Ultrahigh Ion-Selective and Durable Nafion-NdZr Composite Layer Membranes for All-Vanadium Redox Flow Batteries

A thin Nafion-neodymium zirconium oxide nanotube (NdZr) composite (Nafion-NdZr) membrane has been fabricated and further modified by the polycation, poly(diallyldimethylammonium chloride) (PDDA), and polyanion, poly(sodium styrene sulfonate) (PSS). The ion selectivity of the Nafion-NdZr (1%)/[P-S]₂ composite layer membrane was found to be 6.9, 3.5, and 2.3 times higher than those of recast Nafion, Nafion/[P-S]₂ layer, and Nafion-NdZr (1%) composite membranes, respectively. As a result, the vanadium redox flow batteries (VFBs) assembled with Nafion-NdZr (1%) composite and Nafion-NdZr (1%)/[P-S]₂ composite layer membranes have surpassed the VFB performance operated with recast Nafion and Nafion/[P-S]₂ layer membranes. Noticeably, VFB operated with the Nafion-NdZr (1%)/[P-S]₂ composite layer membrane (513.7 h) exhibited a longer self-discharge time than those with Nafion-NdZr (1%) (293.2 h), Nafion/[P-S]₂ (124.1 h), and recast Nafion (32.7 h) membranes. Finally, the single VFB cell constructed with Nafion-NdZr (1%)/[P-S]₂ and Nafion-NdZr (1%) membranes remarkably showed 80.1 and 73.4% capacity retention, respectively, over 200 charge-discharge cycles, whereas recast Nafion exhibited a 41.5% capacity retention over 100 cycles at a 40 mA cm^-2 current density. The structure and morphology of the Nd₂Zr₂O₇ nanotube, Nafion-NdZr composite, and Nafion-NdZr (1%)/[P-S]₂ composite layer membranes were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared, and atomic force microscopy analyses. Longer cyclic performance and excellent oxidative, chemical, and thermal stability further prove the durability of proposed membranes.