2D MXene-based nanomaterials have attracted tremendous attention because of their unique physical/chemical properties and wide range of applications in energy storage, catalysis, electronics, optoelectronics, and photonics. However, MXenes and their derivatives have many inherent limitations in terms of energy storage applications. In order to further improve their performance for practical application, the nanoengineering of these 2D materials is extensively investigated. In this Review, the latest research and progress on 2D MXene-based nanostructures is introduced and discussed, focusing on their preparation methods, properties, and applications for energy storage such as lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. Finally, the critical challenges and perspectives required to be addressed for the future development of these 2D MXene-based materials for energy storage applications are presented.

Nanoengineering of 2D MXene-Based Materials for Energy Storage Applications

The development of pseudocapacitive materials for energy-oriented applications has stimulated considerable interest in recent years due to their high energy-storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery-like to the pseudocapacitive-like behavior. As a result, it becomes challenging to distinguish "pseudocapacitive" and "battery" materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish battery-type and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid-state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state-of-the-art progress in the engineering of active materials is summarized, which will guide for the development of real-pseudocapacitive energy storage systems.

True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

Facial design and synthesis of CoSx/Ni-Co LDH nanocages with rhombic dodecahedral structure for high-performance asymmetric supercapacitors

Supercapacitors (SCs) have attracted much attention as energy storage devices due to their high power density, fast charge/discharge capability, and long cycling life. The core/shell structure design of the electrocapacitive material is one of the effective ways to achieve large surface area and high conductivity for providing more faradaic reaction sites and accelerating the charge transfer, respectively, and therefore to enhance the electrocapacitive performance of SCs. To better understand the core/shell structure, this review paper compares the material category, morphology, and synthesis methods for the core/shell structures as well as their electrochemical performances for the corresponding SCs. The electroactive materials applied in the core/shell structure include carbon materials, conducting polymers, metals, metal hydroxides, metal oxides and metal sulfides, while zero-dimensional, one-dimensional, two-dimensional, and three-dimensional structures are considered for the core/shell material. This review article outlines the most commonly used methods for making the core and shell materials over the past decade (2007–2018), and points out the most efficient combination of the material categories and morphologies for the core/shell structure. By understanding the details of the core/shell materials, more efficient design regarding the choices of material category and morphology can be achieved, and therefore better electrocapacitive performance for the resulting SCs can be realized.

A review of electrode materials based on core–shell nanostructures for electrochemical supercapacitors

Water electrolysis utilizing renewable electricity power is a promising technology to produce green hydrogen energy on a large scale. However, this energy conversion technology is seriously hindered by the high activation barrier of the oxygen evolution reaction (OER), which requires highly active and robust electrocatalysts. Herein, we have developed a novel three-dimensional (3D) core–shell OER electrocatalyst, in which defective and ultrathin NiFe layered double hydroxide (NiFe LDH) nanosheets are rationally decorated on V-doped Ni3S2 nanorod arrays supported on Ni foam (V-Ni3S2@NiFe LDH). The highly conductive V-doped Ni3S2 nanorod cores ensure rapid charge transfer, and the ultrathin NiFe LDH nanosheets with rich defects offer numerous exposed active sites, together with the unique 3D core–shell nanostructures that benefit electrolyte diffusion and gas products releasing, thus our hierarchical catalyst is distinguished by very low overpotentials of 209 and 286 mV to obtain current densities of 10 and 100 mA cm−2, respectively, for OER in 1 M KOH. Impressively, when this 3D core–shell catalyst is paired with V-doped Ni3S2 nanorod arrays for overall water splitting, an outstanding two-electrode electrolyzer is achieved, which only requires 1.55 V to deliver a current density of 10 mA cm−2 in 1 M KOH, even superior to the benchmark of RuO2(+)//Pt(−). Our work provides a novel and effective strategy to rationally design efficient 3D hierarchical catalysts for energy conversion and storage.

Defective and ultrathin NiFe LDH nanosheets decorated on V-doped Ni3S2 nanorod arrays: a 3D core–shell electrocatalyst for efficient water oxidation

Core-shell structured Ni3S2@Co(OH)2 nano-wires grown on Ni foam as binder-free electrode for asymmetric supercapacitors

Rational design and controllable synthesis of nanostructured materials with unique microstructure and excellent electrochemical performance for energy storage are crucially desired. In this paper, a facile method is reported for general synthesis of hierarchically core-shell structured Ni3 S2 @NiMoO4 nanowires (NWs) as a binder-free electrode for asymmetric supercapacitors. Due to the intimate contact between Ni3 S2 and NiMoO4 , the hierarchical structured electrodes provide a promising unique structure for asymmetric supercapacitors. The as-prepared binder-free Ni3 S2 @NiMoO4 electrode can significantly improve the electrical conductivity between Ni3 S2 and NiMoO4 , and effectively avoid the aggregation of NiMoO4 nanosheets, which provide more active space for storing charge. The Ni3 S2 @NiMoO4 electrode presents a high areal capacity of 1327.3 µAh cm-2 and 67.8% retention of its initial capacity when current density increases from 2 to 40 mA cm-2 . In a two-electrode Ni3 S2 @NiMoO4 //active carbon cell, the active materials deliver a high energy density of 121.5 Wh kg-1 at a power density of 2.285 kW kg-1 with excellent cycling stability.

Rational Design of Hierarchically Core-Shell Structured Ni 3 S 2 @NiMoO 4 Nanowires for Electrochemical Energy Storage

High-performance all-solid-state asymmetric supercapacitors based on sponge-like NiS/Ni3S2 hybrid nanosheets

In this study, a simple and one-pot synthesis method is developed to prepare a three-dimensional petal-like Ni3S2/CoNi2S4 hybrid material which can be used as a binder-free electrode in electrochemical energy storage. This material possesses a high areal capacity, such as 758 μA h cm−2 at a current density of 2 mA cm−2. With a 20-fold current density increase, the as-prepared material retains 70% of its original capacity. Furthermore, an asymmetric supercapacitor is assembled by using the Ni3S2/CoNi2S4 hybrid as the cathode and active carbon as the anode in a coin cell. This cell exhibits an outstanding areal power density of 31.212 mW cm−2 at an areal energy density of 0.288 mW h cm−2 and still retains a quite high areal power density of 3.876 mW cm−2 at an areal energy density of 0.364 mW h cm−2. These results show that the 3D petal-like Ni3S2/CoNi2S4 hybrid is a promising cathode material for practical application in energy storage.

Fangshuai Chen

Papers

Core-shell structured Ni3S2@Co(OH)2 nano-wires grown on Ni foam as binder-free electrode for asymmetric supercapacitors

Rational Design of Hierarchically Core-Shell Structured Ni 3 S 2 @NiMoO 4 Nanowires for Electrochemical Energy Storage

High-performance all-solid-state asymmetric supercapacitors based on sponge-like NiS/Ni3S2 hybrid nanosheets

A 3D petal-like Ni3S2/CoNi2S4 hybrid grown on Ni foam as a binder-free electrode for energy storage

Hierarchical core-shell structured CoNi2S4/Ni3S2@Ni(OH)2 nanosheet arrays as electrode for electrochemical energy storage