Study of the production of vanadium electrolytes from ammonium metavanadate

Graphene oxide (GO) has attracted tremendous attention in membrane-based separation field as it can filter ions and molecules. Recently, GO-based materials have emerged as excellent modifiers for vanadium redox flow battery (VRFB) application. Its high mechanical and chemical stability, nearly frictionless surface, high flexibility, and low cost make GO-based materials as proper materials for the membranes in VRFB. In VRFB, a membrane acts as the key component to determine the performance. Therefore, employing low vanadium ion permeability with excellent stability membrane in vanadium electrolytes is important to ensure high battery performance. Herein, recent progress of GO-modified membranes for VRFB is briefly reviewed. This review begins with current membranes used for VRFB, followed by the challenges faced by the membranes. In addition, the transport mechanism of vanadium ion and the stability properties of GO-modified membranes are also discussed to enlighten the role of GO in the modified membranes.

Preparation of dense polybenzimidazole proton exchange membranes with different basicity and flexibility for vanadium redox flow battery applications

An acid pretreatment strategy is developed to enhance the proton transport of polysulfone-polyvinylpyrrolidone (PSF-PVP) membranes for application in vanadium redox flow batteries (VRFB). The acid pretreatment leads to the formation of ionic conducting clusters with a size of around d=15.41 nm in the membrane (p-PSF-PVP). As a result, the proton conductivity and proton/vanadium ion selectivity of the p-PSF-PVP membrane increases to 6.60×10-2 S cm-1 and 10.63×107 S min cm-3 , respectively, values significantly higher than 2.30×10-2 S cm-1 and 6.67×107 S min cm-3 of the pristine PSF-PVP membrane. Moreover, a VRFB assembled with the p-PSF-PVP membrane exhibits a high coulombic efficiency of 98.6 % and an outstanding energy efficiency of 88.5 %. The results indicate that treatment with either sulfuric acid or phosphoric acid leads to an improvement of membrane properties, and the acid pretreatment is a promising strategy to significantly enhance the performance of the PSF-PVP membrane for VRFB application.

The vanadium redox flow battery (VRFB) is one of the most promising energy storage technologies for large scale commercialization. The vanadium electrolyte is the main component, determining the VRFB’s energy density and capacity. The quality of the vanadium electrolyte is key to VRFB’s operation, as presence of soluble impurities will affect VRFB’s performance and durability. Understanding the impact of these impurities may ultimately lead to a specification for the impurity levels a VRFB can tolerate without compromising its performance and durability. Fe is an impurity element typically found in the vanadium ores, which is also present in the commercially available vanadium electrolytes. It was reported that Fe in positive electrolyte in a narrow concentration range (< 0.0286 M or 0.12 wt. %) could slightly affected the VRFB performance [1]. At high concentration (1.0 - 1.4 M Fe), Fe was reported to stabilize the positive electrolyte at high temperature (50oC) [2]. However, systematic studies on the effect of Fe on vanadium redox reactions, over a wider range of concentrations, are needed in order to achieve a good understanding of how it affects the VRFB performance.This work reports on the effect of Fe on vanadium redox reactions, with concentrations ranging from 0.05 wt. % or 0.012 M to 2 wt. % or 0.48 M, investigated by cyclic voltammetry and VRFB single cell cycling. In the present study, CV response of vanadium electrolyte on glassy carbon, Pt disk and graphite electrode was compared. On glassy carbon and Pt disk electrodes, vanadium redox reaction has shown irreversible or partially reversible CV response, while on graphite rod electrode, reversible redox reaction response was observed. Thus, a graphite rod was selected as working electrode for the subsequent investigations. The cyclic voltammogram on graphite rod electrode is dependent on electrode pre-treatment. Redox peak current, and peak separation are different on freshly polished graphite electrode compared to an electrode that has been used for a certain period of time (e.g. half hour). The difference is dependent on Fe concentration. In 0.05 wt. % (or 0.012 M) Fe electrolyte, electrode passivation was observed (Fig. 1), where the freshly polished electrode shows higher redox peak current than the one that has been used for CV for a certain time. However, in electrolytes with higher Fe content (e.g. 0.5 wt. % or 0.12 M), the freshly polished electrode presents lower redox peak current, indicating that electrode activation can occur during CV testing (Fig 2).VRFB performance was further evaluated, and it was found that the effect of Fe on VRFB capacity, capacity change profile with cycling, and efficiency is dependent on Fe concentration. Low Fe concentration affected more on the efficiency, while higher Fe concentration shows significant effect on capacity change during cycling. AC impedance and vanadium crossover were used to diagnosis the VRFB performance and degradation, and it was found that Fe concentration affects VRFB degradation and water transfer. The tolerance level of Fe in vanadium electrolyte can be deduced from this study, which may provide guidance on the design of low purity vanadium electrolyte.

Synthesis and properties of branched sulfonated polyimides for membranes in vanadium redox flow battery application

Nafion membrane has been widely used in fuel cells due to its good proton conductivity, chemical and mechanical stabilities. However, the well-developed water channels in Nafion membrane have restricted its application in all vanadium redox flow batteries owing to the high permeation of vanadium ions and the cost. In this work, the water adsorption rate, resistivity and transference number of vanadium ions of sulfonated-graphene/Nafion composite membrane were investigated and compared with those of Nafion membrane in an effort to overcome the abovementioned drawbacks. It was found that the area resistivity and permeation rate of sulfonated-graphene/Nafion composite membrane were significantly reduced after the modification of Nation membrane by sulfonated grapheme. The results from the all vanadium redox flow battery indicated that the sulfonated-graphene/Nafion composite membrane exhibited better electrochemical performance, demonstrating potential application in all vanadium redox flow batteries.

This paper presents the application of vanadium redox flow battery (VRFB) to grid connected microgrid energy management. The application of an energy storage system could enhance the economic feasibility of the microgrid. VRFB as a microgrid energy storage system has an advantage of low capital cost. However, the efficiency of VRFB is comparatively lower than other energy storage systems. Characteristics of VRFB have to be considered to achieve optimal energy management of the microgrid energy management system. The VRFB operation is restricted by its balance of plant power usages and variable efficiency. The microgrid energy management system considers both limitations and provides an optimal VRFB control schedule. The optimal control schedule of the real VRFB system shows that the microgrid energy management system tries to operate the VRFB near its maximum efficiency regions. Application of VRFB with an energy management system could guarantee both economic feasibility and low capital cost of the microgrid.

Although the Nafion membrane has a high energy efficiency, long service life, and operational flexibility when applied for vanadium redox flow battery (VRFB) applications, its applications are limited due to its high vanadium permeability. In this study, anion exchange membranes (AEMs) based on poly(phenylene oxide) (PPO) with imidazolium and bis-imidazolium cations were prepared and used in VRFBs. PPO with long-pendant alkyl-side-chain bis-imidazolium cations (BImPPO) exhibits higher conductivity than the imidazolium-functionalized PPO with short chains (ImPPO). ImPPO and BImPPO have a lower vanadium permeability (3.2 × 10-9 and 2.9 × 10-9 cm2 s-1) than Nafion 212 (8.8 × 10-9 cm2 s-1) because the imidazolium cations are susceptible to the Donnan effect. Furthermore, under the current density of 140 mA cm-2, the VRFBs assembled with ImPPO- and BImPPO-based AEMs exhibited a Coulombic efficiency of 98.5% and 99.8%, respectively, both of which were higher than that of the Nafion212 membrane (95.8%). Bis-imidazolium cations with long-pendant alkyl side chains contribute to hydrophilic/hydrophobic phase separation in the membranes, thus improving the conductivity of membranes and the performance of VRFBs. The VRFB assembled with BImPPO exhibited a higher voltage efficiency (83.5%) at 140 mA cm-2 than that of ImPPO (77.2%). These results of the present study suggest that the BImPPO membranes are suitable for VRFB applications.

Quaternary ammonium groups grafted polybenzimidazole membranes for vanadium redox flow battery applications

Hybrid inorganic-organic proton-conducting membranes based on SPEEK doped with WO3 nanoparticles for application in vanadium redox flow batteries

Chemically stable anion exchange membranes based on C2-Protected imidazolium cations for vanadium flow battery

Partially fluorinated sulfonated poly(arylene ether) (SFPAE) copolymers were investigated as chemically stable proton exchange membranes for application in vanadium redox flow batteries (VRFB). The membranes' proton conductivity and vanadium ion permeability were quantified and correlated to other membrane properties such as water uptake and tensile modulus to provide insight into the tradeoffs in the design of new membranes for flow battery applications. The SFPAE-1.8 sample with optimized proton conductivity to vanadium permeability selectivity was selected for evaluation in a VRFB device and compared to the performance of a cell with a NAFION® N212 membrane. The VRFB cell with a SFPAE-1.8 membrane had higher coulombic efficiency, voltage efficiency, and energy efficiency compared to a VRFB with a N212 membrane under all tested current densities. The capacity fade of a VRFB with the SFPAE-1.8 membrane was 1.1 mA h per cycle, which was about 7 times lower than the fade experienced for a VRFB with a N212 membrane. The performance characteristics of the device could be correlated directly to the membrane properties and this work demonstrates our progress towards high-performance, low-cost, long-lifetime ion exchange membranes for electrochemical energy storage devices.

Poly(phenyl sulfone) anion exchange membranes with pyridinium groups for vanadium redox flow battery applications

Thermally cross-linked sulfonated poly(ether ether ketone) membranes containing a basic polymer-grafted graphene oxide for vanadium redox flow battery application

Silicon carbide (SiC) was pretreated and functionalized by using α, ω-diaminopropyl polydimethylsiloxane (PDMS) to obtain a novel inorganic filler SiC (PDMS). Then, a series of branched sulfonated polyimide/SiC (PDMS) (bSPI/SiC (PDMS)) composite membranes with different contents of SiC (PDMS) were fabricated for vanadium redox flow battery (VRFB) application. Fourier transform infrared spectra, X-ray diffraction, and field emission scanning electron microscope demonstrate the successful preparation of SiC (PDMS) and bSPI/SiC (PDMS) membranes. The thermogravimetric analysis shows that bSPI/SiC (PDMS)-1.5% membrane has better thermal stability than pure bSPI, bSPI/SiC-1.5%, and Nafion 117 membranes. The ex situ chemical stability test results show that bSPI/SiC (PDMS)-0.5–2.5% composite membranes have better chemical stabilities than pure bSPI membrane. The physicochemical properties of bSPI/SiC (PDMS) membranes, including water uptake, swelling ratio, ion exchange capacity are investigated. Thereinto, bSPI/SiC (PDMS)-1.5% membrane has the highest proton selectivity (S: 2.99 × 105 S min cm−3) and was chosen as an optimum VRFB membrane. And the VRFB assembled with bSPI/SiC (PDMS)-1.5% membrane exhibits better battery performance than that assembled with Nafion 117 membrane during 500-time cyclic charge–discharge test at 20–60 mA cm−2. Above results indicate that as-optimized bSPI/SiC (PDMS)-1.5% membrane has great potential for VRFB application.