Exploration of Charge Storage Behavior of Binder-Free EDL Capacitors in Aqueous Electrolytes.

Charge storage in electrochemical double-layer capacitors (EDLCs) is via the adsorption of electrolyte counterions in their positive and negative electrodes under an applied potential. This study investigates the EDLC-type charge storage in carbon nanotubes (CNT) electrodes in aqueous acidic (NaHSO 4 ), basic (NaOH), and neutral (Na 2 SO 4 ) electrolytes of similar cations but different anions as well as similar anions but different cations (Na 2 SO 4 and Li 2 SO 4 ) in a two-electrode Swagelok-type cell configuration. The physicochemical properties of ions, such as mobility/diffusion and solvation, are correlated with the charge storage parameters. The neutral electrolytes offer superior charge storage over the acidic and basic counterparts. Among the studied ions, SO 4 2- and Li + showed the most significant capacitance owing to their larger solvated ion size. The charge stored by the anions and cations follows the order SO 4 2- > HSO 4 - > OH - and Li + > Na + , respectively. Consequently, the CNT//Li 2 SO 4 //CNT cell displayed outstanding charge storage indicators (operating voltage ∼0-2 V, specific capacitance ∼122 F·g -1 , specific energy ∼67 W h·kg -1 , and specific power ∼541 W·kg -1 at 0.5 A·g -1 ) than the other cells, which could light a red light-emitting diode (2.1 V) for several minutes. Besides, the CNT//Li 2 SO 4 //CNT device showed exceptional rate performance with a capacitance retention of ∼95% at various current densities (0.5-2.5 A·g -1 ) after 6500 cycles. The insights from this work could be used to design safer electrochemical capacitors of high energy density and power density.