  Slurry electrodes are an effective means to improve flow battery performance
by reducing electrode fouling and increasing the active surface area necessary
for electrochemical reactions. It is critical to understand how the slurry
formulation impacts its rheological profile and ultimately battery performance.
We study the linear and nonlinear rheology of activated carbon (AC) based
slurry electrode used in an all-iron flow battery in the presence of a nonionic
surfactant (Triton X-100). Our results show the slurry mimics a colloidal gel
with elasticity remaining constant despite increasing surfactant concentration
until $\alpha$(=C$_{surf.}$/C$_{AC}$) < 0.65. However, at $\alpha \ge$ 0.65,
the slurry abruptly transitions to a fluid with no measurable yield stress.
This critical surfactant concentration at which the rheological profile
undergoes a dynamic change matches the concentration found previously for gel
collapse of this system. Moreover, this transition is accompanied by a complete
loss of electrical conductivity. These data show that site specific adsorption
of surfactant molecules often used in slurry formulation have a significant and
dramatic impact on flow behavior, which should be considered when formulating a
slurry electrode.
