High Performance Membrane For Non-Aqueous Redox Flow Battery

Non-aqueous redox flow battery（NARFB）,the second-generation flow battery based on organic solvent,which has potential applicability in facilitating the harvest of the intermittent renewable power sources,like wind and solar energy.As a crucial component of NARFB,the membrane is served to prevent the crossover of the catholyte and anolyte active materials whilst facilitating the transport of the supporting electrolyte ions.However,the state-of-the-art membranes utilized in NARFBs still need great improvements.For instance,ceramic membrane owns poor mechanical stability and low ionic conductivity,ion exchange membrane shows high swellability and poor chemical stability,while the low ionic selectivity of porous membrane causes serious crossover of electrolytes.Thus,this thesis targets to obtain high performance NARFB membranes by modifying commercial porous membrane as well as synthesizing intrinsic composite membrane.First,the electrochemical performance of NARFBs with different commercial porous memrbanes（Celgard and Daramic）are studied by using2,5-di-tert-butyl-1-methoxy-4-[2’-methoxyethoxy]benzeneascatholyte,2,1,3-benzothiadiazoleasanolyte,andtetraethylammonium bis（trifluoromethylsulfonyl）imide（TEATFSI）as supporting electrolyte.The results show that the battery assembled with porous Daramic 250 membrane exhibits relatively high performance and delivers an average discharge capacity of 1.7 Ah L^-1,with a voltage efficiency（VE）of 87.8%,Coulombic efficiency（CE）of 89.1%,and energy efficiency of 78.3%over 100 cycles at 40 m A cm^-2.In addition,the reasons for the capacity decay over cycling in terms of electrolyte leakage,crossover,and chemical decomposition are discussed.Crossover is the main cause of capacity decay and enhancing the membrane performance is needed.Two-dimensional（2D）metal-organic framework（MOF）nanosheets-modified porous membranes are prepared by the filtration method.N-（ferrocenylmethyl）-N,N-dimethyl-N-ethylammonium bis（trifluoromethane-sulfonyl）imide（Fc1N112-TFSI）and iron（III）acetylacetonate are used as catholyte and anolyte,respectively,and TEATFSI is used as supporting electrolyte.The battery constructed with the 2D MOF nanosheets-modified membrane exhibits substantially higher CE（91.0%vs 82.9%）without obvious compromise on VE（93.7%vs 94.2%）,and larger average discharge capacity(1.30 vs 0.86 Ah L^-1)compared with the pristine Celgard membrane at 4 m A cm^-2.Such performance is originated from the unique microstructure of 2D MOF nanosheets.The interlayer spaces assembled by the nanosheet layers enable the fast transport of supporting electrolyte but impede the crossover of active materials and impose limited effect on the membrane resistance.To further mitigate the crossover of active materials and consider cost-effectiveness,a facile“rolling dough”approach is proposed for preparing flexible free-standing MOF-based mixed-matrix（MMM）membrane,which achieving100%materials utilization and excellent repairability.A Li-based hybrid NARFB using Fc1N112-TFSI as active species with lithium bis（trifluoromethane）sulfonimide as supporting electrolyte is used.The prepared membrane shows a remarkable ionic selectivity of Li⁺/Fc1N112⁺at high ratio of 26.6.The battery constructed with the MMM membrane exhibits high CE of 99.7%and average discharge capacity of 2.26Ah L^-1 with capacity retention of 99.96%per cycle over 200 cycles at 4 m A cm^-2.Such performance is attributed to the MOF nanoparticles and the MMM membrane,which shows high ionic selectivity,cycling performance and chemical stability.To further improve the performance of the membrane.A bifunctional MOF-based free-standing mixed-matrix membrane（MPM）is prepared with MOF and poly（vinylidene fluoride）by solution casting.The Fc1N112-TFSI/Li hybrid NARFB constructed with the MPM membrane exhibits excellent CE of 99.0%and average discharge capacity of 1.95 Ah L^-1 with capacity retention of 99.95%per cycle over500 cycles at 4 m A cm^-2.Furthermore,the VE of battery increases,and resistance decreases as cycling.Such excellent performance is attributed to the membrane not only can mitigate the active material by size sieving,but also the-SO₃^-anion group of the MOF provides a clear ion conduction channel.It can effectively regulate Li⁺ions flux and form a stable well-assembled interphase after long-term Li plating/stripping.