Electrochemical Capacitor Systems Research Articles

Capacitor lifetime prolonged by addition of organic ammonium salt with cyclohexyl substituent and 2,5-dihydroxybenzenesulfonic anion

It is well known that the electrochemical corrosion negatively affects the functional properties of metal and steel materials that constitute a part of systems conducting electric charge. On the other hand, the phenomenon of corrosion is rarely associated with the degradation of chemical power sources and energy storage devices. This article is aimed to fill this gap by describing the effect demonstrated by a substituted ammonium salt with cyclohexyl substituent and 2,5-dihydroxybenzenesulfonate anion used as a corrosion inhibitor in symmetrical electrochemical capacitor system. The accelerated ageing test of capacitor systems has shown that the addition of substituted ammonium salt to capacitor electrolyte allows to extend the service lifetime of the entire system by inhibiting the corrosion of 316L stainless steel current collectors. The inhibitory effect of substituted ammonium salt additives on the electrochemical corrosion of 316L stainless steel surface has been also confirmed by cyclic potentiodynamic polarization (CPP) tests and X-ray photoelectron spectroscopy (XPS) measurements.

Enhancing capacitor lifetime by alternate constant polarization

This paper reports the successful attempts of symmetric aqueous electrochemical capacitor (EC) lifespan extension. A novel exploitation procedure, i.e., alternate floating, is herein proposed. Since only the positive electrode ages remarkably in EC systems with LiNO3 as the electrolyte, it is demonstrated that alternate polarization allows the properties of the negative electrode to be fully exploited. In our approach both electrodes play the role of the positive electrode and the negative one alternately; carbon electrodes were subjected to standard and alternate voltage-holding polarization. To observe the capacitor behavior under alternate floating at different conditions, various electrolyte concentrations of LiNO3 (0.2–5.0 mol L−1) were used. It was found that the utilization of alternate polarization during the aging test allows the lifespan of the capacitor to be noticeably prolonged. Moreover, the lifetime improvement strongly depends on the electrolyte concentration. A maximum improvement of 83% of capacitor operation time is achieved for 0.2 mol L−1 LiNO3, i.e., almost doubling the operation time. The aged electrodes were tested postmortem using SEM, TEM, XPS and N2 sorption at 77 K. The results demonstrated that the alternate floating protocol ages both electrodes almost equally.

Redox Mechanism Contributions to the Behaviour of Electrochemical Capacitor Materials

The mechanisms of charge storage in four typical electrochemical capacitor systems are compared and contrasted. These systems are based on activated carbon, ruthenium dioxide, manganese dioxide and nickel hydroxide. Charge storage is discussed in terms of charge delocalization either on the surface or throughout the electrode material. Electrical double layer formation, such as on activated carbon, is considered an example of charge delocalization, with charge distributed over the electrolyte accessible surface irrespective of the applied potential. Ruthenium dioxide also stores delocalized charge, in this case through the reversible Ru(IV)/Ru(III) redox couple. Manganese dioxide is unique in that in alkaline (battery) electrolytes charge is localized in specific structural domains, while in neutral (capacitor) electrolytes charge is delocalized over the material structure. Nickel hydroxide in an alkaline electrolyte is an example of charge localization when redox cycling due to its two-phase redox mechanism. The impact of these differing charge storage mechanisms on electrochemical performance is discussed.

Electroanalytical characterization of electrochemical capacitor systems

Herein we report on an in-situ electroanalytical technique to investigate electrode-electrolyte behaviour during cycling in a conventional symmetrical electrochemical capacitor system based on an activated carbon (AC) electrodes with 1 M tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (ACN) as the electrolyte. The electrochemical cell is based on a Swagelok T-junction, with the perpendicular port being used for a Ag/Ag+ (0.01 M AgNO3 in ACN) reference electrode. Situated between the opposing AC electrodes, between two separators, is a Pt sensing electrode, the potential of which is either monitored or controlled to explore either electrolyte fluxes during cycling, or the formation of soluble species in the electrolyte, respectively. It was deduced that electrolyte ions are strongly associated with the AC electrodes rather than between the electrodes even without an applied potential. Furthermore, aging of the electrochemical system with cycling (cyclic voltammetry) is also explored. Even with identical electrodes it is noted that the individual electrodes behaved differently as a result of anion versus cation association. Finally, at cell voltages approaching 2 V, oxidation of the positive electrode is observed, liberating species into the electrolyte. The implications of these observations have been discussed.

(Invited) Factors Affecting Electrochemical Capacitor Performance

Electrochemical capacitors are an emerging technology suited for high power pulse applications. The performance of these devices is dependent on their composite materials, including the electroactive materials, conductive additives and electrolyte, as well as the electrode substrates and separators, as well as cell engineering, which includes factors such as anode-to-cathode ratio and the amount of conductor added to each electrode. The combination of each of these factors leads to the overall performance of the device. Many electrochemical methods have been used to explore the behaviour of electrochemical capacitors, with techniques such as cyclic voltammetry and galvanostatic charge-discharge cycling providing information on material performance. Other methods such as electrochemical impedance spectroscopy and potential floating measurements have also been used to provide performance, conductivity and stability information. We have also contributed to the development of electrochemical methods with the application of step potential electrochemical spectroscopy (SPECS) to a variety of electrochemical capacitor systems. Here we will present a range of outcomes arising from the application of SPECS to explore the effects of electrolyte, charge storage mechanism, and conductivity on elctrochemical capacitor performance.

Highly Concentrated Aqueous Electrolyte with a Large Stable Potential Window for Electrochemical Double-Layer Capacitors

Although there has been a major effort, all over the world, to increase an energy density of electrochemical capacitors (ECs) to meet more demands for electric automotive and regenerative energy storage applications; till now how to significantly increase the energy density of ECs is still the biggest challenge facing ECs. In the EC research, much efforts have been focused on the development of electrode materials with high specific capacitance (Cs) in various electrolytes, and on the design of various types of EC systems including battery-like electrode/capacitor-like electrode (hybrid capacitors) [1, 2], and recently reported alkali metal ion capacitors (Li ion battery-like capacitors) [3, 4]. Electrochemical capacitor systems based on active carbon (AC) can offer the advantages of low cost and environmentally benignity, together with a double-layer (DL) mechanism-based high power and super long cyclic life, and thus are expected to provide a various applications to meet ever-increasing energy demands. However, the obvious shortage of low intrinsic energy density makes an impediment of their widely exploration. With a growing interest recently in Lithium bis(trifluoromethane sulfonyl) imide ((LiN(SO2CF3)2, LiTFSI) as conducting salt for Li (or Li-ion) batteries[5, 6], in which a highly concentrated aqueous electrolyte (LiTFSI, i.e., water-in-salt), with expanded potential stable window of ~3.0 volts was reported. To utilize such extended potential window, if it is acceptable or suitable for EDLCs, would be much interesting and profoundly for the improvement of the performance of EDLCs; more obviously the energy density stored in EDLCs would be improved significantly, because the energy density of EDLCs increases quadratically with voltage. Driven by this consideration, in this paper, we try to demonstrate an AC/AC capacitor utilizing LiTFS or LiTFSI aqueous electrolyte. The potential window of such aqueous electrolytes with various salt concentrations is experimentally examined for single AC electrode and AC/AC coin cell, and the performance index of the AC/AC capacitor is also discussed. Fig.1 CV curves of an active carbon electrode (AC) behaviors completely capacitive in both LiTFS and LiTFSI aqueous electrolytes at 100 mV∙s-1(a, b); and an AC/AC coin cell in 20 M LiTFS (a, b) and 20 M LiTFSI aqueous electrolytes at 100 mV∙s-1(c). The experimental results shown in Fig.1 indicate that both LiTFS and LiTFSI aqueous electrolytes have similar electrochemical potential stable window for active carbon electrode, achieving -1.2 – 1.2 V vs.SCE, which overpass that of most traditional aqueous electrolytes. An AC/AC electrochemical capacitor in 20 M LiTFS or LiTFSI aqueous electrolyte present a stable working voltage range of 0 – 2.5 V, which is the largest voltage range for AC-based capacitor in aqueous electrolyte till now.

Study of Redox and Pseudocapacitive Processes of K-Ion Storage Using New Transition Metal Oxides Derived from PBA Type-Materials

The fast growing use of natural energy sources such as solar and wind to generate electricity, requires the development of reliable, affordable, and efficient electric energy storage systems [1]. Thus, electrochemical capacitor systems can offer excellent devices showing high power (>10 kW kg-1), high rate capability and long cycle life (>1 000 000 cycles) [2]. Many kind of materials have been widely investigated as supercapacitor electrodes, such as carbon materials, graphene-based composites of metal hydroxides and metal oxides. In this work, new pseudocapacitive materials were obtained by thermal descomposition of cyanometalates (PBAs) obtained by precipitation over different carbon structures (graphene oxide, carbon nanotubes and mesoporous carbon). The materials were characterized using XRD, Raman, ATR-FTIR, SEM, TEM and XPS. Furthermore, the electrochemical performance of the materials was tested by cyclic voltammetry, step potential electrochemical spectroscopy, electrochemical impedance spectroscopy and charge-discharge curves at different current rates in 1 M KOH. These techniques were applied in order to understand the double layer and pseudocapacitive mechanism of K ion storage. The material formed in presence of graphene oxide exhibited the best electrochemical behavior, maintaining almost constant its initial capacitance after 500 cycles at 1 A g-1.

Ionic shot noise in an electrochemical capacitor system made of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) film and silver-coated polybenzoxazole-stainless steel electrodes on textile fabrics

Unusual noisy discharge profiles were observed from some of our textile-based energy storage devices having asymmetrical electrodes consisting of pure stainless steel and silver-coated polybenzoxazole yarns. Our hypothesis was that it was due to the ionic shot noise behavior that occurred in electrochemical devices and is a well-known phenomenon in electronic devices. The poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) polymer coverage on the cell surface, which functioned as a solid electrolyte in this system, was believed to play an important role. To confirm this assumption, devices made with different drop-coating strategies to obtain thin and thick layers of PEDOT:PSS polymer on the cell surface were prepared. The noisy discharge profiles were observed from devices with thin polymer layers, while those with thick layers exhibited normal discharge profiles. The morphology of cell surfaces was characterized by a stereo microscope. Cross-section analysis of a device with a thick layer was employed to further characterize its film formation and elemental properties using Scanning Electron Microscopy-Electron Dispersive Spectroscopy. Formation of multilayers of PEDOT:PSS polymer on the cell surface allowed a continuous interaction between electrode metals and PEDOT:PSS ions at the interface area. The noisy component was caused by the movement of silver ions hopping from one trap to the next. A qualitative model of the autocorrelation function executed to the data with noisy behavior further confirmed the ionic shot noise phenomenon.

Aqueous based asymmetrical-bipolar electrochemical capacitor with a 2.4 V operating voltage

A novel asymmetrical-bipolar electrochemical capacitor system leveraging the contributions of a Zn-CNT asymmetrical electrode and a KOH-H2SO4 dual-pH electrolyte was developed. The positive and negative electrodes operated in electrolytes with different pH, exploiting the maximum potential of both electrodes, which led to a cell voltage of 2.4 V. The potential tracking of both electrodes revealed that the Zn negative electrode could maintain a potential at −1.2 V, while the CNT positive electrode can be charged to +1.2 V without significant irreversible reactions. A bipolar ion exchange membrane has effectively separated the acid and alkaline from neutralization, which resulted in stable performance of the device with capacitance retention of 94% and coulombic efficiency of 99% over 10,000 cycles. This asymmetrical-bipolar design overcomes the thermodynamic limit of water decomposition, opening a new avenue towards high energy and high power density aqueous-based ECs.

Aluminum-Ion-Intercalation Supercapacitors with Ultrahigh Areal Capacitance and Highly Enhanced Cycling Stability: Power Supply for Flexible Electrochromic Devices.

Electrochemical capacitor systems based on Al ions can offer the possibilities of low cost and high safety, together with a three-electron redox-mechanism-based high capacity, and thus are expected to provide a feasible solution to meet ever-increasing energy demands. Here, highly efficient Al-ion intercalation into W18 O49 nanowires (W18 O49 NWs) with wide lattice spacing and layered single-crystal structure for electrochemical storage is demonstrated. Moreover, a freestanding composite film with a hierarchical porous structure is prepared through vacuum-assisted filtration of a mixed dispersion containing W18 O49 NWs and single-walled carbon nanotubes. The as-prepared composite electrode exhibits extremely high areal capacitances of 1.11-2.92 F cm-2 and 459 F cm-3 at 2 mA cm-2 , enhanced electrochemical stability in the Al3+ electrolyte, as well as excellent mechanical properties. An Al-ion-based, flexible, asymmetric electrochemical capacitor is assembled that displays a high volumetric energy density of 19.0 mWh cm-3 at a high power density of 295 mW cm-3 . Finally, the Al-ion-based asymmetric supercapacitor is used as the power source for poly(3-hexylthiophene)-based electrochromic devices, demonstrating their promising capability in flexible electronic devices.

Characterising the Charge Storage Mechanisms in Electrochemical Capacitors Using a Combination of Electrochemical Impedance Spectroscopy (EIS) and Step Potential Electrochemical Spectroscopy (SPECS)

Electrochemical capacitors are energy storage devices which have been demonstrated to exhibit both high specific capacitance and high specific power. Electrochemical capacitors are a promising energy storage technology due to their unique combination of specific energy and power output in addition to being relatively inexpensive and environmentally friendly. The performance of electrochemical capacitors is largely influenced by the electrode material properties. The specific properties of the electrode will determine both the nature and magnitude of the charge storage processes occurring within the electrode, such as double layer capacitance (non-faradaic) and redox reactions (faradaic; pseudo-capacitance). For example, activated carbons store charge almost exclusively via double layer capacitance, whereas metal oxides, such as ruthenium oxide and manganese oxide, will store charge via a combination of double layer and pseudo-capacitance. In metal oxides, pseudo-capacitance arises due to the reduction and oxidation of the material via proton insertion (and de-insertion), with ruthenium oxide and manganese dioxide exhibiting different charge storage characteristics. Understanding how the charge storage mechanism is influenced by material (and electrolyte) properties is vital for designing electrodes with optimised performance characteristics. However, this requires an understanding of how each charge storage mechanism contributes to capacitive performance. In terms of evaluating electrode performance, conventional electrochemical methods, such as cyclic voltammetry and constant current charge-discharge, cannot differentiate the capacitance contributions from charge storage processes involved. Characterising the different charge storage contributions from double-layer charge storage (non-faradaic) and pseudo-capacitive redox processes (faradaic) is a vital step in relating electrode performance to its material properties. In this work, both electrochemical impedance spectroscopy (EIS) and step potential electrochemical spectroscopy (SPECS) have been applied to electrochemical capacitor electrodes as a performance analysis method to determine the charge storage contributions from different processes. EIS has a number of advantages over other techniques, namely, the different time constants of electrochemical processes can be utilized to separate the different mechanisms occurring at the electrode. The SPECS experiment uses the same principle as EIS, i.e. separating the electrochemical mechanisms based on their different time constants. The SPECS procedure involves applying a small (±25 mV) potential step to the working electrode followed by a long equilibration time (300 s). This process is repeated over and entire charge-discharge cycle. By scanning at such a slow rate, the electrode has time to equilibrate at each potential, and the maximum charge storage capabilities of the electrode can be accessed. Each of the different charge storage processes occurring at the electrode has a unique time- dependent current response, and hence each potential step profile can be fitted to a model describing each of these processes. From this, values for series resistance (RS), double layer capacitance (CDL), diffusion limited capacitance (CD) and residual capacitance (CR) can be extracted. When the potential is stepped over an entire capacitor cycling range, contributions from each process can be determined at each point in the cycle. Additionally, by varying the equilibration time over which the current response is analysed, the scan rate can be effectively increased therefore the electrode behaviour can be analysed over a range of scan rates. This allows the development of a Ragone diagram for the different charge storage processes, indicating how specific charge storage mechanisms contribute to the power and energy characteristics of different electrode materials. This technique has been applied to a range of commonly used electrochemical capacitor systems including activated carbon, manganese dioxide and ruthenium oxides.

Charge storage mechanisms in electrochemical capacitors: Effects of electrode properties on performance

The capacitive behaviour of four commonly studied electrochemical capacitor systems has been analyzed using the step potential electrochemical spectroscopy (SPECS) method. Electrode-electrolyte combinations with different charge storage mechanisms were characterized, including activated carbon in aqueous (H2SO4) and organic (TEABF4 in acetonitrile) electrolytes, manganese dioxide (Na2SO4) and anhydrous ruthenium oxide (H2SO4). The SPECS method was used to separate the charge storage contributions from double layer capacitance (CDL) and diffusion-limited pseudo-capacitance (CD) at scan rates ranging from 0.08 to 125 mV/s. The relative contributions from each process are related to the physicochemical properties of the electrode. Additionally, the effects of these electrode properties on the overall performance of each system, in terms of specific power and energy, are identified.

Theoretical guidelines to designing high performance energy storage device based on hybridization of lithium-ion battery and supercapacitor

We discuss the theoretical approaches for various electrochemical capacitor systems via performance-potential estimation in regard to specific energy and power densities. Typical energy storage systems, such as symmetric capacitor system and asymmetric capacitor system, are classified with the symmetry of the electrodes (symmetric/asymmetric), and the types of electrolytes (aqueous/organic). Energy and power densities of each system are theoretically calculated using various factors and coefficients for performance comparison. Then, theoretical modeling for the BatCap system is conducted to indicate the electrochemical performance of this new concept device followed by consideration of ideal structure of the BatCap electrode material. Conclusively, this study successively indicates the performance of each energy storage system depending on the specified conditions of the electrodes and electrolyte which consist of the energy storage systems.

Utilization of Ionic liquid Salts in Electrochemical Capacitor System

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Electrochemical Capacitor System Life: Reductions Due to Cell Temperature and Voltage Non-uniformity

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Keggin‐Type Heteropolyacids as Electrode Materials for Electrochemical Supercapacitors

An inexpensive electrochemical capacitor system with an asymmetric configuration is demonstrated. The system includes heteropolyacids and a proton‐exchange polymer membrane. Heteropolyacid electrodes show facile redox processes and host large amounts of easily accessible mobile protons required for charge balance. The proton‐conducting polymer electrolyte provides chemical stability of the heteropolyacids which would otherwise dissolve into the aqueous liquid electrolyte. In this context, /Nafion 117/ is a model system. A very reversible specific capacitance of 112 F/g and an energy density of 36 J/g, based on the active materials only, were measured for the optimized cold‐pressed single cell with an operating voltage of 0.8 V. This approach can be extended to other transition‐metal oxides chemically unstable in aqueous solution that have mixed‐valence redox energies between the highest occupied and lowest unoccupied molecular orbital of the electrolyte. An electronic energy diagram for advanced design considerations is presented.