Galvanostatic Charge Discharge Process Research Articles

Synthesis of non-planar graphene-wrapped copper nanoparticles with iron(III) oxide decoration for high performance supercapacitors

The performance of supercapacitors strongly depends on the electrochemical characterizations of electrode materials. Herein, a composite material consisted of iron(III) oxide (Fe2O3) and multilayer graphene-wrapped copper nanoparticles (Fe2O3/MLG-Cu NPs) is fabricated on a flexible carbon cloth (CC) substrate via two-step synthesis process for supercapacitor application. Where, MLG-Cu NPs are prepared on CC by one-step chemical vapor deposition synthesis approach; thereafter, the Fe2O3 is further deposited on the MLG-Cu NPs/CC via successive ionic layer adsorption and reaction method. The related material characterizations of Fe2O3/MLG-Cu NPs are well investigated by scanning electron microscopic, high resolution transmission electron microscopy), Raman spectrometer and X-ray photoelectron spectroscopy; the electrochemical behaviors of the pertinent electrodes are studied by cyclic voltammogram, galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy measurements. The flexible electrode with Fe2O3/MLG-Cu NPs composites exhibits the best specific capacitance of 1092.6 mF cm−2 at 1 A g−1, which is much higher than those of electrodes with Fe2O3 (863.7 mF cm−2), MLG-Cu NPs (257.4 mF cm−2), multilayer graphene hollow balls (MLGHBs, 14.4 mF cm−2) and Fe2O3/MLGHBs (287.2 mF cm−2). Fe2O3/MLG-Cu NPs electrode also exhibits an excellent GCD durability, and its capacitance remains 88% of its original value after 5000 cycles of the GCD process. Finally, a supercapacitor system consisted of four Fe2O3/MLG-Cu NPs/CC electrodes can efficiently power various light-emitting diodes (i.e. red, yellow, green, and blue lights), demonstrating the practical application of Fe2O3/MLG-Cu NPs/CC electrode.

Insight into the failure mechanism of large-scale cylindrical lithium-sulphur cells.

Li-S batteries with a sulphur loading content of 5 mg cm-2 were produced as large-scale 18 650 cylindrical cells. We have found that a key failure mode of cylindrical Li-S battery cells is the severe capacity fading during the galvanostatic charge-discharge process due to the corrosion of the electrodes, the electrolyte decomposition, and the severe polysulphide shuttling effect.

Elucidating MnO2 Reaction Mechanism By Multi-Modal Characterization in Aqueous Zn-MnO2 Batteries

Aqueous Zn-ion batteries has attracted great attention in recent years, as a promising candidate for grid energy storage applications. An aqueous system offers intrinsic safety, high ionic conductivity contributing improved power capability and raw materials that are more earth abundant and environment friendly. Numerous promising reports haven been focusing on the Zn/MnO2 system owing to its low cost, moderate discharge potentials and with improved reversibility in the mild aqueous electrolyte. However, many questions remain unanswered regarding its reaction mechanism. The different reaction mechanisms including Zn+2 insertion, H+ insertion, chemical conversion reaction including the combined intercalation and conversion reaction mechanism, and the dissolution-deposition of the manganese oxide.In this work, we aim to unravel the reaction mechanism by a systematic multimodal synchrotron characterization. This work discusses the galvano-static charge-discharge process of aqueous Zn-MnO2 batteries using operando measurements, which provides us with a direct insight into the phenomenon and can be directly correlated to the battery's electrochemical response. The multimodal techniques include operando X-ray diffraction to study the structural phase change of the cathode active material, operando X-ray absorption spectroscopy to probe the local structure changes and transmission X-ray microscopy studies to observe the key morphological events. Overall, this multimodal approach gives us an insight into the reaction mechanism enabling us to better design Zn-MnO2 batteries for practical applications.

Modeling galvanostatic charge\u2013discharge of nanoporous supercapacitors

Molecular modeling has been considered indispensable in studying the energy storage of supercapacitors at the atomistic level. The constant potential method (CPM) allows the electric potential to be kept uniform in the electrode, which is essential for a realistic description of the charge repartition and dynamics process in supercapacitors. However, previous CPM studies have been limited to the potentiostatic mode. Although widely adopted in experiments, the galvanostatic mode has rarely been investigated in CPM simulations because of a lack of effective methods. Here we develop a modeling approach to simulating the galvanostatic charge–discharge process of supercapacitors under constant potential. We show that, for nanoporous electrodes, this modeling approach can capture experimentally consistent dynamics in supercapacitors. It can also delineate, at the molecular scale, the hysteresis in ion adsorption–desorption dynamics during charging and discharging. This approach thus enables the further accurate modeling of the physics and electrochemistry in supercapacitor dynamics.

Hierarchical Co3O4 decorated nitrogen-doped graphene oxide nanosheets for energy storage and gas sensing applications

Nano-sized cobalt oxide decorated nitrogen-doped graphene oxide (Co3O4@NGO) composite was produced by a feasible and cost-effective hydrothermal route for electrochemical supercapacitors and gas sensor applications. The composite materials formation was ascertained by Raman spectroscopy, X-ray diffraction, and X-ray photo electron spectroscopy analyses. Field emission scanning electron microscopy (FE-SEM) and field emission transmission electron microscopy (FE-TEM) results explored the controlled nanoscale-sized sheet-like morphology for the prepared composite materials. Electrochemical storage properties were studied by cyclic voltammetry (CV), galvanostatic charge–discharge process (GCD), and electrochemical impedance spectroscopy analyses using three-electrode configuration with 3 M KOH electrolyte. The observed results showed ~466 F/g specific capacitance at a current density of 1 A/g for Co3O4@NGO composite structure with the capacity retention of 96 % after 5000 cycles. Further, the synthesized Co3O4@NGO composite revealed improved detection response, cyclability, and linearity for dimethyl methyl phosphonate vapor gas sensing. The synthesized composite also demonstrated excellent selectivity, stability, sensitivity, and rapid response time.

Ca-substituted P3-type NaxNi1/3Mn1/3Co1/3O2 as a potential high voltage cathode active material for sodium-ion batteries

The electrochemical property of the layered sodium transition metal oxide is dependent upon the reversible phase transition during the sodium intercalation/deintercalation process. Here we synthesized Ca-doped P3-type NaxNi1/3Mn1/3Co1/3O2 as a potential cathode active material for sodium-ion batteries. The Ca-doping leads the lattice expansion along the c-axis, due to the electrostatic repulsion between Ca2+ ions and the transition metal ions. Even though the doped Ca2+ ions occupy Na+ sites, the Ca-doped cathode shows lower overpotential during the galvanostatic charge-discharge process. The electrochemical impedance spectroscopy proved that the charge-transfer resistance of the Ca-doped layered oxide is lower than the Ca-free one at low cell voltage. These results indicate that the Ca-doping enhances the mobility of the Na+ ions in the layered structure. The doped Ca2+ ions also suppress the irreversible O′3–O1 phase transition upon the full deintercalation of the sodium ions from the layered structure. The improved stability of the Ca-doped NaxNi1/3Mn1/3Co1/3O2 leads better capacity retention during the galvanostatic cycling.

Electrochemical activation fabrication towards high-capacity nickel hydroxide electrode

Nickel hydroxide (Ni(OH)2), as a representative of highly active materials, has been widely studied and applied in the field of energy storage. However, a typical electrochemical activation process is often required before its practical application. Thus, exploring this key activation process is of great significance for the construction of high-capacity Ni(OH)2 electrodes. In this work, we find that the activation of galvanostatic charge-discharge (GCD) is apparently different from that of cyclic voltammetry (CV). After activation, the composition of Ni(OH)2 deposited on the surface of Ni foam prepared by GCD and CV process is the same, but the morphology is obviously different. The layered porous network Ni(OH)2 nanostructures can be prepared by GCD process, while the spherical stacking nanostructures can be formed by CV process. By combining a wet process, two adhesive Ni(OH)2-coated Ni foam (Ni(OH)2/Ni) electrodes can be successfully fabricated. The results exhibit that the Ni(OH)2/Ni electrode obtained by the GCD process shows super energy storage capacity (2338.9 μA h cm-2 (3402 F g-1)) at a high loading mass of Ni(OH)2, which is almost 1.7 times higher than that of the CV-activated electrode (1380.6 μA h cm-2 (2008 F g-1)).

Ultrahigh “Relative Energy Density” and Mass Loading of Carbon Cloth Anodes for K-Ion Batteries

Mass loading and potential plateau are the two most important issues of potassium (K)-ion batteries (KIBs), but they have long been ignored in previous studies. Herein, we report a simple and scala...

Ta2O5 nanoparticles as an anode material for lithium ion battery

Ta2O5 is one of the promising anode materials for lithium ion battery application undergoing conversion reaction in combination with an extrinsic pseudocapacitance property. We have synthesized Ta2O5 nanoparticles by hydrothermal method using and without using polyethylene glycol (PEG) as co-solvent. Ta2O5 nanoparticles were characterized by PXRD, SEM, EDAX, BET, and TEM analysis. Electrochemical characterizations like Cyclic voltammetry, galvanostatic charge-discharge process, and rate capability were studied for Ta2O5 NPs. A reversible capability of 130 mA h g−1 is retained after 85 cycles at 0.1C rate by Ta2O5 NPs synthesized using PEG, and reversible capability of 127 mA h g−1 is retained after 40 cycles at 0.1C rate by Ta2O5 NPs synthesized without using PEG. Polyethylene glycol as cosolvent induced enhanced porosity favoring the fast lithium ion transport and better reversible capacity upon cycling.

Recrystallization-induced capacity fading of birnessite-type MnO2 nanosheets for pseudocapacitors

Birnessite-type MnO2 materials are widely studied as electrodes for pseudocapacitors. However, the poor cycling stability limits their applications. Here, wrinkle birnessite-type MnO2 nanosheet array has been synthesized and taken as an example to discover the reason of capacity fading for pseudocapacitors. It is discovered that in the the galvanostatic charge-discharge process, the smalle nanosheets blocking the macropores of the array framework have been dissolved and recrystallized on the large wrinkle birnessite nanosheet array framework, finally forming a hierarchical structure of nanosheets@nanosheets of birnessite. The recrystallizaion of the small birnessite nanosheets may decrease the specific capacity through the decreasement of the surface area of the electrode.

Facile construction of 3D porous carbon nanotubes/polypyrrole and reduced graphene oxide on carbon nanotube fiber for high-performance asymmetric supercapacitors

Flexible fiber-based supercapacitors applied in portable energy storage devices and wearable electronics have recently aroused widespread research interests. However, challenges still exist in the pursuit of facile fabrication technique for simultaneously realizing high capacitive performance and excellent mechanical stability. Here, two novel types of core-sheathed hybrid electrodes with carbon nanotube fiber (CNF) as the core and three-dimensional (3D) porous carbon nanotubes/polypyrrole (CNTs/PPy) or reduced graphene oxide (rGO) as the sheath were hierarchically constructed through one-pot electrochemical deposition. An all-solid-state asymmetric fiber-shaped supercapacitor was assembled by wrapping the gel electrolyte coated negative CNF/rGO electrode along the positive CNF/CNTs/PPy electrode. Thanks to the combination of abundant 3D pore structure and synergistic effect of different components in the fiber electrodes, the resulted supercapacitors exhibited a broadened potential window of 1.6 V, a high areal specific capacitance of 58.82 mF cm−2 and a high areal energy density of 20.91 μW h cm−2. It should be noted that 98.6% of the initial capacitance of the supercapacitor device can still be maintained after experiencing 200 reciprocating bending cycles, demonstrating outstanding mechanical stability. Moreover, the supercapacitor exhibited excellent cyclic performance, which was testified by 90% capacity retention after 10000 times of galvanostatic charge-discharge process.

Unraveling the Underlying Redox Mechanism in Na2FeP2O7 Pyrophosphate Cathode for Sodium-Ion Batteries

Polyanionic materials have contributed plethora of electrodes (anode and cathode) in the field of Li-ion and Na-ion battery. Li-ion batteries (LIB) and Na-ion batteries (SIB) are building up in our landfills thus researchers need to develop more efficient, less hazardous and safer electrode materials for these class of rechargeable batteries. In the race of developing novel and safer cathode “pyrophosphate-based” polyanionic compounds has rekindled scientific attention.[1] Recently analysis of phase stabilities reveals that LiFeP2O7 (P21/c) is intrinsically more stable than FePO4 (olivine) against reduction (high temperature). Similar high thermal stability is also observed for Li1.4MnP2O7 which decomposes to Li2MnP2O7, Mn2P2O7, LiPO3, and O2 at 450 °C, much higher than the olivine counterpart MnPO4.[2] considering this fact and probing Li/Na-M-P-O quaternary system, Li2FeP2O7 and Na2FeP2O7 has been reported as a 3.5 V and 3 V cathode material for LIB and SIB (Figure A, B) .[3,4] These materials offer a robust three-dimensional (P2O7)-4-framework with multiple sites for alkali ions and robust chemical/ thermal stability. Based on earth-abundant elements, Na2FeP2O7 can form an economic cathode for grid storage. Thus in this work we focus our interest on Na2FeP2O7. Na2FeP2O7 has triclinic (space group: P-1) structure, where each corner of FeO6 octahedra is bridged to PO4 tetrahedra. Such arrangements of FeO6 octahedra where no edge-sharing can be seen will delimit large channels for migration of charge carrier. During the galvanostatic charge-discharge reversible desodiation reaction occur between pristine Na2Fe2+P2O7 and desodiated NaFe3+P2O7 end-members involving an Fe3+/Fe2+ redox reaction. During this redox process, it goes through various mixed-valence intermediate compositions [Na2-xFe2+/3+P2O7, x = 0-1] with steady structural ordering/ reorientation. These intermediate stages can be traced with a combination of electrochemical techniques and in-situ X-ray diffraction technology.[5] Here, in the present work we have studied the underlying redox mechanism by preparing a custom-made in-situ cell, able to perform both in reflection and transmission mode. Pristine Na2FeP2O7 was synthesized by combustion method using Ferric nitrate (III) precursor. Carbothermal reduction was done using Ascorbic acid (for reduction of Fe+3 into Fe+2) and Urea fuel. Electrodes were prepared by mixing 80 wt% of Na2FeP2O7 with 20 wt% of Carbon super P. 1 M NaClO4 dissolved in propylene carbonate (PC) was used as an electrolyte to form Na half-cell. In-situ X-ray diffraction patterns were recorded (Figure C) at C/25 C-rate during electrochemical testing. XRD pattern were recorded for 3624 seconds at an interval of 2 seconds in the angular domain of 9-60°. Observation of such measurement clearly tells the composition variation of Na into Na2FeP2O7 host during the Galvanostatic charge-discharge process. The exploration of structure-property relationship like de-insertion and re-insertion of Na from the host material is very clear from the phases recorded which actually tells what are the crystal planes involved for charge carrier migration. A close look in the 2θ range of 15-18°, 20-25° and 30-35° reveals the structural changes occurring due to Na de-insertion from the host Na2FeP2O7 giving rise to Na2-xFeP2O7 (at complete charge) shown in red color in Figure C. After complete charge the diffraction peaks are transforming and acquiring the pristine phase, shown in blue color which strongly suggest the re-insertion of Na into Na2-xFeP2O7 giving rise Na2FeP2O7 after the first electrochemical cycle. Volume strain of the crystal during cathode operation can be verified in the 2θ range 30-35° which shows widening of the peak position i.e. some peaks are shifting towards low 2θ, and some are towards high 2θ, possibly coming due to lattice expansion due to Na removal. Removal of Na has resulted stronger repulsive interactions between electronegative oxygens in Na2-xFeP2O7 during charge. No matter how strong is this interaction structure is not collapsing which is due to strong framework built by pyrophosphate units. Thus in-situ operando diffraction based experiments are very beneficial for the researchers as at the same time it gives both electrochemical data and structure insight which will reveal the stability of future cathode materials.

Birnessite manganese oxide nanosheets assembled on Ni foam as high-performance pseudocapacitor electrodes: Electrochemical oxidation driven porous honeycomb architecture formation

It has not been considered in the reported literatures that the current or the cyclic number of galvanostatic charge-discharge process controls the morphology growth of manganese oxide materials. Here, standing birnessite nanosheets assembled porous honeycomb architecture is in situ formed on Ni foam by electrochemical oxidation of hydrothermally synthesized Mn3O4 nanospheres electrodes via a electrochemical cycling technique. The birnessite morphology much depends on the conditions of the electrochemical cycling. By tuning the current and cycle number, Mn3O4 nanospheres are converted to birnessite with optimal morphology, while a high pseudocapacitive performance is achieved. The optimized birnessite electrodes exhibit a high level of specific capacitance (377 F g−1 at 1 A g−1), a superior rate capability (227 F g−1 at 30 A g−1) and a good cycling stability (75% retention after 4000 cycles). This work provides useful strategies for developing high-performance manganese oxide electrodes.

Three-dimensional crisscross porous manganese oxide/carbon composite networks for high performance supercapacitor electrodes

Manganese oxide/carbon (MnOx/C) composites have been successfully prepared via a high temperature heat treatment method followed by the electrochemical oxidation. The presence of carbon not only enhances the electronic conductivity of manganese oxides (MnOx), but also provides more active sites for the transformation of manganese monoxide (MnO) during the galvanostatic charge–discharge process. Simultaneity, the interconnected porous structures of MnOx/C samples are believed to provide a continuous channel for the diffusion of electrolyte ion and shorten the diffusion length of ions involved in the charge/discharge cycling processes. Consequently, these advantages endow the MnOx/C electrode a better capacitance performance, a superior long-term cyclic stability and outstanding rate capability compared with pristine MnOx. More importantly, the composites show a fascinating capacitance of 807Fg−1 at 1Ag−1, which is much higher than the reported hydrous RuO2 electrodes. It can be easily speculated that MnOx/C composites will act as a promising electrode materials for designing high-performance supercapacitors.

The preparation of Co9S8 and CoS2 nanoparticles by a high energy ball-milling method and their electrochemical hydrogen storage properties

Two different Co–S compounds with enhanced hydrogen storage properties, Co9S8 and CoS2, were prepared by ball-milling mixtures of Co metal and S powder. X-ray diffractometry, scanning electron microscopy and transmission electron microscopy were used to show that specific molar ratios of Co:S and ball-milling speeds and times result in pure Co9S8 and CoS2, thus overcoming a long-standing inability to obtain pure Co–S compounds via ball-milling. A galvanostatic charge–discharge process and cyclic voltammetry measurements showed that the as-obtained Co9S8 and CoS2 nanoparticles have enhanced electrochemical hydrogen storage capacities of 1.79 and 1.57 wt% hydrogen, respectively, which are higher than those previously reported. In addition, based on the corresponding X-ray photoelectron spectroscopy and cyclic voltammetry measurements, a new electrochemical hydrogen storage mechanism for the two Co–S compounds was proposed and discussed.

A 4 V-electrochemical capacitor using electrode and electrolyte materials free of metals

A 4 V asymmetric capacitor has been assembled using non-porous activated mesophase carbon microbeads (AMCMB) as the negative electrode, a graphitized carbon (GMCMB) as the positive electrode and quaternary alkyl ammonium based organic electrolyte. AMCMB has been heated under the reduction atmosphere of hydrogen/nitrogen after a chemical activation. It has small specific surface area (24.48 m2 g−1), but delivers a large capacitance value due to the irreversible “electrochemical activation” in the initial galvanostatic charge–discharge process. The storage charge behavior of AMCMB has also been investigated by ex situ XRD, Raman spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. In terms of specific capacity, energy density, power density and cycle ability, the AMCMB/GMCMB capacitor is superior the other capacitors like AC/graphite, AC/AC.

High-energy-density LiMn0.7Fe0.3PO4 nanorods synthesized by microwave-assisted solvothermal method

Microwave-assisted solvothermal method has been developed for synthesizing LiMn0.7Fe0.3PO4 cathode materials with an olivine structure. The obtained LiMn0.7Fe0.3PO4 nanorods were characterized by X-ray diffraction, scanning and transmission electron microscope, Brunauer-Emmett-Teller surface area measurements, and electrochemical tests. Electrochemical tests clearly indicate that the as-made LiMn0.7Fe0.3PO4 nanorods exhibit two redox activities of Fe3+/Fe2+ and Mn3+/Mn2+ couple at galvanostatic charge-discharge process, which is due to the coexistence of Mn2+ with Fe2+ at 4c sites. The as-synthesized materials have high energy density, excellent rate capability and cycling stability.

Preparation and electrochemical properties of x Li2MnO3·(1-x)LiMn2O4 composites

Li2MnO3-doped spinel LiMn2O4 composites were synthesized by sol-gel method to improve the electrochemical performance of LiMn2O4. The microstructures, morphologies and electrochemical performance of the obtained xLi2MnO3·(1-x)LiMn2O4 composites were characterized by X-ray diffraction(XRD), scan electron microscopy(SEM) and a galvanostatic charge-discharge process. It was found that both Li2MnO3 and LiMn2O4 components exist in xLi2MnO3·(1-x)LiMn2O4(0<x<1) composites. The 0.3Li2MnO3·0.7LiMn2O4 composite shows the optimized electrochemical performance, including discharge capacity and cycle stability. It was demonstrated that Li2MnO3-doped spinel LiMn2O4 cathode material can work at wide potential window with quite good capacity retention and considerably larger reversible capacity compared to single-phase LiMn2O4 component.

Electrochemical properties of LiNi0.8Co0.15Al0.05O2–graphene composite as cathode materials for lithium-ion batteries

A LiNi0.8Co0.15Al0.05O2–graphene composite has been synthesized by a high energy mechanical ball milling (HEMM) process. The structural and morphological features of the synthesized samples are characterized by various techniques. Electrochemical studies on the LiNi0.8Co0.15Al0.05O2–graphene composite are conducted using of the galvanostatic charge–discharge process and electrochemical impedance spectroscopy methods by constructing a lithium-ion cell. The LiNi0.8Co0.15Al0.05O2–graphene composite exhibits a high capacity of >180mAh/g with good cyclability and high rate capability. The improved electrochemical performance is attributed to increased electrical contact provided by the graphene layer, which reduces the cell polarization.

High energy ball-milling preparation of Co–B amorphous alloy with high electrochemical hydrogen storage ability

As a new type hydrogen storage material, Co–B hydrogen storage alloy was a hot research topic and has been paid increasing attention in recent years. In this work, a series of Co–B alloys with molar ratio of 1:1, 2:1 and 3:1 were synthesized by high energy ball-milling method. The phase, structure and morphology of the samples have been characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), which demonstrated that the samples ball-milled more than 5h were mainly amorphous phase. Their electrochemical performance as electrochemical hydrogen storage material was investigated by galvanostatic charge–discharge process and cyclic voltammetry (CV) techniques. Electrochemical measurements showed that the Co–B alloys had excellent electrochemical hydrogen storage capacities and exhibited remarkable high rate discharge performance. Their electrochemical hydrogen storage capacity was 771mAh/g, which was much higher than previous reports (238mAh/g). Based on XRD and CV results, the exceptional electrochemical performances were suggested to arise from the electrochemical hydrogen storage reaction on the Co–B materials, which was brought about by the special amorphous structure and high chemical reactivity of the Co–B particles.