Hydrophobic porous membranes exhibit excellent properties including high moisture permeability, high hydrophobicity and low thermal conductivity that are highly desirable for distillation-based seawater desalination application. To date, various modification approaches have been proposed to enhance their performance. Among them, the introduction of carbon-based fillers, such as graphene, in the membrane matrix have been identified as a promising solution for improving the performance of the composite membranes. However, the full performance of TiO2-graphene oxide porous membranes is yet to be achieved because further improvements require a thorough understanding of the heat and mass transfer mechanism inside the membranes, which is not fully explored in the literature.
To this note, a team of researchers at the South China University of Technology: Dr. Si Zeng, Dr. Qianwen Su and led by Professor Li-Zhi Zhang investigated the micro-scale heat and mass transfer mechanism of graphene oxide-polyvinyl pyrrolidone/poly vinylidene fluoride (GO-PVP/PVDF) membranes via molecular-level manipulation technique based on molecular dynamics simulations. The main objective was to improve the performance of GO-PVP/PVDF composite membranes. Their work is currently published in International Journal of Heat and Mass Transfer. Previous methods are confined to black box or macro-scale modeling. This study is the first time that heat and mass transfer in a composite membrane are manipulated by a molecular level methodology. In this way, material transport performance is improved by adjustment of molecule types and compositions.
In their approach, the properties of the GO-PVP/PVDF membrane, i.e. thermal conductivity, hydrophobicity and moisture diffusivity were optimized through atomic modifications inside the material, taking into consideration the bulk and interfacial resistance as well as the molecular and pore structures. Moreover, the molecular dynamics simulations coupled with the macro-scale resistance-in-series/parallel model was used to evaluate the overall bulk performance, moisture diffusivity and heat conductivity. Lastly, they discussed the effects of the functional groups and two-layer thickness on the heat and mass transport performance of GO-PVP/PVDF membranes.
The authors observed that it was difficult for moisture to cross the six-membered ring of graphene oxide directly. This problem was solved by fabricating a novel GO membrane that allows water to penetrate through the graphene oxide pores directly. Unlike commercial poly vinylidene fluoride (PVDF) membrane, the fabricated GO-PVP/PVDF membrane exhibited excellent moisture diffusivity, which was increased by 38%, and a high contact angle. This was attributed to the addition of PVP, which played a significant role in ensuring uniform dispersion of GO in the skin layer as well as making the skin porous and permeable. The chemical bond connecting the GOs and PVDF functioned to prevent any cracking between the skin and substrate layers, thus enhancing the high reliability of the composite membranes.
The transport properties of the prepared membrane could be optimized by adjusting the three main parameters; namely, membrane thickness, GO content and the grafted functional groups. For instance, the graphene oxidation was observed to be directly proportional to the thermal conductivity of the GO-PVP membranes, and the highest diffusivity was achieved at 50% GO content. Furthermore, the overall membrane resistance was largely influenced by the interfaces and the contact faces, especially for thin membranes. For example, at several micro levels for the membrane thickness, the contact faces accounted for 38.5% and 52.4% of the overall membrane resistance for mass and heat transfer, respectively.
In summary, this is also the first study to investigate the molecular heat and mass transport mechanisms through graphene stacks. In a statement to the Advances in Engineering, Professor Li-Zhi Zhang said their study would enable future development of high-performance composite membranes for different applications, by this multi-scale methodology.
Zeng, S., Su, Q., & Zhang, L. (2020). Molecular-level evaluation and manipulation of thermal conductivity, moisture diffusivity and hydrophobicity of a GO-PVP/PVDF composite membrane. International Journal of Heat and Mass Transfer, 152, 119508.