Exploring Advanced Sandwiched Arrays by Vertical Graphene and N‐Doped Carbon for Enhanced Sodium Storage

Smart hybridization of active materials into tailored electrode structure is highly important for developing advanced electrochemical energy storage devices. With the help of sandwiched design, herein a powerful strategy is developed to fabricate three‐layer sandwiched composite core/shell arrays via combined hydrothermal and polymerization approaches. In such a unique architecture, wrinkled MoSe₂ nanosheets are sandwiched by vertical graphene (VG) core and N‐doped carbon (N‐C) shell forming sandwiched core/shell arrays. Interesting advantages including high electrical conductivity, strong mechanical stability, and large porosity are combined in the self‐supported VG/MoSe₂/N‐C sandwiched arrays. As a preliminary test, the sodium ion storage properties of VG/MoSe₂/N‐C sandwiched arrays are characterized and demonstrated with high capacity (540 mA h g^?1), enhanced high rate capability, and long‐term cycling stability (298 mA h g^?1 at 2.0 A g^?1 after 1000 cycles). The sandwiched core/shell structure plays positive roles in the enhancement of electrochemical performances due to dual conductive carbon networks, good volume accommodation, and highly porous structure with fast ion diffusion. The directional electrode design protocol provides a general method for synthesis of high‐performance ternary core/shell electrodes.     

A Novel Sustainable Flour Derived Hierarchical Nitrogen‐Doped Porous Carbon/Polyaniline Electrode for Advanced Asymmetric Supercapacitors

Hierarchically porous nitrogen‐doped carbon (HPC)/polyaniline (PANI) nanowire arrays nanocomposites are synthesized by a facile in situ polymerization. 3D interconnected honeycomb‐like HPC was prepared by a cost‐effective route via one‐step carbonization using urea and alkali‐treated wheat flour as carbon precursor with a high specific surface area (1294 m² g^?1). The specific capacitances of HPC and HPC/PANI (with a surface area of 923 m² g^?1) electrode are 383 and 1080 F g^?1 in 1 m H₂SO₄, respectively. Furthermore, an asymmetric supercapacitor based on HPC/PANI as positive electrode and HPC as negative electrode is successfully assembled with a voltage window of 0–1.8 V in 1 m Na₂SO₄ aqueous electrolyte, exhibiting high specific capacitance (134 F g^?1), high energy density (60.3 Wh kg^?1) and power density (18 kW kg^?1), and excellent cycling stability (91.6% capacitance retention after 5000 cycles).     

Seamlessly Conductive 3D Nanoarchitecture of Core–Shell Ni‐Co Nanowire Network for Highly Efficient Oxygen Evolution

Electrochemical splitting of water is an attractive way to produce hydrogen fuel as a clean and renewable energy source. However, a major challenge is to accelerate the sluggish kinetics of the anodic half‐cell reaction where oxygen evolution reaction (OER) takes place. Here, a seamlessly conductive 3D architecture is reported with a carbon‐shelled Ni‐Co nanowire network as a highly efficient OER electrocatalyst. Highly porous and granular Ni‐Co nanowires are first grown on a carbon fiber woven fabric utilizing a cost‐effective hydrothermal method and then conductive carbon shell is coated on the Ni‐Co nanowires via glucose carbonization and annealing processes. The conductive carbon layer surrounding the nanowires is introduced to provide a continuous pathway for facile electron transport throughout the whole of the integrated 3D catalyst. This 3D hierarchical structure provides several synergistic effects and beneficial functions including a large number of active sites, easy accessibility of water, fast electron transport, rapid release of oxygen gas, enhanced electrochemical durability, and stronger structural integrity, resulting in a remarkable OER activity that delivers an overpotential of 302 mV with a Tafel slope of 43.6 mV dec^?1 at a current density of 10 mA cm^?2 in an alkaline medium electrolyte (1 m KOH).     

Surface Free Energy‐Induced Assembly to the Synthesis of Grid‐Like Multicavity Carbon Spheres with High Level In‐Cavity Encapsulation for Lithium–Sulfur Cathode

Carbon microcapsules with a large interior cavity and porous shell are ideal hosts for guest species, while to maximize in‐cavity volume has always been a challenge. Herein, a surface free energy‐induced assembly approach is proposed for synthesis of multicavity carbon spheres (MCC). When used as a host for lithium–sulfur cathodes, MCC are fully accessible for sulfur—with high level in‐cavity encapsulation ability of grid‐like cavities. The crucial point for this assembly approach is the employment of small sized nanoemulsions with high homogeneity as primary building blocks. Spontaneous aggregation and assembly of substructural units are processing in following hydrothermal synthesis induced by reduction of surface free energy of system. As a result, multicavity structure is formed, where the size and number of cavities can be modulated by changing size of nanoemulsion and concentration of polymer. Confined pyrolysis enables to further enlarge cavity size compared to regular pyrolysis. The carbon–sulfur cathode exhibits excellent cycling stability and rate performance, i.e., high capacity of 943 and 869 mA h g^?1 after 200 cycles at current density of 0.5 and 2.0 C. The strategy has paved the way for custom‐ordered synthesis of nanostructured carbon with keen demands in high loading capacity of guest species.     

A Novel High‐Energy Hybrid Supercapacitor with an Anatase TiO2–Reduced Graphene Oxide Anode and an Activated Carbon Cathode

A hybrid supercapacitor with high energy and power densities is reported. It comprises a composite anode of anatase TiO₂ and reduced graphene oxide and an activated carbon cathode in a non‐aqueous electrolyte. While intercalation compounds can provide high energy typically at the expense of power, the anatase TiO₂ nanoparticles are able to sustain both high energy and power in the hybrid supercapacitor. At a voltage range from 1.0 to 3.0 V, 42 W h kg^?1 of energy is achieved at 800 W kg^?1. Even at a 4‐s charge/discharge rate, an energy density as high as 8.9 W h kg^?1 can be retained. The high energy and power of this hybrid supercapacitor bridges the gap between conventional batteries with high energy and low power and supercapacitors with high power and low energy.     

A Stable Graphitic,Nanocarbon‐Encapsulated,Cobalt‐Rich Core–Shell Electrocatalyst as an Oxygen Electrode in a Water Electrolyzer

The oxygen electrode plays a vital role in the successful commercialization of renewable energy technologies, such as fuel cells and water electrolyzers. In this study, the Prussian blue analogue‐derived nitrogen‐doped nanocarbon (NC) layer‐trapped, cobalt‐rich, core–shell nanostructured electrocatalysts (core–shell Co@NC) are reported. The electrode exhibits an improved oxygen evolution activity and stability compared to that of the commercial noble electrodes. The core–shell Co@NC‐loaded nickel foam exhibits a lower overpotential of 330 mV than that of IrO₂ on nickel foam at 10 mA cm^?2 and has a durability of over 400 h. The commercial Pt/C cathode‐assisted, core–shell Co@NC–anode water electrolyzer delivers 10 mA cm^?2 at a cell voltage of 1.59 V, which is 70 mV lower than that of the IrO₂–anode water electrolyzer. Over the long‐term chronopotentiometry durability testing, the IrO₂–anode water electrolyzer shows a cell voltage loss of 230 mV (14%) at 95 h, but the loss of the core–shell Co@NC–anode electrolyzer is only 60 mV (4%) even after 350 h cell‐operation. The findings indicate that the Prussian blue analogue is a class of inorganic nanoporous materials that can be used to derive metal‐rich, core–shell electrocatalysts with enriched active centers.     

A Stretchable Polymer–Carbon Nanotube Composite Electrode for Flexible Lithium‐Ion Batteries: Porosity Engineering by Controlled Phase Separation

Flexible energy‐storage devices have attracted growing attention with the fast development of bendable electronic systems. However, it still remains a challenge to find reliable electrode materials with both high mechanical flexibility/toughness and excellent electron and lithium‐ion conductivity. This paper reports the fabrication and characterization of highly porous, stretchable, and conductive polymer nanocomposites embedded with carbon nanotubes (CNTs) for application in flexible lithium‐ion batteries. The systematic optimization of the porous morphology is performed by controllably inducing the phase separation of polymethylmethacrylate (PMMA) in polydimethylsiloxane (PDMS) and removing PMMA, in order to generate well‐controlled pore networks. It is demonstrated that the porous CNT‐embedded PDMS nanocomposites are capable of good electrochemical performance with mechanical flexibility, suggesting these nanocomposites could be outstanding anode candidates for use in flexible lithium‐ion batteries. The optimization of the pore size and the volume fraction provides higher capacity by nearly seven‐fold compared to a nonporous nanocomposite.     

Molecular‐Level Design of Pyrrhotite Electrocatalyst Decorated Hierarchical Porous Carbon Spheres as Nanoreactors for Lithium–Sulfur Batteries

Lithium–sulfur batteries (LSBs) are a class of new‐generation rechargeable high‐energy‐density batteries. However, the persisting issue of lithium polysulfides (LiPs) dissolution and the shuttling effect that impedes the efficiency of LSBs are challenging to resolve. Herein a general synthesis of highly dispersed pyrrhotite Fe_1?xS nanoparticles embedded in hierarchically porous nitrogen‐doped carbon spheres (Fe_1?xS‐NC) is proposed. Fe_1?xS‐NC has a high specific surface area (627 m² g^?1), large pore volume (0.41 cm³ g^?1), and enhanced adsorption and electrocatalytic transition toward LiPs. Furthermore, in situ generated large mesoporous pores within carbon spheres can accommodate high sulfur loading of up to 75%, and sustain volume variations during charge/discharge cycles as well as improve ionic/mass transfer. The exceptional adsorption properties of Fe_1?xS‐NC for LiPs are predicted theoretically and confirmed experimentally. Subsequently, the electrocatalytic activity of Fe_1?xS‐NC is thoroughly verified. The results confirm Fe_1?xS‐NC is a highly efficient nanoreactor for sulfur loading. Consequently, the Fe_1?xS‐NC nanoreactor performs extremely well as a cathodic material for LSBs, exhibiting a high initial capacity of 1070 mAh g^?1 with nearly no capacity loss after 200 cycles at 0.5 C. Furthermore, the resulting LSBs display remarkably enhanced rate capability and cyclability even at a high sulfur loading of 8.14 mg cm^?2.     

Dual‐Confined Flexible Sulfur Cathodes Encapsulated in Nitrogen‐Doped Double‐Shelled Hollow Carbon Spheres and Wrapped with Graphene for Li–S Batteries

Batteries with high energy and power densities along with long cycle life and acceptable safety at an affordable cost are critical for large‐scale applications such as electric vehicles and smart grids, but is challenging. Lithium–sulfur (Li‐S) batteries are attractive in this regard due to their high energy density and the abundance of sulfur, but several hurdles such as poor cycle life and inferior sulfur utilization need to be overcome for them to be commercially viable. Li–S cells with high capacity and long cycle life with a dual‐confined flexible cathode configuration by encapsulating sulfur in nitrogen‐doped double‐shelled hollow carbon spheres followed by graphene wrapping are presented here. Sulfur/polysulfides are effectively immobilized in the cathode through physical confinement by the hollow spheres with porous shells and graphene wrapping as well as chemical binding between heteronitrogen atoms and polysulfides. This rationally designed free‐standing nanostructured sulfur cathode provides a well‐built 3D carbon conductive network without requiring binders, enabling a high initial discharge capacity of 1360 mA h g^?1 at a current rate of C/5, excellent rate capability of 600 mA h g^?1 at 2 C rate, and sustainable cycling stability for 200 cycles with nearly 100% Coulombic efficiency, suggesting its great promise for advanced Li–S batteries.     

A Three‐Dimensionally Interconnected Carbon Nanotube–Conducting Polymer Hydrogel Network for High‐Performance Flexible Battery Electrodes

High‐performance flexible energy‐storage devices have great potential as power sources for wearable electronics. One major limitation to the realization of these applications is the lack of flexible electrodes with excellent mechanical and electrochemical properties. Currently employed batteries and supercapacitors are mainly based on electrodes that are not flexible enough for these purposes. Here, a three‐dimensionally interconnected hybrid hydrogel system based on carbon nanotube (CNT)‐conductive polymer network architecture is reported for high‐performance flexible lithium ion battery electrodes. Unlike previously reported conducting polymers (e.g., polyaniline, polypyrrole, polythiophene), which are mechanically fragile and incompatible with aqueous solution processing, this interpenetrating network of the CNT‐conducting polymer hydrogel exibits good mechanical properties, high conductivity, and facile ion transport, leading to facile electrode kinetics and high strain tolerance during electrode volume change. A high‐rate capability for TiO₂ and high cycling stability for SiNP electrodes are reported. Typically, the flexible TiO₂ electrodes achieved a capacity of 76 mAh g^–1 in 40 s of charge/discharge and a high areal capacity of 2.2 mAh cm^–2 can be obtained for flexible SiNP‐based electrodes at 0.1C rate. This simple yet efficient solution process is promising for the fabrication of a variety of high performance flexible electrodes.     

General Dimension‐Controlled Synthesis of Hollow Carbon Embedded with Metal Singe Atoms or Core–Shell Nanoparticles for Energy Storage Applications

Metal–organic framework (MOF) derived carbonaceous nanocomposites have recently received enormous interest due to their intriguing physiochemical properties and diverse energy applications. However, there is a lack of general synthetic approaches that can achieve flexible dimension control while manipulating metal dispersion of MOF derived carbon composites. Herein, the authors present an attractive route for the growth of zeolitic imidazolate frameworks (ZIFs) with different dimensions and types of metal nodes that can be further transformed into either core–shell nanoparticles or metal single atoms. The formation of a ZIF‐8 seed layer on ZnO template is identified as the key step, enabling uniform growth of various ZIF materials (e.g., Zn/Co‐ZIF, Zn/Fe‐ZIF, and ZIF‐7) with different dimensions (1D, 2D, and 3D). Simultaneously, this approach avoids free growth of 0D MOF particles and diminishing of the ZnO template. To demonstrate the importance of dimensional control over the growth of ZIF materials for energy application, the 1D and 2D ZnO@ZIF precursors are converted into carbon nanotube and carbon nanoplate, which are decorated with Co/CoS₂ nanoparticles and Fe single atoms, respectively. Two high dimensional carbon nanocomposites deliver significantly enhanced performances compared to their 0D counterparts when employed as the Li‐ion battery anode and bifunctional oxygen electrocatalyst.     

Graphene Quantum Dots‐Based Advanced Electrode Materials: Design,Synthesis and Their Applications in Electrochemical Energy Storage and Electrocatalysis

Graphene quantum dots (GQDs) have aroused great interest in the scientific community in recent years due to their unique physicochemical properties and potential applications in different fields. To date, much research has been conducted on the ingenious design and rational construction of GQDs‐based nanomaterials used as electrode materials and/or electrocatalysts. Despite these efforts, research on the efficient synthesis and application of GQDs‐based nanomaterials is still in the early stages of development and timely updates of recent research progress on new design concepts, synthetic strategies, and significant breakthroughs in GQDs‐based nanomaterials are highly desired. In light of the above, the effect of synthetic methods on the final product of the GQDs, the GQDs synthesis mechanism, and specific perspectives regarding the effect of the unique surface and structural properties of GQDs (e.g., defects, heteroatom doping, surface/edge state, size, conductivity) on the electrochemical energy‐related systems are discussed in‐depth in this review. Additionally, this review also focuses on the design of GQDs‐based composites and their applications in the fields of electrochemical energy storage (e.g., supercapacitors and batteries) and electrocatalysis (e.g., fuel cell, water splitting, CO₂ reduction), along with constructive suggestions for addressing the remaining challenges in the field.     

Rational Design of Metal‐Organic Framework Derived Hollow NiCo2O4 Arrays for Flexible Supercapacitor and Electrocatalysis

Metal‐organic frameworks (MOFs) are promising porous precursors for the construction of various functional materials for high‐performance electrochemical energy storage and conversion. Herein, a facile two‐step solution method to rational design of a novel electrode of hollow NiCo₂O₄ nanowall arrays on flexible carbon cloth substrate is reported. Uniform 2D cobalt‐based wall‐like MOFs are first synthesized via a solution reaction, and then the 2D solid nanowall arrays are converted into hollow and porous NiCo₂O₄ nanostructures through an ion‐exchange and etching process with an additional annealing treatment. The as‐obtained NiCo₂O₄ nanostructure arrays can provide rich reaction sites and short ion diffusion path. When evaluated as a flexible electrode material for supercapacitor, the as‐fabricated NiCo₂O₄ nanowall electrode shows remarkable electrochemical performance with excellent rate capability and long cycle life. In addition, the hollow NiCo₂O₄ nanowall electrode exhibits promising electrocatalytic activity for oxygen evolution reaction. This work provides an example of rational design of hollow nanostructured metal oxide arrays with high electrochemical performance and mechanical flexibility, holding great potential for future flexible multifunctional electronic devices.     

Electrochemical Performance of Porous Carbon/Tin Composite Anodes for Sodium‐Ion and Lithium‐Ion Batteries

The electrochemical performance of mesoporous carbon (C)/tin (Sn) anodes in Na‐ion and Li‐ion batteries is systematically investigated. The mesoporous C/Sn anodes in a Na‐ion battery shows similar cycling stability but lower capacity and poorer rate capability than that in a Li‐ion battery. The desodiation potentials of Sn anodes are approximately 0.21 V lower than delithiation potentials. The low capacity and poor rate capability of C/Sn anode in Na‐ion batteries is mainly due to the large Na‐ion size, resulting in slow Na‐ion diffusion and large volume change of porous C/Sn composite anode during alloy/dealloy reactions. Understanding of the reaction mechanism between Sn and Na ions will provide insight towards exploring and designing new alloy‐based anode materials for Na‐ion batteries.