Direct visualization of electrochemical reactions and comparison of commercial carbon papers in operando by fluorescence microscopy using a quinone-based flow cell

We demonstrate the use of fluorescence microscopy as a tool for mapping the spatial distribution of fluid flow and electrochemical reactions in an operating aqueous quinone flow cell. 9, 10-anthraquinone-2, 7-disulfonic acid (AQDS) is a reversibly redox active molecule with a reduced form (H₂AQDS) that fluoresces when excited by UV light. Visualization of AQDS/H₂AQDS within commercial porous carbon electrode papers enables a direct quantitative comparison of their performance. In particular, this technique illuminates surprisingly large-scale heterogeneous fluid flow profiles present in several carbon papers, leaving substantial areas of the electrode mass-transport limited. In others, more homogeneous flow distribution is observed, but limitations such as low electronic conductivity and limited accessible electrode surface area limit the performance. These observations provide insights into improving structural properties of porous electrodes for high-power density electrochemical flow cells.