A Recoverable Carbon/Zn Hybrid-Ion Supercapacitor with High Rate Charge Storage, Improved Rate Capability and Superior Gravimetric Power Density Using a Mixed Mono/Multivalent Electrolyte
Authors: Owusu Kwadwo Asare , Mai Liqiang 
Published: February 2020

Carbon aqueous symmetric supercapacitors are attractive supercapacitor devices owing to the low-cost and high conductivity of porous carbons. However, the limited capacitive charge storage process in carbon symmetric capacitors still leads to low energy density even when the cell voltage is extended up to 2.0 V in neutral aqueous electrolytes. Meanwhile, a carbon/metal ion capacitor such as sodium (Na), lithium (Li), Potassium (K) and much recently, zinc (Zn) can easily achieve high energy density from the synergistic contribution of double layer and mono/multivalent faradaic charge storage.
Taking Zn-based devices as an example, the zinc anode presents the advantages of the large theoretical capacity of Zn, low redox potential, excellent stability and lower cost in comparison with Na and Li. The major emphasis on Zn-ion hybrid supercapacitor research has been on the energy density. However, compared with a carbon symmetric supercapacitor, the zinc stripping/de-stripping reaction occurring at the zinc may increase the overall equivalent series resistance leading to lower gravimetric power density. Since the study of these devices is still in its infancy, it is highly necessary to devise an effective strategy to enhance the gravimetric power density as well.
Herein, we successfully report a recoverable activated zinc ion hybrid supercapacitor with enhanced high rate charge storage, improved rate capability and gravimetric power density by utilizing a mixed mono/multivalent electrolyte. A facilely activated carbon fiber clothes with a large specific surface area (615 m2 g-1), rich microporosity and superhydrophilicity serves as the positive electrode, whiles zinc foil serves as anode. At first, a zinc-ion HSC is fabricated using conventional 1 M ZnSO4 electrolyte, and the device exhibits a high voltage drop of 1.21 V at 50 mA cm-2, which would definitely affect the rate capability and maximum gravimetric power density of the device. This voltage drop is almost halved to 0.68 V at 50 mA cm-2 as well as the equivalent series resistance is significantly reduced, using the mono/multivalent aqueous electrolyte. This translates into charge storage at the high current density of 50 mA cm-2, a rate capability of 55 % compared to 38 % in the conventional ZnSO4 electrolyte and a maximum gravimetric energy density of 130.2 kW kg-1, compared to 73.62 kW kg-1 in the conventional ZnSO4 electrolyte. Using ex-situ SEM, XRD and cyclic voltammetry characterizations, we fully confirm that the 1 M Na2SO4 inhibits the formation of ZnO/Zn4SO4(OH)6•5H2O dendrites and serves as the preferential adsorption electrolyte at high rates in the carbon positive electrode. In terms of energy density, the fabricated Zn-ion HSC achieves a maximum energy density of 81.4 Wh kg-1, far exceeding fabricated carbon symmetric capacitors as well as stable cycling performances up to 10000 cycles with ~ 99 % coulombic efficiencies.
Finally, we believe our work will draw research attention into developing high-energy, high-rate and high-power aqueous Zn-ion supercapacitors through electrolyte modification.
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