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 H 3 PO 4 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.

Redox flow batteries (RFBs) have significant potential in grid-level electrical energy storage, generated by renewable sources such as wind or solar power. Of particular interest in this research are aqueous organic electrolyte systems due to their safety, cost, fast kinetics and greater sustainability compared to the use of conventional inorganic electrolytes or organic solvents. To date this area has been limited by both the solubility and long term cycling stability of organic electrolytes.1 The most difficult challenge in this field is the development of stable organic catholytes that have high redox potentials, high energy density, and high battery efficiency. Triarylamines (TAAs) have the potential to meet these criteria because of the ease of functional group modification allowing for variation of the redox potential, and the low reorganisation energy between the neutral and radical states.2,3 TAAs have previously been explored as catholyte materials in organic non-aqueous RFBs, with studies showing that the redox properties of these easily prepared compounds can be tuned by judicious choice of functional groups at the para positions.4 It was also shown that TAAs have high cycling stability compared to other popular catholytes such as TEMPOL.5 However, TAAs have not previously been used as catholytes in aqueous RFBs. In this work, a number of TAAs were explored with various substituents in the para-positions of their aromatic rings, with the aim of promoting good aqueous solubility and cycling stability (two of the main selection criteria for any RFB electrolyte). Tris-4-amino-phenyl amine was found to be the most promising candidate, with reversible redox at high positive potentials, ease of synthesis, and reasonable aqueous solubility. Extensive electrochemical investigations using cyclic voltammetry (CV), impedance and full cell cycling were carried out which provide clues as to how the TAA framework can be modified to improve the redox properties for future catholyte applications. Figure 1: Triarylamine framework containing various substituents and CV of 1mM Tris-4-amino-phenyl amine with a scan rate of 20 mV/s in 1 M HCl with a Ag/AgCl reference electrode, glassy carbon working electrode and platinum counter electrode 1 R. M. Darling, K. G. Gallagher, J. A. Kowalski, S. Ha and F. R. Brushett, Energy Environ. Sci., 2014, 7, 3459–3477. 2 Tohru Nishinaga, Organic Redox Systems, Synthesis, Properties and Applications, Wiley, 2016. 3 J. Wang, K. Liu, L. Ma and X. Zhan, Chem. Rev., 2016, 116, 14675–14725. 4 I. A. V. Romadina, Elena I., K. J. Stevensona and P. A. Troshin, Mater. Chem. A, 2021, 9, 8303–8307. 5 G. Kwon, K. Lee, J. Yoo, S. Lee, J. Kim, Y. Kim, J. E. Kwon, S. Y. Park and K. Kang, Energy Storage Mater., 2021, 42, 185–192. Figure 1