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Exploration of innovative strategies aiming to boost energy densities of supercapacitors without sacrificing the power density and long-term stability is of great importance. Herein, highly porous nitrogen-doped carbon spheres (NPCS) are decorated onto the graphite sheets (GSs) through a hydrothermal route, followed by a chemical activation. The capacitive performance of the NPCS is then enhanced by hydroquinone sulfonic acid (HSQA) incorporation in both cathodic electrolyte and electrode materials. Later, NPCS are decorated with polypyrrole (PPY), in which HSQA takes a versatile role as conjugated polymer dopant and cathodic redox additive. The capacitive performance of the negative electrodes is enhanced by incorporating of alizarin red S (ARS) as anodic redox additive. Finally, PPY(HQSA)@NPCS-GS//NPCS-GS asymmetric supercapacitor is assembled and tested in dual redox electrolyte system containing HQSA-cathodic and ARS-anodic electrolytes. This device delivers a remarkable energy density of 60.37 Wh kg -1 , which is close or even better than lead acid batteries. Thus, the present work provides a novel pathway to develop high energy supercapacitors using redox active electrolytes for next-generation energy storage applications., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)

Carbon materials with hierarchical porous structures hold great potential for redox electrolyte-enhanced supercapacitors. However, restricted by the intrinsic inert and nonpolar characteristics of carbon, the energy barrier of anchoring redox electrolytes on the pore walls is relatively high. As such, the redox process at the interface less occurs, and the rate of mass transfer is impaired, further leading to a poor electrochemical performance. Here, a ferricyanide anion-philic interface made of in situ inserted boron species into carbon rings is constructed for enhanced charge storage in supercapacitors. Profiting from the unique component-driven effects, the polar anchoring sites on the pore wall can be built to grasp the charged redox ferricyanide anion from the bulk electrolyte and promote the redox process; the dynamics process is fastened correspondingly. Especially, the boron atoms in BC 2 O and BCO 2 units with higher positive natural bond orbital values in the carbon skeleton are pinpointed as intrinsic active sites to bind the negatively charged nitrogen atoms in the ferricyanide anion via electrostatic interaction, confirmed by density functional theoretical calculations. This will suppress the shuttle and diffusion effects of the ferricyanide anion from the surface of the electrode to the bulk electrolyte. Finally, the well-designed PC-3 with high content of BC 2 O and BCO 2 units can reach 1099 F g -1 at 2 mV s -1 , which is a more than 2-fold increase over boron-free units of carbon (428 F g -1 ). The work offers a novel version for designing high-performance carbon materials with unique yet reaction species-philic effects.

Multiple oxidation-state metal oxide has presented a promising charge storage capability for aqueous supercapacitors (SCs); however, the ion insert/deinsert behavior in the bulk phase generally gives a sluggish reaction kinetic and considerable volume effect. Herein, iron oxide/poly(3,4-ethylenedioxythiophene) (Fe 2 O 3 /PEDOT) heterointerface was constructed and enabled boosted Faradaic pseudocapacitance by dual-ion-involved redox reactions in Na 2 SO 3 electrolytes. The Fe 2 O 3 /PEDOT interface served as a "bridge" to couple electrode and anion SO 3 2- and exhibited a strong force and stable bonding with SO 3 2- , thus providing an additional Faradaic charge storage contribution for SCs. Significantly, the PEDOT-capsulated Fe 2 O 3 nanorod array (Fe 2 O 3 @PEDOT) electrode presented a specific capacitance of 338 mF cm -2 at 1 mA cm -2 with 1 M Na 2 SO 3 electrolyte, which was twice that of the pristine Fe 2 O 3 nanorod electrode. The boosted interfaced Faradaic reaction of SO 3 2- partially hindered the intercalation of Na + in the Fe 2 O 3 bulk phase, efficiently favoring the electrochemical stability.

Natural salinity gradients are a promising source of so-called "blue energy", a renewable energy source that utilizes the free energy of mixing for power generation. One promising blue energy technology that converts these salinity gradients directly into electricity is reverse electrodialysis (RED). Used at its full potential, it could provide a substantial portion of the world's electricity consumption. Previous theoretical and experimental works have been done on optimizing RED devices, with the latter often focusing on precious and expensive metal electrodes. However, in order to rationally design and apply RED devices, we need to investigate all related transport phenomena─especially the fluidics of salinity gradient mixing and the redox electrolyte at various concentrations, which can have complex intertwined effects─in a fully functioning and scalable system. Here, guided by fundamental electrochemical and fluid dynamics theories, we work with an iron-based redox electrolyte with carbon electrodes in a RED device with tunable microfluidic environments and study the fundamental effects of electrolyte concentration and flow rate on the potential-driven redox activity and power output. We focus on optimizing the net power output, which is the difference between the gross power output generated by the RED device and the pumping power input, needed for salinity gradient mixing and redox electrolyte reactions. We find through this holistic approach that the electrolyte concentration in the electrode rinse solution is crucial for increasing the electrical current, while the pumping power input depends nonlinearly on the membrane separation distance. Finally, from this understanding, we designed a five cell-pair (CP) RED device that achieved a net power density of 224 mW m -2 CP -1 , a 60% improvement compared to the nonoptimized case. This study highlights the importance of the electrode rinse solution fluidics and composition when rationally designing RED devices based on scalable carbon-based electrodes.

As a promising substrate, cellulose fibers were widely investigated in supercapacitors for their low cost and sustainability. However, the low performance created great barrier for the future applications of the cellulosic paper-based supercapacitors. The performance of paper-based supercapaciors may be improved by the addition of redox active molecule. As a plant derived redox active molecule, Alizarin red S was used to improve the performance of PEDOT paper-based electrode via a simple post-treatment process. By combination of the treated paper electrode and the redox electrolyte, a symmetric paper-based supercapacitor with a superior performance of 2191.3 m F/cm 2 (at 5 mA/cm 2 ) and 4.87 mW h/cm 3 (at power density of 36 mW/cm 3 ) were fabricated. The charge and mass transfer mechanisms of paper electrode were detailed discussed. The simple and efficient strategy developed in this work opens up new doors for the development of other cellulose related high performance energy storage devices., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Supercapacitors (SCs) have attracted widespread attention due to their short charging/discharging time, long cycle life, and good temperature characteristics. Electrolytes have been considered as a key factor affecting the performance of SCs. They largely determine the energy density based on their decomposition voltage and the power density from their ionic conductivity. In recent years, redox electrolytes obtained a growing interest due to an additional redox activity from electrolytes, which offers an increased charge storage capacity in SCs. This article summarizes the latest progress in the research of redox electrolytes, and focuses on their properties, mechanisms, and applications based on different solvent types available. It also proposes potential solutions for how to effectively increase the energy density of the SCs while maintaining their high power and long life., (Copyright © 2020 Zhang, Yang, Chang, Zhao, Wang, Yang and Cao.)

It is a big challenge to improve the energy density of the carbon-based supercapacitors for wide applications. In this work, considering the properties of redox electrolytes, functionalized porous carbon has been synthesized with interconnected pores and oxygen functional groups, which is employed to well hold the redox electrolyte ions. As a result, the functionalized porous carbon shows a high capacitance of 454 F g -1 at a current density of 1 A g -1 and can maintain 88% of the initial capacitance after 10 000 charge-discharge cycles at 10 A g -1 . Especially, the as-prepared asymmetric supercapacitor obtains high energy density of 36.9 W h kg -1 at the power density of 225 W kg -1 . This new design strategy by coordinating carbon materials with the redox electrolytes will guide the development of high-energy density supercapacitors.

Insufficient energy density and poor cyclic stability is still challenge for conductive polymer-based supercapacitor. Herein, high performance electrochemical system has been assembled by combining poly (3,4-ethylenedioxythiophene) (PEDOT):poly (styrene sulfonate) (PSS) redox electrode and potassium iodide redox electrolyte, which provide the maximum specific capacity of 51.3 mAh/g and the retention of specific capacity of 87.6% after 3000 cycles due to the synergic effect through a simultaneous redox reaction both in electrode and electrolyte, as well as the catalytic activity for reduction of triiodide of the PEDOT:PSS.

Redox chemistry is the cornerstone of various electrochemical energy conversion and storage systems, associated with ion diffusion process. To actualize both high energy and power density in energy storage devices, both multiple electron transfer reaction and fast ion diffusion occurred in one electrode material are prerequisite. The existence forms of redox ions can lead to different electrochemical thermodynamic and kinetic properties. Here, we introduce novel colloid system, which includes multiple varying ion forms, multi-interaction and abundant redox active sites. Unlike redox cations in solution and crystal materials, colloid system has specific reactivity-structure relationship. In the colloidal ionic electrode, the occurrence of multiple-electron redox reactions and fast ion diffusion leaded to ultrahigh specific capacitance and fast charge rate. The colloidal ionic supercapattery coupled with redox electrolyte provides a new potential technique for the comprehensive use of redox ions including cations and anions in electrode and electrolyte and a guiding design for the development of next-generation high performance energy storage devices., (© 2018 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The cooperative effects between the PANI (polyaniline)/nano-NiO (nano nickel oxide) composite electrode material and redox electrolytes (potassium iodide, KI) for supercapacitor applications was firstly discussed in this article, providing a novel method to prepare nano-NiO by using β-cyelodextrin (β-CD) as the template agent. The experimental results revealed that the composite electrode processed a high specific capacitance (2122.75 F·g -1 at 0.1 A·g -1 in 0.05 M KI electrolyte solution), superior energy density (64.05 Wh·kg -1 at 0.2 A·g -1 in the two-electrode system) and excellent cycle performance (86% capacitance retention after 1000 cycles at 1.5 A·g -1 ). All those ultra-high electrical performances owe to the KI active material in the electrolyte and the PANI coated nano-NiO structure., Competing Interests: The authors declare no conflict of interest.

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.

Redox electrolytes can provide significant enhancement of capacitance for supercapacitors. However, more important promotion comes from the synergetic effect and matching between the electrode and electrolyte. Herein, we report a novel electrochemical system consisted of a polyanilline/carbon nanotube composite redox electrode and a hydroquinone (HQ) redox electrolyte, which exhibits a specific capacitance of 7926 F/g in a three-electrode system when the concentration of HQ in H₂SO₄ aqueous electrolyte is 2 mol/L, and the maximum energy density of 114 Wh/kg in two-electrode symmetric configuration. Moreover, the specific capacitance retention of 96% after 1000 galvanostatic charge/discharge cycles proves an excellent cyclic stability. These ultrahigh performances of the supercapacitor are attributed to the synergistic effect both in redox polyanilline-based electrolyte and the redox hydroquinone electrode.

Carbon beads with sub-micrometer diameter were produced with a self-emulsifying novolac-ethanol-water system. A physical activation with CO2 was carried out to create a high microporosity with a specific surface area varying from 771 (DFT) to 2237 m(2)/g (DFT) and a total pore volume from 0.28 to 1.71 cm(3)/g. The carbon particles conserve their spherical shape after the thermal treatments. The controllable porosity of the carbon spheres is attractive for the application in electrochemical double layer capacitors. The electrochemical characterization was carried out in aqueous 1 M Na2SO4 (127 F/g) and organic 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (123 F/g). Furthermore, an aqueous redox electrolyte (6 M KI) was tested with the highly porous carbon and a specific energy of 33 W·h/kg (equivalent to 493 F/g) was obtained. In addition to a high specific capacitance, the carbon beads also provide an excellent rate performance at high current and potential in all tested electrolytes, which leads to a high specific power (>11 kW/kg) with an electrode thickness of ca. 200 μm.

We report a new electrochemical capacitor with an aqueous KI-KOH electrolyte that exhibits a higher specific energy and power than the state-of-the-art nonaqueous electrochemical capacitors. In addition to electrical double layer capacitance, redox reactions in this device contribute to charge storage at both positive and negative electrodes via a catholyte of IOx-/I- couple and a redox couple of H2O/Had, respectively. Here, we, for the first time, report utilizing IOx-/I- redox couple for the positive electrode, which pins the positive electrode potential to be 0.4-0.5 V vs Ag/AgCl. With the positive electrode potential pinned, we can polarize the cell to 1.6 V without breaking down the aqueous electrolyte so that the negative electrode potential could reach -1.1 V vs Ag/AgCl in the basic electrolyte, greatly enhancing energy storage. Both mass spectroscopy and Raman spectrometry confirm the formation of IO3- ions (+5) from I- (-1) after charging. Based on the total mass of electrodes and electrolyte in a practically relevant cell configuration, the device exhibits a maximum specific energy of 7.1 Wh/kg, operates between -20 and 50 °C, provides a maximum specific power of 6222 W/kg, and has a stable cycling life with 93% retention of the peak specific energy after 14,000 cycles.