Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Ongoing technological advances in diverse fields including portable electronics, transportation, and green energy are often hindered by the insufficient capability of energy-storage devices By taking advantage of two different electrode materials, asymmetric supercapacitors can extend their operating voltage window beyond the thermodynamic decomposition voltage of electrolytes while enabling a solution to the energy storage limitations of symmetric supercapacitors This review provides comprehensive knowledge to this field We first look at the essential energy-storage mechanisms and performance evaluation criteria for asymmetric supercapacitors to understand the wide-ranging research conducted in this area Then we move to the recent progress made for the design and fabrication of electrode materials and the overall structure of asymmetric supercapacitors in different categories We also highlight several key scientific challenges and present our perspectives on enhancing the electrochemical performance of future asymmetric supercapacitors

Design and Mechanisms of Asymmetric Supercapacitors.

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion 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

Supercapacitors have gained a lot of attention due to their unique features like high power, long cycle life and environment-friendly nature. They act as a link for energy-power difference between a traditional capacitor (having high power) and fuel cells/batteries (having high energy storage). In this perspective, a worldwide research has been reported to address this and rapid progress has been achieved in the advancement of fundamental as well as the applied aspects of supercapacitors. Here, a concise description of technologies and working principles of different materials utilized for supercapacitors has been provided. The main focus has been on materials like carbon-based nanomaterials, metal oxides, conducting polymers and their nanocomposites along with some novel materials like metal-organic frameworks, MXenes, metal nitrides, covalent organic frameworks and black phosphorus. The performance of nanocomposites has been analysed by parameters like energy, capacitance, power, cyclic performance and rate capability. Some of the latest supercapacitors such as electrochromic supercapacitor, battery-supercapacitor hybrid device, electrochemical flow capacitor, alternating current line filtering capacitor, micro-supercapacitor, photo-supercapacitor, thermally chargeable supercapacitor, self-healing supercapacitor, piezoelectric and shape memory supercapacitor have also been discussed. This review covers the up-to-date progress achieved in novel materials for supercapacitor electrodes. The latest fabricated symmetric/asymmetric supercapacitors have also been reported.

Review of supercapacitors: Materials and devices

A facile and efficient electrochemical oxidation method to directly activated carbon cloth as an excellent electrode material for supercapacitors is reported. Flexible asymmetric supercapacitor devices based on activated carbon cloth anodes reach a remarkable energy density and excellent long-term durability.

A Novel Exfoliation Strategy to Significantly Boost the Energy Storage Capability of Commercial Carbon Cloth

Energy storage devices are the key components for successful and sustainable energy systems. Some of the best types of energy storage devices right now include lithium-ion batteries and supercapacitors. Research in this area has greatly improved electrode materials, enhanced electrolytes, and conceived clever designs for device assemblies with the ever-increasing energy and power density for electronics. Electrode materials are the fundamental key components for energy storage devices that largely determine the electrochemical performance of energy storage devices. Various materials such as carbon materials, metal oxides and conducting polymers have been widely used as electrode materials for energy storage devices, and great achievements have been made. Recently, metal nitrides have attracted increasing interest as remarkable electrode materials for lithium-ion batteries and supercapacitors due to their outstanding electrochemical properties, high chemical stability, standard technological approach and extensive fundamental importance. This review analyzes the development and progress of metal nitrides as suitable electrode materials for lithium-ion batteries and supercapacitors. The challenges and prospects of metal nitrides as energy storage electrode materials are also discussed.

Recent advances in metal nitrides as high-performance electrode materials for energy storage devices

Three-dimensional (3D) electrodes have been demonstrated to be promising candidates for high-performance supercapacitors because of their unique architectures and outstanding electrochemical properties. However, the fabrication process for current 3D electrodes is not scalable. Herein, a novel and cost-effective activation process has been developed to macroscopically produce 3D porous Ni@NiO core-shell electrodes with enhanced electrochemical properties. The porous Ni@NiO core-shell electrode obtained by activated commercial Ni foam (NF) in a 3 M HCl solution yields an ultrahigh areal capacitance of 2.0 F cm−2 at a high current density of 8 mA cm−2, which is substantially higher than that of most reported 3D NF-based electrodes. Moreover, the activated NF (ANF) electrode exhibited super-long cycling stability. Owing to the increased accessible surface area and continual formation of electrochemically active NiO during cycling, the areal capacitance of the ANF electrode did not exhibit any decay and instead increased from 0.47 to 1.27 F cm−2 after 100 000 cycles at 100 mV s−1. This is the best cycling stability achieved by a 3D NF-based electrode. Additionally, a high-performance asymmetrical supercapacitor (ASC) device based on the as-prepared ANF cathode and a reduced graphene oxide (RGO) anode was also prepared. The ANF//RGO-ASC device was able to deliver a maximum energy density of 1.06 mWh cm−3 and a maximum power density of 0.42 W cm−3. Researchers from China have discovered a cost-effective way to produce supercapacitors on large scales using nickel foam. This three-dimensional porous metal is an ideal electrode for high-capacity energy storage because of its lightweight, corrosion-resistant structure. To achieve supercapacitance, however, researchers must insert active substances, such as graphene, deep into the nickel pores. Xihong Lu and colleagues from Sun Yat-Sen University solved this problem by immersing commercial-grade nickel foam into hot hydrochloric acid for several minutes. The one-step reaction pitted the formerly smooth nickel foam surface and created a thin outer ‘shell’ of nickel oxide that surrounded an inner nickel ‘core’. Electrochemical experiments revealed that the favorable core–shell structure, combined with a more accessible surface area achieved from the acid etching, yielded an energy-dense supercapacitor electrode that was effective for more than 100,000 charge–recharge cycles. A novel and cost-effective activation process has been developed to macroscopically produce three-dimensional (3D) porous Ni@NiO core-shell electrode by activated Ni foam (ANF) in HCl aqueous solution. The ANF electrode yielded a remarkable areal capacitance of 2.0 F cm−2 at a high current density of 8 mA cm−2 and exhibited ultrahigh long-term cycling stability without any decay of capacitance after 100 000 cycles.

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Scalable self-growth of Ni@NiO core-shell electrode with ultrahigh capacitance and super-long cyclic stability for supercapacitors

Due to their unique architectures and outstanding electrical properties, three dimensional graphene-based frameworks (3DGFs) have attracted extensive attention in wide fields. However, recently reported techniques always require complex processes and high cost, which severely limit their large-scale application. In this study, the massive preparation of macroscopically porous 3DGFs from the inherently inexpensive graphite paper is for the first time realized by simply combining the modified Hummer's method with freezing technique. The as-prepared 3DGFs that consist of well exfoliated, high-quality reduced graphene oxide (RGO) exhibit a mesoporous structure and superior conductivity. Such unique features enable the 3DGFs to be directly used as a supercapacitor electrode and as ideal 3D scaffolds to create PANI@3DGFs, Pd@3DGFs, and Pt@3DGFs composites, which hold great potential applications in supercapacitors and catalysts.

Building Three-Dimensional Graphene Frameworks for Energy Storage and Catalysis

In this work, we report the facile synthesis of Fe3O4/reduced graphene oxide (RGO) nanocomposites and their improved lithium storage capability. Fe3O4/RGO composites synthesized by a facile co-precipitation method exhibited outstanding electrochemical performance with good cycling stability. As an anode material for lithium ion batteries (LIBs), the Fe3O4/RGO composites achieved a high reversible capacity of 1637 mA h g−1 (0.1 A g−1) at the 10th cycle, which still remained at 1397 mA h g−1 after 100 cycles. Moreover, the Fe3O4/RGO composites have excellent rate capability. Characterization results reveal that such a large reversible capacity is attributed to the synergistic effect between Fe3O4 and RGO, with the Fe3O4 nanoparticles (NPs) with surface step atoms offering abundant electrochemical active sites for lithium storage. In addition, RGO acts as a volume buffer and electron conductor, and more importantly preserves the electrochemically active surface and avoids the aggregation of the Fe3O4 NPs.

Wang Wang

Papers

A Novel Exfoliation Strategy to Significantly Boost the Energy Storage Capability of Commercial Carbon Cloth

Recent advances in metal nitrides as high-performance electrode materials for energy storage devices

Scalable self-growth of Ni@NiO core-shell electrode with ultrahigh capacitance and super-long cyclic stability for supercapacitors

Building Three-Dimensional Graphene Frameworks for Energy Storage and Catalysis

Fe3O4/reduced graphene oxide with enhanced electrochemical performance towards lithium storage