Enlarged Voltage Window Research Articles

Ferrimagnetic pseudocapacitive MnFe2O4 electrodes and supercapacitor devices

This investigation is motivated by the surge of interest in materials, combining high spontaneous magnetization and pseudocapacitance at room temperature. Ferrimagnetic MnFe2O4 offers benefits of high magnetization. However, the non-pseudocapacitive behavior, low capacitance and high resistance of MnFe2O4 are limiting factors for its applications in magnetic pseudocapacitive devices. This investigation demonstrates that nearly ideal pseudocapacitive behavior can be achieved for MnFe2O4 electrodes in Na2SO4 electrolyte. High pseudocapacitance is observed in positive and negative potential ranges and two different charging mechanisms are proposed. High capacitance is achieved at a low impedance. The ability to achieve comparable and high areal capacitances in the positive and negative potential ranges facilitates the fabrication of a symmetric pseudocapacitive device, containing ferrimagnetic MnFe2O4 as cathode and anode material for operation in enlarged voltage window of 1.6 V. The symmetric device shows capacitance of 0.92 F cm−2 at a current density of 3 mA cm−2. The individual electrodes and device show good cycling stability. The approach is based on the use of murexide and gallocyanine as redox-active dispersants and charge transfer mediators. The analysis of testing results provides an insight into the influence of chemical structure, charge and redox properties of the dispersants on the capacitive behavior. The ability to fabricate a pseudocapacitive device, containing two ferrimagnetic electrodes is promising for energy storage, water purification and novel applications based on magnetocapacitive effects.

Multifunctional MXene–Fe3O4–Carbon Nanotube Composite Electrodes for High Active Mass Asymmetric Supercapacitors

Ti3C2Tx–Fe3O4–carbon nanotube composites were prepared for electrochemical energy storage in the negative electrodes of supercapacitors. The electrodes show a remarkably high areal capacitance of 6.59 F cm−2 in a neutral Na2SO4 electrolyte, which was obtained by the development of advanced nanofabrication strategies and due to the synergistic effect of the individual components. Enhanced capacitance was achieved using the in-situ synthesis method for the Fe3O4 nanoparticles. The superparamagnetic behavior of the Fe3O4 nanoparticles facilitated the fabrication of electrodes with a reduced binder content. Good mixing of the components was achieved using a celestine blue co-dispersant, which adsorbed on the inorganic components and carbon nanotubes and facilitated their co-dispersion and mixing. The capacitive behavior was optimized by the variation of the electrode composition and mass loading in a range of 30–45 mg cm−2. An asymmetric device was proposed and fabricated, which contained a Ti3C2Tx–Fe3O4–carbon nanotube negative electrode and a polypyrrole–carbon nanotube positive electrode for operation in an Na2SO4 electrolyte. The asymmetric supercapacitor device demonstrated high areal capacitance and excellent power-density characteristics in an enlarged voltage window of 1.6 V. This investigation opens a new avenue for the synthesis and design of MXene-based asymmetric supercapacitors for future energy storage devices.

High power aqueous hybrid asymmetric supercapacitor based on zero-dimensional ZnS nanoparticles with two-dimensional nanoflakes CuSe2 nanostructures

Energy storage materials, particularly chalcogenides, are fascinating electrode materials for supercapacitors (SCs) because of their high capacitance, remarkable electrical conductivity, and multiple oxidation states contributed by numerous metal cations. Herein, a novel nanocomposite based on zinc sulfide and copper diselenide, denoted as (ZnS–CuSe2), was prepared via a sonochemical-assisted method. The structural analysis revealed the cubic structure for pure ZnS, orthorhombic for CuSe2, and co-existing cubic and orthorhombic phases for ZnS–CuSe2 nanocomposites with high purity and crystallinity. The ZnS–CuSe2 nanocomposite offered exceptional electrochemical performance with redox peaks from the CV analysis, and coupled with plateaus in the charge/discharge profile, confirming the faradaic energy storage properties with functional reversibility. Similarly, a high conductive feature of the ZnS–CuSe2 composite was revealed by impedance study, with a minor charge transfer resistance than their bulk materials. A hybrid asymmetric supercapacitor (HASCs) composed of ZnS–CuSe2//AC was constructed, which manifested an enlarged voltage window up to 1.7 V with capacitance of 95 F g−1 and a maximum specific energy of 38 Wh kg−1. Also, high power delivery was attained at 3927 Wkg-1 when specific energy goes down to 12 Wh kg− at 5 A g−1. Interestingly, only 81.8% retention was left beneath when cycled to 8000 cycles, specifying decent stability of the ZnS–CuSe2//AC HASCs.

Unveiling the Reversibility and Stability Origin of the Aqueous V2 O5 -Zn Batteries with a ZnCl2 "Water-in-Salt" Electrolyte.

Aqueous V2O5–Zn batteries, an alternative chemistry format that is inherently safer to operate than lithium‐based batteries, illuminates the low‐cost deployment of the stationary energy storage devices. However, the cathode structure collapse caused by H2O co‐insertion in aqueous solution dramatically deteriorates the electrochemical performance and hampers the operation reliability of V2O5–Zn batteries. The real‐time phase tracking and the density functional theory (DFT) calculation prove the high energy barrier that inhibits the Zn2+ diffusion into the bulk V2O5, instead the ZnCl2 “water‐in‐salt electrolyte” (WiSE) can enable the dominant proton insertion with negligible lattice strain or particle fragment. Thus, ZnCl2 WiSE enables the enhanced reversibility and extended shelf life of the V2O5–Zn battery upon the high temperature storage. The improved electrochemical performance also benefits by the inhibition of vanadium cation dissolution, enlarged voltage window, as well as the suppression of the Zn dendrite protrusion. This study comprehensively elucidates the pivotal role of a concentrated ZnCl2 electrolyte to stabilize the aqueous batteries at both the static storage and dynamic operation scenarios.

High-Performance Flexible Asymmetric Supercapacitor Paired with Indanthrone@Graphene Heterojunctions and MXene Electrodes.

The energy density formula illuminated that widening the voltage window and maximizing capacitance are effective strategies to boost the energy density of supercapacitors. However, aqueous electrolyte-based devices generally afford a voltage window less than 1.2 V in view of water electrolysis, and chemically converted graphene yields mediocre capacitance. Herein, multi-electron redox-reversible, structurally stable indanthrone (IDT) π-backbones were rationally coupled with the reduced graphene oxide (rGO) framework to form IDT@rGO molecular heterojunctions. Such conductive agent- and binder-free film electrodes delivered a maximized capacitance of up to 345 F g-1 in a potential range of -0.2 to 1.0 V. The partner film electrode-Ti3C2Tx MXene which worked in the negative potential range of -0.1 to -0.6 V-afforded a capacitance as large as 769 F g-1. Thanks to the perfect complementary potentials of the IDT@rGO heterojunction positive electrode and Ti3C2Tx MXene negative partner, the polyvinyl alcohol/H2SO4 hydrogel electrolyte-based flexible asymmetric supercapacitor delivered an enlarged voltage window of 1.6 V and an impressive energy density of 17 W h kg-1 at a high power density of 8 kW kg-1, plus remarkable rate capability and cycling life (capacitance retention of ∼90% after 10000 cycles) as well as exceptional flexibility and bendability.

Multi-electron redox asymmetric supercapacitors based on quinone-coupled viologen derivatives and Ti3C2Tx MXene

Organic materials are emerging for the pseudocapacitors as they offer high theoretical redox capacitance and can be derived from the renewable sources. They are composed of non-metals, resulting in light weight, flexible, and potentially low-cost devices. While there are hundreds of commercial organic molecules available and nearly unlimited can be synthesized, only a handful of them are suitable for the pseudocapacitive applications. Therefore, the discovery of innovative organic materials beyond conventional pseudocapacitive organic materials (e.g. quinones, conducting polymers, etc.) is much needed for the sustainable pseudocapacitors. Here, for the first time, we report quinone-functionalized viologen molecules as a high capacitance/rate pseudocapacitive organic electrode material on hybridization with reduced graphene oxide sheets. Given the reliable pseudocapacitance of quinone-functionalized viologen-based hybrids under positive potentials, optimized electrodes were paired with two-dimensional titanium carbide (Ti3C2Tx) MXene as negative electrodes to manufacture multi-electron redox asymmetric supercapacitors. The resulting full devices were capable to store charge within enlarged voltage window up to 1.5 V in 3 M H2SO4. In addition, these devices exhibited ultrahigh rate performance (~77% capacitance retention from 10 to 1,000 mV/s), energy density (~20 Wh/kg), and capacitance retention of 80% after 10,000 charge/discharge cycles.

MoS2 Confined MXene Heterostructures as Electrode Material for Energy Storage Application

Two-dimensional (2D) titanium carbide Ti 3 C 2 (MXene) is exemplified as the promising electrode material for supercapacitors. MXene was derived by etching of Al-layer from MAX phase (Ti 3 AlC 2 ), and MoS 2 was confined on MXenes through incipient wet impregnation of MoS 2 precursor. The prepared MXene and MoS 2 /MXene materials were characterized by X-ray diffraction, scanning electron microscope and energy-dispersive X-ray spectroscopy, BET analysis, and X-ray photoelectron spectroscopy. The electrochemical characteristics of MXene and MoS 2 /MXene heterostructures were evaluated by different techniques such as cyclic voltammogram, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. The electrochemical measurements revealed that the maximum specific capacitance of the MoS 2 /Mxene electrodes reaches up to 342 F g −1 at a discharge current density of 0.4 A g −1 in an enlarged voltage window of -1.5 V to 1.5 V. Also, Electrochemical impedance studies show that the incorporation of MoS 2 decreases the charge transfer resistance of MoS 2 /MXene. Overall, the electrochemical performance of MoS 2 /MXene exhibited excellent reversibility, cycle stability, and rate performance. The obtained results uncover MoS 2 /MXene as promising electrode materials for supercapacitors.

MXene coupled with molybdenum dioxide nanoparticles as 2D-0D pseudocapacitive electrode for high performance flexible asymmetric micro-supercapacitors

Recently, two-dimensional (2D) transition metal carbides and carbonitrides (MXenes), have shown great potential in micro-supercapacitors (MSCs). However, the maximum voltage output of symmetric MXene MSCs is limited to 0.6 V due to the oxidation effects at high anodic potentials. Herein, we developed asymmetric micro-supercapacitors (AMSCs) based on titanium carbide MXene (Ti3C2Tx) and MXene-MoO2 electrodes with an enlarged voltage window of 1.2 V, which is twice wider than that of symmetric MXene MSCs. The 2D-0D MXene-MoO2 microelectrode is fabricated by homogenous dispersing zero-dimensional (0D) MoO2 nanoparticles into MXene layers to impede layers stacking and MoO2 nanoparticles aggregation. Notably, the AMSCs delivered good electrochemical performances of areal capacitance of ∼19 mF cm−2 and volumetric capacitance of 63 F cm−3 at a scan rate of 2 mV s−1, and high energy density of 9.7 mW h cm−3 at a power density of 0.198 W cm−3. The AMSCs also presented exceptionally mechanical flexibility under different bending states and excellent cyclic stability, with 88% capacitance retention after 10000 cycles at a discharge current density of 0.5 mA cm−2. For practical application, the serially connected AMSCs are fully affordable to power electronics, which is beneficial for soft and wearable power devices.

Highly porous CNTs knotted cerium oxide hollow tubes with exalted energy storage performance for hybrid supercapacitors

Development of porous transition metal oxide nanostructures and their conductive compounds has attracted widespread attention in hybrid supercapacitors (SCs) because of their large surface area, mesoporosity and good electrochemical activity which significantly enhance the energy storage performance. Herein, we reported highly porous cerium oxide hollow tubes (CeO2 HTs) by a facile biomorphic process with waste cotton fibers as a template. The waste cotton fibers were initially infiltrated with aqueous cerium nitrate solution, followed by solution evaporation and thermal treatment, leading to the formation of highly porous CeO2 HTs. To enhance the electrochemical kinetics, the carbon nanotubes (CNTs) were subsequently decorated on CeO2 HTs via ultrasonication and they were used as a cathode material (CNTs@CeO2 HTs) for hybrid SCs. Owing to large surface area, high porosity, and electrochemical conductivity, the prepared CNTs@CeO2 HTs composite showed a higher capacity of 70.7 μAh cm−2 at 1 mA cm−2 than the bare CeO2 HTs (41.9 μAh cm−2) in alkaline electrolyte. A hybrid SC assembled with the CNTs@CeO2 HTs and activated carbon materials as positive and negative electrodes, respectively exhibited an enlarged voltage window of 1.55 V. The device demonstrated a maximum energy density of 0.041 mWh cm−2 at 1 mA cm−2 with superior cycling stability (86.4%). Utilizing the high energy storage properties, two hybrid SCs connected in series could light up commercial light-emitting diodes for several minutes, indicating their potency for portable electronic applications. This facile convincing method may offer a cost-effective approach for the development of high capacity cathode materials for promising hybrid SCs on a large scale.

Hierarchical porous carbon foam supported on carbon cloth as high-performance anodes for aqueous supercapacitors

Carbon anodes have been widely utilized for the fabrication of high-performance asymmetric supercapacitors. However, they generally suffer from unsatisfactory energy density due to low specific capacitance arising from inferior conductivity and insufficient ionic diffusion rate. Here a surface modification method is conducted after the annealing of ZIF-67 precursor to produce hydrophilic, porous and heteroatom-doped carbon foam. On top of enhanced area capacitance, widened voltage window of −1.3–0 V (vs saturated calomel electrode) can be achieved through electrochemical reduction to suppress the hydrogen evolution reaction. The optimized reduced porous carbon foam on carbon cloth exhibits a maximum area capacitance of 1049 mF/cm2 at an applied current density of 12 mA/cm2 with excellent capacitance retention of 98.4% after 6000 charge-discharge cycles at 15 mA/cm2. By well pairing with hierarchical MnO2/CC cathode, a 2.3 V asymmetric supercapacitor in neutral aqueous Na2SO4 electrolyte is assembled, which delivers an exceptional energy density of up to 10.07 mWh/cm3. The procedure in this paper for carbonaceous material to simultaneously achieve considerable capacitance and enlarged voltage window can open up a wider prospect toward design of anodes for high-performance aqueous supercapacitor.

Iron oxide encapsulated in nitrogen-doped carbon as high energy anode material for asymmetric supercapacitors

A new strategy is proposed for the preparation of Fe3O4 encapsulated in N-doped carbon (denoted as Fe3O4@NC) through a self-assembly of the colloidal FeOOH with polyaniline (PANI) and then a subsequent pyrolysis. Benefiting from the well-designed heterostructure, the resultant Fe3O4@NC shows an ultra-high gravimetric capacitance of 313 F g−1 at 1 A g−1 and still delivers 193 F g−1 at a high current density of 15 A g−1, demonstrating an excellent rate performance. In addition, it has an outstanding volumetric capacitance of 20.25 F cm−3 at 1 A g−1. The asymmetric supercapacitors (ASCs) are constructed by using Fe3O4@NC as anode and layered double hydroxides (LDHs) as cathode, respectively. Due to the enlarged voltage window and matched capacity and kinetics, the obtained CoMn-LDH//Fe3O4@NC ASC device shows a remarkable electrochemical performance with a large energy density up to 35 Wh·kg−1 at a power density of 900 W kg−1 and a high energy density of 24 Wh·kg−1 at a power density of 13.5 kW kg−1 as well as an excellent rate capability and a good cycling stability. The present work may provide an insight into the design of novel anode materials for high-performance ASCs.

Porous interconnected NiCo2O4 nanosheets and nitrogen- and sulfur-codoped reduced graphene oxides for high-performance hybrid supercapacitors

We demonstrate a facile hydrothermal synthesis of the interconnected porous NiCo2O4 nanosheets for hybrid supercapacitor applications. The as-synthesized NiCo2O4 nanosheets show a high specific capacitance of 3137 F g−1 at a current density of 2 A g−1, which is much greater than 1916 and 1251 F g−1 of Co3O4 and NiO, respectively. Interestingly, the total specific capacitance of the NiCo2O4 is almost close to the sum of the specific capacitance of the NiO and Co3O4. Furthermore, a hybrid supercapacitor is configured with the NiCo2O4 nanosheets and the nitrogen- and sulfur-codoped reduced graphene oxide as the positive and negative electrodes, respectively. This hybrid supercapacitor delivers a maximum energy density of 33.64 W h kg−1 at a power density of 1196 W kg−1 and excellent long-term cyclic stability over 12,000 charge/discharge cycles at the enlarged voltage window of 1.5 V. The remarkable supercapacitive performances of the hybrid device are attributed to the interconnected porous structure of NiCo2O4 nanosheets and three-dimensional continuous macropores of codoped reduced graphene oxides.

Microemulsion-Assisted Synthesis of Nanosized LiMnO Spinel Cathodes for High-Rate Lithium-Ion Batteries

Li1.16Mn1.84O4 nanoparticles (50–90 nm) with cubic spinel structure are synthesized by combining a microemulsion process to produce ultrafine Mn(OH)2 nanocrystals (3–8 nm) with a solid-state lithiation step. The nanostructured lithium-rich Li1.16Mn1.84O4 shows stable cycling performance and superior rate capabilities as compared with the corresponding bulk material, for example, the nano-sized Li1.16Mn1.84O4 electrode shows stable reversible capacities of 74 mAh g−1 during the 1000th cycle at a high rate of 40 C between 3.0 and 4.5 V. In addition, Li1.16Mn1.84O4 nanoparticles also show high Li storage properties over an enlarged voltage window of 2.0–4.5 V with high capacities and stable cyclability, for example, delivering discharge capacities of 209 and 114 mAh g−1 at rates of 1 and 20 C, respectively.

Activated Carbon-Coated Carbon Nanotubes for Energy Storage in Supercapacitors and Capacitive Water Purification

Polypyrrole-coated multiwalled carbon nanotubes (PPy-MWCNT) were used for the fabrication of activated carbon-coated MWCNT doped with nitrogen (N-AC-MWCNT). The conceptually new method for the fabrication of non-agglomerated PPy-MWCNT with good coating uniformity allowed the fabrication of uniform and well-dispersed N-AC-MWCNT with high surface area. The use of N-AC-MWCNT allowed the fabrication of supercapacitor electrodes with high mass loading in the range of 15–35 mg cm–2 and with a high active material to current collector mass ratio of 0.21–0.50. The N-AC-MWCNT electrodes showed excellent electrochemical performance in aqueous 0.5 M Na2SO4 electrolyte. The maximum specific capacitance of 3.6 F cm–2 (103.1 F g–1) was achieved for mass loading of 35 mg cm–2 at a scan rate of 2 mV s–1. The aqueous supercapacitor cells, based on N-AC-MWCNT electrodes, exhibited excellent performance with energy density of 16.1 mWh g–1, power density of 14.4 W g–1, and enlarged voltage window of 1.8 V. The individual electrodes and cells showed good capacitance retention at high charge–discharge rates and good cycling stability. Moreover, the N-AC-MWCNT electrodes showed promising performance for capacitive deionization of water. The feasibility of capacitive removal of organic dyes from aqueous solutions has been demonstrated. A quartz crystal microbalance method was used as a tool for the analysis of electrosorption and electrodesorption of ions and charged dyes during charge and discharge.

Synergistic effect of amorphous carbon coverage and enlarged voltage window on the superior lithium storage performance of nanostructured mesoporous anatase TiO2: Emphasis on interfacial storage phenomena

Mesoporous anatase TiO2 nanoparticles coated with an ultrathin layer of amorphous carbon are hydrothermally synthesized. Used as an anode material, it achieves a sustained superior lithium storage performance, presenting a high reversible capacity of 270mAhg−1 up to 300 cycles at a current density of 30mAhg−1 in an enlarged voltage window of 0.01–3V, which is firstly adopted for TiO2 anode material. Remarkably, the carbon coated TiO2 nanoparticles can still maintain a capacity of 171mAhg−1 at 300mAg−1 after 1000 cycles, and even 93mAhg−1 at 600mAg−1 after 1000 cycles. We propose an overall view on the diverse features influencing the electrochemical performance of the high-surface-area mesoporous carbon coated TiO2 nanoparticles and emphasize that the excellent performance is the synergistic result of the enlarged voltage window, which leads to higher interfacial lithium storage, and the uniform amorphous carbon coverage, which not only improves electrical conductivity, minimizes the direct solid-electrolyte interphase (SEI) formation, but also helps to avoid the structure instability arising from the enlargement of the potential window.