High-performance hybrid electrochemical capacitor with binder-free Nb ₂O₅@graphene

Hybrid electrochemical capacitors (HECs) are capable of storing more energy than supercapacitors while providing more power compared to lithium-ion batteries (LIBs). The development of Li-intercalating materials is critical to organic electrolyte based HECs, which generally give larger potential output than aqueous electrolyte based HECs. This article reports on a simple binder-free Nb₂O₅@graphene composite that exhibited excellent HEC performance as compared with other Li intercalating electrode materials. The composite exhibited enhanced cyclability with a capacity retention of 91.2% compared to 74.4% of the pure Nb₂O₅ half-cell when tested at a rate of 2000 mA g^-1 (10 C). The composite displayed a lower polarization effect when cycled at increasing scan rates (1-10 mV s^-1). The enhanced rate capability could be ascribed to the use of a highly conductive graphene support. As a result, the HEC composed of the Nb₂O₅@graphene composite and activated carbon (AC) delivered a maximum energy and power density of 29 W h kg^-1 and 2.9 kW kg^-1. The performance is better than most reported HECs with other Li-intercalating electrode materials.