Silica-polysiloxane nanocomposite membrane via 2-step atmospheric-pressure PECVD for precise molecular-sieving H2 separation

Silica-based membranes exhibit excellent molecular sieving properties and robustness to achieve efficient molecular separation. In recent years, polymer-supported silica-based membranes have been proposed as a cost-effective alternative to ceramic-supported membranes due to their lower cost and ease of scalability. However, fabricating highly permselective silica layers at low temperatures within the limit of the thermal stability of polymeric support remains a significant challenge. In this study, we developed a two-step atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) process for fabricating polymer-supported silica-based membranes with enhanced molecular sieving properties. Our approach involves sequentially depositing a polysiloxane-like layer followed by forming a silica-like layer, allowing precise control over the siloxane network pore sizes. Neither the silica-like layer nor the polysiloxane-like layer alone can achieve high selectivity as the former faces challenges in achieving uniform coverage, while the latter offers better coverage but has a loose siloxane network structure that lacks sufficient molecular sieving properties. On the other hand, by stacking these layers, we achieved a significant improvement in hydrogen selectivity, with H₂/N₂ and H₂/CH₄ permeance ratios of 53.1 and 54.4, respectively, with H₂ permeance of 6.23 × 10^-8 mol/(m² s Pa). Characterization through FTIR and SEM analysis revealed that the first layer not only effectively covered the porous polymeric support but also underwent oxidation, facilitating uniform deposition of the second silica layer to improve selectivity. This scalable process, conducted under ambient conditions, atmospheric pressure and low temperature, offers a cost-effective and versatile platform for fabricating highly selective polymer-supported silica membranes for molecular separation.