Controllable Synthesis of 2D Materials by Electrochemical Exfoliation for Energy Storage and Conversion Application

2D materials have captured much recent research interest in a broad range of areas, including electronics, biology, sensors, energy storage, and others. In particular, preparing 2D nanosheets with high quality and high yield is crucial for the important applications in energy storage and conversion. Compared with other prevailing synthetic strategies, the electrochemical exfoliation of layered starting materials is regarded as one of the most promising and convenient methods for the large-scale production of uniform 2D nanosheets. Here, recent developments in electrochemical delamination are reviewed, including protocols, categories, principles, and operating conditions. State-of-the-art methods for obtaining 2D materials with small numbers of layers—including graphene, black phosphorene, transition metal dichalcogenides and MXene—are also summarized and discussed in detail. The applications of electrochemically exfoliated 2D materials in energy storage and conversion are systematically reviewed. Drawing upon current progress, perspectives on emerging trends, existing challenges, and future research directions of electrochemical delamination are also offered.     

2D Metal Chalcogenides Incorporated into Carbon and their Assembly for Energy Storage Applications

2D metal chalcogenides have become a popular focus in the energy storage field because of their unique properties caused by their single‐atom thicknesses. However, their high surface energy and van der Waals attraction easily cause serious stacking and restacking, leading to the generation of more inaccessible active sites with rapid capacity fading. The hybridization of 2D metal chalcogenides with highly conductive materials, particularly, incorporating ultrasmall and few‐layered metal chalcogenides into carbon frameworks, can not only maximize the exposure of active sites but also effectively avoid their stacking and aggregation during the electrochemical reaction process. Therefore, a satisfactory specific capacity will be achieved with a long cycle life. In this Concept, the representative progress on such intriguing nanohybrids and their applications in energy storage devices are mainly summarized. Finally, an outlook of the future development and challenges of such nanohybrids for achieving an excellent energy storage capability is also provided.     

Confinement Catalysis with 2D Materials for Energy Conversion

The unique electronic and structural properties of 2D materials have triggered wide research interest in catalysis. The lattice of 2D materials and the interface between 2D covers and other substrates provide intriguing confinement environments for active sites, which has stimulated a rising area of “confinement catalysis with 2D materials.” Fundamental understanding of confinement catalysis with 2D materials will favor the rational design of high‐performance 2D nanocatalysts. Confinement catalysis with 2D materials has found extensive applications in energy‐related reaction processes, especially in the conversion of small energy‐related molecules such as O₂, CH₄, CO, CO₂, H₂O, and CH₃OH. Two representative strategies, i.e., 2D lattice‐confined single atoms and 2D cover‐confined metals, have been applied to construct 2D confinement catalytic systems with superior catalytic activity and stability. Herein, the recent advances in the design, applications, and structure–performance analysis of two 2D confinement catalytic systems are summarized. The different routes for tuning the electronic states of 2D confinement catalysts are highlighted and perspectives on confinement catalysis with 2D materials toward energy conversion and utilization in the future are provided.     

Vertical Growth of 2D Amorphous FePO4 Nanosheet on Ni Foam: Outer and Inner Structural Design for Superior Water Splitting

Rational design of highly efficient bifunctional electrocatalysts based on 3D transition‐metal‐based materials for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great importance for sustainable energy conversion processes. Herein, a novel strategy involving outer and inner structural engineering is developed for superior water splitting via in situ vertical growth of 2D amorphous FePO₄ nanosheets on Ni foam (Am FePO₄/NF). Careful experiments and density functional theory calculations show that the inner and outer structural engineering contributing to the synergistic effects of 2D morphology, amorphous structure, conductive substrate, and Ni?Fe mixed phosphate lead to superior electrocatalytic activity toward OER and HER. Furthermore, a two‐electrode electrolyzer assembled using Am FePO₄/NF as an electrocatalyst at both electrodes gives current densities of 10 and 100 mA cm^?2 at potentials of 1.54 and 1.72 V, respectively, which is comparable to the best bifunctional electrocatalyst reported in the literature. The strategies, introduced in the present work, may open new opportunities for the rational design of other 3D transition‐metal‐based electrocatalyst through an outer and inner structural control to strengthen the electrocatalytic performance.     

二维材料MXene的储能性能与应用

能量存储和转化器件是现代社会的重要基础。随着清洁能源、便携式电子设备及电动汽车的快速发展,人们对储能器件性能的要求越来越高。储能材料是决定储能器件性能的重要因素。通常,储能材料需满足具有可逆的氧化还原反应、易于电解液离子脱嵌、尽可能多地提供氧化还原位点、良好的导电能力等要求。近年来,二维材料因比表面积大、离子传输路径短等特点而得到广泛关注,在储能领域也具有巨大的发展潜力。只有原子量级厚度的二维材料,表面活性位点多,力学性能优良,正契合储能器件对电极材料的要求。MXene是一类新型二维材料,通式为Mn+1XnTx,其中M代表过渡族金属元素,X为碳和/或氮,T代表MXene在制备过程中产生的官能团(-F、-OH、-O等),n一般为1～4。自2011年首次报道以来,MXene在储能领域就被寄予厚望。MXene含有碳原子层,所以具有类似石墨烯的良好导电性;而过渡金属层使其表现出类似过渡金属氧化物的性能;同时,表面多样的官能团赋予MXene良好的亲水性。这种独特的性能组合,使得MXene电荷响应速度快,具有赝电容特征且循环性能稳定,成为储能领域的研究焦点。另外,便携式储能器件要求更高的体积容量与体积能量密度,而MXene与碳基电极材料相比堆积密度高,可有效降低器件体积,拓展应用范围。目前,MXene及其复合材料已经在超级电容器、锂/钠/镁离子二次电池、锂硫电池、锌-空气电池、储氢等诸多储能领域展现出实用价值。但是,MXene容易发生塌陷和堆垛,影响其作为电极材料的性能。因此,需将MXene进行插层、改性、掺杂或与其他材料复合,以阻止MXene堆叠,减小离子扩散阻力,并增加离子吸附位点,从而提高其电化学性能。而且,不同的能量存储和转化装置对MXene的合成方法和结构特征有不同的要求,鉴于MXene能源应用相关研究的大量呈现,有必要对其进行全面总结与分析,以期推动MXene在该领域的发展。本文旨在综述MXene在制备、结构、性能及其在储能方面的最新研究动态与发展方向,并讨论面临的挑战。     

Layer-Controlled Perovskite 2D Nanosheet Interlayer for the Energy Storage Performance of Nanocomposites

Polymer-based nanocomposites are desirable materials for next-generation dielectric capacitors. 2D dielectric nanosheets have received significant attention as a filler. However, randomly spreading the 2D filler causes residual stresses and agglomerated defect sites in the polymer matrix, which leads to the growth of an electric tree, resulting in a more premature breakdown than expected. Therefore, realizing a well-aligned 2D nanosheet layer with a small amount is a key challenge; it can inhibit the growth of conduction paths without degrading the performance of the material. Here, an ultrathin Sr_1.8Bi_0.2Nb₃O₁₀ (SBNO) nanosheet filler is added as a layer into poly(vinylidene fluoride) (PVDF) films via the Langmuir–Blodgett method. The structural properties, breakdown strength, and energy storage capacity of a PVDF and multilayer PVDF/SBNO/PVDF composites as a function of the thickness-controlled SBNO layer are examined. The seven-layered (only 14 nm) SBNO nanosheets thin film can sufficiently prevent the electrical path in the PVDF/SBNO/PVDF composite and shows a high energy density of 12.8 J cm⁻³ at 508 MV m⁻¹, which is significantly higher than that of the bare PVDF film (9.2 J cm⁻³ at 439 MV m⁻¹). At present, this composite has the highest energy density among the polymer-based nanocomposites under the filler of thin thickness.     

Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective

2D materials have shown great potential as electrode materials that determine the performance of a range of electrochemical energy technologies. Among these, 2D copper-based materials, such as Cu–O, Cu–S, Cu–Se, Cu–N, and Cu–P, have attracted tremendous research interest, because of the combination of remarkable properties, such as low cost, excellent chemical stability, facile fabrication, and significant electrochemical properties. Herein, the recent advances in the emerging 2D copper-based materials are summarized. A brief summary of the crystal structures and synthetic methods is started, and innovative strategies for improving electrochemical performances of 2D copper-based materials are described in detail through defect engineering, heterostructure construction, and surface functionalization. Furthermore, their state-of-the-art applications in electrochemical energy storage including supercapacitors (SCs), alkali (Li, Na, and K)-ion batteries, multivalent metal (Mg and Al)-ion batteries, and hybrid Mg/Li-ion batteries are described. In addition, the electrocatalysis applications of 2D copper-based materials in metal–air batteries, water-splitting, and CO₂ reduction reaction (CO₂RR) are also discussed. This review also discusses the charge storage mechanisms of 2D copper-based materials by various advanced characterization techniques. The review with a perspective of the current challenges and research outlook of such 2D copper-based materials for high-performance energy storage and conversion applications is concluded.     

Multifunctional Energy Storage and Conversion Devices

Multifunctional energy storage and conversion devices that incorporate novel features and functions in intelligent and interactive modes, represent a radical advance in consumer products, such as wearable electronics, healthcare devices, artificial intelligence, electric vehicles, smart household, and space satellites, etc. Here, smart energy devices are defined to be energy devices that are responsive to changes in configurational integrity, voltage, mechanical deformation, light, and temperature, called self‐healability, electrochromism, shape memory, photodetection, and thermal responsivity. Advisable materials, device designs, and performances are crucial for the development of energy electronics endowed with these smart functions. Integrating these smart functions in energy storage and conversion devices gives rise to great challenges from the viewpoint of both understanding the fundamental mechanisms and practical implementation. Current state‐of‐art examples of these smart multifunctional energy devices, pertinent to materials, fabrication strategies, and performances, are highlighted. In addition, current challenges and potential solutions from materials synthesis to device performances are discussed. Finally, some important directions in this fast developing field are considered to further expand their application.     

Metallic MoS2 for High Performance Energy Storage and Energy Conversion

Metallic phase 2D molybdenum disulfide (MoS₂) is an emerging class of materials with remarkably higher electrical conductivity and catalytic activities. The goal of this study is to review the atomic structures and electrochemistry of metallic MoS₂, which is essential for a wide range of existing and new enabling technologies. The scope of this paper ranges from the atomic structure, band structure, electrical and optical properties to fabrication methods, and major emerging applications in electrochemical energy storage and energy conversion. This paper also thoroughly covers the atomic structure–properties–application relationships of metallic MoS₂. Understanding the fundamental properties of these structures is crucial for designing and manufacturing products for emerging applications. Today, a more holistic understanding of the interplay between the structure, chemistry, and performance of metallic MoS₂ is advancing actual applications of this material. This new level of understanding also enables a myriad of new and exciting applications, which motivated this review. There are excellent reviews already on the traditional semiconducting MoS₂, and this review, for the first time, focuses on the uniqueness of conducting metallic MoS₂ for energy applications and offers brand new materials for clean energy application.     

A High‐Performance Lithium‐Ion Capacitor Based on 2D Nanosheet Materials

Lithium‐ion capacitors (LICs) are promising electrical energy storage systems for mid‐to‐large‐scale applications due to the high energy and large power output without sacrificing long cycle stability. However, due to the different energy storage mechanisms between anode and cathode, the energy densities of LICs often degrade noticeably at high power density, because of the sluggish kinetics limitation at the battery‐type anode side. Herein, a high‐performance LIC by well‐defined ZnMn₂O₄‐graphene hybrid nanosheets anode and N‐doped carbon nanosheets cathode is presented. The 2D nanomaterials offer high specific surface areas in favor of a fast ion transport and storage with shortened ion diffusion length, enabling fast charge and discharge. The fabricated LIC delivers a high specific energy of 202.8 Wh kg^?1 at specific power of 180 W kg^?1, and the specific energy remains 98 Wh kg^?1 even when the specific power achieves as high as 21 kW kg^?1.     

Carbon-Based Nanocages: A New Platform for Advanced Energy Storage and Conversion

Energy storage and conversion play a crucial role in modern energy systems, and the exploration of advanced electrode materials is vital but challenging. Carbon-based nanocages consisting of sp² carbon shells feature a hollow interior cavity with sub-nanometer microchannels across the shells, high specific surface area with a defective outer surface, and tunable electronic structure, much different from the intensively studied nanocarbons such as carbon nanotubes and graphene. These structural and morphological characteristics make carbon-based nanocages a new platform for advanced energy storage and conversion. Up-to-date synthetic strategies of carbon-based nanocages, the utilization of their unique porous structure and morphology for the construction of composites with foreign active species, and their significant applications to the advanced energy storage and conversion are reviewed. Structure–performance correlations are discussed in depth to highlight the contribution of carbon-based nanocages. The research challenges and trends are also envisaged for deepening and extending the study and application of this multifunctional material.     

Economizing Production of Diverse 2D Layered Metal Hydroxides for Efficient Overall Water Splitting

2D layered metal hydroxides (LMH) are promising materials for electrochemical energy conversion and storage. Compared with exfoliation of bulk layered materials, wet chemistry synthesis of 2D LMH materials under mild conditions still remains a big challenge. Here, an “MgO‐mediated strategy” for mass production of various 2D LMH nanosheets is presented by hydrolyzing MgO in metal salt aqueous solutions at room temperature. Benefiting from this economical and scalable strategy, ultrathin LMH nanosheets (M = Ni, Fe, Co, NiFe, and NiCo) and their derivatives (e.g., metal oxides and sulfides) can be synthesized in high yields. More importantly, this strategy opens up opportunities to fabricate hierarchically structured LMH nanosheets, resulting in high‐performance electrocatalysts for the oxygen‐ and hydrogen‐evolution reactions to realize stable overall water splitting with a low cell voltage of 1.55 V at 10 mA cm⁻². This work provides a powerful platform for the synthesis and applications of 2D materials.     

Engineering 2D Nanofluidic Li‐Ion Transport Channels for Superior Electrochemical Energy Storage

Rational surface engineering of 2D nanoarchitectures‐based electrode materials is crucial as it may enable fast ion transport, abundant‐surface‐controlled energy storage, long‐term structural integrity, and high‐rate cycling performance. Here we developed the stacked ultrathin Co₃O₄ nanosheets with surface functionalization (SUCNs‐SF) converted from layered hydroxides with inheritance of included anion groups (OH^?, NO₃^?, CO₃^2?). Such stacked structure establishes 2D nanofluidic channels offering extra lithium storage sites, accelerated Li‐ion transport, and sufficient buffering space for volume change during electrochemical processes. Tested as an anode material, this unique nanoarchitecture delivers high specific capacity (1230 and 1011 mAh g^?1 at 0.2 and 1 A g^?1, respectively), excellent rate performance, and long cycle capability (1500 cycles at 5 A g^?1). The demonstrated advantageous features by constructing 2D nanochannels in nonlayered materials may open up possibilities for designing high‐power lithium ion batteries.     

Ultrathin Porous NiFeV Ternary Layer Hydroxide Nanosheets as a Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Herein, the hydrothermal synthesis of porous ultrathin ternary NiFeV layer double hydroxides (LDHs) nanosheets grown on Nickel foam (NF) substrate as a highly efficient electrode toward overall water splitting in alkaline media is reported. The lateral size of the nanosheets is about a few hundreds of nanometers with the thickness of ≈10 nm. Among all molar ratios investigated, the Ni_0.75Fe_0.125V_0.125‐LDHs/NF electrode depicts the optimized performance. It displays an excellent catalytic activity with a modest overpotential of 231 mV for the oxygen evolution reaction (OER) and 125 mV for the hydrogen evolution reaction (HER) in 1.0 m KOH electrolyte. Its exceptional activity is further shown in its small Tafel slope of 39.4 and 62.0 mV dec^?1 for OER and HER, respectively. More importantly, remarkable durability and stability are also observed. When used for overall water splitting, the Ni_0.75Fe_0.125V_0.125‐LDHs/NF electrodes require a voltage of only 1.591 V to reach 10 mA cm^?2 in alkaline solution. These outstanding performances are mainly attributed to the synergistic effect of the ternary metal system that boosts the intrinsic catalytic activity and active surface area. This work explores a promising way to achieve the optimal inexpensive Ni‐based hydroxide electrocatalyst for overall water splitting.     

Strong Coupling of MoS2 Nanosheets and Nitrogen‐Doped Graphene for High‐Performance Pseudocapacitance Lithium Storage

Layered material MoS₂ is widely applied as a promising anode for lithium‐ion batteries (LIBs). Herein, a scalable and facile dopamine‐assisted hydrothermal technique for the preparation of strongly coupled MoS₂ nanosheets and nitrogen‐doped graphene (MoS₂/N‐G) composite is developed. In this composite, the interconnected MoS₂ nanosheets are well wrapped onto the surface of graphene, forming a unique veil‐like architecture. Experimental results indicate that dopamine plays multiple roles in the synthesis: a binding agent to anchor and uniformly disperse MoS₂ nanosheets, a morphology promoter, and the precursor for in situ nitrogen doping during the self‐polymerization process. Density functional theory calculations further reveal that a strong interaction exists at the interface of MoS₂ nanosheets and nitrogen‐doped graphene, which facilitates the charge transfer in the hybrid system. When used as the anode for LIBs, the resulting MoS₂/N‐G composite electrode exhibits much higher and more stable Li‐ion storage capacity (e.g., 1102 mAh g^?1 at 100 mA g^?1) than that of MoS₂/G electrode without employing the dopamine linker. Significantly, it is also identified that the thin MoS₂ nanosheets display outstanding high‐rate capability due to surface‐dominated pseudocapacitance contribution.     

Porous Graphene Materials for Advanced Electrochemical Energy Storage and Conversion Devices

Combining the advantages from both porous materials and graphene, porous graphene materials have attracted vast interests due to their large surface areas, unique porous structures, diversified compositions and excellent electronic conductivity. These unordinary features enable porous graphene materials to serve as key components in high‐performance electrochemical energy storage and conversion devices such as lithium ion batteries, supercapacitors, and fuel cells. This progress report summarizes the typical fabrication methods for porous graphene materials with micro‐, meso‐, and macro‐porous structures. The structure–property relationships of these materials and their application in advanced electrochemical devices are also discussed.     

Series of Halogen Engineered Ni(OH)2 Nanosheet for Pseudocapacitive Energy Storage with High Energy Density

Ni(OH)₂ nanosheet, acting as a potential active material for supercapacitors, commonly suffers from sluggish reaction kinetics and low intrinsic conductivity, which results in suboptimal energy density and long cycle life. Herein, a convenient electrochemical halogen functionalization strategy is applied for the preparation of mono/bihalogen engineered Ni(OH)₂ electrode materials. The theoretical calculations and experimental results found that thanks to the extraordinarily high electronegativity, optimal reversibility, electronic conductivity, and reaction kinetics could be achieved through F functionalization  . However, benefiting from the largest ionic radius, I Ni(OH)₂ contributes the best specific capacity and morphology transformation, which is a new finding that distinguishes it from previous reports in the literature. The exploration of the interaction effect of halogens (F, I Ni(OH)₂, F, Br Ni(OH)₂, and Cl, I Ni(OH)₂) manifests that F, I Ni(OH)₂ delivers a higher specific capacity of 200.6 mAh g⁻¹ and an excellent rate capability of 58.2% due to the weaker electrostatic repulsion, abundant defect structure, and large layer spacing. Moreover, the F, I Ni(OH)₂//FeOOH@NrGO device achieves a high energy density of 97.4 Wh kg⁻¹ and an extremely high power density of 32426.7 W kg⁻¹, as well as good cycling stability. This work develops a pioneering tactic for designing energy storage materials to meet various demands.