Pseudocapacitive Electrode Materials Research Articles

Flexible Supercapacitors: A Materials Perspective

Flexible supercapacitors are highly attractive for the large number of emerging portable lightweight consumer devices. The novelty of a flexible supercapacitor is the incorporation of flexible electrode or substrate material to combine structural flexibility with the inherently high power density of supercapacitors. Flexible supercapacitors can use non-Faradaic energy storage process as seen in the electric double layer capacitor type or a Faradaic mechanism as seen in the pseudocapacitors. In this review, we account the current progress in pseudocapacitive electrode materials, fabrication techniques and new materials for electric double layer capacitor, and different flexible substrates. Future directions in developing new materials towards improved energy density and cost-effectiveness of the flexible supercapacitors and their usage in combination with lithium-ion batteries are highlighted.

Selenium vacancies enriched the performance of supercapacitors with excellent cycling stability via a simple chemical bath deposition method.

Herein, we report a simple and cost-effective route for the fabrication of NiWO4, NiWO4P, and NiWO4Se nanostructures using the chemical bath deposition method. As a binder-free electrode material for pseudo capacitors (PCs), the fabricated nickel foam-supported NiWO4Se exhibited an excellent cycling life of 3000 cycles with a high specific capacitance of 1115.05 F g-1 at 20 mA cm-2. The desirable electrochemical performance of NiWO4Se was mainly attributed to the large specific area, short ion diffusion distance, and the introduction of selenium, which provide transport pathways for electrons and provide more active sites for electrochemical energy storage. The present simple and scalable synthesis shows great potential for the future development of transition metal selenides for different energy storage and conversion applications.

Phase and morphology evolution of CoAl LDH nanosheets towards advanced supercapacitor applications

Al-Based LDH materials have been considered as promising active electrode materials for pseudocapacitors due to their structural tunability.

Facile synthesis of crystalline RuSe2 nanoparticles as a novel pseudocapacitive electrode material for supercapacitors.

Crystalline RuSe2 nanoparticles were prepared by a facile hydrothermal approach followed by thermal treatment, and utilized as a pseudocapacitive electrode material for supercapacitors for the first time, which exhibited a specific capacitance of 100.8 F g-1 at 0.2 A g-1 with good rate performance and superior cycle stability.

Enhancing Capacitance of Nickel Cobalt Chalcogenide via Interface Structural Design.

Spinel NiCo2X4 (X = O or S), comprising two geometrical cobalt ions, Co2+ in the tetrahedral site (Co2+Td) and Co3+ in the octahedral site (Co3+Oh), has been widely evaluated as a promising pseudocapacitor electrode material. Previous literature mainly demonstrated that much higher specific capacitance of NiCo2S4 than that of NiCo2O4 was ascribed to the higher electronic conductivity. However, we argue that only a small amount of capacitance can be induced by the electronic conductivity, while the significance of electrochemical active species in these system has long been ignored. Here, we propose that geometrical-site-dependent pseudocapacitive activity will generate enhanced specific capacitance through the interface structural design. It reveals that specific capacitance of NiCo2S4 (1862 F g-1 at 4 A g-1) is 50% higher than that of NiCo2O4 (1230 F g-1 at 4 A g-1), which is derived from the designed increase of Co2+Td ions (cobalt ions in the tetrahedral site) in NiCo2S4. These results have significant implications for the design and optimization of the electrochemical properties of transition-metal-based pseudocapacitors.

Facile, cost-effective and eco-friendly green synthesis method of MnO2 as storage electrode materials for supercapacitors

Abstract Orange by-products e.g. orange peel and the extract of orange juice are used as sources for biological antioxidants such as ascorbic acid, flavonoids, phenolic compounds and pectins. In this study these antioxidants were successfully used to prepare nanosized materials of α-MnO2 by cost effective and eco-friendly green chemistry method. The prepared oxides of MnO2 which have unique properties as a storage cathode material were tested as a pseudocapacitor electrode materials in this study. X-ray powder diffraction (XRD) confirmed the structure of α-MnO2 for the prepared samples. Thermal behavior of prepared oxides was tested using thermogravimetric analysis (TGA). Transmission electron microscopy (TEM) showed the nanosized nature of the prepared oxides. N2- adsorption–desorption isotherms and pore-size distributions of prepared oxides showed that the surface areas are 5.63 and 8.40 m2. g−1 for sample prepared from the extract of orange juice (OJ-MnO2) and that prepared from the extract of orange peel (OP-MnO2), respectively. Better electrochemical properties are obtained for OP-MnO2, the capacitance of OP-MnO2 (139 F/g) is more than two times and half that obtained for OJ-MnO2 (50 F/g) at the current density 0.5 A/g.

Advanced sodium-ion pseudocapacitor performance of oxygen-implanted hard carbon derived from carbon spheres

Sodium-ion capacitors (NICs) are rising as a promising candidate for next-generation energy storage systems owing to the abundance of sodium resources and the better performance than that of their lithium-ion capacitor counterparts. The design of the pseudocapacitive electrode materials is of great significance for an integrated device to have high energy and power densities. In this work, activated hard carbon (AHC) with a high surface area and abundant oxygen-containing functional groups is synthesized via the KOH activation of controllable carbon spheres that act as a special type of precursors. The AHC can be used as a supercapacitor-type material because it exhibits excellent properties as anode or cathode of sodium-ion batteries. The integrated symmetrical NIC with the AHC as both anode and cathode is superior to most previously reported all-carbon-based capacitors in terms of electrochemical performance. Benefiting from the oxygen-containing groups on the surface of the AHC, the pseudocapacitor shows an energy density of 43 Wh kg−1 at a power density of 3724 W kg−1 after 5000 cycles. This study not only provides a novel electrode material for NICs but also deepens the understanding of all-carbon-based capacitors.

Preparation of La0.7Sr0.3CoO3-δ (LSC)@MnO2 core/shell nanorods as high-performance electrode materials for supercapacitors

Perovskite oxides have attracted significant attention as capacitor electrode materials owing to their unique physical and electronic properties. In this paper, a novel La0.7Sr0.3CoO3-δ (LSC)@MnO2 core–shell nanorod was synthesized by controlled electrospinning combined with hydrothermal synthesis. The LSC, as a typical perovskite-type material, with excellent stability and ion–electron double conductivity, can perfectly serve as conductive backbone. Grid-like MnO2 nanosheets are grown on LSC to form a unique core/shell nanostructure, could effectively improve the electrochemical performance of MnO2. The grid-like MnO2 nanosheets shell significantly increase the effective area over which the reaction may take place and reduce the ion/electron transmission distance, which is beneficial in that it shifts in ions and electrons, enhancing the electrochemical reaction kinetics thereof. LSC@MnO2 core/shell nanorods demonstrated good electrochemical performance with high capacitance (570 F g−1 at 1 A g−1), and revealed excellent cycling stability (capacitance retention remains at 97.2% after 5000 cycles). The asymmetric supercapacitor device (LSC@MnO2//AC) displayed a desirable energy density of 37.6 W h kg−1 at 375 W kg−1, and still remains at 23.3 W h kg−1 at a high power density of 7489.3 W kg−1, indicating that the LSC@MnO2 nanorods are an outstanding pseudo-capacitive electrode material, with significant potential for application in high-performance supercapacitors.

Effective Utilization of Waste Red Mud for High Performance Supercapacitor Electrodes.

In recent years, metal oxide‐based, inexpensive, stable electrodes are being explored as a potent source of high performance, sustainable supercapacitors. Here, the employment of industrial waste red mud as a pseudocapacitive electrode material is reported. Mechanical milling is used to produce uniform red mud nanoparticles, which are rich in hematite (Fe2O3), and lower amounts of other metal oxides. A comprehensive supercapacitive study of the electrode is presented as a function of ball‐milling time up to 15 h. Ten‐hour ball‐milled samples exhibit the highest pseudocapacitive behavior with a specific capacitance value of ≈317 F g−1, at a scan rate of 10 mV s−1 in 6 m aqueous potassium hydroxide electrolyte solution. The modified electrode shows an extraordinary retention of ≈97% after 5000 cycles. A detailed quantitative electrochemical analysis is carried out to understand the charge storage mechanism at the electrode–electrolyte interface. The formation of uniform nanoparticles and increased electrode stability are correlated with the high performance. This work presents two significant benefits for the environment; in energy storage, it shows the production of a stable and efficient supercapacitor electrode, and in waste management with new applications for the treatment of red mud.

Template-Confined Growth of Poly(4-aminodiphenylamine) Nanosheets as Positive Electrode toward Superlong-Life Asymmetric Supercapacitor.

Two-dimensional conducting polymers nanosheet prepared by the soft-template-assisted technique in aqueous system still has rarely been reported, especially the formation of nanosheets structure by protonic acid assisted controlling the stability of the micelles method. Herein, a facile, one-step, bottom-up approach is developed to synthesize novel and uniform conductive poly(4-aminodiphenylamine) (P(4-ADPA)) nanosheets with controlled morphology via regulating the proton acid concentration during the self-assembly of surfactant lamellar micelles. The as-produced conducting P(4-ADPA) demonstrates a integral nanoframework structure with uniform and intertwined nanosheets, resulting in a satisfactory specific capacitance of 395 F g-1 at the current density of 0.5 A g-1. Furthermore, another pseudocapacitor electrode material, urchin-like W18O49, is selected as a negative electrode material to fabricate a novel P(4-ADPA)//W18O49 asymmetric supercapacitor, which extends the operating voltage window up to 1.5 V, provides an energy density of 24.4 W h kg-1 at a high power density of 1491 W kg-1, and possesses superlong-life of 92% capacitance retention after 10 000 cycles. Therefore, the present work will propose an affordable strategy to design novel conducting polymer nanosheets and open up the possibility of cheap conductive polymers to meet the demands of energy storage.

Recent advances in pseudocapacitor electrode materials: Transition metal oxides and nitrides

Abstract Faraday pseudocapacitors take both advantages of secondary battery with high energy density and supercapacitors with high power density, and electrode material is the key to determine the performance of Faraday pseudocapacitors. Transition metal oxides and nitrides, as the two main kinds of pseudocapacitor electrode materials, can enhance energy density while maintaining high power capability. Recent advances in designing nanostructured architectures and preparing composites with high specific surface areas based on transition metal oxides and nitrides, including ruthenium oxides, nickel oxides, manganese oxides, vanadium oxides, cobalt oxides, iridium oxides, titanium nitrides, vanadium nitrides, molybdenum nitrides and niobium nitrides, are addressed, which would provide important significances for deep researches on pseudocapacitor electrode materials.

Synthesis of NiGa2S4-rGO on nickel foam as advanced electrode for flexible solid-state supercapacitor with superior energy density

Pseudocapacitive electrode materials employed in supercapacitors may bring in high energy density (ED) and specific capacitance (Csc), which are critical for their practical applications. Accordingly, logical design of advanced electrode materials is highly demanded to progress high-performance supercapacitors. Here, for the first time, we suggest a straightforward route for the synthesis of NiGa2S4-rGO as an advanced cathode material supported on nickel foam (NF) for employed in flexible solid-state asymmetric supercapacitors (FSASCs). Due to an abundant ratio of active sites and large surface area of the NiGa2S4-rGO advanced material, the as-prepared NiGa2S4-rGO/NF electrode illustrates considerable electrochemical properties including remarkable specific capacitance (Csc) of 2124.34 F g−1 with excellent rate capability of 73%, and exceptional durability, which are better than NiGa2S4/NF and previously reported transition metal sulfides (TMSs). Furthermore, for the first time a pseudocapacitive advanced anode material of FeSe2-rGO have been successfully fabricated on a nickel foam (NF) substrate by a facile strategy. Element Selenium as the favorable element was offered into the Fe for enhancement and adjustment of the anode material electrochemical performance. The FeSe2-rGO/NF advanced anode electrode presents satisfactory electrochemical properties containing an exceptional specific capacitance (Csc) of 432.40 F g−1, significant rate performance of 57.84% and superior durability, which are better than FeSe2/NF electrode and previously studied Fe-based anode material. Considering the remarkable electrochemical performance of the as-prepared pseudocapacitive advanced electrode materials, a FSASC based on the NiGa2S4-rGO/NF as the cathode electrode and FeSe2-rGO/NF as the anode electrode was assembled. The FSASC device delivers superior Csc of 341.20 F g−1, outstanding energy density (ED) of 121.31 W h kg−1, remarkable cycle stability (only 7.30% damage after 5000 charge/discharge (CD) cycles).

Cr2O3 nanoparticles: a fascinating electrode material combining both surface-controlled and diffusion-limited redox reactions for aqueous supercapacitors

Chromium oxide has emerged as an attractive electrode material to construct high-performance supercapacitor. Despite some available reports on Cr2O3-based supercapacitors, the charge storage of Cr2O3 electrodes in alkaline electrolytes is far from being understood unlike those of other compounds of Ti, V, Mn, Fe, Co, Ni etc in periodic table of elements. Herein, the detail kinetics analyses of electrochemical behaviors of the electrode based on Cr2O3 nanoparticles in alkaline electrolyte were demonstrated in this work. We examine that there are both surface-controlled pseudocapacitance (with a contribution of 76%) and diffusion-limited redox reactions (with a contribution of 24%) for the charge storage in electrode based on Cr2O3 nanoparticles at a low sweep rate of 1 mV s−1. The identifying pseudocapacitive contribution dominates the charge storage in electrode based on Cr2O3 nanoparticles at high sweep rates, reaching up to 96% at 50 mV s−1. In addition, the assembled hybrid supercapacitor based on Cr2O3 nanoparticles as the positive electrode displays an operating voltage of 1.45 V, an energy density of 14.7 Wh kg−1, and high stability of 3000 cycles. This work will contribute to the development of pseudocapacitive electrode materials for supercapacitors.

A high-performance pseudocapacitive electrode material for supercapacitors based on the unique NiMoO4/NiO nanoflowers

Currently, although some progress has already been achieved, the complex and high-cost synthetic route, and low electrochemical properties of supercapacitor still restrict its practical application. Here, NiMoO4/NiO nanoflowers material with a high specific surface area grown on a nickel foam conductive substrate is successfully synthesized via a facile and efficient method, which exhibits an excellent capacitance of 10.3 F cm−2 (1982.3 F g−1), a good rate capability, and a remarkable cycling stability (capacity remain 98.6% after 3000 cycles). In addition, a novel asymmetric supercapacitor with excellent energy density is also fabricated via the NiMoO4/NiO nanoflowers as the positive electrode and a piece of active carbon as the negative electrode in 3 M KOH electrolyte. The optimized asymmetric device operating at 1.7 V also offers a high energy density and an outstanding long-term cycling stability. These impressive results suggest that such NiMoO4/NiO nanoflowers can pave the way for designing and developing next-generation high-performance storage systems.

Designing Three-Dimensional Architectures for High-Performance Electron Accepting Pseudocapacitors.

By storing energy from electrochemical processes at the electrode surface, pseudocapacitors bridge the performance gap between electrostatic double-layer capacitors and batteries. In this context, molecular design offers the exciting possibility to create tunable and inexpensive organic electroactive materials. Here we describe a porous structure composed of perylene diimide and triptycene subunits and demonstrate its remarkable performance as a pseudocapacitor electrode material. The material exhibits capacitance values as high as 350 F/g at 0.2 A/g as well as excellent stability over 10 000 cycles. Moreover, we can alter the performance of the material, from battery-like (storing more charge at low rates) to capacitor-like (faster charge cycling), by modifying the structure of the pores via flow photocyclization. Organic materials capable of stable electron accepting pseudocapacitor behavior are rare and the capacitance values presented here are among the highest reported. More broadly, this work establishes molecular design and synthesis as a powerful approach for creating tunable energy storage materials.

Large‐Scale Conductive Yarns Based on Twistable Korean Traditional Paper (Hanji) for Supercapacitor Applications: Toward High‐Performance Paper Supercapacitors

AbstractA simple and scalable method to fabricate a yarn‐type supercapacitor with a large specific capacitance without the aid of traditional pseudocapacitive electrode materials such as conducting polymers and metal oxides is reported. The yarn‐type supercapacitors are made from twisting reduced graphene oxide (rGO) or/and single‐walled carbon nanotubes (SWNTs)‐coated Korean traditional paper (KTP). The yarn‐type paper supercapacitor displays surprisingly enhanced electrochemical capacitance values, showing synergistic effect between rGO and SWNTs (500 times larger than performance of yarn‐type rGO‐coated paper supercapacitors). Coating rGO or/and SWNTs on KTP gives good morphology to the composite film, in which porosity increases and mean pore diameter decreases. The yarn‐type rGO/SWNT paper supercapacitor shows good mechanical strength, high flexibility, excellent electrochemical performance, and long‐life operation. The yarn‐type supercapacitor has an excellent electrochemical performance with a specific capacitance of 366 F g−1 at scan rate of 25 mV s−1 and high stability without any degradation in electrical performance up to 10 000 charge–discharge cycles. The average capacitance of rGO/SWNT@KTP yarn‐type supercapacitors is seven times higher than that of sheet‐type supercapacitors at scan rate of 500 mV s−1. The lighting of a red light‐emitting diode (LED) is demonstrated by the yarn‐type paper supercapacitor without connecting supercapacitors in series.

Boosting Electrochemistry of Manganese Oxide Nanosheets by Ostwald Ripening during Reduction for Fiber Electrochemical Energy Storage Device.

The poor electronic conductivity of MnO x severely limits the practical application as high-performance electrode materials for faradaic pseudocapacitors. Herein, a facile vapor reduction method is demonstrated for the treatment of MnO x with hydrazine hydrate (HH) to improve the electronic conductivity. The HH vapor treatment without annealing process not only introduces oxygen vacancies to form oxygen-deficient MnO x, but also leads to obvious structural transformation from highly aggregated and poorly crystallized MnO x nanorobs and nanoparticles into uniformly orientated and highly crystallized MnO x nanosheets via the Ostwald ripening process. Compared with pristine MnO x on carbon fiber (CF-MnO x), the reduced CF-MnO x exhibits a highly improved specific capacitance of 1130 mF cm-1 (434 F g-1) with excellent rate capability and cycling stability. Our results have shown that the moderate concentration of oxygen vacancies and highly uniform orientation of reduced MnO x endow the electrode with a fast electron and ion transport, respectively. Moreover, a flexible fiber asymmetric supercapacitor (ASC) device with high-energy and power density based on the as-prepared reduced CF-MnO x as a cathode and electrochemically activated graphene oxide on carbon fiber (CF-ArGO) as an anode is fabricated. The MnO x//ArGO ASC device delivers a high volumetric capacitance of 1.9 F cm-3, a maximum energy density of 1.06 mWh cm-3, and a volumetric power density of 371.3 mW cm-3. The present work opens a new way for oxygen vacancy introduction and structural modification of metal oxide as high-performance materials for energy storage applications.

Melamine-functionalized graphene oxide: Synthesis, characterization and considering as pseudocapacitor electrode material with intermixed POAP polymer

In this study, Melamine functionalized graphene oxide (FGO-Melamine) was synthesized by chemical route. The prepared functionalized graphene oxide was characterized by different analytical techniques such as FT-IR, Raman spectroscopy, thermogravimetry coupled with mass spectrometry (TG-MS), X-Ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS). To improve electrochemical properties of polyorthoaminophenol (POAP), the electropolymerized POAP/FGO-Melamine films, employed as an active electrode regarding electrochemical performance. In terms of electrochemical measurements, galvanostatic charge–discharge evaluation, EIS (electrochemical impedance spectroscopy) and CV (cyclic voltammetry) were employed for conducting an enquiry into supercapacitive performance of nanocomposite of POAP/FGO-Melamine. This graphene-based electrode showed a specific capacitance (SC) by 273 F g−1 and high energy of 37.91 W kg−1 at power density of 500 W kg−1. The novel materials synthesized in the current work show higher efficiency compared to the carbon-based ones concerning redox reactions of capacitors consisting of good stability in the existence of aqueous electrolyte, large active surface area and ease synthesis method.

Facile and scalable synthesis of nanostructured Fe2O3 using ionic liquid-assisted ball milling for high-performance pseudocapacitors

In this study, we report that ionic liquid (IL)-assisted ball milling of iron precursors in the absence of any reagent resulted in the scalable production of IL/Fe2O3 hybrids with a favourable morphology (high surface area of 202 m2/g and pore size of 40nm) for supercapacitor applications. During ball milling process, ILs are played role as structure guiding templates for construction of rod-like pore containing Fe2O3 and surface functionalities for IL/Fe2O3 hybrids, resulting in good wettability and enhanced ion transfer. These attractive features of IL/Fe2O3 hybrids make them excellent candidate pseudocapacitive electrode materials that exhibit a high specific capacitance of 230 F/g at a current density of 1 A/g, fast and reversible charge/discharge rates (∼71% retention at 15 A/g), and excellent long-term cycle stability (∼100% retention over 10000 cycles).

Study on Ba0.5Sr0.5CoxFe1-XO3-Δ As Pseudocapacitive Electrode Material to Improve the Volumetric Energy Density of Supercapacitors

The design and fabrication of high volumetric energy density supercapacitors lead to high density materials to be considered as electrode material [1]. So far, most of the studies have been carried out on mono-cationic oxides such as RuO2, MnO2 or Fe3O4, while polycationic oxides, for which more than a single redox center are potentially involved, have been mostly neglected [2]. As such, we have investigated Ba0.5Sr0.5CoxFe1-xO3-δ phases, with 0.75 ≤ x ≤ 0.90, so-called BSCFs, as pseudocapacitive electrode materials. Our motivations towards these perovskite phases are on their high density (close to 6 g.cm-3) and on both Co and Fe as potentially active redox centers. BSCFs phases have been prepared by a glycine-nitrate process at 850°C and resulting powders show the same structural arrangement and identical morphology characteristics in the whole series of compositions. Electrochemical performances have been evaluated by cyclic voltamperometry at room temperature, in various aqueous electrolytes: 5M and 1M LiNO3, 0.5M Li2SO4, Na2SO4 and K2SO4. Prepared BSCFs showed promising pseudocapacitive behavior as electrode material and performances obtained are dependent on the Co/Fe ratio but also on the electrolyte nature. A maximum capacitance of 87 F.g-1 at 5 mV.s-1 was obtained for the electrode based on the BSCF material with x = 0.80 in 5M LiNO3. This specific capacitance corresponds to a volumetric capacitance of about 500 F.cm-3 (as shown on the figure below) i.e. five times greater than that of activated carbon based electrode. This demonstrates the potential of high density oxides as electrode materials for improving the volumetric energy density of supercapacitors. Cycling ability of this electrode was also evaluated upon 2000 cycles as well as its self-discharge. Figure: Specific and volumetric capacitance of BSCFs based electrode vs. Co content in 5M LiNO3 at 5 mV.s-1. Charge storage mechanism was investigated by in situ X-Ray Absorption Spectroscopy, performed on the ROCK beamline at SOLEIL Synchrotron (France). Experiments were carried out at both Fe and Co K-edges in the aim to determine the Co and Fe valency at various states of charge. Data analysis demonstrates that both Co and Fe are electrochemically involved in the charge storage mechanism. Their respective contribution to the capacitance depends on BSCF composition and on the scan rate, in accordance with the redox kinetics at Fe vs. Co centers. Thanks to these in situ measurements, an advanced description of the involved charge storage mechanism is provided. [1] N. Goubard-Bretesché et al: Improving the volumetric energy density of supercapacitors. Electrochim Acta, 206, 458 (2016). [2] O. Crosnier et al: Polycationic oxides as potential electrode materials for aqueous-based electrochemical capacitors. Submitted to Current Opinion in Electrochemistry, February 2018. Figure 1