Coupled optical-electric monitoring of charge percolation events in carbon flow-electrodes

The industrial application of electrochemical processes is limited by a variety of challenges, such as catalyst stability and selectivity. However, the question of how to efficiently exploit the entire reactor volume for the process is also essential for a viable implementation. Flow electrodes, which consist of conductive particles suspended in the electrolyte, are promising for increasing the active electrode area without increasing the reactor footprint. They are currently being used for different applications, such as energy storage or desalination. However, especially the charge transport mechanisms in flow electrodes remain unclear because the occurring phenomena are fast and highly complex. In this work, we introduce a methodology that combines electrical measurements with high-speed recording of particle movements in flow electrodes, simultaneously. Our algorithm evaluates the video sequences and calculates the detected particle area in the channel. This allows the electrical current to be correlated with the actual amount of particles in the channel. The results reveal that only an aqueous electrolyte can result in a substantial increase in conductivity because of the particles present in the fluid channel. Furthermore, the conductivity increases with the number of particles, regardless of the observed particle-electrode contact. When the particle flow was hydrodynamically forced toward the electrode, we found that the particle-electrode contact increased the conductivity even more strongly than that of isolated particle clusters in an unfocused flow. However, the greatest effect on the electrical conductivity relates to the formation of particle bridges resulting from the electrical percolation networks between the electrodes.