Activated Carbon Negative Electrode Research Articles

Flexible screen-printed supercapacitors with asymmetric PANI/CDC–AC electrodes and aqueous electrolyte

Abstract We report the fabrication and electrical performance of screen-printed flexible supercapacitors based on activated carbon (AC) and polyaniline (PANI)/carbide-derived carbon (CDC) composite electrodes, and neutral aqueous electrolytes. The devices are entirely constructed from safe, low-cost, and non-toxic materials, fabricated through a mass production capable screen printing process, and fully disposable with normal household waste. Symmetric and asymmetric cells with a thin planar face-to-face structure were fabricated on polyethylene terephthalate (PET) substrate, using eco-friendly chitosan binder based electrode inks, and printed graphite current collectors. The asymmetric cell configuration, with a PANI/CDC positive electrode and an AC negative electrode, demonstrated significantly improved electrochemical performance, through increased operating voltage, energy density and power density, improved cyclic stability and rate capability, and decreased equivalent series resistance (ESR) and leakage current, compared to previously reported symmetric PANI/CDC supercapacitors. The fabricated asymmetric devices had an average capacitance of 250–270 mF, ESR of 20–23 Ω, and leakage current of 140–150 µA, depending on the PANI/CDC variant used. Energy densities of 4.8 Wh kg-1 and 4.9 Wh kg-1, power densities of 1.6 kW kg-1 and 1.5 kW kg-1, and capacitance retention rates of 93 % and 97 % after 2,000 charge-discharge cycles, were achieved with PANI/CDC (10:1) and (30:1) variants, respectively.

Polyaniline packed activated carbon as pseudocapacitive negative electrodes

Hybrid supercapacitors (HSCs) are attracting more attentions due to their potential high power and energy density. To exploring high capacity negative electrode materials, especially the capacitive materials, is more critical for further enhancing the energy density of HSC in comparing with more reported advanced battery-type positive materials. For perfect combination of electrochemical double layer and pseudocapacitive capacitance in negative electrode, the method of evenly filling polyaniline on nanopores of activated carbon is developed herein with keeping large surface area, the suitable ion-diffusion structure of activated carbon and further obtaining rich surface redox-active sites. The polyaniline packed activated carbons exhibit high specific capacitance (437.9 F g−1 at 0.5 A g−1) in negative voltage window (0 to −1.0 V), superior rate performance (378.0 F g−1 at 30 A g−1), and excellent capacitance retention even at a high mass loading and a high current density (326.5 F g−1 at 10 mg cm−2 and 20 A g−1). Additionally, an aqueous HSC based on the polyaniline packed activated carbon negative electrode and Ni-Co-LDH positive electrode presents higher energy density of 44.6 Wh kg−1 at a power density of 377.1 W kg−1. The present study presents novel method and materials for developing high performance capacitive negative electrode materials.

High-Performance Supercapacitors Using Hierarchical And Sulfur-Doped Trimetallic NiCo/NiMn Layered Double Hydroxides.

A supercapacitor features high power density and long cycling life. However, its energy density is low. To ensemble a supercapacitor with high power- and energy-densities, the applied capacitor electrodes play the key roles. Herein, a high-performance capacitive electrode is designed and grown on a flexible carbon cloth (CC) substrate via a hydrothermal reaction and a subsequent ion exchange sulfuration process. It has a 3D heterostructure, consisting of sulfur-doped NiMn-layered double hydroxide (LDH) nanosheets (NMLS) and sulfur-doped NiCo-LDH nanowires (NCLS). The electrode with sheet-shaped NMLS and wire-shaped NCLS on their top (NMLS@NCLS/CC) increases the available surface area, providing more pseudocapacitive sites. It exhibits a gravimetric capacity of 555.2 C g-1 at a current density of 1 A g-1 , the retention rate of 75.1% when the current density reaches up to 20 A g-1 , as well as superior cyclic stability. The assembled asymmetric supercapacitor that is composed of a NMLS@NCLS/CC positive electrode and a sulfurized activated carbon negative electrode presents a maximum energy density of 24.2Wh kg-1 and a maximum power density of 16000W kg-1 . In this study, a facile strategy for designing hierarchical LDH materials is demonstrated as well as their applications in advanced energy storage systems.

Charge/discharge activation of CoS/NiMn-hydroxide composite for enhancing the electrochemical performance

Electrochemical activation tactics have been adopted to enhance the electrochemical performance of electrodes. To reveal the charge/discharge activation mechanism of sulfides/hydroxides composites, herein, self-supporting NF/CoS/NiMn-hydroxides (NF/CoS/NiMn-OH) composite is prepared with NiMn-OH nanosheets wrapped rod-like CoS. The activation mechanism is revealed in 1 M KOH electrolyte, as well as the relationship between the charge/discharge cycles, microstructure/composition transition and electrochemical performance. During a charging process, the S2− anions are replaced by OH－ to generate active Co-hydroxides, accompanied by the dissolution of rod-like CoS. When activated for 10 cycles, the capacitance reaches the maximum value of 2980 F g−1 (8.046 F cm−2) at 1 A g−1, much higher than the initial capacitance of 2160 F g−1. The resulting product is verified as highly active CoNiMn–OH. A possible activation mechanism of CoS/NiMn–OH is proposed as the anion exchange reaction of S2−, followed by the generation and reconstruction of CoNiMn–OH nanosheets. Compared with initial composite, the inner CoS is converted into Co-hydroxide. Meanwhile, newly generated tri-metallic CoNiMn-hydroxide contributes to much more pseudocapacitance by playing a synergistic effect. When acting as a positive electrode, the asymmetric supercapacitor with activated carbon negative electrode presents a capacitance of 63.6 F g−1 at 1 A g−1, with a high capacitance retention of 90.6% over 6000 cycles.

Reduced self-discharge of supercapacitors based on surfactant-functionalized NiAl layered double hydroxide

Nickel aluminum layered double hydroxide (NiAl LDH) is an important pseudocapacitive electrode material for its low cost, facile preparation method, and high specific capacity. Due to the lack of chemical stability, however, NiAl LDH-based supercapacitors typically suffer from fast self-discharge. Herein, we report that NiAl LDH functionalized with sodium dodecyl sulfonate (SDS) can show much suppressed self-discharge for supercapacitors assembled from NiAl LDH positive electrodes and activated carbon negative electrodes. Compared to pristine NiAl LDH, the introduction of SDS in NiAl LDH leads to slower decay of open circuit voltage (26.7 vs. 11.6 h for a voltage decay from 1.6 to 0.42 V), reduced leakage current (1.2 vs. 0.17 mA at a charging voltage of 1.6 V), and improved capacitance retention (77 % vs. 49 % after resting for 4 h). The slower self-discharge can be attributed to the negatively charged hydrophobic layer on the surface of SDS-functionalized NiAl LDH particles so that the access of OH− ions to the electrode surface is impeded. As a result, the oxidization of OH− as a main process for the self-discharge of NiAl LDH is largely suppressed.

Active sites-induced decoration of nickel hydroxide nanosheets on copper oxide nanocubes as electroactive material of battery supercapacitor hybrids

Battery supercapacitor hybrids (BSH) comprising battery and supercapacitor-type electrodes can attain high energy/power densities and long cycle life. Copper oxide is regarded as a promising battery-type active material, due to low-cost, high chemical stability and good electrochemical properties. However, poor electrical conductivity and insufficient active sites should be improved to attain high energy storage ability. In this work, a novel battery-type material composed of Ni(OH)2 nanosheets decorated on Cu2O cages is synthesized via a facile solution process for the first time. Less deposition of Ni(OH)2 leads to sparse distribution on Cu2O cages, and full coverage of Ni(OH)2 is achieved via depositing four Ni(OH)2 layers on Cu2O cages. The largest specific capacitance (CF) of 389.1 F/g is achieved at 20 mV/s for the optimized Ni(OH)2 and Cu2O (Ni(OH)2@Cu2O) composite, while the Cu2O electrode shows a smaller CF value of 79.7 F/g. Coating Ni(OH)2 on Cu2O can induce active site formation and enhance electrical conductivity. The Ni(OH)2@Cu2O positive electrode and the active carbon negative electrode are used to assemble a BSH, which presents a maximum energy density of 4.09 Wh/kg at 500 W/kg and excellent cycling ability with CF retention of 70% and Coulombic efficiency higher than 97% after 10,000 charge/discharge cycles.

Tailing copper cobalt sulfide particle-decorated tube-like structure as efficient active material of battery supercapacitor hybrid

Copper cobalt sulfide (CuCo2S4) has received intensive attentions as one of effective battery-type materials of battery supercapacitor hybrids (BSH), because of numerous redox states and large conductivity. Morphology has been widely reported to play important roles on energy storage ability. Tube-like structure is beneficial for providing large surface area at inner/outer surface, and efficient charge transfer paths in one-dimensional directions. In this study, the CuCo2S4 particle-decorated tube-like structure is firstly designed on nickel foam via the hydrothermal process at different temperatures. The effects on morphology, composition and electrochemical performance are examined. The CuCo2S4 synthesized at 160 °C shows the most uniform rod size and regularly distributed particles on surface. The highest specific capacitance (CF) of 831.7 F/g corresponding to the capacity of 115.6 mAh/g is achieved for the optimal CuCo2S4 electrode, due to the largest electrochemical surface area with multiple active sites. A BSH is fabricated by using the CuCo2S4 positive electrode and the activated carbon negative electrode. A wide potential window of 1.5 V and the maximum energy density of 32.1 Wh/kg at power density of 1.1 kW/kg are achieved. The CF retention of 78.8% and Coulombic efficiency of 95.5% are also attained after 10,000 charge/discharge cycles for the BSH.

Fabrication of Co-Fe-Mn composites with a mixed-dimensional structure for applications in high-performance supercapacitors

Reasonably regulating the dimensionality and interface of an electrode material can effectively improve its surface chemical reactivity, charge transfer rate, and ion transport performance, which is crucial to solving several problems with supercapacitors, including low energy density and poor cycle stability. The multidimensional hybrid morphology of Co-Fe carbonate hydroxide (CH) coupled with Co-Mn layered double hydroxide (LDH) is generated directly on the nickel foam substrate by a two-stage process. The prepared multidimensional hybrid materials with pore morphology with the synergistic effect of 1D nanorods, 2D nanosheets, and 3D “porous coral” not only ensure sufficient contact between the electrolyte and electrode material but also enhance the charge transfer and ion diffusion of the electrolyte. The strong coupling effect of Co, Fe, and Mn metals between the heterojunction surface can produce various ionic valence states and high conductivities, resulting in increased redox activity, which thus improves the electrochemical performance. The results of the electrochemical performance test show that when the current density is 10 mA cm−2, the high area-specific capacitance of 7.33 F cm−2 is two times (3.85 F cm−2) that of a Co-Fe CH electrode and five times that of a Co CH electrode. The quasi-solid state asymmetric supercapacitor with a Co-Fe-Mn positive electrode and an active carbon negative electrode displays an energy density of 0.723 mWh cm−2 at a power density of 8.53 mW cm−2. After 5500 cycles, the apparatus exhibits 101.72 % of the initial capacitance, which can effectively drive small electrical appliances. The results indicate that the supercapacitor exhibits excellent cycle stability and application potential in energy storage.

Effect of variation of metals in quaternary metal oxide based electrodes on carbon fiber for super capacitor application

The objective of the present study is to explore novel, cost-effective, environmental friendly electrode materials with enhanced supercapacitor performance. Cobalt and nickel oxide based materials have been widely used for supercapacitor applications. In this work, Nickel-Cobalt-Molybdenum in combination with another metal of group (Ag, Mn, Cu), based quaternary metal oxide (QMO), were directly grown over a carbon fiber via a simple hydrothermal route. Hierarchical 3-D nanostructures of QMOs were synthesized over carbon fiber and characterized to investigate the effect of different metals over electrochemical performance. The Nickel-Cobalt-Molybdenum-Silver oxide electrode attained the best electrocapacitive performance of 530 Fg−1 at current density of 1 Ag−1 owing to its unique structure and high surface area. The fabricated supercapacitor hybrid device composed of the Ni-Co-Mo-Ag oxide positive electrode and activated carbon negative electrode showed capacitance 126 Fg−1 at current density of 1 Ag−1 with a potential window of 0.4–0.5 V and the 97 % capacitance retention. This facile synthesis of novel quaternary metal oxides provides new possibilities for making flexible energy storage devices with higher electro capacitive ability.

Regulating the supercapacitor properties of hollow NiCo-LDHs via morphology engineering

Layered double hydroxides (LDHs) capture wide attention due to their unique properties. However, the easy agglomeration and limited exposure of surface active sites seriously limit the further improvement of their electrochemical performances. Herein, we report a strategy to improve the supercapacitor performance of hollow NiCo-LDHs by facile morphology engineering. By simply adopting multifunctional microbial derived carbon (M-FC) as an inducer, which can not only regulate the size of the outside nanosheets and the internal cavity, but also synergize with NiCo-LDHs via its excellent electrical conductivity and the supercapacitor feature. The optimal NiCo-LDHs/M-FC electrode material exhibits a high specific capacitance (1877 F g-1 at 1 A g-1) and excellent rate ability (up to 10 A g-1). The assembled asymmetric supercapacitor device using NiCo-LDHs/M-FC positive electrode and activated carbon negative electrode exhibits an energy density of 106.7 Wh kg-1 at the power density of 1333.8 W kg-1 at 2 mA cm-2, and still retained 80 Wh kg-1 at the power density of 9931 W kg-1 at 15 mA cm-2. This work demonstrates a facile strategy to regulate the microstructure and properties of LDHs.

Binder-free synthesis of metal organic framework derived cobalt sulfide on carbon cloth for efficient energy storage devices

Cobalt based metal organic framework (Co-MOF) derivatives are regarded as attractive electroactive materials for energy storage, due to its high surface area, excellent pore structure and the possible formation of cobalt compounds. Cobalt sulfide with multiple redox states of Co and high electronegativity of S can produce numerous redox reactions for electron storage and provide high electrical conductivity for charge transfer. In this work, it is the first time to synthesize the novel binder-free sulfurized Co-MOF electrode as energy storage electrode of supercapacitor (SC). Three sulfur sources are used to sulfurize the Co-MOF electrode in hydrothermal process. The sulfurized Co-MOF prepared using thiourea (S67/CC-thiourea) shows wrinkles surface and particle connected aggregations, which can provide high surface area and electrical conductivity, respectively. The highest specific capacitance (CF) of 469.5 F/g at 4 A/g is achieved for the S67/CC-thiourea electrode. The untreated Co-MOF electrode merely shows a CF value of 8.0 F/g. An asymmetric SC is assembled using activated carbon negative electrode and S67/CC-thiourea positive electrode. A maximum energy density of 3.25 Wh/kg at 275 W/kg, and CF retention of 106 % and Coulombic efficiency higher than 91 % after 10,000 charge/discharges cycles are obtained for the asymmetric SC. This work opens new routes for creating novel MOF derivatives directly on conductive substrate using simple experimental processes.

Built-in electric field boosted ionic transport kinetics in the heterostructured ZnCo2O4/ZnO nanobelts for high-performance supercapacitor

Metal oxides are promising electrode candidates for supercapacitor due to their high theoretical capacitance, good reversibility, and low cost. However, they show inferior specific capacitance and power density because of their sluggish ion diffusion kinetics and intrinsically poor electrical conductivity within the solid phase. Herein, heterostructured ZnCo2O4/ZnO nanobelts are successfully prepared by using self-assembled Zn/Co-based nanosized coordination polymers as the precursors. The resulted nanobelts are composed of uniformly distributed ZnCo2O4 and ZnO nanocrystals, which spontaneously develop built–in electric fields in the nanobelts, and thus improve the conductivity and accelerate charge transport. The as-obtained ZnCo2O4/ZnO nanobelts display a high specific capacitance of 481.0 F g−1 at 1 A/g. The asymmetric supercapacitor, with a ZnCo2O4/ZnO positive electrode and an activated carbon negative electrode, deliver an energy of 23.77 Wh kg−1 at the power density of 399.98 W kg−1 and excellent prolonged cycle life. The excellent electrochemical performance benefits from both the special structure and built–in field at the heterostructure interface, which could significantly reduce the ion diffusion resistance and thus promote charge transport. This strategy may blaze a trail for engineering efficient electrode based on earth-abundant materials.

The development of NiCo2O4/PVP/PANI heterogeneous nanocomposites as an advanced battery-type electrode material for high-performing supercapacitor application

Recently, various electroactive materials composed of binary transition metal oxides (TMOs), and conducting polymers have been extensively studied, with the materials demonstrating great potential in high-performing battery-type electrodes. Furthermore, the development of heterogeneous nanocomposites is a versatile approach because of their well-defined surface morphology and the significant synergistic effect of the various species. In this study, NiCo2O4 (NCO) combined with polyvinyl pyrrolidone (PVP) and polyaniline (PANI) hybrid nanocomposites of NCO/PVP/PANI are newly developed using a facile hydrothermal method. The NCO in combination with PVP and PANI species provides the interconnected hierarchical characteristics by offering large electroactive sites that facilitate the charge transfer. The NCO/PVP/PANI1.5 nanocomposites had a high areal capacity of 698.44 µAh cm−2 at a current density of 4 mA cm–2 and a remarkable capacitance retention of 86 % over 4,000 galvanostatic charge/discharge (GCD) cycles owing to its excellent synergistic effect of the rich faradaic redox reaction kinetics of metallic species, and highly conductive materials. Moreover, the proposed NCO/PVP/PANI1.5 battery-type positive electrode was successfully assembled with an activated carbon negative electrode, resulting a hybrid device with a high energy density of 27.60 Wh kg−1 at a power density of 874.9 W kg−1 and excellent cycling stability. This research holds the significant promise for advanced heterogeneous materials in energy storage systems by describing the newly developed nanocomposites.

Two-step hydrothermal synthesis of a fireworks-like amorphous Co3S4 for asymmetric supercapacitors with superior cycling stability

Transition metal sulfides (TMSs) as pseudocapacitive materials have high specific capacitance. But the main concern of the volume variation as a result of insertion/desertion of electrolyte ions into/from TMSs during the electrochemical processes leads to the terrible cycling stability of supercapacitor devices. To solve this concern, this work reports a fireworks-like amorphous Co3S4 prepared via a facile two-step hydrothermal method containing the synthesis of precursor Co2(CO3)(OH)2 in the first step, which is converted into Co3S4 in the second step via an anion-exchange reaction with S2−. The influence of anion-exchange temperature on the structure and electrochemical performances of as-prepared materials is discussed. It is observed that the structure of materials changes from disorder to order with the increment in anion-exchange temperature, while the fireworks-like morphology is kept almost unchanged. When the anion-exchange temperature is 100 °C, the resultant Co3S4–100 is in an amorphous structure. A remarkable specific capacitance (2520.2 F g−1@1 A g−1) is delivered by Co3S4-100, surpassing that of crystalline Co3S4-220 (1675.4 F g−1), which are attributable to the former's larger specific surface area together with the amorphous structure nature. Apart from eminent energy density (52.4 Wh kg−1@1026.3 W kg−1 power density), the asymmetric supercapacitor (Co3S4–100 positive electrode and activated carbon negative electrode) shows exceptionally good cycling stability (retaining 98.4% of the initial capacitance after 10,000 cycles).

Composite electrodes with NiCoAl-LDH coated Ti3C2Tx MXene and incorporated Ag nanowires for screen-printable in-plane hybrid supercapacitors on textiles

Coplanar supercapacitors on textiles have received much concern with the development of flexible and wearable energy-storage-devices because this configuration can remarkably enhance the mechanical deformation and facilitate the integration with other devices as compared with conventional stacked configuration. However, the low energy density limits their applications. Here, in-plane hybrid supercapacitor (IHSC) on textile with special positive electrode is put forward to improve the energy density. The battery-type electrode is incorporated by combining the high capacity of metallic layer double hydroxide (NiCoAl-LDH), the high conductivity of Ti3C2Tx MXene and Ag nanowires, with the skeleton function of Ti3C2Tx MXene, which displays high capacity of 592C g−1 at 1 A g−1, high-rate performance, and long cycle life over 10,000 cycles. Based on this composite material and active carbon (negative electrode), screen-printed IHSC device on textile presents high areal energy density of 22.18 μWh cm−2 and power density of 3.0 mW cm−2, which is outstandingly superior to those conventional carbon-based in-plane supercapacitors on textiles. In addition, the device shows the excellent bending capability. These remarkable properties of textile based IHSCs demonstrate their promising potential as effective and printable power sources used in flexible and wearable electronics.

A high-performance asymmetric supercapacitor achieved by surface-regulated MnO2 and organic-framework–derived N-doped carbon cloth

It is possible to achieve high energy density and power density simultaneously for asymmetric supercapacitors by using pseudocapacitive materials with abundant ion intercalation/de-intercalation sites on the surface. Herein, a positive electrode based on feather-like MnO2 anchored on the activated carbon cloth is prepared, in which oxygen-enriched MnO2 nanorods with a radial sheet-like structure (OMO@AC) further form via electrochemical oxidation. Because of the large contact area with electrolyte and abundant oxidation functional groups on its surface, the OMO@AC displays excellent capacitance of 3,160 mF/cm2 at 1 mA/cm2. For the nitrogen-doped active carbon negative electrode, the capacitance is up to 1,875 mF/cm2 at 4 mA/cm2 due to the increase in disorder and defect on the carbon surface by N-doping. Furthermore, we verify the good electrochemical activity on the OMO@AC electrode surface by first-principles calculations and confirm the good matching degree between the positive and negative electrodes by CV testes. The aqueous oxygen-enriched MnO2// nitrogen-doped active carbon asymmetric supercapacitor exhibits an ultrahigh energy density of 8.723 mWh/cm3 at a power density of 14.248 mW/cm3 and display excellent cycle stability maintaining 95.5% after 10,000 cycles. The facile synthesis method and excellent performance provide a feasible way for the preparation of high-performance electrode materials for energy storage devices.

Dual-ligand modulation approach for improving supercapacitive performance of hierarchical zinc–nickel–iron phosphide nanosheet-based electrode

Mixed nanostructured transition metal-based complex materials with hierarchical and porous architectures, built from interconnected nano-building blocks, are considered as high-performance positive electrode materials in supercapacitors (SCs). Herein, zinc–nickel–iron phosphide (ZnNiFe–P) and Zn–Ni–Fe–hydroxide precursors (ZnNiFe–OH) were combined in a 3D hierarchical and porous structure (ZnNiFe–(P/OH)) to improve their durability and electrochemical activity by incorporating a dual-ligand synergistic modulation strategy. The 3D ZnNiFe–(P/OH) architectures, comprising perfectly aligned nanosheet arrays (NSA), were successfully grown on Ni foam using a facile hydrothermal process followed by partial phosphorization. The dual-ligand ZnNiFe–(P/OH) electrode exhibited excellent specific capacitance/areal capacitance (1708Fg−1/5.64Fcm−2 for 1Ag−1), high rate performance (62% upto 15Ag−1) and good cycle life. Moreover, the ZnNiFe–(P/OH) NSA positive electrode was coupled with an activated carbon negative electrode to design an asymmetric supercapacitor device. The device delivered an excellent capacitance of ∼187Fg−1 at 0.8Ag−1, a superior energy density of ∼58.4Whkg−1 at 600Wkg−1, and an excellent power density of 11250Wkg−1 at 34.4Whkg−1 while maintaining good cycling performance (88% after 5000 cycles).

Polypyrrole-wrapped NiCo2S4 nanoneedles as an electrode material for supercapacitor applications

In this study, dicobalt tetrasulfide (NiCo2S4) nanoneedles were successfully synthesized by a two-step hydrothermal method on nickel foam. A layer of polypyrrole (PPy) was further wrapped on the surface of the NiCo2S4 nanoneedles by in-situ polymerization. The obtained NiCo2S4@PPy composite was investigated for supercapacitor applications, which exhibited a capacitance of 1842.8 F g−1 at 1 A g−1. An asymmetric supercapacitor device fabricated with an activated carbon negative electrode and NiCo2S4@PPy positive electrodes exhibited an energy density of 41.2 Wh kg−1 at 402.2 W kg−1 with a high charge–discharge cycling stability (92.8% after 5000 cycles). These results demonstrate that NiCo2S4@PPy electrodes have broad application prospects as energy storage electrode materials.

Porous oxygen-doped NiCoP nanoneedles for high performance hybrid supercapacitor

The preparation of electrode materials with high performance is of great significance to the practical application of supercapacitors. Here, O-doped NiCoP (ONiCoP) electrode material with porous nanoneedle structure was synthesized in-situ on carbon cloth by hydrothermal process and subsequent phosphorization treatment with high mass loading of 5.07 mg cm−2. The electrode combined the large specific surface area of porous nanoneedle, the high electronic conductivity of phosphide and the electronic structure regulation of oxygen element, which finally showed excellent electrochemical performance. The optimized ONiCoP electrode delivered a high specific area capacity of 3457 mC cm−2 at a current density of 1 mA cm−2 and a high specific mass capacity of 621 C g−1 at a current density of 1 A g−1. The hybrid supercapacitor composed of ONiCoP positive electrode and commercial activated carbon negative electrode exhibited a high energy density of 30.52 Wh kg−1 at a power density of 749.36 W kg−1.

(Invited) A Hybrid Capacitor with LFP-Based Positive and AC-Based Negative Electrodes

1. Introduction EDLCs have been applied to backup power supplies and auxiliary power sources for memories, because of their feature of high input-output density. It is, however, essential to improve energy density for further application. In recent years, therefore, hybrid capacitors exhibiting higher energy density than that of EDLCs have attracted considerable attention as new electric storage devices to replace EDLCs as well as LIBs.In this research, we have focused on hybrid capacitors utilizing a LFP electrode as a positive electrode, which is used in a LIB, and activated carbon (AC) for a negative electrode. As a point of general concern, it has been reported that a typical solvent for EDLC is reductively decomposed at high voltage operation; the decomposed products are deposited on a negative electrode, and then adsorption sites for cations are lost [1]. In this research, therefore, we aimed at high voltage operation of the hybrid capacitor using 1,2-dimethoxyethane (DME) which is a typical solvent with reduction resistance for an electrolyte.2. Experimental As for the negative electrode material, YP50F, which is micro- and meso-porous activated carbon, was used. AB was used as conductive additive. SBR and CMC were applied to a binder and dispersant, respectively. Regarding the positive electrode, lithium iron phosphate (LFP) was used as active material with ketjen black as conductive additive and PVdF as binder. A two-electrode cell was constructed by using LiTFSI dissolved in DME or PC so as to be 1 mol dm–3 as the electrolyte with a microporous polyolefin membrane as a separator. The N/P ratio was normally 0.694. A galvanostatic cycling test was performed; the maximum operating voltage was 2.5 V.3. Results and Discussion Fig. 1 shows the charge and discharge curves at the first cycle, and Fig. 2 indicates the coulombic efficiency and the capacity retention when the galvanostatic test in 50 cycles is carried out. From the charging curve in Fig. 1, regarding the cell with PC, a large irreversible capacity increase can be confirmed at the charge end when compared to the DME cell. In the discharge curve, the plateau can be seen in PC, but the DME system shows a linear profile. This may be explained by the mechanism that the reduction products of PC, which is generated during charge, is oxidized during discharge. Looking at Fig. 2, the coulombic efficiency at the first cycle is about 54% for PC, while it improves to about 80% for DME.These results suggest that PC undergoes a significant irreversible reaction at the AC negative electrode in the initial charge and discharge. After the third cycle, Both DME and PC cells show a high efficiency of ca. 98%. This means that although PC decomposes considerably until the third cycle and irreversible products continue to be generated, the highly resistive characteristics of the decomposition products would suppress further PC decomposition from the third cycle onwards, and then PC cell also looks stable in terms of coulombic efficiency. The capacity retention of DME is twice as large as that of PC. This may be due to the low contribution of irreversible reaction at the negative electrode at the first cycle in the cell containing DME. From the above results, it was found that when DME is used as a solvent, no decomposition occurs and hence a stable cycle can be obtained.Reference1) S. Ishimoto, Y. Asakawa, M. Shina, and K. Naoi, J. Electrochem. Soc. 156 (7), A563-A571 (2009). Figure 1