Flow batteries

Porous electrodes are a crucial hardware for many sustainable electrochemical energy devices. They are broadly used in redox flow batteries and fuel cells as well as in electrochemical synthesis and separation. Although porous electrodes with many different internal fibre architectures have been studied, there is no clear consensus as to what structural features affect electrode performance. Recent results revealing heterogeneities on length scales an order of magnitude greater than the pore size challenge the common assumption that transport in porous electrodes can be approximated by a homogeneous Darcy-like permeability, particularly at the length scales relevant to many electrochemical systems.

In this study, we investigate various porous electrodes with different fiber architectures by visualization of the electrolyte charging process via in-situ confocal microscopy at length scales down to the single digit micron scale. We present fluorescent electrochemical microscopy, a strategy to couple confocal fluorescence microscopy with electrochemical monitoring as a new analytical method to investigate electrochemical flow systems in operando. The fluorescent signature of quinone electrolytes differs for the charged and discharged redox states, which enables us to map the local state of charge in three spatial and one temporal dimension (4D imaging). In addition, we use 4D fluorescent particle tracking to evaluate local velocity fields, fluid streamlines, and spatial heterogeneities. The correlation between flow fields and electrochemical concentration maps shows that not only the surface area limits the electrode efficiency but also the fibre arrangement affecting the overall electrolyte flow.