Yufei Zhang

Binary metal oxide: advanced energy storage materials in supercapacitors

Binary transition metal oxides (BTMOs) possess higher reversible capacity, better structural stability and electronic conductivity, and have been widely studied to be novel electrode materials for supercapacitors. In this review, we present an extensive description of BTMO materials and the most commonly used synthetic methods. Furthermore, we review several notable BTMOs and their composites in application of supercapacitors. With the increasing attention for energy storage, more and more exciting results about BTMO materials will be reported in the future.

Hybrid NiCo2S4@MnO2 heterostructures for high-performance supercapacitor electrodes

Using a simple hydrothermal route coupled with a carbonization treatment, one-dimensional NiCo2S4@MnO2 heterostructures have been fabricated successfully. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) measurements showed that MnO2 nanoflakes uniformly wrapped around the surface of NiCo2S4 nanotubes and formed core–shell heterostructured nanotubes, which combine the advantages of both NiCo2S4 such as excellent cycle stability and MnO2 such as high capacity. Serving as a supercapacitor electrode, the NiCo2S4@MnO2 heterostructures possess a remarkable specific capacitance (1337.8 F g−1 at the current density of 2.0 A g−1) and excellent cycling stability (retaining 82% after 2000 cycles) due to the synergistic effects of NiCo2S4 and MnO2. These unique nanoarchitectures demonstrate potential applications in energy storage electrodes and may inspire researchers to continue to focus on heterostructured materials.

Co9S8/MoS2 Yolk–Shell Spheres for Advanced Li/Na Storage

Uniform sized Co9 S8 /MoS2 yolk-shell spheres with an average diameter of about 500 nm have been synthesized by a facile route. When evaluated as anodes for lithium-ion and sodium-ion batteries, these Co9 S8 /MoS2 yolk-shell spheres show high specific capacities, excellent rate capabilities, and good cycling stability.

Phase-controlled synthesis of α-NiS nanoparticles confined in carbon nanorods for high performance supercapacitors

A facile and phase-controlled synthesis of α-NiS nanoparticles (NPs) embedded in carbon nanorods (CRs) is reported by in-situ sulfurating the preformed Ni/CRs. The nanopore confinement by the carbon matrix is essential for the formation of α-NiS and preventing its transition to β-phase, which is in strong contrast to large aggregated β-NiS particles grown freely without the confinement of CRs. When used as electrochemical electrode, the hybrid electrochemical charge storage of the ultrasmall α-NiS nanoparticels dispersed in CRs is benefit for the high capacitor (1092, 946, 835, 740u2005F g−1 at current densities of 1, 2, 5, 10u2005A g−1, respectively.). While the high electrochemical stability (approximately 100% retention of specific capacitance after 2000 charge/discharge cycles) is attributed to the supercapacitor-battery electrode, which makes synergistic effect of capacitor (CRs) and battery (NiS NPs) components rather than a merely additive composite. This work not only suggests a general approach for phase-controlled synthesis of nickel sulfide but also opens the door to the rational design and fabrication of novel nickel-based/carbon hybrid supercapacitor-battery electrode materials.

Pulsed laser ablation of preferentially orientated ZnO:Co diluted magnetic semiconducting thin films on Si substrates

ZnO:Co thin films with room-temperature ferromagnetism have been synthesized on (001) Si substrates by pulsed laser deposition using a Zn0.95Co0.05O ceramic target. Single-phase wurtzite thin films with (002) preferential orientation were grown at 450°C in vacuum. There is no indication of Co nanocluster formation. However, copious edge dislocations appear to have formed during the film growth. A saturation magnetization of 1.04μB∕Co and a coercivity of 25Oe were obtained at room temperature. In addition to O vacancies, the Zn interstitial induced by edge dislocations may also contribute to the ferromagnetic properties in this oxide-diluted magnetic semiconductor.

Hierarchical carbon@Ni3S2@MoS2 double core–shell nanorods for high-performance supercapacitors

Hierarchical carbon@Ni3S2@MoS2 (C@Ni3S2@MoS2) double core–shell nanorods have been synthesized by a facile hydrothermal method using highly conductive carbon/Ni (C/Ni) nanorods as both the precursor and template. As supercapacitor electrodes, the C@Ni3S2@MoS2 nanorods deliver a specific capacitance as high as 1544 F g−1 at a current density of 2 A g−1 with excellent cycling stability (retaining 92.8% of the capacitance after 2000 cycles at a current density of 20 A g−1). The C/Ni nanorods as the backbone played crucial roles in enhancing the rate performance of the device, in the meanwhile, interconnected MoS2 nanosheets on the shell provided numerous accessible surfaces and contacts with the electrolyte. Our work demonstrated an effective design of robust hierarchical double core/shell nanostructures, which could provide a general and promising approach to fabricate high-performance materials for energy storage applications.

Carbon nanotubes synthesized by biased thermal chemical vapor deposition as an electron source in an x-ray tube

Carbon nanotubes (CNTs) with supreme field emission properties were synthesized by depositing Co-containing amorphous carbon (a-C:Co) composite films using filtered cathodic vacuum arc technique with a 15at% Co-containing graphite target and subsequently growing CNTs using biased thermal chemical vapor deposition at 580°C with the a-C:Co composite film as a catalyst layer. The as-grown CNTs with a thin diameter of about 10nm have a low threshold field of 1.6V∕μm and a stable current density of 2.1mA∕cm2 at 3V∕μm. Thus an x-ray source was built in a diode configuration using the CNTs as its cold electron source showing good potential in x-ray radiography.

Graphene-based three-dimensional hierarchical sandwich-type architecture for high performance supercapacitors

A facile two-step method is developed for large-scale preparation of graphene-based three-dimensional hierarchical sandwich-type architecture (graphene/carbon nanotubes (CNTs)/Mn2O3) for high performance supercapacitor. The synthesis involves a chemical vapor deposition (CVD) method to fabricate sponge-like three-dimensional (3D) graphene/CNTs and an electrodeposition process to deposit Mn2O3 nano-sheets on the surface of 3D graphene/CNTs. With the novel sandwich-type composite as an electrode, the measurements indicate the synergy effects of Mn2O3 and 3D carbonous materials make the electrode present a high specific capacitance. The composite electrode also presents a high reversible capacity, excellent cycle performance and rate capability. It could be concluded that the composite of Mn2O3 with 3D graphene/CNTs displays excellent synergy effects of transition-metal oxide and carbonous materials. This work also inspires in-depth research for the application of 3D graphene-based composites for high performance supercapacitors.

Nanowires assembled from MnCo2O4@C nanoparticles for water splitting and all-solid-state supercapacitor

The rational design of earth-abundant catalysts with excellent water splitting activities is important to obtain clean fuels for sustainable energy devices. In this study, mixed transition metal oxide nanoparticles encapsulated in nitrogendoped carbon (denoted as AB2O4@NC) were developed using a one-pot protocol, wherein a metal–organic complex was adopted as the precursor. As a proof of concept, MnCo2O4@NC was used as an electrocatalyst for water oxidation, and demonstrated an outstanding electrocatalytic activity with low overpotential to achieve a current density of 10 mA·cm−1 (η10 = 287 mV), small Tafel slope (55 mV·dec−1), and high stability (96% retention after 20 h). The excellent electrochemical performance benefited from the synergistic effects of the MnCo2O4 nanoparticles and nitrogen-doped carbon, as well as the assembled mesoporous nanowire structure. Finally, a highly stable all-solid-state supercapacitor based on MnCo2O4@NC was demonstrated (1.5% decay after 10,000 cycles).

MoS2 coated hollow carbon spheres for anodes of lithium ion batteries

With the assistance of resorcinol–formaldehyde, MoS2 coated hollow carbon spheres (C@MoS2) were synthesized through a facile hydrothermal route followed by heat and alkali treatments. The measurements indicate that the hollow carbon spheres with an average diameter of 300 nm and shell thickness of 20 nm. And the hollow core are uniformly covered by ultrathin MoS2 nanosheets with a length increased to 400 nm. The unique hollow structure and the synergistic effect between carbon layer and MoS2 nanosheets significantly enhance the rate capability and electrochemical stability of C@MoS2 spheres as anode material of lithium-ion battery. The synthesized C@MoS2 delivered a capacity of 750 mAh g−1 at a current density of 100 mA g−1. More importantly, the C@MoS2 maintained a reversible capacity of 533 mAh g−1 even at a high current density of 1000 mA g−1. The study indicated that MoS2 coated hollow carbon spheres can be promising anode material for next generation high-performance lithium-ion batteries.