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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 application of vanadium redox flow batteries (VRFBs) has encountered challenges because the most commonly used commercial membrane (perfluorinated sulfonic acid, PFSA) has severe vanadium ion permeation, which yields poor stability of the battery. Herein, a PFSA‐based hybrid membrane with a sandwich structure created using a reinforced polytetrafluoroethylene thin layer with hydrophilic nanohybrid fillers is developed. The tungsten trioxide (WO3) nanoparticles are in situ grown on the surface of graphene oxide (GO) nanosheets to overcome the electrostatic effect, and to enhance the hydrophilicity and dispersibility of GO nanosheets, which is embedded in the PFSA matrix to act as a "barrier" to reduce vanadium ions permeation. In addition, these hydrophilic WO3 nanoparticles on GO nanosheets surface serve as proton active sites to facilitate proton transportation. As a result, at 120 cm−2, the cell of the hybrid membrane displays high Coulombic efficiency (over 98.1%) and high energy efficiency (up to 88.9%), better than the commercial Nafion 212 membrane at the same condition. These performances indicate the proposed hybrid membrane is applicable for VRFB application. Also, this design method of the membrane can be extended to other fields including water treatments and fuel cells. [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 A novel composite membrane blended the sulfonated poly (ether ether ketone) (SPEEK) with modified carbonaceous mudstone (SPEEK/MCM) has been developed for vanadium redox flow battery (VRB). Using a strong acidification treatment, a large amount of hydrophilic oxygen functional groups was introduced on the modified carbonaceous mudstone (MCM) particles, which has been evidenced to enhance the proton conductivity. It is found that the MCM particles help to retard the permeation of vanadium ions through the membrane. With these merits, the VRB single cell based on our novel SPEEK/MCM membrane exhibits outstanding device performance. Notably, the coulombic efficiency (CE) of the cell is as high as more than 99% and the per-cycle capacity loss is only 0.15% under a high current density of 120 mA cm−2. Our work provides a simple method to prepare low cost and high-performance membranes for VRB. More importantly, the SPEEK/MCM membrane has promising potential in other battery applications. Highlights • A composited SPEEK/ carbonaceous mudstone (SPEEK/MCM) membrane is fabricated for vanadium redox flow battery. • The SPEEK/MCM membrane shows much lower vanadium ion permeability compared to Nafion 212 membrane. • A vanadium redox flow battery assembled with a SPEEK/MCM membrane shows excellent cell performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • A PTFE@PFSA/Functionalized-SiC membrane with sandwich structure is fabricated. • The hybrid membrane has low vanadium ion permeation and high proton conductivity. • A VRFB cell with the hybrid membrane shows high efficiency and cyclic stability. As a key component of vanadium redox flow battery (VRFB), ion exchange membrane determines the performance of VRFB, wherein commercial membrane (such as Nafion) exacerbate problems including high permeability of vanadium ions and poor cyclic stability. In this study, a low vanadium ion permeation hybrid membrane is developed by dispersing functionalized silicon carbide nanowires in perfluorinated sulfonic acid (PFSA) matrix and sandwiching an ultrathin porous polytetrafluoroethylene (PTFE) layer. This hybrid membrane greatly improves the performance of VRFB cell exhibiting good Coulombic efficiency (up to 96.2%) and high energy efficiency (up to 87.1%) at 120 mA cm−2, contrasted with the Nafion 212 membrane (91.3% and 79.8%). Furthermore, a single cell with the hybrid membrane possesses ultra-stable cyclicity (1000 cycles, over 233.4 h). This can be assigned to the unique structure of the membrane in which the functionalized nanowires are uniformly dispersed in PFSA matrix to sandwich an ultra-thin PTFE layer to block or lengthen the diffusion paths of vanadium ions, while the functionalized groups on the surface of the rigid nanowires facilitate protons transportation. This strategy of reshaping membrane may serve in relative fields including proton exchange membrane fuel cells and water treatment. [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]

Abstract Graphene deposited on the surface of a carbon felt (CF) using a solution coating method has been developed as a high-performance positive electrode for an all vanadium redox flow battery (VRB). A key to obtain excellent electrochemical activity towards the VO 2 +/VO2+ redox couple is to wrap the CF using the graphene with high specific surface area and superb conductivity. It is found that the dipping times of CF into the graphene/Nafion solution significantly affect its electrochemical activity. The cyclic voltammetry (CV) results indicate that with 5 dipping times, the graphene coated CF (G/CF) exhibits the highest peak current and lowest peak potential difference towards the VO 2 +/VO2+ redox couple. More importantly, the VRB assembled with our novel G/CF cathodic electrode shows a decreased polarization during charge/discharge process compared with the control VRB with the pristine CF. Consequently, both the voltage efficiency and energy efficiency of the VRB with G/CF electrode have increased compared to the one with pristine CF. Our work provides a simple solution coating process to fabricate graphene modified CF electrode for VRB with high performance and this simple method is believed to be promising in other electrocatalysts applications. Highlights • A graphene coated carbon felt electrode (G/CF) for has been fabricated. • The graphene is homogenous dispersed on the surface of fiber of carbon felt. • The G/CF electrode displays high activity towards the VO2+/VO 2 + redox couple. • The G/CF electrode shows much better cell performance compared to pristine carbon felt electrode. [ABSTRACT FROM AUTHOR]