All-Ceramic Asymmetric Membranes for Hydrogen Separation

Abstract

High-temperature hydrogen separation with the all-ceramic asymmetric membrane is a suitable technology for the intensification of important industrial processes, such as H2 separation/purification and H2-related reactions. However, many technological challenges had to be addressed to obtain highly performing membranes with asymmetric architecture. This activity faced all the challenges related to the manufacturing of a thin and dense membrane supported by a porous and tough layer. In addition, the enhanced properties were evaluated through H2 permeation measurements under different conditions. The first part of the thesis was devoted to the individuation of the ceramic-ceramic composite (cer-cer) comprising a proton conducting and an electron conducting phases forming a dual phase inter-percolating network with enhanced H2 permeation properties. Y-doped BaCexZr1-xO3-δ (BCZY) was selected as the proton conducting phase. Then, a screening activity indicated ceria-based - CeO2 and Ce0.8Gd0.2O1.9 (GDC) - as the most promising systems thanks to their chemical and thermomechanical compatibility with BCZY. Afterwards, the attention was devoted to the production of a dense membrane of this composite supported by a porous layer of the same material. This asymmetric architecture was obtained through tape casting and lamination of the support and membrane layers. Dynamic Mechanical Analysis (DMA) showed the possibility to mimic the lamination process, allowing the identification of the best conditions for lamination. The correlation between the temperature and the viscosity of the multilayers during the lamination represents an important breakthrough for the multilayers processing. The sintering atmosphere was found to play a key role in obtaining a thin and dense membrane layer with the required phase composition. In fact, sacrificial Ba-containing powders were necessary to counterbalance the Ba loss from the BCZY phase which occurs at the high temperatures required for the membrane densification. An excessive Ba content, however, promoted the formation of a single-phase material due to Ba dissolution into the BCZY and GDC lattices. Only using the BCZY-GDC as Ba-source, the desired phase composition was achieved, and the suitable microstructure was obtained. The process optimization allowed the production of a sufficiently dense 20 μm-thick cer-cer membrane layer supported by a porous (36 vol. %) 750 μm-thick layer. The obtained asymmetric membranes were planar and crack-free, therefore suitable for H2 separation applications. H2 permeation tests revealed the asymmetric architecture displays remarkable H2 permeation: 0.26 mL min-1 cm-2 at 750°C with both membrane sides humidified feeding 50 vol.% H2 in He. The Pt-catalytic activated cer-cer asymmetric membrane showed the highest hydrogen flux reported in literature for an all-ceramic membrane: 0.47 mL min-1 cm-2 at 750°C with both membrane sides humidified feeding 50 vol.% of H2 in He. The experiments showed also that to obtain high hydrogen fluxes, the phase purity of the membrane is mandatory. In conclusion, this activity showed how the ceramic process optimization can be useful in obtaining engineered architectures with suitable microstructure and phase composition responsible for enhanced performances. The obtained asymmetric BCZY-GDC membrane showed the highest permeation flux known to date for this technology.