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Lithium perchlorate-doped polypyrrole growing on titanium substrate (LiClO4-PPy/Ti) has been fabricated to act as electroactive electrode material for feasible electrochemical energy storage. A theoretical and experimental investigation is adopted to disclose the conductivity, electroactivity properties and interfacial interaction-dependent capacitance of LiClO4-PPy/Ti electrode. The experimental measurement results disclose that LiClO4-PPy/Ti reveals lower ohmic resistance (0.2226 Ω cm−2) and charge transfer resistance (2116 Ω cm−2) to exhibit higher electrochemical conductivity, a more reactive surface, and feasible ion diffusion to present higher double-layer capacitance (0.1930 mF cm−2) rather than LiClO4/Ti (0.3660 Ω cm−2, 65,250 Ω cm−2, 0.0334 mF cm−2). LiClO4-PPy/Ti reveals higher Faradaic capacitance caused by the reversible doping and dedoping process of perchlorate ion on PPy than the electrical double-layer capacitance of LiClO4/Ti caused by the reversible adsorption and desorption process of the LiClO4 electrolyte on Ti. Theoretical simulation calculation results prove that a more intensive electrostatic interaction of pyrrole N···Ti (2.450 Å) in LiClO4-PPy/Ti rather than perchlorate O···Ti (3.537 Å) in LiClO4/Ti. LiClO4-PPy/Ti exhibits higher density of states (57.321 electrons/eV) at Fermi energy and lower HOMO-LUMO molecule orbital energy gap (0.032 eV) than LiClO4/Ti (9.652 electrons/eV, 0.340 eV) to present the enhanced electronic conductivity. LiClO4-PPy/Ti also exhibits a more declined interface energy (−1.461 × 104) than LiClO4/Ti (−5.202 × 103 eV) to present the intensified interfacial interaction. LiClO4-PPy/Ti accordingly exhibits much higher specific capacitances of 0.123~0.0122 mF cm−2 at current densities of 0.01~0.10 mA cm−2 rather than LiClO4/Ti (0.010~0.0095 mF cm−2, presenting superior electroactivity and electrochemical capacitance properties. LiClO4-PPy/Ti could well act as the electroactive supercapacitor electrode for feasible energy storage. [ABSTRACT FROM AUTHOR]

Suitable transitional metal electrode substrate should keep high electroactivity and meanwhile suppress polarization effect. Theoretical calculation and experimental measurement are adopted to investigate interfacial interaction-dependent capacitance of lithium perchlorate-titanium and nickel (LiClO4-Ti, LiClO4-Ni) metal substrates. LiClO4-Ti reveals ideal electrical double layer capacitance due to electrochemical stability and LiClO4-Ni reveals the deviated electrical double layer capacitance due to polarization effect in potential range of 0–0.8 V. LiClO4-Ni reveals lower ohmic resistance, charge transfer resistance and Warburg resistance to present higher conductivity, more active interface, and more feasible ion diffusion properties than LiClO4-Ti. Titanium and nickel substrates achieve mean response current of 0.13–2.47 μA cm−2 and 1.03–22.20 μA cm−2 at 5200 mV s−1. The corresponding specific capacitances are 0.010–0.0095 mF cm−2 and 0.322–0.155 mF cm−2 at 0.01–0.10 mA cm−2. LiClO4-Ni keeps much higher capacitance but relatively lower electrochemical stability than LiClO4-Ti. Theoretical calculation proves interfacial electrostatic adsorption of LiClO4 on Ti and Ni substrates could change charge density distribution. LiClO4-Ni with the enlarged potential difference causes intensified polarization effect. LiClO4-Ni exhibits higher electrical conductivity and lower interface energy than LiClO4-Ti. Furthermore, carbon-coated nickel (C@Ni) was prepared to mostly restrain polarization effect and meanwhile keep high electroactivity of Ni substrate for the promising supercapacitor electrode application. It reveals specific capacitance of 25.9 mF cm−2 and cycling capacitance retention of 94.5% after 1000 charge-discharge cycles at 1.0 mA cm−2 in 1.0 M LiClO4. The C@Ni electrode accordingly exhibits superior electroactivity and electrical double layer capacitance for energy storage application. [ABSTRACT FROM AUTHOR]

Multi-transition metal compounds exhibit greater electroactivity and are preferred as electrode materials for supercapacitors (SC) due to their superior electrochemical properties compared to monometallic compounds. This article reviews the types of redox reactions of multi-transition metal compounds. The pseudocapacitance and potential window are summarized for bimetallic and trimetallic compounds. The simultaneous redox reaction occurring at the same electrode and electrolyte system is defined as matching Faradaic reaction. The matching Faradaic reaction involves simultaneous reversible redox reactions of two or more types of transition metal ions, which increases pseudocapacitance and broadens the potential window of multi-transition metal compounds, thereby achieving higher energy density. The positive effects of utilizing multi-transition metal compounds as SC electrodes are summarized. The matching Faradaic reaction mechanism is proposed for multi-transition metal compounds. The correlation between the valence band potential and redox reaction of multi-transition metal compounds is proposed to enhance the pseudocapacitance of such compounds. An overview is provided on the strategies for increasing capacitance and expanding the potential window of multi-transition metal compounds to enhance the energy density of SCs. Finally, recommendations are proposed to enhance the electrochemical performance of electrodes composed of multi-transition metal compounds in future applications. [ABSTRACT FROM AUTHOR]

The well-controlled graphene was prepared by pre-oxidisation, full-oxidisation and hydrothermal reduction of flake graphite. The graphene oxide (GO) reveals the overlapped few-layer structure and the reduced graphene oxide (RGO) shows well-dispersed few-layer structure. Raman spectra analysis indicates that the RGO shows higher B band/G band ratio value than GO, indicating well-restored SP2 carbon atom arrangement and crystal structure. Both fewer-layer GO and RGO were coated on nickel form for supercapacitor electrode application. The GO with sufficient hydroxyl, epoxy and carboxyl groups reveals low conductivity and low current response, which causes low specific capacitance of 0.36, 0.38 and 0.55 F g−1 at 0.5 A g−1 in Na2SO4, KOH and LiOH electrolyte solution, respectively. The RGO with high conductivity reveals the enhanced specific capacitance of 37.12, 51.80 and 61.31 F g−1 at 0.5 A g−1. The LiOH electrolyte with superior ionic diffusion coefficient leads to higher capacitance performance of fewer-layer GO and RGO than other electrolytes. The RGO shows much higher current response than GO at the same scan rate in cyclic voltammetry measurement, which is ascribed to its higher electroactivity. The similar capacitance enhancement of RGO was achieved through cyclic voltammetry-based capacitance and galvanostatic charge/discharge-based capacitance measurements. Accordingly, fewer-layer RGO reveals the superior electrochemical capacitance performance in LiOH electrolyte for the effective energy-storage application. [ABSTRACT FROM AUTHOR]

Free-standing reduced graphene oxide (RGO) hydrogel was synthesized through two-stepped hydrothermal reduction treatment of aqueous graphene oxide (GO) suspension. GO with high interface affinity for overlapping of cross-linking graphene sheets contributes to forming steady-state RGO hydrogel. RGO hydrogel shows higher B band/G band intensity ratio than GO suspension in Raman spectrum analysis, indicating well-restored hexagonal carbon rings with the enhanced crystal structure. Free-standing RGO hydrogel could be directly acted as supercapacitor electrode without using any substrate material. The specific capacitance decreases from 253.9 to 92.3 F g−1 in 1.0 M Na2SO4 electrolyte and from 275.3 to 125.9 F g−1 in 1.0 M KOH electrolyte when the current density increased from 0.3 to 4.0 A g−1, presenting the capacity retention of 36.4% and 45.7% respectively. Free-standing RGO hydrogel presents specific capacitance in alkaline KOH electrolyte rather than neutral Na2SO4 electrolyte. KOH electrolyte with superior ionic diffusion coefficient of hydroxyl ion leads to higher current response and electrical double-layer capacitance performance. The capacitance retention ratio of 97.8% at 4.0 A g−1 in 1.0 M KOH electrolyte after 1000 charge-discharge cycles presents superior cycling stability. Accordingly, free-standing RGO hydrogel applied well as superior supercapacitor electrode presents the promising energy storage application. [ABSTRACT FROM AUTHOR]

Sodium manganese oxide (NaxMnO2) is supported on nickel foam (NF) substrate to prepare electroactive NaxMnO2/NF electrode material. NaxMnO2 as sodium ion pre-intercalation compound is formed through solid phase reaction of MnO2 and NaOH with optimal mole ratio of 1:2 and calcination treatment at 700 °C for 5 h. NaxMnO2 shows lower Warburg diffusion impedance and charge transfer resistance than MnO2. NaxMnO2/NF shows much higher current response than MnO2/NF due to predominant reversible redox reaction of NaxMnO2. The reversible intercalation/deintercalation of sodium ion at the deep interlayer of NaxMnO2 contributes to Faradaic capacitance. MnO2/NF and NaxMnO2/NF reveal capacitance decay from 18.5 to 4.2 F g−1 and from 133.2 to 65.7 F g−1 in 0.5 M Na2SO4 electrolyte when current density increases from 0.2 to 2.0 A g−1, indicating respective capacity retention of 22.7% and 49.3%. NaxMnO2/NF exhibits higher capacitance and better rate capacitance retention than MnO2/NF, presenting the promising supercapacitor energy storage. [ABSTRACT FROM AUTHOR]

The polyaniline/hydrothermal active carbon fiber (PANI/EACF) is designed and acted as supercapacitor electrode for the electrochemical energy storage application. PANI fiber layer with porous structure is fully covered on the surface of EACF substrate to form electroactive PANI/EACF electrode. The active carbon fiber (ACF) conducts the electrochemical oxidation and reduction activation to form EACF with functional hydroxyl group. The PANI/EACF with intermolecular hydrogen bond interaction exhibits the stronger and more stable bonding interface than PANI/ACF with Van der Waals force interaction, which accordingly promotes the interfacial charge transfer and charge storage capacitance in the charge–discharge process. PANI/EACF exhibits higher response current density than PANI/EACF at the same scan rate, indicating higher conductivity and electroactivity of PANI/EACF. The specific capacitance declines from 78.1 to 36.8 F g−1 for PANI/ACF and from 133.5 to 65.3 F g−1 for PANI/EACF when current density increases from 1 to 10 A g−1, presenting the obviously improved capacity. The corresponding capacitance retention, respectively, achieves 47.1 and 48.9%, presenting the slightly increased rate capability. The capacitance retention ratio of PANI/ACF and PANI/EACF achieves 61.4 and 67.1% at 5 A g−1 after 1000 cycles, presenting the slightly increased cycling stability. PANI/EACF with superior capacitive performance in the electrochemical charge–discharge process presents the promising energy storage application. [ABSTRACT FROM AUTHOR]

An electroactive nitrogen-doping tin dioxide nanorod array (N-SnO2 NRA) is designed as an effective energy-storage electrode material for supercapacitor applications. N-SnO2 supported on a carbon fiber substrate is prepared using SnCl4 as a precursor through hydrolysis, hydrothermal growth, and an NH3-nitriding process. Electroactive N-SnO2 is formed by an N-doping reaction between Sn(OH)4 and NH3, revealing a high nitrogen-doping level of 12.5% in N-SnO2. N-SnO2/carbon fiber reveals a lower ohmic resistance and charge transfer resistance than SnO2/carbon fiber, which is consistent with its higher current response and lower voltage drop in electrochemical measurements. N-SnO2 NRA has an independent nanoarray structure and a small side length of a quadrangular nanorod, contributing to a more accessible interspace, reactive sites, and feasible electrolyte ion diffusion. The N-SnO2/carbon fiber NRA electrode shows higher specific capacitance (105.4 F g−1 at 0.5 A g−1) and rate capacitance retention (45.0% from 0.5 to 5 A g−1) than a SnO2/carbon fiber NRA electrode (58.6 F g−1, 38.4%). Significantly, the cycling capacitance retention after 2000 cycles increases from 78.1% of SnO2/carbon fiber to 98.8% of N-SnO2/carbon fiber, presenting a superior electrochemical cycling stability. The N-SnO2 supercapacitor maintains stable power working at an output voltage of 1.6 V. The specific capacitance decreases from 75.2 to 55.1 F g−1 when the current density increases from 1 to 10 A g−1. The corresponding energy density decreases from 24.23 to 9.81 Wh kg−1, presenting a reasonable rate capability. So, the prepared N-SnO2 nanorod array demonstrates superior capacitance performance for energy-storage applications. [ABSTRACT FROM AUTHOR]

A PPY/TiO2/PPY jacket nanotube array was fabricated by coating PPY layer on the external and internal surface of a tube wall-separated TiO2 nanotube array. It shows coaxial triple-walled nanotube structure with two PPY nanotube layers sandwiching one TiO2 nanotube layer. PPY/TiO2/PPY reveals much higher current response than TiO2. The theoretical calculation indicates PPY/TiO2/PPY reveals higher density of states and lower band gap, accordingly presenting higher conductivity and electroactivity, which is consistent with the experimental result of a higher current response. The electroactivity is highly enhanced in H2SO4 rather than Na2SO4 electrolyte due to feasible pronation process of PPY in an acidic solution. PPY/TiO2/PPY could conduct the redox reaction in H2SO4 electrolyte which involves the reversible protonation/deprotonation and HSO4− doping/dedoping process and accordingly contributes to Faradaic pseudocapacitance. The specific capacitance is highly enhanced from 1.7 mF cm−2 of TiO2 to 123.4 mF cm−2 of PPY/TiO2/PPY at 0.1 mA cm−2 in H2SO4 electrolyte. The capacitance also declines from 123.4 to 31.7 mF cm−2 when the current density increases from 0.1 to 1 mA cm−2, presenting the rate capacitance retention of 26.7% due to the semiconductivity of TiO2. A PPY/TiO2/PPY jacket nanotube with high charge storage capacitance is regarded as a promising supercapacitor electrode material. [ABSTRACT FROM AUTHOR]

The interconnected and independent titania nanotube array (TiO2 NTA) grown on titanium metal foil with tunable nanotube diameter and length is fabricated by ultrasonic bath-assisted anodisation process. The electrical double-layer capacitance is highly dependent on the nanotube microstructure, crystal phase and working electrolyte. Anatase TiO2 NTA shows cyclic voltammetry-based current response and capacitance in H2SO4 rather than NaOH electrolyte, which is due to higher electrical conductivity and more effective proton diffusion rather than hydroxyl ion diffusion. Interconnected and independent TiO2 NTA with a diameter of 60, 120, 200nm and a length of 0.62, 1.69, 1.28μm exhibits galvanostatic charge-discharge based capacitance of 0.76, 1.88 and 2.63 mF cm−2 at 0.1mA cm−2 in 1.0M H2SO4 electrolyte. The superior capacitance performance of independent TiO2 NTA is ascribed to more effective ion diffusion in the enlarged independent nanotube channels. TiO2 NTA with a tunable microstructure is accordingly suitable for supercapacitor electrode substrate application. [ABSTRACT FROM AUTHOR]

The sodium ion pre-intercalation manganese dioxide (Na x MnO 2) is supported on titanium nitride (TiN) substrate to form electroactive Na x MnO2/TiN electrode through an electrodeposition process in Mn(CH3COOH)2/Na2SO4 precursors with high Mn/Na ratio. MnO2 has a tiled leaf-like structure with a wrinkling morphology. Na x MnO2 has a cross-linking nanorod structure with a nanoporous morphology, which is beneficial for electrolyte ion diffusion. The density functional theory (DFT) calculation results indicate that Na x MnO2 reveals the enhanced density of states (DOS) and the lowered band gap than MnO2, which is consistent with higher cyclic voltammetry current response due to superior electroactivity of Na x MnO2. The Faradaic process involves Na + adsorption/desorption on the surface of MnO2 by contributing to electrochemical capacitance and Na + intercalation/deintercalation on the deep interlayer of pre-intercalation Na x MnO2 by contributing to pseudocapacitance. Concerning the electrolyte ion size effect, both MnO2/TiN and Na x MnO2/TiN electrodes have higher capacitive performance in Li2SO4 electrolyte than that in Na2SO4 and K2SO4 electrolyte due to more feasible Li + diffusion. When MnO2 is converted into Na x MnO2, the capacitance at 2.5 mA cm − 2 increases from 351.3 mF cm − 2 to 405.6 mF cm − 2 in Na2SO4 electrolyte and from 376.3 mF cm − 2 to 465.1 mF cm − 2 in Li2SO4 electrolyte. The conductive TiN substrate leads to high rate capacity retention ratio of 50.7% for MnO2/TiN and 49.5% for Na x MnO2/TiN when current density increases from 0.5 mA cm − 2 to 5 mA cm − 2 . So, Na x MnO2/TiN with sodium ion pre-intercalation exhibits the improved capacitive performance in Li2SO4 electrolyte to act well as the promising supercapacitor electrode. CV and GCD curves of NaxMnO2/TiN electrode in Na2SO4 electrolyte show the redox peaks and the charge–discharge potential platforms, indicating the reversible intercalation–deintercalation of sodium ion into/from deep interlayer of NaxMnO2. [ABSTRACT FROM AUTHOR]

The conductive polymer of polypyrrole can be acted as electroactive electrode material of supercapacitor due to reversible redox behavior and high capacitance. It usually suffers from low electrochemical stability due to the breakdown of polymer molecule chain in the long‐term charge and discharge process. The monometallic or bimetallic‐coordinated polypyrrole usually exhibits the improved electrochemical performance. The transition metal ions such as ruthenium, iron, copper and cobalt are adopted for the coordination modification. The transition metal‐coordinated polypyrrole includes the intrachain and interchain coordination structure between transition metal ion and nitrogen atom of pyrrole ring. It is able to reinforce the polymer molecule chain strength to overcome excessive volumetric swelling and shrinking during charge‐discharge process, improving the cycling stability and rate capability of polypyrrole. Accordingly, the transition metal‐coordinated polypyrrole keeps simultaneously high capacitance performance and electrochemical stability, acting as the promising conductive polymer‐based supercapacitor electrode material for effective energy storage. [ABSTRACT FROM AUTHOR]

Interfacial chemical bonding reinforced activated carbon fiber-m-aminobenzenesulfonic acid-polyaniline (ACF-mABSA-PANI) is designed as an electroactive supercapacitor electrode to achieve long cycle life and high capacitive performance. The ACF-mABSA-PANI molecular involves amide bond (–CO–NH–) between mABSA and ACF, as well as sulfonamide bond (–SO 2 –NH–) between mABSA and PANI. ACF-mABSA-PANI forms a π-π conjugated structure with enhanced electron delocalization and accelerated charge transfer, ultimately leading to reduced interfacial resistance, increased cycle life and superior capacitive performance. [Display omitted] • Bridge molecule-reinforced ACF-mABSA-PANI involves chemical bonding interaction. • ACF-mABSA-PANI shows higher cycle life and specific capacitance than ACF-PANI. • ACF-mABSA-PANI shows fast-Faradaic process dominated energy storage mechanism. • ACF-mABSA-PANI electrode is well used for braidable-supercapacitor application. The bridge molecule-reinforced activated carbon fiber-m-aminobenzenesulfonic acid-polyaniline (ACF-mABSA-PANI) is designed and fabricated as braidable-supercapacitor electrode to achieve long cycle life and high capacitive performance. The mABSA acts as bridge molecule to form amide bond between mABSA and ACF, as well as sulfonamide bond between mABSA and PANI, which is confirmed by infrared spectrometer and X-ray photoelectron spectroscopy. The interfacial interaction between ACF and PANI is enhanced by chemical bonding, thus alleviating the shedding of polyaniline and improving the cycle life of ACF-mABSA-PANI. The electrostatic potential and Mulliken charge analysis calculations indicate that ACF-mABSA-PANI has an extended delocalized π-π conjugation system. The bridge molecule of mABSA improves the free electron migration efficiency between PANI and ACF, leading to an increase in the specific capacitance of ACF-mABSA-PANI. As a result, ACF-mABSA-PANI presents much higher specific capacitance of 599.3F/g at 0.5 A/g and much higher capacitance retention ratio of 93.8 % after 2000 charge/discharge cycles at 2 A/g while ACF-PANI only keeps 335.0F/g and 54.4 %. Braidable-supercapacitor based on ACF-mABSA-PANI electrode provides an energy density of 39.83 Wh kg−1 at a power density of 900 W kg−1 and maintains a high capacitance retention of 78.1 % after 2000 charge/discharge cycles at 2 A/g. Well-designed ACF-mABSA-PANI presents the potential and promising application for wearable electrochemical energy storage devices. [ABSTRACT FROM AUTHOR]

Graphene oxide–modified poly (vinyl alcohol)/sodium sulfate-sodium molybdate (GO/PVA-Na2SO4-Na2MoO4, GPSS) gel polymer electrolyte and nickel molybdate (NiMoO4) electrode are integrated to fabricate carbon paper (CP) supercapacitor to improve capacitance performance. GO in PVA gel can introduce an effective ion transport pathway to improve ionic conductivity of gel polymer electrolyte. The ionic conductivity increases from 3.73 mS cm−1 for PVA-Na2SO4 to 6.46 mS cm−1 for GO/PVA-Na2SO4 at optimal GO mass ratio of 0.6% in GO/PVA gel. It also obviously increases from 4.33 mS cm−1 for PVA-Na2SO4-Na2MoO4 to 28.86 mS cm−1 for GO/PVA-Na2SO4-Na2MoO4. Both Na2MoO4 electrolyte and NiMoO4 electrode show reversible redox electroactivity to provide superior pseudocapacitance. Accordingly, the CP supercapacitor using GPSS gel shows specific capacitance of 41.67 mF cm−2 and energy density of 70.02 mWh m−2 at 0.5 mA cm−2, presenting higher performance than 15.91 mF cm−2 and 26.74 mWh m−2 using GO/PVA-Na2SO4 gel. Furthermore, the NiMoO4/CP supercapacitor using GPSS gel shows even higher specific capacitance of 78.18 mF cm−2 and energy density of 131.39 mWh m−2 at 0.5 mA cm−2. It also exhibits high cycling capacitance retention of 85% at 0.5 mA cm−2 for 1000 cycles. The improved capacitance performance of CP supercapacitor using Na2MoO4 gel polymer electrolyte and NiMoO4 electrode is ascribed to reversible redox reaction of Mo(VI)/Mo(V), Mo(VI)/Mo(IV), and Ni(II)/Ni(III). The NiMoO4/CP supercapacitor using GO/Na2MoO4 gel polymer electrolyte becomes desirable for the promising application in energy storage devices. [ABSTRACT FROM AUTHOR]

Abstract CuCo bimetallic-coordinated polypyrrole (PPy-CuCo) is designed as active electrode material of supercapacitor to improve the electrochemical stability of PPy. PPy-CuCo is synthesized by electrochemical polymerization reaction of copper-coordinated pyrrole (Py-Cu) and cobalt-coordinated pyrrole (Py-Co) monomers, which is initially formed by chemical coordination reaction between pyrrole and Cu2+ or Co2+. PPy-CuCo involves the alternating linear chain-like structure of PPy-Cu and tetrahedron structure of PPy-Co. The linear chain-like structure promotes the interchain electron transfer, accordingly improving the rate capability of PPy. The tetrahedron structure reinforces the polymer chain strength of PPy. The volume swelling or shrinkage of PPy during the charge/discharge process is restrained to improve electrochemical cycling stability of PPy. The specific capacitance increases from 249 F g−1 for PPy to 556 F g−1 for PPy-CuCo at 1.0 A g−1. The rate capacitance retention accordingly increases from 40.9% for PPy to 48.2% for PPy-CuCo when the current density increases from 1.0 to 20.0 A g−1. Moreover, the cycling capacitance retention also increases from 79.4% for PPy to 90.0% for PPy-CuCo even at 20.0 A g−1 after 2000 cycles. PPy-CuCo with superior electrochemical stability presents the promising supercapacitor application for electrochemical energy storage. [ABSTRACT FROM AUTHOR]

Ni x Co 2x (OH) 6x decorated NiCo 2 O 4 (Ni x Co 2x (OH) 6x /NiCo 2 O 4 ) is synthesized by hydrothermal treatment and electrochemical deposition to improve electrochemical stability of Ni x Co 2x (OH) 6x during charge-discharge process. NiCo 2 O 4 with unique nanoflower structure and good conductivity is used as suitable supporting skeleton to stabilize Ni x Co 2x (OH) 6x . Ni x Co 2x (OH) 6x /NiCo 2 O 4 exhibits high specific capacitance of 827.1 F g −1 at 1 A g −1 . The long-term cycling stability is obvious improved from 43.3% for Ni x Co 2x (OH) 6x to 97.8% for Ni x Co 2x (OH) 6x /NiCo 2 O 4 after 2000 cycles at 20 A g −1 . Low capacitance decay of 42.9% is achieved for NiCo 2 O 4 /Ni x Co 2x (OH) 6x when current density increased from 1 to 50 A g −1 , indicating the improved rate capability. The mechanism of Ni x Co 2x (OH) 6x /NiCo 2 O 4 with high electrochemical stability is proposed. TiN nanotube array with superior conductivity is adopted as negative electrode to match well with Ni x Co 2x (OH) 6x /NiCo 2 O 4 positive electrode. All-solid-state asymmetrical supercapacitor based on Ni x Co 2x (OH) 6x /NiCo 2 O 4 and TiN electrodes achieves high energy density of 35.1 W h kg −1 at a power density of 900 W kg −1 and an expanded voltage window of 1.8 V, indicating potential application for energy storage. [ABSTRACT FROM AUTHOR]

Ruthenium coordinated polypyrrole electrodeposited on a carbon paper substrate is developed as an active electrode material for supercapacitors. PPy–Ru(12) was formed by electrochemical polymerization of the ruthenium chloride coordinated pyrrole monomer with a saturation coordination time of 12 h. The capacitance of PPy declined from 249 at 1.0 A g−1 to 102 F g−1 at 20.0 A g−1 with a capacitance retention of 40.9%. The capacitance of PPy–Ru(12) declined from 618 at 1.0 A g−1 to 301 F g−1 at 20.0 A g−1 with a capacitance retention of 48.7%, presenting higher rate capability. The coordination process could induce bipolarons to improve the electrochemical conductivity and rate capability. The capacity retention ratio highly increased from 79.4% for PPy to 91.4% for PPy–Ru(12) after 2000 cycles at 20.0 A g−1, presenting the superior cycling stability of PPy–Ru(12). The coordination reaction between the pyrrole monomer and RuCl3 led to the formation of a tetrahedral structure that restrained volume swelling or shrinking of PPy during the charge–discharge process, accordingly causing improved electrochemical cycling stability. In addition, the all-solid-state PPy–Ru(12) supercapacitor showed a capacitance of 122 F g−1 at 2.0 A g−1 and a capacity retention ratio of 82.8% after 500 cycles. This PPy–Ru(12) with good capacitive performance and electrochemical stability presents promising application as a supercapacitor electrode material. [ABSTRACT FROM AUTHOR]

The polyaniline bonding with nitrogen doped MoS 2 (MoS x N y @PANI) was fabricated by hydrothermal vulcanization, nitrogen doping and electropolymerization process, which was designed as the stable structured supercapacitor electrode. The interface bonding interaction between conductive PANI and conductive MoS x N y is proposed to prove the feasibility of the coordination bond enforced hybrid electrode material by using experimental measurement and density functional theory simulation calculation. The MoS x N y involves an intermediate energy level to improve its electrical conductivity by lowering band gap from 1.45 to 0.65 eV. MoS x N y @PANI involves the interface Mo-N coordination bond between N atom of terminal amino group of PANI and the exposed Mo atom of MoS x N y. MoS x N y @PANI shows lower energy barrier (5.07 eV) and lower interface energy (1.41 eV) than MoS 2 @PANI (5.31 eV, 1.86 eV) due to interface Mo-N bonding, contributing to its highly improved structure stability. MoS x N y @PANI exhibits higher pseudocapacitive contribution (61.4%) than MoS x N y (47.4%) and MoS 2 (41.9%), indicating the Mo-N coordination-induced activation of PANI. MoS x N y @PANI electrode achieves high specific capacitance of 577 F g−1 and impressive capacitance retention of 85.3% after 3000 cycles even at high current density of 5 A g−1. Interface Mo-N coordination bonding MoS x N y @PANI with high electrochemical stability and capacitance performance exhibits the promising application as stable energy storage devices. [ABSTRACT FROM AUTHOR]

Dual-defects induced band edge reconstruction of tin dioxide (SnO 2) via metal and non-metal ions co-doping strategy is applied to synthesize cobalt and nitrogen co-doped tin dioxide (Co, N-SnO 2) on activated carbon fiber (ACF) for supercapacitor. N-doping contributes to upward shift of valence band edge through the hybridization of N-2p and O-2p and Co-doping contributes to downward shift of conduction band edge through the hybridization of Co-3d and O-2p. The bandgap is highly narrowed from 2.29 eV for SnO 2 to 0.68 eV for Co, N-SnO 2. The Co, N-SnO 2 /ACF electrode achieves specific capacitance of 361.2 F g−1 at 1 A g−1, rate capability of 61.1% from 1 to 10 A g−1 and cycling stability of 103.3% for 10,000 cycles. The density functional theory proves that Co-doping mainly promotes the capacity due to its easier deprotonation process in acid electrolyte, whereas N-doping mainly promotes the rate capability due to its narrower bandgap and thus facile electron transport. The wearable bracelet-supercapacitor based on Co, N-SnO 2 /ACF electrode delivers an energy density of 70.7 Wh Kg−1 under a wide potential window of 2.0 V. This finding provides a feasible route to regulate the band edge of metal oxide electrode for the promising energy storage. The metal (Co) and non-metal (N) co-doping introduces the dual‐defects in SnO 2 crystal. The hybridization of Co-3d and O-2p causes the downward shift of conduction band edge. Meanwhile, the hybridization of N-2p and O-2p causes upward shift of valence band edge. Accordingly, the face-to-face bidirectional reconstruction of band edge results in highly narrowed bandgap of 0.68 eV for Co, N-SnO 2. [Display omitted] • Dual‐defects induced band edge reconstruction of Co, N-SnO 2 causes narrowed bandgap. • Face-to-face approaching of band edge of Co, N-SnO 2 promotes electron transition. • Co, N-SnO 2 shows much higher capacity and rate capability than Co-SnO 2 and N-SnO 2. • Co, N-SnO 2 /ACF is applied for wearable bracelet-supercapacitor with 70.7 Wh Kg−1. [ABSTRACT FROM AUTHOR]

Conductive CoS 2 and cycling stable MoS 2 are integrated to form the interconnected CoS 2 /MoS 2 nanosheet composite with electroactive interface grown on Mo foil substrate to act as high-performance supercapacitor anode material with electrochemical and mechanical stability. The band gap and density of state of CoS 2 /MoS 2 determined by density functional theory simulation calculation indicates its improved electrical conductivity. The interface binding energy and differential charge density reveal that strong crystal interface interaction between MoS 2 (002) and CoS 2 (111) enables effective charge transfer in interconnected CoS 2 /MoS 2 nanosheet composite. CoS 2 /MoS 2 exhibits higher specific capacitance of 605.77 mF cm−2 than MoS 2 of 218.1 mF cm−2 at 1.0 mA cm−2 in 0.5 M Na 2 SO 4. The rate capacitance retention is improved from 57.8% of MoS 2 to 82.8% of CoS 2 /MoS 2 at 1–10 mA cm−2. The high cycling stability is achieved for MoS 2 (100.2%) and CoS 2 /MoS 2 (104.05%) after 5000 cycles at 10 mA cm−2 due to self-activation of multilayered MoS 2. An asymmetric solid-state supercapacitor using CoS 2 /MoS 2 anode and MnO 2 /carbon paper cathode achieves a capacitance of 4.32 F cm−3 at 1.0 mA cm−2 and an energy density of 1.944 mWh cm−3 at a power density of 18 mW cm−3, 97.8% after 5000 cycles at 5.0 mA cm−2, presenting effective energy storage. [ABSTRACT FROM AUTHOR]

Electroactive FeS 2 -modified MoS 2 (FeS 2 /MoS 2) nanosheet grown on Mo foil is designed as supercapacitor electrode material. FeS 2 /MoS 2 nanosheet is crosslinked to form nanoporous microstructure. Density functional theory calculation reveals FeS 2 /MoS 2 exhibits much narrowed bandgap and high density of states at the Fermi level, suggesting highly improved conductivity. The charge transfers resistance decreases from 1.375 Ω of MoS 2 to 0.4167 Ω of FeS 2 /MoS 2. FeS 2 /MoS 2 exhibits higher specific capacity (495 and 394 mF cm−2) than MoS 2 (132 and 218.1 mF cm−2) and FeS 2 (315 and 286.5 mF cm−2) at 1.0 mA cm−2 in 3.0 M KOH and 0.5 M Na 2 SO 4. The rate capacity retention is enhanced from 57.8% of MoS 2 and 73.3% of FeS 2 to 76.14% of FeS 2 /MoS 2 in 0.5 M Na 2 SO 4 when current density increases from 1 to 10 mA cm−2, indicating its improved rate capability. The cycling capacity retention keeps 100.2% of MoS 2 and 100.7% of FeS 2 /MoS 2 for 5000 cycles at 5 mA cm−2 in 0.5 M Na 2 SO 4 , indicating the similar cycling stability. An asymmetric solid-state supercapacitor using FeS 2 /MoS 2 anode, exfoliated graphite carbon paper cathode and NaMoO 4 –Na 2 SO 4 -PVA gel electrolyte achieves specific capacity of 1.88 F cm−3 (112.8 mF cm−2) at 1.0 mA cm−2, energy density of 0.936 mWh cm−3 (56.56 mWh cm−2), capacity retention of 97.8% after 5000 cycles at 5.0 mA cm−2 and 1.9 V output voltage. • Conductive FeS 2 and stable MoS 2 are integrated to form electroactive FeS 2 /MoS 2 nanosheet. • Density functional theory calculation proves highly improved conductivity of FeS 2 /MoS 2. • FeS 2 /MoS 2 shows superior capacitance of 495 mF cm−2 at 1.0 mA cm−2 in 3.0 M KOH. • FeS 2 /MoS 2 keeps 95% capacity retention at 5 mA cm−2 for 5000 cycles in 3.0 M KOH. [ABSTRACT FROM AUTHOR]

Interspace-expanded molybdenum disulfide (IE-MoS2) has been designed as a supercapacitor electrode material to improve the cycling stability. IE-MoS2 was formed through the ultrasound exfoliation of the interspace-compacted molybdenum disulfide (IC-MoS2), which was initially prepared through hydrothermal synthesis using Na2MoO4 as molybdenum source and CH4N2S as sulfur source. As-formed IE-MoS2 shows a few-layer structure with approximate 8-16 monolayer packing and monolayer distance of 0.83 nm. The MoS2 few-layer distance increased from 12 nm of IC-MoS2 to 20 nm for IE-MoS2. The specific capacitance was determined to be 108 F g−1 for IC-MoS2 to 192 F g−1 for IE-MoS2 at 0.5 A g−1. The improved specific capacitance was ascribed to more active sulfur atom exposed at the edges of IE-MoS2 few-layer to conduct the promoted proton attachment reaction. IE-MoS2 showed the capacity retention of 42% when the current density increased from 0.5 to 10 A g−1, presenting the high-rate capability. IE-MoS2 achieved the capacity retention of 116% at 10 A g−1 after 5000 charge-discharge cycles, which was ascribed to the electro-activation of the few-layer expanded MoS2 in proton acid electrolyte solution. IE-MoS2 exhibited the obviously improved cycling stability in comparison with IC-MoS2. All solid-state IE-MoS2 supercapacitor based on two symmetric IE-MoS2 electrodes and H2SO4-PVA gel electrolyte exhibited the energy density of 18.75 Wh kg−1 and power density of 375 W kg−1 at 0.5 A g−1 and high voltage window of 1.5 V. IE-MoS2 supercapacitor also exhibited the improved capacity retention of 110% after 1000 charge-discharge cycles. Such well-designed IE-MoS2 few-layer with highly improved cycling stability performance presented the promising energy storage application. [ABSTRACT FROM AUTHOR]

Bridge molecule thiourea (TU) enforced polyaniline (PANI) growing on activated carbon fiber (ACF) is applied to synthesize PANI-TU-ACF as flexible supercapacitor electrode material which involves an amide-bonding interface between ACF and TU, a hydrogen bonding interface between TU and PANI. [Display omitted] • Bridge molecular thiourea enforces interfacial interaction between ACF and PANI. • Amide bonding PANI and hydrogen bonding ACF improve cyclability of PANI-TU-ACF. • Bridging-structured PANI-TU-ACF is used for braidable-supercapacitor application. The bridge molecule thiourea (TU) enforced polyaniline (PANI) growing on activated carbon fiber (ACF) is applied to synthesize polyaniline-thiourea-activated carbon fiber (PANI-TU-ACF) through surface activation, hydrothermal treatment and electrochemical deposition processes for wearable braidable-supercapacitor. Experimental measurement and theoretical calculation demonstrate that PANI-TU-ACF involves amide bonding interface between ACF and TU and hydrogen bonding interface between TU and PANI, thus confirming the formation of bridging structure. The absorption spectrum red-shift of N–H vibration peak keeps in agreement with the declined bonding energy of N–H bond achieved by partial atom potential simulation calculation, which proves the formation of hydrogen bonding in PANI-TU-ACF. PANI-TU-ACF exhibits larger N(E) value and lower HOMO-LUMO energy gap than PANI-ACF, indicating its higher electrical conductivity. PANI-TU-PANI shows more lowered interface energy than PANI-ACF, suggesting its higher stability. PANI-TU-ACF electrode delivers excellent capacitance of 563.8F/g at 1 A/g and cycling stability of 89.1 % for 2000 cycles. Wearable braidable-supercapacitor based on PANI-TU-ACF electrode delivers energy density of 62.9 Wh kg−1 at power density of 800 W kg−1, maintains high capacitance retention of 90.13 % after 2000 charging/discharging cycles. The interfacial bonding strengthened PANI-TU-ACF with excellent performance displays potential in wearable supercapacitors. [ABSTRACT FROM AUTHOR]

A three-dimensional criss-crossed CoO-modified NiMoO 4 (NiMoO 4 /CoO) nanohybrid was designed to enhance the capacitance performance of NiMoO 4 via two-step hydrothermal strategies. The NiMoO 4 /CoO nanohybrid demonstrates a higher specific surface area (55 m2 g−1) than that of simplex NiMoO 4 (25 m2 g−1). The presence of CoO enhances the density of reaction active sites of NiMoO 4 /CoO nanohybrid, which contributes to improving the electrochemical performance of NiMoO 4 /CoO nanohybrid. The specific capacitance is obviously improved from 1.99 F cm−2 (1331 F g−1) of NiMoO 4 to 5.36 F cm−2 (2332 F g−1) of NiMoO 4 /CoO nanohybrid at 2 mA cm−2 in 6 M KOH electrolyte. Furthermore, an asymmetric all-solid-state supercapacitor employing NiMoO 4 /CoO nanohybrid and exfoliated graphite carbon paper exhibits superior energy density of 71.4 W h kg−1 at a power density of 750 W kg−1 as well as a considerable cycling durability of 87.1% after 2000 cycles at 5 A g−1. The NiMoO 4 /CoO supported on Ni foam could act as binder-free electrode to provide highly enhanced electrochemical performance and show promising energy storage application. • CoO-modified NiMoO 4 nanohybrid shows a three-dimensional criss-crossed structure. • CoO nanostars increases the specific surface area of NiMoO 4 /CoO nanohybrid. • The presence of CoO increases the concentration of reaction active sites. • NiMoO 4 /CoO nanohybrid exhibits a specific capacitance of 5.36 F cm-2 (2332 F g−1). • The asymmetric supercapacitor shows a high energy density of 71.4 W h kg−1. [ABSTRACT FROM AUTHOR]

• Hybrid membrane involves electroactive PANI layer and full crack-free carbon layer. • FC@NF shows superior electrochemical anti-corrosion performance in H 2 SO 4 electrolyte. • PANI@FC@NF achieves high capacitance and cycling stability in H 2 SO 4 electrolyte. Hybrid membrane of polyaniline layer/full carbon layer coating on nickel foam (PANI@FC@NF) is designed as supercapacitor electrode with superior anti-corrosion and capacitance performance in an acidic electrolyte. PANI@FC@NF is prepared by fully coating crack-free carbon layer on nickel foam (FC@NF) through hydrothermal and thermal carbonization of glucose, and then PANI layer is coated on FC@NF through the method of electrodeposition. FC@NF shows much lower corrosion current density (0.284 mA cm−2) and higher corrosion potential (-0.171 V) in 1.0 M H 2 SO 4 , compared with NF (7.208 mA cm−2 and -0.208 V). PANI@FC@NF electrode achieves specific capacitance of 610 mF cm−2 at 1.0 mA cm−2 and cycling capacitance retention of 77.63 % at 2.0 mA cm−2 after 1000 cycles in 0.5 M H 2 SO 4. All-solid-state symmetrical supercapacitor using PANI@FC@NF electrode and polyvinyl alcohol-H 2 SO 4 gel electrolyte achieves 41 μWh cm−2 at 0.9 mW cm−2 and an excellent cycling performance. [ABSTRACT FROM AUTHOR]

CoMoO 4 with oxygen vacancy (CoMoO 4-x) porous nanosheet is prepared by hydrothermal synthesis and hydrogenation reduction method. The CoMoO 4-x porous nanosheet has a higher specific surface area together with a more diverse pore size distribution than CoMoO 4 nanosheet. The density functional theory calculation result exhibits that CoMoO 4-x has lower energy band gap (0.041 eV) than CoMoO 4 (1.683 eV). The electronic density of states plots reveals that CoMoO 4-x has more electron state distribution at Fermi energy level than CoMoO 4. CoMoO 4-x porous sheet achieves higher specific capacitance (1989 g−1) than CoMoO 4 (917 F g−1) at 2 mA cm−2. It also achieves superior capacitance retention rate (91.3%) than CoMoO 4 (83.9%) with the current density increasing from 2 to 20 mA cm−2. The introduction of oxygen vacancy can increase the carrier density, accelerate the electron transfer, promote the electrical conductivity, and accordingly strengthen the redox reactivity of CoMoO 4-x. An asymmetric supercapacitor is also constructed by using CoMoO 4-x as positive electrode and activated carbon as negative electrode. It achieves high energy density of 62.3 W h kg−1 at a power density of 800 W kg−1, together with a good cyclic life. Both experimental measurement and theorical calculation are applied to prove the promotive role of oxygen vacancy in supercapacitive performance of CoMoO 4-x. [ABSTRACT FROM AUTHOR]

• Excessive nitrogen doping of SnO 2 forms SnO x N y with micro-phase separation of SnN x. • SnN x contributes to high electron transfer and electronic conductivity of SnO x N y. • SnO x N y has high capacity of 637.3 F g−1 at 1 A g−1 & cyclability of 99.2% at 5 A g−1. • Density functional theory calculation proves high capacitive performance of SnO x N y. • Wire-shaped microsupercapacitor is formed using SnO x N y /ACFs and TiN wire electrodes. Tin dioxide (SnO 2) nanorod array has been vertically grown on activated carbon fibers (ACFs) substrate by seed-assisted hydrothermal synthesis. Well-aligned tin oxynitride (SnO x N y) nanorod array can be accordingly formed by excessive nitrogen doping of SnO 2 via nitriding treatment under an ammonia atmosphere. SnO x N y shows the micro-phase seperation of SnN x in SnO 2. The lower band gap energy of SnN x is advantageous to accelerate the electrons transfer and enhance the electronic conductivity of SnO 2 electrode material. The density functional theory calculation results prove that SnO x N y exhibits higher electronic donation at its interface for electrochemical reaction than SnO 2. Hence, the free-standing SnO x N y /ACFs electrode delivers greatly improved electrochemical performance including higher specific capacitance (637.3 F g−1 at 1 A g−1) and cycling stability (99.2% at 5 A g−1 for 2000 cycles) in 1.0 M H 2 SO 4 electrolyte than SnO 2 /ACFs (275.5 F g−1; 85.2%). The asymmetric wire-shaped microsupercapacitor using SnO x N y /ACFs as a positive electrode, TiN wire as a negative electrode and sulfuric acid-polyvinyl alcohol gel as the electrolyte has also achieved high output voltage of 1.6 V, high specific capacitance of 1.2 F cm−3 at 10 mA cm−3 and high energy density of 0.43 W h L−1. So, SnO x N y /ACFs can act as the promissing positive electrode material for effective energy storage application. [ABSTRACT FROM AUTHOR]