A Low-Cost and High-Performance Sulfonated Polyimide Proton-Conductive Membrane for Vanadium Redox Flow/Static Batteries

A Novel Sulfonated Polyimide Composite Membrane Containing a Sulfonated Porous Material for All-Vanadium Redox Flow Batteries

To improve the battery efficiency and cycling stability of sulfonated polyimide (SPI), a polyphosphazene with built-in -SO3H moieties (PP-SO3H), which is a porous covalent organic framework (COF) material, is facilely synthesized by the polymeric combination of hexachlorocyclotriphosphazene (HCCP) and p-diaminobenzenesulfonic acid. Due to its tunable pore size and flexible molecular design, the COF material can address the trade-off between the conductivity and the ion permeability of ion exchange membranes well, thereby improving the ion selectivity of membranes. The experimental results show that the SPI/PP-SO3H composite membrane has an excellent conductivity (up to 114.8 mS cm-1); the ion selectivity of the SPI/2% PP-SO3H membrane is 11.69 × 104 S min cm-3, which is 2.18 times higher than that of the SPI base membrane. PP-SO3H also improves the SPI membrane's mechanical strength, and the effect of PP-SO3H on SPI intermolecular interactions is analyzed by surface electrostatic potential (ESP) theoretical calculations. The Coulombic efficiency (CE) of the SPI/2% PP-SO3H membrane is 98.92%, the energy efficiency (EE) is 84.1% at a current density of 100 mA cm-2, and the self-discharge time of the SPI/2% PP-SO3H membrane is 3.5 times compared with the SPI base membrane. To measure the cycling stability of the composite membrane, the SPI/2% PP-SO3H membrane is cycled in the VRFB for more than 400 cycles, which is more stable than that of the SPI base membrane. These results show that SPI/2% PP-SO3H composite membranes are viable for VRFB applications.

An Ultra-Low Self-Discharge Aqueous|Organic Membraneless Battery with Minimized Br2 Cross-Over

Batteries dissolving active materials in liquids possess safety and size advantages compared to solid-based batteries, yet the intrinsic liquid properties lead to material cross-over induced self-discharge both during cycling and idle when the electrolytes are in contact, thus highly efficient and cost-effective solutions to minimize cross-over are in high demand. An ultra-low self-discharge aqueous|organic membraneless battery using dichloromethane (CH2 Cl2 ) and tetrabutylammonium bromide (TBABr) added to a zinc bromide (ZnBr2 ) solution as the electrolyte is demonstrated. The polybromide is confined in the organic phase, and bromine (Br2 ) diffusion-induced self-discharge is minimized. At 90% state of charge (SOC), the membraneless ZnBr2 |TBABr (Z|T) battery shows an open circuit voltage (OCV) drop of only 42 mV after 120 days, 152 times longer than the ZnBr2  battery, and superior to 102 previous reports from all types of liquid active material batteries. The 120-day capacity retention of 95.5% is higher than commercial zinc-nickel (Zn-Ni) batteries and vanadium redox flow batteries (VRFB, electrolytes stored separately) and close to lithium-ion (Li-ion) batteries. Z|T achieves >500 cycles (2670 h, 0.5 m electrolyte, 250 folds of membraneless ZnBr2  battery) with ≈100% Coulombic efficiency (CE). The simple and cost-effective design of Z|T provides a conceptual inspiration to regulate material cross-over in liquid-based batteries to realize extended operation.

An Economical Composite Membrane with High Ion Selectivity for Vanadium Flow Batteries

The ion exchange membrane of the Nafion series widely used in vanadium flow batteries (VFBs) is characterized by its high cost and high vanadium permeability, which limit the further commercialization of VFBs. Herein, a thin composite membrane enabled by a low-cost microporous polyethylene (PE) substrate and perfluorosulfonic acid (PFSA) resin is proposed to reduce the cost of the membrane. Meanwhile, the rigid PE substrate limits the swelling of the composite membrane, which effectively reduces the penetration of vanadium ions and improves the ion selectivity of the composite membrane. Benefiting from such a rational design, a VFB assembled with the PE/PFSA composite membrane exhibited a higher coulombic efficiency (CE ≈ 96.8%) compared with commercial Nafion212 at 200 mA cm^-2. Significantly, the energy efficiency maintained stability within 200 cycles with a slow decay rate. In practical terms, the thin PE/PFSA composite membrane with low cost and high ion selectivity can make an ideal membrane candidate in VFBs.

A Sulfonated Polyimide/Nafion Blend Membrane with High Proton Selectivity and Remarkable Stability for Vanadium Redox Flow Battery

A sulfonated polyimide (SPI)/Nafion blend membrane composed of a designed and synthesized SPI polymer and the commercial Nafion polymer is prepared by a facile solution casting method for vanadium redox flow battery (VRFB). Similar molecular structures of both SPI and Nafion provide good compatibility and complementarity of the blend membrane. ATR-FTIR, ¹H-NMR, AFM, and SEM are used to gain insights on the chemical structure and morphology of the blend membrane. Fortunately, the chemical stability of the SPI/Nafion blend membrane is effectively improved compared with reported SPI-based membranes for VRFB applications. In cycling charge-discharge tests, the VRFB with the as-prepared SPI/Nafion blend membrane shows excellent battery efficiencies and operational stability. Above results indicate that the SPI/Nafion blend membrane is a promising candidate for VRFB application. This work opens up a new possibility for fabricating high-performance SPI-based blend membrane by introduction of a polymer with a similar molecular structure and special functional groups into the SPI polymer.

Polymer Membranes for All-Vanadium Redox Flow Batteries: A Review

Redox flow batteries such as the all-vanadium redox flow battery (VRFB) are a technical solution for storing fluctuating renewable energies on a large scale. The optimization of cells regarding performance, cycle stability as well as cost reduction are the main areas of research which aim to enable more environmentally friendly energy conversion, especially for stationary applications. As a critical component of the electrochemical cell, the membrane influences battery performance, cycle stability, initial investment and maintenance costs. This review provides an overview about flow-battery targeted membranes in the past years (1995-2020). More than 200 membrane samples are sorted into fluoro-carbons, hydro-carbons or N-heterocycles according to the basic polymer used. Furthermore, the common description in membrane technology regarding the membrane structure is applied, whereby the samples are categorized as dense homogeneous, dense heterogeneous, symmetrical or asymmetrically porous. Moreover, these properties as well as the efficiencies achieved from VRFB cycling tests are discussed, e.g., membrane samples of fluoro-carbons, hydro-carbons and N-heterocycles as a function of current density. Membrane properties taken into consideration include membrane thickness, ion-exchange capacity, water uptake and vanadium-ion diffusion. The data on cycle stability and costs of commercial membranes, as well as membrane developments, are compared. Overall, this investigation shows that dense anion-exchange membranes (AEM) and N-heterocycle-based membranes, especially poly(benzimidazole) (PBI) membranes, are suitable for VRFB requiring low self-discharge. Symmetric and asymmetric porous membranes, as well as cation-exchange membranes (CEM) enable VRFB operation at high current densities. Amphoteric ion-exchange membranes (AIEM) and dense heterogeneous CEM are the choice for operation mode with the highest energy efficiency.

Applications of polyimide coatings: a review

Polyimides, high-performance polymers with superior properties such as high temperature stability, resistance to solvents and high strength, can be used in high-tech applications of the aerospace and aviation, medical or electronics industry in different forms (film, fiber, nanofiber, membrane, foam, adhesive or coating). Among these applications, coating has a special place and is used to develop advanced structures having high temperature resistance, flame retardancy and etc. for high tech industries via an economical and feasible way. Therefore, in this review, we aimed to report the broad application status of polyimide coatings by reviewing publications, patents and commercial products. Thus, this study can assist in selecting suitable polyimide types and production methods for polyimide coating applications and in understanding their applicability for future products.

Bilayer Designed Hydrocarbon Membranes for All-Climate Vanadium Flow Batteries To Shield Catholyte Degradation and Mitigate Electrolyte Crossover

The use of low-cost hydrocarbon membranes in vanadium flow batteries (VFBs) still remains a great challenge because of the strong oxidation of VO₂⁺ catholyte and rapid capacity fading. Here, we report a bilayer design strategy using an antioxidant and dense cross-linked sulfonated polyimide (cSPI) layer as a protective layer for a sulfonated poly(ether ether ketone) (SPEEK) membrane to shield catholyte degradation and mitigate electrolyte crossover. A scalable process is developed to fabricate an integrated bilayer SPEEK/cSPI membrane without delamination by spraying a SPEEK transition layer between the two polymers. The tightly bridged cSPI layer not only protects the SPEEK membrane from degradation but also enhances its mechanical strength, puncture resistance, and proton/vanadium-ion selectivity. When assembled in a VFB, the bilayer SPEEK/cSPI membrane demonstrates excellent rate performance under current densities of 40-200 mA cm^-2, high adaptability at a wide temperature range of -15 to 60 °C, very slow capacity decay rate of 0.054% per cycle at 160 mA cm^-2, and a maximum power density of 480 mW cm^-2. These merits make the bilayer SPEEK/cSPI membrane a promising candidate for the next-generation VFB to achieve low-cost, high-rate, and all-climate energy storage.

Novel Triple Tertiary Amine Polymer-Based Hydrogen Bond Network Inducing Highly Efficient Proton-Conducting Channels of Amphoteric Membranes for High-Performance Vanadium Redox Flow Battery

A novel amphoteric membrane was designed by blending triple tertiary amine-grafted poly(2,6-dimethyl-1,4-phenylene oxide) (PPO-TTA) with sulfonated poly(ether ether ketone) (SPEEK) for vanadium redox flow batteries. An "acid-base pair" effect is formed by the combination of the tertiary amine group and sulfonic group, and extra nonbonding amine groups could be protonated. Both of them constitute a hydrogen bond network, which facilitates proton conduction and also hinders vanadium permeability because of the lowered swelling ratio and Donnan effect. All these contribute to improve the ion selectivity of the membrane while maintaining ionic conductivity. Compared with other amphoteric and SPEEK-based membranes, the membrane exhibits an excellent performance. The amphoteric membrane containing 15% PPO-TTA exhibits an ultralow vanadium permeability of 3.4 × 10^-9 cm² s^-1 and a low area resistance of 0.39 Ω cm^-2. Consequently, the cell assembled with this membrane shows excellent performances far superior to SPEEK and Nafion 212. The Coulombic efficiency and energy efficiency of the cell are 94.3-98.3 and 90.3-77.1% at 40-200 mA cm^-2, respectively, and have no significant reductions after 200 cycles. This performance is at a high level among the amphoteric and SPEEK-based membranes reported in recent years. The cell's open circuit voltage is maintained for up to 165 h. In addition, the membrane's chemical stability is improved by the effective barrier to the vanadium ion.

A highly proton-/vanadium-selective perfluorosulfonic acid membrane for vanadium redox flow batteries

The polar clusters of Nafion are blocked by the incorporation of the nanohybrid, which contributes to suppress vanadium ions crossover.

High Proton Selectivity Sulfonated Polyimides Ion Exchange Membranes for Vanadium Flow Batteries

High proton selectivity is the ultimate aim for the ion exchange membranes (IEMs). In this study, two kinds of sulfonated polyimides (SPI)—non-fluorinated and fluorine-containing polyimide—with about 40% sulfonation degree were synthesized by one-step high temperature polymerization. High proton selectivity IEMs were prepared and applied in vanadium flow batteries (VFB). The chemical structures, physicochemical properties and single cell performance of these membranes were characterized. The results indicate that high molecular weight of SPIs can guarantee the simultaneous achievement of good mechanical and oxidative stability for IEMs. Meanwhile, the proton selectivity of SPI membrane is five times higher than that of Nafion115 membranes due to the introduction of fluorocarbon groups. Consequently, the single cell assembled with SPI membranes exhibits excellent energy efficiency up to 84.8% at a current density of 100 mA·cm⁻², which is 4.6% higher than Nafion115. In addition, the capacity retention is great after 500 charge–discharge cycles. All results demonstrate that fluorinated SPI ion exchange membrane has a bright prospect in new energy field.

Preparation of Polymer Electrolyte Membranes via Radiation-Induced Graft Copolymerization on Poly(ethylene-alt-tetrafluoroethylene) (ETFE) Using the Crosslinker N,N'-Methylenebis(acrylamide)

Polymer electrolyte membranes (PEM) prepared by radiation-induced graft copolymerization are investigated. For this purpose, commercial poly(ethylene-alt-tetrafluoroethylene) (ETFE) films were activated by electron beam treatment and subsequently grafted with the monomers glycidyl methacrylate (GMA), hydroxyethyl methacrylate (HEMA) and N,N′-methylenebis(acrylamide) (MBAA) as crosslinker. The target is to achieve a high degree of grafting (DG) and high proton conductivity. To evaluate the electrochemical performance, the PEMs were tested in a fuel cell and in a vanadium redox-flow battery (VRFB). High power densities of 134 mW∙cm⁻² and 474 mW∙cm⁻² were observed, respectively.

Novel Deep-Eutectic-Solvent-Infused Carbon Nanofiber Networks as High Power Density Green Battery Cathodes

Redox flow batteries (RFBs) have emerged as a promising candidate for large-scale energy storage because of the flexible design for high energy, power, and safety. In this study, FeCl₃·6H₂O/urea composite deep eutectic catholyte (FeU-DEC)-infused self-standing carbon nanofiber (CNF) was synthesized for green and high power density RFB through industrially available processes. FeU-DEC-infused CNF displayed an extremely high power density (874 mW/g) as well as high capacity (27.28 mAh/g) derived from high theoretical capacity of FeU-DEC (89.24 mAh/g) in addition to the advantages of the FeU-DEC characteristics (e.g., nonflammable, biodegradable, facile preparation). This is because of the large electroactive area derived from the high surface area of CNF and superlyophilicity of FeU-DEC on CNFs. Furthermore, we compared the wettability of CNF with other electrodes, as well as the chemical stability and electrode performance, based on topological wetting analysis using parameters of fiber radius, fiber interval, the equilibrium contact angle of FeU-DEC on electrodes, and surface tension of FeU-DEC, giving wetting threshold for FeU-DEC on fibrous electrodes. The wetting analysis are applied not only for FeU-DEC, but also for a wide range of other DECs and deep eutectic anolyte. This work contributes to the further development of green and high-performance RFBs.