Vanadium Redox Flow Battery Single Cell Research Articles

Construction of thermal, chemical and mechanically stable ion exchange membranes with improved ion selectivity for vanadium redox flow batteries applications

As an alternative to expensive Nafion membrane, SPEEK (Sulfonated Poly Ether Ether Ketone) membrane reinforced with hydroxylated boron nitride is investigated as a potential proton exchange membrane for vanadium redox flow battery (VRFB). SPEEK based hybrid membranes are characterized for their thermal, mechanical and chemical stability along with proton conductivity and vanadium permeability. The results reveal that the hydroxylated boron nitride enhances the physicochemical properties of SPEEK and shown to improve selectivity of ions (52.629 *103. S.cm−3.min) with better stability. The improved properties are attributed to hydrogen bond formation between hydroxyl moieties of born nitride and sulfonic acid group in SPEEK. Among the hybrid membrane, SPEEK reinforced with 0.05 wt % of hydroxylated boron nitride exhibits a higher coulombic efficiency of 93.6 % (91.8 % for Nafion), higher self-discharge time of 14.21 h (11.36 h for Nafion) in VRFB single cell performance study. Cyclic performance study in VRFB with hybrid membrane shows stable characteristics during 50 cycles and it reaffirms the structural, thermal and chemical stability of hybrid membranes. Thus, this study provides insight into replacing commercial Nafion with hydroxylated boron nitride reinforced SPEEK hybrid membrane for vanadium redox flow battery.

Tuning Polybenzimidazole‐Derived Crosslinked Interpenetrating Network Membranes for Vanadium Redox Flow Batteries

AbstractNon‐fluorinated ion exchange membranes with high proton selectivity and conductivity are sought as separators for vanadium redox flow batteries (VRFB) to substitute the typically used perfluorosulfonic acid (PFSA) polymer membranes. Polybenzimidazole based membranes offer a promising non‐fluorinated alternative due to their excellent thermomechanical properties, low vanadium crossover, and ionic conductivity in acidic media. In this work, a series of polybenzimidazole‐polyvinylchloride polymer blends were cast and decorated with different quaternary ammonium functionalities. The polymer blends were systematically studied with respect to the blend composition and chemical structure of the quaternary ammonium groups. Assessment of relevant membrane properties for use in an aqueous acidic flow battery was conducted through a combination of VRFB single cell testing, permeability measurements, water/electrolyte uptake, and infrared spectroscopy. The blend with a polybenzimidazole content of 90 % decorated with DABCO (1,4‐diazabicyclo[2.2.2]octane) was found to combine low polarization resistance with low swelling and high stability. The performance was found to be similar to that of a benchmark polybenzimidazole membrane despite being three times thicker. This led to the conclusion that quaternary ammonium functionalized polybenzimidazole‐polyvinylbenzylchloride systems is an excellent candidate for further modification or fabrication of thinner membranes to further reduce the membrane resistance without compromising on vanadium blocking properties.

State of charge estimation for the Vanadium Redox Flow Battery based on Extended Kalman filter using modified parameter identification

ABSTRACT Vanadium Redox Flow Battery (VRFB) is widely utilized in energy storage due to its excellent characteristics. Credible knowledge of the state of charge (SOC) is a pre-condition for the effective health management of batteries. The SOC estimation depends on the second-order RC equivalent circuit model (ECM) parameters identified by the forgetting factor recursive least squares (FF-RLS). Considering the time-varying characteristics of the model parameters, an adaptive forgetting factor recursive least squares (AFF-RLS) based on the gradient descent method is proposed to identify the EMC parameters. The forgetting factor can be obtained based on the error between the terminal voltage measurement and terminal voltage estimation. The proposed joint estimator has been verified by performing charge and discharge experiments for VRFB single cell. The mean error, maximum estimated error and root mean square error of SOC under 6 A pulse discharging current are 1.38 × 10−3, 3.76 × 10−5, and 1.38 × 10−5. The result indicates that AFF-RLS is robust against outliers of model parameters and improve the anti-interference of EKF. In addition, this paper finds that the different value of learning rate can affect the sensitive and anti-interference of EKF.

Polytetrafluoroethylene Modified Nafion Membranes by Magnetron Sputtering for Vanadium Redox Flow Batteries

Commercial Nafion membranes have been widely used for vanadium redox flow batteries (VRFB) but with relatively low ion selectivity. A chemical method is commonly employed to modify the organic membranes, whereas physical approaches are rarely reported in view of less compatibility with the organic species. In this study, an ultrathin polytetrafluoroethylene (PTFE) film of less than 30 nm is deposited onto the Nafion substrates by radio frequency magnetron sputtering to form PTFE@Nafion composite membranes. The PTFE layer of hydrophobic and inert feature enhances the dimensional stability and the ion selectivity of the Nafion membranes. The VRFB single cell with an optimized composite membrane exhibits a better self-discharge property than that of the Nafion 212 (i.e., 201.2 vs. 18.6 h), due to a higher ion selectivity (i.e., 21.191 × 104 vs. 11.054 × 104 S min cm–3). The composite membranes also show better discharge capacity retention than the Nafion 212 over the entire 100 cycles. The results indicate that the magnetron sputtering is an alternative and feasible route to tailor the organic membranes via surface modification and functionalization.

Simulasi Pengaruh Ketebalan Elektroda Terhadap Potensial Elektrolit dan Molaritas Spesies Vanadium Redox Flow Battery (VRFB)

Vanadium Redox Flow Battery (VRFB) is a relative new type of secondary battery that has ability as Energy Storage System (ESS) for renewable energy power plants such as solar cell. 2D VRFB single cell was modeled using COMSOL Multiphysics software and simulated numerically by using Nernst-Planck, secondary current distribution, tertiary current distribution and Buttler-Volmer principle. Simulation was conducted at 293,15 K, with electrode thickness variations of 1 mm (L1), 2 mm (L2), 3 mm (L3), 4 mm (L4) and 5 mm (L5). According to the simulation results, it is known that electrolyte potential in negative electrode is higher than positive electrode and the distribution is tend to decrease for all variations. Surface concentration near collector is dominant during charging compare inlet position and the reverse phenomenon occur during discharging process. VRFB L1 show weak performance and VRFB L2 show the best performance in term of electrolyte potential and species molarity in the electrode surface.

Surface‐engineered Nafion/CNTs nanocomposite membrane with improved voltage efficiency for vanadium redox flow battery

AbstractA carbon nanotubes (CNTs) reinforced Nafion membrane (CNT/N) with reduced interfacial resistance was prepared and served as a promising ion exchange membrane for vanadium redox flow batteries (VRFBs). The reinforcement of CNT effectively enhanced the tensile properties of Nafion membranes. The electrochemical properties of CNT/N membrane were analyzed by electrochemical impedance spectroscopy (EIS) and fitted with an analytical model to investigate the ionic and interfacial resistances. The EIS measurement reveals that the exposed CNT on the composite membrane surface significantly reduced the interfacial resistance of the membrane. The VRFB single cell performance of the CNT/N shows higher voltage efficiency (93% vs 89%) and energy efficiency (86% vs 83%) than recast Nafion at a current density of 40 mA cm−2. The cycling stability measurement showed that the discharge capacity retention of the VRFB cell equipped with CNT/N was greatly enhanced. The results suggest that the incorporation of CNT in ion exchange membrane is an effective approach for achieving lower membrane bulky and interfacial resistance, and thus, improving capacity retention, voltage, and energy efficiency.

Crosslinked sulfonated poly (arylene ether sulfone)/sulfonated poly (vinyl alcohol) membrane formed by in situ casting and reaction for vanadium redox flow battery application

The in situ crosslinked membranes for vanadium redox flow battery (VRFB) application, prepared from sulfonated poly (arylene ether sulfone) (SPAES) and poly (vinyl alcohol) (PVA) as well as sulfonated poly (vinyl alcohol) (SPVA), are detailedly evaluated in this paper. The results of scanning electron microscope (SEM) and X-ray diffraction (XRD) reveal that the cross reaction improved the compatibility between SPAES and SPVA, and reduced the crystallinity in the membrane. The highly homogeneous dispersed structures of the crosslinked membrane are found under the electronic microscope. The crosslinked structure effectively reduces the water uptake, swelling ratio, and vanadium ion permeability, and more importantly improved the mechanical performance and stability of the membranes. Due to the introduction of sulfonic groups into the PVA, the proton selectivity of SPAES crosslinked SPVA (SPAES-C-SPVA) reaches to 3.37×105 S min cm−3 which is 8 times higher than that of Nafion117. The VRFB single cell tests show that the cell assembled with SPAES-C-SPVA membrane significantly displays higher energy efficiency (70.1% vs 60.9% at 100 mA cm−2) and much longer self-discharge time than Nafion117 (98 h vs 41 h). After 100 charge-discharge cycling test, the crosslinked membrane still maintains excellent stability. The above results indicate the crosslinked membranes could be a promising candidate used in VRFB applications.

Finely controlled swelling: A shortcut to construct ion-selective channels in polymer membranes

Finely controlled swelling is reported as a universal and facile method to construct abundant ion-selective channels in polymer membranes. In this work, excellent ion-selectivity of H+/Vn+ is achieved by using a swelling-solidification method based on sulfonated poly(ether sulfone) (SPES) with only 20% sulfonation degree. The swelling degree of the membranes is well controlled by adjusting the solvent ratio of the swelling agent and the swelling time to achieve desired proton conductivity and Vn+ permeability. The as-prepared membrane is successfully applied as highly selective separator for vanadium redox flow batteries (VRFBs). Key properties such as morphology, proton conductivity, vanadium permeability, ion selectivity, tensile strength and durability are comprehensively characterized and studied. Besides, the performance of the single cells assembled with the swollen SPES membranes are tested and compared with the commercial Nafion®212 membrane. The results indicate that, the VRFB single cell with the swollen SPES membrane shows high energy efficiency exceeding 83% at 100 mA cm−2 and excellent cycling stability over 290 charge-discharge cycles. The capacity retention of the cell with the SPES membrane even reaches 98.21% after 100 cycles at 100 mA cm−2, 24.06% higher than that of Nafion®212 membrane.

Energy Efficiency Improvement of Vanadium Redox Flow Battery by Integrating Electrode and Bipolar Plate

An integral electrode-bipolar plate assembly, which is composed of electrode, conductive adhesive film (CAF) and bipolar plate, has been developed and evaluated for application with a vanadium redox flow battery (VRB) to decrease contact resistance between electrode and bipolar plate. The CAF, made of EVA (ethylene-vinyl-acetate) material with carbon black or CNT (Carbon Nano Tube), is applied between the electrode and the bipolar plate to enable an integral assembly by adhesion. In order to evaluate the integral assembly of VRB by adhesive film, the resistivity of integral assembly and the performance of single cell were investigated. Thus, it was verified that the integral assembly is applicable to redox flow battery. Through resistance and contact resistance of bare EVA and CAF films on bipolar plate were changed. Among the adhesive films, CAF film coated with carbon black showed the lowest value in through resistance, and CAF film coated with CNT showed the lowest value in contact resistance, respectively. The efficiency of VRB single cell was improved by applying CAF films coated with carbon black and CNT, resulting in the reduced overvoltage in charging process. Therefore, the energy efficiency of both CAF films, about 84%, were improved than that of blank cell, about 79.5 % under current density at 40 mA cm−2. The energy efficiency of the two cells were similar, but carbon black coated CAF improved the coulomb efficiency and CNT coated CAF improved the voltage efficiency, respectively.

A novel double branched sulfonated polyimide membrane with ultra-high proton selectivity for vanadium redox flow battery

The development of a vanadium redox flow battery (VRFB) urgently requires proton conductive membranes with high proton selectivity. Herein, we synthesized branched anhydride monomer 1,3,5-tris(3,4-anhydride phenoxy)benzene using a three-step method. Furthermore, we prepared successfully novel double branched sulfonated polyimide (dbSPI) membranes with ultra-low vanadium ion permeability (0.4–2.5×10−8 cm2 min−1), one or two orders of magnitude lower than that of the Nafion 212 membrane (7.25×10−7 cm2 min−1). Among all membranes, the dbSPI-50 membrane with the highest proton selectivity (23.25×105 S min cm−3), approximately 68 times higher than that of the Nafion 212 membrane, was chosen for assembling of a VRFB single cell. Because of its ultra-low vanadium ion permeability, the dbSPI-50 membrane exhibited much longer self-discharge time (97 h) and superior capacity retention (82.6–91.3%) compared with the Nafion 212 membrane. Moreover, both coulomb efficiency (99.6–98.5%) and energy efficiency (69.3–80.1%) of the dbSPI-50 membrane are higher than those of the Nafion 212 membrane after 300 times charge-discharge tests at a current density from 200 to 100 mA cm−2. All these results demonstrate that the optimized dbSPI-50 membrane is a promising candidate for applications in VRFBs.

Novel amphoteric ion exchange membranes by blending sulfonated poly(ether ether ketone) with ammonium polyphosphate for vanadium redox flow battery applications

AbstractA novel amphoteric ion exchange membrane for vanadium redox flow battery (VRFB) was explored by blending sulfonated poly(ether ether ketone) (SPEEK) and ammonium polyphosphate (APP). The high‐stability flame retardant of cross‐linked APP with a large number of NH4+ groups was first introduced into SPEEK membrane. It was observed that the addition of APP with special structure could achieve a good balance between proton conductivity and vanadium ions permeability. The abundant NH4+ in APP could block the penetration of vanadium ions by Donnan/Manning exclusion effect and ionic crossing networks due to the ionic bonds between cation and anion groups, and specially a small amount of APP within 5% could remarkably improve the proton conductivity of pristine SPEEK membrane might be ascribed to the unique fast proton transport channels formed by hydrogen bond networks and particular micro‐phase separation as a result of interaction between SPEEK and APP. When 5% APP was blended, the SPEEK/APP‐5% (S/APP‐5%) amphoteric membrane showed a higher selectivity of 20.87 × 104 S min/cm3 (with a good proton conductivity of 0.075 S/cm and a lower VO2+ permeability of 3.45 × 10−7 cm2/ min) and presented better thermal and chemical stability compared to Nafion115 and SPEEK membranes. The VRFB single cell assembled with S/APP‐5% amphoteric membrane exhibited more excellent performance than that of Nafion115 and pristine SPEEK membranes, which revealed a higher coulombic efficiency of 96.3%–98.3%, comparable voltage efficiency of 88.4%–78.7% and higher energy efficiency of 85.1%–77.4% from 40 to 80 mA/cm2, respectively, and showed relatively good stability of the efficiency up to 50 cycles at 60 mA/cm2. The results demonstrated that the designed S/APP amphiprotic membrane of outstanding selectivity, high battery efficiency, and good durability is a prospected VRFB separator.

Investigations on Effect of Fe on Vanadium Redox Reaction and Vanadium Redox Flow Battery Performance

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.

A cost-effective nafion/lignin composite membrane with low vanadium ion permeation for high performance vanadium redox flow battery

Here we report an eco-friendly Nafion/lignin composite membrane with excellent performance for vanadium redox flow battery (VRFB). Through facile solution casting, the Nafion/lignin composite membrane presents the merits of high conductivity and low vanadium ions permeability. Under high current density (120 mA cm−2), the VRFB single cell with the optimized Nafion/lignin composite membrane exhibits superior performance metrics to commercial counterpart: high Coulombic efficiency (~97.4%) and energy efficiency (~82.7%), and prominent cyclability with slower capacity decay for ~52.8% capacity retention after 1000 cycles (vs. Nafion 212 membrane ~34.8% after only 150 cycles). More importantly, compared with the pristine Nafion 212 membrane, the cost of the composite membrane is reduced sharply by reducing the usage amount of Nafion polymer, while maintaining the membrane stability. Thus, the Nafion/lignin composite membrane is one of the competitive candidates for VRFB towards commercialization.

Enhanced performance and durability of composite bipolar plate with surface modification of cactus-like carbon nanofibers

To sufficiently improve the electrical conductivity of composite bipolar plates is crucial for their application in fuel cells and redox flow batteries. Eliminating the resin-rich layer on the surface of composite bipolar plates turns to be an effective approach. In this work, graphite/resin composite bipolar plates with advanced performance and durability are obtained after surface treatment with cactus-like carbon nanofibers which grow from the catalyst cores and stretch out to form cactus-like structures. Morphology and structure of the carbon nanofibers, as well as the morphology, electrical conductivity, electrochemical properties and charge-discharge performance of the bipolar plates in a vanadium redox flow battery (VRFB) are investigated. Results show that surface treatment with graphitized cactus-like carbon nanofibers significantly enhance the conductivity of the composite plates which reaches as high as 198.7 S cm−1, and the area specific resistance can be reduced to 25.4 mΩ cm2. The VRFB single cell with the modified bipolar plate exhibits very high energy efficiency of 86.28%, at 100 mA cm−2, and shows excellent durability in charge-discharge cycling test. With the superior properties above, composite bipolar plates after surface treatment with cactus-like carbon nanofibers are promising candidates for VRFBs.

Advanced Sulfonated Poly(Ether Ether Ketone)/Graphene-Oxide/Titanium Dioxide Nanoparticle Composited Membrane with Superior Cyclability for Vanadium Redox Flow Battery.

The purpose of this study was to improve the repulsion ability of sulfonated poly(ether ether ketone) (SPEEK) membrane for the vanadium ions crossover. For this purpose graphene oxide (GO) nanosheet and titanium dioxide (TiO₂) nanoparticles were employed into the polymer matrix to prepare SPEEK/GO/TiO₂ hybrid membrane via solution-casting method for vanadium redox flow battery (VRFB). The morphology, permeability of vanadium ions and device performance of asprepared membrane were investigated and discussed. It was observed that with the barrier block effect by the filler, the VRFB single cell with the optimized SPEEK/GO/TiO₂ hybrid membrane exhibited high coulombic efficiency (~99%), excellent energy efficiency (~85%) and vigorous cyclability (~97.2% capacity retention after 100 cycles). Moreover, the VRFB cell with this blend membrane showed lower vanadium ions permeability than Nafion 212 or pure SPEEK membranes. These results demonstrated that the comprehensive properties of hybrid membrane have been remarkably improved comparing to pristine SPEEK which suggested that the hybrid membrane was applicable for VRFB energy storage system.

Hybrid Membranes Dispersed with Superhydrophilic TiO2 Nanotubes Toward Ultra‐Stable and High‐Performance Vanadium Redox Flow Batteries

AbstractThe vanadium redox flow battery (VRFB) is a large‐scale energy storage technique and has been regarded as a promising candidate to integrate intermittent renewable energy with the grid. Its long‐term stability has so far been limited by the core component, an ion exchange membrane with low ion selectivity. Here a hybrid membrane with superhydrophilic TiO2 nanotubes dispersed in a Nafion matrix is reported. The VRFB single cell with the hybrid membrane exhibits an impressive performance with high coulombic efficiency (CE, ≈98.3%) and outstanding energy efficiency (EE, ≈84.4%) at 120 mA cm−2, which is higher than that of the commercial Nafion 212 membrane (CE, ≈94.5%; EE, ≈79.2%). More importantly, the cell maintains a discharge capacity of ≈55.7% after 1400 cycles (over 518 h), in obvious contrast to that of ≈20% after only 410 cycles for the one using commercial Nafion 212. This is attributed to the high ion selectivity of the hybrid membrane, because of, 1) the blocked and elongated ion diffusion pathway induced by the dispersed nanotubes and 2) binding and alignment of the sulfonic acid groups on nanotube surface. The high‐performance membranes may also find important applications in other fields, such as fuel cells, dialytic batteries, and water treatment.

Sulfonated poly(ether ether ketone)/polyimide acid-base hybrid membranes for vanadium redox flow battery applications

A series of acid-base hybrid membranes based on sulfonated poly(ether ether ketone) (SPEEK) and polyimide (PI) were prepared and used for vanadium redox flow battery (VRFB). The proton conductivity, vanadium ion permeability, ion-exchange capacity (IEC), and water uptake of the SPEEK/PI (S/PI) hybrid membranes were tested. It was observed that the S/PI hybrid membrane exhibited lower vanadium ion permeability and higher selectivity than SPEEK and Nafion115 membranes. The VRFB single cell with S/PI hybrid membranes exhibited a better overall performance, such as higher coulombic efficiency (CE) of 96%, higher energy efficiency (EE) of 84%, and longer discharge time (DT) of 41 h, than that with the Nafion115 membrane (CE 92%, EE 82%, and DT 27 h) at 60 mA cm−2. The results showed that the S/PI acid-base hybrid membrane is a potential candidate for VRFB.

Novel Sulfonated Aromatic Polymer Membranes for Breaking the Coulombic and Voltage Efficiency Trade-Off Limitation in Vanadium Redox Flow Battery

Vanadium redox flow batteries (VRFBs) have received extensive attention due to its attractive features for the large-scale energy storage because of their design flexibility, absence of intercalation/deintercalation and stress build-up in electrodes, and safe operation. One of the most critical components in VRFB is an ion exchange membrane which can prevent cross mixing of the multivalence vanadium ions, while it still allows the transport of protons (or anions) to complete the circuit. However, the trade-off relationship of proton exchange membrane limits the energy efficiency (EE) of the VRFB due to low coulombic efficiency (CE) caused by vanadium ion cross over through the membrane or high membrane resistances. Anion exchange membrane suffers from poor chemical stability and low voltage efficiency due to low ion conductivity. In this study, we synthesized a series of novel sulfonated aromatic polymer membranes having high proton conductivity as well as selectivity which surpass the proton conductivity-selectivity trade-off relationship. The effect of the pendent group structure on the membrane ion selectivity and battery performance was investigated. It was found that the incorporation of aromatic structures on the pendent groups effectively improved the proton conductivity and selectivity. The membrane with aromatic pendent groups (BPAr-F4) shows 3 times higher vanadium/proton ion selectivity than Nafion 212. As a result, the VRFB single cell equipped with the 80um-thick BPAr-F4 membrane shows significantly better performance in comparison to the cell with Nafion membranes, as high as 97.18% CE, 95.91% VE and 93.20% EE (vs. 75.14% CE, 96.12% VE and 72.22% EE of Nafion 212; 91.30% CE, 95.63% VE and 87.62% EE of Nafion 117) at 20 mA/cm2. Small-angle X-ray scattering (SAXS) studies indicates that their unique morphologies with narrow aqueous ionic domains which are well organized with acidic -SO3H groups results in the high proton selectivity and VRFB performance.

Effects of covalent bond interactions on properties of polyimide grafting sulfonated polyvinyl alcohol proton exchange membrane for vanadium redox flow battery applications

The preparation and characterization of novel carboxyl-containing polyimide (PID) grafting sulfonated polyvinyl alcohol (SPVA) copolymer membranes applied in vanadium redox flow batteries (VRFBs) are presented in this work. The microphase structures of the copolymer membranes are identified by TEM and XRD. Other basic properties are characterized, particularly the oxidative and water stability. The copolymer membranes exhibit better chemical stability than normal SPIs with five-numbered rings due to the covalent bonds between PI and SPVA as well as highly dispersed microphase separated structure. The physico-chemical properties of the copolymer membranes, including IEC, water uptake, swelling ratio, proton conductivity and vanadium ion permeability are evaluated and compared with Nafion 117. The PID30-g-SPVA membrane has the highest proton selectivity (1.33 × 105 S min cm−3), being much higher than that of Nafion 117 (0.41 × 105 S min cm−3). In VRFB single cell tests, the cells assembled with copolymer membranes show higher coulombic efficiency (CE), higher energy efficiency (63.3% vs 74.3% at 90 mA cm−2) and much lower self-discharge rate than with Nafion 117. In addition, the copolymer membranes retain excellent stability after 100 cycles. All the results show that the copolymer membranes have promising prospects for VRFB applications.

Correlations of Through-Plane Cell Voltage Losses, Imbalance of Electrolytes, and Energy Storage Efficiency of a Vanadium Redox Flow Battery.

Correlations of the through-plane voltage losses in the vanadium redox flow battery (VRFB), changes in the posolyte (positive electrolyte) and negolyte (negative electrolyte) conditions, energy storage capacity, and coulomb efficiency were investigated under an operando charge-discharge process. Collective and quantitative in situ studies with Ag/AgCl reference-electrodes inserted into a VRFB single-cell and the state of charge measuring external open-circuit voltage half-cells revealed that the overall performance of a VRFB is predominantly governed by (i) relatively high internal resistance of the anode half-cell and (ii) conditions of the negolyte as the limiting electrolyte. The studies showed that the round-trip coulomb efficiency, which is commonly used for VRFB performance analysis, has limitations in expressing the energy storage efficiency and capacity decay phenomena under a consecutive charge-discharge process. In situ and concurrent analysis of the internal cell overpotential and the electrolytes condition can provide quantitative information on the relative contributions of the coulombic round-trip loss, concentration loss, vanadium crossover, and charge imbalance between the posolyte and negolyte for the overall efficiency and capacity decay phenomena. This operando analysis approach is highly effective for gaining an in-depth and collective understanding of the complex phenomena taking place in the VRFB system.