As flow batteries gain attention for potential deployment for grid-scale energy storage, a deeper understanding of micro-scale phenomena, particularly electrochemical reactions within porous electrodes, is becoming increasingly important to predict and improve performance. In the past few years, aqueous-soluble organic molecules such as quinones have been increasingly studied because of their potential to be abundant, inexpensive active electrolytes for flow batteries. Furthermore, redox-active quinones can also have distinct fluorescence signatures enabling direct, in situ reaction-flow mapping using fluorescence microscopy. In contrast to many other in situ techniques used to characterize flow batteries, fluorescence microscopy enables fast (<100 ms) high resolution imaging enabling detailed understanding of reactions and fluid flow within porous electrodes.

In this study, we use fluorescence microscopy to bridge the micro-scale (<10 μm) and macro-scale (>2 cm) reaction-flow properties of flow batteries by mapping quinone reduction in flow past individual fibers and within bulk electrodes. To gain a deeper understanding of the reaction-advection-diffusion profile around individual fibers, spanning filaments are mounted perpendicular to the fluid flow using 3D-printed supports. The resulting quinones flow profile is imaged while passing a reducing current through the spanning fiber. This information is then correlated to bulk properties of porous electrodes.

Bulk properties of porous electrodes are also evaluated by fluorescence microscopy while operating a quinone-based flow battery at various fluid flow rates and electronic current densities. The abstract image here shows a snapshot of how reaction distributions can vary at low fluid flow rates. The results suggest that microscopically-heterogeneous, macroscopically-homogeneous electrode materials such as porous carbon papers can lack the full utilization of their surface area, and provide an opportunity for exploring improved electrode architectures. The results of this work aim to illuminate possibilities for improving the performance of flow batteries for grid-scale energy storage.