Molybdenum Nitride Nanocrystals Anchored on Phosphorus-Incorporated Carbon Fabric as a Negative Electrode for High-Performance Asymmetric Pseudocapacitor

Metallicity and chemical bonding in anti-anatase Mo2N

Here we present a detailed analysis of the structure, bonding character, and electronic structure of anti-anatase β-Mo2N using density functional theory calculations. We analyze the crystal orbital Hamilton populations, phonon band structure, and electronic structure calculations to explain its low energy transport behavior. We further examine the electronic structures of (anti-)rutile and (anti-)anatase M3-nXn (X = N,O; n = 1,2) M = Ti and Mo nitrides and oxides to show that the atomic structure of anti-anatase leads to metallic behavior independent of the metal and ligand chemistry. Finally, we assess whether these anti-anatase compounds are viable electrides using electron density maps and electron localization functions. Our work shows anti-structures of known binary compounds can expand the phase space of available metallic ceramics beyond layered, hexagonal carbides and nitrides, e.g., Mn+1An (MAX) where n = 1-4.

Experimental and computational investigation on the charge storage performance of a novel Al2O3-reduced graphene oxide hybrid electrode

The advancements in electrochemical capacitors have noticed a remarkable enhancement in the performance for smart electronic device applications, which has led to the invention of novel and low-cost electroactive materials. Herein, we synthesized nanostructured Al₂O₃ and Al₂O₃-reduced graphene oxide (Al₂O₃-rGO) hybrid through hydrothermal and post-hydrothermal calcination processes. The synthesized materials were subject to standard characterisation processes to verify their morphological and structural details. The electrochemical performances of nanostructured Al₂O₃ and Al₂O₃- rGO hybrid were evaluated through computational and experimental analyses. Due to the superior electrical conductivity of reduced graphene oxide and the synergistic effect of both EDLC and pseudocapacitive behaviour, the Al₂O₃- rGO hybrid shows much improved electrochemical performance (~ 15-fold) as compared to bare Al₂O₃. Further, a symmetric supercapacitor device (SSD) was designed using the Al₂O₃- rGO hybrid electrodes, and detailed electrochemical performance was evaluated. The fabricated Al₂O₃- rGO hybrid-based SSD showed 98.56% capacity retention when subjected to ~ 10,000 charge-discharge cycles. Both the systems (Al₂O₃ and its rGO hybrid) have been analysed extensively with the help of Density Functional Theory simulation technique to provide detailed structural and electronic properties. With the introduction of reduced graphene oxide, the available electronic states near the Fermi level are greatly enhanced, imparting a significant increment in the conductivity of the hybrid system. The lower diffusion energy barrier for electrolyte ions and higher quantum capacitance for the hybrid structure compared to pristine Al₂O₃ justify improvement in charge storage performance for the hybrid structure, supporting our experimental findings.

Ultrafast PEDOT:PSS/H2 SO4 Electrical Double Layer Capacitors: Comparison with Polyaniline Pseudocapacitors

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is one of the most widely studied conductive polymers, owing to its excellent electrical, optical, and mechanical properties, with various applications such as organic electrochemical transistors, electrochromics, and flexible/stretchable supercapacitors. The charging mechanism of PEDOT:PSS supercapacitors has been traditionally believed to be faradaic, which involves the transfer of charge across the electrode/electrolyte interface. In the present work, however, robust experimental evidence suggests that the PEDOT:PSS supercapacitors mainly store and deliver charge nonfaradaically. The various electrochemical properties of PEDOT:PSS electrical double layer capacitors (EDLCs) are clearly distinguishable from those of polyaniline (PANI) pseudocapacitors, which store charge faradaically. Owing to the nonfaradaic mechanism, the frequency response of PEDOT:PSS supercapacitors is comparable to that of state-of-the-art ultrafast EDLCs with carbon-based electrodes, making them suitable for high-frequency applications such as 60 Hz AC line filtering. This result is of great importance for the fundamental understanding of the charging mechanism of mixed ionic-electronic conducting polymers, such as PEDOT:PSS, and is expected to contribute to the development of various electrochemical devices based on this type of material.

Facile Synthesis of NiCo2O4 Nanowire Arrays/Few-Layered Ti3C2-MXene Composite as Binder-Free Electrode for High-Performance Supercapacitors

Herein, a 3D hierarchical structure is constructed by growing NiCo₂O₄ nanowires on few-layer Ti₃C₂ nanosheets using Ni foam (NF) as substrate via simple vacuum filtration and solvothermal treatment. Ti₃C₂ nanosheets are directly anchored on NF surface without binders or surfactants, and NiCo₂O₄ nanowires composed of about 15 nm nanoparticles uniformly grow on Ti₃C₂/NF skeleton, which can provide abundant active sites and ion diffusion pathways for enhancing electrochemical performance. Benefiting from the unique structure feature and the synergistic effects of active materials, NiCo₂O₄/Ti₃C₂ exhibits a high specific capacitance of 2468 F g^-1 at a current density of 0.5 A g^-1 and a good rate performance. Based on this, an asymmetric supercapacitor (ASC) based on NiCo₂O₄/Ti₃C₂ as positive electrode and activated carbon (AC)/NF as negative electrode is assembled. The ASC achieves a high specific capacitance of 253 F g^-1 at 1 A g^-1 along with 91.5% retention over 10,000 cycles at 15 A g^-1. Furthermore, the ACS presents an outstanding energy density of 90 Wh kg^-1 at the power density of 2880 W kg^-1. This work provides promising guidance for the fabrication of binder-free, free-standing and hierarchical composites for energy storage application.

Sputter-Deposited Binder-Free Nanopyramidal Cr/γ-Mo2N TFEs for High-Performance Supercapacitors

Due to their outstanding power density, long cycle life and low cost, supercapacitors have gained much interest. As for supercapacitor electrodes, molybdenum nitrides show promising potential. Molybdenum nitrides, however, are mainly prepared as nanopowders via a chemical route and require binders for the manufacture of electrodes. Such electrodes can impair the performance of supercapacitors. Herein, binder-free chromium (Cr)-doped molybdenum nitride (Mo₂N) TFEs having different Cr concentrations are prepared via a reactive co-sputtering technique. The Cr-doped Mo₂N films prepared have a cubic phase structure of γ-Mo₂N with a minor shift in the (111) plane. While un-doped Mo₂N films exhibit a spherical morphology, Cr-doped Mo₂N films demonstrate a clear pyramid-like surface morphology. The developed Cr-doped Mo₂N films contain 0-7.9 at.% of Cr in Mo₂N lattice. A supercapacitor using a Cr-doped Mo₂N electrode having the highest concentration of Cr reveals maximum areal capacity of 2780 mC/cm², which is much higher than that of an un-doped Mo₂N electrode (110 mC/cm²). Furthermore, the Cr-doped Mo₂N electrode demonstrates excellent cycling stability, achieving ~ 94.6% capacity retention for about 2000 cycles. The reactive co-sputtering proves to be a suitable technique for fabrication of binder-free TFEs for high-performance energy storage device applications.

Recent Developments of Transition Metal Compounds-Carbon Hybrid Electrodes for High Energy/Power Supercapacitors

Due to their rapid power delivery, fast charging, and long cycle life, supercapacitors have become an important energy storage technology recently. However, to meet the continuously increasing demands in the fields of portable electronics, transportation, and future robotic technologies, supercapacitors with higher energy densities without sacrificing high power densities and cycle stabilities are still challenged. Transition metal compounds (TMCs) possessing high theoretical capacitance are always used as electrode materials to improve the energy densities of supercapacitors. However, the power densities and cycle lives of such TMCs-based electrodes are still inferior due to their low intrinsic conductivity and large volume expansion during the charge/discharge process, which greatly impede their large-scale applications. Most recently, the ideal integrating of TMCs and conductive carbon skeletons is considered as an effective solution to solve the above challenges. Herein, we summarize the recent developments of TMCs/carbon hybrid electrodes which exhibit both high energy/power densities from the aspects of structural design strategies, including conductive carbon skeleton, interface engineering, and electronic structure. Furthermore, the remaining challenges and future perspectives are also highlighted so as to provide strategies for the high energy/power TMCs/carbon-based supercapacitors.

Theories and models of supercapacitors with recent advancements: impact and interpretations

Supercapacitors provide remarkable eco-friendly advancement in energy conversion and storage with a huge potential to control the future economy of the entire world. Currently, industries focus on the design and engineering aspects of supercapacitors with high performance (high energy), flexibility (by the use of composite polymer based electrolytes), high voltage (ionic liquid) and low cost. The paper reviews the modelling techniques like Empirical modelling, Dissipation transmission line models, Continuum models, Atomistic models, Quantum models, Simplified analytical models etc. proposed for the theoretical study of Supercapacitors and discusses their limitations in studying all the aspects of Supercapacitors. It also reviews the various software packages available for Supercapacitor (SC) modelling and discusses their advantages and disadvantages. The paper also reviews the Experimental advancements in the field of electric double layer capacitors (EDLCs), pseudo capacitors and hybrid/asymmetric supercapacitors and discusses the commercial progress of supercapacitors as well.

Transition metal nitrides for electrochemical energy applications

This review comprehensively summarizes the progress on the structural and electronic modulation of transition metal nitrides for electrochemical energy applications.

Fabrication of a High-Energy Flexible All-Solid-State Supercapacitor Using Pseudocapacitive 2D-Ti3C2Tx-MXene and Battery-Type Reduced Graphene Oxide/Nickel-Cobalt Bimetal Oxide Electrode Materials

Owing to excellent metallic conductivity, hydrophilic surfaces, and surface redox properties, a two-dimensional (2D) metal carbide of Ti₃C₂T_x-MXene could serve as a promising pseudocapacitive electrode material for energy storage devices. Meanwhile, the 2D reduced graphene oxide (rGO) combining with the hierarchical cubic spinel nickel-cobalt bimetal oxide (NiCo₂O₄) nanospikes could control ion diffusion for charge storage, thereby facilitating the improvement of the energy density of a supercapacitor. As per the strategy, the pseudocapacitive 2D Ti₃C₂T_x was loaded on a flexible acid-treated carbon fiber (ACF) backbone to prepare a Ti₃C₂T_x/ACF negative electrode by a convenient drop-casting method. Meanwhile, 2D rGO was deposited on ACF by a simple dip-dry process, which was further decorated by the spinel NiCo₂O₄ nanospikes using a hydrothermal method to obtain a NiCo₂O₄@rGO/ACF positive electrode. The fabricated Ti₃C₂T_x/ACF electrode exhibited an excellent specific capacitance of 246.9 F/g (197.5 mF/cm²) at 4 mA/cm² along with 96.7% capacity retention over 5000 charge/discharge cycles, whereas the NiCo₂O₄@rGO/ACF electrode showed a specific capacitance of 1487 F/g (458.3 mA h/g) at 3 mA/cm² with a cycling stability of 88.2% over 10 000 charge/discharge cycles. As a result, a flexible all-solid-state hybrid supercapacitor (FHSC) device using the pseudocapacitive Ti₃C₂T_x/ACF on the negative side with a widespread voltage window and the battery-type NiCo₂O₄@rGO/ACF on the positive side with high electrochemical activity delivered an excellent volumetric capacitance of 2.32 F/cm³ (141.9 F/g) at a current density of 5 mA/cm² with a high-energy density of 44.36 Wh/kg (0.72 mWh/cm³) at a power density of 985 W/kg (16.13 mW/cm³) along with a cycling stability of 90.48% over 4500 charge/discharge cycles. Therefore, the pseudocapacitive 2D Ti₃C₂T_x/ACF negative electrode could replace carbon-based electrodes and a combination of it with the battery-type NiCo₂O₄@rGO/ACF positive electrode should be a promising way to step up the energy density of a supercapacitor.

Asymmetric Pseudocapacitors Based on Interfacial Engineering of Vanadium Nitride Hybrids

Vanadium nitride (VN) shows promising electrochemical properties as an energy storage devices electrode, specifically in supercapacitors. However, the pseudocapacitive charge storage in aqueous electrolytes shows mediocre performance. Herein, we judiciously demonstrate an impressive pseudocapacitor performance by hybridizing VN nanowires with pseudocapacitive 2D-layered MoS2 nanosheets. Arising from the interfacial engineering and pseudocapacitive synergistic effect between the VN and MoS2, the areal capacitance of VN/MoS2 hybrid reaches 3187.30 mF cm-2, which is sevenfold higher than the pristine VN (447.28 mF cm-2) at a current density of 2.0 mA cm-2. In addition, an asymmetric pseudocapacitor assembled based on VN/MoS2 anode and TiN coated with MnO2 (TiN/MnO2) cathode achieves a remarkable volumetric capacitance of 4.52 F cm-3 and energy density of 2.24 mWh cm-3 at a current density of 6.0 mA cm-2. This work opens a new opportunity for the development of high-performance electrodes in unfavorable electrolytes towards designing high areal-capacitance electrode materials for supercapacitors and beyond.

Two-Dimensional Materials for High-Energy Solid-State Asymmetric Pseudocapacitors with High Mass Loadings

A porous nanostructure and high mass loading are crucial for a pseudocapacitor to achieve a good electrochemical performance. Although pseudocapacitive materials, such as MnO₂ and MoS₂ , with record capacitances close to their theoretical values have been realized, the achieved capacitances are possible only when the electrode mass loading is less than 1 mg cm^-2 . Increasing the mass loading affects the capacitance as electron conduction and ion diffusion become sluggish. Achieving fast ion and electron transport at high mass loadings through all active sites remains a challenge for high-mass-loading electrodes. In this study, 2D MnO₂ nanosheets supported on carbon fibers (MnO₂ @CF) as well as MoS₂ @CF with high mass loadings (6.6 and 7.2 mg cm^-2 , respectively) were used in a high-energy pseudocapacitor. These hierarchical 2D nanosheets yielded outstanding areal capacitances of 1187 and 495 mF cm^-2 at high current densities with excellent cycling stabilities. A pliable pseudocapacitive solid-state asymmetric supercapacitor was designed using MnO₂ @CF and MoS₂ @CF as the positive and negative electrodes, respectively, with a high mass loading of 14.2 mg cm^-2 . The assembled solid-state asymmetric cell had an energy density of 2.305 mWh cm^-3 at a power density of 50 mW cm^-3 and a capacitance retention of 92.25 % over 11 000 cycles and a very small diffusion resistance (1.72 Ω s^-1/2 ). Thus, it is superior to most state-of-the-art reported pseudocapacitors. The rationally designed nanostructured electrodes with high mass loading are likely to open up new opportunities for the development of a supercapacitor device capable of supplying higher energy and power.

Porous Ultrathin NiSe Nanosheet Networks on Nickel Foam for High-Performance Hybrid Supercapacitors

Transition metal selenides (TMSs) with excellent electrochemical activity and high intrinsic electrical conductivity have attracted considerable attention owing to their potential use in energy storage devices. However, the low energy densities of the reported TMSs, which originate from the small active surface area and poor electrolyte ion mobility, substantially restrict their application potential. In this work, porous ultrathin nickel selenide nanosheet networks (NiSe NNs) on nickel foam are fabricated by using a novel, facile method, that is, selenylation/pickling of the pre-formed manganese-doped α-Ni(OH)₂ . Removal of Mn resulted in NNs with a highly porous structure. The 3D framework of the NNs and the inherent nature of the NiSe affords high ion mobility, abundant accessible activated sites, vigorous electrochemical activity, and low resistance. One of the highest specific capacities of TMSs ever reported, that is, 443 mA h g^-1 (807 μAh cm^-2 ) at 3.0 A g^-1 , is achieved with the NNs as electrodes. The assembled NiSe NNs//porous carbon hybrid supercapacitor delivers a high energy density of 66.6 Wh kg^-1 at a power density of 425 W kg^-1 , with excellent cycling stability. This work provides a new strategy for the production of novel electrode materials that can be applied in high-performance hybrid supercapacitors, and a fresh pathway towards commercial applications of hybrid supercapacitors based on TMS electrodes.

Hydrothermally synthesized chalcopyrite platelets as an electrode material for symmetric supercapacitors

A novel chalcopyrite (CuFeS₂) platelet like open-pored micro-flower nanostructure investigated as an electrode material for supercapacitors.