Bridging reaction activity mismatch through asymmetric structure design for all-soluble all-iron aqueous redox flow battery

Abstract

All-soluble all-iron flow batteries are considered a promising technology for low-cost and large-scale energy storage. In the past few years, efforts have been taken to design various iron chelates to enhance the cycling stability of negative electrolytes, while ignoring the kinetic mismatch and the corresponding battery design strategies, which greatly limited the performance of all-soluble all-iron flow batteries. In this regard, combining experimental analysis and numerical simulation, this work analyzed the kinetic performance of iron chelates on the negative side and conducted further investigations on battery asymmetric structure design to balance mass transport and electrochemical reactions within the battery. Results show that the reaction rate constant of the $\mathrm{Fe}\left(\mathrm{II}\right){\left(\mathrm{BIS}-\mathrm{TRIS}\right)}^{2-}$/$\mathrm{Fe}\left(\mathrm{III}\right){\left(\mathrm{BIS}-\mathrm{TRIS}\right)}^{-}$ redox couple is $1.48\times {10}^{-5}$ cm s⁻¹, significantly lower than that of the positive electrolyte ($6.0\times {10}^{-5}$ cm s⁻¹), which limits the performance of the battery. Utilizing an asymmetric electrode design to increase active reaction sites and enhance convection is a critical strategy in achieving balanced mass transport and reaction activity between the positive and negative electrolytes. More notably, the battery with asymmetric geometric characteristics demonstrates a remarkable energy efficiency reaching up to 80.17 % at 80 mA cm⁻², which is 7.95 % higher than that of the symmetric structure. This research provides theoretical guidance for the structural design of key components of batteries and reduces the cost of trial and error.