The energy density formula illuminated that widening the voltage window and maximizing capacitance are effective strategies to boost the energy density of supercapacitors. However, aqueous electrolyte-based devices generally afford a voltage window less than 1.2 V in view of water electrolysis, and chemically converted graphene yields mediocre capacitance. Herein, multi-electron redox-reversible, structurally stable indanthrone (IDT) π-backbones were rationally coupled with the reduced graphene oxide (rGO) framework to form IDT@rGO molecular heterojunctions. Such conductive agent- and binder-free film electrodes delivered a maximized capacitance of up to 345 F g-1 in a potential range of -0.2 to 1.0 V. The partner film electrode-Ti3C2Tx MXene which worked in the negative potential range of -0.1 to -0.6 V-afforded a capacitance as large as 769 F g-1. Thanks to the perfect complementary potentials of the IDT@rGO heterojunction positive electrode and Ti3C2Tx MXene negative partner, the polyvinyl alcohol/H2SO4 hydrogel electrolyte-based flexible asymmetric supercapacitor delivered an enlarged voltage window of 1.6 V and an impressive energy density of 17 W h kg-1 at a high power density of 8 kW kg-1, plus remarkable rate capability and cycling life (capacitance retention of ∼90% after 10000 cycles) as well as exceptional flexibility and bendability.
Keywords: MXene; graphene; heterojunction; pseudocapacitors; supercapacitors.