Selective regulation of nanosheets inside or outside carbon nanoboxes for advanced supercapacitors

Abstract

Rational structure control is an effective route to boost electrochemical performance of energy storage materials and devices. Confining composite materials with core–shell structure in hollow carbon nanomaterils is challenging and valuable in the field of energy storage. Herein, driving inward formation of core–shell structured Fe3O4@MnO2 in hollow carbon nanoboxes is achieved for superior supercapacitor electrodes. It’s found that when small Fe3O4 nanoparticles are reserved in carbon nanoboxes in advance, the MnO2 nanosheets only grow on the internal Fe3O4 nanoparticles to form Fe3O4@MnO2 core–shell structure in the carbon nanoboxes (Fe3O4@MnO2@C NBs) or only on the outer shell of carbon nanoboxes (Fe3O4@C@MnO2) or on both Fe3O4 nanoparticles and the carbon shell (Fe3O4@MnO2@C@MnO2), which can be selectively regulated. The density functional theory (DFT) calculation reveals that Fe3O4@MnO2@C NBs have higher conductivity, higher adsorption energy and lower diffusion energy barrier for sodium ions due to the core–shell structure of Fe3O4@MnO2 in carbon nanoboxes, which could promote the reaction kinetics efficiently. The asymmetric quasi-solid supercapacitor based on Fe3O4@MnO2@C NBs electrode achieves an energy density of 49.75 Wh kg−1 at a power density of 820 W kg−1 and demonstrates excellent stability. The route may provide a method for achieving complex internal structure control in hollow carbon material to enhance the electrochemical properties.