A comparative study on the solubility and stability of p-phenylenediamine-based organic redox couples for non-aqueous flow batteries

• Published in: Journal of power sources 2017-04, Vol.348, p.264-269
• Main Authors: Kim, Hyun-seung, Lee, Keon-Joon, Han, Young-Kyu, Ryu, Ji Heon, Oh, Seung M.
• Format: Article
• Language: English
• Subjects: Chemical stability; Li-flow batteries; Non-aqueous electrolytes; Organic redox couples; Solubility
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2017.03.019

A methyl-substituted p-phenylenediamine (PD), N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), is examined as a positive redox couple with high energy density for non-aqueous Li-flow batteries. Methyl substitution affects the solubility of the redox couple, as the solubility is increased by a factor of ten, to a maximum solubility of 5.0 M in 1.0 M lithium tetrafluoroborate-propylene carbonate supporting electrolyte due to elimination of the hydrogen bonding between the solute molecules. The methyl substitution also enhances the chemical stability of the cation radical and di-cation being generated from PD, as the redox center is shielded by the methyl groups. Furthermore, this organic redox couple demonstrate two-electron redox reactions at 3.2 and 3.8 V (vs. Li/Li+); therefore, the volumetric capacity is twice higher compared to conventional one-electron involved redox couples. In a non-flowing Li/TMPD coin-cell, this organic redox couple demonstrates very stable cycleability as a positive redox couple for non-aqueous flow batteries. •TMPD shows promising behaviors as positive redox couple for flow batteries.•The solubility (5 M) is greatly enhanced by elimination of hydrogen bonding.•The redox stability is improved by shielding effect from methyl substitution.•TMPD is electrochemically/chemically stable to give an excellent cycle performance.