The current problem of the still relatively low energy densities of supercapacitors can be effectively addressed by designing electrodes hierarchically on micro- and nanoscale. Herein, we report the synthesis of hierarchically porous, nanosheet covered submicrometer tube forests on Ni foam. Chemical deposition and thermal treatment result in homogeneous forests of 750 nm diameter FeCo2O4 tubes, which after hydrothermal reaction in KMnO4 are wrapped in MnO2-nanosheet-built porous covers. The covers’ thickness can be adjusted from 200 to 800 nm by KMnO4 concentration. An optimal thickness (380 nm) with a MnO2 content of 42 wt % doubles the specific capacitance (3.30 F cm–2 at 1.0 mA cm–2) of the bare FeCo2O4-tube forests. A symmetric solid-state supercapacitor made from these binder-free electrodes achieves 2.52 F cm–2 at 2 mA cm–2, much higher than reported for capacitors based on similar core–shell nanowire arrays. The large capacitance and high cell voltage of 1.7 V allow high energy and power densities (93.6 Wh kg–1, 10.1 kW kg–1). The device also exhibits superior rate capability (71% capacitance at 20 mA cm–2) and remarkable cycling stability with 94% capacitance retention being stable after 1500 cycles.Keywords: core−shell structure; hierarchical; nanosubmicrometer composites; property optimization; supercapacitors

Template-free chemical growth on Ni foam and thermal treatment results in homogeneous FeCo2O4 submicron-tube arrays which serve as binder-free electrodes with high capacitance, rate-capability and cycling-stability owing to FeCo2O4 conductivity, high porosity, and strong bonding between tubes and Ni foam, all allowing even symmetric devices to have superior energy density.

Silver nanowires (NWs) coated with platinum (Pt) nanoparticles were synthesized via a galvanic partial replacement of Ag NWs in an aqueous K2PtCl6 solution at room temperature. The products were characterized using a combination of electron microscopies, selected area electron diffraction, energy-dispersive X-ray mapping and X-ray diffraction. The surface morphology and Pt/Ag composition ratios are controlled by adjusting the K2PtCl6 concentration. Different concentrations result in various surface morphologies including rough nanoparticle coating, porous and relatively smooth surfaces. The formation mechanism was discussed based on the lattice constants' difference, concentration driven nucleation, consumption of Ag NWs, and stoichiometry of the replacement reaction. The effects of the bimetallic interface on the catalytic activity toward the reduction of 4-nitrophenol by sodium borohydride were studied. The activity of Ag–Pt NWs is highly enhanced over monometallic nanostructures, and optimized by a low Pt loading of 1.34 at.%, which indicates a catalytic role of the inter-metallic interface for the electron transfer.

In order to achieve high energy densities, an asymmetric all-solid-state supercapacitor is developed by synthesizing a novel composite of cobalt carbonate hydroxide (CCH) nanowire covered N-doped graphene (NG) as positive and porous NG as negative electrodes. The CCH–NG composite is obtained from a one-step hydrothermal method, where optimization of the CCH content triples the specific capacitance of porous NG, reaching 1690 F g−1 at 1.0 A g−1. The optimal composite exhibits a remarkable cycling stability retaining 94.2% of the initial capacitance after 10000 cycles, and good rate capability (still 1358 F g−1 at 10 A g−1). The assembled asymmetric supercapacitor based on the optimal composite has a high discharge areal capacitance of 153.5 mF cm−2 (at 1.0 mA cm−2), can cycle reversibly in the high-voltage region of 0–1.9 V, and thus provide superior energy and power densities (0.77 W h m−2 and 25.3 W m−2).

We report a novel method for the large-scale fabrication of porous bulk silver thin sheets (PSTS) built from three-dimensionally interconnected nanoparticles (NPs). The synthesis starts with synthesizing silver sponges via an in situ growth of NPs which assemble into networks. The sponges are pressed into thin sheets before etching in acid. The resulting porosity is nearly homogeneous throughout the whole volume. The dependence on acid concentration was investigated and the average pore diameter can be controlled in a range of 83–145 nm by etching time. Growing metal oxides results in PSTS/Co3O4 composites which can be used directly as binder-free supercapacitor electrodes. The Co3O4 growth is optimized and the optimized composite electrode provides a much higher specific capacitance (1276 F g−1 at 1 A g−1) than previously reported for pure Co3O4 nanostructures with different shapes or those for Ag–Co3O4 composite nanowire array electrodes. The optimal electrode has a superior rate capability (still 986 F g−1 at 10 A g−1). The improvements are attributed to the continuous open porosity of PSTS and a direct contact between Co3O4 and Ag ligaments. The method can be extended to many other metals or alloys, promising wide application.

In the present work, we report the preparation of graphene (G) doped polyacrylic acid/polyaniline (G-PAA/PANI) composites with excellent processibility for ensuring ultrathin, defect-free and highly flexible films, as well as high electrochemical performance. The weight content of PANI is maximized under the constraint of still allowing defect-free films, and the G content is optimized. Interestingly, we combine two steps that both, if taken in isolation as a strategy, worsen the solubility. The PANI and G contents are optimized to be 20 wt% and 1.3 wt%, respectively. The optimal G-PAA/PANI composite film has a gravimetric capacitance of 399 F g−1 at 10 mV s−1, which is more than twice that of pure PANI nanoparticles. Considering the film thickness of only 50 μm, its specific areal and volumetric capacitances are as high as 1.20 F cm−2 and 240 F cm−3. The film still has a gravimetric capacitance of 342 F g−1 at a high scan rate of 100 mV s−1 (86% of that at 10 mV s−1), which promises great potential for applications needing a rapid charge/discharge. An assembled all-solid-state supercapacitor using two such flexible G-PAA/PANI films provides 93 F g−1; an eighteen-fold improvement over that of a previously reported similar device. The capacitor also exhibits excellent electrochemical stability under different bending angles.

Hierarchically porous yet densely packed MnO2 microspheres doped with Fe3O4 nanoparticles are synthesized via a one-step and low-cost ultrasound assisted method. The scalable synthesis is based on Fe2+ and ultrasound assisted nucleation and growth at a constant temperature in a range of 25–70 °C. Single-crystalline Fe3O4 particles of 3–5 nm in diameter are homogeneously distributed throughout the spheres and none are on the surface. A systematic optimization of reaction parameters results in isolated, porous, and uniform Fe3O4–MnO2 composite spheres. The spheres’ average diameter is dependent on the temperature, and thus is controllable in a range of 0.7–1.28 μm. The involved growth mechanism is discussed. The specific capacitance is optimized at an Fe/Mn atomic ratio of r = 0.075 to be 448 F/g at a scan rate of 5 mV/s, which is nearly 1.5 times that of the extremely high reported value for MnO2 nanostructures (309 F/g). Especially, such a structure allows significantly improved stability at high charging rates. The composite has a capacitance of 367.4 F/g at a high scan rate of 100 mV/s, which is 82% of that at 5 mV/s. Also, it has an excellent cycling performance with a capacitance retention of 76% after 5000 charge/discharge cycles at 5 A/g.Keywords: composite materials; hierarchically porous structures; manganese dioxide; supercapacitors

We report a simple hydrothermal synthesis of nanocomposites, constructed by 3D nitrogen-doped graphene (NG) networks with hexagonal Co(OH)2 nanoplates, which are optimized for applications as electrochemical pseudocapacitor materials. Single-crystalline Co(OH)2 plates are distributed homogeneously inside the conductive interconnected NG networks. The 71% Co(OH)2 weight content achieves a capacitance of 952 F g−1 at 1.0 A g−1, more than triple that of the pure NG and nearly four times that of Co(OH)2 plates; moreover, this value exceeds the recently reported values for 2D graphene/Co(OH)2 composites. Capacitance retention over 2000 cycles is still high as 95%. The improvements are attributed to the regular morphology of Co(OH)2 and the 3D porosity, which prevents the stacking of the Co(OH)2 plates as well as the composite, and the continuously connected pores and highly conductive NG networks, which facilitate electron and ion transport.

Template-free chemical growth on Ni foam and thermal treatment results in homogeneous FeCo2O4 submicron-tube arrays which serve as binder-free electrodes with high capacitance, rate-capability and cycling-stability owing to FeCo2O4 conductivity, high porosity, and strong bonding between tubes and Ni foam, all allowing even symmetric devices to have superior energy density.

In order to achieve high energy densities, an asymmetric all-solid-state supercapacitor is developed by synthesizing a novel composite of cobalt carbonate hydroxide (CCH) nanowire covered N-doped graphene (NG) as positive and porous NG as negative electrodes. The CCH–NG composite is obtained from a one-step hydrothermal method, where optimization of the CCH content triples the specific capacitance of porous NG, reaching 1690 F g−1 at 1.0 A g−1. The optimal composite exhibits a remarkable cycling stability retaining 94.2% of the initial capacitance after 10000 cycles, and good rate capability (still 1358 F g−1 at 10 A g−1). The assembled asymmetric supercapacitor based on the optimal composite has a high discharge areal capacitance of 153.5 mF cm−2 (at 1.0 mA cm−2), can cycle reversibly in the high-voltage region of 0–1.9 V, and thus provide superior energy and power densities (0.77 W h m−2 and 25.3 W m−2).