Crossover Of Vanadium Ions Research Articles

On the quantification of coulombic efficiency for vanadium redox flow batteries: Cutoff voltages vs. state-of-charge limits

The most common practice in determining the coulombic efficiency (CE) of vanadium redox flow batteries (VRFBs) is to charge and discharge the VRFB to designated cutoff voltages and then use the corresponding charging and discharging times to compute the CE. A major shortcoming of this technique is that it does not account for the change in the system's capacity during cycling due the crossover of vanadium ions. Rather than using cutoff voltages, an alternative approach is to use state-of-charge (SOC) limits since any change in the system's capacity can be captured by tracking the SOC of the device. In this study, these two techniques were investigated to assess their effectiveness in quantifying the CE in a manner that accurately captures the physical processes governing the device operation. A performance model was utilized to simulate the VRFB operation under different currents and the CE of the device was computed using both techniques. Results indicate that CEs computed using SOC limits were consistently lower than those computed using cutoff voltages, and they were consistent with the change in the chemical state of the electrolytes during cyclic operation.

Preparation of silica nanocomposite anion-exchange membranes with low vanadium-ion crossover for vanadium redox flow batteries

Crossover of vanadium ions through the membranes of all-vanadium redox flow batteries (VRFB) is an issue that limits the performance of this type of flow battery. This paper reports on the preparation of a sol–gel derived silica nanocomposite anion exchange membrane (AEM) for VRFBs. The EDS and FT-IR characterizations confirm the presence and the uniformity of the silica nanoparticles formed in the membrane via an in situ sol–gel process. The properties of the obtained membrane, including the ion-exchange capacity, the area resistance, and the water uptake, are evaluated and compared to the pristine AEM and the Nafion cation exchange membrane (CEM). The experimental results show that the permeability of the vanadium ions through the silica nanocomposite AEM is about 20% lower than that of the pristine AEM, and one order of magnitude lower than that of the Nafion CEM. As a result, the rates of self-discharge and the capacity fading of the VRFB are substantially reduced. The Coulombic and energy efficiencies at a current density of 40mAcm−2 are, respectively, as high as 92% and 73%.

Layered zirconium phosphate sulfophenylphosphonates reinforced sulfonated poly (fluorenyl ether ketone) hybrid membranes with high proton conductivity and low vanadium ion permeability

Sulfonated poly(fluorenyl ether ketone) (SPFEK) membranes were modified with layered proton conductors zirconium phosphate sulfophenylenphosphonates (ZrPSPP) to improve the proton conductivity and mitigate vanadium ions crossover. The effects of ZrPSPP filler loading on the microstructure and chemical–physical property of SPFEK/ZrPSPP composite membranes as well as the vanadium redox flow battery performance are reported. The uniform dispersion of ZrPSPP sheets in the SPFEK matrix is investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Since the sulfonic acid groups on the aromatic main-chains are less flexible, the proton channels in the pristine SPFEK membranes are relatively narrow and not well connected. The intensive sulfonic acid groups on ZrPSPP fillers increase the water uptake and the hydrophobic/hydrophilic microphase separation of the composite membranes, resulting in improved proton conductivity. The impermeable nanoplates demonstrate to be a potential physical barrier for vanadium ion diffusion. The VRB assembled with the SPFEK/5wt%ZrPSPP composite membrane shows longest discharge time and maximal coulombic efficiency among VRBs using the pristine SPFEK and other hybrid SPFEK/ZrPSPP and Nafion 117 membranes.

Covalently incorporating a cationic charged layer onto Nafion membrane by radiation-induced graft copolymerization to reduce vanadium ion crossover

Nafion membrane has been covalently functionalized with a cationic charged layer synthesized by radiation-induced graft copolymerization of N,N-dimethylaminoethylmethacrylate (DMAEMA) into Nafion substrate and subsequent protonation to reduce vanadium ion crossover for the application in vanadium redox flow battery (VRFB). The chemical composition and structure of the modified Nafion were characterized by microscopic Fourier transform infrared spectroscopy and thermogravimetric analysis. The grafting behavior was monitored by changing the experimental parameters including dose, dose rate and monomer concentration. The growth of cationic charged layer was studied by surface composition analysis of the membranes using X-ray photoelectron spectroscopy and atomic force microscope. The conductivity and permeability of vanadium ions of the obtained membranes with varying grafting yields were also measured. The functionalized Nafion membrane with a barrier layers displayed substantial resistance to permeability of vanadium ions accompanied by a relatively lower conductivity in comparison to that of the pristine Nafion membrane. The selectivity of the protonated Nafion-g-PDMAEMA membranes was higher than that of Nafion membrane, indicating the strategy is suitable for modifying Nafion membrane aiming at VRFB system. This work paves the way for the development of a new class of Nafion-based membranes for the application of VRFB.

Sulfonated poly(phthalazinone ether sulfone) membrane as a separator of vanadium redox flow battery

To develop a novel and low-cost membrane as a separator of vanadium redox flow battery, sulfonated poly(phthalazinone ether sulfone) (SPPES) was prepared by sulfonating PPES with fuming sulfuric acid. By testing the sulfonation degree, intrinsic viscosity, and solubility of SPPES, the results showed that sulfonated polymers had higher intrinsic viscosities and excellent solubility in most polar solvents. IR analysis revealed that the –SO3H group was successfully attached to SPPES backbone. DSC and TG results showed that SPPES exhibited higher T g than that of PPES, and T d at the first weight loss of SPPES was about 300 °C. The SPPES membrane (SP-02) showed a dramatic reduction in crossover of vanadium ions across the membrane compared with that of the Nafion membrane. Cell tests identified that VRB with the SPPES membrane exhibited a lower self-discharge rate, higher coulombic efficiency (92.82%), and higher energy efficiency (67.58%) compared with the Nafion system. Furthermore, cycling tests indicated that the single cell with SPPES membrane exhibited a stable performance during 100 cycles.

Nafion/TiO2 hybrid membrane fabricated via hydrothermal method for vanadium redox battery

To improve the performance of Nafion membrane as a separator in vanadium redox battery (VRB) system, a Nafion/TiO2 hybrid membrane was fabricated by a hydrothermal method. The primary properties of this hybrid membrane were measured and compared with the Nafion membrane. The Nafion/TiO2 hybrid membrane has a dramatic reduction in crossover of vanadium ions compared with the Nafion membrane. The results of scanning electron microscope, energy dispersive X-ray spectroscopy, and X-ray diffraction of the hybrid membrane revealed that the TiO2 phase was formed in the bulk of the prepared membrane. Cell tests identified that the VRB with the Nafion/TiO2 hybrid membrane presented a higher coulombic efficiency (CE) and energy efficiency (EE), and a lower self-discharge rate compared with that of the Nafion system. The CE and EE of the VRB with the hybrid membrane were 88.8% and 71.5% at 60 mA cm−2, respectively, while those of the VRB with Nafion membrane were 86.3% and 69.7% at the same current density. Furthermore, cycling tests indicated that the Nafion/TiO2 hybrid membrane can be applied in VRB system.

Cycling performance and efficiency of sulfonated poly(sulfone) membranes in vanadium redox flow batteries

As an alternative to Nafion® ion exchange membrane, an inexpensive commercially-available Radel® polymer was sulfonated, fabricated into a thin membrane, and evaluated for its performance in a vanadium redox flow battery (VRFB). The sulfonated Radel (S-Radel) membrane showed almost an order of magnitude lower permeability of VO 2+ ions (2.07 × 10 −7 cm 2/min), compared to Nafion 117 (1.29 × 10 −6 cm 2/min), resulting in better coulombic efficiency (~ 98% vs. 95% at 50 mA/cm 2) and lower capacity loss per cycle. Even though the S-Radel membrane had a slightly higher membrane resistance, the energy efficiency of the VRFB with the S-Radel membrane was comparable to that of Nafion because of its better coulombic efficiency resulting from the lower vanadium ion crossover. The S-Radel membrane exhibited good performance up to 40 cycles, but a decline in performance at later cycles was observed, likely as a result of membrane degradation.

Nafion/organic silica modified TiO 2 composite membrane for vanadium redox flow battery via in situ sol–gel reactions

To improve the selectivity of Nafion membrane, reduce the crossover of vanadium ions and decrease water transfer across the membranes used in VRB systems, the sol–gel method was employed to obtain a composite membrane of Nafion/organic silica modified TiO 2 (Nafion/Si/Ti hybrid membrane). The preparation technique and characteristics of the composite membrane were described. The water transfer and the permeability of vanadium ions were also measured. Results showed that crossover of vanadium ions and water transfer across the membrane for the composite membrane have a remarkable decrease comparing with the unmodified Nafion membrane. It has been confirmed by EDX and FT-IR analysis of the modified membrane that the Si and Ti elements distributed uniformly in the prepared membrane. As a result, the coulombic efficiency of the VRB single cell with modified Nafion membrane was higher than that of the VRB single cell with pristine Nafion membrane. The ion exchange capacity (IEC), the area resistance and the water uptake of the composite membrane were also evaluated. Furthermore, the open circuit voltage (OCV) of the VRB with modified and unmodified Nafion membrane has been carried out and cycle performance of the VRB with modified membrane proves that it has high stability in strong acid condition.

Nafion/organically modified silicate hybrids membrane for vanadium redox flow battery

In our previous work, Nafion/SiO 2 hybrid membrane was prepared via in situ sol–gel method and used for the vanadium redox flow battery (VRB) system. The VRB with modified Nafion membrane has shown great advantages over that of the VRB with Nafion membrane. In this work, a novel Nafion/organically modified silicate (ORMOSIL) hybrids membrane was prepared via in situ sol–gel reactions for mixtures of tetraethoxysilane (TEOS) and diethoxydimethylsilane (DEDMS). The primary properties of Nafion/ORMOSIL hybrids membrane were measured and compared with Nafion and Nafion/SiO 2 hybrid membrane. The permeability of vanadium ions through the Nafion/ORMOSIL hybrids membrane was measured using an UV–vis spectrophotometer. The results indicate that the hybrids membrane has a dramatic reduction in crossover of vanadium ions compared with Nafion membrane. Fourier transform infrared spectra (FT-IR) analysis of the hybrids membrane reveals that the ORMOSIL phase is well formed within hybrids membrane. Cell tests identify that the VRB with Nafion/ORMOSIL hybrids membrane presents a higher coulombic efficiency (CE) and energy efficiency (EE) compared with that of the VRB with Nafion and Nafion/SiO 2 hybrid membrane. The highest EE of the VRB with Nafion/ORMOSIL hybrids membrane is 87.4% at 20 mA cm −2, while the EE of VRB with Nafion and the EE of VRB with Nafion/SiO 2 hybrid membrane are only 73.8% and 79.9% at the same current density. The CE and EE of VRB with Nafion/ORMOSIL hybrids membrane is nearly no decay after cycling more than 100 times (60 mA cm −2), which proves the Nafion/ORMOSIL hybrids membrane possesses high chemical stability during long charge–discharge process under strong acid solutions. The self-discharge rate of the VRB with Nafion/ORMOSIL hybrids membrane is the slowest among the VRB with Nafion, Nafion/SiO 2 and Nafion/ORMOSIL membrane, which further proves the excellent vanadium ions blocking characteristic of the prepared hybrids membrane.

Self-assembled polyelectrolyte multilayer modified Nafion membrane with suppressed vanadium ion crossover for vanadium redox flow batteries

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.

Modification of Nafion membrane using interfacial polymerization for vanadium redox flow battery applications

In order to reduce the permeation of vanadium ions across the ion exchange membrane during the operation of vanadium redox flow battery (VRB) based on Nafion membrane, the interfacial polymerization was applied to form a cationic charged layer on the surface of Nafion 117 membrane. The area resistance and the permeability of vanadium ions were measured. The results indicate that comparing with the unmodified Nafion membrane, the modification of Nafion membrane results in a dramatic reduction in crossover of vanadium ions across the membrane and a little higher area resistance of the membrane. As a result, the columbic efficiency for the VRB single cell based on the modified Nafion membrane(VRB-modified Nafion), which is related to the concentration of the incubation solution of polyethylenimine (PEI), was increased significantly. The value is 96.2–97.3%, which is higher than that obtained with the VRB single cell based on unmodified Nafion membrane(VRB-Nafion) (around 93.8%). Due to the little higher area resistance caused by the modification, the voltage efficiency of VRB-modified Nafion is lower than that of VRB-Nafion. Furthermore, the water transfer across the modified membrane was also reduced. The ion exchange capacity (IEC) of the modified Nafion membrane was also evaluated. The formation of the thin cationic charged layer on the membrane surface was confirmed by IR spectra analysis.

Research progress of vanadium redox flow battery for energy storage in China

Principle and characteristics of vanadium redox flow battery (VRB), a novel energy storage system, was introduced. A research and development united laboratory of VRB was founded in Central South University in 2002 with the financial support of Panzhihua Steel Corporation. The laboratory focused their research mainly on the selection and preparation of electrode materials, membrane material and modification, stable concentrated electrolyte producing approach, test cell configuration design and optimization. Some relevant foundation problems, such as state of vanadium in sulfurous acid with various additives, the difference of electrochemical reaction rate in anode and in cathode, the crossover of vanadium ions and so on, have been emphasized. The details of these studies have been given and discussed. A 5 kW VRB stack was fabricated in the laboratory and its performances, especially electrochemical performance such as voltage efficiencies, energy efficiencies, and durability, were fully tested. The results will be shown in the talk. The key technologies of developing VRB, such as to improve the activity of its electrode materials, the stability of electrolyte and selectivity of separator, were also discussed. In addition, the research progresses in other laboratories in China were briefly introduced.

Preparation of ETFE-based anion exchange membrane to reduce permeability of vanadium ions in vanadium redox battery

Using γ-radiation technique, dimethylaminoethyl methacrylate (DMAEMA) was grafted onto ethylene–tetrafluoroethylene (ETFE) membrane (ETFE- g-PDMAEMA), and then ETFE- g-PDMAEMA membrane was protonized to prepare anion exchange membrane, while PDMAEMA grafted onto ETFE membrane was changed into poly(methacryloxyethyl dimethyl ammonium chloride) (PMAOEDMAC) (denoted as ETFE- g-PMAOEDMAC anion exchange membrane (AEM)). Micro-FTIR spectra testified that PDMAEMA was successfully grafted onto ETFE membrane. Water uptake and ion exchange capacity (IEC) of the AEM increased with grafting yield (GY), while area resistance decreased. At 40% GY, the AEM showed higher IEC and lower area resistance than Nafion117 membrane. In particular, the AEM had very low permeability of vanadium ions just as expected due to coulomb repulsion between the cation groups of the AEM and vanadium ions. Permeability of vanadium ions through the AEM with 40% GY has been reduced to 1/20 to 1/40 of that through Nafion117 membrane, which is favorable to reduce crossover of vanadium ions in vanadium redox battery (VRB). Finally, the open circuit voltage measurement showed that the VRB assembled with the AEM could maintain voltage for more than 50 h, which was much longer than that with Nafion117 membrane. The improved properties as well as the low cost suggest that the AEM can be good candidate for the VRB.

Nafion/SiO 2 hybrid membrane for vanadium redox flow battery

Sol–gel derived Nafion/SiO 2 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/SiO 2 hybrid membrane shows nearly the same ion exchange capacity (IEC) and proton conductivity as pristine Nafion 117 membrane. ICP-AES analysis reveals that Nafion/SiO 2 hybrid membrane exhibits dramatically lower vanadium ions permeability compared with Nafion membrane. The VRB with Nafion/SiO 2 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/SiO 2 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/SiO 2 hybrid membrane approach is a promising strategy to overcome the vanadium ions crossover in VRB.