Asymmetric Supercapacitor Cell Research Articles

Synthesis and Characterization of α‐­NiMoO4 Nanorods for Supercapacitor ­Application

AbstractWe report the synthesis of electrode materials based on one‐dimensional α‐NiMoO4 nanorods for application as a supercapacitor. The structure and morphology of the electrodes were characterized by powder X‐ray diffraction, Raman analysis, X‐ray photoelectron spectroscopy, and scanning and transmission electron microscopy. The maximum specific capacitance obtained from electrochemical measurements on a standard three‐electrode system was 730 F g–1 at a scan rate of 5 mV s–1. Furthermore, an asymmetric supercapacitor (ASC) was fabricated in which the NiMoO4 nanorods were used as the positive electrode and reduced graphene oxide was used as the negative electrode. The maximum specific energy obtained from the ASC study was 12.31 Wh kg–1 at a specific current of 0.5 A g–1 in an aqueous KOH electrolyte. This ASC cell showed good stability with 85 % capacitance retention up to 2000 cycles. These results reveal that our ASC shows a high performance and specific energy as well as good cycling stability.

Hierarchical Cu2O/CuO/Co3O4 core-shell nanowires: synthesis and electrochemical properties

We prepared hierarchical Cu2O/CuO/Co3O4 core–shell nanowires (NWs) via a facile chemical deposition method followed by calcination for use as the electrode of supercapacitors. The Cu2O/CuO/Co3O4 electrode showed a specific capacitance of 318 F g−1 at a current density of 0.5 A g−1. 80% of the original specific capacitance was retained after 3000 cycles at a current density of 5 A g−1. An asymmetric supercapacitor cell using Cu2O/CuO/Co3O4 NWs as the positive electrode and activated graphene as the negative electrode exhibited a maximum energy density of 12 Wh kg−1. The electrochemical properties of the electrode were strongly related to the hierarchical nanostructure, which not only provided rich active sites but also shortened ion transport pathways.

An Investigation on the Degradation Behavior of an Asymmetric Supercapacitor of α-MnO2//AC

We have investigated the degradation behavior of a typical asymmetric supercapacitor cell operating in an aqueous solution of Na2SO4 with a voltage window of 2 V, where α-MnO2 and activated carbon (AC) were chosen as the active materials for positive and negative electrodes, respectively. In this study, potentials of α-MnO2 and AC electrodes of the asymmetric cell were monitored with respect to an Ag/AgCl reference electrode during repetitive charge/discharge cycling. Since the charges passed at positive and negative electrodes are the same, a change in a certain parameter should be compensated by changes in the other parameters. An irreversible current due to reduction of water on AC electrode triggered capacitance fade in the initial charge/discharge cycling. This phenomenon made the lowest potential more positive to be deviated from the potential region where water reduction can occur, resulting in the following stable operation. Then, α-MnO2 facilitated the oxidation of water, which deactivated α-MnO2 itself probably due to an irreversible increase in reduced Mn sites. The deactivation of α-MnO2 was compensated by expansion of its potential window in the cathodic scan, leading to further capacitance fade due to dissolution.

High-performance asymmetric pseudocapacitor cell based on cobalt hydroxide/graphene and polypyrrole/graphene electrodes

Cobalt hydroxide nanowires network grown on nitrogen modified microwave exfoliated graphite oxide (NMEG) with a specific capacitance of 610 F/g is successfully synthesized by a chemical precipitation method. Asymmetric-type pseudocapacitors are fabricated with Co(OH)2/NMEG and polypyrrole (PPy)/rG-O applied as positive and negative electrodes respectively. The electrochemical properties of the electrodes in three electrode and two electrode systems are systematically investigated in 1 M KOH electrolyte. Various supercapacitor devices, such as rG-O//Co(OH)2/NMEG, NG//NG, rG-O//rG-O, and PPy/rG-O//PPy/rG-O are assembled with electrochemical performance evaluated by cyclic voltammetry and galvanostatic charge/discharge measurements. The Co(OH)2/NMEG//PPy/rG-O asymmetric supercapacitor cells can achieve a high cell voltage of 1.6 V and an energy density up to 24.9 Wh/kg with an active materials loading of ∼5 mg/cm2, significantly higher than that of rG-O//Co(OH)2/NMEG (19.3 Wh/kg), NG//NG (16.4 Wh/kg), rG-O//rG-O (15.3 Wh/kg) and PPy/rG-O//PPy/rG-O (9.4 Wh/kg) supercapacitor devices under the same measurement environment. The PPy/rG-O is a superior negative electrode to match cobalt/nickel oxides/hydroxides based positive electrodes for supercapacitor devices.

Graphenated tantalum(IV) oxide and poly(4-styrene sulphonic acid)-doped polyaniline nanocomposite as cathode material in an electrochemical capacitor

Nanostructured poly(4-styrene sulphonic acid) and tantalum (IV) oxide-doped polyaniline nanocomposite were synthesised and their electro-conductive properties were determined. The oxide was synthesized using a modified sol-gel method and then dispersed in acidic media through sonication and entrapped in-situ into the polymeric matrix during the oxidative chemical polymerization of aniline doped with poly(4-styrene sulphonic acid). The oxides and novel polymeric nanocomposite were characterised with TEM, SEM, EDX, XRD, FTIR, UV-visible to ascertain elemental and phase composition, successful polymerization, doping, morphology and entrapment of the metal oxide nanoparticles. The electro-conductivity of the nanomaterial was interrogated using scanning electrochemical microscopy (SECM) and cyclic voltammetry (CV). The material was then anchored on activated graphitic carbon and used in the design of an asymmetric supercapacitor cell using 6M KOH aqueous electrolyte. Characteristically high specific capacitance values of 318.4 F/g with a corresponding energy and power densities of 1.57 kWh/kg and 0.435kW/kg, respectively, were demonstrated. The cell also showed high coulombic efficiency of 94.9% with a long cycle life and good cycle stability making the nanomaterial suitable for constructing supercapacitor cell electrodes.

Step-by-step assembled poly(3,4-ethylenedioxythiophene)/manganese dioxide composite electrodes: Tuning the structure for high electrochemical performance

A class of novel composite electrodes based on poly(3,4-ethylenedioxythiophene) (PEDOT) and manganese dioxide (MnO2) was assembled by step-by-step anodic deposition on nickel foam for supercapacitor application. The effect of the distribution and loading of PEDOT on the electrochemical performance of composite electrodes was studied carefully. Among these composite electrodes, the optimized PEDOT/MnO2/PEDOT sandwich electrode shows excellent capacitive behavior, and MnO2 species in the optimized sandwich electrode possesses a high specific capacitance of 487.5Fg−1 at 1Ag−1. Furthermore, an asymmetric supercapacitor cell with the optimized sandwich composite as positive electrode and active carbon (AC) as negative electrode also exhibits outstanding performance at a cell voltage of 1.8V in a 0.5M Na2SO4 aqueous electrolyte. It has a high energy density of 30.2Whkg−1 with a power density of 180Wkg−1. And even at a high power density of 2.7kWkg−1, the asymmetric supercapacitor can deliver a high energy density of 13.1Whkg−1. Moreover, the asymmetric supercapacitor displays impressive electrochemical cycling stability with 99.5% initial capacitance remained after 1000 continuous cycles.

High voltage asymmetric supercapacitor based on MnO2 and graphene electrodes

Asymmetric supercapacitor cells based on MnO2 as positive electrode and graphene as negative electrode have been constructed with 1M Na2SO4 aqueous electrolyte. Amorphous MnO2 nanoparticles were prepared by reducing KMnO4 in N,N-dimethylformamid (DMF). Graphene was prepared by hydrothermal reduction of exfoliated graphite oxide sheets. The MnO2/graphene asymmetric supercapacitor could operate reversibly at a high cell voltage of 2.0V, showing an energy density of 25.2Whkg−1 at a power density of 100Wkg−1, which is much higher than those of symmetric supercapacitors based on MnO2/MnO2 (4.9Whkg−1) and graphene/graphene (3.6Whkg−1). Moreover, the asymmetric system exhibits capacitance retention of 96% after 500 cycles.

Ultrathin MnO2 nanofibers grown on graphitic carbon spheres as high-performance asymmetric supercapacitor electrodes

Growing MnO2 nanofibers on graphitic hollow carbon spheres (GHCS) is conducted by refluxing GHCS in a KMnO4 aqueous solution aimed to enhance the electrochemically active surface area of MnO2. The stoichiometric redox reaction between GHCS and MnO4− yields GHCS–MnO2 composites with controllable MnO2 content. It is found that these ultrathin MnO2 nanofibers are vertically grown on the external surface of the GHCS, yielding a composite electrode showing good electron transport, rapid ion penetration, fast and reversible Faradic reaction, and excellent rate performance when used as supercapacitor electrode materials. An asymmetric supercapacitor cell with GHCS–MnO2 as the positive electrode and GHCS as the negative electrode can be reversibly charged/discharged at a cell voltage of 2.0 V in a 1.0 mol L−1 Na2SO4 aqueous electrolyte, delivering an energy density of 22.1 Wh kg−1 and a power density of 7.0 kW kg−1. The asymmetric supercapacitor exhibits an excellent electrochemical cycling stability with 99% initial capacitance and 90% coulombic efficiency remained after 1000 continuous cycles measured using the galvanostatic charge–discharge technique.

Synthesis and electrochemical properties of activated carbons and Li4Ti5O12 as electrode materials for supercapacitors

New activated nanoporous carbons, produced by carbonization of mixtures of coal tar pitch and furfural with subsequent steam activation, as well as electrochemically active oxide Li4Ti5O12, prepared by thermal co-decomposition of oxalates, were tested and characterized as electrode materials for electrochemical supercapacitors. The phase composition, microstructure, surface morphology and porous structure of the materials were studied. Pure carbon electrodes as well as composite electrodes based on these materials obtained were fabricated. Two types of supercapacitor (SC) cells were assembled and subjected to charge–discharge cycling study at different current rates: (1) symmetric sandwich-type SC cells with identical activated carbon electrodes and different organic electrolytes, and (2) asymmetric hybrid SC cell composed by activated graphitized carbon as a negative electrode and activated carbon–Li4Ti5O12 oxide composite as a positive electrode, and an organic electrolyte (LiPF6–dimethyl carbonate/ethylene carbonate (DMC/EC). Four types of carbons with different specific surface area (1,000–1,600 m2 g−1) and texture parameters, as well as three types of organic electrolytes: Et4NBF4–propylene carbonate (PC), LiBF4–PC and LiPF6–DMC/EC in the symmetric SC cell, were tested and compared with each other. Capacitance value up to 70 F g−1 for the symmetric SC, depending on the electrolyte microstructure and conductivity of the carbon material used, and capacitance of about 150 F g−1 for the asymmetric SC cell, with good cycleability for both supercapacitor systems, were obtained.

Electrochemical Performances of Nanoparticle Fe3O4/Activated Carbon Supercapacitor Using KOH Electrolyte Solution

In this study, activated carbon (AC)-Fe3O4 nanoparticles asymmetric supercapacitor cells have been assembled and characterized in 6 M KOH aqueous electrolyte for the first time. The nanostructure Fe3O4 was prepared by the microwave method. It only cost several minutes to prepare magnetite nanoparticles with average particle size of 35 nm. The electrochemical performances of the hybrid AC-Fe3O4 supercapacitor were tested by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge−discharge tests. The results show that the asymmetric supercapacitor has electrochemical capacitance performance within potential range 0−1.2 V. The supercapacitor delivered a specific capacitance of 37.9 F/g at a current density of 0.5 mA/cm2. The result of cyclic characteristic test showed that it also can keep 82% of initial capacity over 500 cycles.

Pharmacological, microbiological and phytochemical studies on roots of Aegle marmelos

Cobalt hydroxide nanowires network grown on nitrogen modified microwave exfoliated graphite oxide (NMEG) with a specific capacitance of 610 F/g is successfully synthesized by a chemical precipitation method. Asymmetric-type pseudocapacitors are fabricated with Co(OH)2/NMEG and polypyrrole (PPy)/rG-O applied as positive and negative electrodes respectively. The electrochemical properties of the electrodes in three electrode and two electrode systems are systematically investigated in 1 M KOH electrolyte. Various supercapacitor devices, such as rG-O//Co(OH)2/NMEG, NG//NG, rG-O//rG-O, and PPy/rG-O//PPy/rG-O are assembled with electrochemical performance evaluated by cyclic voltammetry and galvanostatic charge/discharge measurements. The Co(OH)2/NMEG//PPy/rG-O asymmetric supercapacitor cells can achieve a high cell voltage of 1.6 V and an energy density up to 24.9 Wh/kg with an active materials loading of ∼5 mg/cm2, significantly higher than that of rG-O//Co(OH)2/NMEG (19.3 Wh/kg), NG//NG (16.4 Wh/kg), rG-O//rG-O (15.3 Wh/kg) and PPy/rG-O//PPy/rG-O (9.4 Wh/kg) supercapacitor devices under the same measurement environment. The PPy/rG-O is a superior negative electrode to match cobalt/nickel oxides/hydroxides based positive electrodes for supercapacitor devices.