(166y) Rheological Characterization of Oxidized Carbon Black Suspensions in Aqueous Salt Solutions for Use As Flowable Electrodes in Capacitive Deionization

Climate change, coupled with rising demand on already strained aquifers, threatens water security across the globe. Currently, 1.5 billion people lack access to clean water, and the UN projects this number to rise to 2.8 billion before the end of 2025. In coastal regions like the Middle East and California, desalination has increasingly been utilized to address water scarcity. Whereas most industrial desalination methods rely on thermal distillation or high-pressure reverse osmosis to remove salt from saline and brackish water, capacitive deionization (CDI) has the potential to reduce energy usage while retaining high salt removal, ensuring lower water costs and increased accessibility.

One class of scalable CDI technology is flow capacitive deionization (FCDI). A key component of this technology is a suspension of electrochemically active particles that are continuously fed through a device to perform capacitive deionization of seawater. FCDI promises lower energy usage and expense compared to traditional CDI methods, but the high viscosity of slurry flow electrodes currently limits practical use. To overcome this hurdle, the viscosity of these suspension electrodes must be reduced while maintaining the ability to perform electrochemical work. Carbon black is the most common conductive material within a slurry electrode, and one method to lower viscosity is oxidation of the carbon’s surface.

In this work, we investigate the impact of acid-based surface oxidation on the colloidal stability of carbon black nanoparticles for use in FCDI cells. More specifically, we use dynamic light scattering and zeta potential to measure the stability ratio as a function of pH and ionic concentration and, in turn, its effect on the macroscopic rheology of carbon black in salt solutions. Under harsh oxidation conditions, we find carbon black adopts strong negative (repulsive) charge at high pH. As a result, improved colloidal stability is observed in comparison to the unoxidized form. This can be rationalized within the context of DLVO theory. Furthermore, our findings demonstrate strong surface oxidation provides a way to enhance stability in colloidal suspensions of carbon black in aqueous salt solution, thus reducing flow electrode viscosity and increasing viability for use in desalination technology.