## 1 Introduction  
Flowable electrodes (FEs) or semi-solid electrodes have received significant interest for large-scale applications such as wastewater treatment, seawater desalination, and grid-scale energy storage systems. FE is a material system comprising an active material (electrode) suspended in an electrolyte solution. Advanced technologies such as redox flow batteries [1, 2], fuel cells [3, 4, 5], and electrochemical flow capacitors (EFCs) [6] use FEs to improve the scalability limitations of electrochemical energy storage. In such systems, the continuous flow of semi-solid electrodes provides the cell's steady charge capacity when the uncharged slurry is provided as input [7]. In principle, the charge storage in FE systems originates because of different mechanisms such as electric double-layer formation and faradic reactions [6, 8], surface redox reactions [9, 10], intercalation [11], polymer redox [12, 13], and hybrid mechanisms [14, 15]. The operating performance of the FEs relies on support from the diffusion of the ions [16], electronic charge in and out of the pores via a particle-current collector, particle-particle interactions [17], and the formation of the electrical double layer (EDL) at the pore surface [18, 19]. Under the mild flow condition, the suspended electrode particles and their mutual interaction establish a dynamically varying physical contact for charge transport through the bulk material. Thus, the particle-particle interaction becomes indispensable for the charging/discharging of FEs [20, 21, 22]. A major challenge for FE applications is the accurate prediction of rheological and electrical properties of suspended electrodes [23, 24]. Low electrical conductivity and high cell resistance are the attributes of a high electrolyte concentration compared with the electrode. However, the conductivity can be improved by using a conductive additive [25], optimizing particle size and shape [26], and utilizing a finely dispersed suspension [27]. The design of the EFC cell is critically important to facilitate the FEs (i.e., mitigate clogging) and maximize system performance. A reduction in ohmic losses can be achieved by using shallow channel geometry and supporting the free passage of the FEs without clogging. The design of EFC is also dependent on the properties of the FEs used for capacitive storage. In particular, the concentration of electrode particles, particle size/shape, and carrier-fluid (electrolyte) viscosity have a significant effect on the performance of the FEs and can be altered to obtain improvements in rheological and electrochemical properties for enhanced slurry flow and EFC performance [6]. The charge is stored by forming an electric double layer at the interface of the highly porous electrode and the surrounding electrolyte. Once charged, the FE is pumped into ionically and electronically isolated external reservoirs, where it is held until the stored energy is needed to be recovered (Figure 1). Previous investigations showing the low-current-density operations of an EFC required a better understanding of the relationships between slurry properties, preparation protocols, operating mode, and device design improvements [6; 28]. For instance, slurry preparation by stir-bar and turbo-shear mixing methods was used to study the mixing effects on the non-additive slurry compositions and those containing multi-walled carbon nanotubes (MWCNTs). When more time was allowed for mixing, the electrical conductivity of the FE improved by up to 57% [27]. Additional studies showed that the electronic conductivity of FE relies on the particles' direct physical contact or the surface-to-surface hopping mechanism of electrons [21; 28], and the conductivity was improved through redox-active mediators [29] or high-salt-concentration aqueous electrolytes [30]. Furthermore, a biphasic suspension electrode at 20% vol of active material (LiFePO4) was found to customize the interactions between the active and conducting particles and improve flowability and electronic conductivity [31].