Orientated graphene oxide/Nafion ultra-thin layer coated composite membranes for vanadium redox flow battery

Graphene oxide (GO)/Nafion composite membranes, with orientated GO nanosheets in parallel to the surface of the ultra-thin coating layer (400-440 nm), are prepared by spin coating method and evidenced by electron microscopy analysis. Orientation of GO maximizes the vanadium ions barrier effect of GO. The GO/Nafion membrane (M-2) achieves lower vanadium ion permeability (8.2 × 10−8 cm2 min−1, only 2.64% of the pristine Nafion membrane), and higher coulombic efficiency and energy efficiency (92.9-98.8% and 81.5-88.4%, respectively) comparing with the pristine Nafion membrane (73.3-90.5% and 68.9-79.1%, respectively) at current densities of 20-100 mA cm−2. With the design of orientated GO nanosheets and ultra-thin GO/Nafion coating layer, good balance between vanadium crossover suppression and protons conduction retention is achieved. M-2 exhibits excellent battery performances over 200 charge-discharge cycles. The capacity decay rate is about 0.23% per cycle, much lower than those assembled with Nafion 212 (0.40% per cycle) and the recast Nafion membrane (0.44% per cycle). Spin coating with water suspensor leads to uniform dispersion of GO and good binding between GO/Nafion coating layer and substrate Nafion membrane. Therefore, the composite membrane could be reinforced by GO and keep integration even with 200 cycles operation.