2D transition metal carbides and nitrides, named MXenes, are attracting increasing attentions and showing competitive performance in energy storage devices including electrochemical capacitors, ...lithium‐ and sodium‐ion batteries, and lithium–sulfur batteries. However, similar to other 2D materials, MXene nanosheets are inclined to stack together, limiting the device performance. In order to fully utilize MXenes' electrochemical energy storage capability, here, processing of 2D MXene flakes into hollow spheres and 3D architectures via a template method is reported. The MXene hollow spheres are stable and can be easily dispersed in solvents such as water and ethanol, demonstrating their potential applications in environmental and biomedical fields as well. The 3D macroporous MXene films are free‐standing, flexible, and highly conductive due to good contacts between spheres and metallic conductivity of MXenes. When used as anodes for sodium‐ion storage, these 3D MXene films exhibit much improved performances compared to multilayer MXenes and MXene/carbon nanotube hybrid architectures in terms of capacity, rate capability, and cycling stability. This work demonstrates the importance of MXene electrode architecture on the electrochemical performance and can guide future work on designing high‐performance MXene‐based materials for energy storage, catalysis, environmental, and biomedical applications.
Hollow Ti3C2Tx spheres and 3D macroporous MXene films are fabricated using a sacrificial template approach. The 3D MXene films are free‐standing, flexible, and highly conductive. They can serve directly as electrodes for Na‐ion storage and exhibit high capacities accompanied with excellent stabilities and rate performance.
Two‐dimensional (2D) heterostructured materials, combining the collective advantages of individual building blocks and synergistic properties, have spurred great interest as a new paradigm in ...materials science. The family of 2D transition‐metal carbides and nitrides, MXenes, has emerged as an attractive platform to construct functional materials with enhanced performance for diverse applications. Here, we synthesized 2D MoS2‐on‐MXene heterostructures through in situ sulfidation of Mo2TiC2Tx MXene. The computational results show that MoS2‐on‐MXene heterostructures have metallic properties. Moreover, the presence of MXene leads to enhanced Li and Li2S adsorption during the intercalation and conversion reactions. These characteristics render the as‐prepared MoS2‐on‐MXene heterostructures stable Li‐ion storage performance. This work paves the way to use MXene to construct 2D heterostructures for energy storage applications.
MoS2‐on‐MXene heterostructures were obtained by an in situ sulfidation of Mo2TiC2Tx MXene, which deliver improved Coulombic efficiency and cycling performance for the Li‐ion battery. A computational study shows that the strong Li and Li2S adsorption on 2D heterostructures leads to a stable Li‐ion storage performance.
This review focuses on the recent progresses, mechanisms of action and regulation directions on MXenes-based electrocatalysts in HER including pristine MXenes and MXenes-based composites.
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The growing energy concern all over the world has recognized hydrogen energy as the most promising renewable energy sources. Recently, electrocatalytic hydrogen evolution reaction (HER) by water splitting has been extensively studied with a focus on developing efficient electrocatalysts that can afford HER at overpotential with minimum power consumption. The two-dimensional transition metal carbides and nitride, also known as MXenes, are becoming the rising star in developing efficient electrocatalysts for HER, owing to their integrated chemical and electronic properties, e.g., metallic conductivity, variety of redox-active transition metals, high hydrophilicity, and tunable surface functionalities. In this review, the recent progress about the fundamental understanding and materials engineering of MXenes-based electrocatalysts is summarized in concern with two aspects: i) the regulation of the intrinsic properties of MXenes, which include the composition, surface functionality, and defects; and ii) MXenes-based composites for HER process. In the end, we summarize the present challenges concerning the efficiency of MXenes-based HER electrocatalysts and propose the directions of future research efforts.
Highlights
Room temperature graphene oxide-assisted assembly of 3D Ti
3
C
2
T
x
MXene aerogels have been realized by introducing interfacial mediators (amino-propyltriethoxysilane, Mn
2+
, Fe
2+
, Zn
...2+
, and Co
2+
).
The methodology not only suppresses the oxidation degradation of Ti
3
C
2
T
x
, but also generates porous aerogels with a high Ti
3
C
2
T
x
content (87 wt%) and robustness.
As freestanding electrode of the as-prepared Ti
3
C
2
T
x
-based aerogel with a practical-level mass loading of 12.3 mg cm
-2
still delivers an areal capacity of 1.26 mAh cm
-2
at a current density of 0.1 A g
-1
.
Low-temperature assembly of MXene nanosheets into three-dimensional (3D) robust aerogels addresses the crucial stability concern of the nano-building blocks during the fabrication process, which is of key importance for transforming the fascinating properties at the nanoscale into the macroscopic scale for practical applications. Herein, suitable cross-linking agents (amino-propyltriethoxysilane, Mn
2+
, Fe
2+
, Zn
2+
, and Co
2+
) as interfacial mediators to engineer the interlayer interactions are reported to realize the graphene oxide (GO)-assisted assembly of Ti
3
C
2
T
x
MXene aerogel at room temperature. This elaborate aerogel construction not only suppresses the oxidation degradation of Ti
3
C
2
T
x
but also generates porous aerogels with a high Ti
3
C
2
T
x
content (87 wt%) and robustness, thereby guaranteeing the functional accessibility of Ti
3
C
2
T
x
nanosheets and operational reliability as integrated functional materials. In combination with a further sulfur modification, the Ti
3
C
2
T
x
aerogel electrode shows promising electrochemical performances as the freestanding anode for sodium-ion storage. Even at an ultrahigh loading mass of 12.3 mg cm
−2
, a pronounced areal capacity of 1.26 mAh cm
−2
at a current density of 0.1 A g
−1
has been achieved, which is of practical significance. This work conceptually suggests a new way to exert the utmost surface functionalities of MXenes in 3D monolithic form and can be an inspiring scaffold to promote the application of MXenes in different areas.
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 paper towel derived carbon paper are fabricated as freestanding electrodes for sodium‐ion batteries. Benefiting from the 3D hierarchical structure, the as‐prepared electrodes exhibited high initial Coulombic efficiency, high reversible capacity, high‐rate charging, and a long cycle life. In situ Raman electrospectroscopy is employed to investigate the sodiation/desodiation process.
Ultrathin two‐dimensional (2D) nanostructures have attracted increasing research interest for energy storage and conversion. However, tackling the key problem of lattice mismatch inducing the ...instability of ulreathin nanostructures during phase transformations is still a critical challenge. Herein, we describe a facile and scalable strategy for the growth of ultrathin nickel phosphide (Ni2P) nanosheets (NSs) with exposed (001) facets. We show that single‐layer functionalized graphene with residual oxygen‐containing groups and a large lateral size contributes to reducing the lattice strain during phosphorization. The resulting nanostructure exhibits remarkable hydrogen evolution activity and good stability under alkaline conditions.
Interfacial stress transfer: Ultrathin nickel phosphide (Ni2P) nanosheets with exposed (001) facets were obtained by using single‐layer functionalized graphene with residual oxygen‐containing groups to reduce the lattice strain during phosphorization. The resulting nanostructure exhibits remarkable hydrogen evolution activity and good stability under alkaline conditions.
Sodium‐ion batteries (NIBs) are an emerging technology, which can meet increasing demands for large‐scale energy storage. One of the most promising cathode material candidates for sodium‐ion ...batteries is Na3V2(PO4)3 due to its high capacity, thermal stability, and sodium (Na) Superionic Conductor 3D (NASICON)‐type framework. In this work, the authors have significantly improved electrochemical performance and cycling stability of Na3V2(PO4)3 by introducing a 3D interconnected conductive network in the form of carbon fiber derived from ordinary paper towel. The free‐standing Na3V2(PO4)3‐carbon paper (Na3V2(PO4)3@CP) hybrid electrodes do not require a metallic current collector, polymeric binder, or conducting additives to function as a cathode material in an NIB system. The Na3V2(PO4)3@CP cathode demonstrates extraordinary long term cycling stability for 30 000 deep charge–discharge cycles at a current density of 2.5 mA cm−2. Such outstanding cycling stability can meet the stringent requirements for renewable energy storage.
Na3V2(PO4)3 nanoparticles on 3D interconnected conducting carbon fibers achieve outstanding long term cycling stability without the need of any conducting additives, polymeric binders, or additional current collectors when applied as cathode in sodium‐ion batteries. By using a simple proof‐of‐concept battery assembly, the ease and efficiency of the stand‐alone carbon paper electrode design are demonstrated.
Mesoporous SnO microspheres were synthesised by a hydrothermal method using NaSO4 as the morphology directing agent. Field emission scanning electron microscopy (FESEM), transmission electron ...microscopy (TEM) and high‐resolution transmission electron microscopy (HRTEM) analyses showed that SnO microspheres consist of nanosheets with a thickness of about 20 nm. Each nanosheet contains a mesoporous structure with a pore size of approximately 5 nm. When applied as anode materials in Na‐ion batteries, SnO microspheres exhibited high reversible sodium storage capacity, good cyclability and a satisfactory high rate performance. Through ex situ XRD analysis, it was found that Na+ ions first insert themselves into SnO crystals, and then react with SnO to generate crystalline Sn, followed by Na–Sn alloying with the formation of crystalline NaSn2 phase. During the charge process, there are two slopes corresponding to the de‐alloying of Na–Sn compounds and oxidisation of Sn, respectively. The high sodium storage capacity and good electrochemical performance could be ascribed to the unique hierarchical mesoporous architecture of SnO microspheres.
Let it SnO: Hierarchical mesoporous SnO microspheres were synthesised by a hydrothermal method using NaSO4 as the morphology directing agent. When applied as anode materials for Na‐ion batteries, they exhibited high reversible sodium storage capacity and excellent cyclability. The good electrochemical performance could be ascribed to the unique hierarchical mesoporous architecture of SnO microspheres (see graphic).
A facile microwave method was employed to synthesize NiCo2O4 nanosheets as electrode materials for lithium‐ion batteries and supercapacitors. The structure and morphology of the materials were ...characterized by X‐ray diffraction, field‐emission scanning electron microscopy, transmission electron microscopy and Brunauer–Emmett–Teller methods. Owing to the porous nanosheet structure, the NiCo2O4 electrodes exhibited a high reversible capacity of 891 mA h g−1 at a current density of 100 mA g−1, good rate capability and stable cycling performance. When used as electrode materials for supercapacitors, NiCo2O4 nanosheets demonstrated a specific capacitance of 400 F g−1 at a current density of 20 A g−1 and superior cycling stability over 5000 cycles. The excellent electrochemical performance could be ascribed to the thin porous structure of the nanosheets, which provides a high specific surface area to increase the electrode–electrolyte contact area and facilitate rapid ion transport.
Pore‐table electronics: A facile microwave method was used to synthesize mesoporous NiCo2O4 nanosheets as electrode materials for Li‐ion batteries and supercapacitors. The NiCo2O4 electrodes exhibited a high reversible capacity of 891 mA h g−1 at a current density of 100 mA g−1, good rate capability and stable cycling performance. When used in supercapacitors, NiCo2O4 nanosheets showed superior pseudocapacitive performance and excellent cycling stability (4.8 % loss after 5000 cycles).
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