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The need for enhanced energy storage and improved catalysts has led researchers to explore advanced functional materials for sustainable energy production and storage. Herein, we demonstrate a reductive electrosynthesis approach to prepare a layer-by-layer (LbL) assembled trimetallic Fe-Co-Ni metal-organic framework (MOF) in which the metal cations within each layer or at the interface of the two layers are linked to one another by bridging 2-amino-1,4-benzenedicarboxylic acid linkers. Tailoring catalytically active sites in an LbL fashion affords a highly porous material that exhibits excellent trifunctional electrocatalytic activities toward the hydrogen evolution reaction (eta(j=10) = 116 mV), oxygen evolution reaction (eta(j=10) = 254 mV), as well as oxygen reduction reaction (half-wave potential = 0.75 V vs reference hydrogen electrode) in alkaline solutions. The dispersion-corrected density functional theory calculations suggest that the prominent catalytic activity of the LbL MOF toward the HER, OER, and ORR is due to the initial negative adsorption energy of water on the metal nodes and the elongated O-H bond length of the H2O molecule. The Fe-Co-Ni MOF-based Zn-air battery exhibits a remarkable energy storage performance and excellent cycling stability of over 700 cycles that outperform the commercial noble metal benchmarks. When assembled in an asymmetric device configuration, the activated carbonliFe-Co-Ni MOF supercapacitor provides a superb specific energy and a power of up to 56.2 W h kg(-1) and 42.2 kW kg(-1), respectively. This work offers not only a novel approach to prepare an LbL assembled multimetallic MOF but also provides a benchmark for a multifunctional electrocatalyst for water splitting and Zn-air batteries.

Because of increasing interest in environmentally benign supercapacitors, earth-abundant biopolymers have found their way into value-added applications. Herein, a promising nanocomposite based on an interpenetrating network of polyaniline and sulfonated lignin (lignosulfonate, LS) is presented. On the basis of an appropriate regulation of the nucleation kinetics and growth behavior via applying a series of rationally designed potential pulse patterns, a uniform PANI-LS film is achieved. On the basis of the fast rate of H+ insertion-deinsertion kinetics, rather than the slow SO42- doping-dedoping process, the PANI-LS nanocomposite delivers specific capacitance of 1200 F g-1 at 1 A g-1 surpassing the best conducting polymer-lignin supercapacitors known. A symmetric PANI-LS||PANI-LS device delivers a high specific energy of 21.2 W h kg-1, an outstanding specific power of 26.0 kW kg-1, along with superb flexibility and excellent cycling stability. Thus, combining charge storage attributes of polyaniline and lignosulfonate enables a waste-to-wealth approach to improve the supercapacitive performance of polyaniline.

Despite their low cost, safety, environmentally friendliness, and intrinsic non-flammable nature, the widespread application of aqueous rechargeable zinc-based batteries has been held back by their low coulombic efficiency and the notorious dendritic growth at the zinc-based anodes, along with the fast capacity fading of the cathodes. Herein, an aqueous Zn superbattery that consists of a mixed ZnCO3 MnCO3 grafted onto a graphene aerogel (ZMG) negative electrode and a nanotubular sulfidated Ni-Co-Fe layered double hydroxide (LDHS) positive electrode is reported. The alkaline ZMGLDHS superbattery delivers an excellent capacity and a superb rate capability (356 mA h g(cathode)(-1) (89 mA h g(total mass)(-1)) at 12 A g(-1); 108 mA h g(cathode)(-1) at 300 A g(-1)), extremely high specific energy and power (568 W h kg(cathode)(-1) or 15.8 mW h cm(-3), 429 kW kg(cathode)(-1) or 11.9 W cm(-3)), along with a high output voltage (1.8 V). The device also exhibits unprecedented cycling stability (99.2% capacity retention after 17,000 cycles at 100% depth of discharge) thanks to an electrochemical pulse-driven regenerative mechanism.

Transition-metal chalcogenides have emerged as a promising class of materials for energy storage applications due to their earth abundance, high theoretical capacity, and high electrical conductivity. Herein, we introduce a facile and one-pot electrodeposition method to prepare high-performance nickel selenide NixSey (0.5 Ni3Se2 > NiSe > Ni6Se5. The present study thus provides an appealing approach for tailoring the phase composition of NS as an alternative to the commonly used templated synthesis methods.

The pursuit of new negative electrode materials for redox supercapacitors with a high capacitance, boosted energy, and high rate capability is still a tremendous challenge. Herein, we report a Nile Blue conjugated graphene aerogel (NB–GA) as a negative electrode material with excellent pseudocapacitive performance (with specific capacitance of up to 483 F g–1 at 1 A g–1) in all acidic, neutral, and alkaline aqueous electrolytes. The contribution from capacitive charge storage represents 93.4% of the total charge, surpassing the best pseudocapacitors known. To assess the feasibility of NB–GA as a negative electrode material across the full pH range, we fabricated three devices, namely, a symmetric NB–GA||NB–GA device in an acidic (1.0 M H2SO4) electrolyte, an NB–GA||MnO2 device in a pH-neutral (1.0 M Na2SO4) electrolyte, and an NB–GA||LDH (LDH = Ni–Co–Fe layered double hydroxide) device in an alkaline (1.0 M KOH) electrolyte. The NB–GA||NB–GA device exhibits a maximum specific energy of 22.1 Wh kg–1 and a ...

High power performance with the ability to be charged in a matter of seconds have made supercapacitors the technology of choice for cutting-edge applications. However, improving the energy content of supercapacitors is necessary to contribute to the technological advancement of energy storage devices. Herein, we report the synthesis of two transition metal spinel oxide-reduced graphene oxide (rGO) nanocomposites; namely CuCo2O4-rGO and CoFe2O4-rGO. The CuCo2O4-rGO nanocomposite as a positive electrode exhibits an extremely high specific capacitance of 2064.0 F g−1 at 2 A g−1, whereas the CoFe2O4-rGO nanocomposite as a negative electrode shows a specific capacitance of 261.0 F g−1 at 2 A g−1. The asymmetric CoFe2O4-rGO//CuCo2O4-rGO device exhibits an ultrahigh specific energy of 77.2 Wh kg−1 at 953.0 W kg−1, maintains 8.3 Wh kg−1 under the outstanding specific power of 21.0 kW kg−1, and displays excellent cycling stability (96% capacitance retention after 5000 cycles). We also fabricated an asymmetric AC//CuCo2O4-rGO device (AC = activated carbon) as a control, and realized that the CoFe2O4-rGO nanocomposite not only outperforms AC in terms of specific energy, but also contributes to a higher specific power. The results indicate that these nanocomposites are promising materials for the fabrication of high performance supercapacitors.

The development of power generators that can function in harsh snowy environments and in contact with snow can be beneficial but challenging to accomplish. Herein, we introduce the first snow-based triboelectric nanogenerator (snow-TENG) that can be used as an energy harvester and a multifunctional sensor based on the principle of snow-triboelectrification. In this work, we used a 3D printing technique for the precise design and deposition of the electrode and triboelectric layer, leading to flexible, stretchable and metal-free triboelectric generators. Based on the single electrode mode, the device can generate an instantaneous output power density as high as 0.2 mW/m(2), an open circuit voltage up to 8 V, and a current density of 40 μA/m(2). In addition, the snow-TENG can function as a miniaturized weather station to monitor the weather in real time to provide accurate information about the snowfall rate, snow accumulation depth, wind direction, and speed in snowy and/or icy environments. In addition, the snow-TENG can be used as a wearable power source and biomechanical sensor to detect human body motions, which may prove useful for snow-related sports. Unlike conventional sensor platforms, our design works without the need for batteries or image processing systems. We envision these devices could potentially be integrated into solar panels to ensure continuous power supply during snowy weather conditions.

We report fabrication of flexible all-solid-state transparent electrochromic patterned microsupercapacitors based on two-dimensional layered nanostructured molybdenum oxide (MoO3–x)/poly(3,4-ethyle...

Carbon-based supercapacitors exhibit great rate capability, power density and cycle life, but suffer from relatively low energy density. Polyaniline provides high specific capacitance, but lacks cycling stability. By combining carbon-based materials with tetraaniline, an oligomer of polyaniline, a hybrid composite is formed that demonstrates improved supercapacitor performance relative to either material alone. In this study, the reduced graphene oxide-oligoaniline composites have been synthesized by a one-step hydrothermal process without the need for adding any oxidizing or reducing agents. FTIR, Raman spectroscopy, XPS, and MALDI-TOF mass spectroscopy indicate the successful reduction of GO to rGO and the formation of aniline oligomers. Unlike most polyaniline nanostructures for which charge storage kinetics are limited by slow diffusion-controlled reactions, the majority of oligoaniline in this composite is exposed to the electrolyte and stores charge through fast surface-controlled reactions. The unique microstructure of the rGO-oligoaniline composites facilitates transport of ions and electrons, leading to greater utilization of the active materials, high specific capacitance of 640 F/g at 0.2 mA/cm2 (corresponding to 707 C/g specific capacity), great rate capability and good cycle stability (91% retention after 2000 cycles).

The development of highly sensitive sensors and power generators that could function efficiently in extreme temperatures and contact with fire can be lifesaving but challenging to accomplish. Herein, we report, for the first time, a fire-retardant and self-extinguishing triboelectric nanogenerator (FRTENG), which can be utilized as a motion sensor and/or power generator in occupations such as oil drilling, firefighting or working in extreme temperature environments with flammable and combustible materials. The device takes advantage of the excellent thermal properties of carbon derived from resorcinol-formaldehyde aerogel whose electrical, mechanical and triboelectric properties have been improved via the introduction of Polyacrylonitrile nanofibers and graphene oxide nanosheets. This FRTENG is not flammable even after 90 s of trying, whereas conventional triboelectric materials were entirely consumed by fire under the same conditions. The developed device shows exceptional charge transfer characteristics, leading to a potential difference up to 80 V and a current density up to 25 µA/m2. When integrated into firefighter's shoes, the FRTENG is able to discern the movements of a firefighter in hazardous situations, while providing the high thermal stability missing in conventional TENGs. The fire-retardant and self-extinguishing characteristics offered by the FRTENG makes it a path-breaking device for lifesaving wearable applications.

Micro-supercapacitors (MSCs) are one of the most promising electronic circuit elements in electronic devices as they are designed for use as energy storage components, decoupling capacitors, ripple current filter elements, and in many other applications. The materials used for MSCs have been greatly restricted since only a few materials or material combinations are capable of operating at the required high frequencies with high power and sufficient stability for long-term operation [1]. By using graphene and its derivatives as electrodes, here we demonstrate that MSCs with ultrahigh power density will push further the monolithic integration and the realization of smart multifunctional autonomous system-on-a-chip.

We report our discovery of utilizing perhydroxylated dodecaborate clusters ([B12(OH)12]2-) as a molecular cross-linker to generate a hybrid tungsten oxide material. We further demonstrate how these robust B12-based clusters in the resulting hybrid tungsten oxide material can effectively preserve the specific capacitance up to 4000 cycles and reduce the charge transfer resistance as well as the response time compared to that of pristine tungsten oxide.

Carbon materials are widely used for supercapacitor applications thanks to their high surface area, good rate capability, and excellent cycling stability. However, the development of high energy density carbon supercapacitors still remains a challenge. In this work, hollow Co3O4 nanoboxes have been embedded into three-dimensional macroporous laser-scribed graphene (LSG) to produce composite electrodes with improved electrochemical performance. Here, Co3O4 provides high capacity through fast and reversible redox reactions, while LSG serves as a conductive network to maintain high power. The open nanobox morphology is a unique solution for extracting the maximum capacity from Co3O4, resulting in electrodes whose surfaces, both internal and external, are accessible to the electrolyte. The electrochemical performance of the composite material is promising with a volumetric capacity of 60.0 C/cm3 and a specific capacity of 542.3 C/g, corresponding to 682.0 C/g of the constituent Co3O4. With a low equivalent series resistance of 0.9 Ω, the Co3O4/LSG electrode is able to maintain 113.1% of its original capacity after 10,000 cycles. This work provides new insights into the design of high-performance carbon/metal oxide nanocomposites for next-generation energy storage devices.

Graphene is extensively utilized in energy storage devices because of its high surface area and electronic conductivity as well as ease of electrode fabrication. But graphene sheets often stack themselves in polymeric matrices leading to poor capacitive performance. This problem was addressed herein by developing and inserting respectively two types of nano-sized conducting polymers into graphene interlayer spacing. The resulting hydrogel composite electrodes demonstrated efficient electron transfer for fast and reversible Faradaic reactions at the interface. Theoretical modelling by the density functional theory suggested that the reduction involve 2H + transfer steps from polyaniline to graphene oxide: the first step would be an epoxy-ring opening process after activation of the C-O bond, and the second step would be C-O rupture leading to a de-epoxidation process. This binder-free electrode demonstrated high cycling performance and ultrahigh volumetric capacitance of 612 F cm −3 , being 10 times higher than the activated carbon used in the current industry. The study represents a step forward towards the fabrication of flexible, high-energy density super capacitors. Refereed/Peer-reviewed

With rising energy consumption in the world and the negative environmental and human health impacts of fossil fuels, the demand for renewable energy sources is increasing. The energy generated by renewable energy sources can be stored either in a chemical (water splitting) or an electrochemical (battery or supercapacitor) form, that are two distinct processes. Here, we introduce an integrated solar-powered system for both electrochemical energy storage and water electrolysis. A nickel-cobalt-iron layered double hydroxide (Ni-Co-Fe LDH) was successfully synthesized on nickel foam as a substrate using a fast, one-step electrodeposition approach. The Ni-Co-Fe LDH exhibited excellent electrochemical properties both as an active electrode material in supercapacitors, and as a catalyst in the oxygen evolution reaction (OER). When employed as the positive electrode in a supercapacitor, along with activated carbon as the negative electrode in an asymmetric configuration, the ultrathin and porous Ni-Co-Fe LDH nanoplatelets delivered an ultrahigh specific energy of 57.5 W h kg −1 with an outstanding specific power of 37.9 kW kg −1 and an excellent cycle life. As an OER electrocatalyst, Ni-Co-Fe LDH exhibited superior electrocatalytic performances with a very low overpotential of 0.207 V versus a reference hydrogen electrode (RHE) at 10.0 mA cm −2 , and a small Tafel slope of 31 mV dec −1 . The superior energy storage and catalytic OER properties of the Ni-Co-Fe LDH nanoplatelet array can be attributed to both the synergistic effects among the metal species and the unique mesoporous structure of the LDH that provides facilitated charge/ion diffusion pathways and more available active sites.

While the market for supercapacitors is rapidly growing due to their high power density, their low energy density compared to batteries represents a great barrier for the future of this technology. The poorly understood chemistry of electrode-electrolyte interfaces implies that there is substantial room for improvement through a careful design of the materials involved. Here we present a unique approach for improving the energy density of supercapacitors through redox additive-assisted electrocatalytic in situ regeneration of the electrode active materials. By utilizing a quinone-based redox electrolyte and a nanostructured conjugated polyaniline electrode, we continually regenerate the reactants, resulting in a redox supercapacitor having an extremely high energy density of 1091 Wh kg−1 (based on the total mass of the electrode active materials and the redox additive) and a high power density up to 196 kW kg−1. Considering the other outstanding properties of the polyaniline-naphthoquinone system, such as extreme flexibility (96% capacity retention after bending at an angle of 180° for 1000 cycles), non-flammability, and excellent cycling stability (84% capacity retention after 7000 cycles at 35 A g−1), such a well designed in situ regeneration of the electrode active materials makes this method a very promising approach towards the development of state-of-the-art energy storage devices.

Aqueous supercapacitors based on neutral solutions have the advantages of high-ionic conductivity, being environmentally friendly, safe, and low cost. However, the operating potential window for most aqueous electrolytes is far lower than that of organic electrolytes that are commonly used in commercial supercapacitors. In this work, we report on the fabrication of a wide potential window, high-energy aqueous asymmetric supercapacitor, without sacrificing power, by using a nanostructured LiMn2O4/reduced graphene oxide (LMO–rGO) nanocomposite. We synthesized the uniformly distributed LMO in the LMO–rGO nanocomposite using a co-precipitation route followed by a low-temperature hydrothermal treatment. In a three-electrode cell setup, the specific capacitance of the LMO–rGO nanocomposite electrode at 1 A/g (1.2 mA/cm2) is 268.75 F/g (258 mF/cm2), which shows a dramatic improvement over the sum of the specific capacitances of pristine LMO (162.5 F/g) and pure rGO (29.94 F/g) electrodes in their relative ratios, when used alone. This finding suggests a synergistic coupling of LMO and rGO in the nanocomposite. We also assembled the LMO–rGO nanocomposite, as the positive electrode, with activated carbon, as the negative electrode, into an asymmetric cell configuration. The device shows an ultra-wide potential window of 2.0 V in a neutral aqueous Li2SO4 electrolyte, with a maximum energy density of 29.6 Wh/kg (which approaches the commercial lead-acid batteries), power density of up to 7408 W/kg, and an excellent cycle life (5% loss after 6000 cycles). These findings confirm that an LMO–rGO nanocomposite is a promising material to meet the demands of real world energy storage.

Since their debut in 2012, triboelectric nanogenerators (TENGs) have attained high performance in terms of both energy density and instantaneous conversion, reaching up to 500 W m−2 and 85%, respectively, synchronous with multiple energy sources and hybridized designs. Here, a comprehensive review of the design guidelines of TENGs, their performance, and their designs in the context of Internet of Things (IoT) applications is presented. The development stages of TENGs in large‐scale self‐powered systems and technological applications enabled by harvesting energy from water waves or wind energy sources are also reviewed. This self‐powered capability is essential considering that IoT applications should be capable of operation anywhere and anytime, supported by a network of energy harvesting systems in arbitrary environments. In addition, this review paper investigates the development of self‐charging power units (SCPUs), which can be realized by pairing TENGs with energy storage devices, such as batteries and capacitors. Consequently, different designs of power management circuits, supercapacitors, and batteries that can be integrated with TENG devices are also reviewed. Finally, the significant factors that need to be addressed when designing and optimizing TENG‐based systems for energy harvesting and self‐powered sensing applications are discussed., Self‐powered and integrated triboelectric nanogenerators are comprehensively reviewed based on blue energy, their performance, and hybrid designs under the context of large‐scale systems and Internet of Things applications.

To meet the future need for clean and sustainable energies, there has been considerable interest in the development of triboelectric nanogenerators (TENGs) that scavenge waste mechanical energies. The performance of a TENG at the macroscale is determined by the multifaceted role of surface and interface properties at the nanoscale, whose understanding is critical for the future development of TENGs. Therefore, various protocols from the atomic to the macrolevel for fabrication and tuning of surfaces and interfaces are required to obtain the desired TENG performance. These protocols branch out into three categories: chemical engineering, physical engineering, and structural engineering. Chemical engineering is an affordable and optimal strategy for introducing more surface polarities and higher work functions for the improvement of charge transfer. Physical engineering includes the utilization of surface morphology control, and interlayer interactions, which can enhance the active interfacial area and electron transfer capacity. Structural engineering at the macroscale, which includes device and electrode design/modifications has a considerable effect on the performance of TENGs. Future challenges and promising research directions related to the construction of next-generation TENG devices, taking into consideration “interfaces” are also presented.

In this paper, we demonstrate the use of triboelectric nanogenerator (TENG) as a mean of mechanical light triggering to control InGaN-based light-emitting diodes (LEDs). Light extraction from the LED is two successive steps process. First, the voltage produced by the TENG is used to control the gate-to-source current of a MOSFET transistor through adjusting the transistor channel width and length. The second step is forwarding the drain-source current resulting from MOSFET transistor to the LED as its injection current to induce spontaneous emission from the LED surface to the air. Three LED colors are considered: red, green and blue. Significant emitted power from these InGaN-based LEDs in the RGB wavelength band is observed for both P-MOSFET and N-MOSFET transistor configurations. The emitted optical spectrum is controlled by optimizing the combined TENG-MOSFET-RGB LED geometry; dimensions and the bias voltage between the drain and source terminals of the MOSFET transistor. With recent advances in TENGs as an energy harvesting technology, it is expected that this study offers an approach to enhance the light extraction of various LED devices. With the enhancements in the performance of optoelectronic devices, the field of tribo-phototronics has attracted more attention, and in this work, we introduce the first theoretical framework, to the best of our knowledge, based on finite element modeling. This study provides significant insights into the working principles of tribo-Phototronic devices as well as guidelines for future device design.

The surging interest in high performance, low-cost, and safe energy storage devices has spurred tremendous research efforts in the development of advanced electrode active materials. Herein, the in situ growth of zinc-iron layered double hydroxide (Zn-Fe LDH) on graphene aerogel (GA) substrates through a facile, one-pot hydrothermal method is reported. The strong interaction and efficient electronic coupling between LDH and graphene substantially improve interfacial charge transport properties of the resulting nanocomposite and provide more available redox active sites for faradaic reactions. An LDH-GA||Ni(OH)2 device is also fabricated that results in greatly enhanced specific capacity (187 mAh g-1 at 0.1 A g-1 ), outstanding specific energy (147 Wh kg-1 ), excellent specific power (16.7 kW kg-1 ), along with 88% capacity retention after >10 000 cycles. This approach is further extended to Ni-MH and Ni-Cd batteries to demonstrate the feasibility of compositing with graphene for boosting the energy storage performance of other well-known Ni-based batteries. In contrast to conventional Ni-based batteries, the nearly flat voltage plateau followed by a sloping potential profile of the integrated supercapacitor-battery enables it to be discharged down to 0 V without being damaged. These findings provide new prospects for the design of high-performance and affordable superbatteries based on earth-abundant elements.

Over the past decade, electrochemical energy storage (EES) devices have greatly improved, as a wide variety of advanced electrode active materials and new device architectures have been developed. These new materials and devices should be evaluated against clear and rigorous metrics, primarily based on the evidence of real performances. A series of criteria are commonly used to characterize and report performance of EES systems in the literature. However, as advanced EES systems are becoming more and more sophisticated, the methodologies to reliably evaluate the performance of the electrode active materials and EES devices need to be refined to realize the true promise as well as the limitations of these fast-moving technologies, and target areas for further development. In the absence of a commonly accepted core group of metrics, inconsistencies may arise between the values attributed to the materials or devices and their real performances. Herein, we provide an overview of the energy storage devices from conventional capacitors to supercapacitors to hybrid systems and ultimately to batteries. The metrics for evaluation of energy storage systems are described, although the focus is kept on capacitive and hybrid energy storage systems. In addition, we discuss the challenges that still need to be addressed for establishing more sophisticated criteria for evaluating EES systems. We hope this effort will foster ongoing dialog and promote greater understanding of these metrics to develop an international protocol for accurate assessment of EES systems.

Ongoing technological advances in diverse fields including portable electronics, transportation, and green energy are often hindered by the insufficient capability of energy-storage devices. By taking advantage of two different electrode materials, asymmetric supercapacitors can extend their operating voltage window beyond the thermodynamic decomposition voltage of electrolytes while enabling a solution to the energy storage limitations of symmetric supercapacitors. This review provides comprehensive knowledge to this field. We first look at the essential energy-storage mechanisms and performance evaluation criteria for asymmetric supercapacitors to understand the wide-ranging research conducted in this area. Then we move to the recent progress made for the design and fabrication of electrode materials and the overall structure of asymmetric supercapacitors in different categories. We also highlight several key scientific challenges and present our perspectives on enhancing the electrochemical performance of future asymmetric supercapacitors.

Carbon materials are widely used in supercapacitors because of their high surface area, controlled porosity and ease of processing into electrodes. The combination of carbon with metal oxides results in hybrid electrodes with higher specific capacitance than pure carbon electrodes, which has so far limited the energy density of supercapacitors currently available commercially. However, the preparation and processing of carbon/metal oxide electrodes into supercapacitors of different structures and configurations, especially for miniaturized electronics, has been challenging. Here, we demonstrate a simple one-step process for the synthesis and processing of laser-scribed graphene/RuO 2 nanocomposites into electrodes that exhibit ultrahigh energy and power densities. Hydrous RuO 2 nanoparticles were successfully anchored to graphene surfaces through a redox reaction of the precursors, graphene oxide, and RuCl 3 using a consumer grade LightScribe DVD burner with a 788 nm laser. This binder-free, metal current collector-free graphene/RuO 2 film was then used directly as a hybrid electrochemical capacitor electrode, demonstrating much-improved cycling stability and rate-capability with a specific capacitance up to 1139 F g − 1 . We employed these hybrid electrodes for building aqueous-based symmetric and asymmetric cells that can deliver energy densities up to 55.3 Wh kg −1 , placing them among the best performing hybrid electrochemical capacitors. Furthermore, this technique was used for the direct writing of interdigitated hybrid micro-supercapacitors in a single step for the first time, with great potential for miniaturized electronics. This simple approach provides a general strategy for making a wide range of composite materials for a variety of applications.

Capacitors. Open up your computer and they stick out like rocks on a sandy beach. They're the one kind of electronic device that never made it to Lilliput. If they finally obeyed Moore's Law by squeezing themselves down to the microscale, it would make life a lot easier for electronics engineers.

The search for faster, safer, and more efficient energy storage systems continues to inspire researchers to develop new energy storage materials with ultrahigh performance. Mesoporous nanostructures are interesting for supercapacitors because of their high surface area, controlled porosity, and large number of active sites, which promise the utilization of the full capacitance of active materials. Herein, highly ordered mesoporous CuCo2O4 nanowires have been synthesized by nanocasting from a silica SBA-15 template. These nanowires exhibit superior pseudocapacitance of 1210 F g–1 in the initial cycles. Electroactivation of the electrode in the subsequent 250 cycles causes a significant increase in capacitance to 3080 F g–1. An asymmetric supercapacitor composed of mesoporous CuCo2O4 nanowires for the positive electrode and activated carbon for the negative electrode demonstrates an ultrahigh energy density of 42.8 Wh kg–1 with a power density of 15 kW kg–1 plus excellent cycle life. We also show that two a...

Supercapacitors now play an important role in the progress of hybrid and electric vehicles, consumer electronics, and military and space applications. There is a growing demand in developing hybrid supercapacitor systems to overcome the energy density limitations of the current generation of carbon-based supercapacitors. Here, we demonstrate 3D high-performance hybrid supercapacitors and microsupercapacitors based on graphene and MnO2 by rationally designing the electrode microstructure and combining active materials with electrolytes that operate at high voltages. This results in hybrid electrodes with ultrahigh volumetric capacitance of over 1,100 F/cm(3). This corresponds to a specific capacitance of the constituent MnO2 of 1,145 F/g, which is close to the theoretical value of 1,380 F/g. The energy density of the full device varies between 22 and 42 Wh/l depending on the device configuration, which is superior to those of commercially available double-layer supercapacitors, pseudocapacitors, lithium-ion capacitors, and hybrid supercapacitors tested under the same conditions and is comparable to that of lead acid batteries. These hybrid supercapacitors use aqueous electrolytes and are assembled in air without the need for expensive "dry rooms" required for building today's supercapacitors. Furthermore, we demonstrate a simple technique for the fabrication of supercapacitor arrays for high-voltage applications. These arrays can be integrated with solar cells for efficient energy harvesting and storage systems.

The demand for flexible/wearable electronic devices that have aesthetic appeal and multi-functionality has stimulated the rapid development of flexible supercapacitors with enhanced electrochemical performance and mechanical flexibility. After a brief introduction to flexible supercapacitors, we summarize current progress made with graphene-based electrodes. Two recently proposed prototypes for flexible supercapacitors, known as micro-supercapacitors and fiber-type supercapacitors, are then discussed. We also present our perspective on the development of graphene-based electrodes for flexible supercapacitors.

Using nanoparticles, instead of microparticles, as active electrode materials in lithium ion batteries could provide a solution to slow charging rates due to long ion diffusion pathways in conventional bulk materials. In this work, we present a new strategy for the synthesis of high purity lithium nickel manganese oxide (LiNi0.5Mn1.5O4) nanoparticles as a high-voltage cathode. A sonochemical reaction is used to synthesize nickel hydroxide and manganese dioxide nanoparticles followed by a solid-state reaction with lithium hydroxide. The product shows a single spinel phase and uniform spherical nano-particles under the appropriate calcination conditions. The LiNi0.5Mn1.5O4 exhibits a high voltage plateau at about 4.7–4.9 V in the charge/discharge process and delivers a discharge capacity of more than 140 mA h g−1 and excellent cycling performance with 99% capacity retention after 70 cycles. The synthesized nano-particles show improved electrochemical performance at high rates. This electrode delivers a power density as high as 26.1 kW kg−1 at a discharge rate of 40 C. This power performance is about one order of magnitude higher than traditional lithium ion batteries. These findings may lead to a new generation of high power lithium ion batteries that can be recharged in minutes instead of hours.

One of the fundamental issues with graphene for logic applications is its lack of a band gap. In this issue of ACS Nano, Shim and colleagues introduce an effective approach for modulating the current flow in graphene by forming p-n junctions using lasers. The findings could lead to a new route for controlling the electronic properties of graphene-based devices. We highlight recent progress in the direct laser synthesis and patterning of graphene for numerous applications. We also discuss the challenges and opportunities in translating this remarkable progress toward the direct laser writing of graphene electronics at large scales.

Nanostructures of the conducting polymer poly(3,4-ethylenedioxythiophene) with large surface areas enhance the performance of energy storage devices such as electrochemical supercapacitors. However, until now, high aspect ratio nanofibers of this polymer could only be deposited from the vapor-phase, utilizing extrinsic hard templates such as electrospun nanofibers and anodized aluminum oxide. These routes result in low conductivity and require postsynthetic template removal, conditions that stifle the development of conducting polymer electronics. Here we introduce a simple process that overcomes these drawbacks and results in vertically directed high aspect ratio poly(3,4-ethylenedioxythiophene) nanofibers possessing a high conductivity of 130 S/cm. Nanofibers deposit as a freestanding mechanically robust film that is easily processable into a supercapacitor without using organic binders or conductive additives and is characterized by excellent cycling stability, retaining more than 92% of its initial capacitance after 10,000 charge/discharge cycles. Deposition of nanofibers on a hard carbon fiber paper current collector affords a highly efficient and stable electrode for a supercapacitor exhibiting gravimetric capacitance of 175 F/g and 94% capacitance retention after 1000 cycles.

There is significant interest in the development of methods to create hybrid materials that transform capabilities, in particular for Earth-abundant metal oxides, such as TiO2, to give improved or new properties relevant to a broad spectrum of applications. Here we introduce an approach we refer to as ‘molecular cross-linking’, whereby a hybrid molecular boron oxide material is formed from polyhedral boron-cluster precursors of the type [B12(OH)12]2–. This new approach is enabled by the inherent robustness of the boron-cluster molecular building block, which is compatible with the harsh thermal and oxidizing conditions that are necessary for the synthesis of many metal oxides. In this work, using a battery of experimental techniques and materials simulation, we show how this material can be interfaced successfully with TiO2 and other metal oxides to give boron-rich hybrid materials with intriguing photophysical and electrochemical properties.

Polyaniline (PANi)/graphene nanocomposites have attracted tremendous interest because of their great potential in electrochemical energy storage applications, especially supercapacitors. We herein focus on the composite synthesis, device fabrication and particularly various techniques for the improvement of electrochemical performance. It is imperative to take close control of the interface in these nanostructured composites, which thus would lead to the desired synergistic effects and cyclic stability with the efficient diffusion of electrolyte ions and electrons. Challenges and perspectives are discussed for the development of highly efficient PANi/graphene electrodes for supercapacitors. Refereed/Peer-reviewed

Graphene has recently enabled the dramatic improvement of portable electronics and electric vehicles by providing better means for storing electricity. In this Review, we discuss the current status of graphene in energy storage and highlight ongoing research activities, with specific emphasis placed on the processing of graphene into electrodes, which is an essential step in the production of devices. We calculate the maximum energy density of graphene supercapacitors and outline ways for future improvements. We also discuss the synthesis and assembly of graphene into macrostructures, ranging from 0D quantum dots, 1D wires, 2D sheets and 3D frameworks, to potentially 4D self-folding materials that allow the design of batteries and supercapacitors with many new features that do not exist in current technology. Graphene has now enabled the development of faster and more powerful batteries and supercapacitors. In this Review, we discuss the current status of graphene in energy storage, highlight ongoing research activities and present some solutions for existing challenges.