Construction of rGO-Encapsulated Co3 O4 -CoFe2 O4 Composites with a Double-Buffer Structure for High-Performance Lithium Storage.

Transition metal oxides (TMOs) are promising anode materials for next-generation lithium-ion batteries (LIBs). Nevertheless, their poor electronic and ionic conductivity as well as huge volume change leads to low capacity release and rapid capacity decay. Herein, a reduced graphene oxide (rGO)-encapsulated TMOs strategy is developed to address the above problems. The Co3 O4 -CoFe2 O4 @rGO composites with rGO sheets-encapsulated Co3 O4 -CoFe2 O4 microcubes are successfully constructed through a simple metal-organic frameworks precursor route, in which Co[Fe(CN)5 NO] microcubes are in situ coated by graphene oxide sheets, followed by a two-step calcination process. As anode material of LIBs, Co3 O4 -CoFe2 O4 @rGO exhibits remarkable reversible capacity (1393 mAh g-1 at 0.2 A g-1 after 300 cycles), outstanding long-term cycling stability (701 mAh g-1 at 2.0 A g-1 after 500 cycles), and excellent rate capability (420 mAh g-1 at 4.0 A g-1 ). The superior lithium storage performance can be attributed to the unique double-buffer structure, in which the outer flexible rGO shells can prevent the structure collapse of the electrode and improve its conductivity, while the hierarchical porous cores of Co3 O4 -CoFe2 O4 microcubes can buffer the volume expansion. This work provides a general and straightforward strategy for the construction of novel rGO-encapsulated bimetal oxides for energy storage and conversion application.

Metal-organic framework derived Fe/Fe3C@N-doped-carbon porous hierarchical polyhedrons as bifunctional electrocatalysts for hydrogen evolution and oxygen-reduction reactions.

The development of simple and cost-effective synthesis methods for electrocatalysts of hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) is critical to renewable energy technologies. Herein, we report an interesting bifunctional HER and ORR electrocatalyst of Fe/Fe3C@N-doped-carbon porous hierarchical polyhedrons (Fe/Fe3C@N-C) by a simple metal-organic framework precursor route. The Fe/Fe3C@N-C polyhedrons consisting of Fe and Fe3C nanocrystals enveloped by N-doped carbon shells and accompanying with some carbon nanotubes on the surface were prepared by thermal annealing of Zn3[Fe(CN)6]2·xH2O polyhedral particles in nitrogen atmosphere. This material exhibits a large specific surface area of 182.5 m2 g-1 and excellent ferromagnetic property. Electrochemical tests indicate that the Fe/Fe3C@N-C hybrid has apparent HER activity with a relatively low overpotential of 236 mV at the current density of 10 mA cm-2 and a small Tafel slope of 59.6 mV decade-1. Meanwhile, this material exhibits excellent catalytic activity toward ORR with an onset potential (0.936 V vs. RHE) and half-wave potential (0.804 V vs. RHE) in 0.1 M KOH, which is comparable to commercial 20 wt% Pt/C (0.975 V and 0.820 V), and shows even better stability than the Pt/C. This work provides a new insight to developing multi-functional materials for renewable energy application.

g-C3N4/AgBr nanocomposite decorated with carbon dots as a highly efficient visible-light-driven photocatalyst.

Visible-light-driven photocatalysis as a green technology has attracted intense interest due to its potential applications in environmental remediation. However, the poor visible light utilization and low electron-hole separation efficiency of photocatalysts largely limited their practical application. In this work, a new ternary visible-light driven photocatalyst of g-C3N4/CDs/AgBr has been prepared by the introduction of carbon dots (CDs) onto the surface of g-C3N4, followed by in-situ growth of AgBr nanoparticles on CDs-modified g-C3N4 nanosheets. The g-C3N4/CDs/AgBr nanocomposite exhibits excellent photocatalytic efficiency for organic pollutant degradation, which is about 4.0, 5.3 and 2.3 times higher than that of AgBr, g-C3N4 and g-C3N4/AgBr, respectively. The result indicates the introduction of CDs into g-C3N4/AgBr can largely improve the photocatalytic activity since CDs act as the light absorber and the electron mediator between g-C3N4 and AgBr, which effectively promote the separation of photogenerated charge carriers and the utilization of visible light. Moreover, the photocatalytic activity of g-C3N4/CDs/AgBr has no obvious decrease after four photodegradation cycles, demonstrating a high photocatalytic stability. This study highlights the potential application of highly efficient CDs decorated photocatalysts in waste water purification.