Surface-Modified Approach to Fabricate Nafion Membranes Covalently Bonded with Polyhedral Oligosilsesquioxane for Vanadium Redox Flow Batteries.

An aminopropyl isobutyl polyhedral oligosilsesquioxane (NH 2 -POSS) surface-modified Nafion membrane has been designed by chemical grafting for vanadium redox flow batteries (VRFBs). NH 2 -POSS is a cage-like macromer consisting of an inorganic Si 8 O 12 core surrounded by seven inert isobutyl groups and one active aminopropyl group. The sulfonic acid groups on the surface of Nafion can be activated by 1,1-carbonyldiimidazole for further modification with NH 2 -POSS. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) prove that NH 2 -POSS has been successfully grafted on the surface of a Nafion 115 membrane. Although the proton conductivity decreases slightly, the organic-inorganic hybrid membranes display enhanced ion selectivity and excellent dimensional stability with lower water uptake and swelling ratio than Nafion 115. Moreover, two-dimensional-grazing incidence X-ray diffraction (2D-GIXRD) reveals that the introduction of NH 2 -POSS forms a POSS layer on the surface of the membrane and narrows the space of Nafion clusters, which helps to block VO 2+ permeation. A VRFB with the surface-modified Nafion membrane displays an outstanding performance with an average Coulombic efficiency (CE) of 98.7% and energy efficiency (EE) of 84.5% at a current density of 80 mA cm -2 , superior to those of the Nafion 115 membrane (CE = 95.7%, EE = 81.7%). Furthermore, the cell holds a high capacity retention of 49.2% after 1000 charge-discharge cycles, in contrast to that of 41.9% for the cell with Nafion 115 after only 200 cycles. The results suggest that the surface-modified hybrid membrane is a promising strategy to overcome the vanadium ion crossover in VRFBs.