Yuyue Zhao

High-performance porous uncharged membranes for vanadium flow battery applications created by tuning cohesive and swelling forces

A simple and effective solvent treatment method was developed to prepare porous membranes with a tunable morphology for vanadium flow battery applications. The solvent treatment method can effectively create poly(ether sulfone) (PES) membranes with a well-controllable pore size and pore size distribution. An impressive vanadium flow battery (VFB) performance with a coulombic efficiency of over 99% and an energy efficiency of over 90% was obtained, which are the highest values ever reported for porous uncharged membranes. The concept provides an entirely novel, simple and cost-effective way to fabricate high-performance porous membranes for VFB applications.

Highly stable membranes based on sulfonated fluorinated poly(ether ether ketone)s with bifunctional groups for vanadium flow battery application

A facile strategy for fabricating a sulfonated poly (ether ether ketone) ion exchange membrane with high chemical stability, excellent ion selectivity and battery performance is presented. The essence of this strategy lies in the introduction of benzotrifluoride bifunctional groups that function to protect the ether bond (through the conjugation effect of the benzene ring) and restrict swelling behaviour of the membrane (resulting from the -CF3 hydrophobic groups). With the introduction of benzotrifluoride bifunctional groups, the SFPEEK membranes exhibited excellent ion selectivity and cell performance under vanadium flow battery (VFB) operation conditions, exhibiting a columbic efficiency of 98.32% and an energy efficiency of 87.74% at a current density of 80 mA cm(-2), which is much higher than commercial Nafion 115 (CE = 94.83%, EE = 83.25%). A VFB single cell assembled with a SFPEEK membrane shows a stable performance after continuously running more than 1050 cycles, which is by far the longest cycle life reported on sulfonated aromatic cation exchange membranes. These results demonstrated that the sulfonated poly (ether ether ketone) ion exchange membranes with benzotrifluoride bifunctional groups are promising candidates for VFB systems.

Polypyrrole modified porous poly(ether sulfone) membranes with high performance for vanadium flow batteries

Polypyrrole (PPY) modified porous poly(ether sulfone) (PPY/PES) membranes with excellent ion conductivity and high ion selectivity are prepared and employed in vanadium flow batteries (VFBs). The porous PES membranes are modified through in situ polymerization of pyrrole (PR) by using VO2+ as the oxidizing agent. The positively charged PPY nanoparticles can effectively retain vanadium ions via the Donnan exclusion and afford excellent ion conductivity through the interaction between the sulfuric acid in electrolytes and the nitrogen elements in PPY. As a consequence, the designed PPY/PES porous membranes demonstrate high ion selectivity and excellent ion conductivity along with exceptional chemical stability under VFB operation conditions. The PPY/PES porous membranes exhibited a very prospective performance for vanadium flow applications, showing a coulombic efficiency (CE) of 96.30% and an energy efficiency (EE) of 87.20% at a current density of 80 mA cm−2, which are much better than those of a VFB with a Nafion 115 membrane (coulombic efficiency of 93.16% and energy efficiency of 82.29%). Furthermore, a VFB using the PPY/PES porous membranes delivers a stable battery efficiency after continuously operating for more than 100 cycles, displaying good potential usage in VFB applications.

Advanced charged porous membranes with flexible internal crosslinking structures for vanadium flow batteries

Advanced charged porous membranes with flexible internal crosslinking networks were designed and fabricated for vanadium flow battery application. Flexible 1,4-diaminobutane was introduced in CMPSF spongy porous membranes to build flexible crosslinking networks on the pore walls. The flexible segments could induce relatively highly micro-phase separated structures (hydrophilic and hydrophobic phase) and effectively enhance the mobility of hydrophilic and hydrophobic domains, which is highly beneficial for proton transportation. At the same time, the positively charged crosslinking networks can ensure the high chemical stability of resulting membranes. As a result, porous membranes with flexible crosslinking networks could meet the requirement of VFB application owing to their excellent conductivity, high selectivity and high chemical stability. A VFB single cell with the prepared charged porous membrane exhibits a coulombic efficiency (CE) of more than 99% and an energy efficiency (EE) of about 87% at 80 mA cm−2, showing much higher performance than commercial Nafion 115 (CE = 92.5%, EE = 83.7%). A VFB employing the prepared membrane maintains a stable performance after more than 4000 charge–discharge cycles, showing impressive potential for VFB application.