Surfactant induced catastrophic collapse of carbon black suspensions used in flow battery application.

Hypothesis: Carbon black particles act as electronically conductive additives in the slurry electrodes used in electrochemical redox flow batteries. Modifying the carbon black slurry formulation with the addition of a nonionic surfactant could impart improved particle dispersion, gravitational stability, and flowability leading to better battery performance.

Experiments: Carbon black particles were dispersed in 1 M HSO with volume fractions Φ = 0.01 to 0.06 and a nonionic surfactant (Triton X-100) concentration of c = 0, 0.05, and 0.1 M. Particle size was characterized using microscopy and surfactant adsorption using UV-vis spectroscopy. Sedimentation kinetics was measured using a custom camera set-up that tracks the height of the settling particle bed. Rheology experiments were conducted to measure linear viscoelasticity and shear flow behavior.

Findings: The sedimentation dynamics of the slurry resembled that of a gel collapse. At short times we observed fast sedimentation associated with structural gel collapse and at long times very slow sedimentation associated with compaction of the sediment. Rheological investigations revealed that the slurry indeed behaved like colloidal gels. Addition of nonionic surfactant at α (= (c/c)) < 0.75 improved particle dispersion and increased gel elasticity. However, α> 0.75 led to a weaker gel that exhibits a fast 'catastrophic collapse' under gravity.