Tailoring Dihydroxyphthalazines to Enable Their Stable and Efficient Use in the Catholyte of Aqueous Redox Flow Batteries

Abstract

To enable cost-efficient stationary energy storage, organic active materials are the subject of current investigations with regard to their application in aqueous redox flow batteries. Especially quinones with their beneficial electrochemical properties and natural abundance pose a promising class of compounds for this challenging endeavor. Yet, there are not many active materials available for the catholyte side to realize solely quinone-based systems. Herein we introduce the novel hydroquinone 5,8dihydroxy-2,3-phthalazine together with two of its derivatives and propose it as a promising active material for the catholyte side of aqueous redox flow batteries. We systematically investigate the electrochemical properties as well as the structure−property relationship of this class of compounds. The unmodified dihydroxyphthalazine exhibits a favorably high redox potential of 796 mV vs SHE in acidic solution that is competitive with benzoquinone compounds. Moreover, the introduced dihydroxyphthalazines feature a high electron transfer rate surpassing benzoquinone species by almost one order of magnitude. With regard to stable cycling performance, we further achieved a high resilience against detrimental side reactions such as Michael addition by adding methyl substituents to the base structure. Our experimental findings are supported and extended by theoretical considerations in terms of density functional theory calculations. With this combined approach we outline further promising dihydroxyphthalazine-based materials with regard to performance-relevant quantities like redox potential, cycling stability, and water solubility. This study aims to propel further research in the field of quinone-based active materials for the catholyte of future aqueous redox flow batteries.

Electrochemical Characterization of Aromatic Molecules with 1,4-Diaza Groups for Flow Battery Applications

“…This way, high solubility, suitably placed redox potentials, fast charge transfer kinetics, and chemical stabilities in the order of decades may be achieved [ 4 ]. The most common groups of organic molecules studied for aqueous RFB applications include quinones [ 1 , 2 , 5 , 6 , 7 ] and N -containing organic heterocycles, such as phthalazine [ 7 ], quinoxaline [ 8 ], and phenazine [ 9 , 10 , 11 , 12 ].…”

Substituent Pattern Effects on the Redox Potentials of Quinone‐Based Active Materials for Aqueous Redox Flow Batteries

“…[13] Furthermore, it has been shown for diaza-anthraquinones and dihydroxy-phthalazines that À OMe, À Me and À OH result in redox potential lowering. [14,15] General chemistry concepts indicate that methyl substituents have a positive inductive effect, which should lower the redox potential. Methoxy and hydroxyl substituents show a positive resonance effect and a negative inductive effect which could lead to an increase or decrease of the redox potential.…”

“…This qualitative trend is also in agreement with the available experimental data for Qs with −SO 3 and −OH substituents . Furthermore, it has been shown for diaza‐anthraquinones and dihydroxy‐phthalazines that −OMe, −Me and −OH result in redox potential lowering . General chemistry concepts indicate that methyl substituents have a positive inductive effect, which should lower the redox potential.…”

Molecular redox species for next-generation batteries

“…Given all of the factors discussed above, renewed interest in RFBs has driven a broad search for new CCs across a wide range of molecular species, and recent studies have included the exploration of metal ions, 2,11 clusters, 48,49 metallocenes, 50,51 organic molecules, [52][53][54] and polymeric species. 1,55 In the following we will examine the molecular engineering approaches that have been applied to develop CCs with improved properties and thereby enhanced RFB performance.…”

“…High throughput computational screening methods have also been used to assess the structure-property relationship of quinones and their derivatives. 52 Careful molecular design of quinones for application as positive electrolyte is required to achieve sufficiently positive redox potentials and electrochemical stability. Janek, Schröder and co-workers investigated how the electron density, and therefore redox potential of the molecule is tuned by altering the quinone framework.…”