Wanxing Xu

Anion‐Conductive Membranes with Ultralow Vanadium Permeability and Excellent Performance in Vanadium Flow Batteries

Anion exchange membranes prepared from quaternized poly(tetramethyl diphenyl ether sulfone) (QAPES) were first investigated in the context of vanadium flow battery (VFB) applications. The membranes showed an impressive suppression effect on vanadium ions. The recorded vanadium permeability was 0.02×10(-7)-0.09×10(-7) cm(2) min(-1), which was two orders of magnitude lower than that of Nafion 115. The self-discharge duration of a VFB single cell with a QAPES membrane is four times longer than that of Nafion 115. The morphological difference in hydrophilic domains between QAPES and Nafion was confirmed by TEM. After soaking the membranes in VO(2)(+) solution, adsorbed vanadium ions can barely be found in QAPES, whereas the hydrophilic domains of Nafion were stained. In the ex situ chemical stability test, QAPES showed a high tolerance to VO(2)(+) and remained intact after immersion in VO(2)(+) solution for over 250 h. The performance of a VFB single cell assembled with QAPES membranes is equal to or even better than that of Nafion 115 and remains stable in a long-term cycle test. These results indicate that QAPES membranes can be an ideal option in the fabrication of high-performance VFBs with low electric capacity loss.

Membranes with well-defined ions transport channels fabricated via solvent-responsive layer-by-layer assembly method for vanadium flow battery

In this work we presented a general strategy for the fabrication of membranes with well-defined ions transport channels through solvent-responsive layer-by-layer assembly (SR-LBL). Multilayered poly (diallyldimethylammonium chloride) (PDDA) and poly (acrylic acid) (PAA) complexes were first introduced on the inner pore wall and the surface of sulfonated poly (ether ether ketone)/poly (ether sulfone) (PES/SPEEK) nanofiltration membranes to form ions transport channels with tuned radius. This type of membranes are highly efficient for the separators of batteries especially vanadium flow batteries (VFBs): the VFBs assembled with prepared membranes exhibit an outstanding performance in a wide current density range, which is much higher than that assembled with commercial Nafion 115 membranes. This idea could inspire the development of membranes for other flow battery systems, as well as create further progress in similar areas such as fuel cells, electro-dialysis, chlor-alkali cells, water electrolysis and so on.

A novel solvent-template method to manufacture nano-scale porous membranes for vanadium flow battery applications

A solvent-template method is proposed for manufacturing ion conducting membranes (ICMs) with controlled ion transport channels (in nano-scale) for vanadium flow battery (VFB) application. As a result, the ion transport properties and single cell performance of the ICMs could be easily tuned by this method. The VFB single cell assembled with the prepared ICMs achieved CE over 99% and EE over 92% when operated under a current density of 40 mA cm−2, which is among the excellent performances ever studied for VFB porous ICMs. The solvent-template method proposed in this article is promising for fabricating high performance ICMs for the VFB as well as other potential battery systems.

Morphology and performance of poly(ether sulfone)/sulfonated poly(ether ether ketone) blend porous membranes for vanadium flow battery application

Poly(ether sulfone) (PES) porous membranes with tunable morphology were fabricated via a phase inversion method and applied in vanadium flow batteries (VFBs). The morphology of the PES membrane was adjusted by changing the polymer concentration and blending with hydrophilic sulfonated poly(ether ether ketone) (SPEEK) in the cast solution. The relationship between the membrane morphology and the performance in VFBs was investigated in detail. The results indicated that with increasing polymer concentration of the cast solution, the number of macrovoids gradually decreased and the finger-like pores became larger. A higher coulombic efficiency (CE) can be obtained due to the lower vanadium permeability, while the voltage efficiency (VE) decreased. In addition, the introduction of SPEEK in cast solution will induce the transformation of membrane structures from finger-like to spongy-like pores. The CE decreased with the higher vanadium permeability, while the VE increased due to the increased proton conductivity. As a result, optimized VFB performance of the PES membranes was obtained, showing a CE of 92.8% and an energy efficiency (EE) of 78.4%. The battery assembled with the prepared membranes showed a stable battery performance after running for more than 200 cycles, showing good oxidation stability. This work presents an effective and facile method to fabricate PES membranes with tunable battery performance.