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 x 10(-8) cm(2) min(-1), only 2.64% of the pristine Nafion membrane), and higher coulombic efficiency and energy efficiency (92.9-98.8% and 81.588.4%, respectively) comparing with the pristine Nafion membrane (73.3-90.5% and 68.979.1%, 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. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.