Modulating the electronic structure and pseudocapacitance of δ-MnO2 through transitional metal M (M = Fe, Co and Ni) doping

Abstract

The experimental capacitance of manganese oxide (MnO2) is ordinarily less than 100 F/g due to their poor electronic conductivity and low structural stability. Herein, we demonstrate the design and synthesis of doped δ-MnO2 with enhanced electronic conductivity by the introduction of transitional metal Fe, Co and Ni. Structural characterizations shown that the Raman spectra of doped MnO2 has obviously blue shifted, as well as the separation values of Mn 3s XPS spectra has expanded, which indicate the transitional metal doping successfully changes the crystal lattice of δ-MnO2. The I-V measurements confirm that the electronic conductivity of MnO2 is significantly improved after doping. As a result, the Fe doped δ-MnO2 with a 0.5% doping amount displays the highest specific capacitance of 157 F/g at 0.5 A/g, by increasing 50.4% of the specific capacity than non-doped MnO2. Simultaneously, the Co doped δ-MnO2 exhibits the superb cycling stability (almost no degradation). Furthermore, the assembled 0.5% FeMO//AC, 1% CoMO//AC and 1% NiMO//AC asymmetric supercapacitor provide a specific energy densities of 30.3, 25.2 and 23.6 Wh/kg at a power density of 1000 Wh/kg. The excellent properties of as-prepared MnO2 are due to the enhanced conductivity after doping, which can ascribed to the forming of intermediate bands, or changing the intensity of valence band/conduction band as demonstrated by spin-polarized density functional (DFT) calculations. Thus, the current work will provide a pathway for the development of high-performance pseudocapacitive materials, as well as for other energy storage systems.