Slurry Electrode Properties of Carbon Nanotubes and Graphene for Flow Cell Energy Storage Applications
Authors: Sodiq Ahmed 
Published: June 2019

Redox Flow Batteries (RFBs), among other options, is uniquely suited for large-scale Electrical Energy Storage (EES) due to many advantages over the conventional sealed batteries (e.g., lead-acid and Li-ion batteries). Owing to its design flexibility, flow batteries can meet a wide range of commercial-scale EES applications with varying power-to-energy ratios. In addition, flow batteries have excellent energy efficiency and competitive life-cycle cost and are considered to be inherently safer compared to conventional sealed batteries. Despite its attractiveness and demonstrated performances, flow battery technology still suffers from high system capital cost (which limits its potential deployment into grid storage applications) and low energy density of the reactant fluids (which leads to large footprint and makes it impractical for EES applications with confined spaces). These challenges can be addressed if higher energy density and cheaper battery materials are developed. An attractive approach has been recently proposed, where the active materials are mixed with carbon to create a flowable electrode (otherwise known as slurry electrode). This system helps to operate flow battery with increased concentration of redox species and as a result the energy density is significantly improved.Slurry electrode (shown in Fig. 1) has been defined as “a suspension of particles with large double-layer capacity, such as activated carbon, in an electrolyte solution” [1]. These particles transfer charge from an electrochemical cell to an external reactor, where a substance is oxidized or reduced, and are recharged in the cell [1]. Unlike conventional RFB system, the semi-solid technique uses fluid electrodes that are electronically conductive [2]. Slurry electrodes are characteristically dynamic, particles continuously move and continuously make and break contact with one another. Percolation theory defines a critical loading, fc, above which there are enough particles in the slurry that, at any point in time, a constant network of particles is formed that spans the distance from one boundary to another [3]. It is due to these networks of percolated particles that slurries (with sudden increase in conductivity and viscosity at percolation) are able to conduct electrons out into the electrode and away from the current collector to utilize the high surface area of the particles for electrochemical processes [4, 5]. Other advantages of slurries involve their ability to have high surface areas, simple assembly and ease of maintenance through filtering (for example the carbon materials can be recycled without necessarily opening up the cell). The performance of slurry-electrode-based flow batteries highly depends on the features of the interaction between the particles and current collectors. Parameters such as storage duration or maximum power can be tuned by adjusting the properties of the slurry, being extremely important are the ionic and electrical conductivities of the slurries. Figure 1 In this presentation, the performance of slurry electrodes, based on carbon and redox systems (Vanadium/CNTs and Vanadium/Graphene), focusing in particular on electrochemical performance, conductivity, corrosion, and cell performance will be presented. We achieved high electrochemical activities in the case of Vanadium/CNTs as compared with Vanadium/Graphene as shown in Fig. 2. According to the structural features of the CNTs; the oxygen functional groups improve the hydrophilicity of the electrode. The O = C-OH groups on the CNTs probably behave as active sites, and facilitate the reactions of VO2+/VO2+ redox couple. Figure 2 References [1] B. Kastening, et al., «Design of a slurry electrode reactor system,» Journal of applied electrochemistry, vol. 27, pp. 147-152, 1997. [2] Z. Li, et al., «Aqueous semi-solid flow cell: demonstration and analysis,» Physical Chemistry Chemical Physics, vol. 15, pp. 15833-15839, 2013. [3] L. Gao, et al., «Effective thermal and electrical conductivity of carbon nanotube composites,» Chemical Physics Letters, vol. 434, pp. 297-300, 2007. [4] T. J. Petek, et al., «Characterizing Slurry Electrodes Using Electrochemical Impedance Spectroscopy,» Journal of The Electrochemical Society, vol. 163, pp. A5001-A5009, 2016. [5] H. Parant, et al., «Flowing suspensions of carbon black with high electronic conductivity for flow applications: Comparison between carbons black and exhibition of specific aggregation of carbon particles,» Carbon, vol. 119, pp. 10-20, 2017/08/01/ 2017. [6] H. Yoon, et al., «Pseudocapacitive slurry electrodes using redox-active quinone for high-performance flow capacitors: an atomic-level understanding of pore texture and capacitance enhancement,» Journal of Materials Chemistry A, vol. 3, pp. 23323-23332, 2015.