Energy Density Of Asymmetric Supercapacitor Research Articles

The Mass-Balancing between Positive and Negative Electrodes for Optimizing Energy Density of Supercapacitors.

Supercapacitors (SCs) are some of the most promising energy storage devices, but their low energy density is one main weakness. Over the decades, superior electrode materials and suitable electrolytes have been widely developed to enhance the energy storage ability of SCs. Particularly, constructing asymmetric supercapacitors (ASCs) can extend their electrochemical stable voltage windows (ESVWs) and thus achieve high energy density. However, only full utilization of the electrochemical stable potential windows (ESPWs) of both positive and negative electrodes can endow the ASC devices with a maximum ESVW by using a suitable mass-ratio between two electrodes (the mass-balancing). Nevertheless, insufficient attention is directed to mass-balancing, and even numerous misunderstandings and misuses have appeared. Therefore, in this Perspective, we focus on the mass-balancing: summarize theoretic basis of the mass-balancing, derive relevant relation equations, analyze and discuss the change trends of the specific capacitance and energy density of ASCs with mass-ratios, and finally recommend some guidelines for the normative implementation of the mass-balancing. Especially, the issues related to pseudocapacitive materials, hybrid devices, and different open circuit potentials (OCPs) of the positive and negative electrodes in the mass-balancing are included and emphasized. These analyses and guidelines can be conducive to understanding and performing mass-balancing for developing high-performance SCs.

Design and preparation of hierarchical porous carbon-based materials with bionic “ant nest” structure for high performance asymmetric supercapacitors

Rational design of electrodes to broaden the working potential window is an attractive approach to improve the energy density of asymmetric supercapacitors. Herein, a simple non-solvent-induced phase separation method was used to construct a bionic "ant nest" three-dimensional hierarchical porous carbon (HPC) without any template, realizing a three-dimensional interlaced interconnection network structure similar to that of a natural ant nest, which adsorbs more ions through a larger accessible area and facilitates the bi-continuous transport of ions and electrons. Further, the aqueous asymmetric supercapacitor assembled with MnO2-modified HPC (HPC/MnO2) as the positive electrode and nitrogen-doped HPC (NHPC) as the negative electrode has an energy density as high as 21 Wh kg−1 at a power density of 410 W kg−1, and as high as 17 Wh kg−1 even at a power density of 13 kW kg−1. The device exhibits an excellent electrochemical performance over a potential window of 0–1.6 V.

Direct Ink Writing (DIW) printed high-performance asymmetric supercapacitor based on 0D@2D silver-nanoparticles@MXene as anode and 0D@2D MnO2-nanoparticles@MXene as cathode materials

The electrochemical performance of printed asymmetric supercapacitors can be improved by designing and implementing properly engineered anode and cathode electrode materials with enriched redox kinetics. However, the development of high-performance printed anode/cathode electrodes with three-dimensional (3D) structures is crucial task. Direct Ink Writing (DIW) is a versatile advanced manufacturing technology for developing 3D complex electrode architectures for efficient energy storage devices. In addition, replacing conventional electric double-layer capacitors as anode materials with redox-driven electrodes can significantly improve the specific capacitance and energy density of asymmetric supercapacitors. Herein, for the first time, we report a 0D@2D silver-nanoparticles@Ti3C2 DIW printed anode material for asymmetric supercapacitors. The Ag-NPs@Ti3C2 hybrid material contributes a high-specific capacitance of 368.56 F g−1 at a current density of 1 A g−1 with a minimum specific capacitance of 233.84 F g−1 at 10 A g−1. To assemble the asymmetric supercapacitor device, 0D@2D MnO2@Ti3C2 DIW printed cathode is implemented, which displays excellent electrochemical performance in terms of specific capacitance (474.45 F g−1 at 1 A g−1). In the existence of all-printed advanced anode (Ag@Ti3C2) and cathode (MnO2@Ti3C2) materials, the asymmetric supercapacitor device shows a maximum energy density of 38.16 Wh kg−1 at a power density of 800 W kg−1, with a capacitance retention of up to 91.27 % after 5000 cycles. Thus, the application of DIW printed electrode materials with optimum electrochemical contributions provides new avenues for the development of high-performance energy storage devices.

Hybridization of porous vanadium nitride nanosheets with cobalt-encapsulated nitrogen-doped carbon nanotubes on carbon cloth as an advanced monolithic negative electrode for boosting asymmetric supercapacitors

The low energy density of asymmetric supercapacitors (ASCs) is mainly attributable to the low capacitance of the negative electrode; consequently, developing negative electrodes with high capacitance is crucial to boosting the energy density of ASCs. Vanadium nitride (VN), with excellent theoretical capacitance, is an ideal negative electrode material, but its low conductivity and poor electrochemical stability limit its application in energy storage. Herein, we design a high-performance monolithic negative electrode of ASCs that consists of well-defined porous VN nanosheets stacked with small nanoparticles decorated on a conductive substrate of ultrafine cobalt-encapsulated nitrogen-doped carbon nanotubes on carbon cloth (VN NS/Co@NCNTs/CC). Because the Co@NCNTs/CC substrate can enhance conductivity and facilitate the dispersion of active species and the exposure of more reaction sites, the newly developed porous VN NS/Co@NCNTs/CC negative electrode exhibits a high capacitance of 1016 mF cm−2 (782.3 F g−1) at 2 mA cm−2 and a superior cycle stability (110% capacitance retention after 11,000 cycles). To construct the ASC, a positive electrode of manganese dioxide (MnO2) nanoneedles grown on Co@NCNTs/CC (MnO2 NN/Co@NCNTs/CC) was also built using a similar preparation method. Benefiting from the wide operating voltage and high capacitance of the negative and positive electrodes, the assembled ASC can operate stably at a high voltage of 2 V. More importantly, the ASC can reach an energy density of 29.9 μWh cm−2 (23.9 Wh kg−1) and a capacitance retention of 94.3 % after 20,000 cycles, outperforming most of the reported aqueous ASC devices. This study provides a feasible way to reasonably design ASC electrodes with superior electrochemical performance.

Designing NiCo2S4-Acetylene black engrained nitrogen-doped porous reduced graphene oxide nanocomposites conducting network: As positive/negative electrode combinations for high energy density of asymmetric supercapacitor and hydrogen evolution reaction

The development of two-dimensional composite materials for energy conversion/storage applications is crucial in light of the current high energy demand. Herein, we report the synthesis of nitrogen-doped porous reduced graphene oxide (N-prGO), NiCo2S4-acetylene black (NiCo2S4-AB), and their composite by hydrothermal method for supercapacitor (EC) and hydrogen evolution reaction (HER) applications. The morphology of the N-prGO/NiCo2S4-AB composite appeared as clusters bound in a few thin layers, confirmed by SEM, HR-TEM characterizations. EDAX mapping and elemental analysis confirmed the structural compositions. The synthesized composite materials were used for the fabrication of four different EC combinations of anode and cathode materials to study the performance of symmetric EC (SEC) and Asymmetric EC (AEC). Among them, the ACE type N-prGO (negative)/NiCo2S4-AB (positive) electrode combination showed the highest specific capacitance of 321.75 Fg-1 at 2 mVs−1 high energy density of 59.7 WhKg−1 along with good power density of 450 WKg-1 and time constant of 345 s. Furthermore, the N-prGO/NiCo2S4-AB composite studied as the electrode for HER, showed the lowest overpotential (198 mV) and Tafel slope (98 mV dec−1) values, thus indicating a potential electrode for hydrogen production. The synergetic effect between morphology and mesoporosity of the N-prGO/NiCo2S4-AB composite plays a vital role in improving charge transfer which ultimately showed the superior performance of the N-prGO/NiCo2S4-AB composite towards dual applications.

One-step preparation of Ni3S4 quantum dots composite graphene/carbon nanotube conductive network for asymmetric supercapacitor

In this reported work, Ni3S4 quantum dots (QDs) composite graphene/carbon nanotube (G/CNTs) composites were prepared using a one-step method. Among them, graphene is produced by carbon nanotubes and Ni catalyst. G/CNTs can not only prevent the agglomeration of Ni3S4 QDs, but also preclude this insulating drawback caused by the interface of QDs. XPS analysis data showed that after adding G/CNTs to Ni3S4 QDs, the electron interaction was stronger, which promotes the electron transport in the material. The electrochemical test results of the electrode material showed that G/CNTs increased the specific capacitance of the Ni3S4 QDs by 4.5 times and its rate capability from 22% to 82%. The energy density of asymmetric supercapacitor assembled with electrodes containing the Ni3S4 QDs@G/CNTs and activated carbon achieved 50 Wh kg−1. This work provides a reasonable way for improving the electrochemical performance of supercapacitor electrode materials.

Asymmetric supercapacitors based on La-doped MoO3 nanobelts as advanced negative electrode and VOR nanosheets as positive electrode

The energy density of asymmetric supercapacitors (ASCs) is limited by the negative electrodes; thus, it is imperative to design negative electrodes to optimize the device and increase the energy density of ASCs. Recently, doping has been shown to be an effective technique to regulate the band gap and increase the excitation of charge carriers to the conduction band and improve the capacitive performance for further energy conversion. Herein, by doping heteroelement metal ions, the La-doping strategy reduced the band gap of MoO3 and improved the electric conductivity, as proof by experiments and DFT calculations. The specific capacitance of the LaMoO3-4 sample exhibits an obvious improvement (from 116.5 to 605.6 F g−1 at a current density of 1 A g−1). Aqueous ASCs with a working voltage of 2 V are assembled based on VOR nanosheets as the positive electrode and LaMoO3-4 as the negative electrode. At a power density of 1000 W kg−1, the energy density of the assembly is 123 Wh kg−1; it also has an excellent cycle stability, and the capacitance retention rate after 6 000 cycles is 95.4%. A promising research route to achieve a high power density and high energy density is demonstrated herein. La is doped into the MoO3 nanobelts and the as-obtained LaMoO3 shows narrower band gap (1.91 eV) than pure MoO3 (2.48 eV) leading to an effective enhancement of electron transport and much better electrochemical performance (605.6 F g-1 at a current density of 1 A g-1). ASCs with a working voltage of 2 V based on the LaMoO3 as negative electrode and VOR as positive electrode exhibit a high energy density of 123 Wh kg-1.

Boosted charge transfer in p-n heterojunctions LaMoO3/GQDs negative electrode for all-solid-state asymmetric supercapacitor

The low energy density of asymmetric supercapacitor (ASC) is limited by the negative electrodes; thus, to further increase the energy density of ASCs, it is necessary to optimize the design of the negative electrode and the device. Constructing p-n heterojunctions may be a viable approach for enhancing the performance of the negative electrode, as it enhances the driving force for the built-in charge transfer and thereby it is beneficial for the charge transfer dynamics. Herein, we attempted to configure graphene quantum dots (GQDs) and LaMoO3 as a p-n heterojunction via one-step hydrothermal method, which shows outstanding capacitance performance. The specific capacitance of the LaMoG-3 (1396 F g−1 at 1 A g−1) was higher than the theoretical value for molybdenum trioxide (1256 F g−1). Moreover, the ASC is successfully assembled all-solid-state ASC with GQDs/Mn3O4 as the positive electrode and LaMoG-3 as the negative electrode. The as-fabricated ASC exhibited an ultra-high density of 235.8 Wh kg−1 and a power density of ~1000 W kg−1. One device can light the LED indicator for more than 11 min after only charging for 5 min. The above-mentioned results will open up promising research pathways for achieving excellent energy density and high power density.

Atomic Modulation Triggering Improved Performance of MoO3 Nanobelts for Fiber-Shaped Supercapacitors.

Asymmetric supercapacitors (ASCs) are emerging as a new class of energy storage devices that could potentially meet the increasing power and energy demand for next-generation portable and flexible electronics. Yet, the energy density of ASC is severely limited by the low capacitance of the anode side, which commonly uses the carbon-based nanomaterials. Here, the demonstration of sulfur-doped MoO3- x nanobelts (denoted as S-MoO3- x ) as the anode for high-performance fiber-shaped ASC are reported. The Mo sites in MoO3 are intentionally modulated at the atomic level through sulfur doping, where sulfur could be introduced into the MoO6 octahedron to intrinsically tune the covalency character of bonds around Mo sites and thus boost the charge storage kinetics of S-MoO3- x . Moreover, the oxygen defects are occurring along with sulfur-doping in MoO3 , enabling efficient electron transport. As expected, the fiber-shaped S-MoO3- x achieves outstanding capacitance with good rate capability and long cycling life. More impressively, the fiber-shaped ASC based on S-MoO3- x anode delivers extremely high volumetric capacitance of 6.19 F cm-3 at 0.5 mA cm-1 , which makes it promising as one of the most attractive candidates of anode materials for high-performance fiber-shaped ASCs.

Boosting the Energy Density of Flexible Asymmetric Supercapacitor with Three Dimensional Fe2O3 Composite Brush Anode

Flexible asymmetric supercapacitor is fabricated with three dimensional(3D) Fe2O3/Ni(OH)2 composite brush anode and Ni(OH)2/MoO2 honeycomb cathode. Particularly for 3D composite brush anode, a layer of thin Fe2O3 film is firmly adhered on a 3D Ni brush current collector with the assist of Ni(OH)2, functioning as both adherence layer and pseudocapacitive active material. The unique 3D Ni brush current collector possesses large surface area and stretching architecture, which facilitate to achieve the composite anode with high gravimetric capacitance of 2158 F/g. In terms of cathode, Ni(OH)2 and MoO2 have a synergistic effect to improve the specific capacitance, and the resulting Ni(OH)2/MoO2 honeycomb cathode shows a very high gravimetric capacitance up to 3264 F/g. The asymmetric supercapacitor(ASC) has balanced cathode and anode, and exhibits an ultrahigh gravimetric capacitance of 1427 F/g and an energy density of 476 Wh/kg. The energy density of ASC is 3–4 times higher than those of other reported aqueous electrolyte-based supercapacitors and even comparable to that of commercial lithium ion batteries. The device also shows marginal capacitance degradation after 1000 cycles’ bending test, demonstrating its potency in the application of flexible energy storage devices.

Polypyrrole-coated Fe2O3 nanotubes constructed from nanoneedles as high-performance anodes for aqueous asymmetric supercapacitors.

Asymmetric supercapacitors (ASCs) show promising potential for electrochemical energy storage applications. However, the energy density of ASCs is limited by the poor electrochemical performance of anodes. To achieve high-performance ASCs, herein, Fe2O3 nanotubes constructed from Fe2O3 nanoneedles were fabricated by employing MnO2 nanotubes as a self-sacrificing template, and then a layer of polypyrrole (PPy) was coated through an in situ chemical oxidative polymerization method to enhance their performance. The electrochemical tests indicate that the resultant PPy-coated Fe2O3 nanotubes (Fe2O3@PPy) exhibit a high areal capacitance of 530 mF cm-2 at 1 mA cm-2 and good cycling stability, which are superior to those of the Fe2O3 nanotubes. The superior performance of the Fe2O3@PPy nanotubes can be attributed to the synergistic effect between the PPy shell and Fe2O3 core, in which the conducting PPy shell not only works as a superhighway for charge transport, but also stabilizes the Fe2O3 nanotubes during charge-discharge processes. When the Fe2O3@PPy nanotubes were assembled with MnO2 nanotubes, the as-assembled ASCs possess a high cell voltage of 2.0 V and deliver a high energy density of up to 51.2 Wh kg-1 at a power density of 285.4 W kg-1 with an excellent cycling stability (83.5% capacitance retention over 5000 cycles).

Co9S8-Ni3S2/CuMn2O4-NiMn2O4 and MnFe2O4-ZnFe2O4/graphene as binder-free cathode and anode materials for high energy density supercapacitors

An essential route to improve the energy density of asymmetric supercapacitor (ASC) is to develop unique, smart and highly-efficient positive and negative current collectors with hierarchical combination of various electroactive materials. Herein, we developed a multicomponent integration of hierarchical Co9S8-Ni3S2 nanoparticles anchored on CuMn2O4-NiMn2O4 nanosheet arrays (Co-Ni-S NPs/Cu-Ni-Mn-O NSAs) and rhombus-like shaped MnFe2O4-ZnFe2O4 nanocrystals are grown on the graphene-ink nanosheets (Mn-Zn-Fe-O/G-ink), which were effectively applied as a superior binder-free cathode and anode electrodes for ASCs. The hierarchical Co-Ni-S NPs/Cu-Ni-Mn-O NSAs and Mn-Zn-Fe-O/G-ink electrode architectures show large surface area, high conductivity and provide rich active sites for redox reactions. As a result, the electrochemical properties of Co-Ni-S NPs/Cu-Ni-Mn-O NSAs and Mn-Zn-Fe-O/G-ink electrodes deliver that both have excellent specific capacities (263 and 149.44 mA h g−1 at 2 A g−1), superior rate capabilities (86.54% and 92.64% even at 20 A g−1) and remarkable cycling stabilities (97.39% and 94.83% over 5000 cycles), respectively. Furthermore, an asymmetric supercapacitor (ASC) assembled using the Co-Ni-S NPs/Cu-Ni-Mn-O NSAs as positive electrode and Mn-Zn-Fe-O/G-ink as negative electrode with an aqueous KOH electrolyte. The energy and power densities of the ASC are calculated based on the weight of the electroactive materials. As a result, the ASC delivers ultrahigh energy and power densities of 75.65 W h kg−1 and 6629.53 W kg−1 as well as achieve exceptional cycling stability of 96.89% retention with 98.26% of columbic efficiency over 5000 cycles. These attractive results of binder-free advanced current collectors with composites of various active materials and favorable architectures paves a path for the engineering of new class collectors for high-performance ASCs.

Structure-designed fabrication of all-printed flexible in-plane solid-state supercapacitors for wearable electronics

The current development of wearable electronics has raised the requirement for developing matchable supercapacitors. Currently, most researches have been focused on the electrode materials, but few studies have been carried out on the structure of supercapacitors. Herein, we report an efficient fully-printing fabrication approach to construct flexible in-plane solid-state supercapacitors. Utilization of the screen printing technique, a series of factors which affect the performance of planar supercapacitors have been studied to construct smart planar supercapacitors, including dimensions, finger interspaces, functional layers of active materials and asymmetric structure, which is firstly systematically investigated for efficiently increasing the performance of planar supercapacitors. More interestingly, the planar supercapacitors can be fabricated as various patterns with artistic design on various flexible substrates, demonstrating that the screen printing process is facile and easy scalable for practical production. The optimized symmetric supercapacitors exhibit good mechanical flexibility, outstanding areal capacitance of 35.3 mF cm−2 and superior cycling performance. Compare with the symmetric supercapacitors, higher energy density of asymmetric supercapacitors can be achieved (from 0.00177 mWh cm−2 to 0.00687 mWh cm−2). We envision that this strategy of constructing fully-printed flexible planar supercapacitors with structural design paves the way for the improvement of flexible energy storage devices.

ZnCo2O4@MnCo2O4 heterojunction structured nanosheets for high-performance supercapacitor

ZnCo2O4@MnCo2O4 heterojunction structured nanosheets were successfully prepared by simple and effective hydrothermal reaction. The obtained heterojunction structured had large specific surface area, good structural stability, and excellent electrical conductivity. Therefore, they were expected to become ideal electrochemical materials. As expected, a series of electrochemical tests showed ZnCo2O4@MnCo2O4 heterojunction structured nanosheets demonstrate high specific capacitance (254 F g−1 at the current density of 1 A g−1). Good stability of charge–discharge, when the current density was 10 A g−1, the capacity can still maintain 186 F g−1. Especially, the maximum energy density of asymmetric supercapacitors based on ZnCo2O4@MnCo2O4 and activated carbon at 750 W Kg−1 was 19.12 Wh Kg−1, and the cycle stability was good, after 2000 cycles, the capacitance retention rate was 98.5%. For practice application, the commercial LED lights can be lighted by two supercapacitor units in series. It shows that the synthetic electrode material has great potential in the energy storage equipment.

The perfect matching between the low-cost Fe2O3 nanowire anode and the NiO nanoflake cathode significantly enhances the energy density of asymmetric supercapacitors

A novel low-cost anode of homogenous Fe2O3 nanowires on CFP was developed, achieving a high specific capacitance in 0 to −1.35 V. Matching it with the NiO nanoflake cathode, an extremely high energy density of 105 W h kg−1 is obtained at 1400 W kg−1.

Enhanced energy density of asymmetric supercapacitors via optimizing negative electrode material and mass ratio of negative/positive electrodes

An asymmetric supercapacitor based on manganese dioxide/Au/nickel foam (MANF) electrode as positive electrode and graphene or commercial activated carbons (AC) as negative electrode was fabricated. The effect of different negative electrode materials and mass ratios of negative/positive electrodes on the electrochemical properties of the asymmetric supercapacitor was carefully investigated. The results suggest that the mass ratio of negative/positive electrode has a significant impact on the specific capacitance of the asymmetric supercapacitor. Especially, it is found that the optimal mass ratio of the negative/positive electrode is slightly lower than that calculated according to charge balance. On the other hand, the asymmetric supercapacitor with commercialized AC as negative electrode possesses higher specific capacitance and better rate capability than that of the asymmetric supercapacitor with graphene as negative electrode. The negative material has slight impact on the cycle stability of the asymmetric supercapacitor. In addition, the optimized asymmetric supercapacitor with MANF composite as positive electrode and AC as negative electrode can obtain an energy density as high as 65.9 Wh kg−1 at a power density of 180 W kg−1 and a cell voltage of 1.8 V in the neutral Na2SO4 aqueous solution.