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.     

Robust proton exchange membrane for vanadium redox flow batteries reinforced by silica-encapsulated nanocellulose

High ion selectivity and mechanical strength are critical properties for proton exchange membranes in vanadium redox flow batteries. In this work, a novel sulfonated poly(ether sulfone) hybrid membrane reinforced by core-shell structured nanocellulose (CNC-SPES) is prepared to obtain a robust and high-performance proton exchange membrane for vanadium redox flow batteries. Membrane morphology, proton conductivity, vanadium permeability and tensile strength are investigated. Single cell tests at a range of 40–140 mA cm⁻² are carried out. The performance of the sulfonated poly(ether sulfone) membrane reinforced by pristine nanocellulose (NC-SPES) and Nafion® 212 membranes are also studied for comparison. The results show that, with the incorporation of silica-encapsulated nanocellulose, the membrane exhibits outstanding mechanical strength of 54.5 MPa and high energy efficiency above 82% at 100 mA cm⁻², which is stable during 200 charge-discharge cycles.     

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.     

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.     

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.     

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 containing densely alkyl sulfide sulfonated side chains for vanadium redox flow battery

Due to further increase the performance of aromatic sulfonated proton exchange membrane (PEM) and make it play a better role in vanadium redox flow battery (VRFB), a series of poly(aryl ether sulfone)s containing eight alkyl sulfide sulfonated side chains (8SPAES-xx) are designed and synthesized. Their molecular structure, phase morphology and some selective properties were investigated in detail, respectively. It is confirmed that 8SPAES-xx membranes have clear hydrophilic/hydrophobic phase separation morphology. These membranes with the ion exchange capacity values of 1.08–1.61 mmol/g exhibit excellent ionic conductivity as well as moderate water uptake and good dimensional stability, and their values are in the range of 25–96 mS/cm, 8–28% and 5–17% at 30 °C, respectively. Among them, the proton conductivity of 8SPAES-12 membrane is 82 mS/cm at 30 °C, which exceeds the ionic conductivity of Nafion 117 (79 mS/cm). The membrane also shows high ion selectivity and excellent battery performance. At current density of 60 mA/cm², the highest energy efficiency of VRFB with 8SPAES-12 membrane is 87.3%, which is higher than that of Nafion 117 (83.8%). Furthermore, the efficiency of VRFB with 8SPAES-12 membrane remains good cycle stability.     

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.     

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.     

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.     

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.     

Development of three-dimensional model for the analysis of the mass transport in vanadium redox flow batteries

It is practical to equip the renewable energy system with the vanadium redox flow battery (VRFB) to improve energy utilization efficiency. A steady-stated, three-dimensional model is developed to study the flow and mass transfer in VRFB with serpentine flow field (SFF), and the corresponding experiments are also executed. The effect of the inlet flow rate on VRFB is analyzed by simulating the charge-discharge process, in which the uniformity factors, pressure drop, overpotential, protons concentration, and the battery efficiency are compared as indicators. Two split-SFFs are proposed to improve the pressure drop in the conventional SFF. The results show the efficiency of VRFB increase with the increase of the flow rate, but when the flow rate is higher than a critical flow rate, the performance on the VRFB is no longer sensitive to the flow rate. Compared to SFF, SFF(SY) performs well on pressure drop but poorly on discharge.     

Dynamic modelling of the effects of ion diffusion and side reactions on the capacity loss for vanadium redox flow battery

The diffusion of vanadium ions across the membrane along with side reactions can have a significant impact on the capacity of the vanadium redox flow battery (VFB) over long-term charge-discharge cycling. Differential rates of diffusion of the vanadium ions from one half-cell into the other will facilitate self-discharge reactions, leading to an imbalance between the state-of-charge of the two half-cell electrolytes and a subsequent drop in capacity. Meanwhile side reactions as a result of evolution of hydrogen or air oxidation of V²⁺ can further affect the capacity of the VFB. In this paper, a dynamic model is developed based on mass balances for each of the four vanadium ions in the VFB electrolytes in conjunction with the Nernst Equation. This model can predict the capacity as a function of time and thus can be used to determine when periodic electrolyte remixing or rebalancing should take place to restore cell capacity. Furthermore, the dynamic model can be potentially incorporated in the control system of the VFB to achieve long term optimal operation. The performance of three different types of membranes is studied on the basis of the above model and the simulation results together with potential operational issues are analysed and discussed.     

Technical and economic comparison of grid supportive vanadium redox flow batteries for primary control reserve and community electricity storage in Germany

Primary control reserve and maximising self‐consumption are currently two of the main applications for large‐scale battery storage systems. Although being currently the most profitable application for large‐scale batteries in Germany, storage systems applying primary control reserve have not been implemented in a grid supportive manner in distribution grids yet. Despite a current unfavourable regulatory framework and reimbursement scheme for community electricity storages in Germany, they are potentially more profitable than residential storages, which is mainly due to their economy of scale, and thus they may become the major large scale battery application in the future. The two applications: primary control reserve and maximising self‐consumption, are combined with a grid supportive behaviour by providing reactive power control and/or peak shaving and are fitted to a vanadium redox flow battery prototype, which is installed in a distribution grid in southern Germany. Based on measured data from the prototype, two battery models for two different time resolutions (1s, 1min) are presented in detail along with their respective operation models. The operation strategy model for primary control reserve comprises the so‐called degrees of freedom used to reduce the energy needed to recharge the battery. The operation strategy to maximise self‐consumption is based on a persistence forecast. The model for the operation strategy for a grid supportive primary control reserve was validated in a field test revealing a relative error of 2.5 % between the simulated and measured state of charge of the battery for a multi‐week time period. The technical assessment of both applications shows that the use of the degrees of freedom can reduce the energy to recharge the battery by 20 %; and in the case of self‐consumption, the curtailment losses can be kept under 1 %. The economic assessment, however, indicates that even for the most promising primary control reserve case, the investment costs of vanadium redox flow batteries must be reduced by at least 30 % in order to break even. Finally, the encouraging key finding is that the negative impact of a grid supportive behaviour, additionally to its primary purpose, is less than 1 % of the revenues. This may encourage distribution grid and battery operators to consider the integration of large scale batteries in distribution grids as part of the solution of a rising share of a decentralised renewable energy generation.     

Progress in poly(phenylene oxide) based cation exchange membranes for fuel cells and redox flow batteries applications

In fuel cell technologies, low-temperature proton exchange membrane fuel cells (LT-PEMFC), high-temperature proton exchange membrane fuel cells (HT- PEMFC), and direct methanol fuel cells (DMFC) are gained significant attention as a promising energy system for practical applications. The developments of cost-effective membrane materials with excellent physicochemical properties are indispensable for replacing the high cost of commercial membranes and achieving the higher performance of fuel cell systems. This review focuses on the developments and modifications of cost-effective poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) as a cation exchange membrane for LT-PEMFC, HT- PEMFC and DMFC. Notably, this review bridges the understanding of PPO based membranes, current advancements, structure, physicochemical properties and fuel cell performances. Progressive developments and a systematic overview of PPO-based membrane developments are explained in detail in terms of functionalization, blend, composite, acid-base, cross-linking, copolymerization, coated and reinforcement. Moreover, the changes in physicochemical properties and fuel cell performances in the membrane are deeply reviewed. Additionally, the utilization of PPO based membranes in different kinds of redox flow battery systems are reviewed. Overall, this review provides an exclusive vision and perspectives to develop the PPO based advanced, cost-effective, and high-performance membranes for fuel cell technologies and redox flow battery systems.     

Preparation and characterization of imidazolium-functionalized poly (ether sulfone) as anion exchange membrane and ionomer for fuel cell application

Imidazolium-functionalized anion exchange membranes (AEMs) for anion exchange membrane fuel cells (AEMFCs) were synthesized by functionalization of chloromethylated poly (ether sulfone) (PES) with 1-alkylimidazole. The properties of AEMs can be controlled by the degree of chloromethylation of PES. Moreover, with the increment of the alkyl line length on the imidazolium group, the water uptake, swelling ratio and solubility of AEMs increased, whereas the hydroxide conductivity declined. By dissolving AEMs in the mixture of ethanol and water, IM-based anion exchange ionomers (AEIs) can be obtained. Electrochemical studies revealed that the catalytic activities of Pt/C towards oxygen reduction and hydrogen oxidation in the presence of imidazolium-functionalized AEIs were almost the same with that of commercial quaternary ammonium-based ionomers. The fabricated AEM and AEI were utilized to assemble H₂/O₂ AEMFC, yielding a peak power density of ∼30 mW cm⁻² with open circuit potential larger than 1.0 V. The results obtained indicate that imidazolium-functionalized AEMs and AEIs may be candidates which are worth further investigation for the application in the AEMFCs.     

Composite membranes based on sulfonated poly(aryl ether ketone)s containing the hexafluoroisopropylidene diphenyl moiety and poly(amic acid) for proton exchange membrane fuel cell application

Composite membranes based on sulfonated poly(aryl ether ketone)s containing the hexafluoroisopropylidene diphenyl moiety and poly(amic acid) with oligoaniline in the main chain have been prepared and immersed in H₃PO₄ to obtain acid-doped composite films. As expected, the water uptake values and methanol permeability of the composite membranes decrease with the increase of the weight fraction of PAA in the membrane matrix. Notably, the SPEEK-6F/PAA-15 shows a water uptake of 13.2% and a methanol permeability of 0.9 × 10⁻⁷ cm² s⁻¹, which are much lower than those of the Nafion (28.6% and 15.5 × 10⁻⁷ cm² s⁻¹, respectively). Although the proton conductivities decrease after the addition of PAA, higher selectivity values are obtained with the composite membranes. Therefore, the SPEEK-6F/PAA blend membranes, with the improved proton conductivity, methanol resistance and good thermal stability, can be used as a good alternative for proton conductive membranes with potential application in proton exchange membrane fuel cells (PEMFCs).