Notably, many significant breakthroughs for a new generation of supercapacitors have been reported in recent years, related to theoretical understanding, material synthesis and device designs. Herein, we summarize the state-of-the-art progress toward mechanisms, new materials, and novel device designs for supercapacitors. Firstly, fundamental understanding of the mechanism is mainly focused on the relationship between the structural properties of electrode materials and their electrochemical performances based on some in situ characterization techniques and simulations. Secondly, some emerging electrode materials are discussed, including metal–organic frameworks (MOFs), covalent organic frameworks (COFs), MXenes, metal nitrides, black phosphorus, LaMnO3, and RbAg4I5/graphite. Thirdly, the device innovations for the next generation of supercapacitors are provided successively, mainly emphasizing flow supercapacitors, alternating current (AC) line-filtering supercapacitors, redox electrolyte enhanced supercapacitors, metal ion hybrid supercapacitors, micro-supercapacitors (fiber, plane and three-dimensional) and multifunctional supercapacitors including electrochromic supercapacitors, self-healing supercapacitors, piezoelectric supercapacitors, shape-memory supercapacitors, thermal self-protective supercapacitors, thermal self-charging supercapacitors, and photo self-charging supercapacitors. Finally, the future developments and key technical challenges are highlighted regarding further research in this thriving field.

Latest advances in supercapacitors: from new electrode materials to novel device designs.

Owing to high specific energy, low cost, and environmental friendliness, lithium–sulfur (Li–S) batteries hold great promise to meet the increasing demand for advanced energy storage beyond portable electronics, and to mitigate environmental problems. However, the application of Li–S batteries is challenged by several obstacles, including their short life and low sulfur utilization, which become more serious when sulfur loading is increased to the practically accepted level above 3–5 mg cm−2. More and more efforts have been made recently to overcome the barriers toward commercially viable Li–S batteries with a high sulfur loading. This review highlights the recent progress in high-sulfur-loading Li–S batteries enabled by hierarchical design principles at multiscale. Particularly, basic insights into the interfacial reactions, strategies for mesoscale assembly, unique architectures, and configurational innovation in the cathode, anode, and separator are under specific concerns. Hierarchy in the multiscale design is proposed to guide the future development of high-sulfur-loading Li–S batteries.

Review on High-Loading and High-Energy Lithium–Sulfur Batteries

Flexible solid-state supercapacitors (FSSCs) are frontrunners in energy storage device technology and have attracted extensive attention owing to recent significant breakthroughs in modern wearable electronics In this study, we review the state-of-the-art advancements in FSSCs to provide new insights on mechanisms, emerging electrode materials, flexible gel electrolytes and novel cell designs The review begins with a brief introduction on the fundamental understanding of charge storage mechanisms based on the structural properties of electrode materials The next sections briefly summarise the latest progress in flexible electrodes (ie, freestanding and substrate-supported, including textile, paper, metal foil/wire and polymer-based substrates) and flexible gel electrolytes (ie, aqueous, organic, ionic liquids and redox-active gels) Subsequently, a comprehensive summary of FSSC cell designs introduces some emerging electrode materials, including MXenes, metal nitrides, metal–organic frameworks (MOFs), polyoxometalates (POMs) and black phosphorus Some potential practical applications, such as the development of piezoelectric, photo-, shape-memory, self-healing, electrochromic and integrated sensor-supercapacitors are also discussed The final section highlights current challenges and future perspectives on research in this thriving field

Towards flexible solid-state supercapacitors for smart and wearable electronics

Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Applications of 2D MXenes in energy conversion and storage systems

Electrochemical water splitting is a promising technology for sustainable conversion, storage, and transport of hydrogen energy. Searching for earth-abundant hydrogen/oxygen evolution reaction (HER/OER) electrocatalysts with high activity and durability to replace noble-metal-based catalysts plays paramount importance in the scalable application of water electrolysis. A freestanding electrode architecture is highly attractive as compared to the conventional coated powdery form because of enhanced kinetics and stability. Herein, recent progress in developing transition-metal-based HER/OER electrocatalytic materials is reviewed with selected examples of chalcogenides, phosphides, carbides, nitrides, alloys, phosphates, oxides, hydroxides, and oxyhydroxides. Focusing on self-supported electrodes, the latest advances in their structural design, controllable synthesis, mechanistic understanding, and strategies for performance enhancement are presented. Remaining challenges and future perspectives for the further development of self-supported electrocatalysts are also discussed.

Self-Supported Transition-Metal-Based Electrocatalysts for Hydrogen and Oxygen Evolution

High-performance electrode materials are the key to advances in the areas of energy conversion and storage (e.g., fuel cells and batteries). In this Review, recent progress in the synthesis and electrochemical application of transition metal carbides (TMCs) and nitrides (TMNs) for energy storage and conversion is summarized. Their electrochemical properties in Li-ion and Na-ion batteries as well as in supercapacitors, and electrocatalytic reactions (oxygen evolution and reduction reactions, and hydrogen evolution reaction) are discussed in association with their crystal structure/morphology/composition. Advantages and benefits of nanostructuring (e.g., 2D MXenes) are highlighted. Prospects of future research trends in rational design of high-performance TMCs and TMNs electrodes are provided at the end.

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Transition Metal Carbides and Nitrides in Energy Storage and Conversion

A new and generic strategy to construct interwoven carbon nanotube (CNT) branches on various metal oxide nanostructure arrays (exemplified by V2 O3 nanoflakes, Co3 O4 nanowires, Co3 O4 -CoTiO3 composite nanotubes, and ZnO microrods), in order to enhance their electrochemical performance, is demonstrated for the first time. In the second part, the V2 O3 /CNTs core/branch composite arrays as the host for Na(+) storage are investigated in detail. This V2 O3 /CNTs hybrid electrode achieves a reversible charge storage capacity of 612 mAh g(-1) at 0.1 A g(-1) and outstanding high-rate cycling stability (a capacity retention of 100% after 6000 cycles at 2 A g(-1) , and 70% after 10 000 cycles at 10 A g(-1) ). Kinetics analysis reveals that the Na(+) storage is a pseudocapacitive dominating process and the CNTs improve the levels of pseudocapacitive energy by providing a conductive network.

Generic Synthesis of Carbon Nanotube Branches on Metal Oxide Arrays Exhibiting Stable High-Rate and Long-Cycle Sodium-Ion Storage

The low utilization of active sites and sluggish reaction kinetics of MoSe2 severely impede its commercial application as electrocatalyst for hydrogen evolution reaction (HER). To address these two issues, the first example of introducing 1T MoSe2 and N dopant into vertical 2H MoSe2/graphene shell/core nanoflake arrays that remarkably boost their HER activity is herein described. By means of the improved conductivity, rich catalytic active sites and highly accessible surface area as a result of the introduction of 1T MoSe2 and N doping as well as the unique structural features, the N-doped 1T-2H MoSe2/graphene (N-MoSe2/VG) shell/core nanoflake arrays show substantially enhanced HER activity. Remarkably, the N-MoSe2/VG nanoflakes exhibit a relatively low onset potential of 45 mV and overpotential of 98 mV (vs RHE) at 10 mA cm−2 with excellent long-term stability (no decay after 20 000 cycles), outperforming most of the recently reported Mo-based electrocatalysts. The success of improving the electrochemical performance via the introduction of 1T phase and N dopant offers new opportunities in the development of high-performance MoSe2-based electrodes for other energy-related applications.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.202006055

Directional Construction of Vertical Nitrogen-Doped 1T-2H MoSe2 /Graphene Shell/Core Nanoflake Arrays for Efficient Hydrogen Evolution Reaction.

The advancement of electrochemical energy storage is closely bound up with the breakthrough of controllable fabrication of energy materials. Inspired by a popcorn fabrication from corn raw, herein a unique porous macrocellular carbon composed of cross-linked nano/microsheets by a powerful puffing of rice precursor is described. The rice is directly puffed with a volume enlargement of ≈20 times when it is instantaneously released from a sealed environment with a high pressure of 1.0 MPa at 200 °C. Interestingly, when metal (e.g., Ni) nanoparticles are embedded in the puffed rice derived carbon (PRC), high-quality PRC/metal composites are achieved with attractive properties of a high electrical conductivity of ≈7.2 × 104 S m−1, a large porosity of 85.1%, and a surface area of 1492.2 m2 g−1. The PRC/Ni are employed as a host in lithium–sulfur batteries. The designed PRC/Ni/S electrode exhibits a high reversible capacity of 1257.2 mA h g−1 at 0.2 C, a prolonged cycle life (821 mA h g−1 after 500 cycles), and enhanced rate capability, much better than other counterparts (PRC/S and rGO/S). The excellent properties are attributed to the advantages of PRC/Ni network with a high electrical conductivity, strong adsorption/blocking ability for polysulfides, and interconnected porous framework.

Popcorn Inspired Porous Macrocellular Carbon: Rapid Puffing Fabrication from Rice and Its Applications in Lithium–Sulfur Batteries

Confining Sulfur in Integrated Composite Scaffold with Highly Porous Carbon Fibers/Vanadium Nitride Arrays for High-Performance Lithium–Sulfur Batteries

Yu Zhong

Papers

Transition Metal Carbides and Nitrides in Energy Storage and Conversion

Generic Synthesis of Carbon Nanotube Branches on Metal Oxide Arrays Exhibiting Stable High-Rate and Long-Cycle Sodium-Ion Storage

Directional Construction of Vertical Nitrogen-Doped 1T-2H MoSe2 /Graphene Shell/Core Nanoflake Arrays for Efficient Hydrogen Evolution Reaction.

Popcorn Inspired Porous Macrocellular Carbon: Rapid Puffing Fabrication from Rice and Its Applications in Lithium–Sulfur Batteries

Confining Sulfur in Integrated Composite Scaffold with Highly Porous Carbon Fibers/Vanadium Nitride Arrays for High-Performance Lithium–Sulfur Batteries