Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.

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Graphene-MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics

The development of sodium-ion batteries for large-scale applications requires the synthesis of electrode materials with high capacity, high initial Coulombic efficiency (ICE), high rate performance, long cycle life, and low cost. A rational design of freestanding anode materials is reported for sodium-ion batteries, consisting of molybdenum disulfide (MoS2) nanosheets aligned vertically on carbon paper derived from paper towel. The hierarchical structure enables sufficient electrode/electrolyte interaction and fast electron transportation. Meanwhile, the unique architecture can minimize the excessive interface between carbon and electrolyte, enabling high ICE. The as-prepared MoS2@carbon paper composites as freestanding electrodes for sodium-ion batteries can liberate the traditional electrode manufacturing procedure, thereby reducing the cost of sodium-ion batteries. The freestanding MoS2@carbon paper electrode exhibits a high reversible capacity, high ICE, good cycling performance, and excellent rate capability. By exploiting in situ Raman spectroscopy, the reversibility of the phase transition from 2H-MoS2 to 1T-MoS2 is observed during the sodium-ion intercalation/deintercalation process. This work is expected to inspire the development of advanced electrode materials for high-performance sodium-ion batteries.

MoS2 Nanosheets Vertically Aligned on Carbon Paper: A Freestanding Electrode for Highly Reversible Sodium Ion Batteries

In this study, we propose a versatile method for synthesizing uniform three-dimensional (3D) metal carbides, nitrides, and carbonitrides (MXenes)/metal–organic frameworks (MOFs) composites (Ti3C2TX/Cu-BTC, Ti3C2TX/Fe,Co-PBA, Ti3C2TX/ZIF-8, and Ti3C2TX/ZIF-67) that combine the advantages of MOFs and MXenes to enhance stability and improve conductivity. Subsequently, 3D hollow Ti3C2TX/ZIF-67/CoV2O6 composites with excellent electron- and ion-transport properties derived from Ti3C2TX/ZIF-67 were synthesized. The specific capacitance of the Ti3C2TX/ZIF-67/CoV2O6 electrode was 285.5 F g−1, which is much higher than that of the ZIF-67 and Ti3C2TX/ZIF-67 electrode. This study opens a new avenue for the design and synthesis of MXene/MOF composites and complex hollow structures with tailorable structures and compositions for various applications. 

In Situ Growth of Three‐Dimensional MXene/Metal‐Organic Framework Composites for High‐Performance Supercapacitors

In this study, we propose a versatile method for synthesizing uniform three-dimensional (3D) metal carbides, nitrides, and carbonitrides (MXenes)/metal-organic frameworks (MOFs) composites (Ti3 C2 TX /Cu-BTC, Ti3 C2 TX /Fe,Co-PBA, Ti3 C2 TX /ZIF-8, and Ti3 C2 TX /ZIF-67) that combine the advantages of MOFs and MXenes to enhance stability and improve conductivity. Subsequently, 3D hollow Ti3 C2 TX /ZIF-67/CoV2 O6 composites with excellent electron- and ion-transport properties derived from Ti3 C2 TX /ZIF-67 were synthesized. The specific capacitance of the Ti3 C2 TX /ZIF-67/CoV2 O6 electrode was 285.5 F g-1 , which is much higher than that of the ZIF-67 and Ti3 C2 TX /ZIF-67 electrode. This study opens a new avenue for the design and synthesis of MXene/MOF composites and complex hollow structures with tailorable structures and compositions for various applications. 

In Situ Growth of Three‐Dimensional MXene/Metal–Organic Framework Composites for High‐Performance Supercapacitors

Limited by single metal active sites and low electrical conductivity, designing nickel‐based metal–organic framework (MOF) materials with high capacity and high energy density remains a challenge. Herein, a series of bi/multimetallic MOF‐74 family materials in situ grown on carbon cloth (CC) by doping Mx+ ions in Ni‐MOF‐74 is fabricated: NiM‐MOF@CC (M = Mn2+, Co2+, Cu2+, Zn2+, Al3+, Fe3+), and NiCoM‐MOF@CC (M = Mn2+, Zn2+, Al3+, Fe3+). The type and ratio of doping metal ions can be adjusted while the original topology is preserved. Different metal ions are confirmed by X‐ray absorption fine structure (XAFS). Furthermore, these Ni‐based MOF electrodes are directly utilized as cathodes for aqueous nickel–zinc batteries (NZBs). Among all the as‐prepared electrodes, NiCo‐MOF@CC‐3 (NCM@CC‐3), with an optimized Co/Ni ratio of 1:1, exhibits the best electrical conductivity, which is according to the density functional theory (DFT) theoretical calculations. The NCM@CC‐3//Zn@CC battery achieves a high specific capacity of 1.77 mAh cm–2, a high areal energy density of 2.97 mWh cm–2, and high cycling stability of 83% capacity retention rate after 6000 cycles. The synthetic strategy based on the coordination effect of metal ions and the concept of binder‐free electrodes provide a new direction for the synthesis of high‐performance materials in the energy‐storage field.

In Situ Synthesis of MOF‐74 Family for High Areal Energy Density of Aqueous Nickel–Zinc Batteries

MXenes have attracted great interests as supercapacitors due to their metallic conductivity, high density, and hydrophilic nature. Herein we report Ti3 C2 -Cu/Co hybrids via molten salt etching in which the existence of metal atoms and their interactions with MXene via surficial O atoms were elucidated by XAFS for the first time. The electrochemical investigation of Ti3 C2 -Cu electrode demonstrated the pseudocapacitive contribution of Cu and a splendid specific capacitance of 885.0 F g-1 at 0.5 A g-1 in 1.0 M H2 SO4 . Symmetric supercapacitor Ti3 C2 -Cu//Ti3 C2 -Cu was demonstrated with operating voltage of 1.6 V, areal capacitance of 290.5 mF cm-2 at 1 mA cm-2 , and stability over 10 000 cycles. It delivered an areal energy density of 103.3 μWh cm-2 at power density of 0.8 mW cm-2 , based on which a supercapacitor pouch was fabricated. It provides deeper insights into the molten salt mechanism and strategies for designing MXene-based materials for electrochemical energy storage.

MXene-Copper/Cobalt Hybrids via Lewis Acidic Molten Salts Etching for High Performance Symmetric Supercapacitors.

MoS2/graphene composites: Fabrication and electrochemical energy storage

Metal-Organic Frameworks Nanocomposites with Different Dimensionalities for Energy Conversion and Storage

Two-dimensional (2D) materials with structures having diverse features are promising for application in energy conversion and storage. A stronger layered orientation can guarantee fast charge transfer along 2D planes and contributes to enhancing the pseudocapacitive properties of the material. Here, we demonstrate a novel one-pot co-precipitation approach to synthesize a hybrid structure comprising a layered transition metal hydroxide (LTMH)/zeolitic imidazolate framework-67 (ZIF-67) (LTMH/ZIF) of ultrathin porous sheet-like nanostructures. The LTMH/ZIF hybrid structures were synthesized with Co2+ as the main metal source and Mn2+/Ni2+/Zn2+ as dopants. The ultrathin 2D topography can shorten the path of electron transfer, reduce the diffusion resistance of the electrolyte, and provide more active sites. Among the bimetallic LTMH/ZIF hybrids, α-CoMn0.05(OH)x/ZIF-67 displayed a specific capacitance of 689 F g−1 at 0.5 A g−1 in a three-electrode configuration. The as-assembled asymmetric supercapacitor (ASC) (α-CoMn0.05(OH)x/ZIF-67//activated carbon (AC)) delivered an outstanding energy density of 79.1 W h kg−1 at a power density of 1350 W kg−1 and reached a high power density of 13 500 W kg−1 with the energy density maintained at 70.5 W h kg−1. The ASC also retained 96.2% of its initial capacity over 3500 cycles. Overall, the proposed strategy enables the preparation of 2D ultrathin LTMH/ZIF hybrid nanostructures, which have great prospects for diverse applications in energy storage.

Controllable synthesis of ultrathin layered transition metal hydroxide/zeolitic imidazolate framework-67 hybrid nanosheets for high-performance supercapacitors

Chunli Liu

Papers

MXene-Copper/Cobalt Hybrids via Lewis Acidic Molten Salts Etching for High Performance Symmetric Supercapacitors.

MoS2/graphene composites: Fabrication and electrochemical energy storage

Metal-Organic Frameworks Nanocomposites with Different Dimensionalities for Energy Conversion and Storage

Controllable synthesis of ultrathin layered transition metal hydroxide/zeolitic imidazolate framework-67 hybrid nanosheets for high-performance supercapacitors

MXenes nanocomposites for energy storage and conversion