Polyoxometalate Clusters Confined in Reduced Graphene Oxide Membranes for Effective Ion Sieving and Desalination

Efficient 2D membranes play a critical role in water purification and desalination. However, most 2D membranes, such as graphene oxide (GO) membranes, tend to swell or disintegrate in liquid, making precise ionic sieving a tough challenge. Herein, the fabrication of the polyoxometalate clusters (PW₁₂) intercalated reduced graphene oxide (rGO) membrane (rGO-PW₁₂) is reported through a polyoxometalate-assisted in situ photoreduction strategy. The intercalated PW₁₂ result in the interlayer spacing in the sub-nanometer scale and induce a nanoconfinement effect to repel the ions in various salt solutions. The permeation rate of rGO-PW₁₂ membranes are about two orders of magnitude lower than those through the GO membrane. The confinement of nanochannels also generate the excellent non-swelling stability of rGO-PW₁₂ membranes in aqueous solutions up to 400 h. Moreover, when applied in forward osmosis, the rGO-PW₁₂ membranes with a thickness of 90 nm not only exhibit a high-water permeance of up to 0.11790 L m^-2 h^-1 bar^-1 and high NaCl rejection (98.3%), but also reveal an ultrahigh water/salt selectivity of 4740. Such significantly improved ion-exclusion ability and high-water flux benefit from the multi-interactions and nanoconfinement effect between PW₁₂ and rGO nanosheets, which afford a well-interlinked lamellar structure via hydrogen bonding and van der Waals interactions.