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.

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

Concerns over air quality reduction resulting from burning fossil fuels have driven the development of clean and renewable energy sources. Supercapacitors, batteries and solar cells serve as eco-friendly energy storage and conversion systems vitally important for the sustainable development of human society. However, many diverse elements influence the performance of energy storage and conversion systems. The overall efficiency of systems depends on the specific structure and properties of incorporated functional materials. Carbon materials, such as graphene, are especially promising for materials development in the energy storage and conversion fields. Graphene, a two-dimensional (2D) carbon material only a single atom thick, has massless Dirac fermions (electron transport is governed by Dirac's equation), displays outstanding electrical conductivity, superior thermal conductivity and excellent mechanical properties. 2D free-standing graphene films and powders have paved the way for promising energy applications. Recently, much effort has been spent trying to improve the number of active sites in electrode materials within 3D network/aerogel structures derived from graphene. This is because graphene aerogels are promising materials for energy systems due to their porous hierarchical structure which affords rapid electron/ion transport, superior chemical and physical stability, and good cycle performance. This review aims to summarize the synthetic methods, mechanistic aspects, and energy storage and conversion applications of novel 3D network graphene, graphene derivatives and graphene-based materials. Areas of application include supercapacitors, Li-batteries, H2 and thermal energy storage, fuel cells and solar cells.

Graphene aerogels for efficient energy storage and conversion

Hydrogen (H2) production via solar water splitting is one of the most ideal strategies for providing sustainable fuel because this requires only water and sunlight. In achieving high-yield production of hydrogen as a recyclable energy carrier, the nanoscale design of semiconductor (SC) materials plays a pivotal role in both photoelectrochemical (PEC) and photocatalytic (PC) water splitting reactions. In this context, the advent of two-dimensional (2D) materials with remarkable electronic and optical characteristics has attracted great attention for their application to PEC/PC systems. The elaborate design of combined 2D layered materials interfaced with other SCs can markedly enhance the PEC/PC efficiencies via bandgap alteration and heterojunction formation. Three classes of 2D materials including graphene, transition metal dichalcogenides (TMDs), and graphitic carbon nitride (g-C3N4), and their main roles in the photoelectrocatalytic production of H2, are discussed in detail herein. We highlight the various roles of these 2D materials, such as enhanced light harvesting, suitable band edge alignment, facilitated charge separation, and stability during the water splitting reaction, in various SC/2D photoelectrode and photocatalytic systems. The roles of emerging 2D nanomaterials, such as 2D metal oxyhalides, 2D metal oxides, and layered double hydroxides, in PEC H2 production are also discussed.

Two-dimensional materials in semiconductor photoelectrocatalytic systems for water splitting

Supercapacitors (SCs) have shown great promise as a possible solution to the increasing world demand for efficient energy storage Two types of mechanisms for SCs exist (double-layer and pseudocapacitive), and each type utilizes a wide variety of materials In this review, a detailed overview of the mechanisms employed by SCs is provided in the introduction, and many studies are compared in order to determine which materials produce electrodes with high capacitance and cyclability in SCs, and to summarize and gauge the state of such research The types of materials looked at include graphene and graphene nanocomposites, activated carbons from renewable materials, conducting polymers, and transition metal dichalcogenides Additionally, different methods of activation that are meant to increase specific capacitance are examined Among the dozens of materials found in the literature during this study, the ones that exhibited the highest specific capacitances are rGO/PANI (Reduced Graphene Oxide/Polyaniline), and PANI-NFS/GF (Polyaniline Nanofiber Sponge Filled Graphene Foam) demonstrated impressive performances These materials all exceeded the current expectations of SCs by remarkable amounts, and more research into similar materials is highly encouraged As more fundamental studies carried out for understanding the mechanisms of SCs, energy density and specific capacitance values continue to improve Production of SCs from renewable materials encourage optimism for environmentally friendly options soon becoming feasible for use on larger scales

Materials for energy storage: Review of electrode materials and methods of increasing capacitance for supercapacitors

Cadmium sulfide (CdS)-based photocatalysts have attracted extensive attention owing to their strong visible light absorption, suitable band energy levels, and excellent electronic charge transportation properties. This review focuses on the recent progress related to the design, modification, and construction of CdS-based photocatalysts with excellent photocatalytic H2 evolution performances. First, the basic concepts and mechanisms of photocatalytic H2 evolution are briefly introduced. Thereafter, the fundamental properties, important advancements, and bottlenecks of CdS in photocatalytic H2 generation are presented in detail to provide an overview of the potential of this material. Subsequently, various modification strategies adopted for CdS-based photocatalysts to yield solar H2 are discussed, among which the effective approaches aim at generating more charge carriers, promoting efficient charge separation, boosting interfacial charge transfer, accelerating charge utilization, and suppressing charge-induced self-photocorrosion. The critical factors governing the performance of the photocatalyst and the feasibility of each modification strategy toward shaping future research directions are comprehensively discussed with examples. Finally, the prospects and challenges encountered in developing nanostructured CdS and CdS-based nanocomposites in photocatalytic H2 evolution are presented.

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Xiaoyu Cai

Papers

In-situ construction of hierarchical CdS/MoS2 microboxes for enhanced visible-light photocatalytic H2 production

MnO2 decorated on carbon sphere intercalated graphene film for high-performance supercapacitor electrodes

Free-standing graphene/carbon nanotubes/CuO aerogel paper anode for lithium ion batteries

Nitrogen and carbon co-doped Ni-TiO 2 spindles for high performance electrochemical capacitor electrodes

Facile in situ synthesis of Ag and MnO2 anchored on carbon microtubes for high-performance asymmetric supercapacitor applications