Carbon Materials as a Flowable Electrode in Electrochemical Flow Capacitors

Investigation of Carbon Materials for Use as a Flowable Electrode in Electrochemical Flow Capacitors
Jonathan W. Campos, Majid Beidaghi, Kelsey B. Hatzell, Christopher R. Dennison, Benjamin Musci, Volker Presser, Emin C. Kumbur, Yury Gogotsi
http://dx.doi.org/10.1016/j.electacta.2013.03.037

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Novel electrical energy storage concept, the electrochemical flow capacitor (EFC), holds much promise for grid-scale energy storage applications. The EFC combines the principles behind the

Carbon Slurry; Electrochemical Flow Capacitor; Flow Battery; Supercapacitor

Fig. 1. Schematic of the operation of an EFC system. Slurries are charged in the flow cell and stored in separate containers. They are pumped back to the cell for discharge.

operation of flow batteries and supercapacitors, and enables rapid charging/discharging and decoupled energy/power ratings. Electrical charge is stored in a flowable carbon slurry composed of low-cost and abundantly available carbon particles in pH-neutral, aqueous electrolyte. Charge storage and transfer is analogous to solid carbon electrodes in conventional supercapacitors. Here, the effects of carbon particle solid fraction, shape, and size on the electrochemical and rheological properties of slurry electrodes are investigated. A static cell configuration is utilized for studying the electrochemical properties of the flowable electrodes. The electrochemical properties of the slurry electrodes tested in a static cell are found to be similar to that of solid electrodes in conventional supercapacitors for both, large spherical and anisometric activated carbons. Flow properties of the slurry electrodes are obtained for shear rates corresponding to pumping shear rates by rheometry. Results indicate that electrochemical and rheological properties of slurries depend on their concentration, shape and size of the carbon particles used in the slurries. For a range of concentrations, slurries based on spherical carbon particles show lower viscosities compared to anisometric activated carbon based slurries while performing similar electrochemically.

Fig. 2. (a-d) SEM micrographs of (a) CB1, (b) CB2, (c) CB3 and (d) AC, and (e) pore size distributions of the porous carbon materials used in this study.

$Fig. 3. (a) Dependency of the average specific capacitance calculated from cyclic voltammetry (CV) on solid fraction of carbon particles; error bars show the standard deviation. The Cyclic voltammograms of carbon beads (CB) and activated carbon (AC) slurries (in 1 M Na2SO4) charged from 0 V to 0.75 V show capacitive behavior as demonstrated by rectangular shapes at low scan rates. CVs were recorded at (b) 5 mV s−1, (c) 20 mV s−1, and (d) 10 mV s−1.$
Fig. 3. (a) Dependency of the average specific capacitance calculated from cyclic voltammetry (CV) on solid fraction of carbon particles; error bars show the standard deviation. The Cyclic voltammograms of carbon beads (CB) and activated carbon (AC) slurries (in 1 M Na2SO4) charged from 0 V to 0.75 V show capacitive behavior as demonstrated by rectangular shapes at low scan rates. CVs were recorded at (b) 5 mV s−1, (c) 20 mV s−1, and (d) 10 mV s−1.

$Fig. 4. Specific capacitances at varying solid fractions (see legend in a) for scan rates from 2-100 mV s−1 calculated by CV for (a) CB1, (b) CB2, (c) CB3, and (d) AC.$
Fig. 4. Specific capacitances at varying solid fractions (see legend in a) for scan rates from 2-100 mV s−1 calculated by CV for (a) CB1, (b) CB2, (c) CB3, and (d) AC.

$Fig. 5. (a,b) Fifth galvanostatic cycle (200 mA g−1) after pre-cycling shows low resistance and symmetry of bead and AC slurries. (c) Dependency of the average specific capacitance and (d) ESR on solid fraction of porous carbon calculated from GC.$
Fig. 5. (a,b) Fifth galvanostatic cycle (200 mA g−1) after pre-cycling shows low resistance and symmetry of bead and AC slurries. (c) Dependency of the average specific capacitance and (d) ESR on solid fraction of porous carbon calculated from GC.

Fig. 6. Cyclic voltammograms of 23 wt% CB2 slurry charging to 0.9 - 1.5 V at 10 mV s−1. Coulombic efficiency drops slightly from 99.2 to 98.5% for the extended voltage window.

Fig. 8. Rheograms of slurry electrodes, 20 and 23 wt% solid, in 1 M Na2SO4 with fits to raw data. Shear rates for a flow rate of 1-10 mL min−1 in the current lab scale EFC range from 40-400 s−1.