Triarylamines as Catholytes in Aqueous Organic Redox Flow Batteries

In the flow: A series of triarylamines was explored as catholytes in aqueous redox flow batteries for assessing the effect of substituents and supporting electrolyte on performance. Tris(4-aminophenyl)amine is found to be the strongest candidate with promising solubility and initial electrochemical performance while the mixed supporting electrolyte further improves its capacity and cycling stability.

Abstract

A series of triarylamines was synthesised and screened for their suitability as catholytes in redox flow batteries using cyclic voltammetry (CV). Tris(4-aminophenyl)amine was found to be the strongest candidate. Solubility and initial electrochemical performance were promising; however, polymerisation was observed during electrochemical cycling leading to rapid capacity fade prescribed to a loss of accessible active material and the limitation of ion transport processes within the cell. A mixed electrolyte system of H₃PO₄ and HCl was found to inhibit polymerisation producing oligomers that consumed less active material reducing rates of degradation in the redox flow battery. Under these conditions Coulombic efficiency improved by over 4 %, the maximum number of cycles more than quadrupled and an additional theoretical capacity of 20 % was accessed. This paper is, to our knowledge, the first example of triarylamines as catholytes in all-aqueous redox flow batteries and emphasises the impact supporting electrolytes can have on electrochemical performance.

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