Fe-triethanolamine is a promising candidate as anolyte for redox flow batteries (RFBs), owning to its low potential, high solubility and low cost. We report here a new dinuclear structure of this complex at solid state when prepared with a stoichiometric amount of triethanolamine and iron in basic medium, whereas more than two equivalents of ligands are usually used to prepare Fe-triethanolamine for RFBs application. We achieve a calibration curve to estimate Fe(III) concentration in solution and coulometric experiments highlight a oneelectron reduction process per iron atom, corresponding to the reduction of the two Fe(III) atoms of the dinuclear complex into Fe(II). A solubility higher than 1.2 mol dm-3 can be reached for Fetriethanolamine with the new synthesis proposed in this work. All-Fe alkaline RFBs implemented with Fe-triethanolamine exhibit good performances in terms of coulombic, voltage and energy efficiencies, and is stable over a hundred of cycles. A power density around 80-120

The performance of a redox compound in redox flow batteries (RFB) highly depends on the electrolytic medium and operating conditions. It is exemplified in this work with the commercially available and relatively low-cost dye 3,4-dihydroxy-9,10-anthraquinone-2-sulfonic acid (ARS), which was used as negolyte in basic medium. At high pH, the ARS behavior revealed interesting features for RFB applications, such as a low halfwave potential of À 0.99 V (vs Ag/AgCl), negatively shifted by phenolate groups, and an improved solubility compared with acidic medium depending on the nature of the cations. For the highly soluble ARS potassium salt (ARSK), a maximum power density of 117 mW cm À 2 and a demonstrated energy density of 20 Wh L À 1 were obtained with K 4 [Fe(CN) 6 ] as posolyte. The capacity slightly decreased during cycling, reaching 90 % after 325 cycles. A long cycling of ARS sodium salt (ARSNa) over 11 operating months was demonstrated in this work. A slow chemical degradation was highlighted giving rise to the formation of 3-hydroxy-9,10-anthraquinone-2-sulfonic acid (HAQS) as the main degradation product due to hydrodeoxygenation reaction. Interestingly, this compound exhibited high performance in RFB and a good stability with a loss of capacity of 0.29 % per day.