Pristine and transition-metal-doped Mn3O4 nanocrystals shaped in octahedrons have been synthesized by hydrothermal reduction of potassium permanganate and characterized by SEM/TEM, X-ray diffraction, X-ray photoelectron spectroscopy, and electrochemical experiments. The results reveal that a multistep reduction process is taking place, and the introduction of doping ions causes a direct synthesis of single-phase Mn3O4 nanocrystals. To assess the properties of Mn3O4 nanocrystals for their use in supercapacitors, cyclic voltammetry and galvanostatic charging-discharging measurements are performed. The phase stability during cycling and charge-transfer behavior are greatly improved by doping with transition metal, and Cr-doped Mn3O4 nanocrystals exhibit a maximum specific capacitance of 272 F g(-1) at a current density of 0.5 A g(-1). These doped Mn3O4 nanocrystals could be a promising candidate material for high-capacity, low-cost, and environmentally friendly electrodes for supercapacitors. In addition, these results have verified the ability of doping to improve capacitive performances of spinel-structured transition-metal oxides.

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A facile and new anion exchange process is presented, which involves the conversion of NiCo‐CO3 layered double hydroxide (LDH) nanosheet arrays in an alkaline solution. The anion exchange between CO32− and OH− results in the construction of a reservoir for OH− anions, and the decoration of thin nanoflakes on the surface of nanosheets effectively enlarges the surface area of NiCo LDH nanoarrays. The capacitance of the as‐soaked NiCo LDH nanoarrays electrode increases from 1.78 F cm−2 (684 F g−1) to 6.22 F cm−2 (2391 F g−1) at 2 mA cm−2 after soaking for 12 h. Moreover, the soaked NiCo‐OH LDH electrode exhibits an enhanced rate capacity, high coulombic efficiency, and good cycling stability compared with the Ni–Co‐S nanosheet electrode synthesized through a hydrothermal sulfuration process. The as‐assembled all‐solid‐state NiCo LDH//active carbon asymmetric supercapacitor shows a maximum energy density of 83.4 W h kg−1 at a power density of 1066 W kg−1.

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