Sulfonated tryptanthrin anolyte increases performance in pH neutral aqueous redox flow batteries

Aqueous organic redox flow batteries (AORFBs) hold great promise as low-cost, environmentally friendly and safe alternative energy storage media. Here we present aqueous organometallic and all-organic active materials for RFBs with a water-soluble active material, sulfonated tryptanthrin (TRYP-SO3H), working at a neutral pH and showing long-term stability. Electrochemical measurements show that TRYP-SO3H displays reversible peaks at neutral pH values, allowing its use as an anolyte combined with potassium ferrocyanide or 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt monohydrate as catholytes. Single cell tests show reproducible charge-discharge cycles for both catholytes, with significantly improved results for the aqueous all-organic RFB reaching high cell voltage (0.94 V) and high energy efficiencies, stabilized during at least 50 working cycles.

battery studies using ferrocyanide and BQDS catholytes were reported. The battery data are not very exciting compared to status of the AORFB field. However, the effort to develop new redox active compounds for energy storage is appreciated. This manuscript can be considered after addressing the comments below.

As the focus of this study is on pH neutral AORFBs, I would like the authors to
provide a more informative summary on the status of pH neutral AORFBs, particularly viologen AORFBs which represent the state of the art AORFBs. I agree with the team that it is more attractive to develop pH neutral AORFBs compared to acidic and alkaline AORFBs which are subject to chemical degradation under corrosive conditions. In this regard, the author should highlight/compare the performance of pH neutral, acidic, and alkaline AORFBs in the introduction. I would recommend the authors to read and cite a very valuable review article on organic flow batteries (ACS Energy Lett. 2019, 4, 2220-2240.).
Reply from the authors: We thank the reviewer for his/her comments. Following the reviewer suggestions the introduction was carefully revised and the reference indicated incorporated as well as others that were used form that revision work. In particular, in the revised version of the manuscript the following text was added: "To overcome the limitation of the corrosive electrolytes (acidic or alkaline media), development of neutral aqueous RFBs has emerged over the years. 9,20,25,28,29 Neutral AORFBs are more ecofriendly and have outstanding advantages with non-corrosive electrolytes and inexpensive simple salts (e.g. KCl, NaCl) as supporting electrolytes. In addition, neutral pH electrolytes suppress undesired side reactions for active species caused by protons and hydroxides at acidic and alkaline conditions. 28,30 So far methyl viologen (MV) aqueous RFBs have demonstrated the most stable cycling performances with capacity retention up to 99.99% in neutral media. 19,28,29,31 Typically MV is employed as anolyte and ferrocene or (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) derivatives as catholyte to develop high-voltage and stable pH neutral aqueous RFBs. 19,29,30 Therefore, the search and development of new water-soluble active materials for improvement of neutral pH battery storage systems is increasingly significant and will continue to growth in the future."

I don't find the solubility of the tryptanthrin in the text. This data should be provided and discussed.
Reply from the authors: We thank the referee for pointing this out. The solubility tests were in fact only in the Supplementary Information in Table SI2 (data below).
Following the reviewer suggestion, the data is now provided and discussed in the main text of the new version of the manuscript.
The following sentences were incorporated in the new version of the manuscript: "Solubility measurements showed that TRYP-SO 3 H has a solubility of approximately 0.12 M in 1.0 M KCl at 293 K (see Table SI2 in SI). Therefore, the concentration of active materials used is relatively low to avoid the possible precipitation of active materials during the cycling." "In order to study the effect of the concentration of the all-organic active materials in the stability and performance of the RFB, the concentration of the redox pair was increased to 0.1 M, a concentration close to the solubility limit of TRYP-SO 3 H (0.12 M in 1.0 M KCl, see Table SI2 in SI)." Data from Table SI2:  Figure 5, capacity data should be provided and discussed in the main text.

In
Reply from the authors: The reviewer suggestion was accepted. Capacity data was provided and discussed in the revised manuscript. The paragraph below was changed (highlighted text) accordingly: there is a decrease in the charge and discharge energy density and capacity, with capacity retention falling to 38% of its original value (6.309 to 2.409 C) over fifty cycles (see Fig. SI11a in SI). In general, non-capacity-related coulombic efficiency loss (capacity fades over the cycles but coulombic efficiency increases, Fig. SI11a) mainly arises from electrolyte side reactions. 58 In the allorganic active materials cell (Fig. 4b) the values steadily increase until ~10 cycles and further stabilizes to a total number of fifty cycles (~29 h). Stable capacity retention was observed with more than 98% total capacity retention after forty cycles (Fig SI11b)." "It can be seen that at neutral pH the aqueous all-organic active materials flow cell presents a notable cycling retention (98%) and coulombic efficiencies above ~90% through several cycles (see Fig. SI11b in SI)." Figure SI11 was incorporated in SI: