Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond.

In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

One-pot synthesis of CoFe2O4/rGO hybrid hydrogels with 3D networks for high capacity electrochemical energy storage devices.

CoFe2O4/reduced graphene oxide (CoFe2O4/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach. The three-dimensional (3D) network structure consists of well-dispersed CoFe2O4 nanoparticles on the surfaces of graphene sheets. As a binder-free electrode material for supercapacitors, the electrochemical properties of the CoFe2O4/rGO hybrid hydrogel can be easily adjusted by changing the concentration of the graphene oxide (GO) precursor solution. The results indicate that the hybrid material made using 3.5 mg mL-1 GO solution exhibits an outstanding specific capacitance of 356 F g-1 at 0.5 A g-1, 68% higher than the pure CoFe2O4 counterpart (111 F g-1 at 0.5 A g-1), owing to the large specific surface area and good electric conductivity. Additionally, an electrochemical energy storage device based on CoFe2O4/rGO and rGO was assembled, which exhibits a high energy density of 17.84 W h kg-1 at a power density of 650 W kg-1 and an excellent cycling stability with 87% capacitance retention at 5 A g-1 after 4000 cycles. This work takes one step further towards the development of 3D hybrid hydrogel supercapacitors and highlights their potential application in energy storage devices.