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Commercial Nafion membranes have been widely used for vanadium redox flow batteries (VRFB) but with relatively low ion selectivity. A chemical method is commonly employed to modify the organic membranes, whereas physical approaches are rarely reported in view of less compatibility with the organic species. In this study, an ultrathin polytetrafluoroethylene (PTFE) film of less than 30 nm is deposited onto the Nafion substrates by radio frequency magnetron sputtering to form PTFE@Nafion composite membranes. The PTFE layer of hydrophobic and inert feature enhances the dimensional stability and the ion selectivity of the Nafion membranes. The VRFB single cell with an optimized composite membrane exhibits a better self-discharge property than that of the Nafion 212 (i.e., 201.2 vs. 18.6 h), due to a higher ion selectivity (i.e., 21.191 × 104 vs. 11.054 × 104 S min cm–3). The composite membranes also show better discharge capacity retention than the Nafion 212 over the entire 100 cycles. The results indicate that the magnetron sputtering is an alternative and feasible route to tailor the organic membranes via surface modification and functionalization. [ABSTRACT FROM AUTHOR]

The vanadium redox flow battery (VRFB) is a large‐scale energy storage technique and has been regarded as a promising candidate to integrate intermittent renewable energy with the grid. Its long‐term stability has so far been limited by the core component, an ion exchange membrane with low ion selectivity. Here a hybrid membrane with superhydrophilic TiO2 nanotubes dispersed in a Nafion matrix is reported. The VRFB single cell with the hybrid membrane exhibits an impressive performance with high coulombic efficiency (CE, ≈98.3%) and outstanding energy efficiency (EE, ≈84.4%) at 120 mA cm−2, which is higher than that of the commercial Nafion 212 membrane (CE, ≈94.5%; EE, ≈79.2%). More importantly, the cell maintains a discharge capacity of ≈55.7% after 1400 cycles (over 518 h), in obvious contrast to that of ≈20% after only 410 cycles for the one using commercial Nafion 212. This is attributed to the high ion selectivity of the hybrid membrane, because of, 1) the blocked and elongated ion diffusion pathway induced by the dispersed nanotubes and 2) binding and alignment of the sulfonic acid groups on nanotube surface. The high‐performance membranes may also find important applications in other fields, such as fuel cells, dialytic batteries, and water treatment. [ABSTRACT FROM AUTHOR]

Abstract The commercial Nafion proton exchange membranes are most widely used in vanadium redox flow batteries (VRBs). However, their poor ion selectivity and high cost significantly limits the further development of the VRB. Herein, a green biopolymer, lignin, was firstly employed as the additive in the pristine SPEEK membrane to replace the Nafion membrane for the VRB applications. It is found that the abundant hydroxyl groups in lignin not only facilitate the dispersion of the lignin in the SPEEK matrix, but also significantly improve the proton conductivity and ion selectivity of the membrane. With these merits, the VRB single cell based on the SPEEK/lignin membrane exhibits 95.95% and 71.47% capacity retention after 100 and 300 cycles under 120 mA cm−2, respectively. Moreover, its ultrahigh coulombic efficiency (over 99.5%) and energy efficiency (over 83.5%) indicate the outstanding capability of ion selectivity and the superb protons conductivity of the membrane. The excellent stability, low cost, and remarkable device performance of the composite membrane make it a promising candidate for the next generation larger-scale VRB applications. Graphical abstract A green sulfonated poly(ether ether ketone)/lignin membrane with excellent ion selectivity is prepared to enable low cost and ultra-stable vanadium redox flow battery. fx1 Highlights • A green sulfonated poly(ether ether ketone)/lignin membrane (SPEEK/lignin) is fabricated. • The SPEEK/lignin membrane shows higher ion selectivity compared to Nafion 212 membrane. • A vanadium redox flow battery assembled with low cost SPEEK/lignin membrane shows impressive cycle life. [ABSTRACT FROM AUTHOR]

Abstract The ions exchange membrane is the key component in the redox flow battery (RFB), which determines the cycle life and the cost of RFB. Herein, we successfully prepared a low-cost K+ formed sulfonated poly(ether ether ketone) (SPEEK-K) membrane for the neutral Zn-Fe redox flow battery (ZFB) by solution casting method. The ZFB based on our SPEEK-K membrane shows outstanding performance with the coulombic efficiency and energy efficiency over 95% and 78%, respectively, at a high current density of 40 mA cm−2. The cost of SPEEK-K membrane for ZFB is also investigated, compared with commercial Nafion 117 membrane. The comprehensive performance of the SPEEK-K membrane for high energy density ZFB makes this membrane as one of the promising candidates for next generation large-scale energy storage system. Highlights • A low-coat and high-performance SPEEK-K membrane has been developed for Zn-Fe flow battery. • The stability and conductivity of the SPEEK-K is higher than that of the pristine SPEEK and commercial Nafion 117 membrane. • The Zn-Fe flow battery based on the SPEEK-K membrane exhibits high coulombic efficiency and energy efficiency. [ABSTRACT FROM AUTHOR]

Here we report an eco-friendly Nafion/lignin composite membrane with excellent performance for vanadium redox flow battery (VRFB). Through facile solution casting, the Nafion/lignin composite membrane presents the merits of high conductivity and low vanadium ions permeability. Under high current density (120 mA cm−2), the VRFB single cell with the optimized Nafion/lignin composite membrane exhibits superior performance metrics to commercial counterpart: high Coulombic efficiency (~97.4%) and energy efficiency (~82.7%), and prominent cyclability with slower capacity decay for ~52.8% capacity retention after 1000 cycles (vs. Nafion 212 membrane ~34.8% after only 150 cycles). More importantly, compared with the pristine Nafion 212 membrane, the cost of the composite membrane is reduced sharply by reducing the usage amount of Nafion polymer, while maintaining the membrane stability. Thus, the Nafion/lignin composite membrane is one of the competitive candidates for VRFB towards commercialization. Image 1 • A cost-effective Nafion/lignin composite membrane is fabricated. • The Nafion/lignin membrane shows lower vanadium ion permeability than Nafion 212. • The VRFB shows ultra-long cycle life with ultra-low capacity decay rate. [ABSTRACT FROM AUTHOR]