Manganese dioxides (MnO2) are considered one of the most attractive materials as an oxygen evolution reaction (OER) electrode due to its low cost, natural abundance, easy synthesis, and environmental friendliness. Here, metal‐ion (Fe, V, Co, and Ni)‐doped MnO2 ultrathin nanosheets electrodeposited on carbon fiber paper (CFP) are fabricated using a facile anodic co‐electrodeposition method. A high density of nanoclusters is observed on the surface of the carbon fibers consisting of doped MnO2 ultrathin nanosheets with an approximate thickness of 5 nm. It is confirmed that the metal ions (Fe, V, Co, and Ni) are doped into MnO2, improving the conductivity of MnO2. The doped MnO2 ultrathin nanosheet/CFP and the IrO2/CFP composite electrodes for OER achieve a low overpotential of 390 and 245 mV to reach 10 mA cm−2 in 1 m KOH, respectively. The potential of the doped composite electrode for long‐term OER at a constant current density of 20 mA cm−2 is much lower than that of the pure MnO2 composite electrode.

A nitrogen‐doped foam‐like 3D carbon plate is prepared by carbonizing a selected biomass precursor of pomelo peel. The carbon plate is composed of interwoven connected carbon tubes and shows a unique 3D interpenetrating macroporous network with binary‐channel structure. Owing to the proper pore structures, short ion‐diffusion paths, high conductivity, and high structure stability, the carbon plate exhibits excellent electrochemical performance as an electrode for supercapacitors directly. A high specific capacitance of up to 338 F g−1 at 1 A g−1, good rate capability with a capacitance retention of 59 % at 20 A g−1, and high cycling stability with only 4 % capacitance loss after 5000 cycles at 20 A g−1 can be achieved in 6 m KOH electrolyte in a three‐electrode system. Furthermore, a symmetric supercapacitor fabricated by using the carbon plates delivers an energy density of 11.05 Wh kg−1 at a power density of 250 W kg−1, and an outstanding cycling stability with only 2.4 % capacitance decay over 5000 cycles at 5 A g−1 in 6 m KOH electrolyte. These very attractive electrochemical properties indicate that the carbon plate derived from pomelo peel is a promising electrode material for supercapacitors.

N-Doped hierarchical porous carbon (NHPC) was obtained from waste boat-fruited sterculia seed by hydrothermal carbonization and KOH activation.

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To further improve the electrochemical performance of electrode materials for supercapacitors, we have achieved a significant increase of the micropore volume of a MnO2 hybrid film on IrO2 nano-wedges grown on a pre-treated Ti plate by the co-doping of vanadium and iron (V+Fe). X-ray diffraction and microstructural analyses demonstrate that the V+Fe co-doped MnO2 hybrid films consist of the lamellar structure that is composed of γ-MnO2 nanocrystallites jointed by disordered interface. Nitrogen sorption analysis confirms that the pore characteristics of the MnO2 film change from mesopore-dominant to micropore-dominant after the co-doping, accompanied by increases in the pore volume and specific surface area. The specific capacitance of the V+Fe co-doped MnO2 hybrid film electrode declines from 426 F g−1 to 314 F g−1 with a relatively limited loss of 26% when the galvanostatic (GV) charging-discharging rate is increased from 0.2 to 5 A g−1. In particular, the doped MnO2 film electrode has a loss of only 6% in the specific capacitance after 9000 cycles at a charge-discharge current density of 10 A g−1.