Faradaic and Non-Faradaic Self-Discharge Mechanisms in Carbon-Based Electrochemical Capacitors.

Electrochemical capacitors (ECs) play a crucial role in electrical energy storage, offering great potential for efficient energy storage and power management. However, they face challenges such as moderate energy densities and rapid self-discharge. Addressing self-discharge necessitates a fundamental understanding of the underlying processes. This review sets itself apart from other reviews by focusing on the basic principles of self-discharge processes in carbon-based ECs, particularly examining the nature of the process and the involvement of redox reactions. This study delineates the potential conditions for various self-discharge processes and proposes plausible criteria for differentiation, complemented by mathematical modeling. Additionally, the model selection, curve fitting, and effective tuning methods are explored to control self-discharge processes.

Laminating Layered Structures of 2D MXene and Graphene Oxide for High-Performance All-Solid-State Supercapacitors.

Graphene oxide (GO)-based all-solid-state supercapacitors (SCs) provide an important complement to liquid- and gel-electrolyte-based SCs in a variety of applications, including flexible electronics. Still, their mediocre capacitance and complex fabrication methods hold back the realization of their full potential. Here, a simple fabrication of all-solid-state SCs with layered GO as a solid electrolyte and MXene as electrodes is demonstrated. The resultant SCs show excellent energy storage capacitance comparable to other MXene-based SCs using liquid electrolytes. The outperformance is attributed to extra interlayer spacing expansion and improved ion transport kinetics thanks to a synergistic water-absorbing effect due to the hydrophilicity of both MXene and GO in combination, which interestingly satisfies the intrinsic surface-dominated pseudocapacitive behavior of MXene. The application of this SC in humidity sensing has also been demonstrated to be fast responsive. The findings describe in this work provide a means of improving the capacitance performance using GO as a solid electrolyte with MXene as the electrodes and exploit the potential application as electronic elements for smart devices.

Hollow Carbon Nanospheres with Developed Porous Structure and Retained N Doping for Facilitated Electrochemical Energy Storage.

Development of highly porous carbons with abundant surface functionalities and well-defined nanostructure is of significance for many important electrochemical energy storage systems. However, porous carbons suffer from a compromise between porosity, doped functionality, and nanostructure that have thus far restricted their performances. Here, we report the design of highly porous, nitrogen-enriched hollow carbon nanospheres (PN-HCNs) by an interfacial copolymerization strategy followed by NH3-assisted carbonization, and further demonstrate their significance and effectiveness in enhancing the electrochemical performances. The PN-HCN simultaneously delivers a large surface area (1237 m2 g-1) and high N functionalities (6.25 atom %) with a remarkable efficiency of the surface area increase to N loss ratio enabled by NH3 treatment while inheriting the hollow nanospherical structure. Accordingly, owing to the enhanced surface area and retained N doping, the prepared PN-HCN demonstrates outstanding electrochemical performances as a cathode host in lithium-sulfur batteries, including a near-to-theoretical capacity of 1620 mAh g-1, high rate capability and good cycling stability (789 mAh g-1 at 0.5C after 200 cycles). These results are superior to those of HCN without NH3 treatment. Also, PN-HCN exhibits superior capacitances (203 F g-1) and fast ion transport ability in supercapacitors. Our finding shows the simultaneous achievement of both highly porous structures and sufficient N functionalities for high-performance applications.

Suppressing Dendritic Lithium Formation Using Porous Media in Lithium Metal-Based Batteries.

Because of its ultrahigh specific capacity, lithium metal holds great promise for revolutionizing current rechargeable battery technologies. Nevertheless, the unavoidable formation of dendritic Li, as well as the resulting safety hazards and poor cycling stability, have significantly hindered its practical applications. A mainstream strategy to solve this problem is introducing porous media, such as solid electrolytes, modified separators, or artificial protection layers, to block Li dendrite penetration. However, the scientific foundation of this strategy has not yet been elucidated. Herein, using experiments and simulation we analyze the role of the porous media in suppressing dendritic Li growth and probe the underlying fundamental mechanisms. It is found that the tortuous pores of the porous media, which drastically reduce the local flux of Li+ moving toward the anode and effectively extend the physical path of dendrite growth, are the key to achieving the nondendritic Li growth. On the basis of the theoretical exploration, we synthesize a novel porous silicon nitride submicron-wire membrane and incorporate it in both half-cell and full-cell configurations. The operation time of the battery cells is significantly extended without a short circuit. The findings lay the foundation to use a porous medium for achieving nondendritic Li growth in Li metal-based batteries.

Normalized Lithium Growth from the Nucleation Stage for Dendrite-Free Lithium Metal Anodes.

Inducing uniform deposition of lithium from the stage of metal crystallization nucleation is of vital importance to achieve dendrite-free lithium anodes. Herein, using experiments and simulation, homogenization of Li nucleation and normalization of Li growth can be achieved on PNIPAM polymer brushes with lithiophilic functional groups modified Cu substrates. The lithiophilic functional groups of amide O can homogenize ion mass transfer and induce the uniform distribution of Li nucleation sites. What is more, the ultra-small space between each brush can act as the channels for Li transportation and normalization growth. Owing to the synergistic effect of homogenization and normalization of electrodeposited Li, the obtained planar columnar Li anode exhibits excellent cycle stability at an ultra-high current density of 20 mA cm-2 .

Superior Potassium Ion Storage via Vertical MoS2 "Nano-Rose" with Expanded Interlayers on Graphene.

Potassium has its unique advantages over lithium or sodium as a charge carrier in rechargeable batteries. However, progresses in K-ion battery (KIB) chemistry have so far been hindered by lacking suitable electrode materials to host the relatively large K+ ions compared to its Li+ and Na+ counterparts. Herein, molybdenum disulfide (MoS2 ) "roses" grown on reduced graphene oxide sheets (MoS2 @rGO) are synthesized via a two-step solvothermal route. The as-synthesized MoS2 @rGO composite, with expanded interlayer spacing of MoS2 , chemically bonded between MoS2 and rGO, and a unique nano-architecture, displays the one of the best electrochemical performances to date as an anode material for nonaqueous KIBs. More importantly, a combined K+ storage mechanism of intercalation and conversion reaction is also revealed. The findings presented indicate the enormous potential of layered metal dichalcogenides as advanced electrode materials for high-performance KIBs and also provide new insights and understanding of K+ storage mechanism.

Chaotic region of elastically restrained single-walled carbon nanotube.

The occurrence of chaos in the transverse oscillation of the carbon nanotube in all of the precise micro-nano mechanical systems has a strong impact on the stability and the precision of the micro-nano systems, the conditions of which are related with the boundary restraints of the carbon nanotube. To generalize some transverse oscillation problems of the carbon nanotube studied in current references, the elastic restraints at both ends of the single-walled carbon nanotube are considered by means of rotational and translational springs to investigate the effects of the boundary restraints on the chaotic properties of the carbon nanotube in this paper. Based on the generalized multi-symplectic theory, both the generalized multi-symplectic formulations for the governing equation describing the transverse oscillation of the single-walled carbon nanotube subjected to the transverse load and the constraint equations resulting from the elastic restraints are presented firstly. Then, the structure-preserving scheme with discrete constraint equations is constructed to simulate the transverse oscillation process of the carbon nanotube. Finally, the chaotic region of the carbon nanotube is captured, and the oscillations of the two extreme cases (including simply supported and cantilever) are investigated in the numerical investigations. From the numerical results, it can be concluded that the relative bending stiffness coefficient and the absolute bending stiffness coefficients at both ends of the carbon nanotube are two important factors that affect the chaotic region of the carbon nanotube, which provides guidance on the design and manufacture of precise micro-nano mechanical systems. In addition, the different routes to the chaos of the carbon nanotube in two extreme cases are revealed.

Hybrid nanostructures of metal/two-dimensional nanomaterials for plasmon-enhanced applications.

Hybrid nanostructures composed of graphene or other two-dimensional (2D) nanomaterials and plasmonic metal components have been extensively studied. The unusual properties of 2D materials are associated with their atomically thin thickness and 2D morphology, and many impressive structures enable the metal nanomaterials to establish various interesting hybrid nanostructures with outstanding plasmonic properties. In addition, the hybrid nanostructures display unique optical characteristics that are derived from the close conjunction of plasmonic optical effects and the unique physicochemical properties of 2D materials. More importantly, the hybrid nanostructures show several plasmonic electrical effects including an improved photogeneration rate, efficient carrier transfer, and a plasmon-induced "hot carrier", playing a significant role in enhancing device performance. They have been widely studied for plasmon-enhanced optical signals, photocatalysis, photodetectors (PDs), and solar cells. In this review, the developments in the field of metal/2D hybrid nanostructures are comprehensively described. Preparation of hybrid nanostructures is first presented according to the 2D material type, as well as the metal nanomaterial morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then described. Lastly, possible future research in this promising field is discussed.

Correction: Hybrid nanostructures of metal/two-dimensional nanomaterials for plasmon-enhanced applications.

Correction for 'Hybrid nanostructures of metal/two-dimensional nanomaterials for plasmon-enhanced applications' by Xuanhua Li et al., Chem. Soc. Rev., 2016, 45, 3145-3187.

Au NPs@MoS2 Sub-Micrometer Sphere-ZnO Nanorod Hybrid Structures for Efficient Photocatalytic Hydrogen Evolution with Excellent Stability.

MoS2 shows promising applications in photocatalytic water splitting, owing to its uniquely optical and electric properties. However, the insufficient light absorption and lack of performance stability are two crucial issues for efficient application of MoS2 nanomaterials. Here, Au nanoparticles (NPs)@MoS2 sub-micrometer sphere-ZnO nanorod (Au NPs@MoS2 -ZnO) hybrid photocatalysts have been successfully synthesized by a facile process combining the hydrothermal method and seed-growth method. Such photocatalysts exhibit high efficiency and excellent stability for hydrogen production via multiple optical-electrical effects. The introduction of Au NPs to MoS2 sub-micrometer spheres forming a core-shell structure demonstrates strong plasmonic absorption enhancement and facilitates exciton separation. The incorporation of ZnO nanorods to the Au NPs@MoS2 hybrids further extends the light absorption to a broader wavelength region and enhances the exciton dissociation. In addition, mutual contacts between Au NPs (or ZnO nanorods) and the MoS2 spheres effectively protect the MoS2 nanosheets from peeling off from the spheres. More importantly, efficiently multiple exciton separations help to restrain the MoS2 nanomaterials from photocorrosion. As a result, the Au@MoS2 -ZnO hybrid structures exhibit an excellent hydrogen gas evolution (3737.4 µmol g-1 ) with improved stability (91.9% of activity remaining) after a long-time test (32 h), which is one of the highest photocatalytic activities to date among the MoS2 based photocatalysts.

Anomalous capacitive behaviors of graphene oxide based solid-state supercapacitors.

Substantial differences in charge storage mechanisms exist between dielectric capacitors (DCs) and electrochemical capacitors (ECs), resulting in orders of magnitude difference of stored charge density in them. However, if ionic diffusion, the major charge transport mechanism in ECs, is confined within nanoscale dimensions, the Helmholtz layers and diffusion layers will overlap, resulting in dismissible ionic diffusion. An interesting contradiction between appreciable energy density and unrecognizable ionic diffusion is observed in solid-state capacitors made from reduced graphene oxide films that challenge the fundamental charge storage mechanisms proposed in such devices. A new capacitive model is proposed, which combines the two distinct charge storage mechanisms of DCs and ECs, to explain the contradiction, of high storage capacity yet undetectable ionic diffusion, seen in graphene oxide based supercapacitors.

Three-dimensional nitrogen-doped multiwall carbon nanotube sponges with tunable properties.

A three-dimensional (3D) nitrogen-doped multiwall carbon nanotube (N-MWCNT) sponge possessing junctions induced by both nitrogen and sulfur was synthesized by chemical vapor deposition (CVD). The formation of "elbow" junctions as well as "welded" junctions, which are attributed to the synergistic effect of the nitrogen dopant and the sulfur promoter, plays a critically important role in the formation of 3D nanotube sponges. To the best of our knowledge, this is the first report showing the synthesis of macroscale 3D N-MWCNT sponges. Most importantly, the diameter of N-MWCNT can be simply controlled by varying the concentration of sulfur, which in turn controls both the sponge's mechanical and its electrical properties. It was experimentally shown that, with increasing diameter of N-MWCNT, the elastic modulus of the sponge increased while the electrical conductivity decreased. The mechanical behaviors of the sponges have also been quantitatively analyzed by employing strain energy function modeling.

Facile decolorization of methylene blue with flower-like manganese wads.

Flower-like manganese wads (MWs) were synthesized via a simple and inexpensive ultrasonic irradiation method for the first time. MWs were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray and transmission electronic microscopy. The decolorization efficiency of MWs for methylene blue (MB) azo dye was examined as a function of pH, stirring time, MW dosage and initial concentration of the MB solution. Results show that MWs have excellent decolorization performance for MB with a higher efficiency (and without using H2O2 or other devises such as UV light and ultrasonic irradiation) compared to other catalysts, such as the mixture of Mn3O4 and H2O2 (with a maximum decolorization rate of 99.7% in 3 h), ZnS and CdS under light irradiation (with a maximum decolorization rate of 73% in 6 h), and sulfate modified titania under solar radiation (with a maximum decolorization rate of nearly 100% in 4 h).

High Density 3D Carbon Tube Nanoarray Electrode Boosting the Capacitance of Filter Capacitor.

Electric double-layer capacitors (EDLCs) with fast frequency response are regarded as small-scale alternatives to the commercial bulky aluminum electrolytic capacitors. Creating carbon-based nanoarray electrodes with precise alignment and smooth ion channels is crucial for enhancing EDLCs' performance. However, controlling the density of macropore-dominated nanoarray electrodes poses challenges in boosting the capacitance of line-filtering EDLCs. Herein, a simple technique to finely adjust the vertical-pore diameter and inter-spacing in three-dimensional nanoporous anodic aluminum oxide (3D-AAO) template is achieved, and 3D compactly arranged carbon tube (3D-CACT) nanoarrays are created as electrodes for symmetrical EDLCs using nanoporous 3D-AAO template-assisted chemical vapor deposition of carbon. The 3D-CACT electrodes demonstrate a high surface area of 253.0 m2 g-1, a D/G band intensity ratio of 0.94, and a C/O atomic ratio of 8. As a result, the high-density 3D-CT nanoarray-based sandwich-type EDLCs demonstrate a record high specific areal capacitance of 3.23 mF cm-2 at 120 Hz and exceptional fast frequency response due to the vertically aligned and highly ordered nanoarray of closely packed CT units. The 3D-CT nanoarray electrode-based EDLCs could serve as line filters in integrated circuits, aiding power system miniaturization.

Flexible all solid-state supercapacitors based on chemical vapor deposition derived graphene fibers.

Flexible all-solid-state supercapacitors based on graphene fibers are demonstrated in this study. Surface-deposited oxide nanoparticles are used as pseudo-capacitor electrodes to achieve high capacitance. This supercapacitor electrode has an areal capacitance of 42 mF cm(-2), which is comparable to the capacitance for fiber-based supercapacitors reported to date. During the bending and cycling of the fiber-based supercapacitor, the stability could be maintained without sacrificing the electrochemical performance, which provides a novel and simple way to develop flexible, lightweight and efficient graphene-based devices.

Dynamic and galvanic stability of stretchable supercapacitors.

Stretchable electronics are emerging as a new technological advancement, since they can be reversibly stretched while maintaining functionality. To power stretchable electronics, rechargeable and stretchable energy storage devices become a necessity. Here, we demonstrate a facile and scalable fabrication of full stretchable supercapacitor, using buckled single-walled carbon nanotube macrofilms as the electrodes, an electrospun membrane of elastomeric polyurethane as the separator, and an organic electrolyte. We examine the electrochemical performance of the fully stretchable supercapacitors under dynamic stretching/releasing modes in different stretching strain rates, which reveal the true performance of the stretchable cells, compared to the conventional method of testing the cells under a statically stretched state. In addition, the self-discharge of the supercapacitor and the electrochemical behavior under bending mode are also examined. The stretchable supercapacitors show excellent cyclic stability under electrochemical charge/discharge during in situ dynamic stretching/releasing.

Na Storage in Sb2S3@C Composite: A Synergistic Capacity Contribution Mechanism with Wider Temperature Adaptability.

The practical applications of metallic anodes are limited due to dendritic growth, propagation in an infinite volume during the plating process, and parasitic interfacial reactions between sodium (Na) and the electrolyte. Herein, we developed Sb2S3 microrods as a template to regulate the nucleation of metallic Na. Additionally, the propagation of the deposited metal could be spatially regulated via a "nanoconfinement effect", that is, within the conformal hard carbon (C) layer of nanothickness. Moreover, we carefully studied the seed effect of the in situ-formed Na-Sb and Na-S alloys within the hard C sheath during the Na plating process. The symmetrical cells of the Sb2S3@C composite anode achieved dendrite-free cycling at 1 mA cm-2 for 1100 h at a high capacity loading of 1 mA h cm-2 and considerably mitigated a nucleation overpotential of 20 mV. Pairing a NaVPO4F (NVPF) cathode (4.6 mg cm-2) with an in situ presodiation Sb2S3@C composite (2*Na excess) prototype delivered a high energy density and a high power density of 173.75 W h kg-1 and 868.57 W kg-1, respectively. Therefore, this study provides tremendous possibilities for employing the proposed hybrid storage mechanism in low-cost and practical applications of high-energy-density Na metal batteries.

Boosting large-current-density water oxidation activity and stability by phytic acid-assisted rapid electrochemical corrosion.

Corrosion engineering is an efficient strategy to achieve durable oxygen evolution reaction (OER) catalysts at high current densities beyond 500 mA cm-2. However, the spontaneous electrochemical corrosion has a slow reaction rate, and most of them need to add large amounts of salts (such as NaCl) to accelerate the corrosion process. In this report, a novel and effective phytic acid (PA)-assisted in situ electrochemical corrosion strategy is demonstrated to accelerate the the corrosion process and form bimetallic active catalysts to show excellent OER performance at large current densities. In situ rapid electrochemical corrosion of nickel foam substrate and PA ligands etching realize localized high concentrations of Ni and Fe ions. High concentrations of metal ions will combine with hydroxyl to effectively form defects-enriched NiFe layered double hydroxides porous nanosheets tightly anchoring on the underneath substrate. Remarkably, the activated electrode exhibits excellent OER catalytic activities with ultralow overpotentials of 289 and 315 mV to reach high current densities of 500 and 1000 mA cm-2, respectively. When coupled with Ni-Mo-N hydrogen evolution reaction catalysts, the two-electrode cell merely requires 1.87 V to deliver 1000 mA cm-2. The ligands-assisted rapid electrochemical corrosion strategy provides a fresh perspective for facile, cost-effective, and scale-up production of superior OER catalysts at large current densities.

Near Zero-Threshold Voltage P-N Junction Diodes Based on Super-Semiconducting Nanostructured Ag/Al Arrays.

Semiconductor devices are currently one of the most common energy consumption devices. Significantly reducing the energy consumption of semiconductor devices with advanced energy-efficient technologies is highly desirable. The discovery of super-semiconductors (SSCs) based on metallic bi-layer shell arrays provides an opportunity to realize ultra-low-power consumption semiconductor devices. As an example, the achievement of near zero-threshold voltage in p-n junction diodes based on super-semiconducting nanostructured Ag/Al arrays is reported, realizing ultra-low-power p-n junction diodes: ≈3 W per trillion diodes with a working voltage of 1 V or 30 mW per trillion diodes with an operating voltage of 0.1 V. In addition, the p-n junction diodes exhibit a high breakdown field of ≈1.1 × 106  V cm-1 , similar to that of SiC and GaN, due to a robust built-in field driven by infrared light photons. The SSC p-n diodes with near zero-threshold voltage and high breakdown field allow access to ultra-low-power semiconducting transistors, integrated circuits, chips, etc.

In situ photocatalytically enhanced thermogalvanic cells for electricity and hydrogen production.

High-performance thermogalvanic cells have the potential to convert thermal energy into electricity, but their effectiveness is limited by the low concentration difference of redox ions. We report an in situ photocatalytically enhanced redox reaction that generates hydrogen and oxygen to realize a continuous concentration gradient of redox ions in thermogalvanic devices. A linear relation between thermopower and hydrogen production rate was established as an essential design principle for devices. The system exhibited a thermopower of 8.2 millivolts per kelvin and a solar-to-hydrogen efficiency of up to 0.4%. A large-area generator (112 square centimeters) consisting of 36 units yielded an open-circuit voltage of 4.4 volts and a power of 20.1 milliwatts, as well 0.5 millimoles of hydrogen and 0.2 millimoles of oxygen after 6 hours of outdoor operation.