In this paper, a hierarchical NiCo2S4@polypyrrole core-shell heterostructure nanotube array on Ni foam (NiCo2S4@PPy/NF) was successfully developed as a bind-free electrode for supercapacitors. NiCo2S4@PPy-50/NF obtained under 50 s PPy electrodeposition shows a low charge-transfer resistance (0.31 Ω) and a high area specific capacitance of 9.781 F/cm(2) at a current density of 5 mA/cm(2), which is two times higher than that of pristine NiCo2S4/NF (4.255 F/cm(2)). Furthermore, an asymmetric supercapacitor was assembled using NiCo2S4@PPy-50/NF as positive electrode and activated carbon (AC) as negative electrode. The resulting NiCo2S4@PPy-50/NF//AC device exhibits a high energy density of 34.62 Wh/kg at a power density of 120.19 W/kg with good cycling performance (80.64% of the initial capacitance retention at 50 mA/cm(2) over 2500 cycles). The superior electrochemical performance can be attributed to the combined contribution of both component and unique core-shell heterostructure. The results demonstrate that the NiCo2S4@PPy-50 core-shell heterostructure nanotube array is promising as electrode material for supercapacitors in energy storage.

Usually, there is a trade-off relationship between C wt and ρ e which is closely associated with the accessible surface area and pore structure, respectively. [8,9] Hence, a balance between the accessible surface area and the pore structure is essential to maximize the volumetric performance of EDLCs for carbon architectures. In recent years, we developed a new kind of hierarchical carbon nanocages (hCNC), which exhibited superior supercapacitive performance owing to the large specific surface area (SSA) and coexisting micro-meso-macropore structure. [10,11] By N-doping to improve the wettability and decrease the equivalent series resistances (R ESR), the C wt was effectively increased from the "highland" (226 F g −1 @1 A g −1) to the "summit" (313 F g −1 @1 A g −1) in alkaline electrolyte. [12] However, the low ρ e of the hierarchical carbon-based nanocages led to the mediocre C vol and E vol. Very recently, by eliminating the surplus mesoand macropores by capillary compression, the collapsed carbon nanocages (cCNC) with a large SSA of 1788 m 2 g −1 and high density of 1.32 g cm −3 were obtained, which presented the state-of-the-art volumetric performance either in an alkaline or in ionic-liquid electrolyte. [13] It can be expected that the heteroatom doping into cCNC could further improve the volumetric performance. As known, N doping can effectively improve the wettability and decrease R ESR , [12] and S doping can modify spin densities of the sp 2 carbon and introduce hydrophilic oxygen-containing functional groups (C-SO x), [14,15] both of which are favorable for the improvement of capacitive performance. With this consideration, herein we combine the N,S dual-doping with the capillary compression strategy, and prepare the collapsed N,S dual-doped carbon nanocages (cNS-CNC) with large SSA and high ρ e. The optimized cNS-CNC exhibits the state-of-the-art volumetric performance either in KOH aqueous or in ionic-liquid electrolytes. Specifically, the C vol at 1 A g −1 reaches 299 and 246 F cm −3 , with ≈52% and ≈25% enhancement relative to the undoped one, respectively, accompanied by the superior rate performance and cycling stability. The excellent stack volumetric energy density (E vol-stack) of 75.3 Wh L −1 at 1 A g −1 is realized in ionic liquid for carbon-based EDLCs, along with the maximum average stack volumetric power density (P vol-stack) of 112 kW L −1. This study

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In this work, γ-MnS/reduced graphene oxide composites (γ-MnS/rGO) were prepared using a facile one-pot hydrothermal method. As an electrode material for supercapacitors, the γ-MnS/rGO-60 composite obtained under dosages of graphene oxide  was 60 mg and exhibited an enhanced specific capacitance of 547.6 F g at a current density of 1 A g, and outstanding rate capability (65% capacitance retention at 20 A g), with superior cycling stability and electrochemical reversibility. An asymmetric supercapacitor assembled from γ-MnS/rGO-60 composite and rGO (γ-MnS/rGO-60//rGO) showed a voltage window of 0-1.6 V and delivered a high energy density of 23.1 W h kg at a power density of 798.8 W kg, and 15.9 W h kg at 4.5 kW kg. Moreover, two such 1.0 × 1.0 cm devices connected together in series easily light up a group of LED lights, showing its potential practical application as an attractive energy storage device.

As a choke point in water electrolysis, the oxygen evolution reaction (OER) suffers from the severe electrode polarization and large overpotential. Herein, the porous hierarchical hetero-(Ni 3−x Fe x )FeN/Ni catalysts are in situ constructed for the efficient electrocatalytic OER. X-ray absorption fine structure characterizations reveal the strong Ni-Fe bimetallic interaction in (Ni 3−x Fe x )FeN/Ni. Theoretical study indicates the heterojunction and bimetallic interaction decrease the free-energy change for the rate-limiting step of the OER and the overpotential thereof. In addition, the high conductivity and porous hierarchical morphology favor the electron transfer, electrolyte access and O 2 release. Consequently, the optimized catalyst achieves a low overpotential of 223 mV at 10 mA•cm −2 , a small Tafel slope of 68 mV•dec −1 , and a high stability. The excellent performance of the optimized catalyst is also demonstrated by the overall water electrolysis with a low working voltage and high Faradaic efficiency. Moreover, the correlation between the structure and performance is well established by the experimental characterizations and theoretical calculations, which confirms the origin of the OER activity from the surface metal oxyhydroxide in situ generated upon applying the current. This study suggests a promising approach to the advanced OER electrocatalysts for practical applications by constructing the porous hierarchical metal-compound/metal heterojunctions.