Discovering efficient pseudocapacitive charge storage materials has become one of the grand challenges to reduce the gap between high energy density batteries and high power density and durable ...electrical double‐layer capacitors. This research direction is facilitated by the introduction of redox‐active species that add Faradaic charge storage to the system. However, the astonishing abilities of organic redox species to increase energy density are insufficient to compensate for their poor electrical conductivity and inferior cyclability. Herein, it is proposed that these challenges can be simultaneously met by thoughtful selection of a redox species, thionine, that can be conjugated to a 3D graphene aerogel as a substrate via π–π interactions. The as‐fabricated metal‐free symmetric device exhibits a very high specific capacitance of 384 F g−1 at 1 A g−1. Moreover, the device shows an ultrawide potential window of 2.0 V in pH‐neutral aqueous electrolytes and delivers a maximum specific energy of 32.6 Wh kg−1, specific power of up to 12.8 kW kg−1, outstanding flexibility, and an excellent capacitance retention of 91% after 10 000 charge–discharge cycles at 10 A g−1. This device design provides an effective strategy to fabricate high‐performance aqueous supercapacitors and facilitates progress toward a sustainable energy future.
π‐π stacking conjugation of thionine to graphene aerogel: Thionine as a redox‐active dye intercalates in between the graphene aerogel nanosheets and tightly stacks on them. The rationally designed thionine–graphene supramolecular complex, as a metal‐free porous material, affords exceptional capacitive performance in addition to superior energy storage capability. Such a design mitigates leakage of the redox moieties out of the active materials.
The rapid development of miniaturized electronic devices has increased the demand for compact on-chip energy storage. Microscale supercapacitors have great potential to complement or replace ...batteries and electrolytic capacitors in a variety of applications. However, conventional micro-fabrication techniques have proven to be cumbersome in building cost-effective micro-devices, thus limiting their widespread application. Here we demonstrate a scalable fabrication of graphene micro-supercapacitors over large areas by direct laser writing on graphite oxide films using a standard LightScribe DVD burner. More than 100 micro-supercapacitors can be produced on a single disc in 30 min or less. The devices are built on flexible substrates for flexible electronics and on-chip uses that can be integrated with MEMS or CMOS in a single chip. Remarkably, miniaturizing the devices to the microscale results in enhanced charge-storage capacity and rate capability. These micro-supercapacitors demonstrate a power density of ~200 W cm-3, which is among the highest values achieved for any supercapacitor.
The global supercapacitor market has been growing rapidly during the past decade. Today, virtually all commercial devices use activated carbon. In this work, it is shown that laser treatment of ...activated carbon electrodes results in the formation of microchannels that can connect the internal pores of activated carbon with the surrounding electrolyte. These microchannels serve as electrolyte reservoirs that in turn shorten the ion diffusion distance and enable better interaction between the electrode surfaces and electrolyte ions. The capacitance can be further increased through fast and reversible redox reactions on the electrode surface using a redox‐active electrolyte, enabling the operation of a symmetric device at 2.0 V, much higher than the thermodynamic decompostion voltage of water. This simple approach can alleviate the low energy density of supercapacitors which has limited the widespread use of this technology. This work represents a clear advancement in the processing of activated carbon electrodes toward the next‐generation of low‐cost supercapacitors.
Laser treatment of activated carbon electrodes results in micro‐channels that connect internal pores of activated carbon with the surrounding electrolyte. These micro‐channels serve as electrolyte reservoirs shortening the ion diffusion distance and enabling better interaction between electrode surfaces and electrolyte ions. The capacitance can be further increased through fast and reversible redox reactions using a redox active electrolyte.
3D cellular graphene films with open porosity, high electrical conductivity, and good tensile strength, can be synthesized by a method combining freeze‐casting and filtration. The resulting ...supercapacitors based on 3D porous reduced graphene oxide (RGO) film exhibit extremely high specific power densities and high energy densities. The fabrication process provides an effective means for controlling the pore size, electronic conductivity, and loading mass of the electrode materials, toward devices with high energy‐storage performance.
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.
Electrochemical energy storage (EES) materials and devices should be evaluated against clear and rigorous metrics to realize the true promises as well as the limitations of these fast-moving technologies.
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.
Although electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, charge and discharge faster than batteries, they are still limited by low energy densities and slow rate ...capabilities. We used a standard LightScribe DVD optical drive to do the direct laser reduction of graphite oxide films to graphene. The produced films are mechanically robust, show high electrical conductivity (1738 Siemens per meter) and specific surface area (1520 square meters per gram), and can thus be used directly as EC electrodes without the need for binders or current collectors, as is the case for conventional ECs. Devices made with these electrodes exhibit ultrahigh energy density values in different electrolytes while maintaining the high power density and excellent cycle stability of ECs. Moreover, these ECs maintain excellent electrochemical attributes under high mechanical stress and thus hold promise for high-power, flexible electronics.
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.
The recent advances in developing graphene-based materials for flexible supercapacitors are summarized in this review.
3D architectures based on graphene have triggered a great deal of interest in energy, sensing, and environmental applications. This work describes, a simple electrochemical approach for the direct ...deposition of a functionalized graphene framework by utilizing electrostatic interactions between graphene oxide (GO) and a cationic surfactant. The surfactant improves the adsorption of GO sheets on the electrode surface, allowing the integration of individual graphene sheets into 3D structures with large electrochemically active surface area. Without using binders or conductive additives, the current approach leads to supercapacitors with high specific capacitance (320 F g−1) and areal capacitance (≈400 mF cm−2 for two electrode cells with single‐sided coatings), which compares favorably to commercial activated carbon supercapacitors. Moreover, the supercapacitors demonstrate low internal resistance (≈1 Ω cm−2), excellent cycling stability (no loss observed after 10 000 cycles) while also maintaining superior rate capability. In another application, these frameworks were successfully implemented as an electrochemical sensor for the simultaneous determination of small biomolecules including ascorbic acid, dopamine and uric acid with high sensitivity, selectivity and reproducibility. This work provides a simple, yet effective, strategy for the fabrication of macroporous electrodes with superior chemical, structural, and electrical properties that are desirable for a broad range of applications.
This study describes a simple strategy for the direct deposition of macroporous graphene films onto conductive substrates for use in electrochemical applications. The result is a 3D graphene framework with a highly effective surface area, which is successfully applied in the construction of fast and reliable supercapacitors and for the simultaneous analysis of various biomolecules with high sensitivity and selectivity.
Significance Batteries run just about everything portable in our lives such as smartphones, tablets, computers, etc. Although we have become accustomed to the rapid improvement of portable ...electronics, the slow development of batteries is holding back technological progress. Thus, it is imperative to develop new energy storage devices that are compact, reliable, and energy dense, charge quickly, and possess both long cycle life and calendar life. Here, we developed hybrid supercapacitors that can store as much charge as a lead acid battery, yet they can be recharged in seconds compared with hours for conventional batteries.
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 MnO ₂ 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 MnO ₂ 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.
PDF
