A versatile iron [1-(naphthalen-2-ylmethyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole]3 metal complex redox active material for energy conversion and storage systems

Novel Fe2+/3+ [npbi]3 redox electrolytes contributed to competitive performances in both DSC and SC applications.

Electrochemical Capacitors with Confined Redox Electrolytes and Porous Electrodes

Electrochemical capacitors (ECs), including electrical-double-layer capacitors and pseudocapacitors, feature high power densities but low energy densities. To improve the energy densities of ECs, redox electrolyte-enhanced ECs (R-ECs) or supercapbatteries are designed through employing confined soluble redox electrolytes and porous electrodes. In R-ECs the energy storage is based on diffusion-controlled faradaic processes of confined redox electrolytes at the surface of a porous electrode, which thus take the merits of high power densities of ECs and high energy densities of batteries. In the past few years, there has been great progress in the development of this energy storage technology, particularly in the design and synthesis of novel redox electrolytes and porous electrodes, as well as the configurations of new devices. Herein, a full-screen picture of the fundamentals and the state-of-art progress of R-ECs are given together with a discussion and outlines about the challenges and future perspectives of R-ECs. The strategies to improve the performance of R-ECs are highlighted from the aspects of their capacitances and capacitance retention, power densities, and energy densities. The insight into the philosophies behind these strategies will be favorable to promote the R-EC technology toward practical applications of supercapacitors in different fields.

Lignin-Derived Quinone Redox Moieties for Bio-Based Supercapacitors

Because of their rapid charging and discharging, high power densities, and excellent cycling life stabilities, supercapacitors have great potential for use in electric vehicles, portable electronics, and for grid frequency modulation. The growing need for supercapacitors that are both efficient and ecologically friendly has generated curiosity in developing sustainable biomass-based electrode materials and electrolytes. Lignin, an aromatic polymer with remarkable electroactive redox characteristics and a large number of active functional groups, is one such candidate for use in renewable supercapacitors. Because its chemical structure features an abundance of quinone groups, lignin undergoes various surface redox processes, storing and releasing both electrons and protons. Accordingly, lignin and its derivatives have been tested as electroactive materials in supercapacitors. This review discusses recent examples of supercapacitors incorporating electrode materials and electrolytes derived from lignin, focusing on the pseudocapacitance provided by the quinone moieties, with the goal of encouraging the use of lignin as a raw material for high-value applications. Employing lignin and its derivatives as active materials in supercapacitor electrodes and as a redox additive in electrolytes has the potential to minimize environmental pollution and energy scarcity while also providing economic benefits.

High-voltage aqueous symmetric supercapacitors based on 3D bicontinuous, highly wrinkled, N-doped porous graphene-like ultrathin carbon sheets

Biomass-derived 3D bicontinuous, highly-wrinkled, N-doped porous graphene-like ultrathin carbon sheets for high-performance aqueous symmetric supercapacitor.

Improvement of Quasi-Solid-State Supercapacitors Based on “Water-in-Salt” Hydrogel Electrolyte by Introducing Redox-Active Ionic Liquid and Carbon Nanotubes

In comparison with supercapacitors (SCs) with aqueous solution electrolytes, quasi-solid-state SCs (QSCs) based on hydrogel electrolytes (HEs) exhibit more extensive application prospect due to the advantages for example easier encapsulation...

Visualising electrochemical reaction layers: mediated vs. direct oxidation

Electrochemical treatments are widely used for 'clean up' in which toxic metals and organic compounds are removed using direct or mediated electrolysis. Herein we report novel studies offering proof of concept that spectrofluorometric electrochemistry can provide important mechanistic detail into these processes. A thin layer opto-electrochemical cell, with a carbon fibre (radius 3.5 μm) working electrode, is used to visualise the optical responses of the oxidative destruction of a fluorophore either directly, on an electrode, or via the indirect reaction of the analyte with an electrochemically formed species which 'mediates' the destruction. The optical responses of these two reaction mechanisms are first predicted by numerical simulation followed by experimental validation of each using two fluorescent probes, a redox inactive (in the electrochemical window) 1,3,6,8-pyrenetetrasulfonic acid and the redox-active derivative 8-hydroxypyrene-1,3,6-trisulfonic acid. In the vicinity of a carbon electrode held at different oxidative potentials, the contrast between indirect electro-destruction, chlorination, and direct oxidation is very obvious. Excellent agreement is seen between the numerically predicted fluorescence intensity profiles and experiment.

Electrolyte selection for supercapacitive devices: a critical review

Electrolytes are one of the vital constituents of electrochemical energy storage devices and their physical and chemical properties play an important role in these devices' performance, including capacity, power density, rate performance, cyclability and safety. This article reviews the current state of understanding of the electrode-electrolyte interaction in supercapacitors and battery-supercapacitor hybrid devices. The article discusses factors that affect the overall performance of the devices such as the ionic conductivity, mobility, diffusion coefficient, radius of bare and hydrated spheres, ion solvation, viscosity, dielectric constant, electrochemical stability, thermal stability and dispersion interaction. The requirements needed to design better electrolytes and the challenges that still need to be addressed for building better supercapacitive devices for the competitive energy storage market have also been highlighted.

Redox-Mediator-Enhanced Electrochemical Capacitors: Recent Advances and Future Perspectives

Supercapacitors deliver exceptional power densities, high cycling stability, and inherent safety but suffer from low energy densities. Many methods to enhance the energy density are based on exploring electrode materials with well-developed structures and designing asymmetric systems with wide voltage windows. The energy density is substantially enhanced at the compromise of power density by utilizing the sluggish kinetics of pseudocapacitive materials. Redox-active electrolytes can contribute additional pseudocapacitance from the reactions of redox mediators at the interface, which have attracted increasing attention of researchers. Redox-mediator-enhanced supercapacitors deliver high energy densities while retaining high power densities. This Minireview highlights the recently prominent progresses of single-, dual-, and ambipolar-redox-mediator-enhanced supercapacitors, the challenges they face, and approaches to suppress self-discharge and develop high-concentration redox-active electrolytes for performance promotion.

Fabrication of Low-Cost and Ecofriendly Porous Biocarbon Using Konjaku Flour as the Raw Material for High-Performance Supercapacitor Application

Low-cost and ecofriendly porous biocarbons were fabricated from konjaku flour via precarbonization and potassium hydroxide (KOH) activation. The obtained biocarbon ACK-5 derived from a precarbonized carbon/potassium hydroxide (KOH) mass ratio of 1:5 possessed an ultrahigh specific surface area of 1403 m2 g-1 and hierarchical porous structures with the existence of micro- to macropores. When ACK-5 was employed as a supercapacitor electrode in 6 M KOH, it showed a high specific capacitance of 216 F g-1 and excellent cycling stability with capacitance retention remaining 93.7% after 5000 cycles. Moreover, the ACK-5 sample acquired a supramaximal specific capacitance of 609 F g-1, and the high energy density of ACK-5//ACK-5 symmetrical cells reached up to 9.2 Wh kg-1 when p-phenylenediamine serving as a redox electrolyte was added into KOH electrolyte. The reported simple fabrication strategy would leverage a green biomass precursor for the preparation of supercapacitors.

Polyaniline-based carbon nanospheres and redox mediator doped robust gel films lead to high performance foldable solid-state supercapacitors

As-fabricated foldable solid-state supercapacitors are suitable for highly fold-tolerant high-energy-density energy storage device applications.

Superior performance of an active electrolyte enhanced supercapacitor based on a toughened porous network gel polymer

A primary challenge of gel electrolytes in the development of flexible and wearable devices is their weak mechanical strength and poor electrochemical performances.

High Performance Hybrid Energy Storage with Potassium Ferricyanide Redox Electrolyte

We demonstrate stable hybrid electrochemical energy storage performance of a redox-active electrolyte, namely potassium ferricyanide in aqueous media in a supercapacitor-like setup. Challenging issues associated with such a system are a large leakage current and high self-discharge, both stemming from ion redox shuttling through the separator. The latter is effectively eliminated when using an ion exchange membrane instead of a porous separator. Other critical factors toward the optimization of a redox-active electrolyte system, especially electrolyte concentration and volume of electrolyte, have been studied by electrochemical methods. Finally, excellent long-term stability is demonstrated up to 10 000 charge/discharge cycles at 1.2 and 1.8 V, with a broad maximum stability window of up to 1.8 V cell voltage as determined via cyclic voltammetry. An energy capacity of 28.3 Wh/kg or 11.4 Wh/L has been obtained from such cells, taking the nonlinearity of the charge-discharge profile into account. The power performance of our cell has been determined to be 7.1 kW/kg (ca. 2.9 kW/L or 1.2 kW/m(2)). These ratings are higher compared to the same cell operated in aqueous sodium sulfate. This hybrid electrochemical energy storage system is believed to find a strong foothold in future advanced energy storage applications.

The effects of amine/nitro/hydroxyl groups on the benzene rings of redox additives on the electrochemical performance of carbon-based supercapacitors

In this work, a series of porous carbon materials with hierarchical porosities have been synthesized via a template carbonization method, in which cheap CaCO3 serves as a template and glucose as a carbon precursor. During the carbonization process, CO2 produced by the decomposition of the CaCO3 template can act as an internal activating agent, significantly improving microporosity and mesoporosity. All the carbon materials obtained by regulating the ratio of glucose to CaCO3 exhibit the amorphous features with a low graphitization degree. Among them, the carbon-1 : 2 sample shows a high BET surface area of up to 818.5 m(2) g(-1) and a large total pore volume of 1.78 cm(3) g(-1) as well as a specific capacitance of 107.0 F g(-1) at 1 A g(-1). In addition, a series of hydroquinone (HQ), p-aminophenol (PAP) and p-nitrophenol (PNP) as novel redox additives that can produce pseudo-capacitances have been added into the KOH electrolyte for promoting the total capacitive performances via redox reactions at the electrode-electrolyte interface. As expected, a 2.5-fold increase in the galvanostatic capacitance of 240.0 F g(-1) in the HQ-0.5 electrolyte occurs, compared with the conventional KOH electrolyte. Similarly, the PAP-0.5 electrolyte and the PNP-0.5 electrolyte also show a high specific capacitance of 184.0 F g(-1) at 2 A g(-1) (156.6 F g(-1) at 3 A g(-1)) and 153.0 F g(-1) at 3 A g(-1), respectively. Additionally, the three kinds of electrolytes exhibit excellent cyclic stability. The remarkable improvement of supercapacitors is attributed to the quick reversible Faradaic reactions of amine and hydroxyl groups adhering to the phenyl rings, which largely accelerates electron migration and brings additional pseudocapacitive contribution for carbon-based supercapacitors.

A review of electrolyte materials and compositions for electrochemical supercapacitors

Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).

Toughened redox-active hydrogel as flexible electrolyte and separator applying supercapacitors with superior performance

The as-fabricated supercapacitor with PVA–H₂SO₄–AQSS gel polymer under fold or in-series conditions exhibits superior electrochemical performance.

High energy density of quasi-solid-state supercapacitor based on redox-mediated gel polymer electrolyte

A novel redox-mediated gel polymer (PVA–H₂SO₄–ARS) is prepared, and a symmetric supercapacitor using the gel polymer as electrolyte and activated carbon as electrode is also assembled.

Synergistic effect of novel redox additives of p-nitroaniline and dimethylglyoxime for highly improving the supercapacitor performances

In present work, we demonstrate a simple but effective strategy for high-performance supercapacitors by adding the p-nitroaniline (PNA) into an alkaline electrolyte of KOH.