In-situ and ex-situ degradation of sulfonated polyimide membrane for vanadium redox flow battery application

Construction of High-Performance Membranes for Vanadium Redox Flow Batteries: Challenges, Development, and Perspectives

While being a promising candidate for large-scale energy storage, the current market penetration of vanadium redox flow batteries (VRFBs) is still limited by several challenges. As one of the key components in VRFBs, a membrane is employed to separate the catholyte and anolyte to prevent the vanadium ions from cross-mixing while allowing the proton conduction to maintain charge balance in the system during operation. To overcome the weakness of commercial membranes, various types of membranes, ranging from ion exchange membranes with diverse functional groups to non-ionic porous membranes, have been designed and reported to achieve higher ionic conductivity while maintaining low vanadium ion permeability, thus enhancing efficiency. In addition, besides overall efficiency, stability and cost-effectiveness of the membrane are also critical aspects that determine the practical applicability of the membranes and thus VRFBs. In this article, we have offered comprehensive insights into the mechanism of ion transportation in membranes of VRFBs that contribute to the challenges and issues of VRFB applications. We have further discussed optimal strategies for solving the trade-off between the membrane efficiency and its durability in VRFB applications. The development of state-of-the-art membranes through various material and structure engineering is demonstrated to reveal the relationship of properties-structure-performance.

A High-Performance Polyimide Composite Membrane with Flexible Polyphosphazene Derivatives for Vanadium Redox Flow Battery

A sulfonated polyimide, S-F-abSPI, with alkyl sulfonic acid side chains, and a polyphosphonitrile derivative, poly[4-methoxyphenoxy (4-fluorophenoxy) phosphazene] (PFMPP), were designed and synthesized. Composite modification of the S-F-abSPI membrane was carried out using PFMPP, resulting in the preparation of composite membranes with different composite ratios, which were then subjected to performance testing and characterization. Experimental results revealed a significant enhancement in the proton conductivity of the S-F-abSPI membrane, reaching 0.116 S cm-1, slightly higher than that of the N212 membrane. The S-F-abSPI/1% PFMPP composite membrane exhibited the optimal comprehensive performance, with a surface resistance as low as 0.54 Ω cm2, comparable to that of the N212 membrane. At a high current density of 200 mA cm-2 during charge-discharge, the composite membrane achieved a voltage efficiency (VE) of 83.12% and an energy efficiency (EE) of 81.95%. Cycling tests over 200 cycles demonstrated the composite membrane's excellent long-term cycling stability. The alkyl sulfonic acid side chains enhanced the proton conductivity of the membrane, while electrostatic potential distribution calculations indicated strong interactions between PFMPP and the base membrane, enhancing the membrane's mechanical strength, reducing vanadium ion permeability, and improving chemical stability and vanadium ion selectivity. This composite membrane holds promise for high-performance VRFB applications.

Swelling-Induced Quaternized Anthrone-Containing Poly(aryl ether ketone) Membranes with Low Area Resistance and High Ion Selectivity for Vanadium Flow Batteries

A vanadium flow battery (VFB) is one of the most promising electrochemical energy storage technologies. However, membranes for VFBs still suffer from high cost or low conductivity and poor stability. Here, we report new quaternized anthrone-containing poly(aryl ether ketone) (QAnPEK) membranes for VFBs. QAnPEK membranes with moderate ion exchange capacity (1.26 mmol g^-1) were swelling-induced in H₃PO₄ (50 wt %) to form wider ion transport pathways that significantly enhanced membrane conductivity (e.g., 0.49 Ω cm² for the QAnPEK-virgin membrane and 0.12 Ω cm² for the swelling-induced QAnPEK-90 membrane). The bulky rigid anthrone-containing backbone provided high swelling resistance and enabled QAnPEK membranes to have high ion selectivity. As a result, QAnPEK membranes displayed low area resistance, high ion selectivity, and robust mechanical strength. The QAnPEK-90 membrane yielded excellent energy efficiencies (92.4% at 80 mA cm^-2, 85.1% at 200 mA cm^-2, and 80.3% at 280 mA cm^-2). Moreover, QAnPEK membranes exhibited outstanding in situ and ex situ stability, for example, the VFB with the QAnPEK-40 membrane demonstrated highly stable battery performance for 3000 cycles at 160 mA cm^-2. QAnPEK membranes are attractive candidates for VFB application.

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.

Separation of Gases by Membranes: The Effects of Pollutants on the Stability of Membranes

The long-term stability of membranes is determined mainly by their sensitivity to pollutants. Their stability was tested using a novel, multichannel measuring system, which is based on pressure differences. This measuring system is suitable to determine the changes in permeability of polymer membranes. The damaging effects of H2S, BTX and n-dodecane were investigated in terms of polyimide gas separation membranes using nitrogen gas.

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

A novel side-chain-type fluorinated sulfonated polyimide (s-FSPI) membrane is synthesized for vanadium redox batteries (VRBs) by high-temperature polycondensation and grafting reactions. The s-FSPI membrane has a vanadium ion permeability that is over an order of magnitude lower and has a proton selectivity that is 6.8 times higher compared to those of the Nafion 115 membrane. The s-FSPI membrane possesses superior chemical stability compared to most of the linear sulfonated aromatic polymer membranes reported for VRBs. Also, the vanadium redox flow/static batteries (VRFB/VRSB) assembled with the s-FSPI membranes exhibit stable battery performance over 100- and 300-time charge-discharge cycling tests, respectively, with significantly higher battery efficiencies and lower self-discharge rates than those with the Nafion 115 membranes. The excellent physicochemical properties and VRB performance of the s-FSPI membrane could be attributed to the specifically designed molecular structure with the hydrophobic trifluoromethyl groups and flexible sulfoalkyl pendants being introduced on the main chains of the membrane. Moreover, the cost of the s-FSPI membrane is only one-fourth that of the commercial Nafion 115 membrane. This work opens up new possibilities for fabricating high-performance proton-conductive membranes at low costs for VRBs.