Surfactant Induced Catastrophic Collapse of Carbon Black Suspension used in Flow Battery Application

TL;DR

This study investigates how adding a nonionic surfactant to carbon black suspensions in flow battery electrodes affects their stability, revealing that excessive surfactant causes gel collapse and reduces viscosity, impacting battery performance.

Contribution

It demonstrates that surfactant concentration critically influences gel stability and induces catastrophic collapse in carbon black suspensions used in flow batteries.

Findings

Surfactant improves dispersion and gel elasticity below a threshold.

Above the threshold, gels weaken and collapse catastrophically.

High surfactant levels lower suspension viscosity.

Abstract

Carbon black particles act as electronically conductive additives in the slurry electrodes used in electrochemical redox flow batteries. Stability and dispersion of the carbon black particles in a slurry electrode are critical parameters for its storage and the efficient functioning of the battery. Modifying the carbon black slurry formulation with the addition of a nonionic surfactant could potentially impart desired properties such as good particle dispersion, gravitational stability and flowability imparting better performance of the flow battery. Matching the typical slurry electrode formulation, we dispersed carbon black particles in 1 M H$_2$SO$_4$ with volume fraction $\phi$ = 0.01 to 0.06 and c$_{surf.}$ = 0, 0.05 and 0.1 M. Rheological investigation reveals that the carbon black suspensions behave like colloidal gels. Sedimentation kinetics of the process was measured by tracking the height of the particle bed over time in a cuvette using a custom camera set-up. The sedimentation dynamics here clearly resembled that of a gel collapse. At short times we observe fast sedimentation associated with structural collapse of the gel and at long times very slow sedimentation associated with compaction of the sediment. Addition of a nonionic surfactant (Triton X-100) to the solvent above CMC at $\alpha$ (= c$_{surf.}$/c$_{CB}$) $<$ 0.7 improves particle dispersion and increases gel elasticity. However, for $\alpha >$ 0.7 leads to the formation of a weaker gel that exhibits 'catastrophic collapse' under gravity and has a lower viscosity.