Boron-doped graphene/TiO2 nanotube-based aqueous lithium ion capacitors with high energy density

Graphene and its derivatives have gained tremendous research interest for energy storage because of their high capacitance and structural stability. However, the synthesis of graphene nanosheets with high electrochemical activity remains a great challenge. Herein, boron-doped graphene (BG)/TiO2 nanotube array/Ti multilayer films were constructed by growing BG sheets on the TiO2 nanotubes. The high electrochemical activity of BG sheets and electronic transfer capability of TiO2 nanotubes led to good structural stability and energy-storage capability. The as-fabricated capacitors with two BG/TiO2/Ti electrodes in a CMC/LiCl gel electrolyte could realize a wide operational voltage of 2.6 V and deliver a high energy density of 221.8 Wh∙kg−1 at 5.98 kW kg−1, which still remains 102.4 Wh∙kg−1 as the power density increased to 35.1 kW kg−1. These devices show very high electrochemical lifespans, with approximately 91.3% retention after 10000 charge/discharge cycles. [Display omitted] •Boron-doped graphene was grown on TiO2 nanotubes (BG/TiO2) by in situ CVD.•A BG/TiO2-based aqueous lithium ion capacitor with operating voltage of 2.6 V was constructed.•A specific capacitance of 134.7 F g−1 at a current density of 30 A g−1 is achieved.•The capacitor shows a high energy density of 221.8 Wh∙kg−1 at 5.98 kW kg−1.