Supercapacitors (Chemistry)

High surface area carbon materials have high double layer capacitances because of their enhanced internal surface area and hence are attractive materials for supercapacitor applications. In this work, we demonstrate that utilizing a simple shaking experiment, hydroquinone can be physisorbed inside the pores of activated charcoal and the material can be used as a supercapacitor having a highest specific capacitance of ~200 F/g in 1 M H2SO4. Nearly 40% of the specific capacitances were pseudocapacitance in nature because of the observed reversible redox chemistry of hydroquinone/benzoquinone couples where hydroquinone underwent proton-coupled electron transfer (2H+/2e-) to form benzoquinone. The redox chemistry of hydroquinone/quinone is chemically irreversible but the same chemistry has been found to be chemically reversible and fast electron transfer kinetics at the electrode surface was observed in this study presumably because of proton-coupled electron transfers that were catalyzed by oxide sites present on the activated charcoal. Due to the observed reversible electrochemistry, the material also showed excellent capacitance retention in the long term cyclic tests. The approach presented in this study conceptually brings a new dimension to improve the chemistry of energy storage systems by simultaneous introduction of physisorption and proton-coupled electron transfer.

Biomass pyrolysis is a promising renewable sustainable source of fuels and petrochemical substitutes. It may help in compensating the progressive consumption of fossil-fuel reserves. The present article outlines biomass pyrolysis. Various types of biomass used for pyrolysis are encompassed, e.g., wood, agricultural residues, sewage. Categories of pyrolysis are outlined, e.g., flash, fast, and slow. Emphasis is laid on current and future trends in biomass pyrolysis, e.g., microwave pyrolysis, solar pyrolysis, plasma pyrolysis, hydrogen production via biomass pyrolysis, co-pyrolysis of biomass with synthetic polymers and sewage, selective preparation of high-valued chemicals, pyrolysis of exotic biomass (coffee grounds and cotton shells), comparison between algal and terrestrial biomass pyrolysis. Specific future prospects are investigated, e.g., preparation of supercapacitor biochar materials by one-pot one-step pyrolysis of biomass with other ingredients, and fabricating metallic catalysts embedded on biochar for removal of environmental contaminants. The authors predict that combining solar pyrolysis with hydrogen production would be the eco-friendliest and most energetically feasible process in the future. Since hydrogen is an ideal clean fuel, this process may share in limiting climate changes due to CO 2 emissions.
Keywords Sustainable and renewable energy source; Fossil-fuel alternatives; Biomass pyrolysis; Biofuel (bio-oil, biogas, biochar); Charcoal (activated carbon); Hydrogen fuel

Supercapacitors are known for their rapid energy charge–discharge properties, often ten to a hundred times faster than batteries. However, there is still a demand for supercapacitors with even faster charge–discharge characteristics to fulfill the requirements of emerging technologies. The power and rate capabilities of supercapacitors are highly dependent on the morphology of their electrode materials. An electrically conductive 3D porous structure possessing a high surface area for ions to access is ideal. Using a flash of light, a method to produce highly interconnected 3D graphene architectures with high surface area and good conductivity is developed. The flash converted graphene is synthesized by reducing freeze-dried graphene oxide using an ordinary camera flash as a photothermal source. The flash converted graphene is used in coin cell supercapacitors to investigate its electrode materials properties. The electrodes are fabricated using either a precoating flash conversion or a postcoating flash conversion of graphene oxide. Both techniques produce supercapacitors possessing ultra-high power (5–7 × 105 W kg−1). Furthermore, optimized supercapacitors retain >50% of their capacitance when operated at an ultrahigh current density up to 220 A g−1.

Nickel doped zinc oxide (Ni/ZnO) nanostructures have the potential to improve the performance of electrochemical capacitors. This study investigates the preparation of Ni/ZnO nanomaterials by facile co-precipitation (CPM) and hydrothermal (HTM) methods. The effect of the synthesis methods on the optical, structural, chemical and morphological properties on ZnO products is investigated using ultra violet (UV)-visible spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray spectrometry (EDX), room temperature photoluminescence (PL), Fourier transform infrared (FTIR) and Raman spectroscopy. Finally, the electrochemical performance of the synthesized nanorods were examined by fabrication of a supercapacitor using standard three electrode cell configuration and tested with a cyclic voltammogram (CV) and galvanostatic charge-discharge (GCD) measurements. The results shown that the samples synthesized by HTM exhibited improved electrochemical capacitance performance with higher current density. The discharge curves are linear in the total range of potential with constant slopes, showing perfect capacitance. In conclusion, Ni/ZnO nanoparticles synthesized by this method with further optimization have the potential to lead to a high-efficiency supercapacitors.

Since its discovery a decade ago, dozens of potential uses for graphene have been proposed, from faster computer chips and flexible touchscreens to hyper-efficient solar cells and desalination membranes. One exciting property that has sparked significant interest is its ability to store electrical charge. A single sheet of graphene sufficient in size to cover an entire American football field would weigh just a fraction of a gram. This huge surface area associated with this small amount of graphene can be squeezed inside an AA battery, enabling the design of new energy-storage devices with the ability to store massive amounts of charge. In this Review, we discuss the inherent properties of graphene and what it has to offer for energy storage. Much of the Review covers the synthesis and assembly of graphene into macrostructures that exploit the unique features of individual graphene sheets to build new materials for various applications. Particular attention is paid to the processing of graphene into electrodes, which is an essential step in the production of devices. Graphene can be useful by itself, but it is also promising — as we discuss — for composites with superior performance compared with existing materials. Currently, graphene is the most studied material for charge storage and the results from many laboratories confirm its potential to change today's energy-storage landscape. Specifically, graphene could present several new features for energy-storage devices, such as smaller capacitors, completely flexible and even rollable energy-storage devices, transparent batteries, and high-capacity and fast-charging devices. These and other features are explored in this Review. Despite notable progress, the future of graphene in the energy-storage market is uncertain because of several challenges. We discuss and propose solutions to these challenges and also briefly discuss the potential of other emerging 2D materials for energy-storage applications. Graphene for energy storage The fundamental properties of graphene make it promising for a multitude of applications. In particular, graphene has attracted great interest for supercapacitors because of its extraordinarily high surface area of up to 2,630 m 2 g −1. Recently, the intrinsic capacitance of single-layer graphene was reported to be ~21 μF cm −2 ; this value sets the upper limit for electric double-layer (EDL) capac-itance for all carbon-based materials 1. Thus, supercapac-itors based on graphene could, in principle, achieve an EDL capacitance as high as ~550 F g −1 if the entire surface area can be fully utilized. However, to understand the limits of graphene in supercapacitors, it is important to know the energy density of a fully packaged cell and not just the capacitance of the active material. In addition to the capacitance of graphene, the maximum energy density of graphene-based supercapacitors depends on several other parameters, such as the thickness and density of the graphene film and other cell components, including the current collector and the separator, the nature and density of the electrolyte, the operating voltage window of the cell and the packaging efficiency. As illustrated in FIG. 1, when using a standard current collector, separator and acetonitrile-based electrolyte, the two key parameters that control the energy density of graphene supercapacitors are the density of the graphene film and the voltage of the cell. For an electrochemical cell using 200-μm-thick graphene electrodes with a density of 1.5 g cm −3 and an operating voltage of 4 V, the maximum theoretical energy density is ~169 Wh kg −1 Abstract | 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. NATURE REVIEWS | MATERIALS ADVANCE ONLINE PUBLICATION | 1 REVIEWS © 2 0 1 6 M a c m i l l a n P u b l i s h e r s L i m i t e d. A l l r i g h t s r e s e r v e d .

The hybrid approach allows for a reinforcing combination of properties of dissimilar components in synergic combinations. From hybrid materials to hybrid devices the approach offers opportunities to tackle much needed improvements in the performance of energy storage devices. This paper reviews
the different approaches and scales of hybrids, materials, electrodes and devices striving to advance
along the diagonal of Ragone plots, providing enhanced energy and power densities by combining
battery and supercapacitor materials and storage mechanisms. Furthermore, some theoretical aspects
are considered regarding the possible hybrid combinations and tactics for the fabrication of optimized
final devices. All of it aiming at enhancing the electrochemical performance of energy 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.

Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti 3 C 2 , the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti 3 AlC 2 MAX. Since then, material scientists have either determined or predicted the stable phases of 4200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electro-chemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium-and sodium-ion batteries, lithium–sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Metal cobaltites have promising electrochemical properties for their application as an energy storage
medium.In this paper, usefulness of MgCo2O4
as a supercapacitor electrode is demonstrated and
compared its performance with two other cobaltites, MnCo2O4
and CuCo2O4. The materials are synthesized
using molten salt method and characterized by X-ray diffraction, scanning electron microscopy,
BET surface area, cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical
impedance spectroscopy techniques. The MgCo2O4 electrodes show superior charge storage
properties in 3 M LiOH among a diverse choice of electrolytes. The MgCo2O4 show higher theoretical (
3122 F/g) and practically achieved capacitance (
320 F/g), larger coulombic efficiency, and cycling stability than the other two; therefore, it could be developed as a low-cost energy storage
medium.

Supercapacitors are increasingly used for energy conversion and storage systems in sustainable nanotechnologies. Graphite is a conventional electrode utilized in Li-ion-based batteries, yet its specific capacitance of 372 mA h g −1 is not adequate for supercapacitor applications. Interest in supercapacitors is due to their high-energy capacity, storage for a shorter period and longer lifetime. This review compares the following materials used to fabricate supercapacitors: spinel ferrites, e.g., MFe 2 O 4 , MMoO 4 and MCo 2 O 4 where M denotes a transition metal ion; perovskite oxides; transition metals sulfides; carbon materials; and conducting polymers. The application window of perovskite can be controlled by cations in sublattice sites. Cations increase the specific capacitance because cations possess large orbital valence electrons which grow the oxygen vacancies. Electrodes made of transition metal sulfides, e.g., ZnCo 2 S 4 , display a high specific capacitance of 1269 F g −1 , which is four times higher than those of transition metals oxides, e.g., Zn-Co ferrite, of 296 F g −1. This is explained by the low charge-transfer resistance and the high ion diffusion rate of transition metals sulfides. Composites made of magnetic oxides or transition metal sulfides with conducting polymers or carbon materials have the highest capacitance activity and cyclic stability. This is attributed to oxygen and sulfur active sites which foster electrolyte penetration during cycling, and, in turn, create new active sites.

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/RuO2 nanocomposites into electrodes that exhibit ultrahigh energy and power densities. Hydrous RuO2 nanoparticles were successfully anchored to graphene surfaces through a redox reaction of the precursors, graphene oxide, and RuCl3 using a consumer grade LightScribe DVD burner with a 788 nm laser. This binder-free, metal current collector-free graphene/RuO2 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.

In present investigation, microflowers like hydrous cobalt phosphate is prepared via a facile single-step hydrothermal method on stainless steel substrate. The microflowers like morphology of hydrous cobalt phosphate thin film consists of microplates and further microplates converted to flakes, by means of a change in length, width and thickness, with urea variation.
Hydrous cobalt phosphate thin film electrode demonstrates a high specific capacitance of 800 F g-1
at 2 mA cm-2 with 33.62 Wh kg-1 energy density and 3.12 kW kg-1 power density. By taking advantages of hydrous cobalt phosphate thin film (as a cathode electrode) and copper sulfide thin film (as an anode electrode), the asymmetric devices (aqueous/all-solid-state) are fabricated. Aqueous asymmetric device shows a high specific capacitance of 163 F g-1 at 2 mA cm-2 with an energy density of 58.12 Wh kg-1 and power density of 3.52 kW kg-1. Moreover, all-solid-state
asymmetric supercapacitor device delivers a high specific capacitance of 70 F g-1 at 2 mA cm-2 with 24.91 Wh kg-1 energy density and 2.63 kW kg-1 power density in PVA-KOH gel electrolyte. The long-term cyclic stability (94 % after 3000 cycles) and actual practical demonstration (lightning 65 red LEDs) suggesting an industrial application of all-solid-state asymmetric device.

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 decomposition 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.

Most methods for improving supercapacitor performance are based on developments of electrode materials to optimally exploit their storage mechanisms, namely electrical double layer capacitance and pseudocapacitance. In such cases, the electrolyte is supposed to be electrochemically as inert as possible so that a wide potential window can be achieved. Interestingly, in recent years, there has been a growing interest in the investigation of supercapacitors with an electrolyte that can offer redox activity. Such redox electrolytes have been shown to offer increased charge storage capacity, and possibly other benefits. There are however some confusions, for example, on the nature of contributions of the redox electrolyte to the increased storage capacity in comparison with pseudocapacitance, or by
expression of the overall increased charge storage capacity as capacitance. This report intends to provide a brief but critical review on the pros and cons of the application of such redox electrolytes in supercapacitors, and to advocate development of the relevant research into a new electrochemical energy storage device in parallel with, but not the same as that of supercapacitors.

Silver (Ag) nanoparticles were synthesized through an electroless deposition providing high surface coverage of porous material. The structural and morphological characteristics were investigated along with electrochemical studies to evaluate the charge storage performance. The nanoparticle morphology provides more active sites and diffusion paths for the electrolyte ions resulting in maximum utilization of active electrode material. Ag nanoparticles based electrode not only exhibited a maximum specific capacitance of 452 F/g (at scan rate of 2 mV/s) and specific energy of 27.8 Wh/kg, but also achieved a tremendous gain over cyclic stability by retrieving 63% capacitance after 10,000 cycles. Such attractive electrochemical behavior of the formed electrode is suitable for the manufacturing of the good quality supercapacitors by utilizing a roll-to-roll industrial production technology.

The electrochemical characteristics of supercapacitors consisting of molybdenum carbide derived carbon C(Mo2C) electrodes in 1M NaPF 6 solutions in various mixtures (0.5–5%) of vinylene carbonate (VC) with propylene carbonate (PC) and ethyl acetate (EA) (1:1 by volume) have been studied using cyclic voltammetry, constant current charging/discharging and electrochemical impedance spectroscopy methods. The specific capacitance and series resistance values dependencies on the used solvent mixture and applied temperature (from −40 • C to 60 • C) have been established. The region of ideal polarizability has been established for C(Mo 2 C) electrodes in all electrolytes and temperatures investigated, except at T ≥ 40 • C. Specific conductivity values have been measured and correlated with electrochemistry data. Limiting capacitance, calculated characteristic time constant and complex power values depend noticeably on the solvent mixture used in the electrolyte, i.e. on the viscosity and specific conductivity of the electrolyte solution. The studied electrolytes are potential candidates for low-temperature supercapacitors.

Nanowires of a composite, Mn 2 O 3-SnO 2 , are synthesized at 100 g scale. The composite nanowires showed beneficial properties of the constituents. They showed significantly high charge storability and cyclability than the constituents. The superior charge storage are attributed to lower characteristic resistances. a b s t r a c t Large scale production of electrochemical materials in non-conventional morphologies such as nanowires has been a challenging issue. Besides, functional materials for a given application do not often offer all properties required for ideal performance; therefore, a composite is the most sought remedy. In this paper, we report large scale production of a composite nanowire, viz. Mn 2 O 3-SnO 2 , and their constituent binary nanowires by a large scale electrospinning pilot plant consisting of 100 needles. Electrochemical characterization of thus produced composite nanowires showed nearly threefold increase in the discharge capacity compared to their single component counterparts: Mn 2 O 3-SnO 2 $53 mA h g À1 (specific capacitance, C S $384 F g À1); Mn 2 O 3 $18 mA h g À1 (C S $164 F g À1); and SnO 2 $14 mA h g À1 (C S $128 F g À1) at 1 A g À1 in 6 M KOH. The EIS studies showed that the characteristic resistances and time of the composite electrode are appreciably lower than their constituents. Owing to the scalability of the synthesis processes and promising capacitive properties achieved would lead the composite material as a competitive low-cost and high-performance supercapacitor electrode.

Orange by-products e.g. orange peel and the extract of orange juice are used as sources for biological anti-oxidants such as ascorbic acid, flavonoids, phenolic compounds and pectins. In this study these antioxidants were successfully used to prepare nanosized materials of α-MnO 2 by cost effective and eco-friendly green chemistry method. The prepared oxides of MnO 2 which have unique properties as a storage cathode material were tested as a pseudocapacitor electrode materials in this study. X-ray powder diffraction (XRD) confirmed the structure of α-MnO 2 for the prepared samples. Thermal behavior of prepared oxides was tested using thermo-gravimetric analysis (TGA). Transmission electron microscopy (TEM) showed the nanosized nature of the prepared oxides. N 2-adsorption-desorption isotherms and pore-size distributions of prepared oxides showed that the surface areas are 5.63 and 8.40 m 2. g −1 for sample prepared from the extract of orange juice (OJ-MnO 2) and that prepared from the extract of orange peel (OP-MnO 2), respectively. Better electrochemical properties are obtained for OP-MnO 2 , the capacitance of OP-MnO 2 (139 F/g) is more than two times and half that obtained for OJ-MnO 2 (50 F/g) at the current density 0.5 A/g.

Prussian blue-polypyrrole (PB-PPy) nanocomposite was deposited directly on glassy carbon electrode (GCE) by self-assembly (SA) based on multiple sequential adsorption of FeCl 3-K 3 [Fe(CN) 6 ] and pyrrole. This approach offers easy preparation of electrodes, which allows control of its electrochemical behaviour and sensitivity towards hydrogen peroxide (H 2 O 2). PB-PPy nanocomposite was also electrodeposited in one-pot aqueous solution for the comparison purpose in terms of morphology and its electrochemical and electrocatalytic behaviours. Field-emission scanning microscope (FESEM) presented the morphology SA PB-PPy nanocomposites was irregular whereas electrodeposited PB-PPy was in nanocubic framework. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to study the electrochemical behaviour of fabricated PB-PPy films. The SA PB-PPy electrode with 10 deposition cycles demonstrated good sensing behaviour towards H 2 O 2 with good selectivity whereas capacitive property was observed when the deposition cycles increased to 20.