Molecular Separation and Power Generation by Carbon Nanomembranes
Creators: name: Qi, Yubo
Publisher: name: Universität Bielefeld
Year: 2022
Subjects: subject: 530

description: Mass transport in two-dimensional (2D) membranes has been at the forefront of materials research for nearly a decade. It is believed that 2D membranes can achieve high permeation and selectivity due to their nanometer-scale thickness and narrow pore size distribution. Herein, we focused on the construction of confined nano-structures and mass transport through these nano-porous membranes. In this thesis, the mass transport theory and the manufacturing mechanism are investigated. First, a regular synthesis of carbon nanomembranes (CNMs) from thiol molecules and a facile fabrication of CNMs from polycyclic aromatic hydrocarbons (PAHs) are illustrated. The thickness of pyrene CNMs can be controlled between 3.0 to 6.5 nm by varying the residence time in the reaction chamber. The electron-induced carbonization is shown to result in continuous membranes on arbitrary substrates, and the material is characterized with a number of spectroscopic and microscopic techniques. <br> The permeation measurements of free-standing membranes with water and noble gases are studied by two methods, i.e., mass loss and vacuum mass spectrometry. The result revealed that the CNMs were intrinsically microporous. While the permeance of helium remains almost the same for 6.5 and 3.0 nm thick CNMs, the water permeance increases by two orders of magnitude. The permeance of saturated water vapor decreased from more than 10–4 mol·m–2·s–1·Pa–1 at 3.0 nm to 10–6·mol·m–2·s–1·Pa–1 at 6.5 nm. By rationalizing the membrane performance with the help of kinetic modelling and vapor adsorption experiments, we recognize the thickness dependence as a breakdown of the collective water motion.<br> Additionally, electro–dialysis experiments with intrinsically porous CNMs are also performed. The selective passage of metal ions is studied in pyrene CNMs. CNMs can screen the hydrated cations in the order of K+ &gt; Na+ &gt; Li+ &gt; Ca2+ &gt; Mg2+, while the K+/Mg2+, Na+/Mg2+, Li+/Mg2+ selectivity amounts to 220, 86, and 13 respectively in 3 nm thick pyrene CNMs. The osmotic energy conversion of CNMs at a 10-fold gradient of KCl concentration was studied, revealing a 0.5 W·m–2 output power density. In addition, a new type of 2D material-bilayer silica dioxide with relative higher ion conductance was investigated for blue energy generation. Under artificial seawater and river water conditions, 5.4 W·m–2 power density was achieved. This finding indicates that two essential elements needed for energy conversion are high ion conductance and excellent ion selectivity.<br> Although intrinsically porous planar nanomaterials are promising for separation applications, their large-scale application is still a challenge. To address the problem of cracks caused by transfer, various reinforcement layers were fabricated to protect the CNMs during the transfer process. Finally, a novel composite membrane was fabricated using CNMs as a selective layer and porous block co-polymer as a protective layer. The composite membrane showed good separation performance for dye solutions. This opens up a new avenue for large-scale production of CNMs for water treatment.<br> CNMs made from different precursors enrich the processes and open the avenue for scaling-up and wide application. In addition, the mechanical strength and high water permeability of CNMs prepared from pyrene promise advances in their scalability and can potentially benefit applications in areas such as ion exchange, electro-dialysis, and energy harvesting membranes.