Ke Ye

Molten salt synthesis of nitrogen doped porous carbon: a new preparation methodology for high-volumetric capacitance electrode materials

To meet the ever-increasing need for high-efficiency energy storage in modern society, porous carbon materials with large surface areas are typically employed for electrical double-layer capacitors to achieve high gravimetric performances. However, their poor volumetric performances come from low packing density and/or high pore volume resulting in poor volumetric capacitance, which would limit their further applications. Here, a novel and one-step molten salt synthesis of a three-dimensional, densely nitrogen-doped porous carbon (NPC) material by using low-cost and eco-friendly tofu as the nitrogen-containing carbon source is proposed. Hierarchically porous carbon with a specific surface area of 1202 m2 g−1 and a high nitrogen content of 4.72 wt% and a bulk density of ∼0.84 g cm−3 is obtained at a carbonation temperature of 750 °C. As the electrode material for a supercapacitor, the NPC electrode shows both ultra-high specific volumetric and gravimetric capacitances of 360 F cm−3 and 418 F g−1 at 1 A g−1 (based on a three-electrode system), respectively, and excellent cycling stability without capacitance loss after 10 000 cycles at a high charge current of 10 A g−1 in KOH electrolyte. Moreover, the as-assembled symmetric supercapacitor exhibits not only an excellent cycling stability with 97% capacitance retention after 10 000 cycles, but also a high volumetric energy density up to 27.68 W h L−1 at a current density of 0.2 A g−1, making this new method highly promising for compact energy storage devices with simultaneous high volumetric/gravimetric energy and power densities.

One-step synthesis of copper compounds on copper foil and their supercapacitive performance

Nanowire-like Cu(OH)2 arrays, microflower-like CuO standing on Cu(OH)2 nanowires and hierarchical CuO microflowers are directly synthesized via a simple and cost-effective liquid–solid reaction. The specific capacitance of Cu(OH)2, CuO/Cu(OH)2 and CuO are 511.5, 78.44 and 30.36 F g−1, respectively, at a current density of 5 mA cm−2. Therefore, the Cu(OH)2/Cu-foil electrode displays the best supercapacitive performance. The capacitance retention reaches up to 83% after 5000 charge/discharge cycles with the columbic efficiency of ∼98%. More importantly, the nanowire Cu(OH)2 transformed into stable nanosheet CuO after about 600 constant current charge–discharge cycles. Additionally, we fabricate an asymmetric supercapacitor with nanowire Cu(OH)2/Cu-foil as a positive electrode, activated carbon (AC) as a negative electrode and 6 mol dm−3 KOH as electrolyte, which exhibits an energy density of 18.3 W h kg−1 at a power density of 326 W kg−1.

Preparation of porous cadmium sulphide on nickel foam: a novel electrode material with excellent supercapacitor performance

Large surface area, high electrical conductivity, and abundant channels have been recognized to favor faradic capacitors, but their realization at the same time by a facile preparation process is still a great challenge. Here, we synthesized porous cadmium sulphide freely standing on nickel foam (CdS/NF) via a one-step hydrothermal method which possesses high specific capacitance, good rate capability and outstanding cycling stability. The CdS/NF microspheres present pores with a mean size of ∼3 nm, resulting in fast diffusion of ions, facile transportation of electrons and high activity, which make great synergistic contributions to reversible redox reactions. In the resulting supercapacitors, a specific capacitance of 909 F g−1 is achieved at a current density of 2 mA cm−2 with excellent rate capability that 88% of the original capacitance is retained at 50 mA cm−2. After 5000 charge–discharge cycles at current densities as large as 50 mA cm−2, 104% of initial capacitance is maintained. Finally, asymmetric supercapacitors with a high energy density of 28 W h kg−1 at a power density of 160 W kg−1 were obtained.

Preparation of binder-free CuO/Cu2O/Cu composites: a novel electrode material for supercapacitor applications

Cuprous(I) oxide (Cu2O) carries high theoretical specific capacitance (2247.6 F g−1), however, the amount of research about the supercapacitive performance of Cu2O is relatively small compared with other transition metal oxides. A composite of metal and metal oxide could improve the electrochemical performance efficiently. In this work, the results of XRD and XPS demonstrate that CuO/Cu2O/Cu is prepared successfully via a facile, eco-friendly, one-step template-free growth process. SEM figures show that cubic CuO/Cu2O/Cu uniformly and densely covers a skeleton of nickel foam. The binder-free CuO/Cu2O/Cu electrode exhibits excellent supercapacitive performance with a high specific capacitance of 878 F g−1 at a current density of 5 mA cm−2 (1.67 A g−1), when the current density is enlarged ten times (50 mA cm−2 (16.7 A g−1)), the specific capacitance still remains at 545 F g−1. Furthermore, we have first successfully constructed a CuO/Cu2O/Cu//AC asymmetric supercapacitor, which can achieve an energy density of 42 W h kg−1 at a power density of 0.44 kW kg−1. The good electrochemical performance and simple accessibility prove that the as-prepared CuO/Cu2O/Cu/NF electrode has a potential application in electrochemical capacitors.

Facile preparation of three-dimensional Ni(OH)2/Ni foam anode with low cost and its application in a direct urea fuel cell

Three-dimensional Ni(OH)2/Ni foam electrodes with low cost are simply fabricated via a template-free growth method and employed as efficient anodes for a direct urea–hydrogen peroxide fuel cell (DUPFC). The surface morphologies of Ni(OH)2 catalysts on the electrodes can be easily controlled by altering the reaction temperatures. The nano-sheet (NS) Ni(OH)2/Ni foam electrode exhibits highest catalytic activity towards urea electro-oxidation among the four electrodes. The oxidation current density of the NS Ni(OH)2/Ni foam electrode reaches 337 mA cm−2 at 0.45 V (vs. Ag/AgCl) with a low onset oxidation potential in 0.6 mol L−1 urea and 5 mol L−1 KOH solutions. The DUPFC using NS Ni(OH)2/Ni foam anode shows an open circuit voltage of 0.86 V and high peak power density of 19.7 mW cm−2 and 28.8 mW cm−2 at 20 °C and 50 °C, respectively, which is much higher than the performance of direct urea fuel cells reported previously. The outstanding cell performance using a cheap NS Ni(OH)2/Ni foam anode indicates DUPFC is a promising new type of fuel cell.

Synthesis of Hierarchically Porous Sandwich-Like Carbon Materials for High-Performance Supercapacitors

For the first time, hierarchically porous carbon materials with a sandwich-like structure are synthesized through a facile and efficient tri-template approach. The hierarchically porous microstructures consist of abundant macropores and numerous micropores embedded into the crosslinked mesoporous walls. As a result, the obtained carbon material with a unique sandwich-like structure has a relatively high specific surface (1235 m2  g-1 ), large pore volume (1.30 cm3  g-1 ), and appropriate pore size distribution. These merits lead to a comparably high specific capacitance of 274.8 F g-1 at 0.2 A g-1 and satisfying rate performance (87.7 % retention from 1 to 20 A g-1 ). More importantly, the symmetric supercapacitor with two identical as-prepared carbon samples shows a superior energy density of 18.47 Wh kg-1 at a power density of 179.9 W kg-1 . The asymmetric supercapacitor based on as-obtained carbon sample and its composite with manganese dioxide (MnO2 ) can reach up to an energy density of 25.93 Wh kg-1 at a power density of 199.9 W kg-1 . Therefore, these unique carbon material open a promising prospect for future development and utilization in the field of energy storage.

A novel three-dimensional gold catalyst prepared by simple pulse electrodeposition and its high electrochemical performance for hydrogen peroxide reduction

Novel Au nanoparticles (NP), Au pinecones (PC) and Au nanodendrites (ND) supported on carbon coated titanium dioxide (C@TiO2) nanoarrays were successfully obtained through a facile chemical vapor deposition of three-dimensional (3D) C@TiO2 substrate, followed by potential pulse electrodeposition of Au electrocatalysts. The morphology and structure of the open 3D Au–C@TiO2 electrodes was characterized by scanning electron microscopy and X-ray diffractometry. The different morphology of electrodeposited Au can be easily controlled by the applied potential (Eo). Electrochemical methods, including cyclic voltammetry, linear sweep voltammetry and chronoamperometry, were used to examine the catalytic activity of the electrode for H2O2 electroreduction in H2SO4 solution. The Au ND–C@TiO2 electrode exhibited the largest effective specific surface area among the Au–C@TiO2 electrodes, owing to its open nanodendritic structure allowing the full utilization of Au surface active sites. A nearly constant reduction current density of 0.655 A cm−2 was successfully achieved on the Au ND–C@TiO2 electrode at the potential of 0 V in 2.0 mol L−1 H2O2 + 2.0 mol L−1 H2SO4 solution, which was significantly higher than the catalytic activity of H2O2 electroreduction achieved previously with precious metals as catalysts.

Uniformly grown PtCo-modified Co3O4 nanosheets as a highly efficient catalyst for sodium borohydride electrooxidation

A facile hydrothermal synthetic method, followed by in situ reduction and galvanic replacement processes, is used to prepare PtCo-modified Co3O4 nanosheets (PtCo/Co3O4 NSs) supported on Ni foam. The prepared nanomaterial is used as an electrocatalyst for NaBH4 oxidation in alkaline solution. The morphology and phase composition of PtCo/Co3O4 NSs are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The catalytic performance of PtCo/Co3O4 NSs is investigated by cyclic voltammetry (CV) and chronoamperometry (CA) in a standard three-electrode system. Current densities of 70 and 850 mA·cm–2 were obtained at–0.4 V for Co/Co3O4 and PtCo/Co3O4 NSs, respectively, in a solution containing 2 mol·L–1 NaOH and 0.2 mol·L–1 NaBH4. The use of a noble metal (Pt) greatly enhances the catalytic activity of the transition metal (Co) and Co3O4. Besides, both Co and Co3O4 exhibit good B–H bond breaking ability (in NaBH4), which leads to better electrocatalytic activity and stability of PtCo/Co3O4 NSs in NaBH4 electrooxidation compared to pure Pt. The results demonstrate that the as-prepared PtCo/Co3O4 NSs can be a promising electrocatalyst for borohydride oxidation.

Fabric-based flexible electrode with multi-walled carbon nanotubes@Ni network structure as a novel anode for hydrogen peroxide electrooxidation

A simple method involving dyeing and electrodeposition is introduced to fabricate a three-dimensional Ni@multi-walled carbon nanotubes flexible electrode on wearable fabric. The as-prepared Ni@multi-walled carbon nanotubes/Fabric (Ni@MWNTs/Fabric) electrode was characterized by scanning electron microscopy and X-ray diffraction spectrometry. The catalytic activity of the Ni@MWNTs/Fabric electrode for hydrogen peroxide electrooxidation was tested by means of cyclic voltammetry and chronoamperometry. Such a three-dimensional hybrid electrode structure allows a large specific surface area and a large mass loading, which lead to a high areal current density of 720 mA cm−2 at 0.5 V in 2 mol dm−3 NaOH and 2.5 mol dm−3 hydrogen peroxide. The electrode shows great promise as the anode of a direct peroxide fuel cell due to its being flexible, wearable, and environmentally friendly.

From biomass with irregular structures to 1D carbon nanobelts: a stripping and cutting strategy to fabricate high performance supercapacitor materials

One-dimensional (1D) nanostructures have been identified as the most viable structures for high-performance supercapacitors from the view of high ion-accessible surface area and rapid electron transport path as well as excellent mechanical properties. Herein, we report a “stripping and cutting” strategy to produce 1D carbon nanobelts (CNB) from tofu with irregular structures through a molten salts assisted technique. It is a completely novel and green avenue for constructing 1D carbon materials from biomass, showing large commercial potential. The resultant CNB electrode delivers a high specific capacitance (262 F g−1 at 0.5 A g−1) and outstanding cycling stability with capacitance retention up to 102% after 10 000 continuous charging/discharging cycles. Additionally, a CNB//CNB symmetric supercapacitor and CNB//MnO2–CNB asymmetric supercapacitor are assembled and reach energy densities of 18.19 and 29.24 W h kg−1, respectively. Therefore, such a simple, one-pot and low-cost process may have great potential for preparing eco-friendly biomass-derived carbon materials for high-performance supercapacitor electrodes.