Aqueous lithium and sodium ion capacitors with boron-doped graphene/BDD/TiO2 anode and boron-doped graphene/BDD cathode exhibiting AC line-filtering performance

Abstract

This paper reports our investigation of aqueous sodium and lithium ion capacitors (SIC and LIC, respectively) with a boron-doped graphene (BDG)/boron-doped diamond (BDD) cathode and BDG/nanocrystal BDD/TiO2 nanotube array anode. Vertically grown BDG nanosheets formed a porous three-dimensional morphology rich in charge storage sites with a short ion-diffusion distance and, high structural stability. In an aqueous NaCl solution, the potential windows are 0–1.1 V and −1.2–0 V for the cathode and anode, respectively, and their specific capacitances are 66.76 and 23.86 mF cm−2 at a current density of 3 mA cm−2, respectively. The devices exhibit high energy densities (SIC: 47.8 μWh cm−2; LIC: 33.9 μWh cm−2) and high power densities (SIC: 77.7 mW cm−2; LIC: 65.3 mW cm−2) at an operating voltage of 2.5 V. The SIC and LIC exhibited 100% coulombic efficiency over 10,000 cycles, with a capacitance retention of 99.5% and 97.8%, respectively, at a high current density of 90 mA cm−2 over 2000 cycles. Moreover, the capacitors exhibit effective AC current filtering over a wide frequency range of 50–10,000 Hz. The coulombic efficiencies and long-term stabilities of these aqueous devices are better at high current densities, and large energy densities are retained at high power densities, which is advantageous for high-frequency filtering.