Densely Quaternized Fluorinated Poly(fluorenyl ether)s with Excellent Conductivity and Stability for Vanadium Redox Flow Batteries

In Situ Crosslinking of Tröger Base-Based Membranes with Improved Vanadium Flow Battery Property

The high conductivity of anion exchange membrane (AEM) remains a great challenge in achieving high-performance vanadium flow batteries. In this work, this is achieved by designing a series of microporous crosslinked quaternary ammonium membranes (QDTTB-Xs), which is synthesized by in situ reacting of iodomethane with a series of novel crosslinked microporous Tröger base membranes (DTTB-Xs) that prepared by condensation of 2, 6 (7)-diamino-triptycene and 2, 6 (7)-13-triamino-triptycene through in situ crosslinking. Compared with linear microporous QDTTB-0, the crosslinked QDTTB-X membranes showed higher conductivity. The QDTTB-35 membrane displays both higher coulombic efficiency and voltage efficiency, and 80% of energy efficiency is realized at 200 mA cm-2. Outperforming N117 and other reported anion exchange membranes. This is due to the increased triamino-triptycene molar ratio in the membrane resulting in both higher N+ concentration and improved micropores concentration. Moreover, positively charged N+ groups combined with the low swelling ratio also help in restricting the vanadium ions permeation. These results give great perspectives in designing high-performance AEMs for VRFB applications.

Aryl ether-free polymer electrolytes for electrochemical and energy devices

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

Aspergillus Niger Derived Wrinkle-Like Carbon as Superior Electrode for Advanced Vanadium Redox Flow Batteries

The scarcity of high electrocatalysis composite electrode materials has long been suppressing the redox reaction of V(II)/V(III) and V(IV)/V(V) couples in high performance vanadium redox flow batteries (VRFBs). Herein, through ingeniously regulating the growth of Aspergillus Niger, a wrinkle-like carbon (WLC) material that possesses edge-rich carbon, abundant heteroatoms, and nature wrinkle-like structure is obtained, which is subsequently successfully introduced and uniform dispersed on the surface of carbon fiber of graphite felt (GF). This composite electrode presents a lower overpotential and higher charge transfer ability, as the codoped multiheteroatoms increase the electrocatalysis activity and the wrinkled structure affords more abundant reaction area for vanadium ions in the electrolyte when compared with the pristine GF electrode, which is also supported by the density functional theory (DFT) calculations. Hence, the assembled battery using WLC electrodes achieves a high energy efficiency of 74.5% for 300 cycles at a high current density of 200 mA cm-2 , as well as the highest current density of 450 mA cm-2 . The WLC material not only uncovers huge potential in promoting the application of VRFBs, but also offers referential solution to synthesis microorganism-based high-performance electrode in other energy storage systems.

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.

Preparation of Sulfonated Poly(arylene ether)/SiO2 Composite Membranes with Enhanced Proton Selectivity for Vanadium Redox Flow Batteries

Proton exchange membranes (PEMs) are an important type of vanadium redox flow battery (VRFB) separator that play the key role of separating positive and negative electrolytes while transporting protons. In order to lower the vanadium ion permeability and improve the proton selectivity of PEMs for enhancing the Coulombic efficiency of VRFBs, herein, various amounts of nano-sized SiO2 particles were introduced into a previously optimized sulfonated poly(arylene ether) (SPAE) PEMs through the acid-catalyzed sol-gel reaction of tetraethyl orthosilicate (TEOS). The successful incorporation of SiO2 was confirmed by FT-IR spectra. The scanning electron microscopy (SEM) images revealed that the SiO2 particles were well distributed in the SPAE membrane. The ion exchange capacity, water uptake, and swelling ratio of the PEMs were decreased with the increasing amount of SiO2, while the mechanical properties and thermal stability were improved significantly. The proton conductivity was reduced gradually from 93.4 to 76.9 mS cm−1 at room temperature as the loading amount of SiO2 was increased from 0 to 16 wt.%; however, the VO2+ permeability was decreased dramatically after the incorporation of SiO2 and reached a minimum value of 2.57 × 10−12 m2 s−1 at 12 wt.% of SiO2. As a result, the H+/VO2+ selectivity achieved a maximum value of 51.82 S min cm−3 for the composite PEM containing 12 wt.% of SiO2. This study demonstrates that the properties of PEMs can be largely tuned by the introduction of SiO2 with low cost for VRFB applications.