Summary An all‐iron aqueous redox flow battery using iron (Fe) 2,2‐bis(hydroxymethyl)‐2,2′,2′‐nitrilotriethanol (BIS‐TRIS) complex (Fe(BIS‐TRIS)) and Ferrocyanide (Fe[CN]6) as redox couple is newly suggested. The redox potential of Fe(BIS‐TRIS) is −1.11 V (vs Ag/AgCl) and this makes Fe(BIS‐TRIS) appropriate as active material for anolyte, while Fe(CN)6 is proper for catholyte due to its excellent redox reactivity, redox potential, and cheap cost. According to quantitative evaluations, Fe(BIS‐TRIS) does not produce any side reactions and is more stable than Fe triethanolamine (TEA) (Fe(TEA)) complex that is conventionally considered for the purpose. This fact is confirmed by computational analysis using density functional theory. In the calculation, energy barrier of Fe(BIS‐TRIS) suppressing the occurrence of undesirable side reactions is higher than that of other Fe‐ligand complexes, indicating that desirable redox reaction of Fe(BIS‐TRIS) occurs more stably. In redox flow battery (RFB) tests, RFBs using Fe(BIS‐TRIS) do not show any side reactions even after 250 cycles with excellent performances, such as capacity of 11.7 Ah L−1 and coulombic efficiency and capacity retention rate of 99.8 and 99.9%, respectively. This corroborates that RFBs using Fe(BIS‐TRIS) have excellency in both performance and stability, while the cheap cost of BIS‐TRIS and Fe(CN)6 enhances the economic benefit of RFBs.

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Summary In this study, to improve the performance of vanadium redox flow battery (VRFB), Fe‐triethanolamine (Fe(TEA)) is used as a precursor of carbon etching to form two‐scale mesoporous structure on graphite felt (GF) electrode. Specifically, large‐scale mesopores (~0.5 μm) and small‐scale mesopores (~10 nm) are produced on the fibers of GF electrode (Fe(TEA)‐TGF). With the mesopores, the specific surface area and the number of active site of GF electrode are significantly increased, and the overvoltage sacrificed on GF electrode is reduced. When the performances of VRFBs using the treated GF electrodes are measured, energy efficiency (EE) of VRFB using Fe(TEA)‐TGF is 86.1%, which is 8.2% and 2.6% higher than that of VRFBs using pristine GF (79.6%) and TGF (83.9%) at 80 mA cm−2. In addition, in a long‐term test of VRFB performed at a high current density of 200 mA cm−2, VRFB using Fe(TEA)‐TGF shows the improved performances with voltage efficiency and EE of 69.5% and 68.0%. Based on that, this is proved that Fe(TEA)‐TGF can be used to improve the performances of VRFB even at high current density ranges.

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