Improved energy density of reduced graphene oxide based aqueous symmetric supercapacitors in redox-active and “water-in-salt” electrolytes

Abstract

This work demonstrates an eco-friendly and sustainable approach for developing high-energy-density aqueous supercapacitors. Herein, the energy density of reduced graphene oxide-based aqueous symmetric supercapacitors (rGO-SCs) was augmented using two facile strategies, viz. redox-active electrolyte and water-in-salt electrolyte. The rGO-SCs with commercial-level electrode mass loading (9–10 mg cm−2), exhibited high gravimetric specific capacitance, excellent cyclic stability, and rate capability in three aqueous electrolytes, viz. 1 M H2SO4 (0–1 V), 6 M KOH (0–1 V), and 1 M Li2SO4 (0–1.8 V). The rGO-SCs demonstrated a superior energy density of 15.39 W h kg−1 (at 180 W kg−1) in neutral Li2SO4 electrolyte, which is ~2.6–3× higher compared to the acidic and alkaline electrolyte. Incorporating 0.1 M Na2MoO4 as the redox-additive in Li2SO4 enhanced the energy density of the device to 21.42 W h kg−1 (at 180 W kg−1) via reversible redox reactions at the electrode surface. In addition, the redox-enhanced supercapacitor displayed high cycle stability of 99.3% even after 5000 cycles. Furthermore, a high cell voltage of 2.1 V was achieved for the rGO-SC using a water-in-salt electrolyte (11 M NaNO3). This high voltage supercapacitor exhibited an energy density of 22.87 W h kg−1 (at 210 W kg−1) and 98.1% capacitance retention over 5000 cycles.