Process For Supercapacitors Research Articles

Porosity and tortuosity: Keys for accurate modeling of porous electrodes in supercapacitors

Modeling of porous electrodes in supercapacitors (SCs) is rather important for both characterizing and improving the charging/discharging process of SCs, but the current models fail to balance accuracy, flexibility, and detail characterization of the electrode structure, especially the porosity and tortuosity. In this work, we develop a porous tortuous electrode (PTE) model consisting of a stack of multiple parallel plate capacitors with adjustable structural parameters for accurate modeling of SCs. By solving the equivalent circuit of the PTE model, the influence of the electrode structures on electric capacity and relaxation time are characterized with experimental validation, signifying the keys for accurate modeling of SCs as tortuosity and porosity and offering a general formula for describing the relaxation time of SCs. Based on the adjustable parameters in the PTE model, the spatial heterogeneity of the electrodes enables more intricate electrode structure designs. The PTE model facilitates various electrode designs and provides insight into the charging and discharging kinetics of porous electrodes.

A unified framework for exploring hysteresis between charge and discharge processes in supercapacitors

In supercapacitor technology, the determination of the largest voltage range useable for the charge storage is a key parameter to optimize their energy. For this purpose, “acceptable” coulombic and energy efficiencies are commonly used as cut-off values to delineate a safe operational potential window in opening-window charge-discharge experiments. Because of the arbitrariness of such an approach, attempts have been made over the past three decades to rationalize the determination of the stability window, but differentiate between capacitive and faradaic currents remains challenging. Assuming that it’s better to examine what is lost during the charge period to investigate the degradation of electrochemical interfaces, part of the problem is that we have not the correct perspective on stability with coulombic and energy efficiencies, since they give an indication of the way in which performance are retained during the charge process, as they represent the capacitive fractions of the measured quantities. Here, we propose a new formalism based on non-capacitive fractions to focus on what is lost during the charge period, in order to place the heterogeneous kinetics of the electrochemical degradation processes at the center of the debate on stability of the electrode-electrolyte interfaces in supercapacitors.

Super-capacitor energy storage system to recuperate regenerative braking energy in elevator operation of high buildings

<span>In operating phases of elevators, accelerating, braking modes occur frequently, so braking energy recuperation of elevators has contributed considerably to decrease the total electric energy consumption for operating elevators in multi-floor buildings. In this paper, the supercapacitor energy storage system is used to recover regenerative braking energy of elevators when they operate down full-load and up no-load, reducing fluctuation of voltage on DC bus as well. Therefore, super-capacitor energy storage system (SCESS) will be parallel with line utility to recuperate regenerative braking energy in braking phase and support energy for acceleration phase. The surplus energy will be stored in the supercapacitors thanks to a DC-DC converter capable of exchanging energy bidirectionally in buck/boost modes, and designing control strategy including two control loops. Inner loop-current loop: controlling charge/discharge process of supercapacitors by current iL complying with operation characteristic of elevator; Outer loop-voltage loop: managing UDC-link at a fixed value. Simulation results with elevator system of the ten-floor building, Hanoi, Vietnam installed SCESS have been verified on MATLAB Simulink, SimPowerSystem with saving energy level about 30%.</span>

Ribbon-like Nickel Cobaltite with Layer-by-Layer-Assembled Ordered Nanocrystallites for Next-Generation All-Solid-State Hybrid Supercapatteries.

In the context to develop ultra-efficient electrode materials with good physicoelectrochemical and electrostructural properties, for their application in high-performance supercapatteries, herein, a facile tartrate-mediated inhibited crystal growth method is reported to engineer thoroughly uniform ribbon-like nickel cobaltite (NiCo2O4) microstructure with unique layer-by-layer-assembled nanocrystallites. This material demonstrates significant kinetic reversibility, good rate efficiency and bulk diffusibility of the electroactive ions, and a predominant semi-infinite diffusion mechanism during the redox-based charge storage process. This material also shows bias-potential-independent equivalent series resistance, very low charge-transfer resistance, and diagonal Warburg profile, corresponding to the ion diffusion occurring during the electrochemical processes in supercapacitors and batteries. Further, the fabricated NiCo2O4-based all-solid-state supercapattery (NiCo2O4||N-rGO) delivers excellent rate-specific capacity, very low internal resistance, good electrochemical and electrostructural stability (∼94% capacity retention after 10,000 charge-discharge cycles), energy density (31 W h kg-1) of a typical rechargeable battery, and power density (13,003 W kg-1) of an ultra-supercapacitor. The ultimate performance of the supercapattery is ascribed to low-dimensional crystallites, ordered inter-crystallite and channel-type bulk and boundary porosity, multiple reactive equivalents, enhanced electronic conductivity, and "ion buffering pool" like behavior of ribbon-like NiCo2O4, supplemented with enhanced electronic and ionic conductivities of N-doped rGO (negative electrode) and PVA/KOH gel (electrolyte separator), respectively.

Electrochemical Supercapacitors: From Mechanism Understanding to Multifunctional Applications

AbstractElectrochemical supercapacitors (SC) with high power and long cycle life have been extensively studied and applied in certain areas. However, a majority of the efforts have been devoted to developing SCs with improved performance through novel electrode/electrolytes design. The full mechanistic understanding of SCs based on different electrode materials has not yet been realized. In addition, exploration of new functions for SCs to widen their applications must be accelerated. In this essay, the use of advanced characterization methods (in situ X‐ray diffraction, in situ X‐ray scattering, in situ atomic force microscopy, in situ nuclear magnetic resonance, in situ Raman/infrared spectroscopy, electrochemical quartz crystal microbalance, scanning electrochemical microscopy, etc.) to unveil the electrochemical process of SCs from different aspects will be discussed. The working principles, information to be extracted, and case studies of respective methods will be presented. The multipronged mechanism studies of electrode properties inspire and enable exploration of extra functions within the same electrochemical SCs. Realization of mechanically deformable, low‐temperature, color tunable, self‐healable, and self‐chargeable SCs; integrated SC‐sensors; and SC‐actuators with adoption of new electrode/electrolyte/current collectors/configurations are showcased. The remaining issues hindering the wide exploitation of SCs and the future development trend of SCs are also discussed.

Supercapacitive properties of MnO2 and underlying kinetics by distribution of relaxation time method

Charging and discharging processes in supercapacitors and secondary batteries are critically dependent on elementary rate determining sub-processes (ERDSPs) with different relaxation time (τ) at different frequencies. In this work, distribution of relaxation time (DRT) method is employed to investigate the ERDSPs and relevant kinetics of electrodeposited MnO2 electrodes. Two ERDSPs are identified: ion diffusion at low frequencies (100-0.1Hz), and the mechanism of ERDSP at high frequencies (105-102 Hz) is found to be charge transfer across the MnO2/electrolyte interface. The rate capability is found to reduce with the increasing electrode thickness (l), which is attributed to the increasing relaxation time of diffusion (τd), proportional to l2. Oxygen vacancies (VO••) can lead to an increased specific capacitance. Meanwhile, VO•• with positive effective charge may also lead to repulsion to cations, and thus increase the activation energy (Ea) of charge transfer (Ea-ct) and diffusion (Ea-diff) processes. Enhancing cycling stability with reducing Mn3+ and VO•• concentration may result from reducing Mn element loss due to disproportionation reaction of Mn3+, and alleviating polarization and increasing reversibility due to reducing Ea-ct and Ea-diff.

Enhanced electrochemical performances of heteroatom-enriched carbon with hierarchical pores prepared by trehalose as a pore-forming agent and a simple one-step carbonization/activation process for supercapacitors

It is highly desired to simultaneously introduce active heteroatoms and abundant hierarchical pore structures for enhanced electrochemical performances on carbon materials. Herein, trehalose as a pore-forming agent was added into polyvinylpyrrolidone/melamine formaldedyde resin mixture with high concentrations of nitrogen and oxygen. Then a simple one-step carbonization/activation process was adopted and heteroatom-enriched carbon with hierarchical pores (HPC) was fabricated successfully. HPC/HPC symmetric supercapacitors were assembled using KOH electrolyte. It is clearly demonstrated that due to the pore-forming action of trehalose HPC shows the porous honeycomb, interconnected and worm-like pore structure, which is favorable to enhance the double-layer capacitance. It is confirmed that in our system the three active species of pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5) and quinone type oxygen (O-I) are responsible for the pseudocapacitive behavior. Based on XPS, nitrogen adsorption/desorption isotherms and electrochemical impedance spectroscopy, it is deduced that the ratio-optimized HPC-T30 exhibits high concentration of three active species (8.17 at.%), increased specific area (351.26 m2 g−1) and tuned hierarchical pore structures with substantial micropores (micropore area of 321.68 m2 g−1) and a small amount of mesopores and macropores, which lead to decrease of charge transfer resistance, increase of transfer rate of electrolyte ions in the pores and excellent electrochemical performances. In cyclic voltammetry tests of three-electrode system and galvanostatic charge/discharge tests of two-electrode system, HPC-T30 displays high specific capacitance, 46% and 1.2-time enhancement compared to untreated HPC-T0, respectively. The optimized HPC-T30/HPC-T30 supercapacitor delivers the energy density of 6.69 W h kg−1 in 6 M KOH electrolyte. Furthermore, the supercapacitor shows a capacitance retention of 91.16% up to 6000 cycles and the coulombic efficiency reaches nearly 100% for each charged/discharge cycle, demonstrating its good cyclic stability.

In situ electrochemical electron paramagnetic resonance spectroscopy as a tool to probe electrical double layer capacitance.

Electron paramagnetic resonance (EPR) spectroscopy is applied in situ to monitor the electrochemical capacitance of activated carbon in aqueous solutions, thereby revealing aspects of the charge storage mechanism. The EPR signal of activated carbon increases during the charging process and returns reversibly when discharged. Simulation of the spectral lineshape and its temperature dependence indicate that two kinds of spins exist: spin at defects giving rise to a narrow signal, and spins associated with surface-bound aromatic moieties causing a broad signal. A potential-dependent response of the narrow feature is seen in each of the electrolyte solutions used, while changes of the broad feature occur only at higher electrolyte concentrations. The results suggest that the observed increase of unpaired electron density on activated carbon is due to the formation of radical species due to reduction of functional groups. The potential dependence of the broad feature at higher electrolyte concentrations may be related to the further adsorption of ions into the deep porous structure of activated carbon.

Diagnostics of supercapacitors

The paper represents the mathematical model for diagnostics of supercapacitors. The research objectives are the problem of determining a supercapacitor technical condition during its operation. The general reliability of diagnostics is described as the methodological and instrumental reliabilities of diagnostics. The instrumental diagnostic reliability of supercapacitor includes the probabilities of errors of the first and second kind, α and β respectively. The methodological approach to increasing the reliability of supercapacitor diagnostic has been proposed, in terms of multi-parameter supercapacitor diagnostic by applying nonlinear, frequency dependent mathematical models of supercapacitors that take into account nonlinearity, frequency dispersion of parameters and the effect of transient processes in supercapacitors. The more frequencies, operating voltages and currents are applied in the supercapacitor diagnostics, the more methodological reliability of diagnostics will increase in relation to the methodological reliability of supercapacitor diagnostics when only one frequency, voltage and current are applied.

Enhanced electrochemical performance of ordered mesoporous carbons by a one-step carbonization/activation treatment

Abstract Activated ordered mesoporous carbon (AOMC) material was prepared by a one-step carbonization/activation method using SBA-15 as hard template, furfuryl alchohol (FA) as carbon source and ZnCl 2 as activation agent. The as-synthesized AOMC possesses hierarchical micro/mesoporous structure and high BET surface area (up to 1465.7 m 2 g − 1 ), which accelerate the ion-transport and charge-transfer during charging/discharging process for supercapacitors. A maximum specific capacitance value of 270 F g − 1 for AOMC electrode at a current density of 1 A g − 1 in 6 M KOH aqueous solution was obtained from the charge–discharge curves. Meanwhile, it also exhibited outstanding rate performance (~ 80% capacity retention from 0.5 A g − 1 to 20 A g − 1 ) and excellent cycling stability (∼ 94% retention after 5000 continuous charge–discharge cycles). The assembled AOMC-based symmetric cell delivers a maximum energy density of 11.8 Wh kg − 1 at a power density of 450.1 W kg − 1 under a voltage range of 0–1.8 V in 0.5 M Na 2 SO 4 aqueous electrolyte.

Solving the Capacitive Paradox of 2D MXene using Electrochemical Quartz‐Crystal Admittance and In Situ Electronic Conductance Measurements

Fast ion adsorption processes in supercapacitors enable quick storage/delivery of significant amounts of energy, while ion intercalation in battery materials leads to even larger amounts of energy stored, but at substantially lower rates due to diffusional limitations. Intercalation of ions into the recently discovered 2D Ti3C2Tx (MXene) occurs with a very high rate and leads to high capacitance, posing a paradox. Herein, by characterizing the mechanical deformations of MXene electrode materials at various states‐of‐charge with a variety of cations (Li, Na, K, Cs, Mg, Ca, Ba, and three tetra­alkylammonium cations) during cycling by electrochemical quartz‐crystal admittance (EQCA, quartz‐crystal microbalance with dissipation monitoring) combined with in situ electronic conductance and electrochemical impedance, light is shone on this paradox. Based on this work, it appears that the capacitive paradox stems from cationic insertion, accompanied by significant deformation of the MXene particles, that occurs so rapidly so as to resemble 2D ion adsorption at solid‐liquid interfaces. The latter is greatly facilitated by the presence of water molecules between the MXene sheets.