Multilayer membranes of graphene oxide nanosheets on cellulose filter papers for high-flux water filtration and high proton conductivity.

Two-dimensional graphene oxide (GO) nanosheets have emerged as multifunctional units for synthesizing nanostructured membranes for water filtration and proton transport technologies. In this research, advanced GO synthesis were conducted to produce GO dispersions at appropriate concentrations for building primary and secondary layers on cellulose filter papers (Cel). The primary GO layer on the largely porous Cel was formed by vacuum filtration of a GO dispersion 240 ppm. Subsequently, a secondary GO layer was built using a GO dispersion 60 ppm, covering microscale cracks on the primary layer. The multilayer GO membrane on the Cel substrate was compressed and nanostructured for high water permeance (147-240 L m-2 h-1 bar-1), efficient removal of organic dyes (96.7-99.9%), and good performance stability after 15 days in water. Desalination experiments using a Cel/GO/Cel sandwich membrane showed a decrease in NaCl concentration from 2.8% to 0.9% after 12 filtration cycles. Electrochemical impedance spectroscopy measurements were performed to calculate membrane resistances and proton conductivities at 100% RH and various temperatures, leading to the estimation of activation energies. The proton conductivity increased from 1.08-4.82 mS cm-1 for Cel/Cel membranes to 17.59-44.83 mS cm-1 for Cel/GO/Cel membranes at 25-45 °C. Nanochannels in the GO membranes had a d-spacing of 8.5 Å, a water layer, and hydrogen-bond networks for fast proton transport, leading to lower activation energy barrier and higher proton conductivity. The novel multilayer Cel/GO/Cel membrane provides the advantages of low cost, biodegradable substrate, and high performance for water purification and proton conduction.