Flow field design and visualization for flow-through type aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs), which exploit the reversible redox reactions of water-soluble organic electrolytes to store electricity, have emerged as a promising electrochemical energy storage technology. Organic electrolytes possess fast electron-transfer rates that are two or three orders of magnitude faster than those of their inorganic or organometallic counterparts; therefore, their performance at the electrode is limited by mass transport. Direct adoption of conventional cell stacks with flow fields designed for inorganic electrolytes may compromise AORFB performance owing to severe cell polarization. Here, we report the design of a flow field for flow-through type AORFBs based on three-dimensional multiphysics simulation, to realize the uniform distribution of electrolyte flow and flow enhancements within a porous electrode. The electrolyte flow is visualized by operando imaging. Our results show that multistep distributive flow channels at the inlet and point-contact blocks at the outlet are crucial geometrical merits of the flow field, significantly reducing local concentration overpotentials. The prototype pH-neutral TEMPTMA/MV cell at 1.5 M assembled with the optimized flow field exhibits a peak power density of 267.3 mW cm^-2. The flow field design enables charging of the cell at current densities up to 300 mA cm^-2, which is unachievable with the conventional serpentine flow field, where immediate voltage cutoff of the cell occurs. Our results highlight the importance of AORFB cell stack engineering and provide a method to visualize electrolyte flow, which will be appealing to the field of aqueous flow batteries.