Low Vanadium Ion Permeability Research Articles

A soluble highly-sulfonated polybenzimidazole with high molecular weight as membrane for vanadium flow battery with enhanced performance and long-term stability

A series of novel sulfonated polybenzimidazole (SPBI) membranes containing flexible double ether segments, having high ion exchange capacity (IEC), good solubility as well as high molecular weight, are designed for vanadium flow batteries (VFBs). The bis-ether structure of self-synthesized sulfonic acid monomer, 4,4'-((2-sulfo-1,4-phenylene)bis(oxy))dibenzoic acid, improves the solubility of produced SPBI and the microphase separation structure of prepared membrane by disrupting the regularity of the backbone through a non-planar structure and attenuating the interchain stacking effect of the polymer backbone. Such a highly reactive bis-ether monomer also leads to high molecular weights (2.37−2.82 dL g−1). The copolymerization using another sulfonic acid monomer, Monosodium-5-Sulfoisophthalate monomer, leads to a high IEC of up to 2.08 mmol g−1. As a result, the SPBI membrane shows a low area resistance of 0.21 Ω cm2 and a low vanadium ion permeability of 3.36 × 10−9 cm2 s−1. The VFB cell employing SPBI membrane achieves a very high energy efficiency exceeding 82% at 200 mA cm−2. In addition, an ultra-long lifetime of over 3400 cycles is accomplished.

Fabrication of Tri-Directional Poly(2,5-benzimidazole) Membrane Using Direct Casting for Vanadium Redox Flow Battery.

In vanadium redox flow batteries (VRFBs), simultaneously achieving high proton conductivity, low vanadium-ion permeability, and outstanding chemical stability using electrolyte membranes is a significant challenge. In this study, we report the fabrication of a tri-directional poly(2,5-benzimidazole) (T-ABPBI) membrane using a direct casting method. The direct-cast T-ABPBI (D-T-ABPBI) membrane was fabricated by modifying the microstructure of the membrane while retaining the chemical structure of ABPBI, having outstanding chemical stability. The D-T-ABPBI membrane exhibited lower crystallinity and an expanded free volume compared to the general solvent-cast T-ABPBI (S-T-ABPBI) membrane, resulting in enhanced hydrophilic absorption capabilities. Compared to the S-T-ABPBI membrane, the enhanced hydrophilic absorption capability of the D-T-ABPBI membrane resulted in a decrease in the specific resistance (the area-specific resistance of S-T-ABPBI and D-T-ABPBI membrane is 1.75 and 0.98 Ωcm2, respectively). Additionally, the D-T-ABPBI membrane showed lower vanadium permeability (3.40 × 10-7 cm2 min-1) compared to that of Nafion 115 (5.20 × 10-7 cm2 min-1) due to the Donnan exclusion effect. Owing to the synergistic effects of these properties, the VRFB assembled with D-T-ABPBI membrane had higher or equivalent coulomb efficiencies (>97%) and energy efficiencies (70-91%) than Nafion 115 at various current densities (200-40 mA cm-2). Furthermore, the D-T-ABPBI membrane exhibited stable performance for over 300 cycles at 100 mA cm-2, suggesting its outstanding chemical stability against the highly oxidizing VO2+ ions during practical VRFB operation. These results indicate that the newly fabricated D-T-ABPBI membranes are promising candidates for VRFB application.

Long side-chain imidazolium functionalized poly(vinyl chloride) membranes with low cost and high performance for vanadium redox flow batteries

Developing polymer membranes with low price, superior ion conduction and good vanadium ion transport resistance is currently a hot topic in vanadium redox flow batteries (VRFBs). In the present work, long side-chain imidazolium functionalized poly(vinyl chloride) (PVC) membranes are fabricated to provide a versatile strategy for the ion transport and selectivity trade-off in VRFBs. PVC is employed as the membrane matrix owing to its low price, high tensile strength and natural chlorine groups. 1-(3-Aminopropyl) imidazole (APIm) is grafted into PVC via a facile nucleophilic reaction, which provides the basic groups as the interaction sites for sulfonic acid, resulting in low area resistance. Meanwhile, due to the electrostatic repulsion effect of positively charged imidazolium cations between vanadium ions, APIm-PVC membranes display low vanadium ion permeability. Consequently, the 80 %APIm-PVC membrane has both low area resistance of 0.268 Ω∙cm2 and low vanadium ion permeability of 9.0 × 10−8 cm2∙min−1 simultaneously, achieving a 5 times higher ion selectivity (2.65 × 105 S min cm−3) than Nafion 115 (0.52 × 105 S min cm−3). The VRFB equipped with the 80 %APIm-PVC membrane shows higher battery efficiencies than the cell with Nafion 115 ranging from 100 to 160 mA∙cm−2, and also possesses excellent cycle constancy, indicating that low-cost x%APIm-PVC membranes have a great application potentiality for VRFBs.

Perfect capacity retention of all-vanadium redox flow battery using Nafion/polyaniline composite membranes

Crossover of vanadium ions through proton conducting membranes in all-vanadium redox flow batteries (VRFBs) causes considerable engineering problems and deteriorates VRFB performance by reducing capacity retention over cycles. In this study, we have succeeded in fabrication of multi-layered polyaniline/Nafion (PANI-Nafion) composite membranes that exhibit a very low vanadium ion permeability and, therefore, can significantly improve VRFB performance by preserving electric capacity almost perfectly. The multi-layered PANI-Nafion membrane is fabricated by sequentially coating a PANI layer followed by Nafion layer on Nafion 115 membrane using a dip-coating method. The fabricated membrane outperforms pristine Nafion 115 membrane in terms of VRFB performance by exhibiting a very high coulombic efficiency of ∼ 99 % and an energy efficiency of ∼ 91 % at 50 mA cm−2. Particularly, it shows a remarkable improvement in durability and capacity retention with a 100 % discharge capacity retention over 100 cycles of charge/discharge operation as compared with that of pristine N115 (63 %). In-depth analyses have been conducted to characterize the PANI-Nafion composite membrane and to understand the mechanism of how it enhances the performance of VRFB. This study has shown a possibility to make VRFBs having a perfect capacity retention using PANI-coated composite membranes, which may help expedite full-fledged commercialization of VRFBs.

Anionic conductive group tunable amphoteric polybenzimidazole ion conductive membrane for vanadium redox flow battery

Anionic conductive group repels vanadium ions but is difficult to be incorporated into the selectively ionic conductive channels of the amphoteric ion conductive membrane with tunable ratio. Herein, the novel hydroxyl bromopropane anionic precursor side chain is proposed, in which the hydrophilic hydroxyl groups aggregate into ionic clusters with the sulfate ester cationic conductive side chains of polybenzimidazole via hydrogen bond during membrane casting. The subsequent conversion of the bromopropane precursor to quaternary ammonium can easily tune the anionic exchange capacity and integrate cations into the hydroxyl involved ionic conductive channels for selectively repelling vanadium ions. Compared with Nafion 212 membrane, the obtained amphoteric membrane exhibits much lower vanadium ion permeability (1.9 × 10−9 cm2s−1) and slightly lower area resistance (0.25 Ω cm2). Excellent performance of vanadium redox flow battery (coulombic efficiency, 99.2%, energy efficiency, 85.9% at 100 mA cm−2 and 0.31% discharge decay rate) is achieved, and efficiencies keep stable during the 500 cycles test. The performance of VRFB is surpass than that of most reported amphoteric ion conductive and Nafion 212 membranes.

Pyridine-extended proton sponge enabling high-performance membrane for flow batteries

The development of high-performance and low-cost membrane is of significance towards vanadium flow battery (VFB). The membrane usually suffer from a trade-off challenge between conductivity and selectivity especially in long-term operation of flow battery. Herein, using a proton sponge drived by the proton absorption capacity of extended pyridine groups, a highly ion-selective membrane is fabricated. During acid-assisted doping behaviors, transport nanochannels with highly proton-selective conductivity are constructed based on well-defined nanophase separation structure. The protonated nanochannels display effectively acidic absorption during hydration process for enabling fast proton conduction (a low area resistance of 0.33 Ω cm2) and repelling vanadium ion (a low vanadium ion permeability of 3.04 × 10−10 cm2 s−1). Consequently, prepared membrane with proton sponge exhibits remarkable performance of high energy efficiency of 85.5%, together with excellent chemical durability over 1000 charge-discharge cycles (more than 1100 h) at 120 mA cm−2 in VFB application. These notable properties make prepared membrane as a potential alternative used in large-scale energy storage systems.

High-Performance and Low-Cost Membranes Based on Poly(vinylpyrrolidone) and Cardo-Poly(etherketone) Blends for Vanadium Redox Flow Battery Applications

Energy storage systems have aroused public interest because of the blooming development of intermittent renewable energy sources. Vanadium redox flow batteries (VRFBs) are the typical candidates owing to their flexible operation and good cycle durability. However, due to the usage of perfluorinated separator membranes, VRFBs suffer from both high cost and serious vanadium ions cross penetration. Herein, we fabricate a series of low-budget and high-performance blend membranes from polyvinylpyrrolidone (PVP) and cardo-poly(etherketone) (PEKC) for VFRB. A PEKC network gives the membrane excellent mechanical rigidity, while PVP endows the blend membranes with superior sulfonic acid uptake owing to the present N-heterocycle and carbonyl group in PVP, resulting in low area resistance. Meanwhile, blend membranes also display low vanadium ion permeability resulting from the electrostatic repulsion effect of protonated PVP polymer chains towards vanadium ions. Consequently, the 50%PVP-PEKC membrane has a high ionic selectivity of 1.03 × 106 S min cm−3, while that of Nafion 115 is nearly 17 times lower (6.03 × 104 S min cm−3). The VRFB equipped with 50%PVP-PEKC membrane has high coulombic efficiencies (99.3–99.7%), voltage efficiencies (84.6–67.0%) and energy efficiencies (83.9–66.8%) at current densities of 80–180 mA cm−2, and possesses excellent cycle constancy, indicating that low-cost x%PVP-PEKC blend membranes have a great application potentiality for VRFBs.

Highly ion selective composite proton exchange membranes for vanadium redox flow batteries by the incorporation of UiO-66-NH2 threaded with ion conducting polymers

Proton exchange membranes (PEMs) with both high proton conductivity and low vanadium ion permeability are highly desirable for the vanadium redox flow batteries (VFBs) to achieve high performance. In this work, we prepared the UiO-66-NH2 threaded with polystyrene sulfonic acid (PSSA) (S-UiO) by in situ polymerization of the sulfonated monomer impregnated in the pores of UiO-66-NH2. The incorporation of S-UiO into the sulfonated poly(ether ether ketone) (SPEEK) increased the size of hydrophilic domains and the extent of phase separation for the SPEEK/S-UiO composite membranes, resulting in the significant enhancement of proton conductivity. Furthermore, S-UiO also acted as the barriers for the vanadium ion permeation. As a result, compared to that of the SPEEK control membrane, the SPEEK/S-UiO membrane with 15 wt% S-UiO exhibited 63% higher proton conductivity and 83% lower vanadium permeability. This led to much improved VFB performance for the composite membrane with an excellent EE of 83.9% under 120 mA/cm2, much higher than that of the SPEEK control membrane (77.3%).

Nafion/functionalized metal–organic framework composite membrane for vanadium redox flow battery

Although vanadium redox flow batteries hold great promise for energy storage systems, Nafion (being a conventional membrane material) requires improvement in terms of the crossover phenomenon. Herein, a nanoporous metal–organic framework (MOF) is applied as a membrane for vanadium redox flow batteries via addition of the MOF to a Nafion solution to prepare Nafion/MOF composite membranes via casting. Composite membranes are fabricated into a Nafion matrix by introducing small amounts of CAU-10-X (X = –OH, –CH 3 , –OS1, –OS2), an Al-based MOF synthesized with ligands having hydroxyl, methyl, and a combination of hydroxyl and sulfonic acid functional groups. The membrane physicochemical properties (such as water uptake, swelling ratio, ion exchange capacity, proton conductivity, and electrochemical properties from charge–discharge experiments on a single cell) are compared and evaluated. In 100 charge–discharge cycles of the single cell at a current density of 160 mA cm −2 , the composite membrane N/CAU-10-OS1 with 0.6 wt% dispersed MOF and a combination of hydroxyl and sulfonic acid functional groups exhibits capacity retention of 80.34% and energy efficiency of 77.56% at approximately 50 μm thickness. These values are better those achieved by commercial separation membrane Nafion 115 with 127 μm thickness tested under identical conditions and the proposed membrane is thus expected to be economically advantageous. • Using casting, Nafion/metal organic framework (N/MOF) composite membrane prepared. • Small amount of functionalized MOF in Nafion improves membrane performance in energy efficiency and cycle retention. • N/MOF membrane has lower vanadium ion permeability and thickness than Nafion 115. • N/MOF membrane shows high capacity retention (80.34%) at 100 cycles and 160 mA cm −2 .

An Advanced Composite Membrane for the All-Vanadium Redox Flow Battery

Redox flow batteries (RFBs) are a promising technology for grid scale stationary energy storage to complement renewable energy systems. These batteries have a relatively low energy density; however, they offer important advantages, including: long life-time; decoupled energy (arbitrarily large electrolyte volume) and power (electrode area); high round-trip efficiency; scalability and design flexibility; fast response; and low environmental impacts. These advantages make them superior to many energy storage technologies for stationary applications [1-4]. Among the various types of RFBs, vanadium RFBs (VRFBs) are an emerging technology for grid scale energy storage and the integration of renewable energy generation [5]. The membrane is a key component of a VRFB that separates the two half-cell electrolytes and prevents cross-mixing, while allowing the transport of ions during charge-discharge cycles [6]. The VRFB membrane should exhibit low vanadium ion permeability to minimize self-discharge, low cost, and long‐term chemical stability under normal operating conditions. A high proton conductivity and low vanadium ion crossover are known to improve the efficiency of VRFBs [6-7]. In this study, we present a novel composite Nafion based membrane that results in a significant increase in the VRFB performance. The composite membrane has been characterized for its chemical, structural, and thermal properties using appropriate analytical techniques. The battery performance was evaluated in a flow cell using a ‘zero-gap’ design with an electrode area of 5 cm2. The electrolytic solution, 1.6 M VOSO4 in 3 M H2SO4, was circulated through the cell. Thermally treated carbon papers were used as the cathode and anode electrodes. For charge-discharge experiments, a constant current density (10 to 80 mA cm−2) was applied with upper and lower voltage cut-offs of 1.65 and 0.8 V, respectively. The stability of the battery using the composite membrane was evaluated over 100 cycles. Figures 1 and 2 show the energy efficiency and capacity retention during 100 charge-discharge cycles. The results reveal that the energy efficiency was improved from 51% to 63% by using the composite membrane. In addition, the charge-discharge capacity and capacity retention improved by around 200% and 25%, respectively. This improvement can be attributed to a higher proton conductivity and lower vanadium permeability of the composite membrane.

Durable and highly selective ion transport of a sulfonated Diels Alder Poly(phenylene) for vanadium redox flow batteries

To improve the durability and ion selectivity of a hydrocarbon membrane for vanadium redox flow batteries (VRFBs) a polymer was rationally designed with an external hydrophobic shell and internal hydrophilic core. The polymer was designed to prevent hydrophilic polymer chain aggregation by functionalizing the external polymer shell with hydrophobic side chains and attaching acid moieties onto the polymer backbone. The hydrophobic shell is the result of pentafluorobenzoyl functionalization on the pendent aryl rings in a Diels Alder poly(phenylene) backbone. The internal polymer chain contains sulfonic acid moieties to impart hydrophilic character. The physical and electrochemical characteristics of the membrane were systematically studied and compared with widely used Nafion-212. The synthesized membranes were found to be superior to Nafion-212 not only in terms of lower vanadium ion permeability (4.25 × 10 −8 vs. Nafion-212: 6.0 × 10 −6 cm 2 min −1 ) but also higher VRFB performance (EE = 89% and vs. Nafion-212: 85.5% at 40 mA/cm 2 ). The membrane showed over three times higher capacity retention than Nafion-212 (0.075% vs. 0.25% capacity loss per cycle respectively) and excellent ex situ and in situ chemical stability. • Unique polymer design of an external hydrophobic shell and internal hydrophilic core. •50x higher proton/vanadium transport selectivity relative to Nafion-212. •Durable in 1.5 M VO 2 + for 60 days and 800 cycles in vanadium flow battery testing.

New crosslinked membranes based on cardo-poly(etherketone) and poly(ethylene imine) for the vanadium redox flow battery

• Low-cost membranes for VFRBs are synthesized by crosslinking PEI with PEKC. • x %PEI-PEKC membranes exhibt low AR and low VO 2+ permeability simultenously. • The 50%PEI-PEKC membrane displays superior VRFB efficiencies and cycling stability. Developing polymer membranes with low cost, high ionic conduction and low vanadium ion permeability simultaneously for the vanadium redox flow battery (VRFB) is a huge challenge to macromolecule design and engineering. Herein, a series of high-performance and low-cost macromolecule cross-linked membranes for VRFB are synthesized from cardo-poly(etherketone) (PEKC) and poly(ethylene imine) (PEI) through a straightforward lactamization reaction between the primary amines in PEI and lactones in PEKC. Owing to the acid-base interaction between amino groups of PEI and sulfonic acid (SA), crosslinked x %PEI-PEKC membranes exhibit excellent SA adsorption capability and low area resistance. Meanwhile, the formed macromolecule multi-crosslinked networks significantly improve the membrane-forming property of PEI and endow membranes with superior mechanical rigidity and low vanadium ion permeability simultaneously. Consequently, the 50%PEI-PEKC membrane exhibit a nearly 90 times higher ion selectivity of 24.71 × 10 5 S min cm −3 than Nafion 115. The assembled VRFB with 50%PEI-PEKC displays higher battery efficiencies than Nafion 115 at 60–160 mA cm −2 , which also possesses superior cycling stability at 100 mA cm −2 .

Poly(arylene ether sulfone) Membrane Crosslinked with Bi‐Guanidinium for Vanadium Flow Battery Applications

AbstractA membrane with low vanadium ion permeability and excellent electrochemical stability is crucial for a vanadium flow battery (VFB). Herein, a highly stable anion exchange membrane based on a bi‐guanidinium crosslinking quaternized poly(arylene ether sulfone) (PSF) is reported. The positively charged guanidinium groups can not only form continuous and smooth ion transport channels but also prevent the mutual penetration of vanadium electrolyte at positive and negative sides, respectively, based on the Donnan exclusion effect. Moreover, the crosslinking structure endows the membrane high mechanical strength to meet the requirements of the harsh environment of VFBs. As a result, the anion exchange membrane fabricated by the poly(arylene ether sulfone) crosslinked with bi‐guanidinium (BG‐PSF) shows an impressive VFB performance with a coulombic efficiency of 99.8% and an energy efficiency of above 85.8% at a current density of 80 mA cm−2, which is much higher than that of Nafion115. Furthermore, this battery can continuously work for over 200 cycles at a current density of 80 mA cm−2, indicating good stability of the membrane. Therefore, the rationally designed BG‐PSFs are promising for VFB applications.

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.

Clariant expands its capacity for emission control catalysts to meet growing global demand

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.

Alkyl Spacer Grafted ABPBI Membranes with Enhanced Acid-Absorption Capabilities for Use in Vanadium Redox Flow Batteries

Achieving high proton conductivity, low vanadium ion permeability, and high chemical stability using a single material remains a key challenge for hydrocarbon-based membranes for use in vanadium redox flow batteries (VRFBs). Herein, we report amorphous poly(2,5-benzimidazole) (ABPBI) membranes with alkyl spacers which can meet these requirements. Spacer-grafted poly(2,5-benzimidazole)s (ABPBIs) with different grafting ratios were synthesized via an N-substitution reaction and denoted as ABPBI-0, -10, -25, -40, and -50, respectively. The structure to battery performance relationship was investigated through spectroscopic and electrochemical analysis. As the grafting ratio increased, the microstructure of the ABPBI derivative membranes was transformed from a semicrystalline to an amorphous structure because of reduced chain packing, increasing H2SO4-absorption capability and consequently reducing area-specific resistances. Also, they exhibited lower vanadium permeabilities compared with Nafion 115 owing to Donnan exclusion effect. As a combined effect of these characteristics, the membranes outperformed Nafion 115 membrane with notably high coulomb efficiencies and energy efficiencies. Furthermore, a grafted ABPBI membrane showed stable battery cycling performance more than 500 cycles at 200 mAcm–2, and 1000 h continuous operation at 1C-rate, demonstrating excellent chemical stability against highly oxidizing VO2+ solution.

Novel sulfonated polyimide membrane blended with flexible poly[bis(4-methylphenoxy) phosphazene] chains for all vanadium redox flow battery

To improve SPI stability, poly[bis(4-methylphenoxy) phosphazene] (PMPP) was designed and synthesized using polyphosphazene's unique and strong flexibility. A novel sulfonated polyimide (SPI) composite membrane embedded with flexible PMPP chains was prepared for all vanadium redox flow battery applications. SPI/PMPP composite membrane characterizations, such as proton conductivity, vanadium ion permeability, stability, and single cell performance, were thoroughly investigated. PMPP's unique properties not only allow the membrane to exhibit excellent anti-hydrolysis and anti-oxidation stability, but also maintain high proton conductivity and low vanadium ion permeability (as low as 4.81 × 10−7 cm2 min−1). It can also form a rigidity-flexibility network with SPI to improve membrane mechanical properties. It can be deduced that introducing PMPP has achieved many things in one stroke. Results show that the hydrolysis and oxidation stabilities of SPI/PMPP membrane were significantly improved. The SPI/2% PMPP membrane exhibits higher columbic efficiency (CE: 97.52%), voltage efficiency (VE: 84.88%), and energy efficiency (EE: 82.21%) than that of the N212 membrane (CE: 94.97%, VE: 82.04%, EE: 76.99%) at a high current density of 100 mA cm−2. In addition, the charging capacity loss of a battery with a SPI/2% PMPP membrane has also significantly decreased by about 206.70 mAh after 110 cycles at 100 mA cm−2 with the same electrolyte volume compared to a pristine SPI. Meanwhile, the high temperature resistance of SPI/PMPP membrane also shows the possibility of being applied with fuel cells.

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.

Custom-made sulfonated poly (vinylidene fluoride-co-hexafluoropropylene) nanocomposite membranes for vanadium redox flow battery applications

Sulfonated poly (vinylidene fluoride-co-hexafluoropropylene) (SPVDF-co-HFP) based nanocomposite proton exchange membranes (PEM) are fabricated by simple solution casting method using polydopamine coated exfoliated molybdenum disulfide (PDA-MoS2) nanosheets as an alternative for Nafion® in vanadium redox flow batteries (VRFBs). PDA-MoS2 is synthesized by the etching of exfoliated MoS2 nanosheets with dopamine molecule by self-polymerization method. Various characteristic results clearly demonstrated that the incorporated PDA-MoS2 nanosheets homogeneously distributed into the SPVDF-co-HFP matrix and the presence of NH/NH2 group electrostatically interacts with SPVDF-co-HFP to form a strong acid-base pair and thus enhances the proton transport via Grotthuss type mechanism. Besides, the improvement in surface hydrophilicity provides the vehicle type conduction also. As a result, SPVDF-co-HFP/PM nanocomposite membranes showed higher proton conductivity in comparison with the pristine membrane. Especially SPVDF-co-HFP/PM-1 membrane demonstrated the excellent proton conductivity of 5.24 × 10−3 Scm−1 at 25 °C, lower vanadium-ion permeability of 1.05 × 10−8 cm2min−1 and highest membrane selectivity of 49.9 × 104 Scm−3min. On the other hand, vanadium-ion stability of the membrane increased by adding the PD-MoS2 content is attributed to their strong electrostatic attraction towards the polymer matrix. Overall results suggested that the SPVDF-co-HFP/PM-1 nanocomposite membrane is found to be a better alternative for commercially costly Nafion in VRFB applications.

A Cost-effective Nafion Composite Membrane as an Effective Vanadium-Ion Barrier for Vanadium Redox Flow Batteries.

Ion exchange membranes play a key role in all vanadium redox flow batteries (VRFBs). The mostly available commercial membrane for VRFBs is Nafion. However, its disadvantages, such as high cost and severe vanadium-ion permeation, become obstacles for large-scale energy storage. It is thus crucial to develop an efficient membrane with low permeability of vanadium ions and low cost to promote commercial applications of VRFBs. In this study, graphene oxide (GO) has been employed as an additive to the Nafion 212 matrix and a composite membrane named rN212/GO obtained. The thickness of rN212/GO has been reduced to only 41 μm (compared with 50 μm Nafion 212), which indicates directly lower cost. Meanwhile, rN212/GO shows lower permeability of vanadium ions and area-specific resistance compared to the Nafion 212 membrane due to the abundant oxygen-containing functional groups of GO additives. The VRFB cells with the rN212/GO membrane show higher Coulombic efficiencies and lower capacity decay than those of VRFB cells with the Nafion 212 membrane. Therefore, the cost-effective rN212/GO composite membrane is a promising alternative to suppress migration of vanadium ions across the membrane to set up VRFB cells with better performances.