Structural Modifications of Some Nitrogen-containing Redox Couples for Improvement of Non-aqueous Flow Battery Performances

Abstract

As redox couples for non-aqueous flow batteries, the electrochemical and physicochemical properties of several nitrogen-containing molecules are examined. Nitrogen atom have lone pair electrons, which is applicable as ligand and redox center

thereby applying as redox couple design. Moreover, the negative charged nitrogen atom is nucleophile, resulting readily functional group substitution by SN2 reaction. Thus, nitrogen atom can be applied as ligand molecule, redox center and the attachment center of aliphatic group for structural modification of redox couples. While non-aqueous flow battery is highlighted by its energy density from wide electrochemical stability window of organic electrolyte, the solubility drawback is remained problem for practical application of non-aqueous system. Since the nitrogen can be applied with diverse manner for molecule design, the limitation of solubility can be resolved by rational design of nitrogen-containing redox couples. At first, an azamacrocyclic ligand-based complex cation, nickel(II)-1,4,8,11-tetraazacyclotetradecane (cyclam) is examined as a single redox couple for non-aqueous flow batteries. Single redox couple has advantageous feature for practical application because permanent loss of active material from cross-contamination at dual electrolyte-comprised cell is completely prevented. The energy density of this complex cation is tailored by easily dissociative counter anions and using highly dielectric solvents. The nickel(II)-chelated complex cation demonstrates high solubility (0.8 M) and working voltage (2.55 V) with bis(trifluoromethane)sulfonimide anion, resulting the energy density of 27.3 W h L-1. Secondly, nitrogen atom is used as two redox centers in p-phenyldiamines (PD) as positive redox couple. Two amine groups (-NH2) in PD offers two redox reactions with single organic molecule. Thus, the demonstrated volumetric capacity from PD is twice higher than conventional one electron-involved redox couples at same concentration

thereby alloying cost advantages. Nevertheless, the solubility and chemical/electrochemical stability drawbacks are remained for flow battery applications in PD redox couple. Methyl substitution affects the solubility and chemical reversibility of redox couple because the methyl groups eliminates hydrogen bonding and shields foreign attack from electrolyte components. Fully-methyl substituted N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) delivers high energy density with 938.0 W h L-1, which is from 5.0 M of solubility and 3.2 and 3.8 V (vs. Li/Li+) of working voltages. Furthermore, TMPD has facile diffusion rate, which is desirable for good rate capability. Finally, butyl-substituted, N-butylphthalimide (BPI) is proposed as negative redox couple for non-aqueous flow batteries. The ten-fold increase of solubility (5.0 M) and decrease of melting point is from the less-packed asymmetric structure by attaching butyl groups on nitrogen atom. The strong correlation between maximum solubility and melting point implies this result. The electron-donating effect and solvation change of butyl groups shift the working voltage (0.1 V), resulting higher energy density. Consequently, BPI/TMPD comprised flow cell demonstrates promising electrochemical performance as all-organic flow batteries and the theoretical energy density of this cell is 120.6 W h L-1. The nitrogen-containing redox couple design is accomplished by directional approaching based on ideal solubility equation. Designed redox couples have higher energy density than conventional aqueous electrolyte ones

for instance, all-vanadium redox flow batteries, 25.0 W h L-1. Thus, the high energy density non-aqueous flow batteries are designed by comprising nitrogen-containing molecules.