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

A recent approach for solar-to-hydrogen generation has been water electrolysis using efficient, stable, and inexpensive bifunctional electrocatalysts within strong electrolytes. Herein, the direct growth of 1D NiCo2S4 nanowire (NW) arrays on a 3D Ni foam (NF) is described. This NiCo2S4 NW/NF array functions as an efficient bifunctional electrocatalyst for overall water splitting with excellent activity and stability. The 3D-Ni foam facilitates the directional growth, exposing more active sites of the catalyst for electrochemical reactions at the electrode–electrolyte interface. The binder-free, self-made NiCo2S4 NW/NF electrode delivers a hydrogen production current density of 10 mA cm–2 at an overpotential of 260 mV for the oxygen evolution reaction and at 210 mV (versus a reversible hydrogen electrode) for the hydrogen evolution reaction in 1 m KOH. This highly active and stable bifunctional electrocatalyst enables the preparation of an alkaline water electrolyzer that could deliver 10 mA cm–2 under a cell voltage of 1.63 V. Because the nonprecious-metal NiCo2S4 NW/NF foam-based electrodes afford the vigorous and continuous evolution of both H2 and O2 at 1.68 V, generated using a solar panel, they appear to be promising water splitting devices for large-scale solar-to-hydrogen generation.

Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions

While the synthesis of hollow structures of transition metal oxides is well established, it is extremely challenging to fabricate complex hollow structures for mixed transition metal sulfides. Here we report an anion exchange method to synthesize a complex ternary metal sulfides hollow structure, namely nickel cobalt sulfide ball-in-ball hollow spheres. Uniform nickel cobalt glycerate solid spheres are first synthesized as the precursor and subsequently chemically transformed into nickel cobalt sulfide ball-in-ball hollow spheres. When used as electrode materials for electrochemical capacitors, these nickel cobalt sulfide hollow spheres deliver a specific capacitance of 1,036 F g(-1) at a current density of 1.0 A g(-1). An asymmetric supercapacitor based on these ball-in-ball structures shows long-term cycling performance with a high energy density of 42.3 Wh kg(-1) at a power density of 476 W kg(-1), suggesting their potential application in high-performance electrochemical capacitors.

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Formation of nickel cobalt sulfide ball-in-ball hollow spheres with enhanced electrochemical pseudocapacitive properties

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

As new members of carbon material family, carbon and graphene quantum dots (CDs, GQDs) have attracted tremendous attentions for their potentials for biological, optoelectronic, and energy related applications. Among these applications, bio-imaging has been intensively studied, but optoelectronic and energy devices are rapidly rising. In this Feature Article, recent exciting progresses on CD- and GQD-based optoelectronic and energy devices, such as light emitting diodes (LEDs), solar cells (SCs), photodetctors (PDs), photocatalysis, batteries, and supercapacitors are highlighted. The recent understanding on their microstructure and optical properties are briefly introduced in the first part. Some important progresses on optoelectronic and energy devices are then addressed as the main part of this Feature Article. Finally, a brief outlook is given, pointing out that CDs and GQDs could play more important roles in communication- and energy-functional devices in the near future.

Carbon and Graphene Quantum Dots for Optoelectronic and Energy Devices: A Review

A 3D highly conductive urchin-like NiCo2S4 nanostructure has been successfully prepared using a facile precursor transformation method. Remarkably, the NiCo2S4 electroactive material demonstrates superior electrochemical performance with ultrahigh high-rate capacitance, very high specific capacitance, and excellent cycling stability.

Highly conductive NiCo2S4 urchin-like nanostructures for high-rate pseudocapacitors

In situ growth of NiCo2S4 nanotube arrays on Ni foam for supercapacitors: Maximizing utilization efficiency at high mass loading to achieve ultrahigh areal pseudocapacitance

Nickel cobalt sulphides with different stoichiometric nickel and cobalt contents have been synthesized and used as pseudocapacitive materials for supercapacitors. The as-employed polyol method is robust enough to use a one-pot synthesis of nickel cobalt sulphides with similar porous morphology and crystal structure at different nickel and cobalt ratios. The electrochemical performance of the nickel cobalt sulphides can be easily tuned by the varying the Ni and Co content. Owing to the combined contributions from both Ni and Co ions, the bimetallic Ni–Co sulphides show a superior pseudocapacitance compared to monometallic Co and Ni sulphides in terms of high specific capacitance, excellent rate capability and long cycle stability. In particular, the Ni1.5Co1.5S4 sample shows the highest specific capacitance of 1093 F g−1 at 1 A g−1, superior rate capability of 69% capacitance retention after a 50-fold increase in current densities, and longer cycle stability with increased specific capacitance of 108% of capacitance retention after 2000 cycles. In addition, the Ni1.5Co1.5S4 was also used to assemble an asymmetric supercapacitor with reduced graphene oxide and attains excellent capacitive performance with high specific capacitance (113 F g−1 at 1 A g−1), high energy density (37.6 W h kg−1 at 775 W kg−1) and high power density (23.25 kW kg−1 at 17.7 W h kg−1).

One-pot synthesis of porous nickel cobalt sulphides: tuning the composition for superior pseudocapacitance

NiCo2S4 porous nanotubes are synthesised by a sacrificial template method based on the Kirkendall effect. The as-prepared NiCo2S4 nanotube electrode shows a specific capacitance of 1093 F g−1 at a current density of 0.2 A g−1 (933 F g−1 at 1 A g−1).

Haichao Chen

Papers

Highly conductive NiCo2S4 urchin-like nanostructures for high-rate pseudocapacitors

In situ growth of NiCo2S4 nanotube arrays on Ni foam for supercapacitors: Maximizing utilization efficiency at high mass loading to achieve ultrahigh areal pseudocapacitance

One-pot synthesis of porous nickel cobalt sulphides: tuning the composition for superior pseudocapacitance

NiCo2S4 porous nanotubes synthesis via sacrificial templates: high-performance electrode materials of supercapacitors

Facilely synthesized porous NiCo2O4 flowerlike nanostructure for high-rate supercapacitors