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

In our previous work, Nafion/SiO₂ 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/SiO₂ 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/SiO₂ 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/SiO₂ 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/SiO₂ and Nafion/ORMOSIL membrane, which further proves the excellent vanadium ions blocking characteristic of the prepared hybrids membrane.     

Investigations on transfer of water and vanadium ions across Nafion membrane in an operating vanadium redox flow battery

Diffusion coefficients of the vanadium ions across Nafion 115 (Dupont) in a vanadium redox flow battery (VRFB) are measured and found to be in the order of V²⁺ > VO²⁺ > VO₂⁺ > V³⁺. It is found that both in self-discharge process and charge-discharge cycles, the concentration difference of vanadium ions between the positive electrolyte (+ve) and negative electrolyte (−ve) is the main reason causing the transfer of vanadium ions across the membrane. In self-discharge process, the transfer of water includes the transfer of vanadium ions with the bound water and the corresponding transfer of protons with the dragged water to balance the charges, and the transfer of water driven by osmosis. In this case, about 75% of the net transfer of water is caused by osmosis. In charge-discharge cycles, except those as mentioned in the case of self-discharge, the transfer of protons with the dragged water across the membrane during the electrode reaction for the formation of internal electric circuit plays the key role in the water transfer. But in the long-term cycles of charge-discharge, the net transfer of water towards +ve is caused by the transfer of vanadium ions with the bound water and the transfer of water driven by osmosis.     

Orientated graphene oxide/Nafion ultra-thin layer coated composite membranes for vanadium redox flow battery

Graphene oxide (GO)/Nafion composite membranes, with orientated GO nanosheets in parallel to the surface of the ultra-thin coating layer (400–440 nm), are prepared by spin coating method and evidenced by electron microscopy analysis. Orientation of GO maximizes the vanadium ions barrier effect of GO. The GO/Nafion membrane (M-2) achieves lower vanadium ion permeability (8.2 × 10^?8 cm² min^?1, only 2.64% of the pristine Nafion membrane), and higher coulombic efficiency and energy efficiency (92.9–98.8% and 81.5–88.4%, respectively) comparing with the pristine Nafion membrane (73.3–90.5% and 68.9–79.1%, respectively) at current densities of 20–100 mA cm^?2. With the design of orientated GO nanosheets and ultra-thin GO/Nafion coating layer, good balance between vanadium crossover suppression and protons conduction retention is achieved. M-2 exhibits excellent battery performances over 200 charge-discharge cycles. The capacity decay rate is about 0.23% per cycle, much lower than those assembled with Nafion 212 (0.40% per cycle) and the recast Nafion membrane (0.44% per cycle). Spin coating with water suspensor leads to uniform dispersion of GO and good binding between GO/Nafion coating layer and substrate Nafion membrane. Therefore, the composite membrane could be reinforced by GO and keep integration even with 200 cycles operation.     

Nafion/polyvinylidene fluoride blend membranes with improved ion selectivity for vanadium redox flow battery application

Nafion/PVDF blends are employed to prepare the ion exchange membranes for vanadium redox flow battery (VRB) application for the first time. The addition of the highly crystalline and hydrophobic PVDF effectively confines the swelling behavior of Nafion. In VRB single cell test, the Nafion/PVDF binary membranes exhibit higher columbic efficiency than recast Nafion at various current densities. The blend membrane with 20 wt% of PVDF (N_0.8P_0.2) shows energy efficiency of 85% at 80 mA cm⁻², which is superior to that of recast Nafion. N_0.8P_0.2 membrane also possesses twice longer duration in OCV decay test and much lower permeation of VO²⁺ compared with recast Nafion. These results indicate that the addition of PVDF is a simple and efficient way to improve the ion selectivity of Nafion, and the polymer blends with optimized mass fraction of PVDF show good potential for VRB application.     

Preparation and properties of sulfonated poly(fluorenyl ether ketone) membrane for vanadium redox flow battery application

In order to develop novel membranes for vanadium redox flow battery (VRB) with low self-discharge rate and low cost, sulfonated poly(fluorenyl ether ketone) (SPFEK) was synthesized directly via aromatic nucleophilic polycondensation of bisphenol fluorene with 60% sulfonated difluorobenzophenone and 40% difluorobenzophenone. The SPFEK membrane shows the lower permeability of vanadium ions. The open circuit voltage evaluation demonstrates that the SPFEK membrane is superior to Nafion 117 membrane in self-discharge test. Both energy efficiencies (EE) and power densities of the VRB single cell based on the SPFEK membrane are higher than those of the VRB with Nafion 117 membrane at the same current densities. The highest coulombic efficiency (CE) of VRB with SPFEK membrane is 80.3% while the highest CE of the VRB with Nafion 117 membrane is 77.0%. The SPFEK membrane shows the comparative stability to Nafion 117 membrane in VO₂⁺ electrolyte. The experimental results suggest that SPFEK membrane is a promising ion exchange membrane for VRB.     

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.     

Sulfonated poly (fluorenyl ether ketone) membrane with embedded silica rich layer and enhanced proton selectivity for vanadium redox flow battery

A series of novel organic-inorganic hybrid membranes with special microstructure, based on sulfonated poly (fluorenyl ether ketone) ionomer (SFPEK, IEC = 1.92 mequiv. g⁻¹) and SiO₂ or sulfonic acid group containing SiO₂ (SiO₂-SO₃H), has been successfully designed and prepared for vanadium redox flow battery (VRB) application. The SiO₂-SO₃H is synthesized by co-condensation of tetraethoxysilane and γ-propyl mercaptotrimethoxysilane via sol-gel process to control the same IEC with neat SPFEK. The hybrid membranes are prepared by simply adding the inorganic particles into the SPFEK solution in N,N′-dimethylacetamide, followed by ultrasonic dispersion, casting and profiled temperature drying process. The morphology is examined by SEM-EDX which is applied to the top surface, bottom surface and cross-section of the hybrid membranes. The water uptake, oxidative stability, thermal property, mechanical property, proton conductivity, VO²⁺ permeability and single cell performance are investigated in detail in order to understand the relationship between morphology and property of the membranes. All the hybrid membranes show dramatically improved proton selectivity at 20 °C and 40 °C when compared with Nafion117. The VRB assembled with the SPFEK/3%SiO₂ and SPFEK/9%SiO₂ membranes exhibit higher coulombic efficiency and average discharge voltage than the VRB assembled with the SPFEK membrane at all the tested current densities.     

Proton exchange membranes with ultra-low vanadium ions permeability improved by sulfated zirconia for all vanadium redox flow battery

Suppressing vanadium ions crossover is a top priority in the development of membranes for vanadium redox flow battery (VRFB). One method is to dope inorganic fillers into polymer matrix, which usually decreases membrane's ion conductivity. In this work, sulfated zirconia (SZrO₂) is synthesized as a novel additive doped in sulfated poly (ether sulfone) (SPES) to simultaneously enhance the proton conduction and inhibit vanadium migration of the membranes. Membrane characterizations including battery test are carried out to reveal the effects of SZrO₂ on the membrane performance. The SPES/SZrO₂ composite membranes show vanadium permeability one order of magnitude lower than that of Nafion 212 and enhanced proton conductivity, which lead to superior cell performance. The columbic efficiency and energy efficiency of the VRFB reach 98.89% and 86.78%, respectively, at 100 mA cm⁻². Cycling test is carried out to evaluate the chemical and electrochemical stability of the membrane. Energy efficiency above 86% is maintained after70 charge-discharge cycles at 100 mA cm⁻².     

Development of perfluorosulfonic acid polymer-based hybrid composite membrane with alkoxysilane functionalized polymer for vanadium redox flow battery

A new kind of alkoxy silane functionalized polymer (ASFP) is synthesized by selectively functionalized carboxyl groups as a novel inorganic precursor polymer to prepare organic-inorganic hybrid membrane for vanadium redox flow battery (VRFB) system. The novel hybrid membrane has been fabricated by interconnection between hydrophilic domains of Nafion and ASFP functional group. The effective concentration of ASFP for hybrid membrane is 25% (wt/wt). The proton conductivity and selectivity of the hybrid membrane are comparable with those of the Nafion212 membrane, which is mainly attributed by the presence of additional hydrophilic domains in the hybrid membrane. The proton conductivity and ion exchange capacity of the Nafion-ASFP (75:25) membrane is 0.061 S/cm and 0.68 meq/g, respectively. Remarkably, the Nafion-ASFP membrane shows a low vanadium permeability (1.259 × 10⁻⁷ cm²/min) and high selectivity, which is an excellent advantage. As a result, the hybrid membrane shows comparable efficiency performance with Nafion212 over 50 cycles. Notably, the VRFB unit cell with Nafion-ASFP membrane achieves higher coulombic efficiency than Nafion212. The hybrid membrane reveals a new route to develop an alternative fluorinated polymer membrane with numerous advantages especially cost-effectiveness, homogeneous dispersion of inorganic silica precursor materials in the hybrid membrane without deterioration of mechanical strength, and lower vanadium ion crossover for VRFB system.     

A significantly improved membrane for vanadium redox flow battery

A novel sandwich-type sulfonated poly(ether ether ketone) (SPEEK)/tungstophosphoric acid (TPA)/polypropylene (PP) composite membrane for a vanadium redox flow battery (VRB) has been developed with improved properties: the permeability of vanadium ions is greatly reduced and the performance of the VRB cell is greatly increased. The membrane is based on a traditional SPEEK membrane embedded with TPA but PP is used to enhance the membrane for the first time. Although its voltage efficiency (VE) is a little lower than that of a Nafion 212 membrane, it is expected to have good prospects for VRB systems because of its low cost and good performance.     

Amphoteric ion exchange membrane synthesized by direct polymerization for vanadium redox flow battery application

Novel sulfonated poly (fluorenyl ether ketone) with pendant quaternary ammonium groups (SPFEKA) was successfully synthesized by one-pot copolymerization of bis(4-fluoro-3-sulfophenyl)sulfone disodium salt, 4,4′-difluorobenzophenone, bisphenol fluorene and 2,2′-dimethylaminemethylene-9,9′-bis(4-hydroxyphenyl) fluorene (DABPF). The chemical structures were confirmed by FT-IR, and ¹H NMR. The thermal properties were fully investigated by TGA. The synthesized copolymers SPFEKAs are soluble in aprotic solvents, and can be cast into membranes on a glass plate from their N,N′-dimethylacetamide (DMAc) solution. A new kind of amphoteric ion exchange membrane (AIEM) was obtained by immersed SPFEKA into 1 M sulfuric acid. The proton conductivities of these membranes are comparable to the most reported sulfonated polymers under the same conditions. The permeability of vanadium ions in vanadium redox flow battery (VRB) was effectively suppressed by introducing quaternary ammonium groups for Donnan exclusion effect. AIEM-20% possess a only 4.4% vanadium ion permeability of Nafion 115. Cell performance tests showed that the VRB assembled with AIEM-20% shows the highest coulombic efficiency (CE) at the current density of 50 mA/cm², because of its lowest VO²⁺ permeability. In conclusion, these ionomers could be promising candidates for ion-exchange membranes for VRB applications.     

Shunt current loss of the vanadium redox flow battery

The shunt current loss is one of main factors to affect the performance of the vanadium redox flow battery, which will shorten the cycle life and decrease the energy transfer efficiency. In this paper, a stack-level model based on the circuit analog method is proposed to research the shunt current loss of the vanadium redox flow battery, in which the SOC (state of charge) of electrolyte is introduced. The distribution of shunt current is described in detail. The sensitive analysis of shunt current is reported. The shunt current loss in charge/discharge cycle is predicted with the given experimental data. The effect of charge/discharge pattern on the shunt current loss is studied. The result shows that the reduction of the number of single cells in series, the decrease of the resistances of manifold and channel and the increase of the power of single cell will be the further development for the VRFB stack.     

Sulfonated poly(tetramethydiphenyl ether ether ketone) membranes for vanadium redox flow battery application

Sulfonated poly(tetramethydiphenyl ether ether ketone) (SPEEK) with various degree of sulfonation is prepared and first used as ion exchange membrane for vanadium redox flow battery (VRB) application. The vanadium ion permeability of SPEEK40 membrane is one order of magnitude lower than that of Nafion 115 membrane. The low cost SPEEK membranes exhibit a better performance than Nafion at the same operating condition. VRB single cells with SPEEK membranes show very high energy efficiency (>84%), comparable to that of the Nafion, but at much higher columbic efficiency (>97%). In the self-discharge test, the duration of the cell with the SPEEK membrane is two times longer than that with Nafion 115. The membrane keeps a stable performance after 80-cycles charge-discharge test.     

Preparation and characterization of a novel layer-by-layer porous composite membrane for vanadium redox flow battery (VRB) applications

By the solution casting method, a novel porous membrane has been prepared for VRB by doping sulfonated poly(fluorenyl ether ketone) (SPFEK) with imidazole, and then imidazole was washed out by extraction with solution. The proton conductivity of porous membrane increased with increasing the content of imidazole, but proton/vanadium ion (H/V) selectivity decreased. Layer-by-layer (LbL) technique was used to improve the porous membrane with high selectivity. Moreover, the performance of VRB using SPFEK-20.7imidazole-(PDDA/PSS)₈ membrane which is doped with 20.7 wt.% content of imidazole and then removed imidazole, and then deposited with eight LbL bilayers exhibits the highest columbic efficiency (CE) of 92.5% at 30 mA cm⁻².     

Novel branched sulfonated polyimide membrane with remarkable vanadium permeability resistance and proton selectivity for vanadium redox flow battery application

A series of novel branched sulfonated polyimide (bSPI-x) membranes with 8% branched degree are developed for application in vanadium redox flow battery (VRFB). The sulfonation degrees of bSPI-x membranes are precisely regulated for obtaining excellent comprehensive performance. Among all bSPI-x membranes, the bSPI-50 membrane shows strong vanadium permeability resistance, which is as 8 times as that of commercial Nafion 212 membrane. At the same time, the bSPI-50 membrane has remarkable proton selectivity, which is four times as high as that of Nafion 212 membrane. The bSPI-50 membrane possesses slower self-discharge speed than Nafion 212 membrane. Furthermore, the bSPI-50 membrane achieves stable VRFB efficiencies during 200-time charge-discharge cycles at 120–180 mA cm^?2. Simultaneously, the bSPI-50 membrane exhibits excellent capacity retention compared with Nafion 212 membrane. All results imply that the bSPI-50 membrane possesses good application prospect as a promising alternative separator of VRFB.     

A novel long-side-chain sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) membrane for vanadium redox flow battery

A novel long-side-chain sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (S-L-PPO) membrane was successfully prepared for VRFB applications. The long side chains were introduced onto the PPO backbones by a simple and controllable acylation with 4-fluorobenzoyl chloride and a subsequent condensation with sodium 4-hydroxybenzenesulfonate. The introduction of long side chains drives the formation of a good hydrophilic/hydrophobic micro-phase separation structure, which is evidenced by AFM. The S-L-PPO membrane with a degree of sulfonation (DS) of 51% showed an ultralow vanadium permeability (4.3 × 10^?9 cm² s^?1) and a decent proton conductivity (44 mS cm^?1). As a result, the energy efficiency of VRFB with S-L-PPO-51% membrane was up to 81.8% that was higher than that with Nafion 212 (78.0%) at a current density of 120 mA cm^?2. In addition, the self-discharge duration of the cell with S-L-PPO-51% membrane was 222 h that was nine times longer than that of Nafion 212 (23 h). All these indicate that the membrane prepared here is promising for the application in VRFB.     

Branched sulfonated polyimide/s-MWCNTs composite membranes for vanadium redox flow battery application

A novel sulfonated multi-wall carbon nanotubes (s-MWCNTs) filler is synthesized by ring-opening reaction. And then, a series of branched sulfonated polyimide (bSPI)/s-MWCNTs composite membranes are also prepared for application in vanadium redox flow batteries (VRFBs). The optimized bSPI/s-MWCNTs-2% composite membrane has lower vanadium ion permeability (2.01 × 10⁻⁷ cm² min⁻¹) and higher proton selectivity (1.06 × 10⁵ S min cm⁻³) compared to those of commercial Nafion 212 membrane. Moreover, the VRFB with bSPI/s-MWCNTs-2% composite membrane exhibits higher coulombic efficiencies (CEs: 96.0–98.2%) and energy efficiencies (EEs: 79.7–69.5%) than that with Nafion 212 membrane (CEs: 86.5–92.5% and EEs: 78.5–67.6%) at 80–160 mA cm⁻². The VRFB with bSPI/s-MWCNTs-2% composite membrane has stable battery performance over 400 cycles at 100 mA cm⁻², whose EE value is in the top level among previously reported SPI-based composite membranes. The results show that the bSPI/s-MWCNTs-2% composite membrane has a great prospect in VRFB application.     

State of charge monitoring methods for vanadium redox flow battery control

During operation of redox flow batteries, differential transfer of ions and electrolyte across the membrane and gassing side reactions during charging, can lead to an imbalance between the two half-cells that results in loss of capacity. This capacity loss can be corrected by either simple remixing of the two solutions, or by chemical or electrochemical rebalancing. In order to develop automated electrolyte management systems therefore, the state-of-charge of each half-cell electrolyte needs to be known. In this study, two state-of-charge monitoring methods are investigated for use in the vanadium redox flow battery. The first method utilizes conductivity measurements to independently measure the state-of-charge of each half-cell electrolyte. The second method is based on spectrophotometric principles and uses the different colours of the charged and discharged anolyte and catholyte to monitor system balance and state-of charge of each half-cell of the VRB during operation.     

Nuclear magnetic resonance studies on vanadium(IV) electrolyte solutions for vanadium redox flow battery

The vanadium(IV) electrolyte solutions with various vanadium concentrations are studied by variable temperature ¹H and ¹⁷O nuclear magnetic resonance (NMR) spectroscopy. The structure and kinetics of vanadium(IV) species in the electrolyte solutions are explored with respect to vanadium concentration and temperature. It was found that the vanadium(IV) species exist as hydrated vanadyl ion, i.e. [VO(H₂O)₅]²⁺ forming an octahedral coordination with vanadyl oxygen in the axial position and the remaining positions occupied by water molecules. This hydrated vanadyl ion structure is stable in vanadium concentrations up to 3 M and in the temperature range of 240-340 K. The sulfate anions in the electrolyte solutions are found to be weekly bound to this hydrated vanadyl ion and occupies its second-coordination sphere. The possible effects of these sulfate anions in proton and water exchange between vanadyl ion and solvent molecules are discussed based on ¹H and ¹⁷O NMR results.     

A novel electrode-bipolar plate assembly for vanadium redox flow battery applications

A novel electrode-bipolar plate assembly has been developed and evaluated for application in the vanadium redox flow battery (VRB). It is composed of three parts: a graphite felt (electrode), an adhesive conducting layer (ACL) and a flexible graphite plate (bipolar plate). The ACL connects the electrode with the bipolar plate to an assembly. By the evaluations of cost, resistivity, surface morphology, electrolyte permeation and single cell performance, this novel assembly demonstrates its applicability in VRB as evident in the following outcomes: (1) lowers the cost and area resistivity to about 10% and 40% of the conventional setups, respectively; (2) improves electrical conductivity to 4.97 mΩ cm as compared to over 100 mΩ cm of the carbon-plastic composite bipolar plate; (3) attains zero electrolyte permeation; and (4) achieves a higher energy efficiency of 81% at a charge/discharge current density of 40 mA cm⁻² when employed in a VRB single cell, which is 73% for the conventional setup. All these indicate that the novel electrode-bipolar plate assembly is a promising candidate for VRB applications.