High Performance of Anion Exchange Blend Membranes Based on Novel Phosphonium Cation Polymers for All-Vanadium Redox Flow Battery Applications

Abstract

The deployment of alkaline anion exchange membranes (AEMs) in flow battery applications has the advantage of a low cationic species crossover rate. However, the alkaline stability conjugated to the low conductivity of hydroxide ions of anion exchange membranes (AEMs) still represents a major drawback for the large deployment of such technology. In this study, three types of tetraarylpolyphosphonium (pTAP)-based copolymers (namely, CP1, CP2, and CP3) are synthesized and blended with chitosan and polyvinylidene fluoride (PVDF) for the fabrication of AEMs. Chitosan, a green biopolymer, was employed as a blend to enhance the water uptake of the base ionomer matrix. It is proposed that the abundancy of hydroxyl groups in chitosan improves considerably the ionic conductivity, water transport, and ion selectivity of the membrane, together with facilitating the dispersion of the chitosan in the pTAP copolymer matrix. The purpose of blending PVDF is instead to provide stable mechanical strength to the composite blend. The chemical, mechanical, and thermal stabilities of the three fabricated composite-blend membranes (i.e., CM1, CM2, and CM3) were characterized. All the membranes exhibited a high water retaining capacity of up to 36.26% (recorded for CM2) along with a hydroxyl ion conductivity of 17.39 mS cm −1 . Due to the strong interactions between pTAP copolymers, chitosan, and PVDF polymers (confirmed also by Fourier transform infrared spectroscopy), the studied anion exchange membranes are able to retain up to 97% of the original OH conductivity after 1 M KOH treatment at room temperature for 100 h. The three membranes, namely, CM1, CM2, and CM3, have vanadium ion permeabilities measured at 20 °C of 1.775 × 10 −8 , 1.718 × 10 −8 , and 1.648 × 10 −8 cm 2 /s, respectively, which are lower than that for the commercially available Nafion. The good stability and remarkable cell performance of the composite-blend membranes reported here make them definitely excellent candidates for the future generation of vanadium redox flow batteries.

Stable Anion Exchange Membrane Bearing Quinuclidinium for High‐performance Water Electrolysis

“…[2] However, the extensive industrialization and the sustainable deployment of AEM-WEs are thought to be hampered by the unavailability of highly durable and high-performance AEMs (like Nafion ® utilized in PEM-WEs). Since the last few years, there has been a great interest in the development and exploration of synthetic pathways of novel AEMs for applications in electrochemical energy storage and conversion devices such as AEM-WEs, metal-air batteries, [3] redox flow batteries, [4] and anion exchange membrane fuel cells. [5] AEMs are composed of a polymer backbone, functional cationic groups, and free counterions.…”

Fabricating a Partially Fluorinated Hybrid Cation-Exchange Membrane for Long Durable Performance of Vanadium Redox Flow Batteries

“…Vanadium redox flow batteries (VRFBs) are fascinating because of their durability, high consistency, rapid response, and decoupled capacity configuration. − For VRFB, the key component is the membrane to separate the anolyte (V 2+ /V 3+ ) and catholyte (VO 2+ /VO 2 + ), along with the accomplishment of the circuit to conduct protons. − A well-performing membrane should possess elevated stabilities, high ionic conductivity, truncated vanadium ion permeability, and a cost-effective nature. , Various cation-exchange membranes (CEMs) based on sulfonated nonfluorinated polymers suffered because of deprived dimensional, mechanical, and chemical stabilities in the hydrated state. − Fluorinated polymer-based CEMs like Nafion are widely used in VRFB because of their high chemical stability and high conductivity. In contrast, the Nafion separator experiences some stern drawbacks like high cost, severe self-discharge, large capacity loss, electro-osmotic drag, reduced ion selectivity, and swift vanadium crossover that are responsible for reduced battery performance. − Therefore, considerable effort has been expended to adapt the fluorinated CEMs for VRFB application. − Poly­(vinylidene fluoride- co -hexafluoropropylene) (PVDF- co -HFP) is a block copolymer and is regarded as a material with a strong C–F bond that gives outstanding stabilities and a crystalline structure for reducing vanadium-ion penetration. , Various macromolecular amendments of PVDF- co -HFP either by grafting or by blending of ionic moieties have been reported. − A high degree of functionality can often be achieved by the blending of miscible polymers as a result of synergistic effects; , nevertheless, ionic moieties percolated, resulting in the deterioration of membrane conductivity. , Thus, grafting of a functional monomer has been considered a viable option for constructing CEMs . It has been found that the grafting of aromatic moieties such as 4-hydroxybenzenesulfonic acid further degrades membrane stability. , …”

“…23,24 Various macromolecular amendments of PVDF-co-HFP either by grafting or by blending of ionic moieties have been reported. 25−27 A high degree of functionality can often be achieved by the blending of miscible polymers as a result of synergistic effects; 28,29 nevertheless, ionic moieties percolated, resulting in the deterioration of membrane conductivity. 30,31 Thus, grafting of a functional monomer has been considered a viable option for constructing CEMs.…”

Physically and Chemically Stable Anion Exchange Membranes with Hydrogen-Bond Induced Ion Conducting Channels

“…Anion exchange membranes (AEMs) have drawn great interest from researchers because of their essential role in a wide range of electrochemical devices, including anion exchange membrane fuel cells (AEMFCs) [1,2], redox flow batteries [3] and water electrolyzers [4]. The items determine the performance of these electrochemical devices directly [5].…”