Highly selective ion transport by freestanding Zn-Imidazole complex intercalated graphene oxide membrane for enhanced blue energy harvesting

Generating electricity from seawater by utilizing nanofluidic structures is an appealing strategy for achieving zero-emission energy goals. A nanofluidic channel that is atomically small, tunable, and mechanically stable is the key to solving this problem. In this paper, for the first time, we report the construction of centimeter-sized freestanding zinc-imidazole complex (ZnIm) intercalated graphene oxide (GO) membrane (GO-ZnIm) having uniform 2D nanofluidic channels with interlayer spacing of 9\AA. The pristine GO membrane rapidly disintegrates in an aqueous medium due to membrane swelling; however, treatment with an aqueous zeolitic imidazole framework (ZIF-8) suspension, rendering ZnIm complex, provides long-term stability by restricting the excessive interlayer separation of GO sheets. From ion transport, diffusion, and variable load studies, it is observed that, besides stabilizing the GO membrane, intercalation of ZnIm significantly improves average cation selectivity (from 0.46 for GO to 0.74 for GO-ZnIm, measured up to 1000 concentration gradient) and experimental power density (from 1.7 Wm−2 for GO to 10.8 Wm−2 for GO-ZnIm, at 1000 concentration gradient). Moreover, the practical applicability of the GO-ZnIm membrane has also been investigated under artificial estuary conditions at ambient temperature, rendering an osmotic power density of 4.5 Wm−2, which is very close to the commercial benchmark of 5 Wm−2. A mechanically robust and flexible membrane made up of cost-effective components (GO and ZnIm), able to deliver an osmotic power density close to commercial benchmark at ambient temperature is the main highlight of the present GO-ZnIm membrane, which can be a promising candidate for applications such as separation, desalination, and blue energy harvesting.