Development of organic redox-active materials in aqueous flow batteries: Current strategies and future perspectives

Aqueous redox flow batteries,by using redox-active molecules dissolved in nonflammable water solutions as electrolytes,are a promising technology for grid-scale energy storage.Organic redox-active materials offer a new opportunity for the construction of advanced flow batteries due to their advantages of potentially low cost,extensive structural diversity,tunable electrochemical properties,and high natural abundance.In this review,we present the emergence and development of organic redox-active materials for aqueous organic redox flow batteries(AORFBs),in particular,molecular engineering concepts and strategies of organic redox-active molecules.The typical design strategies based on organic redox species for high-capacity,high-stability,and high-voltage AORFBs are outlined and discussed.Molecular engineering of organic redox-active molecules for high aqueous solubility,high chemical/electrochemical stability,and multiple electron numbers as well as satisfactory redox potential gap between the redox pair is essential to realizing high-performance AORFBs.Beyond molecular engineering,the redoxtargeting strategy is an effective way to obtain high-capacity AORFBs.We further discuss and analyze the redox reaction mechanisms of organic redox species based on a series of electrochemical and spectroscopic approaches,and succinctly summarize the capacity degradation mechanisms of AORFBs.Furthermore,the current challenges,opportunities,and future directions of organic redox-active materials for AORFBs are presented in detail.

Evidence for a Bulky Unit of a Pillar[5]arene Flipping in the Solid State

It is well known that pillar[5]arenes have two most stable conformations(pS and pR)in their crystal structures.Because of the intramolecular H-bonding interactions,substituents,temperature,solvent and so on,the rotational behaviors of the phenolic units on pillararenes are also common.This paper showed some other kinds of conforma-tions in the functionalized pillar[5]arenes and gave evidence for a bulky unit(1,4-methoxycarbonylmethoxybenzene unit)flipping in the solid state.The presence of hydrogen bonding facilitated the intermolecular self-assembly in terms of energy-minimized packing in the crystals.Thus,the main driving force for the flipping of this bulky unit might be both the intramolecular hydrogen bonding between the phenolic units on pillararenes and quadrupolar hy-drogen bonding between the host and water.This paper helps us to have a better understanding on the conforma-tions of pillar[5]arenes.

Synthesis of a Pillar[5]arene with Both Hydroxyl and Methoxycarbonyl-Methoxy Groups and Its Host-Guest Complexation with a Bis(imidazolium)Salt

By the introduction of methoxycarbonyl-methoxy groups and hydroxyl groups into a pillar structure,a pillararene entirely with two types of functional groups was successfully prepared,which can form a stable 1∶1 complex with a bis(imidazolium)salt in CHCl3/acetone solution(V∶V=1∶1).