Simultaneous Optical and Electrical Monitoring of Charge Percolation in Flow Electrodes

Abstract

Electrochemical reactions are limited to the surface of the electrode, which makes the construction and intensification of electrochemical processes challenging. To increase the space-time yield of electrochemical cells, the electrochemically active surface area has to be enlarged. This can be achieved by either increasing the surface of a fixed electrode (e.g., porous electrodes) or by so-called flow electrodes. Flow electrodes consist of conductive particles suspended in the electrolyte. They have the advantage of increasing the active surface area of the electrode and also enabling continuous recycling and exchange of the active electrode material. They have been successfully applied, for example, for energy storage in redox flow batteries and desalination in flow-electrode capacitive deionization (FCDI).In flow electrodes the conductive particles dynamically increase the active area of the electrode by collision with the electrode; however, the exact mechanism of charge transport in such suspensions, also called charge percolation, is not completely understood. Yet, the successful implementation of flow electrodes in electrochemical processes requires a deeper understanding of how these particles interact within the electrochemical system. Charge percolation in flow electrodes is highly dynamic, and the conductivity of the suspensions depends on multiple factors like the amount and position of the particles, the shape and accessibility of the current collector, and especially the contact between particles and current collector. Literature presents different approaches which deal with charge transport in conductive suspensions, either by analyzing the electrical properties of flow electrodes in flow or by examining the structure and position of the particles visually in static conditions. We combine visual and electrical in-situ monitoring of charge percolation events in a single experimental setup. We developed a transparent electrochemical flow cell, made of transparent polydimethylsiloxane (PDMS), allowing for in-situ measurements. A constant voltage is applied to the current collector by a potentiostat. The resulting electrical current is measured simultaneously by recoding the particle movements using an optical microscope and a high-speed camera. The design of the electrochemical cell enables different positions of the particles within the channel in different flow modes: Contact between particles and the current collector can be either prevented or enforced. A motion-tracking algorithm analyses the videos to determine the area of particles in the electric field and the area of particles contacting the current collector. This allows for the correlation of particle behavior and electrical current.We investigated the influence of electrolyte, particle fraction, and the contact probability of particles and current collector on charge transport. Our results show that contact between particles and the current collector is a main contributor to charge transport in flow electrodes. But particles cluster can also increase the current, even if they do not touch the current collectors. The highest increase in current was achieved for large particle fraction short circuiting both current collectors. These results show that the way the particle suspension interacts with the electrode and by guiding the flow the electrochemical processes can be improved dramatically.