Investigating the Effects of Slurry Preparation Protocol on Electrochemical Performance of Carbon Based Flowable Electrodes

In recent years, a new concept of electrode design known as flowable electrodes have been proposed as a novel approach for many stationary applications including grid-scale energy storage and water desalination [1]. To date, variety of chemistries and charge storage mechanisms regarding flowable electrodes have been reported including intercalation [2], double layer/faradic [3], precipitation/dissolution [4], and polymer redox [5]. Though major differences are present in their mechanism for charge storage, all flowable electrodes depend on inter-particle interactions and extent of formed percolation networks to conduct electrons to/from the current collectors. Currently, one major challenge that limits widespread utilization of flowable electrodes is their relatively lower electrical conductivity in comparison to conventional film electrodes. As much of the electrode mixture consists of highly resistive electrolyte phase, well dispersion of particles and the nature of percolation networks formed play a major role in final conductivity and performance of the flowable electrodes. In this study, effects of dispersion time and mixing methodology on electrochemical performance of flowable carbon electrodes have been investigated. Specifically, 20 different cases have been tested by systematically changing mixing time (1.5, 3, 4.5, 8, 15 minutes), mixing methodology (stir-bar vs. high-speed shear mixing), and electrode composition (activated carbon (AC) only and AC plus multi-walled carbon nanotube (MWCNT) slurries) to study a wide variety of cases. Each case was then subjected to direct current (DC) conductivity and cyclic voltammetry (CV) measurements to identify the effects on electrochemical performance. Constant current and particle size measurements were also conducted to support the findings in this study. From the data collected, conductivity differences up to 60% have been observed depending on the mixing time and mixing methodology for the same electrode composition (Fig.1). Additionally, capacity differences as high as 77% percent have been observed for the slurries containing MWCNTs. Also, as mixing time increased, closer performance gap between tested cases have been observed suggesting a common well-dispersed state in flowable electrodes. These observations highlight the significance of the degree of dispersion on the performance of capacitive flowable electrodes and suggest that higher mixing speeds and longer mixing times are necessary for better electrochemical performance in carbon based flowable electrodes. References: [1] K. B. Hatzell, M. Boota and Y. Gogotsi, Chemical Society Reviews , 2015, 44, 8664-8687. [2] M. Duduta, B. Ho, V. C. Wood, P. Limthongkul, V. E. Brunini, W. C. Carter and Y.-M. Chiang, Advanced Energy Materials , 2011, 1, 511-516. [3] V. Presser, C. R. Dennison, J. Campos, K. W. Knehr, E. C. Kumbur and Y. Gogotsi, Advanced Energy Materials , 2012, 2, 895-902. [4] F. Y. Fan, W. H. Woodford, Z. Li, N. Baram, K. C. Smith, A. Helal, G. H. McKinley, W. C. Carter and Y.-M. Chiang, Nano Letters , 2014, 14, 2210-2218. [5] Y. Zhao, S. Si and C. Liao, Journal of Power Sources , 2013, 241, 449-453. Figure 1