•rGO supported MnS nanotubes (MnS-NT@rGO) hybrid was synthesized.•Nano-tubular structure endows MnS-NT@rGO high accessible surface area for ORR.•MnS-NT@rGO exhibits comparative catalytic activity for ORR to Pt/C.•MnS-NT@rGO exhibits higher activity for ORR than MnO@rGO and Mn(OH)2@rGO.•MnS-NT@rGO exhibits higher activity than rGO supported MnS nanoparticles.Electronic structure of Mn cations, electric conductivity of active materials and three dimensional structure for mass transport play vital roles in the electrocatalytic activity of Mn-based electrocatalysts for oxygen reduction reaction (ORR). To construct efficient and robust Mn-based electrocatalysts, MnS nanotubes anchored on reduced graphene oxide (MnS-NT@rGO) hybrid was synthesized and used as a novel non-precious metal electrocatalyst for ORR. The formation of nano-tubular structure, which offers more active sites and suitable channels for mass transport to enhance the electrocatalytic activity towards ORR, are carefully illustrated based on the core-dissolution/shell-recrystallization type Ostwald ripening effect. Tuned electronic structure of Mn cations, enhanced electric conductivity and suitable nano-tubular structure endow MnS-NT@rGO electrocatalyst comparative catalytic activity to commercial 20 wt % Pt/C in alkaline electrolyte. The MnS-NT@rGO electrocatalyst exhibits higher catalytic activity than rGO supported MnS nanoparticles (MnS-NP@rGO) and MnS nanotubes without rGO substrate (MnS-NT), as well as rGO supported Mn(OH)2 (Mn(OH)2@rGO) and rGO supported MnO (MnO@rGO). Moreover, the MnS-NT@rGO electrocatalyst shows superior durability and methanol tolerance to commercial Pt/C.Download high-res image (271KB)Download full-size image

•A TiO2 NWAs electrode is firstly used as negative electrode for supercapacitor.•As-assembled supercapacitor exhibits high cell voltage and high capacitance.•Only one single cell of the supercapacitor light a red LED indicator.To enhance the energy density, a novel asymmetric supercapacitor with a high cell voltage of 2.3 V is firstly assembled with a binder-free TiO2 nanowire arrays (NWAs) electrode as the negative electrode and a home-made porous carbon electrode as the positive electrode. The binder-free TiO2 NWAs electrode anchored on the titanium substrate is prepared via a two step method combining a chemical oxidation (with melamine as soft-template) process with a pyrolysis process. The TiO2 NWAs precursor is obtained via the dissolution of titanium to titania sol and the recrystallization to titania nanowire arrays gel assisted by the melamine. Due to the good electrochemical stability and the high over-potential for hydrogen evolution, the TiO2 NWAs electrode exhibits ultra-negative potential range in the aqueous electrolyte (0 ∼ −1.4 V vs.SCE), making it ideal for the negative electrode of asymmetric supercapacitor. The as-assembled HMPC//TiO2-NWAs asymmetric supercapacitor exhibits ultrahigh cell voltage (2.3 V) and high volumtric energy density (6.27 mWh cm−3). Only one single cell of the asymmetric supercapacitor can light a red light emitting diode (LED) indicator for 10 min, implying its promising practical application.Download high-res image (193KB)Download full-size image

To construct a suitable structure for both electronic conduction and ionic transport towards supercapacitors, peanut-like hierarchical manganese carbonate (MnCO3) microcrystals assembled with floss-like nanowires are synthesized via a hydrothermal process and primarily used as an active material for supercapacitors. The formation mechanism is illustrated by means of a dissolution–recrystallization process and magnetically driven self-assembly. The electrode with peanut-like hierarchical MnCO3 microcrystals exhibits a high specific capacitance of 293.7 F g−1 and a superior cycle stability of 71.5% retention after 6000 cycles, which are higher than those of the reported Mn-based active materials in alkaline electrolytes. The asymmetric supercapacitor, assembled with the peanut-like MnCO3 electrode as the positive electrode and a home-made porous carbon electrode as the negative electrode, exhibits an energy density of 14.7 W h kg−1 at a power density of 90.2 W kg−1 and an energy density of up to 11.0 W h kg−1 at 3.3 kW kg−1. An as-assembled all-solid-state supercapacitor series can light up a LED indicator for 10 min, indicating a promising practical application of peanut-like MnCO3 microcrystals.

To construct a suitable three-dimensional structure for ionic transport on the surface of the active materials for a supercapacitor, porous hollow nickel cobalt sulfides are successfully synthesized via a facile and efficient cation-exchange reaction in a hydrothermal process involving the Kirkendall effect with γ-MnS nanorods as a sacrificial template. The formation mechanism of the hollow nickel cobalt sulfides is carefully illustrated via the tuning reaction time and reaction temperature during the cation-exchange process. Due to the ingenious porous hollow structure that offers a high surface area for electrochemical reaction and suitable paths for ionic transport, porous hollow nickel cobalt sulfide electrodes exhibit high electrochemical performance. The Ni1.77Co1.23S4 electrode delivers a high specific capacity of 224.5 mAh g–1 at a current density of 0.25 A g–1 and a high capacity retention of 87.0% at 10 A g–1. An all-solid-state asymmetric supercapacitor, assembled with a Ni1.77Co1.23S4 electrode as the positive electrode and a homemade activated carbon electrode as the negative electrode (denoted as NCS//HMC), exhibits a high energy density of 42.7 Wh kg–1 at a power density of 190.8 W kg–1 and even 29.4 Wh kg–1 at 3.6 kW kg–1. The fully charged as-prepared asymmetric supercapacitor can light up a light emitting diode (LED) indicator for more than 1 h, indicating promising practical applications of the hollow nickel cobalt sulfides and the NCS//HMC asymmetric supercapacitor.Keywords: cation-exchange reaction; hollow structure; hydrothermal synthesis; Kirkendall effect; nickel cobalt sulfide; supercapacitor;

•rGO anchored nickel cobalt sulfides with sandwich-like structure are synthesized.•The synergistic effects of Ni and Co on the performance of binary sulfides are illustrated.•The Ni1.5Co1.5S4/rGO electrode exhibit high electrochemical performance.Reduced graphene oxide (rGO) anchored spinel nickel cobalt binary sulfides nanosheets (NixCo3-xS4/rGO) with sandwich-like structure are synthesized via a facile hydrothermal process. The synergistic effect of nickel and cobalt on the superior electrochemical properties of the NixCo3-xS4/rGO nanocomposites is firstly illustrated in detail through the comparison of the phase structures, metal valent states and electrochemical behaviors of the nickel doped cobalt sulfide (Ni0.3Co2.7S4/rGO), cobalt doped nickel sulfide (Ni2.7Co0.3S4/rGO) and nickel cobalt binary sulfide (Ni1.5Co1.5S4/rGO). Results indicate that the fast valence change of nickel species mainly contributes the faradaic reaction of the active materials, while the cobalt species offer the high electronic conductivity and assist the charge transfer process. Due to the synergistic effect of cobalt and nickel, the Ni1.5Co1.5S4/rGO electrode with sandwich-like structure exhibits superior electrochemical performances, such as high specific capacity (347.5 mAh g−1 based on metal sulfide), high areal specific capacity (4.3 mAh cm−2), high cyclic stability (85.0% retention after 2000 cycles). An asymmetric device, with the Ni1.5Co1.5S4/rGO as positive material and a home-made activated carbon as negative material, delivers high energy density of 61.6 Wh kg−1 at the power density of 563.1 W kg−1 and still remains 49.3 Wh kg−1 even at high power density of 11.0 kW kg−1.

Graphene oxide (GO) anchored porous manganese sulfide nanocrystals (MnS/GO-NH3) were obtained via a facile hydrothermal method based on the Kirkendall effect. The honeycomb-like manganese sulfide nanocrystals (40–80 nm) and the three-dimensional sandwich structure endow the MnS/GO-NH3 with high supercapacitive performance when it was used as a supercapacitor material. The MnS/GO-NH3 electrode exhibits high specific capacitance (390.8 F g−1 at 0.25 A g−1), high rate capacity (78.7% retention at 10 A g−1) and stable cycle life (81.0% retention after 2000 cycles), which are superior to those of GO anchored MnS floccules (MnS/GO) and manganese hydroxide (Mn(OH)2/GO). As a novel material for supercapacitors, the charge–discharge mechanism of the MnS/GO-NH3 composite is proposed via detailed investigation. Asymmetric supercapacitors, assembled with MnS/GO-NH3 as the positive material and activated carbon as the negative electrode, reveal a high specific capacitance (73.63 F g−1), a high energy density of 14.9 W h kg−1 at 66.5 W kg−1 and even 12.8 W h kg−1 at a high power density of 4683.5 W kg−1.

Monodispersed hollow spindle-like nanosphere (HS-NS) and tetrapod nanorod (TP-NR) MnS nanocrystals are obtained via a facile template-free hydrothermal process. The MnS nanocrystals are used as supercapacitor materials and they exhibit high performances. The TP-NR nanocrystals show a higher specific capacitance of 704.5 F g−1 compared to the HS-NS nanocrystals, and both show higher values compared to manganese oxide.

•Cobalt nickel sulfide dendrite/quasi-spherical nanocomposite is synthesized.•Co1.5Ni1.5S4 electrode exhibits the highest areal specific capacitance.•Co1.5Ni1.5S4-AC asymmetric supercapacitor exhibits high energy density.In this study, a spinel binary transition metal sulphide Co1.5Ni1.5S4 dendrite/quasi-spherical nanocomposite with high electronic conductivity and a suitable porous structure was synthesized via a facile hydrothermal process. The morphology and structure of the Co1.5Ni1.5S4 nanocomposite were easily manipulated by tuning the hydrothermal reaction time. To the best of our knowledge, due to its good electronic conductivity and suitable structure for the electrochemical redox reaction, the Co1.5Ni1.5S4 electrode exhibited the highest areal specific capacitance of 41.0 F cm−2. The electrochemical impedance spectroscopy (EIS) results demonstrate that Co1.5Ni1.5S4 possesses a higher electronic conductivity and faster charge transfer than single-component sulphides (CoS and NiS) or cobalt-nickel hydroxides and oxides. These characteristics contribute to the high specific capacitance of the Co1.5Ni1.5S4 electrode even at high material loading, corresponding to an ultrahigh areal specific capacitance. An asymmetric supercapacitor, which was assembled with Co1.5Ni1.5S4 as the positive electrode active material and HNO3-treated activated carbon as the negative electrode active material, exhibited a superior energy density of 32.4 Wh kg−1 at a power density of 103.4 W kg−1 and 25.0 Wh kg−1 at a high power density of 5.5 kW kg−1.

•NixCo(1−x)(OH)2 nanoflowers are synthesized by a template-free method•The formation mechanism of flower-like structure is demonstrated in detail•The NixCo(1−x)(OH)2 exhibit high performance as battery materials.•The NixCo(1−x)(OH)2 battery materials are used in high performance supercapacitor.In this work, various nickel cobalt double hydroxide nanoflowers with different Ni/Co ratios (denoted as NixCo(1−x)(OH)2), assembled by filmy nanoflakes, are prepared via a facile template-free hydrothermal process. The sizes of these nanoflowers are easily tuned by the Ni/Co ratio in the precursors. The formation mechanism of flower-like nickel cobalt hydroxide, based on the synergistic effect of ammonia complexation, Ni/Co ratios, precipitators and solvents in the template-free hydrothermal system, is demonstrated in detail. As the battery materials, the as prepared flower-like nickel cobalt double hydroxides exhibit excellent specific capacities and high rate performance. Ni0.28Co0.72(OH)2 displays the highest capacity of 206.7 mA h g−1 at 1 mV s−1 and 174.3 mA h g−1 at 1 A g−1, respectively. The capacity retention of Ni0.28Co0.72(OH)2 is 59.1% (from 206.7 to 122.2 mA h g−1) at the potential scan rate from 1 to 25 mV s−1. Due to the high rate performance corresponding to high power energy, Ni0.28Co0.72(OH)2 is used as positive material to assemble the hybrid device (asymmetric supercapacitor) with activated carbon as negative material. The as-prepared asymmetric supercapacitor exhibits 19.4 Wh kg−1 at 80.5 W kg−1, and even 20.6 Wh kg−1 at 3.93 kW kg−1.

Floss-like Ni–Co binary hydroxides (FL-NCOH), assembled by whisker-like nanowires, are synthesized via a facile hydrothermal process with sodium dodecyl benzene sulfonate (SDBS) as the soft template. The forming process of FL-NCOH is clarified by tuning the hydrothermal reaction time. The result indicates that floccule-like nickel–cobalt hydroxide nanoclusters gradually become longer and slenderer nanowires to form FL-NCOH with the increase of reaction time. The dissolution–recrystallization of hydroxide plays an important role in the morphology control of nickel–cobalt hydroxide. The high specific surface area (106.5 m2 g−1) and the suitable 3D structure endow the as-prepared FL-NCOH material high specific capacitance (up to 918.9 F g−1 at the current density of 0.2 A g−1), good high-rate performance (594.2 F g−1 even at 10.0 A g−1), and long cycle life (98.7 % capacitance retention after 3000 charge–discharge cycles at 2.0 A g−1).

Carbon supported Pt-Pd (Pt-Pd/C) catalysts with different Pt/Pd ratios were prepared by microwave assisted ethylene glycol method and characterized by X-ray diffraction (XRD), transmission electron spectroscopy (TEM) and electrochemical measurements. The results reveal that the metal nanoparticles of Pt-Pd/C catalysts are uniformly dispersed on the carbon support and exist as alloys with face centered cubic (FCC) structures. The effects of both preparation process and Pt/Pd ratios on the structure and catalytic activity of the Pt-Pd/C catalysts were investigated. With the increase of Pd content, the catalytic activity for ORR is deteriorated due to the increased metal particle size of Pt-Pd/C catalysts, while improving the methanol tolerance. Among the Pt-Pd/C catalysts with different Pt/Pd ratios, the Pt3Pd/C catalyst, which have the highest Pt/Pd ratios, exhibits higher specific mass activities toward oxygen reduction reaction (ORR) than that of the commercial Pt/C catalyst.

Three commercial carbon materials for supercapacitor were investigated by physicochemical characterization, electrochemical measurements and surface treatment to explore the effects of specific surface area, electrolyte and surface functional groups on the specific capacitance, charge storage mode and high rate performance of carbon materials. Results indicate that the specific surface area of carbon material plays dominate role in the specific capacitance. The electrolytes have remarkable effects on the specific capacitance and high rate performance. Investigation of HNO3 treated Vulcan XC-72 carbon material reveals that the treatment can increase the specific surface area and surface functional groups, which observably improve the specific capacitance of the XC-72 carbon material. The surface functional groups contribute to the pseudo-capacitance of the carbon material.

•Monodisperse cobalt hydroxide nanocubes and nanowires are synthesized.•Morphology and size of Co(OH)2 are tuned by CTAB content and reaction time.•Co(OH)2 nanowires electrode exhibits high performance and long cycle life.•As-prepared asymmetric supercapacitor shows high voltage and energy density.A facile hydrothermal process with hexadecyltrimethyl ammonium bromide (CTAB) as the soft template is proposed to tune the morphology and size of cobalt hydroxide (Co(OH)2). Monodisperse β-phase Co(OH)2 nanowires with uniform size are obtained by controlling the CTAB content and the reaction time. Due to the uniform well-defined morphology and stable structure, the Co(OH)2 nanowires material exhibits high capacitive performance and long cycle life. The specific capacitance of the Co(OH)2 nanowires electrode is 358 F g−1 at 0.5 A g−1, and even 325 F g−1 at 10 A g−1. The specific capacitance retention is 86.3% after 5000 charge–discharge cycles at 2 A g−1. Moreover, the asymmetric supercapacitor is assembled with Co(OH)2 nanowires and nitrite acid treated activated carbon (NTAC), which shows an energy density of 13.6 Wh kg−1 at the power density of 153 W kg−1 under a high voltage of 1.6 V, and 13.1 Wh kg−1 even at the power density of 1.88 kW kg−1.

•Flower-like nickel hydroxide was synthesized by a facile hydrothermal process.•The as-prepared nickel hydroxide exhibits high specific capacitance.•The as-prepared nickel hydroxide exhibits excellent high rate performance.To construct suitable nanostructures for electronic and ionic transport in the electrode of a supercapacitor, a flower-like nanostructured nickel hydroxide (Ni(OH)2) was synthesized by a facile hydrothermal process in this study. For comparison, an additional two Ni(OH)2 samples were synthesized to investigate the formation mechanism of the flower-like Ni(OH)2. Physicochemical characterizations indicate that the Ni(OH)2 nanoflower was formed by stacked hexagonal β-phase of the Ni(OH)2 nanoflakes. The dissolution-recrystallization of Ni(OH)2 and the stacking of nanoflakes play important roles in the formation of Ni(OH)2 nanoflowers. Due to the higher conductivity and the suitable macropores for ionic transport, the nanoflower-like Ni(OH)2 exhibits a high specific capacitance of 2653.2 F g−1 at 2 A g−1 and 1998.5 F g−1 at 40 A g−1. An asymmetric supercapacitor, which was assembled with Ni(OH)2 as the positive material and HNO3-treated activated carbon as the negative material, exhibited a high cell voltage of 1.6 V. Due to the high specific capacitance and high cell voltage, the as-prepared asymmetric supercapacitor exhibited a high energy density of 32.7 Wh kg−1 at 71.5 W kg−1 and 25.5 Wh kg−1 at 1.28 kW kg−1.

•Hierarchical porous NiO is synthesized through the biotemplate assisted method.•The potential application as a supercapacitor electrode is tested.•A ~93% capacitance retention is achieved from 0.3 to 10 A g−1.•A ~96 % capacitance retention after 2000 charge/discharge cycles is achieved.•This work inspires an idea for future structure design of other metal oxides.A novel 3D hierarchical porous nickel oxide (NiO) was fabricated by a facile biotemplate assisted method. Physicochemical characterizations indicate that the as prepared hierarchical porous NiO is assembled by multiple-layered porous nanosheets, of which the pore structure is highly ordered. The electrochemical performance of the as prepared hierarchical porous NiO was carried out in 6 M KOH, exhibited relatively high specific capacitance value of 493 F g−1 at a current density of 0.3 A g−1, good rate performance (~93% capacity retention from 0.3 A g−1 to 10 A g−1), as well as good cycling stability (~96% retention upon 2000 charge/discharge cycles at 10 A g−1).

The contact between Ni(OH)2 and graphene oxide (GO) determines the specific capacitance, high-rate performance, and stability of Ni(OH)2-GO composites when they were used as capacitive materials with high/ultrahigh material loading. To improve this contact, the exfoliated GO from Hummers’ method is oxidized twice for anchoring Ni(OH)2 nanoflakes. The X-ray photoelectron spectroscopy (XPS) results reveal that the further oxidation process increases the carbonyl (C–O) groups and oxygen content on the GO surface. The Ni(OH)2-GO composites were obtained through a simple hydrothermal process. Morphology and microstructure characterizations indicate that the further oxidation of GO improves the affinity of Ni(OH)2 and GO via the increased surface groups on the GO. Due to the high conductivity and suitable structure, the Ni(OH)2 anchored on the treated GO (Ni(OH)2/TGO) exhibits good capacitive performance and high areal specific capacitance. The Ni(OH)2/TGO exhibits high specific capacitance of 1236.4 F g–1 at 1.0 mV s–1 and 1374.8 F g–1 at 0.1 A g–1, respectively, which is higher than that of Ni(OH)2 on the untreated GO. The capacitance retention of Ni(OH)2/TGO is 52.2% even at 10 A g–1, which is higher than that of Ni(OH)2/GO (48.8%). For the high conductivity, the specific capacitance is still 996.2 F g–1 at 1.0 A g–1 even with ultrahigh material loading of 12.48 mg cm–2, which can be transferred to 12.06 F cm–2 calculated by areal specific capacitance. Furthermore, low deterioration is observed in Ni(OH)2/TGO (8.8% loss) after 1000-cycle charge–discharge test at 1.0 A g–1, which is lower than that of Ni(OH)2/GO (19.5% loss). The asymmetric supercapacitor, using the Ni(OH)2/TGO and activated carbon as the positive material and negative material, respectively, exhibits high energy density of 22.5 Wh kg–1 at 86.3 W kg–1 and 17.8 Wh kg–1 even at 4.05 kW kg–1.

•Polyhedral Pd–Pt nanocrystallines were synthesized without any surfactants.•Low-Pt polyhedral Pd–Pt/C catalysts exhibit comparable ORR to TKK Pt/C catalyst.•The as prepared Pd–Pt/C show excellent methanol tolerance.•Polyhedral Pd–Pt/C with low ECSA show 1.56-fold mass activity to spherical Pd–Pt/C.Carbon supported polyhedral Pd–Pt nanocrystallines (Pd–Pt/C) are synthesized using a facile surfactant-free process. The morphology of nanocrystallines is tailored by ammonia complexation and Pd/Pt ratios of the metallic precursors. The as-prepared Pd–Pt/C catalysts with low Pt content exhibit oxygen reduction reaction (ORR) activity comparable to that of the commercial Pt/C catalyst, while having much higher methanol tolerance. Compared with the spherical Pd6Pt/C catalyst, the polyhedral Pd6Pt/C catalyst shows 1.56-fold higher mass specific activity for ORR, while its electrochemical surface area (ECSA) is only half of the former. The superior ORR activity may be attributed to the more exposed regular (111) planes, the higher ratio of metallic form and the surface enrichment of Pt atoms.

To construct a suitable structure for both electronic conduction and ionic transport towards supercapacitors, peanut-like hierarchical manganese carbonate (MnCO3) microcrystals assembled with floss-like nanowires are synthesized via a hydrothermal process and primarily used as an active material for supercapacitors. The formation mechanism is illustrated by means of a dissolution–recrystallization process and magnetically driven self-assembly. The electrode with peanut-like hierarchical MnCO3 microcrystals exhibits a high specific capacitance of 293.7 F g−1 and a superior cycle stability of 71.5% retention after 6000 cycles, which are higher than those of the reported Mn-based active materials in alkaline electrolytes. The asymmetric supercapacitor, assembled with the peanut-like MnCO3 electrode as the positive electrode and a home-made porous carbon electrode as the negative electrode, exhibits an energy density of 14.7 W h kg−1 at a power density of 90.2 W kg−1 and an energy density of up to 11.0 W h kg−1 at 3.3 kW kg−1. An as-assembled all-solid-state supercapacitor series can light up a LED indicator for 10 min, indicating a promising practical application of peanut-like MnCO3 microcrystals.

Graphene oxide (GO) anchored porous manganese sulfide nanocrystals (MnS/GO-NH3) were obtained via a facile hydrothermal method based on the Kirkendall effect. The honeycomb-like manganese sulfide nanocrystals (40–80 nm) and the three-dimensional sandwich structure endow the MnS/GO-NH3 with high supercapacitive performance when it was used as a supercapacitor material. The MnS/GO-NH3 electrode exhibits high specific capacitance (390.8 F g−1 at 0.25 A g−1), high rate capacity (78.7% retention at 10 A g−1) and stable cycle life (81.0% retention after 2000 cycles), which are superior to those of GO anchored MnS floccules (MnS/GO) and manganese hydroxide (Mn(OH)2/GO). As a novel material for supercapacitors, the charge–discharge mechanism of the MnS/GO-NH3 composite is proposed via detailed investigation. Asymmetric supercapacitors, assembled with MnS/GO-NH3 as the positive material and activated carbon as the negative electrode, reveal a high specific capacitance (73.63 F g−1), a high energy density of 14.9 W h kg−1 at 66.5 W kg−1 and even 12.8 W h kg−1 at a high power density of 4683.5 W kg−1.

Monodispersed hollow spindle-like nanosphere (HS-NS) and tetrapod nanorod (TP-NR) MnS nanocrystals are obtained via a facile template-free hydrothermal process. The MnS nanocrystals are used as supercapacitor materials and they exhibit high performances. The TP-NR nanocrystals show a higher specific capacitance of 704.5 F g−1 compared to the HS-NS nanocrystals, and both show higher values compared to manganese oxide.