Vanadium Redox Flow Battery Single Cell Research Articles

Ultra-High Proton/Vanadium Selectivity of Polybenzimidazole Membrane by Incorporating Phosphotungstic Acid Functionalized Nanofibers for Vanadium Redox Flow Battery

Proton exchange membrane（PEM） is a key component of vanadium redox flow battery（VRB）, and its proton/vanadium selectivity plays an important role in the performance of a VRB single cell. Commercially available perfluorosulfonic acid（Nafion） membranes have been widely used due to their excellent proton conductivity and favorable chemical resistance. However, the large pore size micelle channels formed by the pendant sulfonic acid groups lead to the excessive penetration of vanadium ions, which seriously affects the coulombic efficiency（CE） of the single cell and accelerates the self-discharge rate of the battery. Additionally, the expensive cost of Nafion is also an important reason to limit its large-scale application. In this paper, the dense and lowcost hydrocarbon polymer polybenzimidazole（PBI） is used as the matrix material of the PEM, which is doped with phosphotungstic acid（PWA） to acquire excellent proton conductivity, and the intrinsic high resistance of PBI for vanadium ions is helpful to obtain high proton/vanadium selectivity. Considering the enormous water solubility of PWA and its easy leaching from membrane, organic polymer nano-Kevlar fibers（NKFs） are utilized as the anchoring agent of PWA, which achieves good anchoring effect and solves the problem of the poor compatibility between inorganic anchoring agent and the polymer matrix. The formation of PWA functionalized NKFs was characterized by scanning electron microscope（SEM） and Fourier transform infrared（FT-IR） spectroscopy. The anchoring stability of NKFs for PWA was evaluated by UV-Vis spectroscopy. The characterizations including water uptake, swelling ratio, ion exchange capacity, proton conductivity, vanadium ion permeability and ion selectivity were performed to evaluate the basic properties of the membranes. At the same time, the charge-discharge, self-discharge and cycle performance of single cell assembled with the composite membrane and recast Nafion were tested at various current densities from 40 to 100 m A·cm-2. Simple tuning for the filling amount of NKFs@PWA gives the composite membrane superior ion selectivity including an optimal value of 3.26 × 105 S·min·cm-3, which is 8.5 times higher than that of recast Nafion(0.34 × 105 S·min·cm-3). As a result, the VRB single cell assembled with the composite membrane exhibits higher CE and significantly lower self-discharge rate compared with recast Nafion. Typically, the CE of the VRB based on PBI-（NKFs@PWA）-22.5% membrane is 97.31% at 100 mA·cm-2 while the value of recast Nafion is only 90.28%. The open circuit voltage（VOC） holding time above 0.8 V of the single cell assembled with the composite membrane is 95 h, which is about 2.4 times as long as that of recast Nafion-based VRB. The utilization of PBI as a separator for VRB can effectively suppress the penetration of vanadium ions, achieve higher proton/vanadium selectivity and superior battery performance as well as reduce the cost of the PEM, which will play an active role in the promotion of VRB applications.

A complex four-point method for the evaluation of ohmic and faradaic losses within a redox flow battery single-cell

We propose a complex 4-point method for characterization of flow batteries. The distribution of ohmic and faradaic losses within a single-cell is evaluated from electrochemical impedance spectra and load curves of positive and negative half-cells measured with platinum wire pseudo-reference electrodes positioned in respective electrode compartment. The developed method can be used e.g., for the component screening and in-situ durability studies on single-cell scale. The method was validated on a vanadium redox flow battery single-cell; however, it can be analogically employed for various chemistries of flow battery.•Complex 4-point method for characterization of flow battery single-cell was developed.•Method is based on electrochemical impedance spectra and load curve measurements.•Direct evaluation of ohmic and faradaic losses distribution within battery single-cell by the method.

Improved vanadium barrier properties of perfluorinated sulfonic acid membranes for vanadium redox flow battery

The search for highly vanadium selective membrane for vanadium redox flow battery (VRFB) is fast growing to accelerate the commercialization of VRFB. Currently, VRFB is challenged by high vanadium permeability through the membranes lowering the battery performance. In this paper, a simple and effective strategy for improving the vanadium barrier permeability of the membranes, without altering the desired proton conductivity, was reported by applying layer(s) of polyelectrolytes. A commercial perfluorinated sulfonic acid membrane, GN115 was modified by layer of positively charged poly(diallyldimethylammonium chloride) (PDDA) or consecutive layers of PDDA and negatively charged poly(sodium styrene sulfonate) (PSS). Upon surface modification, the membranes exhibited a reduced vanadium permeability with a slight decreased in proton conductivity values compared with the pristine GN115 and N117. As a result, higher coulombic efficiency was obtained in VRFB single cell with the modified membranes. This confirms that the addition of the polyelectrolyte layers can suppress the vanadium permeability of the membranes.

A promising SPEEK/MCM composite membrane for highly efficient vanadium redox flow battery

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.

A green SPEEK/lignin composite membrane with high ion selectivity for vanadium redox flow battery

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.

Effect of the redox reactivity of vanadium ions enhanced by phosphorylethanolamine based catalyst on the performance of vanadium redox flow battery

Phosphorylethanolamine (PHOS) doped carbon nanotube (CNT) is newly suggested as a catalyst for enhancing the redox reactions of vanadium ions and vanadium redox flow battery (VRFB) performance. For synthesizing the catalyst, CNT is linked to the phosphate group of the PHOS, forming phosphate functionalized carbon nanotube (POH-CNT). Its catalytic activity is then compared with those of pure CNT and carboxylic acid functionalized CNT (CA-CNT) catalysts. Regarding the redox reactivity of vanadium ions, POH-CNT exhibits superior catalytic activity and reaction reversibility for VO2+/VO2+ reaction to CNT and CA-CNT because of (i) the chelation ability of phosphate group, (ii) its low electron delocalization capability and (iii) its high acid dissociation constant. By the role as chelating agent of phosphate group, the density of active sites for reaction of VO2+ and VO2+ ions in POH-CNT increase. Also, the low electron delocalization and high acid dissociation constant induce effective adsorption and desorption of VO2+ and VO2+ ions. Such effects facilitate the VO2+/VO2+ reaction. In contrast, regarding V2+/V3+ reaction, POH-CNT shows equivalent reactivity to CA-CNT because phosphate and carboxyl groups form similar active sites for the V2+/V3+ reaction. The above results are supported by calculating kinetic parameters, such as charge transfer resistance and diffusion coefficient, while chemical bonding of the catalysts is examined by XPS. Finally, as the POH-CNT is used as catalyst for positive electrode prompting the VO2+/VO2+ reaction, performance of VRFB single cell is improved.

Amphiprotic Side-Chain Functionalization Constructing Highly Proton/Vanadium-Selective Transport Channels for High-Performance Membranes in Vanadium Redox Flow Batteries.

A novel amphiprotic side-chain-functionalized membrane was for the first time designed for vanadium redox flow battery (VFB). Different from frequently used blending amphiprotic membranes, the one proposed here is allowed to possess high anion-exchange capacity (IECa) without sacrificing the cation-exchange capacity (IECc) because both IECa and IECc increased with the grafting degree of side chains. Having a high IECa, the membrane prepared here exhibits an ultralow vanadium permeability (<10-8 cm2 s-1), which leads to very high Coulombic efficiencies (97-98% at 40-200 mA cm-2) of VFB and good cell self-discharge durability. Moreover, the high IECc contributes to a decent ionic conductivity (area resistance: 0.5 Ω cm-2), which ensures a high-voltage efficiency of the cell. On the basis of these good properties, the VFB single cell with this membrane achieves a high energy efficiency (e.g., 77.4% at 200 mA cm-2) that is higher than those of Nafion 212 and other reported amphiprotic membranes. These results indicate that the approach proposed here is an ideal option to prepare amphiprotic membranes for VFBs with high efficiency and good durability.

SPEEK Membrane of Ultrahigh Stability Enhanced by Functionalized Carbon Nanotubes for Vanadium Redox Flow Battery.

Proton exchange membrane is the key factor of vanadium redox flow battery (VRB) as their stability largely determine the lifetime of the VRB. In this study, a SPEEK/MWCNTs-OH composite membrane with ultrahigh stability is constructed by blending sulfonated poly(ether ether ketone) (SPEEK) with multi-walled carbon nanotubes toward VRB application. The carbon nanotubes disperse homogeneously in the SPEEK matrix with the assistance of hydroxyl group. The blended membrane exhibits 94.2 and 73.0% capacity retention after 100 and 500 cycles, respectively in a VRB single cell with coulombic efficiency of over 99.4% at 60 mA cm−2 indicating outstanding capability of reducing the permeability of vanadium ions and enhancing the transport of protons. The ultrahigh stability and low cost of the composite membrane make it a competent candidate for the next generation larger-scale vanadium redox flow battery.

Sulfonated polyimide/phosphotungstic acid composite membrane for vanadium redox flow battery applications

A novel sulfonated polyimide (SPI) membrane embedded with the phosphotungstic acid (SPI/PWA membrane) for vanadium redox flow battery (VRB) has been prepared with low capacity loss, low cost, and high energy efficiency (EE); the proportion of PWA in the composite membrane is 15%. The mechanical strength, vanadium ions permeability, and performance of the membrane in the VRB single cell were characterized. Results showed that the SPI/PWA membrane possessed low permeability of vanadium ions, accompanied by higher mechanical strength than the Nafion117 membrane. The VRB single cell with SPI/PWA composite membrane showed 7.6% higher coulombic efficiency, 4.6% higher EE, but lower capacity loss in comparison with the one with Nafion117 membrane at the current density 40 mA cm−2. The good cell performance, low capacity loss, and high vanadium ions barrier properties of the blend membrane is of significant interest for VRB applications.

Mesoporous tungsten oxynitride as electrocatalyst for promoting redox reactions of vanadium redox couple and performance of vanadium redox flow battery

For enhancing the performance of vanadium redox flow battery (VRFB), a sluggish reaction rate issue of V2+/V3+ redox couple evaluated as the rate determining reaction should be addressed. For doing that, mesoporous tungsten oxide (m-WO3) and oxyniride (m-WON) structures are proposed as the novel catalysts, while m-WON is gained by NH3 heat treatment of m-WO3. Their specific surface area, crystal structure, surface morphology and component analysis are measured using BET, XRD, TEM and XPS, while their catalytic activity for V2+/V3+ redox reaction is electrochemically examined. As a result, the m-WON shows higher peak current, smaller peak potential difference, higher electron transfer rate constant and lower charge transfer resistance than other catalysts, like the m-WO3, WO3 nanoparticle and mesoporous carbon, proving that it is superior catalyst. Regarding the charge-discharge curve tests, the VRFB single cell employing the m-WON demonstrates high voltage and energy efficiencies, high specific capacity and low capacity loss rate. The excellent results of m-WON are due to the reasons like (i) reduced energy band gap, (ii) reaction familiar surface functional groups and (ii) greater electronegativity.

Highly selective sulfonated poly(ether ether ketone)/titanium oxide composite membranes for vanadium redox flow batteries

To lower the cost and enhance the selectivity of membranes used in vanadium redox flow batteries (VRFBs), sulfonated poly(ether ether ketone) (SPEEK)/ titanium oxide (TiO2) composite membranes with different degrees of sulfonation (DS) (prepared at different time durations of 3h, 5h and 7h) and various TiO2 nanoparticles loadings (3%, 5% and 7%) were prepared. Optimum DS and TiO2 nanoparticles loading were confirmed by different characterization techniques including 1H NMR, scanning electron microscopy, water uptake, swelling ratio, proton conductivity, vanadium ion permeability and thermal stability. Results show that 5h SPEEK/5% TiO2 composite membrane possessed comprehensive merits, especially high ion selectivity (72.55 × 103Smincm−3) due to the blocking effect of TiO2 nanoparticles and good proton conductivity. TiO2 nanoparticles combined very well with SPEEK due to strong interaction, as deduced from flux tests. The chemical and oxidative stability were sufficiently good for applications of VRFB. The VRFB single cell with 5h SPEEK/5% TiO2 composite membrane exhibited excellent performance including high coulombic efficiency (98.3%), high energy efficiency (82.9%), long self-discharge time and low capacity loss compared to Nafion 117. Hence, SPEEK/TiO2 composite membranes can potentially be promising substitute for Nafion in VRFBs.

Sulfonated poly(ether ether ketone) composite membranes containing microporous layered silicate AMH-3 for improved membrane performance in vanadium redox flow batteries

Sulfonated poly(ether ether ketone) (SPEEK) is a potential polymer for replacing Nafion in vanadium redox flow batteries (VRBs). However, at a high degree of sulfonation, SPEEK displays high swelling, poor mechanical stability, and high vanadium crossover. In this study, to improve membrane performance, composite membranes of SPEEK and finely ground microporous AMH-3 (G-AMH-3) are prepared with various G-AMH-3 contents and investigated. The physicochemical and mechanical properties, vanadium permeability, and VRB single cell performance of these SPEEK/G-AMH-3 composite membranes are evaluated using various characterization techniques. Interactions between SPEEK and G-AMH-3, and the permselective property of G-AMH-3, result in the composite membranes exhibiting good mechanical properties and low vanadium crossover. Optimal composite membranes gave a VRB that produced a higher charge–discharge capacity, higher cell efficiency, and better capacity retention than that using Nafion. These results indicate that SPEEK-based composite membranes with improved membrane performance, lower vanadium crossover, and good single cell performance were successfully prepared by incorporating G-AMH-3.

Sulfonated poly(ether ether ketone)/TiO2 double-deck membrane for vanadium redox flow battery application

A novel sulfonated poly(ether ether ketone) (SPEEK)/TiO2 double-deck membrane which consists of a layer of SPEEK and a layer of TiO2 was prepared and investigated for vanadium redox flow battery (VRB) application for the first time. The physicochemical properties of the SPEEK/TiO2 membrane, including the water uptake, swelling ratio, ion exchange capacity, proton conductivity, VO2+ permeability and ion selectivity are evaluated in detail, compared to the pristine SPEEK membrane and Nafion 117 membrane. The scanning electron microscopy images reveal its double-deck structure and the structural stability with no delamination, and thermogravimetric analysis (TG) identifies its thermal stability. Among all membranes, the SPEEK/TiO2 double-deck membrane possesses lowest vanadium ion permeability (6.66×10−7cm2min−1) and highest ion selectivity (9.46×104Smincm−3). The VRB single cell with SPEEK/TiO2 double-deck membrane shows higher coulombic efficiency (97.0% vs 93.3%) and energy efficiency (85.8% vs 83.7%) compared to that with Nafion 117 membrane at 60mAcm−2. Furthermore, the SPEEK/TiO2 double-deck membrane exhibits highly stable cell performance after 60 times of cycling tests at 60mAcm−2 and lower capacity decay rate than that of Nafion 117 membrane. Therefore, the SPEEK/TiO2 double-deck membrane exhibits good potential usage in VRB systems.

Sulfonated poly(ether sulfone)/poly(vinylidene fluoride) hybrid membrane for vanadium redox flow battery

Sulfonated poly(ether sulfone)/poly(vinylidene fluoride) (SPES/PVDF) blends are prepared and employed as the separator for vanadium redox flow battery (VRB) to evaluate the vanadium ions permeability and cell performance. The SPES/PVDF membranes exhibit dramatically vanadium ions permeability and cell performance compared with pristine SPES and Nafion115 membrane. The vanadium ion permeability of SPES/PVDF membrane is 1 order of magnitude lower than that of Nafion115 membrane. The low-cost composite membrane exhibits a better performance than Nafion115 membrane at the same operating condition. VRB single cell with SPES/PVDF membrane shows significantly lower capacity loss, higher coulombic efficiency (&gt;98%) and higher energy efficiency (&gt;84%) than that with Nafion115 membrane. In the self-discharge test, S0.7P0.3 membrane shows twice longer duration in the open circuit decay than that with Nafion115 membrane. With all the good properties and low cost, the SPES/PVDF membrane is expected to have excellent commercial prospects as an ion exchange membrane for VRB systems.

SPEEK/PVDF/PES Composite as Alternative Proton Exchange Membrane for Vanadium Redox Flow Batteries

A membrane consisting of a blend of sulfonated poly(ether ether ketone) (SPEEK), poly(vinylidene fluoride) (PVDF), and poly(ether sulfone) (PES) has been fabricated and used as an ion exchange membrane for application in vanadium redox flow batteries (VRBs). The vanadium ion permeability of the SPEEK/PVDF/PES membrane was one order of magnitude lower than that of Nafion 117 membrane. The low-cost composite membrane exhibited better performance than Nafion 117 membrane at the same operating condition. A VRB single cell with SPEEK/PVDF/PES membrane showed significantly lower capacity loss, higher coulombic efficiency (>95%), and higher energy efficiency (>82%) compared with Nafion 117 membrane. In the self-discharge test, the duration of the cell with the SPEEK/PVDF/PES membrane was nearly two times longer than that with Nafion 117 membrane. Considering these good properties and its low cost, SPEEK/PVDF/PES membrane is expected to have excellent commercial prospects as an ion exchange membrane for VRB systems.

Titanium nitride as an electrocatalyst for V(II)/V(III) redox couples in all-vanadium redox flow batteries

Titanium nitride nanoparticles (TiN NPs) are proposed as a novel catalyst towards the V(II)/V(III) redox pair for the negative electrode in vanadium redox flow batteries (VRFB). Electrochemical properties of TiN NPs were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results show that TiN NPs demonstrate better electrochemical activity and reversibility for the processes of V(II)/V(III) redox couples as compared with the graphite NPs. TiN NPs facilitate the charge transfer in the V(II)/V(III) redox reaction. Performance of a VRFB using a TiN NPs coated carbon paper as a negative electrode is much higher than that of a VRFB with a raw carbon paper electrode. The columbic efficiency (CE), the voltage efficiency (VE) and the energy efficiency (EE) of the VRFB single cell at charge-discharge current density of 30mA/cm2 are 91.74%, 89.11% and 81.74%, respectively. During a 50 charge-discharge cycles test, the CE values of VRFB with TiN NPs consistently remain higher than 90%.

Sulfonated Poly(Ether Ether Ketone)/Functionalized Carbon Nanotube Composite Membrane for Vanadium Redox Flow Battery Applications

A novel sulfonated poly(ether ether ketone) (SPEEK) membrane embedded with the short-carboxylic multi-walled carbon nanotube (we name it as SPEEK/SCCT membrane) for vanadium redox flow battery (VRB) has been prepared with low capacity loss, low cost and high energy efficiency. The mechanical strength, vanadium ions permeability and performance of the membrane in the VRB single cell were characterized. Results showed that the SPEEK/SCCT membrane possessed low permeability of vanadium ions, accompanied by higher mechanical strength than the Nafion 212 membrane. The VRB single cell with SPEEK/SCCT membrane showed 7% higher coulombic efficiency (CE), 6% higher energy efficiency (EE) but lower capacity loss in comparison with the one with Nafion 212. The good cell performance, low capacity loss and high vanadium ions barrier properties of the blend membrane is of significant interest for VRB applications.

CeO2decorated graphite felt as a high-performance electrode for vanadium redox flow batteries

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.

Properties Investigation of Sulfonated Poly(ether ether ketone)/Polyacrylonitrile Acid–Base Blend Membrane for Vanadium Redox Flow Battery Application

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.

Characterization of sulfonated poly(ether ether ketone)/poly(vinylidene fluoride-co-hexafluoropropylene) composite membrane for vanadium redox flow battery application

Sulfonated poly(ether ether ketone) (SPEEK) and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) composite membranes are prepared and investigated in detail for vanadium redox flow battery (VRFB) application. With the high hydrophobicity and stability of P(VDF-co-HFP), the properties of composite membranes such as mechanical property and vanadium ion permeability are effectively improved, showing good trends with the increasing of P(VDF-co-HFP) mass ratio. The VRFB single cell assembled with the composite membrane of 15 wt.% P(VDF-co-HFP) (SPEEK-15% membrane) exhibits higher coulombic efficiency (CE, 95.4%) and energy efficiency (EE, 83.8%) than that assembled with Nafion 117 membrane (CE 91.1% and EE 78.4%) at the current density of 80 mA cm−2. Furthermore, the SPEEK-15% membrane maintains a stable performance during 100 cycles at the current density of 80 mA cm−2. Therefore the SPEEK/P(VDF-co-HFP) composite membrane could be used as low-cost and high-performance membrane for VRFB application.