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Conducting polymers (CPs) have been widely studied for electrochemical energy storage. However, the dopants in CPs are often electrochemically inactive, introducing "dead‐weight" to the materials. Moreover, commercial‐level electrode materials with high mass loadings (e.g., >10 mg cm−2) often encounter the problems of inferior electrical and ionic conductivity. Here, a redox‐active poly‐counterion doping concept is proposed to improve the electrochemical performance of CPs with ultra‐high mass loadings. As a study prototype, heptamolybdate anion (Mo7O246−) doped polypyrrole (PPy) is synthesized by electro‐polymerization. A 2 mm thick PPy electrode with mass loading of ≈192 mg cm−2 reaches a record‐high areal capacitance of ≈47 F cm−2, competitive gravimetric capacitance of 235 F g−1, and volumetric capacitance of 235 F cm−3. With poly‐counterion doping, the dopants also undergo redox reactions during charge/discharge processes, providing additional capacitance to the electrode. The interaction between polymer chains and the poly‐counterions enhances the electrical conductivity of CPs. Besides, the poly‐counterions with large steric hindrance could act as structural pillars and endow CPs with open structures for facile ion transport. The concept proposed in this work enriches the electrochemistry of CPs and promotes their practical applications. [ABSTRACT FROM AUTHOR]

Various cathode materials have been proposed for high‐performance rechargeable batteries. Vanadyl phosphate is an important member of the polyanion cathode family. VOPO4 has seven known crystal polymorphs with tunneled or layered frameworks, which allow facile cation (de)intercalations. Two‐electron transfer per formula unit can be realized by using VV/VIV and VIV/VIII redox couples. The electrochemical performance is closely related to the structures of VOPO4 and the types of inserted cations. This Review outlines the crystal structures of VOPO4 polymorphs and their lithiated phases. The research progress of vanadyl phosphate cathode materials for different energy storage systems, including lithium‐ion batteries, sodium‐ion batteries, potassium‐ion batteries, multivalent batteries, and supercapacitors, as well as the related mechanism investigations are summarized. It is hoped that this Review will help with future directions of using vanadyl phosphate materials for energy storage. [ABSTRACT FROM AUTHOR]

A Na2SO4 + H2SO4 mixed electrolyte is demonstrated for a tungsten bronze pseudocapacitive electrode. The Na2SO4 supporting salt allows a large potential window while H+ effectively suppresses phase transformation. The electrode delivers a capacitance of 860 mF cm−2 with a −0.9 V–0 V window and 98% capacitance retention over 30 000 cycles. [ABSTRACT FROM AUTHOR]

Immobilization of phosphotungstate is achieved through doping in polypyrrole via the electrochemical deposition of polypyrrole (PPy) in the presence of the phosphotungstate anions (PW12) on carbon nanotube film. Electrochemical stability in neutral solution is realized for PW12 in the prepared ECNT-PPy/5PW12, owing to the proton sponge provided by PPy. ECNT-PPy/5PW12 shows a high areal capacitance of 1300.0 mF cm−2 in LiCl solution at 1 mA cm−2. ECNT-PPy/5PW12 also displays high rate capability and good cyclic stability due to the interaction of incorporated PW12 and the polymer matrix. ECNT-PPy/5PW12 can retain 78.6% and 92.0% of its capacitance with 40-fold current increase and after 10 000 galvanostatic charge/discharge cycles, respectively. An asymmetric supercapacitor with the ECNT-PPy/5PW12 anode and CNT-MnO2 cathode demonstrates high volumetric energy density (4.71 mW h cm−3 at 13.85 mW cm−3) and long cycle life (90.0% capacitance retention after 10 000 charge/discharge cycles). [ABSTRACT FROM AUTHOR]

Abstract: Design and synthesis of metal oxide‐based pseudocapacitive materials to simultaneously achieve high mass loading (e.g., up to 10 mg cm−2) and excellent rate capability for electrochemical capacitors is a long‐lasting challenge. These two characteristics are usually mutually exclusive due to the poor ion diffusion kinetics of most metal oxides. Here, a glucose‐assisted hydrothermal method to prepare thick hematite film (>1 µm) with engineerable mesopore size through controlled variation of glucose concentration is demonstrated. The capability of controlling the size of mesopores offers a unique opportunity to investigate for the first time the interplay between mesopore size and electrochemical performance of hematite films. The hematite film with an average mesopore size of 3 nm at an ultrahigh loading of 10 mg cm−2 exhibits an areal capacitance of 1502 mF cm−2 at 1 mA cm−2, and retains 871.2 mF cm−2 at 50 mA cm−2. Such performance, to the best of the authors' knowledge, is at the top of the reported hematite electrodes with comparable or even lower mass loadings. The strategy demonstrated herein may be extended to fabricate diverse types of mesoporous metal oxide architectures with improved ion diffusion kinetics, which is critical for a broad range of devices for energy storage and conversion. [ABSTRACT FROM AUTHOR]

Abstract: Vanadium oxide is a promising pseudocapacitive electrode, but their capacitance, especially at high current densities, requires improvement for practical applications. Herein, a VOx@MoO3 composite electrode is constructed through a facile electrochemical method. Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy demonstrate a modification on the chemical environment and electronic structure of VOx upon the effective interaction with the thin layer of MoO3. A careful investigation of the electrochemical impedance spectroscopy data reveals much enhanced power capability of the composite electrode. More charge storage sites will also be created at/near the heterogeneous interface. Due to those synergistic effects, the VOx@MoO3 electrode shows excellent electrochemical performance. It provides a high capacitance of 1980 mF cm−2 at 2 mA cm−2. Even at the high current density of 100 mA cm−2, it still achieves 1166 mF cm−2 capacitance, which doubles the sum of single electrodes. The MoO3 layer also helps to prevent VOx structure deformation, and 94% capacitance retention over 10 000 cycles is obtained for the composite electrode. This work demonstrates an effective strategy to induce interactions between heterogeneous components and enhance the electrochemical performance, which can also be applied to other pseudocapacitive electrode candidates. [ABSTRACT FROM AUTHOR]

Abstract: Electrode materials with low cost, large theoretical capacity, and fast ion/electron transport rate are highly desirable for capacitive applications. Pseudocapacitive iron oxide materials (e. g., Fe2O3, Fe3O4) satisfy the first two aspects, but are electrically/ionically insulating, hindering their practical applications in energy storage. Here, hybrid mesoporous iron oxide (Fe2O3/Fe3O4) materials are integrated with a three‐dimensional electrochemically exfoliated graphite substrate (EG) using an electrodeposition coupled with a post‐chemical transition strategy. The hybrid Fe2O3/Fe3O4 materials on EG exhibit an ultrahigh specific capacitance of 1530 F g−1 at current density of 1 A g−1. This material with such high capacitance ranks on the top of the reported iron oxide materials for capacitive applications. In addition, the assembled device with EG@Fe2O3/Fe3O4 as anode can deliver a high gravimetric energy density of 21.6 Wh kg−1 at a power density of 225 W kg−1, based on the total mass of the two electrodes (including the mass of the current collectors). This system can power various electronic devices, suggesting its great potential for energy storage applications. [ABSTRACT FROM AUTHOR]

Interconnected H 0.12 WO 3 ⋅H 2 O nanosheets with high electrochemical performances are fabricated on partial exfoliated graphite substrate (Ex-GF) by potential-limited pulse galvanostatic method (PLPG). The dead volume problem of bulk pesudocapacitive materials is addressed by the novel interconnected nanosheets structure, enabling a large specific capacitance of 5.95 F cm −2 (495.8 F g −1 ) at 2 mA cm −2 . Merited from the fluent electrolyte penetration channels established by the plenty voids among nanosheets, as well as fast electron transportation in the electronic conductive tungsten bronze which is directly grown from graphite substrate, the obtained WO 3 /Ex-GF demonstrates excellent rate capability. The material can maintain 60.0% of its capacitance when the discharge current density increases from 2 to 100 mA cm −2 . Moreover, WO 3 /Ex-GF doesn't show capacitance decay after 5000 galvanostatic charge-discharge cycles, displaying its super stability. Furthermore, a high performance asymmetric supercapacitor assembled by using WO 3 /Ex-GF and electrochemical fabricated MnO 2 /Ex-GF as negative and positive electrodes, respectively displays a high energy density of 2.88 mWh cm −3  at the power density of 11.1 mW cm −3 , demonstrating its potential application for energy storage. [ABSTRACT FROM AUTHOR]

Polypyrrole (PPy) is a promising pseudocapacitive material for supercapacitor electrodes. However, its poor cycling stability is the major hurdle for its practical applications. Here a two-prong strategy is demonstrated to stabilize PPy film by growing it on a functionalized partial-exfoliated graphite (FEG) substrate and doping it with β-naphthalene sulfonate anions (NS−). The PPy electrode achieves a remarkable capacitance retention rate of 97.5% after cycling between −0.8 and 0 V versus saturated calomel electrode for 10 000 cycles. Moreover, an asymmetric pseudocapacitor using the stabilized PPy film as anode also retains 97% of capacitance after 10 000 cycles, which is the best value reported for PPy-based supercapacitors. The exceptional stability of PPy electrode can be attributed to two factors: 1) the flexible nature of FEG substrate accommodates large volumetric deformation and 2) the presence of immobile NS− dopants suppresses the counterion drain effect during charge-discharge cycling. [ABSTRACT FROM AUTHOR]

Abstract: An electrochemical synthesis method of reducing graphene oxide (GO) under constant potential is reported. Electrochemical technique offers control over reaction parameters such as the applied voltage, electrical current and reduction time; whereas the desired size and thickness of the film can be pre-determined by controlling the amount of precursor GO deposited on the electrode with defined shape and surface area. This synthesis technique produces high quality electrochemically reduced GO (ERGO) film with controllable size and thickness. Electrochemical symmetrical supercapacitors based on ERGO films achieved a specific capacitance of 128F/g with an energy density of 17.8Wh/kg operating within a potential window of 1.0V in 1.0M NaNO3. The supercapacitor was shown to be stable, retaining ca. 86% of the original specific capacitance after 3500 charge–discharge cycles. The results indicate that this simple synthesis technique for providing graphene-like materials has great potential in various applications such as energy storage. [Copyright &y& Elsevier]

Keywords: cathode materials; polyanions; rechargeable batteries; supercapacitors; vanadyl phosphate EN cathode materials polyanions rechargeable batteries supercapacitors vanadyl phosphate 1 1 1 07/04/20 20200702 NES 200702 B The vanadyl phosphate materials b have various crystal structures, which allow reversible cation (de)intercalation. Cathode materials, polyanions, rechargeable batteries, supercapacitors, vanadyl phosphate. [Extracted from the article]

Highlights: Recent progress of active materials in supercapacitors synthesized by electrochemical techniques is reviewed. Electrochemically synthesized nanostructures of various dimensions, compositions, and electrochemical properties are discussed. The advantages and challenges of electrochemical technologies in preparing nano-/microstructured materials for electrochemical energy storage devices are summarized. The article reviews the recent progress of electrochemical techniques on synthesizing nano-/microstructures as supercapacitor electrodes. With a history of more than a century, electrochemical techniques have evolved from metal plating since their inception to versatile synthesis tools for electrochemically active materials of diverse morphologies, compositions, and functions. The review begins with tutorials on the operating mechanisms of five commonly used electrochemical techniques, including cyclic voltammetry, potentiostatic deposition, galvanostatic deposition, pulse deposition, and electrophoretic deposition, followed by thorough surveys of the nano-/microstructured materials synthesized electrochemically. Specifically, representative synthesis mechanisms and the state-of-the-art electrochemical performances of exfoliated graphene, conducting polymers, metal oxides, metal sulfides, and their composites are surveyed. The article concludes with summaries of the unique merits, potential challenges, and associated opportunities of electrochemical synthesis techniques for electrode materials in supercapacitors. [ABSTRACT FROM AUTHOR]

Commercial electrodes need high mass loadings to realize superior energy and power densities. However, good electrochemical properties are usually achieved in the electrodes with ultrathin active materials (e.g., <1 mg cm−2). Good performance and high mass loading are often mutually exclusive characteristics. Herein, a unique 3D exfoliated carbon paper (EC) is demonstrated using a facile electrochemical method to support high mass loading MnO2 materials. The 3D‐interconnected graphene/graphite network, highly porous structure, as well as the strong interaction between the active materials and the substrate, allow efficient charge transport in the composite electrode, addressing the traditional limitations in high mass loading electrodes. The deposited MnO2 (mass loading: 9.5–10 mg cm−2) achieves a remarkable capacitive performance with high areal and gravimetric capacitances of 5.1 F cm−2 and 537 F g−1, respectively. A Zn–MnO2 battery is also assembled using MnO2/EC as the cathode. An excellent specific capacity of 368 mAh g−1 is also delivered. This MnO2/EC outperforms most of the reported MnO2‐based electrodes with similar loadings for capacitive and Zn‐ion battery applications, highlighting the great application potential of the 3D carbon paper support in electrochemical energy storage. [ABSTRACT FROM AUTHOR]

Pyrolyzing metal–organic frameworks (MOFs) typically yield composites consisting of metal/metal oxide nanoparticles finely dispersed on carbon matrices. The blend of pseudocapacitive metal oxides and conductive metals, as well as highly porous carbon networks, offer unique opportunities to obtain supercapacitor electrodes with mutually high capacitances and excellent rate capabilities. Herein, we demonstrate nitrogen-doped carbon nanocuboid arrays grown on carbon fibers and incorporating cobalt metal and cobalt metal oxides. This composite was synthesized via pyrolysis of a chemical bath deposited MOF, cobalt-containing zeolite imidazole framework (Co–ZIF). The active materials for charge storage are the cobalt oxide and nitrogen-doped carbon. Additionally, the Co metal and the nanoporous carbon network facilitated electron transport and the rich nanopores in each nanocuboid shortened ion diffusion distance. Benefited from these merits, our Co–ZIF-derived electrode delivered an areal capacitance of 1177 mF cm−2 and excellent cycling stability of ~94% capacitance retained after 20,000 continuous charge–discharge cycles. An asymmetric supercapacitor prototype having the Co–ZIF-derived hybrid material (positive electrode) and activated carbon (negative electrode) achieved a maximal volumetric energy density of 1.32 mWh cm−3 and the highest volumetric power density of 376 mW cm−3. This work highlights the promise of metal–metal oxide–carbon nanostructured composites as electrodes in electrochemical energy storage devices. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • FC NWA substrate fabrication as a 3D conductive carbon scaffold. • Electrochemical growth of vanadium oxide nanowires on FC NWA substrate. • Enlarged charge storage window for the composite through vanadium valence tuning. • The assembled symmetric supercapacitor works with a high voltage of 2 V. Abstract Vanadium oxides have great potentials in high voltage symmetric supercapacitors, benefiting from the available multiple oxidation states of vanadium. In this work, charge storage in a wide potential window of −1.1 to 0.9 V vs. SCE is achieve for vanadium oxide through vanadium valence tuning and incorporation in functionalized carbon nanowire array. The functionalized carbon nanowire array provides a good three dimensional conductive scaffold for the oxide nanowires, leading to high electron and ion transportation for the pseudo-capacitive materials. A high areal capacitance of 1.31 F cm−2 is achieved for the obtained vanadium oxide electrode (at 1 mA cm−2 in 5 M LiCl aqueous solution). It also displays good rate capability, demonstrating a quite high areal capacitance of 687 mF cm−2 at 100 mA cm−2. The assembled symmetric supercapacitor can be operate with a large voltage of 2 V and so can deliver a high volumetric energy density of 3.61 mWh cm−3 at 0.01 W cm−3. The supercapacitor also shows good stability, it can retain 91% of its capacitance after 10 000 galvanostatic charge-discharge cycles. [ABSTRACT FROM AUTHOR]

Manganese oxide (MnO x ), as a benchmark pseudocapacitive material, has aroused great interest in electrochemical energy storage community. However, ion transport is seriously hindered in the densely packed bulk materials of highly loaded MnO x electrodes. Here, the structural engineering for the electrodeposited MnO x materials is realized by tuning the concentration of the complexing agents used in the electrodeposition process. The fabricated highly porous nanostructure in the thick oxide layer can minimize the ion diffusion distances to the interior electrode surface, and the optimized content of the hydrous species can facilitate the solid-phase diffusion of the electrolyte ions in the oxide lattice. The deposited MnO x electrode with a high mass loading of 7.02 mg cm −2 exhibits excellent rate capability due to the dual-tuning effect. An excellent specific capacitance of 161.2 F g −1 (1.13 F cm −2 ) at a high current density of 20 mA cm −2 can be obtained, which is comparable to the capacitance delivered by the low mass loading electrode at the same current density (214.8 F g −1 for 0.54 mg cm −2 sample), indicating its high material utilization. The performance of the fabricated electrode ranks on the top of the reported MnO x materials with high mass for capacitive applications. [ABSTRACT FROM AUTHOR]

In this work, a core-shell structured pseudocapacitive anode is fabricated through electrochemical deposition of highly loaded (3.89 mg cm −2 ) polypyrrole (PPy) on individual carbon tubes in electrochemically pretreated carbon nanotube film (ECNT) to afford ECNT/PPy electrode. The core-shell structure significantly promotes the charge transfer for energy storage reaction and buffers the volume change of PPy during charge/discharge cycles, leading to much improved pseudocapacitive behaviors for ECNT/PPy. It can retain 75.2% of its capacitance (965.3 mF cm −2 at 1 mA cm −2 ) when the discharge current increased 40 times to 40 mA cm −2 . After 10,000 galvanostatic charge/discharge cycles, ECNT/PPy can maintain 89.1% of its initial capacitance. The asymmetric supercapacitor (ASC) assembled by using ECNT/PPy and CNT/MnO 2 as anode and cathode, respectively demonstrate a high volumetric energy density of 3.63 mWh cm −3 at the power density of 13.86 mW cm −3 . After 10,000 galvanostatic charge/discharge cycles, the ASC can retain 89.0% of its capacitance, showing its good cyclic stability. [ABSTRACT FROM AUTHOR]

Research on anode materials with high capacitive performance is lagging behind that of cathode materials, which has severely hindered the development of high-efficient energy storage devices. Compared with other anode materials, Fe 3 O 4 exhibits highly desirable advantages due to its low cost, high theoretical capacity and preferable electronic conductivity of ∼10 2 S cm −1 . Herein, hierarchical honeycomb Fe 3 O 4 is integrated on functionalized exfoliated graphite through electrochemical deposition and the following chemical conversion. The hierarchical honeycomb Fe 3 O 4 is constructed by the oxide nanorods, which are assembled by a number of nanoparticles. This unique porous structure not only ensures fast ion diffusion in the electrode, but also provides large amount of active sites for electrochemical reactions. The exfoliated graphene atop the graphite base can act as 3D conductive scaffold to facilitate the electron transport of the electrode. Therefore, FEG/Fe 3 O 4 exhibits large specific capacitances of 327 F g −1 @1 A g −1 and 275 F g −1 @10 A g −1 . Good cycling stability is also achieved due to the flexibility of the graphene substrate. The assembled asymmetric device using FEG/Fe 3 O 4 as anode can deliver a high energy density of 54 Wh kg −1 . [ABSTRACT FROM AUTHOR]

Abstract: Carbon cloth was functionalized through cyclic voltammetric scans in 2M H2SO4 to afford functionalized carbon cloth (FC). The FC was characterized by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Self-doped polyaniline (SPAN) was deposited on FC through electro-co-polymerization of aniline and metanilic acid to get SPAN/FC which showed a specific capacitance of 408Fg−1 in 0.5M Na2SO4, based on a constant charge–discharge experiment at a current density of 1Ag−1. A symmetric model capacitor, SPAN/FC-SPAN/FC was assembled by using SPAN/FC as both of the electrodes and 0.5M Na2SO4 as the electrolyte. The charge storage property and charge–discharge cycling stability of SPAN/FC-SPAN/FC were investigated by galvanostatic experiment. [Copyright &y& Elsevier]

Abstract: We report for the first time the enhanced capacitance of multi-walled carbon nanotubes (MWCNTs) after exfoliation. Transmission electron microscopy studies confirmed that the MWCNTs were partially exfoliated with improved effective surface area. Carbon cloth electrode deposited with partially exfoliated carbon nanotubes (Ex-CNTs) yielded specific capacitances in a range of 130–165Fg−1 at charging/discharging current densities from 5 to 0.5Ag−1, with coulombic efficiencies of ∼98%. The specific capacitance of Ex-CNTs was an order of magnitude higher than untreated MWCNTs, and comparable to graphene at all charging/discharging current densities we studied. The enhanced capacitance can be attributed to improved effective surface area and increased defect density of the exfoliated tubular structure. The results declared that Ex-CNTs are promising electrode materials for high-capacitance supercapacitors. [Copyright &y& Elsevier]

Abstract: Composites of polyaniline (PANI) and a unique ordered mesoporous carbon MPC which possesses small pores inside the carbon walls have been prepared. The monomer (aniline) of PANI is firstly soaked in the small pores inside the carbon walls and then confined in the primary mesochannels as well with increasing the concentration of aniline in the soaking solution. After the following in situ polymerization, composites of PANI and MPC with bicontinuous interpenetrating framework are formed. The composites show significant redox activities at potentials negative to 0V vs. Hg/HgO in a strong alkaline solution, which is unusual for PANI and PANI based composites as PANI usually loses its electrochemical activity at pH>4. The specific capacitance of the composites is as high as 400Fg−1 at a high current density of 1Ag−1 in the potential range of −0.7 to 0V, which is twice higher than that of the host MPC. The capacitance only decreases about 9% when the discharge current density increases from 1 to 20Ag−1, indicative of excellent rate capability. Therefore, the composites are very promising for the application as electrode materials in supercapacitors, especially as materials for negative electrode in hybrid supercapacitors. [ABSTRACT FROM AUTHOR]

Abstract: We report the synthesis and pseudocapacitive studies of a composite film (PANI-ND-MnO2) of polyaniline (PANI) and manganese oxide (MnO2) nanoparticles. To enhance the interaction of MnO2 and PANI, the surfaces of MnO2 nanoparticles were modified by a silane coupling reagent, triethoxysilylmethyl N-substituted aniline (ND42). The composite film was obtained via controlled electro-co-polymerization of aniline and N-substituted aniline grafted on surfaces of MnO2 nanoparticles (ND-MnO2) on a carbon cloth in a electrolyte of 0.5M H2SO4 and 0.6M (NaPO3)6. In comparison to similarly prepared PANI film, the incorporation of MnO2 nanoparticles substantially increases the effective surface area of the film by reducing the size of rod-like PANI aggregates and avoiding the entanglement of these PANI nanorods. Significantly, we observed significant enhancement of specific capacitance in PANI-ND-MnO2 film compared to PANI–MnO2 film prepared in a similar condition, indicating that the presence of the coupling reagent can improve the electrochemical performance of PANI composite film. A symmetric model capacitor has been fabricated by using two PANI-ND-MnO2 nanocomposite films as electrodes. The PANI-ND-MnO2 capacitor showed an average specific capacitance of ∼80Fg−1 and a stable coulombic efficiency of ∼98% over 1000 cycles. The results demonstrated that PANI-ND-MnO2 nanocomposites are promising materials for supercapacitor electrode and the importance of designing and manipulating the interaction between PANI and MnO2 for fundamentally improving capacitive properties. [Copyright &y& Elsevier]

Molybdenum oxides are promising pseudo-capacitive electrode materials, but their capacitance and cycling stability require improvement. Herein, we demonstrate a facile electrochemical method to fabricate a high mass loading (14 mg cm−2) MoO x electrode. It provides a high areal capacitance of 3.3 F cm−2 at 5 mA cm−2 and maintains 1.9 F cm−2 at the high current density of 100 mA cm−2. The preferential selection of charge storage redox couples – realized via tuning the cycling potential – effectively promotes an ultra-long cycle life. In the optimal potential window of −1 V to −0.4 V, 101% of the initial capacitance is maintained after 30 000 cycles, in direct contrast to the −1 V to 0 V window which shows a fast degradation. X-ray photoelectron spectroscopy (XPS) reveals the good reversibility of the Mo4+/Mo5+ redox couple within −1 V to −0.4 V window, while Mo6+ accumulates above −0.4 V. Electrochemical impedance spectroscopy (EIS) suggests the poorer electric and more sluggish ion diffusion with the increase amount of Mo6+-components in electrodes, which is responsible for the poorer electrochemical activity and shorter cycle life. Our work demonstrates the utilization of desirable redox couples can effectively enhance the cycling stability of molybdenum oxide electrodes, which could also be applied to other pseudo-capacitive electrode materials. Image 1 • Electrochemical deposition of MoO x with mass loading of 13.8 mg cm−2. • 101% of initial capacitance over 30000 cycles within −1 to −0.4 V (vs. SCE). • Mo4+/Mo5+ redox couple worked within −1 to −0.4 V (vs. SCE) have good reversibility. • Mo6+ accumulates above −0.4 V (vs. SCE) resulting in capacitance decay. [ABSTRACT FROM AUTHOR]