Vacancy-engineered MoO3 and Na+-preinserted MnO2in situ grown N-doped graphene nanotubes as electrode materials for high-performance asymmetric supercapacitors

Abstract

Developing advanced negative and positive electrode materials for asymmetric supercapacitors (ASCs) as electrochemical energy storage devices can enable the device to reach high energy/power densities resulting from the cooperative effect of the two electrodes. In this work, a nanochain-like hybrid nanostructured composite of MoO₃ with rich oxygen vacancies (MoO_3−x) in situ grown on N-doped graphene nanotubes (N-GNTs) is synthesized as the negative electrode for an ASC. It can deliver a large specific capacitance (591 F g⁻¹ at 2 A g⁻¹), as well as an outstanding rate capability. Simultaneously, N-GNTs@Na⁺-preinserted MnO₂ (Na_xMnO₂) nanosheet arrays are fabricated as the positive electrode of the ASC, which can exhibit an enhanced specific capacitance of 429 F g⁻¹ with a wide potential window of 0–1.2 V. When these two different electrodes are assembled in an ASC, a maximum energy density of 105 W h kg⁻¹ at 2.2 kW kg⁻¹, and an ultralong lifespan with 95.4% capacitance retention after 10 000 cycles are achieved, which are superior to those of the reported ASC devices to date. Density functional theory (DFT) calculations show that benefitting from the designed vacancy defects and unique N-GNT skeleton, the electronic structure and adsorption energy can be adjusted to greatly boost the electrical conductivity and ion adsorption efficiency and subsequently enhance the capacitive properties of the electrodes in the ASC.