Layer-by-layer modification of Nafion membranes for increased life-time and efficiency of vanadium/air redox flow batteries

A highly-selective layer-by-layer membrane modified with polyethylenimine and graphene oxide for vanadium redox flow battery

A selective composite membrane for vanadium redox flow battery (VRFB) was successfully prepared by layer-by-layer (LbL) technique using a perfluorosulfonic sulfonic acid or Nafion 117 (N117). The composite membrane referred as N117-(PEI/GO)n, was obtained by depositing alternating layers of positively charged polyethylenimine (PEI) and negatively charged graphene oxide (GO) as polyelectrolytes. The physicochemical properties and performance of the pristine and composite membranes were investigated. The membrane showed an enhancement in proton conductivity and simultaneously exhibited a notable 90% reduction in vanadium permeability. This, in turn, results in a well-balanced ratio of proton conductivity to vanadium permeability, leading to high selectivity. The highest selectivity of the LbL membranes was found to be 19.2 × 104 S.min/cm3, which is 13 times higher than the N117 membrane (n = 0). This was translated into an improvement in the battery performance, with the n = 1 membrane showing a 4-6% improvement in coulombic efficiency and a 7-15% improvement in voltage efficiency at current densities ranging from 40 to 80 mA/cm2. Furthermore, the membrane displays stable operation over a long-term stability at around 88% at a current density of 40 mA/cm2, making it an attractive option for VRFB applications using the LbL technique. The use of PEI/GO bilayers maintains high proton conductivity and VE of the battery, opening up possibilities for further optimization and improvement of VRFBs.

Anion Exchange Membranes Based on Bis-Imidazolium and Imidazolium-Functionalized Poly(phenylene oxide) for Vanadium Redox Flow Battery Applications

Although the Nafion membrane has a high energy efficiency, long service life, and operational flexibility when applied for vanadium redox flow battery (VRFB) applications, its applications are limited due to its high vanadium permeability. In this study, anion exchange membranes (AEMs) based on poly(phenylene oxide) (PPO) with imidazolium and bis-imidazolium cations were prepared and used in VRFBs. PPO with long-pendant alkyl-side-chain bis-imidazolium cations (BImPPO) exhibits higher conductivity than the imidazolium-functionalized PPO with short chains (ImPPO). ImPPO and BImPPO have a lower vanadium permeability (3.2 × 10^-9 and 2.9 × 10^-9 cm² s^-1) than Nafion 212 (8.8 × 10^-9 cm² s^-1) because the imidazolium cations are susceptible to the Donnan effect. Furthermore, under the current density of 140 mA cm^-2, the VRFBs assembled with ImPPO- and BImPPO-based AEMs exhibited a Coulombic efficiency of 98.5% and 99.8%, respectively, both of which were higher than that of the Nafion212 membrane (95.8%). Bis-imidazolium cations with long-pendant alkyl side chains contribute to hydrophilic/hydrophobic phase separation in the membranes, thus improving the conductivity of membranes and the performance of VRFBs. The VRFB assembled with BImPPO exhibited a higher voltage efficiency (83.5%) at 140 mA cm^-2 than that of ImPPO (77.2%). These results of the present study suggest that the BImPPO membranes are suitable for VRFB applications.

Mathematical Modeling of the Selective Transport of Singly Charged Ions Through Multilayer Composite Ion-Exchange Membrane during Electrodialysis

Abstract

The deposition of several alternating anion- and cation-exchange surface layers (layer-by-layer method) is a promising technique for the modification of ion-exchange membranes, which makes it possible to essentially increase their selectivity to singly charged ions. This paper presents a one-dimensional model, which is based on the Nernst–Planck–Poisson equations and describes the competitive transfer of singly and doubly charged ions through a multilayer composite ion-exchange membrane. It has been revealed for the first time that, as in the earlier studied case of a bilayer membrane, the dependence of the specific permselectivity coefficient (P1/2) of a multilayer membrane on the electrical current density passes through a maximum $$\left( {P_{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}}}^{{\max }}} \right).$$ It has been shown that an increase in the number of nanosized modification bilayers n leads to the growth of $$P_{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}}}^{{\max }},$$ but the flux of a preferably transferred ion decreases in this case. It has been established that $$P_{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}}}^{{\max }}$$ is attained at underlimiting current densities and relatively low potential drop. The simulated dependences $$P_{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}}}^{{\max }}$$(n) qualitatively agree with the known literature experimental and theoretical results.

Surface-Modified Approach to Fabricate Nafion Membranes Covalently Bonded with Polyhedral Oligosilsesquioxane for Vanadium Redox Flow Batteries

An aminopropyl isobutyl polyhedral oligosilsesquioxane (NH₂-POSS) surface-modified Nafion membrane has been designed by chemical grafting for vanadium redox flow batteries (VRFBs). NH₂-POSS is a cage-like macromer consisting of an inorganic Si₈O₁₂ core surrounded by seven inert isobutyl groups and one active aminopropyl group. The sulfonic acid groups on the surface of Nafion can be activated by 1,1-carbonyldiimidazole for further modification with NH₂-POSS. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) prove that NH₂-POSS has been successfully grafted on the surface of a Nafion 115 membrane. Although the proton conductivity decreases slightly, the organic-inorganic hybrid membranes display enhanced ion selectivity and excellent dimensional stability with lower water uptake and swelling ratio than Nafion 115. Moreover, two-dimensional-grazing incidence X-ray diffraction (2D-GIXRD) reveals that the introduction of NH₂-POSS forms a POSS layer on the surface of the membrane and narrows the space of Nafion clusters, which helps to block VO²⁺ permeation. A VRFB with the surface-modified Nafion membrane displays an outstanding performance with an average Coulombic efficiency (CE) of 98.7% and energy efficiency (EE) of 84.5% at a current density of 80 mA cm^-2, superior to those of the Nafion 115 membrane (CE = 95.7%, EE = 81.7%). Furthermore, the cell holds a high capacity retention of 49.2% after 1000 charge-discharge cycles, in contrast to that of 41.9% for the cell with Nafion 115 after only 200 cycles. The results suggest that the surface-modified hybrid membrane is a promising strategy to overcome the vanadium ion crossover in VRFBs.

Electric vehicles fire protection during charge operation through Vanadium-air flow battery technology

During the last decade, electric vehicles had a remarkable diffusion caused by Li-ion batteries lowered prices and improved performances. Meanwhile, incidents involving fire were reported for electric vehicles during charge operation or simple parking. The potential drawbacks of this technology could play a role on safety in the next years, especially for household or underground charge. This paper presents the development of a novel system concept based on a Vanadium-air flow battery, applied to provide charge and fire safety of electric vehicles through oxygen reduction in a sealed box. When the vehicle is parked inside the box and the passengers are outside of it, a nitrogen injection is operated to reduce the fire risk quickly, during the subsequent vehicle charge operation the oxygen is consumed from the box atmosphere by the cathode of the Vanadium-air battery that supplies energy, then the nitrogen reserve is restored consuming oxygen from the external ambient and the energy output can be supplied to smart grids. The system is mainly composed by the Vanadium-air flow battery, the protection box and the nitrogen reserve, it is sized relatively to the most diffused road and commercial electric vehicles for different values of on-board battery capacity and charge power Moreover, it can be integrated into vehicle-to-grid energy systems improving intersectoral flexibility.

Stable and Highly Ion-Selective Membrane Made from Cellulose Nanocrystals for Aqueous Redox Flow Batteries

The design of chemically stable ion-exchange membranes with high selectivity for applications in an aqueous redox flow battery (RFB) at high acid concentrations remains a significant challenge. Herein, this study designed a stable and highly ion-selective membrane by utilizing proton conductive cellulose nanocrystals (CNCs) incorporated in a semicrystalline hydrophobic poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrix. The high hydrophobicity of the PVDF-HFP matrix mitigates crossover of the electrolytes, whereas the abundant and low-cost CNCs derived from wood provide high proton conductivity. The fundamental contributors for CNCs' excellent proton conductivity are the hydroxyl (-OH) functional groups, highly acidic sulfonate (-SO₃H) functional groups, and the extensive intramolecular hydrogen bonding network. In addition, CNCs exhibit a mechanically and chemically stable structure in the harsh acidic electrolyte attributed to the high crystallinity (crystalline index of ∼86%). Therefore, because of the high proton conductivity, excellent ion selectivity, high chemical stability, and structural robustness, the vanadium redox flow battery (VRFB) assembled with the homogeneous CNCs and PVDF-HFP (CNC/PVDF-HFP) membrane achieved a Coulombic efficiency (CE) of 98.2%, energy efficiency (EE) of 88.2%, and a stable cycling performance for more than 650 cycles at a current density of 100 mA cm^-2. The obtained membrane possesses excellent flexibility, high mechanical tensile strength, and superior selectivity. Meanwhile, the applied casting method is scalable for large-scale manufacturing.

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.

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

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

Polybenzimidazole/Nafion hybrid membrane with improved chemical stability for vanadium redox flow battery application

In order to increase the chemical stability of polybenzimidazole (PBI) membrane against the highly oxidizing environment of a vanadium redox flow battery (VRFB), PBI/Nafion hybrid membrane was developed by spray coating a Nafion ionomer onto one surface of the PBI membrane. The acid–base interaction between the sulfonic acid of the Nafion and the benzimidazole of the PBI created a stable interfacial adhesion between the Nafion layer and the PBI layer. The hybrid membrane showed an area resistance of 0.269 Ω cm² and a very low vanadium permeability of 1.95 × 10⁻⁹ cm² min⁻¹. The Nafion layer protected the PBI from chemical degradation under accelerated oxidizing conditions of 1 M VO₂⁺/5 M H₂SO₄, and this was subsequently examined in spectroscopic analysis. In the VRFB single cell performance test, the cell with the hybrid membrane showed better energy efficiency than the Nafion cell with 92.66% at 40 mA cm⁻² and 78.1% at 100 mA cm⁻² with no delamination observed between the Nafion layer and the PBI layer after the test was completed.

Novel polybenzimidazole (PBI)/Nafion hybrid membranes for the VRFB are made by spray coating a Nafion layer to protect PBI from chemical degradation.

Modification of Nafion® Membrane via a Sol-Gel Route for Vanadium Redox Flow Energy Storage Battery Applications

Nafion 117(N-117)/SiO2-SO3H modified membranes were prepared using the 3-Mercaptopropyltrimethoxysilane (MPTMS) to react with H2O2via in situ sol-gel route. Basic properties including water uptake, contact angle, ion exchange capacity (IEC), vanadium ion permeability, impedance, and conductivity were measured to investigate how they affect the charge-discharge characteristics of a cell. Furthermore, we also set a vanadium redox flow energy battery (VRFB) single cell by the unmodified/modified N-117 membranes as a separated membrane to test its charge/discharge performance and compare the relations among the impedance and efficiency. The results show that the appropriate amount of SiO2-SO3H led into the N-117 membrane contributive to the improvement of proton conductivity and vanadium ion selectivity. The permeability was effectively decreased from original 3.13 × 10−6 cm2/min for unmodified N-117 to 0.13 × 10−6 cm2/min for modified membrane. The IEC was raised from original 0.99 mmol/g to 1.24 mmol/g. The modified membrane showed a good cell performance in the VRFB charge/discharge experiment, and the maximum coulombic efficiency was up to 94%, and energy efficiency was 82%. In comparison with unmodified N-117, the energy efficiency of modified membrane had increased more than around 10%.