Impact of Surfactant and Flow Rate on the Electrical Properties of Activated Carbon Black Suspensions

Carbon black slurries are a key component in redox flow batteries, as the large surface area provided by the particles allows an increase in the battery capacity without facing limitations posed by many solid-state batteries, such as safety hazards or cost. However, these conductive slurries often have complex mechanical and electrical responses because of the heterogeneous nature of the suspensions. Utilization of these slurries in a redox flow battery requires an understanding of how additives impact the material responses and associated battery performance. This work focuses on the electrochemical performance of the slurry under flow rate conditions matching those of battery operation. In addition, the impact of a nonionic surfactant (Triton X-100) on the conductivity and capacitance of the slurry was measured. Experimental results show that the full capacitive contribution of the carbon black particles can only be measured at low flow rates and low scan rates, while the conductive contribution can be measured at all scan rates under flowing conditions. Upon the addition of surfactant, there is a gradual decrease in the electrochemical performance with increasing surfactant concentration, until the surface of the carbon black particle is saturated. Once saturated, the conductive carbon particles no longer contribute to the electronic conductivity of the slurry. Results presented herein on the electrochemical response of the slurry to the addition of a surfactant are in stark contrast to the mechanical response. While previous work has shown a smaller change in the response, followed by a step-change at a critical surfactant concentration, electrochemical data show a gradual transition. Comparison of these behaviors suggests a difference in the mechanisms for how mechanical networks form in comparison with charge transfer networks for this particular slurry chemistry.