•A method for rapid measurement of the positive side reactions in VFB is presented.•The SOC of positive electrolytes can be detected with resolution of 0.002%.•Side reaction ratios at different charge currents, flow rates are obtained.We present an optical detection method for rapid measurement of the positive side reactions in vanadium flow batteries (VFB). By measuring the transmittance of the positive electrolytes in VFB, the states of charge (SOC) of the positive electrolytes can be detected at very high resolution (better than 0.002% in the SOC range from 98% to 100%), due to the nonlinear transmittance spectra caused by the interactions between V(IV) and V(V) ions. The intensity of the positive side reactions of a VFB can be rapidly measured by a few steps, attributing to the fact that the positive side reactions occur only during the high voltage charging process. The ratios of the positive side reactions at different charge currents and different flow rates are obtained while causing no damage to the battery. This optical detection method can rapidly determine the optimal parameters of the VFB system, providing new means for studying the electrochemical reactions in the VFB system and rapid test in industrial production of VFBs.Download high-res image (75KB)Download full-size image

We report a novel three-dimensional architecture, consisting of tungsten carbide nanocrystals which are intimately riveted to graphite felt fabrics by carbon nanosheets (CNS@WC/GF). The as-prepared CNS@WC/GF monolith is utilized as a binder-free hydrogen evolution reaction electrode in both acidic and alkaline solutions. It demonstrates remarkable activity as well as durability.

Developing cost-effective and efficient hydrogen evolution reaction (HER) electrocatalysts for hydrogen production is of paramount importance to attain a sustainable energy future. Reported herein is a novel three-dimensional hierarchical architectured electrocatalyst, consisting of platinum–copper–nickel nanoparticles-decorated carbon nanofiber arrays, which are conformally assembled on carbon felt fabrics (PtCuNi/CNF@CF) by an ambient-pressure chemical vapor deposition coupled with a spontaneous galvanic replacement reaction. The free-standing PtCuNi/CNF@CF monolith exhibits high porosities, a well-defined geometry shape, outstanding electron conductivity, and a unique characteristic of localizing platinum–copper–nickel nanoparticles in the tips of carbon nanofibers. Such features render PtCuNi/CNF@CF as an ideal binder-free HER electrode for hydrogen production. Electrochemical measurements demonstrate that the PtCuNi/CNF@CF possesses superior intrinsic activity as well as mass-specific activity in comparison with the state-of-the-art Pt/C catalysts, both in acidic and alkaline solutions. With well-tuned composition of active nanoparticles, Pt42Cu57Ni1/CNF@CF showed excellent durability. The synthesis strategy reported in this work is likely to pave a new route for fabricating free-standing hierarchical electrodes for electrochemical devices.Keywords: carbon nanofiber arrays; chemical vapor deposition; free-standing electrocatalysts; hydrogen evolution reaction; ternary platinum−copper−nickel nanoparticles

To improve the electrochemical performance of graphite felt (GF) electrodes in vanadium flow batteries (VFBs), we synthesize a series of ZrO2-modified GF (ZrO2/GF) electrodes with varying ZrO2 contents via a facile immersion-precipitation approach. It is found that the uniform immobilization of ZrO2 nanoparticles on the GF not only significantly promotes the accessibility of vanadium electrolyte, but also provides more active sites for the redox reactions, thereby resulting in better electrochemical activity and reversibility toward the VO2+/VO2+ and V2+/V3+ redox reactions as compared with those of GF. In particular, The ZrO2/GF composite with 0.3 wt % ZrO2 displays the best electrochemical performance with voltage and energy efficiencies of 71.9% and 67.4%, respectively, which are much higher than those of 57.3% and 53.8% as obtained from the GF electrode at 200 mA cm–2. The cycle life tests demonstrate that the ZrO2/GF electrodes exhibit outstanding stability. The ZrO2/GF-based VFB battery shows negligible activity decay after 200 cycles.

Sulfur has been investigated as a candidate cathode material for the next generation lithium battery due to its high specific capacity (1675 mA h g−1). Moreover, sulfur is abundant, cheap and environmental friendly. However, the insulation behaviour of sulfur and lithium sulfide as well as the solubility of polysulfides hinders the commercial application of lithium sulfur batteries. In this article, a novel sulfur host matrix prepared by the deposition of mono-layer carbon into and carbon nanotubes outside silicon dioxide molecular sieve channels and particle surfaces is proposed. The mono-layer carbon and carbon nanotubes can simultaneously improve the conductivity of the composite and decrease the polarization phenomenon. The composite not only keeps the mesoporous structure belonging to the silicon dioxide molecular sieve intact, but also possesses high electron conductivity due to the deposition of carbon. After sulfur impregnation, the composite exhibits a high initial discharge capacity of 500 mA h g−1 and good cycle stability of 250 mA h g−1 after 100 cycles at a rate of 0.1C, corresponding to a low capacity decay rate of about 0.81% per cycle.

We report a method to eliminate the irreversible capacity of 0.4Li2MnO3·0.6LiNi0.5Mn0.5O2(Li1.17Ni0.25Mn0.583O2) by decreasing lithium content to yield integrated layered-spinel structures. XRD patterns, High-resolution TEM image and electrochemical cycling of the materials in lithium cells revealed features consistent with the presence of spinel phase within the materials. When discharged to about 2.8 V, the spinel phase of LiM2O4 (M=Ni, Mn) can transform to rock-salt phase of Li2M2O4 (M=Ni, Mn) during which the tetravalent manganese ions are reduced to an oxidation state of 3.0. So the spinel phase can act as a host to insert back the extracted lithium ions (from the layered matrix) that could not embed back into the layered lattice to eliminate the irreversible capacity loss and increase the discharge capacity. Their electrochemical properties at room temperature showed a high capacity (about 275 mAh g-1 at 0.1 C) and exhibited good cycling performance.

•N-doped carbon nanospheres are decorated on graphite felt electrode (NCS/GF) as electrocatalyst.•NCS/GF exhibits excellent electrolyte wettability and superior activity towards VO2+/VO2+ and V2+/V3+ redox reactions.•NCS/GF shows outstanding cycling performance and excellent durability at current density up to 300 mA cm−2.•NCS/GF can operate in a broad temperature range from −15 °C to 50 °C with high efficiency.Fabricating cost effective and high-performance electrodes is essential to the development of vanadium flow battery (VFB). Moreover, improving the stability of electrodes in acidic electrolyte remains key issues. In this work, we describe a simple method to prepare novel electrodes composed of N-doped carbon nanospheres (NCS) grown on graphite felt (GF) fibers. Dopamine monomers are used as both carbon and nitrogen source. Physical and electrochemical results reveal that the as-prepared NCS/GF electrode exhibits excellent electrocatalytic activity as well as wettability for vanadium ion redox reactions. The single cell tests at current densities of 50–300 mA cm−2 demonstrate superior battery performance in terms of energy efficiency and capacity retention. Exceptional durability of the NCS catalyst is confirmed by long-term cycles at a higher current density of 150 mA cm−2. NCS/GF electrode also shows excellent temperature adaptability from −15 °C to 50 °C. The facile approach reported in this study can pave a new route to fabricate high-performance electrodes for VFB.

•SPEEK membranes with various DS and casting solvents are fully evaluated for VRFB.•Optimum DS of 67% is due to suitable water uptake and high ion selectivity.•Optimum casting solvent of DMF is due to proper polymer–solvent interaction.•S67-DMF membrane shows higher VRFB efficiencies and better cycle-life performance.The properties of sulfonated poly(ether ether ketone) (SPEEK) membranes with various degree of sulfonation (DS) and casting solvent are investigated for vanadium redox flow battery (VRFB). The optimum DS of SPEEK membrane is firstly confirmed by various characterizations such as physicochemical properties, ion selectivity, and VRFB single-cell performance. Subsequently the optimum casting solvent is selected for the optimum DS SPEEK membrane within N,N′-dimethylformamide (DMF), N,N′-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), and dimethylsulfoxide (DMSO). The different performance of SPEEK membranes prepared with various casting solvents can be attributed to the different interaction between solvent and –SO3H group of SPEEK. In the VRFB single-cell test, the optimum SPEEK membrane with DS of 67% and casting solvent of DMF (S67-DMF membrane) exhibits higher VRFB efficiencies and better cycle-life performance at 80 mA cm−2. The investigation of various DS and casting solvent will be effective guidance on the selection and modification of SPEEK membrane towards VRFB application.

A series of novel composite membranes, based on sulfonated poly(ether ether ketone) (SPEEK) with various graphene oxide (GO) loadings, were employed and investigated in vanadium redox flow battery (VRFB) for the first time. The scanning electron microscopy images of the composite membranes revealed the uniform dispersion of GO nanosheets in the polymer matrix due to the interaction between GO and SPEEK, as confirmed by Fourier transform infrared spectra. The mechanical and thermal parameters of the composite membranes increased, while the VO2+ permeability decreased with increasing GO content. Random embedding of GO nanosheets in the membranes can serve as effective barriers to block the transport of vanadium ion, resulting in a significant decrease of vanadium ion permeability. The VRFB assembled with the composite membrane exhibited highly improved cell parameters and strikingly long cycling stability compared with commercial Nafion 117 membrane. With the protection of porous PTFE substrate, the pore-filling SPEEK/GO composite membrane based on VRFB ran for 1200 cycles with relatively low capacity decline.

Acid–base blend membrane prepared from sulfonated poly(ether ether ketone) (SPEEK) and polyacrylonitrile (PAN) was detailedly evaluated for vanadium redox flow battery (VRFB) application. SPEEK/PAN blend membrane exhibited dense and homogeneous cross-section morphology as scanning electron microscopy and energy-dispersive X-ray spectroscopy images show. The acid–base interaction of ionic cross-linking and hydrogen bonding between SPEEK and PAN could effectively reduce water uptake, swelling ratio, and vanadium ion permeability, and improve the performance and stability of blend membrane. Because of the good balance of proton conductivity and vanadium ion permeability, blend membrane with 20 wt % PAN (S/PAN-20%) showed higher Coulombic efficiency (96.2% vs 91.1%) and energy efficiency (83.5% vs 78.4%) than Nafion 117 membrane at current density of 80 mA cm–2 when they were used in VRFB single cell. Besides, S/PAN-20% membrane kept a stable performance during 150 cycles at current density of 80 mA cm–2 in the cycle life test. Hence the SPEEK/PAN acid–base blend membrane could be used as promising candidate for VRFB application.Keywords: acid−base interaction; blend membrane; polyacrylonitrile; sulfonated poly(ether ether ketone); vanadium redox flow battery

•SPEEK/SBA-15 (S/SBA-15) hybrid membranes are used in vanadium redox flow battery.•S/SBA-15 hybrid membranes are dense and homogeneous with no visible hole.•Membranes show good property trends for the interaction between SPEEK and SBA-15.•S/SBA-15 20 membrane shows highest efficiency and highly stable cycle performance.Hybrid membranes of sulfonated poly(ether ether ketone) (SPEEK) and mesoporous silica SBA-15 are prepared with various mass ratios for vanadium redox flow battery (VRB) application and investigated in detail. The hybrid membranes are dense and homogeneous with no visible hole as the SEM and EDX images shown. With the increasing of SBA-15 mass ratio, the physicochemical property, VO2+ permeability, mechanical property and thermal stability of hybrid membranes exhibit good trends, which can be attributed to the interaction between SPEEK and SBA-15. The hybrid membrane with 20 wt.% SBA-15 (termed as S/SBA-15 20) shows the VRB single cell performance of CE 96.3% and EE 88.1% at 60 mA cm−2 due to its good balance of proton conductivity and VO2+ permeability, while Nafion 117 membrane shows the cell performance of CE 92.2% and EE 81.0%. Besides, the S/SBA-15 20 membrane shows stable cell performance of highly stable efficiency and slower discharge capacity decline during 120 cycles at 60 mA cm−2. Therefore, the SPEEK/SBA-15 hybrid membranes with optimized mass ratio and excellent VRB performance can be achieved, exhibiting good potential usage in VRB systems.The left side of graphical abstract image is the combination of SPEEK/SBA-15 hybrid membrane photograph and stable efficiency in cycle life test. The right side of graphical abstract image is the schematic of the interaction between SPEEK and SBA-15.

•SPEEK/Graphene (S/G) composite membrane is first used in vanadium redox flow battery.•S/G composite membrane is homogeneous accompanied with high ion selectivity.•High VRB single cell efficiencies are obtained by using S/G composite membrane.•VRB with S/G composite membrane shows stable cycling performance and slow capacity decline during 500 cycles.A novel composite membrane of sulfonated poly(ether ether ketone) (SPEEK) and graphene was prepared and investigated in this paper. The scanning electron microscopy images of the composite membrane revealed the presence and the uniformity dispersion of graphene in the SPEEK matrix. The properties of the obtained composite membrane, including the water uptake, swelling ratio, ion exchange capacity (IEC), area resistance, proton conductivity, VO2+ permeability, ion selectivity, single cell performance, chemical stability and mechanical properties are evaluated in detail, compared to the pristine SPEEK membrane and Nafion 117 membrane. Self-discharge test proved that the composite membrane possessed the longest discharge time (61 h above 0.8 V). At 80 mA cm−2, a higher coulombic and energy efficiency for SPEEK/Graphene membrane was obtained, compared with Nafion 117 (96.4% vs 92.8% and 83.8% vs 79.5% respectively). Furthermore, the SPEEK/Graphene composite membrane exhibited high stability in 500 cycle's long time running.Graphene sheets act as a physical barrier of vanadium ion diffusion in SPEEK/Graphene (S/G) composite membrane resulting in good selectivity, therefore higher coulombic and energy efficiencies and better cycle performance are achieved.

In this work, CeO2 nanoparticle decorated graphite felts (CeO2/GFs) were prepared by a facile precipitation method. The corresponding CeO2/GF composites containing different contents of CeO2, i.e. 0.1, 0.2, 0.3, 0.5 wt% were synthesized individually as electrodes for vanadium redox flow battery (VRFB) application. Scanning electron microscopy and X-ray diffraction analysis indicated the homogeneous dispersion of CeO2 nanoparticles on GF. The cyclic voltammetry results revealed that the CeO2/GFs exhibited higher activity and better reversibility towards the VO2+/VO2+ redox reaction compared with the pristine GF. Among all the electrodes, 0.2 wt% CeO2/GF demonstrated the best electrochemical properties, thus nominating CeO2 content of 0.2 wt% as an optimum content. The VRFB single cell tests indicated that 0.2 wt% CeO2/GF showed the highest energy efficiency of 64.7% at the current density of 200 mA cm−2, which was significantly higher than that of the pristine GF (53.9%). Furthermore, the cycle life test of a VRFB single cell demonstrated the outstanding stability of the CeO2/GFs electrode.

Blend membranes of sulfonated poly(ether ether ketone) (SPEEK) and poly(vinylidene fluoride) (PVdF) are prepared with various mixing mass ratios for vanadium redox flow battery application for the first time. The SPEEK/PVdF blend membranes are characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The water uptake, swelling ratio, ion exchange capacity, proton conductivity, VO2+ permeability, ion selectivity, single cell performance and mechanical property of blend membranes are detailed evaluated. The blend membranes are dense and uniform when PVdF mass ratio ranges from 5 wt.% to 20 wt.%. The blend membrane with 15 wt.% PVdF (denoted as S/P 15) is further investigated for its good balance of proton conductivity and ion selectivity. The cell with S/P 15 membrane shows higher coulombic efficiency and energy efficiency compared with Nafion 117 membrane (98.0% vs. 92.0% and 81.0% vs. 75.8%, respectively). Furthermore, no obvious efficiency declines are observed after 80 cycles cell test accompanied with a lower discharge capacity decay rate.Graphical abstractHighlights► SPEEK/PVdF (S/P) blend membranes are first employed in vanadium redox flow battery. ► S/P blend membranes are dense and uniform accompanied with high ion selectivity. ► High VRB single cell efficiencies are obtained by using S/P blend membranes. ► VRB with S/P 15 shows stable cycling performances and slow capacity decline.

In this work, the electrochemical activation of graphite felt electrode for vanadium redox flow battery (VRB) was studied. Graphite felt (GF) electrode was oxidized at a range of electrochemical oxidation degrees in H2SO4 solution. The electrochemical performance of the treaded GF was discussed, and the law of the surface properties of GF which changed along with the electrochemical oxidation degree was proposed. The structure, composition, surface tension and electrochemical properties of the oxidized GF (OGF) were characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle measurements, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The GF oxidized at 560–840 mAh g−1 exhibited the best activity toward VO2+/VO2+ redox reaction, according with the highest COH and COOH content (ca. 34%) on its surface. The mechanisms of VO2+/VO2+ redox reaction on OGF were also discussed. VRB single cell with pristine GF and OGF as the electrode were test at various charge–discharge current densities, respectively. The columbic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the cell using OGF electrode are much higher than the cell using pristine GF, suggested that the electro-oxidation method is a promising technology for the activation of GF electrode.

A new type anhydrous PEM material based on Poly (ethylene oxide) (PEO)/Amino Trimethylene Phosphonic Acid (ATMP) composite was prepared. In this study, PEO assumed to “grab” protons via hydrogen bond between PEO and ATMP. Based on this point, the PEO/ATMP composites were prepared firstly as the preliminary study to verify this proton conducting system. Then, PVDF was added to enhance the membrane's stability. The PVDF/PEO/ATMP composite membranes were thermally stable up to 200 °C in the studied composition ranges. The membrane had relatively compact structure by SEM images. Proton conductivity of 59% PVDF/29% PEO/12% ATMP was up to 6.71 × 10 −3 S cm−1 at 86 °C after doping with 7.9 wt% phosphoric acid without extra humidification.

A novel sulfur-coated multi-walled carbon nanotubes composite material (S-coated-MWCNTs) was prepared through capillarity between the sulfur and multi-walled carbon nanotubes. The results of the TEM and XRD measurements reveal that S-coated-MWCNTs have a typical core-shell structure, and the MWCNTs serve as the cores and are dispersed individually into the sulfur matrices. The charge–discharge experiments of the lithium/sulfur cells demonstrated that the S-coated-MWCNTs cathode could maintain a reversible capacity of 670 mAh g−1 after 60 cycles, showing a greatly enhanced cycle ability as compared with the sulfur cathode with simple MWCNTs addition (S/MWCNTs) and the cathode using sulfur-coated carbon black composite (S-coated-CB). The EIS and SEM techniques were used to define and understand the impact of the microstructure of the composite electrode on its electrochemical performance. Derived from these studies, the main key factors to the improvement in the cycle life of the sulfur cathode were discussed.

The crossover of vanadium ions through proton-exchange membranes such as those of Nafion is the chief reason that results in the low energy efficiency and high self-discharge rate of vanadium redox flow batteries (VRB). With respect to applicability, the ideal proton-exchange membrane used in VRB should possess simultaneously high proton conductivity and low vanadium ion permeability. Here, we report a novel approach using a polyelectrolyte layer-by-layer self-assembly technique to fabricate a barrier layer onto the surface of Nafion membrane by alternate adsorption of polycation poly(diallyldimethylammonium chloride) (PDDA) and polyanion poly(sodium styrene sulfonate) (PSS), which can suppress the crossover of vanadium ions. The Nafion–[PDDA-PSS]n membrane (n = the number of multilayers) obtained shows much lower vanadium ion permeability compared with plain Nafion membrane. Accordingly, the VRB with Nafion–[PDDA-PSS]n membrane exhibits a higher coulombic efficiency (CE) and energy efficiency (EE) together with a slower self-discharge rate than that of Nafion system. The highest CE of 97.6% and EE of 83.9% can be achieved at charge–discharge current density of 80 mA cm−2 and 20 mA cm−2, respectively.

The aim of this report is to investigate the particle size-effect of Pt on the catalytic activities for high surface area catalysts. Series of Pt/MWCNTs catalysts with average particle size of 1.7, 2.4, and 4.0 nm were fabricated. Size-effect on their catalytic activity towards CO as well as alcohol electrooxidation reactions was studied by CO-stripping, cyclic voltammetry, and chronoamperometry. We for the first time investigated the size-effect on the apparent activation energy for the reaction of CO electrooxidation. The results showed that the catalytic activities had a strong dependence on their Pt particle size. (1) The reaction of CO electrooxidation on the smaller Pt nanoparticles was found occurring at the higher overpotential and apparent activation energy than on the larger counterparts. The intrinsic nature of those observed phenomena were due to the stronger CO adsorption on the smaller Pt nanoparticles. (2) As the size decreased, the mass activity towards alcohol electrooxidation increased; considering the discount of this increase for the smaller particles, we concluded that the deactivation arising from their more intense CO-poisoning, which limited the beneficial effect on their higher mass activity in this case.

A very simple and promising method to design the anode catalyst architecture for direct alcohol fuel cells by physically mixing Pt/C catalyst with transition-metal oxide nanoparticles is presented and electrochemical measurements confirm that this unique catalyst structure has excellent activity toward alcohol and CO electro-oxidation.

Sol–gel derived Nafion/SiO2 hybrid membrane is prepared and employed as the separator for vanadium redox flow battery (VRB) to evaluate the vanadium ions permeability and cell performance. Nafion/SiO2 hybrid membrane shows nearly the same ion exchange capacity (IEC) and proton conductivity as pristine Nafion 117 membrane. ICP-AES analysis reveals that Nafion/SiO2 hybrid membrane exhibits dramatically lower vanadium ions permeability compared with Nafion membrane. The VRB with Nafion/SiO2 hybrid membrane presents a higher coulombic and energy efficiencies over the entire range of current densities (10–80 mA cm−2), especially at relative lower current densities (<30 mA cm−2), and a lower self-discharge rate compared with the Nafion system. The performance of VRB with Nafion/SiO2 hybrid membrane can be maintained after more than 100 cycles at a charge–discharge current density of 60 mA cm−2. The experimental results suggest that the Nafion/SiO2 hybrid membrane approach is a promising strategy to overcome the vanadium ions crossover in VRB.

SBA-15, due to its ordered mesoporous structure and potential for advanced applications in separation technologies, catalysis, and nanoscience, has attracted much attention in the past few years. In this work, a novel PEO-based composite polymer electrolyte has been developed by using SBA-15 as the filler. The interactions between SBA-15 and PEO matrix are studied by XRD, DSC, and FT-IR techniques in detail. The effects of SBA-15 on the electrochemical properties of the PEO-based polymer electrolyte, such as ionic conductivity, lithium ion transference number, and interfacial stability with lithium electrode, are studied by electrochemical impedance spectroscopy and steady-state current method. The experiment results show that the addition of SBA-15 can enhance the ionic conductivity and increase the lithium ion transference number of PEO-based polymer electrolyte at the same time. The excellent performances such as high mechanical stability, good compatibility with lithium metal electrode, and broad electrochemical stability window suggest that PEO–LiClO4/SBA-15 composite polymer electrolyte can be used as candidate electrolyte material for lithium polymer batteries.

A very simple and promising method to design the anode catalyst architecture for direct alcohol fuel cells by physically mixing Pt/C catalyst with transition-metal oxide nanoparticles is presented and electrochemical measurements confirm that this unique catalyst structure has excellent activity toward alcohol and CO electro-oxidation.

We report a novel three-dimensional architecture, consisting of tungsten carbide nanocrystals which are intimately riveted to graphite felt fabrics by carbon nanosheets (CNS@WC/GF). The as-prepared CNS@WC/GF monolith is utilized as a binder-free hydrogen evolution reaction electrode in both acidic and alkaline solutions. It demonstrates remarkable activity as well as durability.

The crossover of vanadium ions through proton-exchange membranes such as those of Nafion is the chief reason that results in the low energy efficiency and high self-discharge rate of vanadium redox flow batteries (VRB). With respect to applicability, the ideal proton-exchange membrane used in VRB should possess simultaneously high proton conductivity and low vanadium ion permeability. Here, we report a novel approach using a polyelectrolyte layer-by-layer self-assembly technique to fabricate a barrier layer onto the surface of Nafion membrane by alternate adsorption of polycation poly(diallyldimethylammonium chloride) (PDDA) and polyanion poly(sodium styrene sulfonate) (PSS), which can suppress the crossover of vanadium ions. The Nafion–[PDDA-PSS]n membrane (n = the number of multilayers) obtained shows much lower vanadium ion permeability compared with plain Nafion membrane. Accordingly, the VRB with Nafion–[PDDA-PSS]n membrane exhibits a higher coulombic efficiency (CE) and energy efficiency (EE) together with a slower self-discharge rate than that of Nafion system. The highest CE of 97.6% and EE of 83.9% can be achieved at charge–discharge current density of 80 mA cm−2 and 20 mA cm−2, respectively.

A series of novel composite membranes, based on sulfonated poly(ether ether ketone) (SPEEK) with various graphene oxide (GO) loadings, were employed and investigated in vanadium redox flow battery (VRFB) for the first time. The scanning electron microscopy images of the composite membranes revealed the uniform dispersion of GO nanosheets in the polymer matrix due to the interaction between GO and SPEEK, as confirmed by Fourier transform infrared spectra. The mechanical and thermal parameters of the composite membranes increased, while the VO2+ permeability decreased with increasing GO content. Random embedding of GO nanosheets in the membranes can serve as effective barriers to block the transport of vanadium ion, resulting in a significant decrease of vanadium ion permeability. The VRFB assembled with the composite membrane exhibited highly improved cell parameters and strikingly long cycling stability compared with commercial Nafion 117 membrane. With the protection of porous PTFE substrate, the pore-filling SPEEK/GO composite membrane based on VRFB ran for 1200 cycles with relatively low capacity decline.