Influence of Surface Chemistry on Rheological and Electrochemical Properties in Flowable Electrodes

Abstract

Recently there has been a renaissance of new flow-assisted electrochemical devices for grid energy storage and desalination (capacitive deionization) [1-5]. Among these new technologies is a new family of flow systems that rely on flowable suspensions for charge storage. Unlike traditional flow-batteries which utilize insulating redox-electrolytes, suspension electrodes (flowable electrodes) rely on percolation networks of contacting active materials to facilitate charge storage. Capacitive suspension electrodes (CSEs) are one type of flowable electrode that typically utilize a porous active materials [6,7], and store charge electrostatically in an electric double layer. Unlike traditional supercapacitors, the flowing nature of this system offers the ability for continuous and scalable charge storage. From a materials viewpoint, it is important to understand how the active material’s surface heteroatoms effects the electrochemical, kinetic, and rheological performance of flowable electrodes. Surface functionalization has been widely studied in the electrochemical capacitor literature as a means for achieving pseudocapactive charge storage [8], however in a suspension electrode the surface functional groups will play a role in how active materials interact in percolation networks, suspension stability, and ultimately the flow behaviour [9]. Thus, in this study the combined pseudocapacitive and rheological properties of CSEs based on activated carbon enriched with oxygen heteroatoms was examined. Oxidation of the carbon led to an increase in the rise potential and a decrease in the accessible voltage window of the suspension electrode. Furthermore, it was shown that the oxidized carbon suspension electrodes demonstrated lower viscosities than suspension electrodes based on unoxided activated carbon. The increased flowability can be attributed to a greater degree of electrostatic stabilization achieved as the oxygen-rich activated carbon exhibited a greater surface charge. These result demonstrate that high-mass loading can be achieved in flowable electrode through careful design of the active material surface functional groups. The ability to achieve high mass-loading in suspension electrodes will lead to higher energy density and facilitate more contact points within the electrode for efficient ion and charge transport.