Hierarchical metal-organic framework-functionalized PVDF tree-like nanofiber membrane for high-efficient photothermal membrane distillation

The development of membrane with strong sunlight absorption and high energy efficiency is the key to improve the water production efficiency in photothermal membrane distillation (PMD). In this paper, a hierarchical Cu-based metal–organic framework-functionalized polyvinylidene fluoride (PVDF) tree-like nanofiber membrane (Cu-CAT@TLNM-TLNMs) was developed for high-efficient PMD. The Cu-CAT@TLNM-TLNM was prepared by in-situ growth of Cu-CAT on the surface of PVDF tree-like nanofiber membrane (TLNM) as photothermal layer and PVDF TLNM as support layer, which exhibited excellent photothermal conversion performance, high energy utilization and high permeate flux. The PVDF TLNM with high specific surface area provided more active sites for the growth of Cu-CAT, thus forming a uniform Cu-CAT nanoarray. The spinous structure of the Cu-CAT photothermal layer can increase the length of the light-matter interaction and improve light absorption. In addition, the tree-like structure of the support layer with high porosity and small pore size can improve the permeate flux and long-term stability. Under the illumination of 1 kW·m−2, the surface temperature of the Cu-CAT@TLNM-TLNMs reached about 80 °C within 50 s, with a permeate flux of 1.37 kg·m−2·h−1, a salt rejection of >99.99 %, and a photothermal conversion efficiency of 76 %. More importantly, the permeate flux of the Cu-CAT@TLNM-TLNMs was still stable at 0.79 kg·m−2·h−1 with a salt rejection of >99.99 % after 84 h of continuous operation. This photothermal nanofiber composite membrane can be applied to solve the problem of freshwater shortage in areas where freshwater and fuel resources are scarce and solar energy is abundant.