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.

Two dimensional layered inorganic nanomaterials (2D-LINs) have recently attracted huge interest because of their unique thickness dependent physical and chemical properties and potential technological applications. The properties of these layered materials can be tuned via both physical and chemical processes. Some 2D layered inorganic nanomaterials like MoS2, WS2 and SnS2 have been recently developed and employed in various applications, including new sensors because of their layer-dependent electrical properties. This article presents a comprehensive overview of recent developments in the application of 2D layered inorganic nanomaterials as sensors. Some of the salient features of 2D materials for different sensing applications are discussed, including gas sensing, electrochemical sensing, SERS and biosensing, SERS sensing and photodetection. The working principles of the sensors are also discussed together with examples.

Recent developments in 2D layered inorganic nanomaterials for sensing

In this study, a general and effective phosphorization strategy is successfully demonstrated to enhance supercapacitor performance of various transition metals oxide or hydroxide, such as Ni(OH)2, Co(OH)2, MnO2, and Fe2O3. For example, a 3D networked Ni2P nanosheets array via a facile phosphorization reaction of Ni(OH)2 nanosheets is grown on the surface of a Ni foam. The Ni foam-supported Ni2P nanosheet (Ni2P NS/NF) electrode shows a remarkable specific capacitance of 2141 F g−1 at a scan rate of 50 mV s−1 and remains as high as 1109 F g−1 even at the current density of 83.3 A g−1. The specific capacitance is much larger than those of Ni(OH)2 NS/NF (747 F g−1 at 50 mV s−1). Furthermore, the electrode retains a high specific capacitance of 1437 F g−1 even after 5000 cycles at a current density of 10 A g−1, in sharp contrast with only 403 F g−1 of Ni(OH)2 NS/NF at the same current density. The similar enhanced performance is observed for Ni2P powder, which eliminates the influence of nickel foam. The enhanced supercapacitor performances are attributed to the 3D porous nanosheets network, enhanced conductivity, and two active components of Ni2+ and Pδ− with rich valences of Ni2P.

Ultrahigh‐Performance Pseudocapacitor Electrodes Based on Transition Metal Phosphide Nanosheets Array via Phosphorization: A General and Effective Approach

Flexible electrochemical energy storage devices have attracted extensive attention as promising power sources for the ever-growing field of flexible and wearable electronic products. However, the rational design of a novel electrode structure with a good flexibility, high capacity, fast charge–discharge rate and long cycling lifetimes remains a long-standing challenge for developing next-generation flexible energy-storage materials. Herein, we develop a facile and general approach to three-dimensional (3D) interconnected porous nitrogen-doped graphene foam with encapsulated Ge quantum dot/nitrogen-doped graphene yolk-shell nano architecture for high specific reversible capacity (1,220 mAh g−1), long cycling capability (over 96% reversible capacity retention from the second to 1,000 cycles) and ultra-high rate performance (over 800 mAh g−1 at 40 C). This work paves a way to develop the 3D interconnected graphene-based high-capacity electrode material systems, particularly those that suffer from huge volume expansion, for the future development of high-performance flexible energy storage systems. The development of materials for energy storage hinges on the design of electrodes with large capacity, flexibility, fast charge–discharge rate and long cycling lifetime. Here, the authors develop electrodes based on nitrogen doped graphene with encapsulated Ge quantum dots with yolk-shell architecture.

3D nitrogen-doped graphene foam with encapsulated germanium/nitrogen-doped graphene yolk-shell nanoarchitecture for high-performance flexible Li-ion battery

The graphitic carbon nitride (g-C3N4) which is a two-dimensional conjugated polymer has drawn broad interdisciplinary attention as a low-cost, metal-free, and visible-light-responsive photocatalyst in the area of environmental remediation. The g-C3N4-based materials have excellent electronic band structures, electron-rich properties, basic surface functionalities, high physicochemical stabilities and are “earth-abundant.” This review summarizes the latest progress related to the design and construction of g-C3N4-based materials and their applications including catalysis, sensing, imaging, and white-light-emitting diodes. An outlook on possible further developments in g-C3N4-based research for emerging properties and applications is also included.

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Recent Advances of Graphitic Carbon Nitride-Based Structures and Applications in Catalyst, Sensing, Imaging, and LEDs

NiO nanoflakes are created with a simple hydrothermal method on 3D (three-dimensional) graphene scaffolds grown on Ni foams by microwave plasma enhanced chemical vapor deposition (MPCVD). Such as-grown NiO-3D graphene hierarchical composites are then applied as monolithic electrodes for a pseudo-supercapacitor application without needing binders or metal-based current collectors. Electrochemical measurements impart that the hierarchical NiO-3D graphene composite delivers a high specific capacitance of ≈1829 F g−1 at a current density of 3 A g−1 (the theoretical capacitance of NiO is 2584 F g−1). Furthermore, a full-cell is realized with an energy density of 138 Wh kg−1 at a power density of 5.25 kW kg−1, which is much superior to commercial ones as well as reported devices in asymmetric capacitors of NiO. More attractively, this asymmetric supercapacitor exhibits capacitance retention of 85% after 5000 cycles relative to the initial value of the 1st cycle.

Hierarchical Composite Electrodes of Nickel Oxide Nanoflake 3D Graphene for High-Performance Pseudocapacitors

Anodic nanoporous SnO2 grown on Cu foils as superior binder-free Na-ion battery anodes

A facile electrochemical method to selectively remove the outer walls of anodic TiO(2) nanotubes by leaving the as-anodized nanotubes in the same electrolyte and applying an electric field parallel to the anodic film for several minutes is reported. The better-separated single-walled TiO(2) nanotubes thus obtained show significantly improved photocatalytic efficiency compared with their non-etched counterparts.

Selective removal of the outer shells of anodic TiO2 nanotubes.

Electrochemical dealloying using pulsed voltage waveforms and its application for supercapacitor electrodes

We report on Ni-modified TiO2 nanotubes with improved photocatalytic properties. Using the as-anodized TiO2 nanotubes as templates, Ni was electrodeposited using pulsed current waveforms. It was found that the Ni deposition was first initiated at the bottoms of the intertubular voids and then grew upward, resulting in a Ni/TiO2 coaxial nanostructure with Ni wrapping around the TiO2 nanotubes. Moreover, the tube inside was kept empty and tube openings unclogged for the fabricated Ni/TiO2 nanocomposites. Further photodegradation tests using methyl red revealed that the fabricated Ni/TiO2 nanocomposites possess higher photocatalytic efficiency than the counterparts of pristine TiO2 nanotubes. The observed improved photocatalytic efficiency is ascribed to the Schottky barriers formed between Ni and TiO2.

Jie Zhang

Papers

Hierarchical Composite Electrodes of Nickel Oxide Nanoflake 3D Graphene for High-Performance Pseudocapacitors

Anodic nanoporous SnO2 grown on Cu foils as superior binder-free Na-ion battery anodes

Selective removal of the outer shells of anodic TiO2 nanotubes.

Electrochemical dealloying using pulsed voltage waveforms and its application for supercapacitor electrodes

Selective electrodeposition of Ni into the intertubular voids of anodic TiO2 nanotubes for improved photocatalytic properties