Mechanisms and Designs of Asymmetrical Electrochemical Capacitors

Pulsed Laser Ablation of Oxygen deficiency Enriched Superlattice Vanadium Pentoxide (V2O5) Ultrathin Nextrode aiming for Flexible Binder-less Tandem Energy Harvesting Devices

For the initial instance, oxygen deficiency-enriched vanadium pentoxide (O─V2O5@500) thin film electrodes are tuned by the Pulsed Laser Ablation technique. The O─V2O5@500 thin film electrode shows remarkable electrochemical performances confirming the greater potential window of -0.4 to 0.9 V versus Hg/HgO in an alkaline electrolyte; also, the O─V2O5@ 500 thin film electrode exhibits a noteworthy volumetric capacity of 167.7 mAh cm-3 (areal capacity of 73.3 µAh cm-2). Additionally, Density Functional Theory (DFT) theory calculations are carried out for oxygen-deficient V2O5. From the partial density of states (pDOS) and partial charge density analysis, it is clear that oxygen vacancy improves the electrical conductivity due to the higher degree of electron delocalization of V─O─V near the vacancy and enhances the redox properties due to the formation of in-gap states. Further, it is reported that a O─V2O5@ 500 ||PVA-KOH|| Bi2O3 A-650 thin film supercapbattery (TFSCB) device attains an exceptional discharge volumetric capacitance of 182.85 F cm-3 (equal volumetric capacity of 124.5 mAh cm-3). Furthermore, the TFSCB device exhibits an extraordinary maximum volumetric energy (power) density of 14.28 mWh cm-3 (1.66 W cm-3); TFSCB succeeds in supreme capacity retention of 86% with outstanding coulombic efficiency of 94.4% after 21 000 cycles.

Design and construction of hollow metal sulfide/selenide core-shell heterostructure arrays for hybrid supercapacitor

Transition metal sulfides and selenides are common electrode materials in supercapacitors. However, the slow redox kinetics and structural collapse during charge-discharge cycles of single-component materials have impeded their electrochemical performance. In this study, hollow Co9S8 nanotubes were synthesized through a rational morphology design approach. Subsequently, NiSe2 or Co0.85Se was electrodeposited onto the Co9S8 nanotubes, yielding two core-shell heterostructure arrays, namely, NiSe2@Co9S8 and Co0.85Se@Co9S8. By fully leveraging the advantages and synergistic effects of these dual-phase heterostructures, the NiSe2@Co9S8 and Co0.85Se@Co9S8 configurations demonstrated outstanding areal capacitances of 12.54 F cm-2 and 9.61 F cm-2, respectively, at 2 mA cm-2. When integrated with activated carbon in hybrid supercapacitors, the NiSe2@Co9S8//AC and Co0.85Se@Co9S8//AC devices exhibited excellent energy storage performance, with energy densities of 0.959 mW h at 1.681 mW and 0.745 mW h at 1.569 mW, respectively. Additionally, these hybrid supercapacitors demonstrated remarkable cycling stability, with capacitance retention of 87.5% and 89.5% after 5000 cycles for NiSe2@Co9S8//AC and Co0.85Se@Co9S8//AC, respectively. This study provides a novel approach to the synthesis of multiphase core-shell heterostructures based on metal sulfides and selenides, opening new avenues for future research.

Application of Copper-Sulfur Compound Electrode Materials in Supercapacitors

Supercapacitors (SCs) are a novel type of energy storage device that exhibit features such as a short charging time, a long service life, excellent temperature characteristics, energy saving, and environmental protection. The capacitance of SCs depends on the electrode materials. Currently, carbon-based materials, transition metal oxides/hydroxides, and conductive polymers are widely used as electrode materials. However, the low specific capacitance of carbon-based materials, high cost of transition metal oxides/hydroxides, and poor cycling performance of conductive polymers as electrodes limit their applications. Copper-sulfur compounds used as electrode materials exhibit excellent electrical conductivity, a wide voltage range, high specific capacitance, diverse structures, and abundant copper reserves, and have been widely studied in catalysis, sensors, supercapacitors, solar cells, and other fields. This review summarizes the application of copper-sulfur compounds in SCs, details the research directions and development strategies of copper-sulfur compounds in SCs, and analyses and summarizes the research hotspots and outlook, so as to provide a reference and guidance for the use of copper-sulfur compounds.

Supercapatteries as Hybrid Electrochemical Energy Storage Devices: Current Status and Future Prospects

Among electrochemical energy storage (EES) technologies, rechargeable batteries (RBs) and supercapacitors (SCs) are the two most desired candidates for powering a range of electrical and electronic devices. The RB operates on Faradaic processes, whereas the underlying mechanisms of SCs vary, as non-Faradaic in electrical double-layer capacitors (EDLCs), Faradaic at the surface of the electrodes in pseudo-capacitors (PCs), and a combination of both non-Faradaic and Faradaic in hybrid supercapacitors (HSCs). EDLCs offer high power density but low energy density. HSCs take advantage of the Faradaic process without compromising their capacitive nature. Unlike batteries, supercapacitors provide high power density and numerous charge-discharge cycles; however, their energy density lags that of batteries. Supercapatteries, a generic term that refers to hybrid EES devices that combine the merits of EDLCs and RBs, have emerged, bridging the gap between SCs and RBs. There are numerous articles and reviews on EES, and many of those articles have emphasized various aspects of HSCs and supercapatteries. However, there are no recent reviews that dealt with supercapatteries in general. Here, we review recently published critically selected articles on supercapatteries. The review discusses different EES devices and how supercapatteries are different from others. Also discussed are properties, design strategies, and future perspectives on supercapatteries.

Analyzing synthesis routes for BaCuPO4: implications for hydrogen evolution and supercapattery performance

In recent years, energy storage and conversion tools have evolved significantly in response to rising energy demands. Owing to their large surface area, superior electric and chemical stabilities, and thermal conductivities, barium copper phosphate (BaCuPO4) materials are promising electrode materials for electrochemical energy storage systems. In this study, the synthesis of nanostructures (NSs) using hydrothermal and chemical precipitation methods and exploring the electrochemical characteristics of BaCuPO4 in asymmetric supercapacitors provides a comparative investigation. Systematic characterization shows that nanomaterials prepared by applying the hydrothermal method have a more crystalline and large surface area than chemical precipitation. In the three cell arrangements, the hydrothermally prepared BaCuPO4 NSs delivered a high specific capacity (764.4 C g-1) compared to the chemical precipitation route (660 C g-1). Additionally, the supercapattery associated with the two electrode assemblages delivers an optimum specific capacity of 77 C g-1. The energy and power density of BaCuPO4//AC NSs were 52.13 W h kg-1 and 950 W kg-1, respectively. A durability test was also performed with BaCuPO4//AC NSs for 5000 consecutive cycles. Further, the coulombic efficiency and capacity retention of BaCuPO4//AC after 5000 cycles were 81% and 92%, respectively. Bimetallic phosphate is comparatively suggested for future perspectives towards HER to overcome the performance of single metal phosphate materials. This is the first approach, we are aware of, for investigating the electrochemical behavior of BaCuPO4, and our results suggest that it may be useful as an electrode material in electrochemical systems requiring high energy and rate capabilities.

An inorganic-organic hybrid nanomaterial with a core-shell structure constructed by using Mn-BTC and Ag5[BW12O40] for supercapacitors and photocatalytic dye degradation

Creating inorganic-organic hybrids with polyoxometalates (POMs) and metal-organic frameworks (MOFs) as energy storage and dye-degradation materials remains challenging. Here, a new hybrid nanomaterial Mn-BTC@Ag₅[BW₁₂O₄₀] is synthesized by using Ag₅[BW₁₂O₄₀] and Mn₃(BTC)₂(H₂O)₆ (Mn-BTC, BTC = 1,3,5-benzenetricarboxylic acid) through a plain grinding method. The structure and morphology characterization by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and transmission electron microscopy (TEM) shows that the synthetic products have core-shell construction. Due to its unique structure wherein the core is Mn-BTC and the shell is Ag₅[BW₁₂O₄₀], it exhibits excellent capacitance performance. In a three-electrode system where nickel foam is a collector, at a current density of 1 A g^-1, its specific capacitance is 198.09 F g^-1; after 5000 cycles, the capacitance retention rate is 94.4%. When the power density is 503.1 W kg^-1, the symmetrical supercapacitor reveals a high energy density which is 10.9 W h kg^-1. At the same time, the capacitance retention is 92.9% after 5000 cycles which showed good cycle stability. The photocatalytic degradation efficiencies of rhodamine B (RhB), methyl orange (MO) and methylene blue (MB) dyes exceed 90% after 140 min, and the degradation results remained unchanged after five photocatalytic cycles. The photocatalytic degradation mechanism shows that ˙OH has a major effect. The results show that this research provides a fresh idea for the development of energy storage and dye photocatalytic degradation materials.

Supercapatteries as High-Performance Electrochemical Energy Storage Devices

Abstract

The development of novel electrochemical energy storage (EES) technologies to enhance the performance of EES devices in terms of energy capacity, power capability and cycling life is urgently needed. To address this need, supercapatteries are being developed as innovative hybrid EES devices that can combine the merits of rechargeable batteries with the merits of supercapacitors into one device. Based on these developments, this review will present various aspects of supercapatteries ranging from charge storage mechanisms to material selection including electrode and electrolyte materials. In addition, strategies to pair different types of electrode materials will be discussed and proposed, including the bipolar stacking of multiple supercapattery cells internally connected in series to enhance the energy density of stacks by reducing the number of bipolar plates. Furthermore, challenges for this stack design will also be discussed together with recent progress on bipolar plates.

Graphic Abstract

Supercapattery is an innovated hybrid electrochemical energy storage (EES) device that combines the merit of rechargeable battery and supercapacitor characteristics into one device. This article reviews supercapatteries from the charge storage mechanisms to the selection of materials including the materials of electrodes and electrolytes. Strategies for pairing different kinds of electrode materials and device engineering are discussed.

Hierarchically hollow structured NiCo2S4@NiS for high-performance flexible hybrid supercapacitors

Hierarchical nanostructures with outstanding electrochemical properties and mechanical stability are ideal for constructing flexible hybrid supercapacitors. Herein, hierarchically hollow NiCo2S4@NiS nanostructures were designed and synthesized by sulfurizing the hierarchical NiCo double hydroxides (DHs) coated with nickel hydroxide nanostructures on carbon fabrics (NiCo-DHs@Ni(OH)2/CF), which trigger excellent electrochemical performances. The NiCo2S4@NiS/CF exhibits a high specific capacity of 1314.0 C g-1 at a current density of 1 A g-1, and maintains the rate performance at about 79.2% of the initial capacity at 30 A g-1. The hybrid supercapacitors of NiCo2S4@NiS//AC display a high energy density of 62.4 W h kg-1 at a power density of 800 W kg-1 with a remarkable cycling stability (96.2% of initial capacitance after 5000 cycles) and robust mechanical flexibility (no obvious decay of specific capacitance during various deformations). Consequently, NiCo2S4@NiS electrodes are expected to be a promising candidate for new smart energy storage devices with high security, stability and flexibility.

A binder-free Ni2P2O7/Co2P2O7 nanograss array as an efficient cathode for supercapacitors

Binder-free Ni₂P₂O₇/Co₂P₂O₇ cathode of nanograss morphology, delivered an energy and power density of 33.2 W h kg⁻¹ and 257.8 W kg⁻¹ respectively. Through power law, the contribution of each type of mechanism in charge storage process was calculated.

Engineering thermally activated NiMoO4 nanoflowers and biowaste derived activated carbon-based electrodes for high-performance supercapatteries

NiMoO₄ nanoflowers having pure crystalline phases with slight amorphous surface exhibited excellent battery-like electrochemical performance and potential for supercapattery positive electrodes.

Formation of needle-like porous CoNi2S4-MnOOH for high performance hybrid supercapacitors with high energy density

Seeking for suitable electrode materials and designing rational porous structures are great challenges for developing high performance supercapacitors. Herein, needle-like porous CoNi₂S₄-MnOOH (denoted as NCS-MO) were prepared via a simple two steps solvothermal method and used as battery-type electrode of supercapacitor for the first time. Owing to the multiple oxidation states of needle-like porous NCS-MO and the inherent porous structure, the electrode delivers outstanding electrochemical capacitive properties with a high gravimetric specific capacitance of 1267.7 F g^-1 at the scan rate of 1 mV s^-1. To further assess the practical electrochemical performances, we assembled a hybrid supercapacitor using the as-synthesized porous NCS-MO as cathode and active carbon as anode. The device exhibits excellent performance with a high energy density of 47.1 Wh kg^-1 at the power density of 998 W kg^-1 in an extended voltage range of 1.6 V and outstanding cycling stability. These results demonstrate that the needle-like porous NCS-MO could be promising potential electrode material for high performance supercapacitor.

Transition-Metal-Doped Molybdenum Diselenides with Defects and Abundant Active Sites for Efficient Performances of Enzymatic Biofuel Cell and Supercapacitor Applications

We have demonstrated the synthesis of defect-rich Ni-doped MoSe₂ nanoplates (NiMoSe₂) and their application as an efficient electrocatalyst for enzymatic biofuel cells and electrochemical pseudocapacitors. In this study, a new type of interpretation is proposed that a defective surface facilitates the effective entrapment of enzymes (glucose oxidase (GOD), laccase) for biofuel cells and additional ion diffusion for Faradic charge-discharge reaction. The transmission electron microscopy and UV-vis spectroscopy techniques scrutinized the formation of defects/distortions and the resultant successful entrapment of enzymes. The performed electrochemical characterizations of enzyme-immobilized NiMoSe₂/nickel foam (NF) bioanode (NiMoSe₂/GOD/NF) and biocathode (NiMoSe₂/laccase/NF) exhibited better direct charge conductive behavior at the interface of enzymes and electrode material. Herein, the assembled biofuel cells exhibited an open-circuit voltage ( V_OC = 0.6 V) and a short-circuit current density ( J_SC = 8.629 mA cm^-2) with a maximum power density ( P_max) of 1.2 mW cm^-2. For the electrochemical pseudocapacitor application, the proposed NiMoSe₂/NF exhibited excellent specific capacitance (535.74 F g^-1), with 86.7% rate performance. Finally, this work suggests new insights into both enzymatic biofuel cell and supercapacitor applications.

Pseudocapacitance electrode and asymmetric supercapacitor based on biomass juglone/activated carbon composites

A novel electrode material incorporating renewable biomass-derived juglone biomolecules with commercial activated carbon (AC) granules has been through simple ultrasonic dispersion and dissolution–recrystallization and was found to exhibit good electrochemical performance. The juglone biomolecules are prepared by an ultrasound-assisted extraction method from abandoned walnut peel, which decreases pollution and increases economic efficiency. Through the dissolution–recrystallization process with AC, a hierarchical structure with nanosized juglone particles was obtained, and the AC particles worked as scaffolding to strengthen the slight biomolecules, thus expanding the active sites and effectively reducing the dissolution of the active materials. The pseudocapacitance fading mechanism was investigated by ex situ FTIR measurement and the porous structure ensures that the composite electrode has an enhanced specific capacitance of 248 F g⁻¹ compared to 172.8 and 62.5 F g⁻¹ for the respective AC and juglone samples. Besides, the excellent cyclic stability (retained 75% after 3000 charge–discharge cycles) was demonstrated. The highest area-specific capacitance of the composites was 1300 mF cm⁻². An asymmetric supercapacitor based on this composite electrode was assembled with an AC electrode as the counter electrode and exhibited good cyclic performance at a voltage of 1.2 V (retained 77% after 3000 charge–discharge cycles), which provides a high energy density of 12 W h kg⁻¹ at a power density of 0.18 kW kg⁻¹ and a high power density of 2 kW kg⁻¹ at an energy density of 9 W h kg⁻¹. This work explores the application of biomolecule-based composites in energy storage devices and provides a potential strategy for constructing environmentally friendly electrodes.

A strategy for transforming abandoned walnut peel to excellent pseudocapacitance material. The activated carbon reshapes and anchors the juglone, which combined the EDLC and pseudocapacitance to achieve high electrochemical performance.

Fabrication of g-C3N4 Nanomesh-Anchored Amorphous NiCoP2O7: Tuned Cycling Life and the Dynamic Behavior of a Hybrid Capacitor

Developing a novel electrode material with better electrochemical behavior and extended cyclability is a major issue in the field of hybrid capacitors. In this work, we propose a novel strategy for the facile synthesis of nickel-cobalt pyrophosphate nanoparticles anchored on graphitic carbon nitride (NiCoP₂O₇/g-C₃N₄) via the simple solvothermal method. Field emission scanning electron microscopy and transmission electron microscopy analysis revealed the uniform anchoring of NiCoP₂O₇ nanocomposite on g-C₃N₄ nanosheets. Benefitting from the effect of amorphous nature and a conductive matrix of the NiCoP₂O₇/g-C₃N₄ (NP3) composite, the material achieves a specific capacitance of 342 F g^-1 at a scan rate of 5 mV s^-1. Impressively, the electrode shows long-term cycling stability with 100% capacitance retention over 5000 cycles. Employing activated carbon as an anode and as-prepared NP3 as a cathode, the assembled asymmetric hybrid cell exhibits high-energy density and exceptional cyclability (specific capacitance retention over 10 000 cycles). The outstanding electrochemical and cyclic stability is attributed to the shortest electron-ion pathway with effective interfacial interaction. The low electronic resistance of the NiCoP₂O₇/g-C₃N₄ nanocomposite is revealed by varying the bias voltage variation in the electrochemical impedance spectroscopy. Our results promise better utilization of the bimetallic pyrophosphate-anchored g-C₃N₄ matrix as a potential electrode for high-performance energy storage devices.

Design and Mechanisms of Asymmetric Supercapacitors

Ongoing technological advances in diverse fields including portable electronics, transportation, and green energy are often hindered by the insufficient capability of energy-storage devices. By taking advantage of two different electrode materials, asymmetric supercapacitors can extend their operating voltage window beyond the thermodynamic decomposition voltage of electrolytes while enabling a solution to the energy storage limitations of symmetric supercapacitors. This review provides comprehensive knowledge to this field. We first look at the essential energy-storage mechanisms and performance evaluation criteria for asymmetric supercapacitors to understand the wide-ranging research conducted in this area. Then we move to the recent progress made for the design and fabrication of electrode materials and the overall structure of asymmetric supercapacitors in different categories. We also highlight several key scientific challenges and present our perspectives on enhancing the electrochemical performance of future asymmetric supercapacitors.

Redox Additive-Improved Electrochemically and Structurally Robust Binder-Free Nickel Pyrophosphate Nanorods as Superior Cathode for Hybrid Supercapacitors

For several decades, one of the great challenges for constructing a high-energy supercapacitor has been designing electrode materials with high performance. Herein, we report for the first time to our knowledge a novel hybrid supercapacitor composed of battery-type nickel pyrophosphate one-dimensional (1D) nanorods and capacitive-type N-doped reduced graphene oxide as the cathode and anode, respectively, in an aqueous redox-added electrolyte. More importantly, ex situ microscopic images of the nickel pyrophosphate 1D nanorods revealed that the presence of the battery-type redox additive enhanced the charge storage capacity and cycling life as a result of the microstructure stability. The nickel pyrophosphate 1D nanorods exhibited their maximum specific capacitance (8120 mF cm^-2 at 5 mV s^-1) and energy density (0.22 mWh cm^-2 at a power density of 1.375 mW cm^-2) in 1 M KOH + 75 mg K₃[Fe(CN)₆] electrolyte. On the other side, the N-doped reduced graphene oxide delivered an excellent electrochemical performance, demonstrating that it was an appropriate anode. A hybrid supercapacitor showed a high specific capacitance (224 F g^-1 at a current density of 1 A g^-1) and high energy density (70 Wh kg^-1 at a power density of 750 W kg^-1), as well as a long cycle life (a Coulombic efficiency of 96% over 5000 cycles), which was a higher performance than most of those in recent reports. Our results suggested that the materials and redox additive in this novel design hold great promise for potential applications in a next-generation hybrid supercapacitor.