (ii) a pressure gradient across the membrane transports the protons through the solid-state membrane-lattice, where the protons
associate with oxygen sites (OHo.) and hops between adjacent sites, and (iii) a solid-gas interfacial reaction for the protons to re-combine and form H2. The current membrane geometries (i.e. disks and tubes) are considerably thick (~2 mm) and have low H2 permeation rates. In order to enhance the H2 permeation rate, it is essential to control the membrane thickness. Since the permeation rate is inversely proportional to the membrane thickness, dense and thin (sub-micron thick) membrane films supported on the porous substrates should lead to much higher H2 permeation rates. The H2 preparation of dense and thin membrane films have been reported earlier using polymeric precursors and utilizing both the spin and the sol-gel dip-coating techniques on silicone and sapphire substrates. Work is still needed to enhance and produce high quality thin membrane films that are smooth, homogeneous, and free of pin holes and cracks. Hence, from this perspective, the goal of the present study is aimed to prepare and characterize strontium cerate (SrCe0.95Tb0.05O3-δ) (SCT) thin membrane films by spin-coating using a polymeric precursor containing ceramic cations. Continuous homogenous SrCe0.95Tb0.05O3-δ membrane films having thickness within the range of 200 nm–2 µm, with neither pin-holes nor cracks, are reported here. The polymeric precursor and the micro structure of the SrCe0.95Tb0.05O3-δ membranes are characterized using several techniques including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), and focused ion-beam (FIB) microscopy. |
