Owing to the outstanding advantages as electrical energy storage system, supercapacitors have attracted tremendous research interests over the past decade. Current research efforts are being devoted to improve the energy storage capabilities of supercapacitors through either discovering novel electroactive materials or nanostructuring existing electroactive materials. From the device point of view, the energy storage performance of supercapacitor not only depends on the electroactive materials themselves, but importantly, relies on the structure of electrode whether it allows the electroactive materials to reach their full potentials for energy storage. With respect to utilizing nanostructured electroactive materials, the key issue is to retain all advantages of the nanoscale features for supercapacitors when being assembled into electrodes and the following devices. Rational design and fabrication of self-supported nanoelectrodes is therefore considered as the most promising strategy to address this challenge. In this review, we summarize the recent advances in designing and fabricating self-supported nanoelectrodes for supercapacitors towards high energy storage capability. Self-supported homogeneous and heterogeneous nanoelectrodes in the forms of one-dimensional (1D) nanoarrays, two-dimensional (2D) nanoarrays, and three-dimensional (3D) nanoporous architectures are introduced with their representative results presented. The challenges and perspectives in this field are also discussed.

Recent Advances in Designing and Fabricating Self-Supported Nanoelectrodes for Supercapacitors.

Free-standing single crystalline semiconductor membranes have gained intensive attention over the last few years due to their versatile usage in many applications. This material platform possesses a high level of material quality similar to their bulk counterparts because single crystallinity is maintained. Si, Ge, and III–V based membranes have been widely studied for flexible electronic and optoelectronic devices such as thin-film transistors and photodetectors. However, the current status of research and development on free-standing single crystalline wide band-gap membranes is at a relatively early stage compared to IV and III–V based membranes. This review highlights recent advances in free-standing wide band-gap membranes, including GaN, SiC, ZnO, β-Ga2O3, and diamond and their applications. Fabrication techniques of each membrane are presented with material characterization. Some prospects for new research opportunities and challenges are also discussed.

Recent advances in free-standing single crystalline wide band-gap semiconductors and their applications: GaN, SiC, ZnO, β-Ga2O3, and diamond

Facile synthesis of 2D TiO2@MXene composite membrane with enhanced separation and antifouling performance

Single-crystalline GaN-based light-emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid-state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high-quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high-brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal-organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and Inx Ga1-x N/GaN multiple quantum well structures. The as-fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one-step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high-brightness LEDs.

High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer.

Supercapacitors, also known as electrochemical capacitors, have attracted more and more attention in recent decades due to their advantages of higher power density and long cycle life. For the real application of supercapacitors, there is no doubt that cyclic stability is the most important aspect. As the core component, electrode materials determine the cyclic stability of supercapacitors, although the other components like electrolyte and separator are also important. In this review, we sum up the cyclic stability of supercapacitors according to type of electrode material and its energy storage mechanism, discuss the strategies to boost the stability of those electrode materials, and indicate several key significant considerations in measurement of cyclic stability. The purpose is to obtain safe, long-lasting, and high-performance supercapacitor devices.

Cyclic stability of supercapacitors: materials, energy storage mechanism, test methods, and device

Novel Designed MnS-MoS2 Heterostructure for Fast and Stable Li/Na Storage:Insights into the Advanced Mechanism Attributed to Phase Engineering

The progress in nitrides technology is widely believed to be limited and hampered by the lack of high-quality gallium nitride wafers. Though various epitaxial techniques like epitaxial lateral overgrowth and its derivatives have been used to reduce defect density, there is still plenty of room for the improvement of gallium nitride crystal. Here, we report graphene or hexagonal boron nitride nanosheets can be used to improve the quality of GaN crystal using hydride vapor phase epitaxy methods. These nanosheets were directly deposited on the substrate that is used for the epitaxial growth of GaN crystal. Systematic characterizations of the as-obtained crystal show that quality of GaN crystal is greatly improved. The fabricated light-emitting diodes using the as-obtained GaN crystals emit strong electroluminescence under room illumination. This simple yet effective technique is believed to be applicable in metal–organic chemical vapor deposition systems and will find wide applications on other crystal growth.

Improving the quality of GaN crystals by using graphene or hexagonal boron nitride nanosheets substrate.

Single-crystal gallium nitride (GaN) membranes have great potential for a variety of applications. However, fabrication of single-crystalline GaN membranes remains a challenge owing to its chemical inertness and mechanical hardness. This study prepares large-area, free-standing, and single-crystalline porous GaN membranes using a one-step high-temperature annealing technique for the first time. A promising separation model is proposed through a comprehensive study that combines thermodynamic theories analysis and experiments. Porous GaN crystal membrane is processed into supercapacitors, which exhibit stable cycling life, high-rate capability, and ultrahigh power density, to complete proof-of-concept demonstration of new energy storage application. Our results contribute to the study of GaN crystal membranes into a new stage related to the elelctrochemical energy storage application.

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Baoguo Zhang

Papers

Novel Designed MnS-MoS2 Heterostructure for Fast and Stable Li/Na Storage:Insights into the Advanced Mechanism Attributed to Phase Engineering

Improving the quality of GaN crystals by using graphene or hexagonal boron nitride nanosheets substrate.

One-step fabrication of porous GaN crystal membrane and its application in energy storage.

Hollow submicrospheres of trimetallic selenides for high-capacity lithium and sodium ion batteries

Type II cuprous oxide/graphitic carbon nitride p-n heterojunctions for enhanced photocatalytic nitrogen fixation