All iron aqueous redox flow batteries using organometallic complexes consisting of iron and 3-[bis (2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid ligand and ferrocyanide as redox couple

The Organometallic complex consisting of iron and 3-[bis (2-hydroxyethyl) amino]-2-hydroxypropanesulfonic acid (DIPSO) ligand (Fe(DIPSO)) is newly suggested as negative active species for redox flow battery (RFB), while ferrocyanide (Fe(CN)₆) is used as positive active species. When the two active species are used, cell voltage of the RFB reaches 1.37 V. In a comparison of Fe(DIPSO) and Fe-triethanolamine (Fe(TEA)), redox potential of the two complex is similar as −1.05 V (vs. Ag/AgCl), but the Fe(DIPSO) does not suffer from the reduction to metallic iron (Fe(s)) that is the general problem of iron based organometallic materials, meaning that Fe(DIPSO) has a strong resistance against the reduction to Fe(s) and this can induce obvious enhancements in the performance and stability of RFB using the complex. Actually, when RFB using 0.5 M Fe(DIPSO) and Fe(CN)₆ is operated at 80 mA cm⁻², its discharge capacity is 14.4 Ah L⁻¹, power density is 93.2 mW cm⁻² and energy efficiency is 70% and even after 100 cycle, while the capacity preserves well with the decay rate of 0.12% per cycle. In contrast, in the RFB using 0.5 M Fe(TEA) and Fe(CN)₆, although initial discharge capacity is 13.4 Ah L⁻¹, decay rate is very high as 0.96% per cycle with a rapid decrease of columbic efficiency from 99.5 to 95.4% for the initial 25 cycle. This is due to the conversion of ferrous/ferric core to Fe(s) occurring at Fe(TEA) during cycle. Based on that, it is revealed that the RFB using 0.5 M Fe(DIPSO) and Fe(CN)₆ shows excellent performance and stability demonstrated as high power and energy densities.