Hybrid Electrochemical Capacitor Research Articles

Hybrid electrochemical capacitor with high performance built by Co2.1Ni2.1Zn1.8(CO3)2(OH)8·H2O ternary basic carbonate

Metal basic carbonate materials (M-CH), which are employed as precursors of metal hydroxides/oxides, have recently popularity as battery-type electrodes due to their exceptional energy storage performance. However, a unitary metal basic carbonate is insufficient to meet today’s challenges. To make hybrid electrochemical capacitors, multiple-metals basic carbonate materials must to be designed and synthesized. Herein, a one-step hydrothermal method is used to produce Co basic carbonate doped with Ni and Zn (Co, Ni, Zn-CH). The results of X-ray diffractometer refinement demonstrate that following Ni, Zn co-substitution for Co ions, the crystal cell parameters of Co-CH drop. In a three-electrode system, such changes facilitate the electronic transfer, resulting in a high specific capacity of 91.80 mAh g−1 at 1 A g−1, a high rate performance of 52.42% at 20 A g−1, and a 5000-cycle stability of 91.85% at 10 A g−1. Furthermore, an assembled hybrid electrochemical capacitor (HEC) can provide an energy density of 36.10 Wh kg−1 at a power density of 1009.65 W kg−1 and a capacity retention of up to 91.93% after 5000 cycles. Two homemade hybrid electrochemical capacitors connected in series impressively maintain eight LEDs glowing for 30 min, demonstrating superior energy storage capability.

Alternatingly stacked thin film electrodes-based compact aqueous hybrid electrochemical capacitors for hundred-volts AC line filtering

Alternatingly stacked thin film electrodes-based compact aqueous hybrid electrochemical capacitors for hundred-volts AC line filtering

Aqueous hybrid electrochemical capacitors with ultra-high energy density approaching for thousand-volts alternating current line filtering

Filtering capacitors with wide operating voltage range are essential for smoothing ripples in line-powered system, which are still unsatisfactory due to low energy density and limited working voltage scopes. Herein, we report an aqueous hybrid electrochemical capacitor with areal specific energy density of 1.29 mF V2 cm−2 at 120 Hz, greater than common aqueous ones. Interestingly, it can be easily integrated at scale to show excellent flexibility, controllable and stable filtering performance, in which an integrated device (e.g., seven units in series) exhibits fluctuation of 96 mV, 10 times smaller than an aluminum electrolytic capacitor with similar capacitance. A record-high 1,000 V can also be achieved after integrating 670 units, exceeding those reported so far, and about 1.5 times of commercial bulk aluminum electrolytic capacitors (~700 V). This work opens up a new insight for promising applications in multiple electricity transmission systems that requiring high smoothness under harsh voltage.

Eight‐Electron Redox Cyclohexanehexone Anode for High‐Rate High‐Capacity Lithium Storage

AbstractReplacing inorganic anodes with organic electrode materials is an attractive direction for future green Li‐ion batteries (LIBs). Carbonyl compounds are being explored as leading anode candidates for organic LIBs. In particular, cyclohexanehexanone (C6O6), as a perfect structure composed entirely of six CO groups, can theoretically contribute to the most reactive sites and the highest specific capacity, but has not been used as an anode material so far owing to its high solubility in carbonate‐based electrolytes and extremely low electronic conductivity. Herein, C6O6 is first revealed as an ultra‐high capacity and high‐rate anode material through a total eight‐electron redox electrochemical process by effectively constructing an insoluble and highly conductive C6O6‐polymeric binder‐carbon network architecture. Experimental characterizations combined with first‐principles calculations elucidate that CO bonds in C6O6 can be lithiated to Li+ enolate (Li6C6O6) through a reversible six‐Li‐ion electrochemical process and further converted to Li8C6O6 dimers via a reversible two‐electron pseudocapacitive Li+ intercalation reaction. As such, the C6O6 anode shows an ultrahigh capacity of up to 1404 mAh g−1 at 200 mA g−1 and an extraordinary high‐rate durability (814 mAh g−1 after 700 cycles at 5.0 A g−1). A 4.3 V high energy/power density Li‐ion hybrid electrochemical capacitor based on the C6O6 anode is thus derived.

Engineering hierarchical porous S-doped Defective nickel Cobaltite/carbon hybrids to boost efficient asymmetric electrochemical capacitor

Developing energy materials with sufficient active sites, large surface area, high conductivity, and good stability through low-cost preparation routes are vital to the promotion of energy storage technologies. However, it is not easy for the longing component-structure features converged on a single component, the hybrid strategy provides a possible approach for the thorny issue. Herein, micro/mesoporous S–NiCo2O4/AC with abundant vacancy defects is rationally designed via intermolecular bonding of S–NiCo2O4 anchored onto carbon nanosheets through a facile hydrothermal and low-temperature pyrolysis method. The constructed hybrids possess superior structural durability, good electrical conductivity, and rich active sites due to the strong interfacial conjunction and favorable synergistic effect between the porous AC and S–NiCo2O4. Consequently, the S–NiCo2O4/AC hybrid demonstrates high conductivity, reasonable specific surface area, and superior specific capacitance, and the assembled hybrid electrochemical capacitor (S–NiCo2O4/AC//NAC) reveals an ideal energy density of 74.8 Wh kg−1 and power density of 31.4 kW kg−1, which is superior to the reported devices. This work not only paves the strategy toward efficient energy materials but also provides new insight into the relationship between each phase of the hybrids.

SILAR synthesized dysprosium selenide (Dy2Se3) thin films for hybrid electrochemical capacitors

As the necessity of energy storage is continuously increasing, new materials have been investigated for electrochemical energy storage, especially for electrochemical capacitors. These storage devices are rapidly convertible as well as air pollution free. Therefore, a number of materials have been explored as electrode materials for supercapacitors to fulfill different requirements of electrochemical energy storage. Herewith, dysprosium selenide (Dy2Se3) films were prepared using the simple successive ionic layer adsorption and reaction (SILAR) method and characterized using different physico-chemical techniques. The specific capacitance (Cs) of 92 F g−1 was obtained at the current density of 2.85 A g−1 in 1 M LiClO4 electrolyte with a retention of 85% over 5000 galvanostatic charge-discharge (GCD) cycles performed at a current density of 4 A g−1. The flexible solid-state hybrid electrochemical capacitor of configuration Dy2Se3/LiClO4-PVA/MnO2 showed Cs of 83 F g−1 and specific energy of 18 Wh kg−1 at a specific power of 2.7 kW kg−1. This hybrid device retained 92% of capacitance at a device bending angle of 160°. These results demonstrate the facile synthesis of Dy2Se3 and its possible use in electrochemical energy storage applications.

Conducting Polymer‐Based Hybrid Electrochemical Capacitor Utilizing Potassium Iodide Redox Electrolyte with Controlled Self‐Discharge

AbstractThis paper reports on the application of poly (3,4‐ethylenedioxythiophene)/carbon nanotubes (PEDOT/CNT) material for the construction of a hybrid supercapacitor utilizing iodide redox‐active electrolyte. To address the advantages in terms of electrochemical performance, comparison has been made to the system using conventional activated carbon (AC) electrodes. Despite limited specific surface area of the polymer‐based material, the PEDOT/CNT cell demonstrates reasonably high electrical parameters (vs. highly porous AC), especially those normalized per volume or total mass of the device. Interestingly, it also exhibits more stable self‐discharge characteristics over operation time which shall be assigned to smaller fraction of free polyiodides generated in the cell in the presence of the low‐surface‐area polymer electrode, in addition to their strong confinement in the polymer network, thus suppressing redox‐shuttling. Pursuing the mechanistic studies, we have managed to detect (using the electrocatalytic WO3‐based voltammetric probe) the presence of undesirable iodates but at lower concentrations in a case of the PEDOT/CNT‐based capacitor.

Three-dimensional carbon dots/Prussian blue analogues nanocubes /nickel foams as self-standing electrodes for high-performance hybrid electrochemical capacitors

Developing the high-performance supercapacitors is overwhelmingly dependent on the composition design and structure tailoring of electrode materials. By a one-step solution method, the composite of carbon dots/Prussian blue analogues nanocubes-incorporated three-dimensional Ni foams was prepared and used as a self-standing positive electrode for hybrid electrochemical capacitors (HEC). Aside from the role of Ni source for Prussian blue analogues (PBA), Ni foams acts as 3D conductive supports, making electrolytes more accessible to the internal surface of electrode. Meanwhile, carbon dots can be absorbed for the formation of carbon dots/PBA nanocubes on Ni foams surfaces, offering optimized interfaces for the interactions between electrodes and electrolytes and relieving the decomposition of PBA in alkaline electrolyte. With these merits, the carbon dots/Prussian blue analogues nanocubes-Ni foams electrode in the hybrid electrolyte of 0.5 M KOH and 1.3 M Na2SO4 exhibits a maximum specific capacity of 659 C g−1 at current density of 0.5 A g−1 and 344 C g−1 even under large current density of 5 A g−1. An extended working potential window of 1.8 V, high energy density of 65 Wh kg−1 and high power density of 4.052 kW kg−1 as well as improved cyclic stability are also achieved in the assembled HEC. This study builds a boulevard to improve the application potential of PBA in HEC, which will be beneficial for the development of supercapacitors.

Mechanisms of charge accumulation of the electrochemical system LaMnO3 / AC

In this work, the electrochemical behavior of LaMnO3 perovskite material and nanoporous carbon material in an aqueous solution of lithium sulfate are studied. The regularities of the expediency of the joint functioning of these materials as electrodes for a hybrid electrochemical capacitor are determined. It was found that the value of the specific capacity of the investigated electrochemical system of LaMnO3 / electrolyte / AC is 52 F/g during the discharge of the system to 1 V and the value of specific energy is 112.1 J /g at a discharge current of 1 mA.

0.2CNT/NiSex composite derived from CNT/MOF-74 as electrode material for electrochemical capacitor and electrochemical sensor

Selenides derived from MOFs have attracted attention as the materials with excellent electrochemical properties. CNT/MOF-74 composites are successfully synthesized by a simple hydrothermal method. Then CNT/Ni-MOF-74 materials are selected as the precursors for further selenization to prepare CNT/NiSex. The results show that 0.2CNT/NiSex derived from CNT/Ni-MOF-74 prepared in the presence of 0.2 g CNTs has the best electrochemical performance as electrochemical capacitor electrode, which exhibits the highest specific capacitance of 2136 F·g−1 at a current density of 1 A·g−1. In addition, a hybrid electrochemical capacitor (HEC) is assembled with 0.2CNT/NiSex and activated carbon (AC). The obtained 0.2CNT/NiSex//AC HEC shows a high energy density of 59 Wh·kg−1 at a power density of 800 W·kg−1. In order to expand the application of the composition material, a 0.2CNT/NiSex/GCE electrochemical sensor is prepared. The results show that 0.2CNT/NiSex/GCE electrochemical sensor can be used for simultaneous determination of acetaminophen (AMP) and dopamine (DA).

Comparative Study of Carbon and Conducting Polymer-Based Hybrid Electrochemical Capacitors Using Potassium Iodide Redox Electrolyte

A key challenge for the existing electrochemical energy storage systems, enabling versatile and advanced applications in modern electronics, is related with the improvement of their performance in terms of gravimetric and volumetric energy and power, durability, safety and exploitation costs. However, neither batteries, nor electrochemical capacitors (ECs) technologies do not meet all these demands. Hence, the concept of hybrid capacitor has been developed allowing to combine the main advantages of batteries (high energy) and ECs (high power, long lifespan). Among various strategy of development of such hybrid devices is the incorporation of electrochemically active redox species in addition, or instead of a conventional double-layer supporting electrolyte. To date, it has been widely reported that the electron transitions between redox centres present in the electrolyte at various oxidation states result in the increased capacity and energy in comparison to the simple electrical double-layer-type systems. By the fact that these processes should be fast and highly reversible, the rate of charging/discharging remains on a level expected for high power systems.It is worth to emphasize that most of reported active electrolyte-based hybrid ECs rely on carbonaceous materials enabling low electrodes resistance, providing a well-developed surface area and a high number of active sites for the electrochemical reaction. In the present work, we propose an alternative electrode material based on a conducting polymer (poly(3,4-ethylenedioxythiophene (PEDOT)) to support the redox processes of the iodide-based active electrolyte. The performance evaluation is based on the comparison to the the system fabricated from the conventional high surface area activated carbon electrodes (AC). The structure, morphology and porosity of investigated materials are analysed with various physicochemical characterization methods (SEM, XRD, Raman, UV/Vis and N2 physisorption) and electroanalytical techniques. The electrochemical studies revealed that despite a low specific surface area of a PEDOT (S BET equal to 40 m2 g-1), the gravimetric cell capacitance is similar to obtained for AC-based cell (S BET=2600 m2 g-1). Interestingly, despite a lower potential window of PEDOT system (1.2 V vs. 1.5 V for the AC), the volumetric capacitance and energy is even higher due to the high packing density of PEDOT electrodes. A better capacitance/energy responses were also observed upon complete parameters normalization, i.e. including the mass of electrolyte present in the pores and void spaces of the electrodes. It should be noted that such approach, although of a great importance in a viewpoint of realistic industrial application, and is often neglected by the researchers.The performance of both systems is also discussed in terms of stability and the effect of continuous operation (under galvanostatic cycling or potentiostatic ageing) on the coulombic efficiency and the self-discharge rate, where the latter has been found to be significantly lower for PEDOT-type cell, with respect to the AC counterpart. The presence and the amount of undesired reaction by-products, i.e. iodates, is analysed ex-situ using electrochemical sensing methods. As a result, a mechanism of parasitic charge loss under open circuit, leading to self-discharge, is described and named as a “dual redox-shuttle effect”, mainly attributed to the reactions taking place at the negative electrode. Acknowledgement Financial support was provided by the National Centre for Research and Development (NCBR, Poland) under Techmatstrateg Grant no. 347431/14/NCBR/2018.

Design principles and direct applications of cobalt-based metal-organic frameworks for electrochemical energy storage

Abstract Recently, pristine cobalt-based metal-organic frameworks (Co-based MOFs) have received widespread research interest for electrochemical energy storage owing to their tunable pore sizes, structural versatility, huge surface areas, and unique electrochemical properties involved Co central nodes. In this review, the synthetic engineering strategies, such as choosing judicious bridging ligands with multiple redox-active sites, suitable solvents, sophisticated tuning of particle sizes and pore types, and introduction of conductive matrices to Co-based MOFs are comprehensively summarized, in order to construct significantly improved electrochemical performance in energy storage devices. Subsequently, a broad overview of their diversity and complexity of charge storage mechanisms during charge/discharge cycling processes are presented. Moreover, the recent significant advancements of Co-based MOFs with the focus on their direct applications in electrochemical energy storage fields, namely, lithium-ion batteries, supercapacitors, hybrid electrochemical capacitors as well as lithium-sulfur batteries are elaborated in detail. The further existing challenges and perspectives of pristine Co-based MOFs for their applications in electrochemical energy storage devices are highlighted. This review is expected to shed light on the exploitation and rational design of Co-based MOFs, and even new MOF families for advanced energy storage devices in future.

Heazlewoodite, Ni3S2: An electroactive material for supercapacitor application

Engineering of nanostructured materials with a unique and uniform morphological design is considered to be a competent candidate for a diverse range of electrochemical energy applications. However, the construction of ball-in-ball transition metal sulfide via the vaporization process remains a big challenge of today's research community. In this work, carbon encapsulated nickel sulfide (Ni3S2@C) with a complex hollow interior was synthesized and further studied with nitrogen-doped carbon encapsulated nickel sulfide (Ni3S2@NC) for comparison basis. We used metal salt (nickel nitrate) and organic linker (trimesic acid, TMA) as a precursor to synthesize Ni-TMA via a solvothermal method. Next, the sulfidation process was done under 5% H2 balanced argon gas in a tubular furnace to convert into Ni3S2 with encapsulation of carbon. The prepared hybrid material showed ball-in-ball morphology and heazlewoodite mineral phase structure. Furthermore, the prepared materials were examined for electrochemical energy storage properties owing to their well-known faradaic dominant (battery-type) charge storage features and key positive electroactive material for today's growing hybrid electrochemical capacitor. The nitrogen-doped carbon encapsulated nickel sulfide has demonstrated good electrochemical features like a specific capacity of 442.45 F g−1 at 5 mV s−1 of scan rate and 373.52 F g−1 at 2 A g−1 of current density. Moreover, the material showed excellent capacitance retention of 81% and 70% after 5000 and 10,000 cycles run with 100% of coulombic efficiency at 8 A g−1.

Past, present and future of electrochemical capacitors: Technologies, performance and applications

Over the past decades, many research efforts are focused on electrochemical capacitors (ECs) along with materials utilized in them. The two generalstrategies for improving the specific energy (Wh/kg) or energy density (Wh/L) are to increase the cell voltage of the device and to increase the specific capacitance (F/g) of its electrodes. Herein, we provide an overview of the electrochemical performance of various electrode active materials includingnano-structured carbon materials, metal oxides, and conducting polymers. The focus of the paper is to evaluate from available research results the potential of the advanced electrode materials and ionic liquid electrolytes to improve EC performance in the case of complete cells. This is not a simple task because the research data are given in terms of F/g and A/g based on active weight of the electrode or cell. A method is included in the paper that uses the research data to determine inputs to a calculation of complete cell performance in terms of energy and pulsed power capability based on cell voltage and resistance. Considerable test data for commercially available hybrid ECs are included in the paper to assist in evaluating their present performance and future potential. The history of the development of modern ECs starting in about 1990 is also reviewed.This paper is a companion paper to the paper “Past, Present, and Future of Electrochemical Capacitors: Pseudocapacitance, Aging Mechanisms and Service Life Estimation”, P. Kurzweil, J. Schottenhauer, and C. Schell. That paper treats in detail the science of electrochemical capacitors and this paper reviews the technology and development in recent years.

Mechanisms of charge accumulation in electrochemical systems formed based on of nanoporous carbon and manganese oxide

In this work, the processes occurring in electrochemical systems based on nanoporous carbon material and manganese oxide in an aqueous solution of lithium sulfate are analyzed. Furthermore, it is shows the feasibility of these materials combination cycling as electrodes of a hybrid electrochemical capacitor. The combination of electrode materials with different mechanisms of charge accumulation was determined. Consequently, an increase in the accumulated energy by more than 25% by the formation of an electric double layer and the occurrence of redox reactions based on carbon and manganese oxide respectively. The laboratory sample of an aqueous electrolyte hybrid electrochemical capacitor was formed. Moreover, the laboratory sample is electrochemically stable at an operating voltage of 2 V.

Review on supercapacitors: Technologies and performance evaluation

At best uncertainties related to the performance metrics and evaluation of electrochemical capacitors is summarized and a method that uses the material properties to determine the performance of complete cell performance is developed. The development of electrochemical capacitors (i.e. supercapacitors) have attracted a lot of attention in recent years because of the increasing demand for efficient, high-power energy storage. Electrochemical capacitors (ECs) are particularly attractive for transportation and renewable energy generation applications, taking advantage of their superior power capability and outstanding cycle life. Over the past decade, various advanced electrode materials and cell design are being studied to improve the energy density of ECs. Hybrid Li-ion capacitors and pseudo-capacitors that utilize fast surface redox reactions of metal oxide and doped polymers are the prime candidates being considered. This paper is concerned with the metrics being used to describe the performance of ECs and how the metrics are evaluated by testing devices and how the data from the testing are best interpreted. Emphasize is on relating testing of advanced ECs using materials more complex than activated carbons to testing electric double-layer capacitors (EDLCs) using carbon in both electrodes. A second focus of the paper is projecting the potential of the advanced materials and ionic liquid electrolytes for the development of complete EC cells having an energy density more than a factor of ten greater the energy density of the EDLC devices currently on the market. This potential was evaluated by calculating the performance (energy and power) of a series of ECs that utilize the advanced materials that have been studied by electrochemists over the past 10–15 years. The capacitance and resistance of the advanced ECs were calculated utilizing specific capacitance (F/g or F/cm 3 ) and porosity data for the electrode materials and ionic conductivity of the electrolytes. It was concluded that hybrid ECs can be developed with energy densities of at least 50 Wh/kg, 70 Wh/L with efficient power greater than 3 kW/kg. Continued research on micro-porous carbons with specific capacitance of 200F/g and greater is needed.to achieve these EC performance goals.

Comparative Study of Carbon and Conducting Polymer-Based Electrochemical Capacitors Utilizing Potassium Iodide Redox Electrolyte

A key challenge for the existing electrochemical charge storage systems, enabling versatile and advanced applications in modern electronics, is related with the improvement of their performance in terms of gravimetric and volumetric energy and power, durability, exploitation costs and safety. However, neither batteries, nor electrochemical capacitors (ECs) technologies do not meet all these demands. Hence, the concept of hybrid capacitor has been developed allowing to combine the main advantages of batteries (high energy) and ECs (high power, long lifespan). Among various strategy of development of such hybrid devices is the incorporation of electrochemically active redox species in addition or instead of a conventional double-layer supporting electrolyte. To date, it has been widely reported that the electron transitions between redox centers present in the electrolyte at various oxidation states result in the increased capacity and energy in comparison to the simple electrical double-layer-type systems. By the fact that these processes are fast and highly reversible, the rate of charging/discharging remains on a level expected for high power systems. In this respect, it is important to refer to works describing electrocatalytic properties of certain systems (Pt, PEDOT, nanostructured carbons) toward electron transfers within the iodine/iodide redox couple with respect to dye sensitized solar cell applications [1, 2].It is worth to emphasize that most of reported active electrolyte-based hybrid ECs rely on carbonaceous materials enabling low electrodes resistance, well-developed surface area and providing high number of active sites for the electrochemical reaction. In the present work, we propose an alternative electrode material based on a conducting polymer (poly(3,4-ethylenedioxythiophene (PEDOT)) to support the redox processes of an iodide-based active electrolyte, i.e. one of the most widely considered so far. The performance evaluation was based on the comparison to the to the system fabricated from the conventional high surface area activated carbon electrodes (AC). The structure, morphology and porosity of investigated materials were analysed by comparative physicochemical characterization (SEM, XRD, Raman, UV/Vis and N2 adsorption/desorption techniques). The electrochemical studies revealed that despite a low specific surface area of a PEDOT (S BET=40 m2 g-1), the gravimetric cell capacitance is similar to obtained for AC-based cell (S BET=2600 m2 g-1). Interestingly, despite the lower potential window of PEDOT system (1.2 V vs. 1.5 V for AC), the volumetric capacitance and energy is even much higher due to the high packing density of PEDOT electrodes. A better capacitance/energy responses were also observed upon complete parameters normalization, i.e. including the mass of electrolyte present in the pores and void spaces of the electrodes. It should be noted that such approach, although of a great importance in a viewpoint of realistic industrial application, and is often neglected by researchers.The performance of both systems is also discussed in terms of stability and the effect of continuous operation (under galvanostatic cycling or potentiostatic ageing) on the self-discharge rate. As a result, a mechanism of parasitic charge loss under open circuit is described as a dual redox-shuttle effect and has been mainly attributed to the side reactions taking place at the negative electrode. Acknowledgement Financial support was provided by the National Centre for Research and Development (NCBR, Poland) under Techmatstrateg Grant no. 347431/14/NCBR/2018[1]. I. A. Rutkowska, M. Marszalek, J. Orlowska, W. Ozimek, S. M. Zakeeruddin, P. J. Kulesza, M. Graetzel, ChemSusChem 8 (2015) 2560[2]. R. Trevisan, M, Döbbelin, P. P. Boix, E. M. Barea, R. Tena‐Zaera, I. Mora‐Seró, J. Bisquert, Adv. Energy Mater. 1 (2011) 781–784

Polypyrrole derived porous carbon for high-performance Li ion capactiors

Herein, we fabricate polypyrrole-derived activated porous carbon as cathode for high-performance Li-ion hybrid electrochemical capacitor (Li-HEC). The porous carbon is fabricated by the relatively low-temperature pyrolyzation of polypyrrole, and followed by activating with KOH, which shows high surface area, favorable nanowires framework, hierarchical pores structure, and N, O-codoping. The Li-HEC device consisting of porous carbon and Li4Ti5O12 delivers the maximum energy density of about 87 Wh kg−1, which is superior to commercially activated carbon. Furthermore, the Li-HEC device also shows remarkable capacitance retention of 90.2% over 3000 cycles.

Hybrid electrochemical capacitors in aqueous electrolytes: Challenges and prospects

Abstract In an internal hybrid capacitor, at least one electrode displays battery-like charge/discharge and the other electrode stores charge reversibly at the electric double-layer (EDL). Recently, a plethora of hybrid cells in aqueous electrolytes have been proposed by coupling an EDL electrode with a battery electrode, the latter made from a variety of redox-active/redox-mediator species either dissolved in the electrolyte or adsorbed/immobilized in nanoporous electrodes. This review presents current opinions, discusses challenges, and supplies recommendation about the hybrid cells with aqueous electrolytes and carbon electrodes.

Towards an optimized hybrid electrochemical capacitor in iodide based aqueous redox-electrolyte: Shift of equilibrium potential by electrodes mass-balancing

Considering the cost-effectiveness, safety, and environmental friendliness for energy storage and delivery at high rates, hybrid electrochemical capacitors in aqueous electrolytes containing redox-active species are attractive alternatives to expensive organic electrolyte based electric double-layer capacitors (EDLCs). Here, the influence of electrode mass-balancing on the equilibrium potential of hybrid cells in aqueous sodium nitrate + sodium iodide (5 mol L−1 NaNO3 + 0.5 mol L−1 NaI) has been investigated. The shift of equilibrium potential determines, whether the positive electrode behaves fully battery-like (charge/discharge strictly in the iodide redox potential range) or shows a mixed battery-like and EDL capacitive behavior. With an appropriate mass-balancing of the positive and negative electrodes (mass ratio = 1:2), the equilibrium potential shows a negligible shift during galvanostatic charge/discharge cycles at 0.5 A g−1, which results in full battery-like behavior of the positive electrode. Consequently, the hybrid cell exhibits stable electrochemical performance. By contrast, an equal or higher mass of the positive compared to the negative electrode, leads to a shift of the equilibrium potential resulting in two different charge storage mechanisms at the positive electrode. As a result, the overall performance of the hybrid cell deteriorates. We show by thermogravimetric analysis and Raman spectroscopy that the formation of polyiodides (I3− and I5−) is controlled by the oxidation of iodide (I−) anions to molecular iodine in nanoporous carbon based positive electrode, and that more polyiodides are produced, if the positive electrode operates strictly within the iodide/iodine redox potential range.