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The design of ion conductive functional side chains is critical to tune the selectivity of protons to vanadium ions in ion conductive membranes for vanadium redox flow batteries (VRFBs); however, it is far from satisfactory. Herein, the concept of a quinone convertible sulfated ion conductive side chain is proposed with tunable Donnan effect and strong proton conducting ability. The electrically neutral phenol group in the side chain will be chemically converted to quinone of lower electronegativity in situ in the highly acidic VRFB operating environment, which leads to about a 5 fold increase in zeta potential to repel vanadium ions. Meanwhile the sulfate ester cationic exchange group in the side chain dominates the well-formed ion clusters (7.85 nm) and hydrogen bonding for fast proton conduction. With a quinone group capacity (IECQ) of 2.43 mmol g−1 in the polybenzimidazole (PBIPhOSO3-q-2.43), the low area resistance (0.23 Ω cm2) and vanadium permeability (3.2 × 10−9 s cm−2) contribute to about a 16.5 fold increase in H+/Vn+ selectivity as compared with the Nafion 212 membrane. The corresponding VRFB shows a low discharge capacity decay rate (0.31% per cycle) and high energy efficiency (87.7%) at 100 mA cm−2 and remains stable during the 500 cycle test. The performance is superior to that of the most recently reported amphiprotic ion conductive membranes. [ABSTRACT FROM AUTHOR]

Designing membranes with highly ion-selective conductivity is one of the key questions for the large-scale application of redox flow batteries, which can unlock the potential of the renewable energy industry. The selectivity of ion exchange membranes is usually limited by the larger size of the nanochannels than that of redox-active ions. Herein, sub-1-nanometer channels of membranes were constructed through covalently bonding the polysilsesquioxane nanocages with multiple branched amino groups onto a polymer material. The synergistic effect of super hydrophilicity, extra functional amino groups and matched sub-1-nanometer sieve structure accomplished the sieving of proton to redox-active ions and ensured fast proton conduction. The ultrahigh proton-selective conductivity of 1.18 × 1011 mS s cm−3 enabled high electrochemical performance and surprisingly excellent durability over 7000 cycles (>4000 deep cycles for energy applications) even in the harsh environment of a vanadium flow battery. These results indicated that the new membrane design with multi-scale ion-sieving nanochannels is promising for high-efficiency and durable redox flow batteries. [ABSTRACT FROM AUTHOR]

A hydrophilic–hydrophobic microphase separation structure is essential for anion exchange membranes (AEMs) to balance ion conduction and mechanical strength. However, continuous hydrophobic microphase will inevitably become a barrier for ion conduction between dispersed hydrophilic ionic clusters. In this study, throughout ion conductive highway networks are constructed in quaternized polybenzimidazole (PBI) AEMs, in which OH− could break through the hydrophobic polymer backbone by forming dense hydrogen bond networks with pyridine nitrogen (–N＝) in PBI repeating units, and a tri-cationic/non-cationic ether pendant side chain is proposed to greatly improve ionic aggregation that was still difficult to achieve in reported quaternized PBI membranes. The molecular dynamics simulation indicates comparable OH− coordination through ionic bonds in hydrophilic microphase and hydrogen bonds in the hydrophobic microphase. The as-prepared membranes exhibit excellent toughness to fabricate ultrathin self-supporting AEMs (10 μm) with low ohmic resistance. H2/O2 fuel cells achieves a high peak power density of about 1162.3 mW cm−2 at a large current density of 2749.5 mA cm−2, which is the highest value among recently reported PBI-based AEMs and multi-cation side-chain type AEMs. The design of throughout conductive highway network makes the highly stable commercial PBI material one of the best candidates for AEMFCs. [ABSTRACT FROM AUTHOR]

Poly(ether ether ketone) (PEEK) with multiple quaternary ammonium groups on pendent side chains is synthesized through the chloromethylation di-quaternization route, using bi-functional 1,4-diazabicyclo [2,2,2]octane (DABCO) as quaternization reagent. The materials are made into tough and transparent anion exchange membranes (AEMs) by solvent casting. The purpose is to promote efficient hydroxide ion conductive channels, which are particularly important and challenging to improve hydroxide conductivity of AEMs due to the inherently low mobility of hydroxide ions. Transmission electron microscopy (TEM) images show ionic clusters of bigger size scattering in the di-quaternized membranes compared with the mono-quaternized membranes. Given similar ion exchange capacities (IECs), the di- quaternized membranes exhibit much higher values of effective hydroxide ion mobility (~3.5 × 10-4 cm2 s-1 V-1) than the mono-quaternized membranes (~1.7 × 10-4 cm2 s-1 V-1). The structure of multiple quaternary ammonium groups on the pendent side chain promotes hydrophilic-hydrophobic micro-phase separation and efficient hydroxide ion conductive channels in the membranes. As a result, hydroxide conductivity of the di-quaternized membranes is about 2 to 3 fold higher than that of the mono-quaternized membranes with similar IEC, exhibiting a high value of about 35.3 mS cm-1 at 25 °C. At a certain IEC, the di-quaternized membranes have fewer pendent side chains on the polymer backbone, which also benefits the mechanical and chemical stabilities of the AEMs. [ABSTRACT FROM AUTHOR]