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[en] For miniaturized capacitive energy storage, volumetric and areal capacitances are more important metrics than gravimetric ones because of the constraints imposed by device volume and chip area. Typically used in commercial supercapacitors, porous carbons, although they provide a stable and reliable performance, lack volumetric performance because of their inherently low density and moderate capacitances. In this paper, we report a high-performing electrode based on conductive hexaaminobenzene (HAB)-derived two-dimensional metal-organic frameworks (MOFs). In addition to possessing a high packing density and hierarchical porous structure, these MOFs also exhibit excellent chemical stability in both acidic and basic aqueous solutions, which is in sharp contrast to conventional MOFs. Submillimetre-thick pellets of HAB MOFs showed high volumetric capacitances up to 760 F cm-3 and high areal capacitances over 20 F cm-2. Furthermore, the HAB MOF electrodes exhibited highly reversible redox behaviours and good cycling stability with a capacitance retention of 90% after 12,000 cycles. In conclusion, these promising results demonstrate the potential of using redox-active conductive MOFs in energy-storage applications.
[en] Flexible supercapacitors with electrodes coated on inexpensive fabrics by the dipping technique. This paper present details of the design, fabrication and characterisation of fabric supercapacitor. The sandwich structured supercapacitors can achieve specific capacitances of 11.1F/g, area capacitance 105 mF.cm−2 and maintain 95% of the initial capacitance after cycling the device for more than 15000 times
[en] Highlights: • The performances degradation of supercapacitors during power cycling ageing tests have been quantified. • The performances recovery phenomena of supercapacitors is highlighted and modelled. • The impact of the rest conditions (temperature and cut-off voltage) on the recovery behaviour is quantified. • An accurate ageing model able to predict the performances degradation of supercapacitors in power cycling is proposed. - Abstract: During accelerated ageing tests of supercapacitors (SC), a decay in their performance is reflected by a decrease in capacitance and an increase in equivalent series resistance ESR. In power cycling, when electric solicitations of the SC are interrupted for the purposes of real use or characterisation, performance recovery is observed, mainly in terms of an increase in capacitance. This phenomenon is due to a redistribution of electrical charges, balancing of impurities inside the porous carbon electrodes, and the cell’s return to thermodynamically steady-state conditions. A repetitive long rest period during cycling appears to slow down the ageing process, and to reduce the decay in performance. The impacts on capacitance recovery during rest time, of both cut-off voltage and temperature, are studied. A nonlinear analytical expression is used to predict the capacitance decay for several durations and test interruption periodicities; this is also used to model the capacitance during rest time, taking the cut-off voltage, rest time and temperature into account
[en] Highlights: • Current-voltage reciprocity of an Rs-CPE-equivalent supercapacitor is verified. • The accumulated charge on a supercapacitor depends on the charging waveform. • Effective capacitances due to voltage and current charging are derived and compared. • Voltage-charging of a supercapacitor is more efficient than current-charging. - Abstract: The focus in supercapacitor research typically falls into one of two categories: (i) the rational design and engineering of electrode materials and electrolyte formulation to achieve high performance devices at competitive costs, and (ii) the modeling of their resulting behavior in response to constant-current charging/discharging, cyclic voltammetry or impedance spectroscopy. However, less work has been dedicated to new ways for charging these devices. In this work we show that charging a supercapacitor, modeled as a constant phase element with a series resistor, using a linear voltage ramp results in higher stored charge and higher effective capacitance value than when using a linear current ramp. This is despite the reciprocity of the device, as we proved analytically. The theoretical analysis and numerical simulations are in excellent agreement with the experimental results carried out on a commercial supercapacitor. The findings can be viewed as a step towards finding the optimum charging waveforms for these devices that would maximize their effective capacitance.
[en] Herein, nanocage MnCo2O4 is synthesized by using a dual metal zeolitic imidazolate framework (ZIF, Mn-Co-ZIF) as both precursor and template, which shows great potential for high performance supercapacitor. The as-obtained MnCo2O4 exhibits hollow polyhedral nanostructure, that inherit from the dual Mn-Co-ZIF. Moreover, the nanocage MnCo2O4 is composed by numerous nanoparticles, which help to short ion diffusion pathway, provide porous structure, and collaborative electronic transmission. The MnCo2O4 electrodes exhibit excellent supercapacitor performances, that possess a high specific capacitance of 1763 F g−1 (1 A g−1), and a reversible capacitance of 840 F g−1 (10 A g−1). After 4500 cycles at 1 A g−1, the MnCo2O4 electrode shows a capacitance retention of 95%, which demonstrates its superior cycle stability. The nanocage MnCo2O4 exhibits good electrochemical properties, due to its unique hierarchical hollow structure can be maintained during the electrochemical cycling.
[en] Highlights: • Graphene-AgVO3 nanocomposites were synthesized for supercapacitor application. • The specific capacitance, cycle stability, and rate capability can be enhanced. • Both the power and energy densities could be improved with the incorporation of AgVO3.
[en] Highlights: • Pyridinic–rich nitrogen-doped graphene nanoplatelets (PRGOs) are prepared. • The nitrogen content in PRGOs is readily controllable. • PRGOs show supercapacitors with high capacity and cycling stability. - Abstract: Pyridinic–rich nitrogen-doped graphene nanoplatelets (PRGOs) were synthesized using an acid–catalyzed dehydration reaction between GO and 3,4-diaminopyridine. Specific pyridinic nitrogen configurations can be efficiently introduced to graphitic network through newly formed imine and pyrizaine linkages. The structure of PRGOs was confirmed by various microscopic and spectroscopic analyses. In addition, the nitrogen content in PRGOs can be readily controlled over a wide range (1.9–9.1 wt.%) by simple change in the feed ratios of reactants. Owing to the simultaneous structural restoration and nitrogen doping onto graphitic structure, PRGOs show high performance of supercapacitors, including enhanced specific capacitance and long-term stability. Furthermore, the effect of nitrogen contents on specific capacitance has also been investigated to optimize doping levels of nitrogen atoms in PRGOs. The highest specific capacitance of 214 F g−1 at current density of 0.1 A g−1 has been achieved from PRGO-5 with a moderate nitrogen content of 5.3 wt.%, due to its balanced electric double layer capacitance and pseudocapacitance.
[en] Graphene oxide (GO)/polyaniline (PANI) composite nanoflakes were fabricated by chemical oxidative polymerization of aniline onto GO nanoflakes. The electrochromic properties and the electrochemical capacitive behaviors of the GO/PANI composite nanoflake film were investigated. The GO/PANI nanocomposite film exhibits excellent electrochromic and supercapacitive performances. In comparison with neat PANI film, the GO/PANI composite nanoflake film shows larger optical modulation, faster response speed, larger coloration efficiency and better cycling stability. Moreover, an areal capacitance of (137 mF/cm2) is obtained for GO/PANI nanoflake film at a scanning rate of 5 mV/s, higher than that of neat PANI film (36 mF/cm2). The GO/PANI composite nanoflake film also shows enhanced electrochromic and supercapacitive durability. And the electrochromism also could be utilized as an indicator for the capacitance. The high-performance GO/PANI nanocomposite film shows promising features for multifunctional devices combining energy storage and electrochromism.
[en] Highlights: • Nanoporous MnO2 nanoflakes on carbon cloth by electrochemical deposition. • Enlarged potential window for MnO2/CC (0–1.1 V) and MnO2//rGO (0–2.2 V). • Structure water enhance the long cycle stability. • Large energy density of 49.8 Wh/kg for MnO2//rGO supercapacitors. Manganese oxide (MnO2) has been comprehensively studied as one high-voltage electrode material in the neutral aqueous supercapacitors, whereas the working potential window (WPW) for the device hasn’t exceeded 2.0 V in most reports. Here, interlaced ultrathin MnO2 nanoflakes were vertically aligned on the carbon cloth (CC) via a facial potentiostatic electrochemical deposition method. A stable WPW for the MnO2/CC electrodes in three electrolytes of 0.5 M A2SO4 (A = Li, Na, K) was defined as 0–1.1 V, which was attributed to the nanoporous morphology of the MnO2 nanosheets and high content of structure water within it. Furthermore, the MnO2 nanoflakes with 0.5 M Na2SO4 have achieved a high specific capacitance of 272.2 F/g at 2 mV/s, and the retention of the performance was 83.8% (5 A/g) after 5000 cycling tests. As for the flexible asymmetric MnO2 supercapacitors (FAMSC), the MnO2/CC electrode was used as a cathode and a self-assembled rGO film was developed as an anode. Working in an extended WPW of 0–2.2 V, the FASMC exhibited a large energy density of 49.8 Wh/kg and the retention of the performance was 88.7% at 5 A/g after 5000 cycles. This work provides a stable WPW for the MnO2 electrodes, which may promote further development of the high-voltage neutral aqueous supercapacitors.