(51b) Stability of Carbon Black Slurry Used in Flow Battery Applications

Carbon black particles act as an additive that conducts electrons in the slurry used in electrochemical flow batteries. Stability of the carbon black slurry is a critical parameter for its storage and the efficient functioning of the battery. As a result of particle aggregation, sedimentation and deposition onto the device surface, carbon black concentration changes throughout the flow geometry. This not only prevents electrodes from being recycled but also increases the probability of flow channels being clogged. Modifying the carbon black slurry formulation and the flow behavior with addition of a surfactant would improve the suspension stability and the performance of the flow battery. Matching the typical flow battery formulation, we suspended carbon black particles in 1M H₂SO₄ in concentrations ranging from 1-5 vol%. Sedimentation kinetics of the process were measured by tracking the height of the sedimentation bed over time in a cuvette using an in-house camera set-up. Our study reveals that while initial concentration of the carbon black suspension exhibits different sedimentation kinetics, the final packed bed volume fraction is independent of the initial slurry concentration. We attribute this to the formation of a stable gel phase due to the interparticle attractions between the carbon black particles. Furthermore, we modify the interparticle attractions by adding a nonionic surfactant (Triton X-100) above, at, and below the critical micelle concentration (CMC) and study the effect on sedimentation kinetics and gelation volume fraction. Finally, we propose a phase diagram identifying the stable and unstable regimes as a function of particle volume fraction and surfactant concentration.