How Do Pseudocapacitors Store Energy? Theoretical Analysis and Experimental Illustration

Mesoporous NiCo2Se4 tube as an efficient electrode material with enhanced performance for asymmetric supercapacitor applications

• One-component NiCo₂Se₄ is synthesized. • The unique mesoporous tubular micro-nanostructure greatly improves the electrochemical performance. • Selenium with high electrical conductivity is beneficial for improving the energy density and power density.

Pseudocapacitive Charge Storage in MXene-V2O5 for Asymmetric Flexible Energy Storage Devices

Pseudocapacitive asymmetric supercapacitors are promising candidates for achieving high energy density in flexible energy storage devices. However, seeking suitable positive electrode materials that are compatible with negative electrode materials remains a considerable challenge. In the current study, a pseudocapacitive Ti₃C₂T_x MXene used as negative electrodes is rationally compatible with redox-type V₂O₅ as positive electrodes, resulting in the assembly of an all-pseudocapacitive Ti₃C₂T_x MXene//V₂O₅ asymmetric flexible energy storage device. The solid-state asymmetric device can deliver an energy density of 8.33 mW h cm^-3 at a current density of 0.5 A g^-1. Moreover, it can operate in an expanded voltage window of 1.5 V, with dominant surface-capacitive charge-storage mechanisms. Additionally, the device can power a yellow light-emitting diode for up to 7 s, indicating the potential of the device for use in practical applications. This study demonstrates the possibility of using other two-dimensional transition-metal carbide nanosheets for high-energy density flexible energy storage devices.

Electrochemical Energy Storage: Questioning the Popular v/v1/2 Scan Rate Diagnosis in Cyclic Voltammetry

The v/v^1/2 scan rate diagnosis in electrochemical energy storage devices is based on application of the relationship i = k₁v + k₂v^1/2 (where k₁ and k₂ are two constants independent of the scan rate v) to the variation of the cyclic voltammetric responses with v. Several examples show that application of this scan rate diagnosis procedure leads to absurd results because the procedure is inappropriate under these conditions. It follows that the best approach is to simply forget about this v/v^1/2 scan rate diagnosis, concentrate on the maximum number of experimental observations of the scan rate dependency, and build models able to reproduce these data in each case.

Selective Ammonium Removal from Synthetic Wastewater by Flow-Electrode Capacitive Deionization Using a Novel K2Ti2O5-Activated Carbon Mixture Electrode

Ammonium (NH₄⁺) in wastewater is both a major pollutant and a valuable resource. Flow-electrode capacitive deionization (FCDI) is a promising technology for chemical-free and environmentally friendly NH₄⁺ removal and recovery from wastewater. However, the coexisting sodium (Na⁺) in wastewater, with a similar hydrated radius to NH₄⁺, competes for the adsorption sites, resulting in low NH₄⁺ removal efficiency. Here, potassium dititanate (K₂Ti₂O₅ or KTO) particles prepared by the electrospray method followed by calcination were mixed with activated carbon (AC) powder to form a novel KTO-AC flow-electrode for selective NH₄⁺ removal over Na⁺. The mixed KTO-AC electrode exhibits a much higher specific gravimetric capacitance in NH₄Cl solution than in NaCl solution. Compared with the pure AC electrode in the FCDI tests on NH₄⁺ removal from synthetic wastewater, 25 wt % KTO addition in the electrode mixture increases the adsorption selectivity from 2.3 to 31 toward NH₄⁺ over Na⁺, improves the NH₄⁺ removal from 28.5% to 64.8% and increases the NH₄⁺ desorption efficiency from 35.6% to over 80%, achieving selective NH₄⁺ recovery and effective electrode regeneration. Based on DFT calculations, NH₄⁺ adsorption on the K₂Ti₂O₅ (0 0 1) surface is more thermodynamically favorable than that of Na⁺, which contributes to the high NH₄⁺ adsorption selectivity observed.

Fe2O3 Nanoparticles Anchored on the Ti3C2Tx MXene Paper for Flexible Supercapacitors with Ultrahigh Volumetric Capacitance

Ti₃C₂T_x MXene, with high conductivity and flexibility, has drawn great attention in the wearable energy storage devices. However, the easy nanoflake-restacking phenomenon greatly restricts the achievable electrochemical performance of Ti₃C₂T_x-based supercapacitors, in particular volumetric capacitance. Herein, we report a flexible hybrid paper consisting of Fe₂O₃ nanoparticles (NPs) anchored on Ti₃C₂T_x (Fe₂O₃ NPs@MX) via electrostatic self-assembly and annealing treatments. The interlayer spacing of Ti₃C₂T_x nanoflakes is effectively enlarged through the incorporation of Fe₂O₃ NPs, allowing more electrochemical active sites to store charge. Meanwhile, Ti₃C₂T_x nanoflakes form a continuous metallic skeleton and inhibit the volume expansion of Fe₂O₃ NPs during the charging/discharging process, enhancing the cycling stability. The flexible, ultrathin (4.1 μm) Fe₂O₃ NPs@MX hybrid paper shows considerably improved electrochemical performances compared to those of pure Ti₃C₂T_x and Fe₂O₃, including a wide potential window of 1 V, an ultrahigh volumetric capacitance of ∼2607 F cm^-3 (584 F g^-1), and excellent capacitance retention after 13,000 cycles. Besides, the as-assembled symmetric solid-state supercapacitor exhibits an energy density of 29.7 Wh L^-1 and excellent mechanical flexibility. We believe that the present nanostructure design, decorating NPs within a two-dimensional metallic network, has general applicability and could be used to fabricate highly efficient composites for advanced energy storage devices.

The role of carbon nanotubes in enhanced charge storage performance of VSe2: experimental and theoretical insight from DFT simulations

Herein, we report the hybrid structure of metallic VSe₂ and multi-walled carbon nanotube (MWCNT) based hybrid materials for high performance energy storage and high power operation applications. The dominance of capacitive energy storage performance behaviour of VSe₂/MWCNT hybrids is observed. A symmetric supercapacitor cell device fabricated using VSe₂/80 mg MWCNT delivered a high energy density of 46.66 W h kg^-1 and a maximum power density of 14.4 kW kg^-1 with a stable cyclic operation of 87% after 5000 cycles in an aqueous electrolyte. Using density functional theory calculations we have presented structural and electronic properties of the hybrid VSe₂/MWCNT structure. Enhanced states near the Fermi level and higher quantum capacitance for the hybrid structure contribute towards higher energy and power density for the nanotube/VSe₂.

Synthesis and Dual-Mode Electrochromism of Anisotropic Monoclinic Nb12O29 Colloidal Nanoplatelets

Transition metal oxide nanocrystals with dual-mode electrochromism hold promise for smart windows enabling spectrally selective solar modulation. We have developed the colloidal synthesis of anisotropic monoclinic Nb₁₂O₂₉ nanoplatelets (NPLs) to investigate the dual-mode electrochromism of niobium oxide nanocrystals. The precursor for synthesizing NPLs was prepared by mixing NbCl₅ and oleic acid to form a complex that was subsequently heated to form an oxide-like structure capped by oleic acid, denoted as niobium oxo cluster. By initiating the synthesis using niobium oxo clusters, preferred growth of NPLs over other polymorphs was observed. The structure of the synthesized NPLs was examined by X-ray diffraction in conjunction with simulations, revealing that the NPLs are monolayer monoclinic Nb₁₂O₂₉, thin in the [100] direction and extended along the b and c directions. Besides having monolayer thickness, NPLs show decreased intensity of Raman signal from Nb-O bonds with higher bond order when compared to bulk monoclinic Nb₁₂O₂₉, as interpreted by calculations. Progressive electrochemical reduction of NPL films led to absorbance in the near-infrared region (stage 1) followed by absorbance in both the visible and near-infrared regions (stage 2), thus exhibiting dual-mode electrochromism. The mechanisms underlying these two processes were distinguished electrochemically by cyclic voltammetry to determine the extent to which ion intercalation limits the kinetics, and by verifying the presence of localized electrons following ion intercalation using X-ray photoelectron spectroscopy. Both results support that the near-infrared absorption results from capacitive charging, and the onset of visible absorption in the second stage is caused by ion intercalation.

High Power Energy Storage via Electrochemically Expanded and Hydrated Manganese-Rich Oxides

Understanding the materials design features that lead to high power electrochemical energy storage is important for applications from electric vehicles to smart grids. Electrochemical capacitors offer a highly attractive solution for these applications, with energy and power densities between those of batteries and dielectric capacitors. To date, the most common approach to increase the capacitance of electrochemical capacitor materials is to increase their surface area by nanostructuring. However, nanostructured materials have several drawbacks including lower volumetric capacitance. In this work, we present a scalable “top-down” strategy for the synthesis of EC electrode materials by electrochemically expanding micron-scale high temperature-derived layered sodium manganese-rich oxides. We hypothesize that the electrochemical expansion induces two changes to the oxide that result in a promising electrochemical capacitor material: (1) interlayer hydration, which improves the interlayer diffusion kinetics and buffers intercalation-induced structural changes, and (2) particle expansion, which significantly improves electrode integrity and volumetric capacitance. When compared with a commercially available activated carbon for electrochemical capacitors, the expanded materials have higher volumetric capacitance at charge/discharge timescales of up to 40 s. This shows that expanded and hydrated manganese-rich oxide powders are viable candidates for electrochemical capacitor electrodes.

Unveiling Pseudocapacitive Charge Storage Behavior in FeWO4 Electrode Material by Operando X-Ray Absorption Spectroscopy

In nanosized FeWO₄ electrode material, both Fe and W metal cations are suspected to be involved in the fast and reversible Faradaic surface reactions giving rise to its pseudocapacitive signature. In order to fully understand the charge storage mechanism, a deeper insight into the involvement of the electroactive cations still has to be provided. The present paper illustrates how operando X-ray absorption spectroscopy is successfully used to collect data of unprecedented quality allowing to elucidate the complex electrochemical behavior of this multicationic pseudocapacitive material. Moreover, these in-depth experiments are obtained in real time upon cycling the electrode, which allows investigating the reactions occurring in the material within a realistic timescale, which is compatible with electrochemical capacitors practical operation. Both Fe K-edge and W L₃ -edge measurements point out the involvement of the Fe³⁺ /Fe²⁺ redox couple in the charge storage while W⁶⁺ acts as a spectator cation. The result of this study enables to unambiguously discriminate between the Faradaic and capacitive behavior of FeWO₄ . Beside these valuable insights toward the full description of the charge storage mechanism in FeWO₄ , this paper demonstrates the potential of operando X-ray absorption spectroscopy to enable a better material engineering for new high capacitance pseudocapacitive materials.

Transport Phenomena: Challenges and Opportunities for Molecular Catalysis in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are appealing heterogeneous support matrices that can stabilize molecular catalysts for the electrochemical conversion of small molecules. However, moving from a homogeneous environment to a porous film necessitates the transport of both charge and substrate to the catalytic sites in an efficient manner. This presents a significant challenge in the application of such materials at scale, since these two transport phenomena (charge and mass transport) would need to operate faster than the intrinsic catalytic rate in order for the system to function efficiently. Thus, understanding the fundamental kinetics of MOF-based molecular catalysis of electrochemical reactions is of crucial importance. In this Perspective, we quantitatively dissect the interplay between the two transport phenomena and the catalytic reaction rate by applying models from closely related fields to MOF-based catalysis. The identification of the limiting process provides opportunities for optimization that are uniquely suited to MOFs due to their tunable molecular structure. This will help guide the rational design of efficient and high-performing catalytic MOF films with incorporated molecular catalyst for electrochemical energy conversion.

Constructing an Adaptive Heterojunction as a Highly Active Catalyst for the Oxygen Evolution Reaction

Electrochemical water splitting is of prime importance to green energy technology. Particularly, the reaction at the anode side, namely the oxygen evolution reaction (OER), requires a high overpotential associated with OO bond formation, which dominates the energy-efficiency of the whole process. Activating the anionic redox chemistry of oxygen in metal oxides, which involves the formation of superoxo/peroxo-like (O₂ )^{n -} , commonly occurs in most highly active catalysts during the OER process. In this study, a highly active catalyst is designed: electrochemically delithiated LiNiO₂ , which facilitates the formation of superoxo/peroxo-like (O₂ )^{n -} species, i.e., NiOO*, for enhancing OER activity. The OER-induced surface reconstruction builds an adaptive heterojunction, where NiOOH grows on delithiated LiNiO₂ (delithiated-LiNiO₂ /NiOOH). At this junction, the lithium vacancies within the delithiated LiNiO₂ optimize the electronic structure of the surface NiOOH to form stable NiOO* species, which enables better OER activity. This finding provides new insight for designing highly active catalysts with stable superoxo-like/peroxo-like (O₂ )^{n -} for water oxidation.

Role of Nitrogen and Oxygen in Capacitance Formation of Carbon Nanowalls

Supercapacitors based on carbon nanomaterials are attracting much attention because of their high capacitance enabled by large specific surface area. The introduction of heteroatoms such as N or O enhances the specific capacitance of these materials. However, the mechanisms that lead to the increase in the specific capacitance are not yet well-studied. In this Letter, we demonstrate an effective method for modification of the surface of carbon nanowalls (CNWs) using DC plasma in atmospheres of O₂, N₂, and their mixture. Processing in the plasma leads to the incorporation of ∼4 atom % nitrogen and ∼10 atom % oxygen atoms. Electrochemical measurements reveal that CNWs functionalized with oxygen groups are characterized by higher capacitance. The specific capacitance for samples with oxygen reaches 8.9 F cm^-3 at a scan rate of 20 mV s^-1. In contrast, the nitrogen-doped samples demonstrate a specific capacitance of 4.4 F cm^-3 at the same scan rate. The mechanism of heteroatom incorporation into the carbon lattice is explained using density functional theory calculations.

Surveying Manganese Oxides as Electrode Materials for Harnessing Salinity Gradient Energy

The potential energy contained in the controlled mixing of waters with different salt concentrations (i.e., salinity gradient energy) can theoretically provide a substantial fraction of the global electrical demand. One method for generating electricity from salinity gradients is to use electrode-based reactions in electrochemical cells. Here, we examined the relationship between the electrical power densities generated from synthetic NaCl solutions and the crystal structures and morphologies of manganese oxides, which undergo redox reactions coupled to sodium ion uptake and release. Our aim was to make progress toward developing rational frameworks for selecting electrode materials used to harvest salinity gradient energy. We synthesized 12 manganese oxides having different crystal structures and particle sizes and measured the power densities they produced in a concentration flow cell fed with 0.02 and 0.5 M NaCl solutions. Power production varied considerably among the oxides, ranging from no power produced (β-MnO₂) to 1.18 ± 0.01 W/m² (sodium manganese oxide). Power production correlated with the materials' specific capacities, suggesting that cyclic voltammetry may be a simple method to screen possible materials. The highest power densities were achieved with manganese oxides capable of intercalating sodium ions when their potentials were prepoised prior to power production.

Water-Transferred, Inkjet-Printed Supercapacitors toward Conformal and Epidermal Energy Storage

Rapid growth of the internet of things and health monitoring systems have stimulated the development of flexible, wearable, and conformal embedded electronics with the unprecedented need for energy storage systems fully adaptable to diverse form factors. Conventional fabrication methods, such as photolithography for electronics and electrode winding/stacking for energy storage systems, struggle as fabrication strategies to produce devices with three-dimensional, stretchable, and conformal form factors. In this study, we demonstrate the fabrication of supercapacitors on 3D objects through inkjet and water-transfer printing. The devices are initially printed on a water-soluble substrate, which is then placed on the surface of water. Once the substrate is dissolved, the level of water is lowered until the devices are transferred on to the submerged 3D object. As a proof of concept, planar supercapacitors constituted of a silver nanoparticle-based current collector, nickel(II) oxide (NiO) nanoparticle-based active electrodes, and ultraviolet-cured triacrylate polymer-based solid-state electrolyte were used as model materials. The conformal supercapacitors showed a maximum areal capacitance of 87.2 mF·cm^-2 at a voltage window of 0-1.5 V. Moreover, the concept of water transfer was further explored with a particular focus on wearable applications by transferring the supercapacitors onto the skin of a human subject to realize epidermal energy storage. This new class of conformal electrochemical energy storage offers a new alternative approach toward monolithically integrated/object-tailored energy storage systems that are essential for complex-shaped devices for internet of things and flexible/on-skin electronic applications.

Hierarchical NiMoO4@Co3V2O8 hybrid nanorod/nanosphere clusters as advanced electrodes for high-performance electrochemical energy storage

Herein, a synergistic strategy to construct hierarchical NiMoO₄@Co₃V₂O₈ (denoted as NMO@CVO) hybrid nanorod/nanosphere clusters is proposed for the first time, where Co₃V₂O₈ nanospheres (denoted as CVO) have been uniformly immobilized on the surface of the NiMoO₄ nanorods (denoted as NMO) via a facile two-step hydrothermal method. Due to the surface recombination effect between NMO and CVO, the as-prepared NMO@CVO effectively avoids the aggregation of CVO nanosphere clusters. The unique hybrid architecture can make the most of the large interfacial area and abundant active sites for storing charge, which is greatly beneficial for the rapid diffusion of electrolyte ions and fast electron transport. The optimized NMO@CVO-8 composite nanostructure displays battery-like behavior with a maximum specific capacity of 357 C g^-1, excellent rate capability (77.8% retention with the current density increasing by 10 times) and remarkable cycling stability. In addition, an aqueous asymmetric energy storage device is assembled based on the NMO@CVO-8 hybrid nanorod/nanosphere clusters and activated carbon. The device shows an ultrahigh energy density of 48.5 W h kg^-1 at a power density of 839.1 W kg^-1, good rate capability (20.9 W h kg^-1 even at 7833.7 W kg^-1) and excellent cycling stability (83.5% capacitance retention after 5000 cycles). More notably, two charged devices in series can light up a red light-emitting diode (LED) for 20 min, demonstrating its potential in future energy storage applications. This work indicates that the hierarchical NiMoO₄@Co₃V₂O₈-8 hybrid nanorod/nanosphere clusters are promising energy storage materials for future practical applications and also provides a rational strategy for fabricating novel nanostructured materials for high-performance energy storage.

Unravelling the role of temperature in a redox supercapacitor composed of multifarious nanoporous carbon@hydroquinone

Role of temperature has been investigated for multifarious nanoporous carbon having physisorbed hydroquinone.

Towards fast-charging technologies in Li+/Na+ storage: from the perspectives of pseudocapacitive materials and non-aqueous hybrid capacitors

Since the discovery of the pseudocapacitive behavior in RuO₂ by Sergio Trasatti and Giovanni Buzzanca in 1971, materials with pseudocapacitance have been regarded as promising candidates for high-power energy storage. Pseudocapacitance-involving energy storage is predominantly based on faradaic redox reactions, but at the same time the charge storage is not limited by solid-state ion diffusion. Besides the search for pseudocapacitive materials, their implementation into non-aqueous hybrid capacitors stands for the strategy to increase power density by a rational design of the battery structure. Composed of a battery-type anode and a capacitor-type cathode, such devices show great promise to integrate the merits of both batteries and capacitors. Today, the availability of fast-charging technologies is of fundamental importance for establishing electric vehicles on a mass scale. Therefore, from the perspective of materials and battery design, understanding the basics and the recent developments of pseudocapacitive materials and non-aqueous hybrid capacitors is of great importance. With this goal in mind, we introduce here the fundamentals of pseudocapacitance and non-aqueous hybrid capacitors. In addition, we provide an overview of the latest developments in this fast growing research field.

High-performance supercabatteries using graphite@diamond nano-needle capacitor electrodes and redox electrolytes

Supercabatteries have the characteristics of supercapacitors and batteries, namely high power and energy densities as well as long cycle life. To construct them, capacitor electrodes with wide potential windows and/or redox electrolytes are required. Herein, graphite@diamond nano-needles and an aqueous solution of Fe(CN)63-/4- are utilized as the capacitor electrode and the electrolyte, respectively. This diamond capacitor electrode has a nitrogen-doped diamond core and a nano-graphitic shell. In 0.05 M Fe(CN)63-/4- + 1.0 M Na2SO4 aqueous solution, the fabricated supercabattery has a capacitance of 66.65 mF cm-2 at a scan rate of 10 mV s-1. It is stable over 10 000 charge/discharge cycles. The symmetric supercabattery device assembled using a two-electrode system possesses energy and power densities of 10.40 W h kg-1 and 6.96 kW kg-1, respectively. These values are comparable to those of other energy storage devices. Therefore, diamond supercabatteries are promising for many industrial applications.

Structural, Impedance, and EDLC Characteristics of Proton Conducting Chitosan-Based Polymer Blend Electrolytes with High Electrochemical Stability

In this report, a facile solution casting technique was used to fabricate polymer blend electrolytes of chitosan (CS):poly (ethylene oxide) (PEO):NH4SCN with high electrochemical stability (2.43V). Fourier transform infrared (FTIR) spectroscopy was used to investigate the polymer electrolyte formation. For the electrochemical property analysis, cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) techniques were carried out. Referring to the FTIR spectra, a complex formation between the added salt and CS:PEO was deduced by considering the decreasing and shifting of FTIR bands intensity in terms of functional groups. The CS:PEO:NH₄SCN electrolyte was found to be electrochemically stable as the applied voltage linearly swept up to 2.43V. The cyclic voltammogram has presented a wide potential window without showing any sign of redox peaks on the electrode surface. The proved mechanisms of charge storage in these fabricated systems were found to be double layer charging. The EIS analysis showed the existence of bulk resistance, wherein the semicircle diameter decreased with increasing salt concentration. The calculated maximum DC conductivity value was observed to be 2.11 × 10⁻⁴ S/cm for CS:PEO incorporated with 40 wt% of NH₄SCN salt. The charged species in CS:PEO:NH₄SCN electrolytes were considered to be predominantly ionic in nature. This was verified from transference number analysis (TNM), in which ion and electron transference numbers were found to be t_ion = 0.954 and t_el = 0.045, respectively. The results obtained for both ion transference number and DC conductivity implied the possibility of fabricating electrolytes for electrochemical double layer capacitor (EDLC) device application. The specific capacitance of the fabricated EDLC was obtained from the area under the curve of the CV plot.

Untapped Potential of Polymorph MoS2: Tuned Cationic Intercalation for High-Performance Symmetric Supercapacitors

Supercapacitors have been the key target as energy storage devices for modern technology that need fast charging. Although supercapacitors have large power density, modifications should be done to manufacture electrodes with high energy density, longer stability, and simple device structure. The polymorph MoS₂ has been one of the targeted materials for supercapacitor electrodes. However, it was hard to tune its phase and stability to achieve the maximum possible efficiency. Herein, we demonstrate the effect of the three main phases of MoS₂ (the stable semiconductor 2H, the metastable semiconductor 3R, and the metastable metallic 1T) on the capacitance performance. The effect of the cation intercalation on the capacitance performance was also studied in Li₂SO₄, Na₂SO₄, and K₂SO₄ electrolytes. The performance of the electrode containing the metallic 1T outperforms those of the 2H and 3R phases in all electrolytes, with the order 1T > 3R > 2H. The 1T/2H phase showed a maximum performance in the K₂SO₄ electrolyte with a specific capacitance of 590 F g^-1 at a scan rate of 5 mV s^-1. MoS₂ showed a good performance in both positive and negative potential windows allowing the fabrication of symmetric supercapacitor devices. The 1T MoS₂ symmetric device showed a power density of 225 W/kg with an energy density of 4.19 Wh/kg. The capacitance retention was 82% after 1000 cycles, which is an outstanding performance for the metastable 1T-containing electrode.

Sb2S3 Nanoparticles Anchored or Encapsulated by the Sulfur-Doped Carbon Sheet for High-Performance Supercapacitors

The specific capacitance and energy density of antimony trisulfide (Sb₂S₃)@carbon supercapacitors (SCs) have been limited and are in need of significant improvement. In this work, Sb₂S₃ nanoparticles were selectively encapsulated or anchored in a sulfur-doped carbon (S-carbon) sheet depending on the use of microwave-assisted synthesis. The microwave-triggered Sb₂S₃ nanoparticle growth resulted in core-shell hierarchical spherical particles of uniform diameter assembled with Sb₂S₃ as the core and an encapsulated S-carbon layer as the shell (Sb₂S₃-M@S-C). Without the microwave mediation, the other nanostructure was found to comprise fine Sb₂S₃ nanoparticles widely anchored in the S-carbon sheet (Sb₂S₃-P@S-C). Structural and morphological analyses confirmed the presence of encapsulated and anchored Sb₂S₃ nanoparticles in the carbon. These two materials exhibited higher specific capacitance values of 1179 (0 to +1.0 V) and 1380 F·g^-1 (-0.8 to 0 V) at a current density of 1 A·g^-1, respectively, than those previously reported for Sb₂S₃ nanomaterials in considerable SCs. Furthermore, both materials exhibited outstanding reversible capacitance and cycle stability when used as SC electrodes while retaining over 98% of the capacitance after 10 000 cycles, which indicates their long-term stability. Furthermore, a hybrid Sb₂S₃-M@S-C/Sb₂S₃-P@S-C device was designed, which delivers a remarkable energy density of 49 W·h·kg^-1 at a power density of 2.5 kW·kg^-1 with long-term cycle stability (94% over 10 000 cycles) and is comparable to SCs in the recent literature. Finally, a light-emitting diode (LED) panel comprising 32 LEDs was powered using three pencil-type hybrid SCs in series.

Nature of Electronic Conduction in "Pseudocapacitive" Films: Transition from the Insulator State to Band-Conduction

The transition between the insulator state and the band-conducting state is investigated by means of cyclic voltammetry in cobalt oxide porous film electrodes in phosphate-buffered solutions. It is shown that a proton-coupled faradaic oxidative process starting in the insulator region eventually builds an ohmic conduction mode upon anodic polarization. This model allows one to understand the origin of the authentic capacitive behavior of conductive metal oxide films rather than the so-called "pseudocapacitive" behavior. The particular example of cobalt oxide serves to illustrate the way in which, more generally, the behavior of "pseudocapacitors", long ascribed to the superposition of faradaic reactions, is in fact that of true capacitors, once band-conduction has been established upon oxidation of the material.

Energy storage: pseudocapacitance in prospect

This question and its implications are discussed in detail.

The two main types of charge storage devices – batteries and double layer charging capacitors – can be unambiguously distinguished from one another by the shape and scan rate dependence of their cyclic voltammetric current–potential (CV) responses. This is not the case with “pseudocapacitors” and with the notion of “pseudocapacitance”, as originally put forward by Conway et al. After insisting on the necessity of precisely defining “pseudocapacitance” as involving faradaic processes and having, at the same time, a capacitive signature, we discuss the modelling of “pseudocapacitive” responses, revisiting Conway's derivations and analysing critically the other contributions to the subject, leading unmistakably to the conclusion that “pseudocapacitors” are actually true capacitors and that “pseudocapacitance” is a basically incorrect notion. Taking cobalt oxide films as a tutorial example, we describe the way in which a (true) electrical double layer is built upon oxidation of the film in its insulating state up to an ohmic conducting state. The lessons drawn at this occasion are used to re-examine the classical oxides, RuO₂, MnO₂, TiO₂, Nb₂O₅ and other examples of putative “pseudocapacitive” materials. Addressing the dynamics of charge storage—a key issue in the practice of power of the energy storage device—it is shown that ohmic potential drop in the pores is the governing factor rather than counter-ion diffusion as often asserted, based on incorrect diagnosis by means of scan rate variations in CV studies.

Graphene-Graphite Polyurethane Composite Based High-Energy Density Flexible Supercapacitors

Energy autonomy is critical for wearable and portable systems and to this end storage devices with high-energy density are needed. This work presents high-energy density flexible supercapacitors (SCs), showing three times the energy density than similar type of SCs reported in the literature. The graphene-graphite polyurethane (GPU) composite based SCs have maximum energy and power densities of 10.22 µWh cm-2 and 11.15 mW cm-2, respectively, at a current density of 10 mA cm-2 and operating voltage of 2.25 V (considering the IR drop). The significant gain in the performance of SCs is due to excellent electroactive surface per unit area (surface roughness 97.6 nm) of GPU composite and high electrical conductivity (0.318 S cm-1). The fabricated SCs show stable response for more than 15 000 charging/discharging cycles at current densities of 10 mA cm-2 and operating voltage of 2.5 V (without considering the IR drop). The developed SCs are tested as energy storage devices for wide applications, namely: a) solar-powered energy-packs to operate 84 light-emitting diodes (LEDs) for more than a minute and to drive the actuators of a prosthetic limb; b) powering high-torque motors; and c) wristband for wearable sensors.

Toward Low-Cost and Sustainable Supercapacitor Electrode Processing: Simultaneous Carbon Grafting and Coating of Mixed-Valence Metal Oxides by Fast Annealing

There is a rapid market growth for supercapacitors and batteries based on new materials and production strategies that minimize their cost, end-of-life environmental impact, and waste management. Herein, mixed-valence iron oxide (FeO_x) and manganese oxide (Mn₃O₄) and FeO_x-carbon black (FeO_x-CB) electrodes with excellent pseudocapacitive behavior in 1 M Na₂SO₄ are produced by a one-step thermal annealing. Due to the in situ grafted carbon black, the FeO_x-CB shows a high pseudocapacitance of 408 mF cm⁻² (or 128 F g⁻¹), and Mn₃O₄ after activation shows high pseudocapacitance of 480 mF cm⁻² (192 F g⁻¹). The asymmetric supercapacitor based on FeO_x-CB and activated-Mn₃O₄ shows a capacitance of 260 mF cm⁻² at 100 mHz and a cycling stability of 97.4% over 800 cycles. Furthermore, due to its facile redox reactions, the supercapacitor can be voltammetrically cycled up to a high rate of 2,000 mV s⁻¹ without a significant distortion of the voltammograms. Overall, our data indicate the feasibility of developing high-performance supercapacitors based on mixed-valence iron and manganese oxide electrodes in a single step.

Core-Shell Structured Cobalt Sulfide/Cobalt Aluminum Hydroxide Nanosheet Arrays for Pseudocapacitor Application

The direct synthesis of nanostructured electrode materials on three-dimensional substrates is important for their practical application in electrochemical cells without requiring the use of organic additives or binders. In this study, we present a simple two-step process to synthesize a stable core-shell structured cobalt sulfide/cobalt aluminum hydroxide nanosheet (LDH-S) for pseudocapacitor electrode application. The cobalt aluminum layered double hydroxide (CoAl-LDH) nanoplates were synthesized in basic aqueous solution with a kinetically-controlled thickness. Owing to the facile diffusion of electrolytes through the nanoplates, thin CoAl-LDH nanoplates have higher specific capacitance values than thick nanoplates. The as-grown CoAl-LDH nanoplates were transformed into core-shell structured LDH-S nanosheets by a surface modification process in Na2 S aqueous solution. The chemically robust cobalt sulfide (CoS) shell increased the electrochemical stability compared to the sulfide-free CoAl-LDH electrodes. The LDH-S electrodes exhibited high electrochemical performance in terms of specific capacitance and rate capability with a galvanostatic discharge of 1503 F g-1 at a current density of 2 A g-1 and a specific capacitance of 91 % at 50 A g-1 .