Non-porous activated (mesophase carbon microbeads) MCMBs have been prepared by calcinations at 1100 °C and then activated with KOH at 800 °C in a nitrogen flow. This spherical carbon material could store a large number of ions after having been subjected to an “extra” electrochemical activation. It has been used as the negative electrode for asymmetric electrochemical capacitors of MCMB/activated carbon. The charge storage mechanism at the MCMB negative electrode has been investigated by XRD and TEM. The results demonstrate that the intercalation of quaternary alkyl ammonium cations from the electrolytes causes an irreversible expansion of interlayer spaces between adjacent carbon layers, which provides accommodation for subsequent ion adsorption. A series of quaternary alkyl ammonium-based electrolytes have been tested in the capacitors and the effect of the cation has been studied. Spiro-(1,1′)-bipyrrolidinium increases the storage ability most significantly although it is not the lightest quaternary alkyl ammonium cation.Highlights► Non-porous activated MCMB as a negative electrode in electrochemical capacitors. ► It elevates both energy and power densities. ► The initial cation intercalation into MCMB causes irreversible structure change. ► Spiro-(1,1′)-bipyrrolidinium enlarges the storage capability of MCMB remarkably.

•A facile and scalable chemical synthesis strategy is proposed.•The NiCo2O4 materials display a high surface area and porosity.•The NiCo2O4 electrode shows a high specific capacitance and rate capability.•The AC-NiCo2O4 capacitor exhibits a high Ragone behavior and high cycling stability.Highly porous nickel cobaltite (NiCo2O4) materials have been synthesized via a facile and scalable chemical synthesis route. The obtained NiCo2O4 material displays a typical secondary submicron/micron-sized (0.1–2 μm) agglomerate morphology, exhibiting large surface area (190.1 m2 g−1) and high porosity (1.136 cm3 g−1). The fabricated NiCo2O4 electrode shows high specific capacitance (351 F g−1 at 1 A g−1) and high-rate capability (82.1% capacitance retention at 8 A g−1), which is superior to many reported NiCo2O4 materials. Further, the assembled AC-NiCo2O4 aqueous hybrid capacitor exhibits high power and energy densities (2805 W kg−1, 6.8 Wh kg−1 at 8 A g−1) and high cycling stability (15% loss after 5000 cycles at 1.5 A g−1). The high-performance of the NiCo2O4 materials is attributed to their large surface area and highly porous structure which contribute to rich surface electroactive sites and easy ions transport pathways for facile electrochemical reactions.

Mesoporous nickel cobaltite (NiCo2O4) nanoparticles were synthesized via a hydrothermal and soft-templating method through quasi-reverse-micelle mechanism. The physicochemical properties of the NiCo2O4 materials were characterized via X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectra, and nitrogen sorption isotherms measurements. The electrochemical performances of the NiCo2O4 electrode were investigated by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy tests. The obtained NiCo2O4 materials exhibit typical mesoporous structures, with an average particle size of about 200 nm, a specific surface area of 88.63 m2 g−1, and a total pore volume of 0.337 cm3 g−1. The facile electrolytes penetration for the mesoporous structures favors high-performance of the NiCo2O4 electrode. The NiCo2O4 electrode shows a high specific capacitance (591 F g−1 at 1 A g−1), high-rate capability (248 F g−1 at 20 A g−1), and a good cycling behavior for tested 3,000 cycles, indicating a promising application for electrochemical capacitors.

The lithium-based energy storage technology is currently being considered for electric automotive industry and even electric grid storage. However, the hungry demand for vast energy sources in the modern society will conflict with the shortage of lithium resources on the earth. The first alternative choice may be sodium-related materials. Herein, we propose an electric energy storage system (sodium-ion capacitor) based on porous carbon and sodium titanate nanotubes (Na-TNT, Na+-insertion compounds) as positive and negative electrode materials, respectively, in conjunction with Na+-containing non-aqueous electrolytes. As a low-voltage (0.1–2 V) sodium insertion nanomaterial, Na-TNT was synthesized via a simple hydrothermal reaction. Compared with bulk sodium titanate, the predominance of Na-TNT is the excellent rate performance, which exactly caters to the need for electrochemical capacitors. The sodium-ion capacitors exhibited desirable energy density and power density (34 Wh kg–1, 889 W kg–1). Furthermore, the sodium-ion capacitors had long cycling life (1000 cycles) and high coulombic efficiency (≈ 98 % after the second cycle). More importantly, the conception of sodium-ion capacitor has been put forward.Keywords: capacitors; materials; nanotubes; negative electrode; sodium titanate; sodium-ion;

As a promising antifreeze, Ethylene glycol (EG) was employed for preparation of new types of antifreezing Ag/AgCl reference electrodes. Different proportions of EG were used as antifreezing additives in the inner electrolyte of Ag/AgCl reference electrode. The potential shifts of the resultant reference electrodes are less than 10 mV in the wide range of temperatures (25 ∼ −40 °C), which extends their application at low temperatures. In addition, the proposed antifreezing electrodes maintain non-polarizable and reversible nature of the initial Ag/AgCl ones even in low-temperature environment. Finally, to ensure their stability at low temperatures, the antifreezing reference electrodes were applied in various low-temperature electrochemical systems.Highlights► Antifreezing Ag/AgCl reference electrode was fabricated. ► Ethylene glycol was adopted as antifreezing additive for electrode’s electrolyte. ► The antifreezing reference electrode possesses constant potential even at low temperature. ► The reference electrode can be applied in low-temperature electrochemical system.

Besides presenting the intrinsic characteristics of manganese dioxide electrode materials, the worldwide research progress in their preparation and application for electrochemical capacitor are introduced in this article. Moreover, the future development trend and prospects are discussed.

Hierarchical porous nickel cobaltite (NiCo2O4) nanomaterials were synthesized via a hard-templating route. The obtained materials consist of nanostructured cubic NiCo2O4 spinels and a spot of cubic NiO nanoparticles, and the materials display a typical hierarchical porous structure. The NiCo2O4 electrode displays quasireversible dynamics characteristics, mainly Faradaic capacitance behavior and capacitance relaxation feature. The NiCo2O4 electrode exhibits an excellent long cycling behavior with no capacitance decays during 5,000 cycles at a current density of 2 A g−1 in 1 M KOH electrolytes, and the NiCo2O4 electrode exhibits both high power and energy performances even after 5,000 cycles with respective value of 1,758 W kg−1 and 8.3 W h kg−1 in 1 M KOH electrolytes, indicating that the NiCo2O4 nanomaterials are promising candidates for electrochemical capacitors.

A mild hydrolysis method was proposed to synthesize TiO2 nanoparticles (NPs), in which polyoxometalate adjusted the hydrolyzation reaction rate and controlled the size of TiO2. The resultant TiO2 NPs were fully characterized via XRD, SEM, TEM, XPS. The electrochemical investigations demonstrated that the as-obtained TiO2 NPs delivered higher specific capacitance than that of bulk and commercial samples in lithium-ion aqueous solution. Furthermore, the charge storage mechanism of TiO2 NPs was also studied. More importantly, the aqueous hybrid capacitors based on activated carbon (AC) and TiO2 were constructed and their electrochemical performance were fully investigated in aqueous solutions.Keywords: electrochemical capacitors; hybrid; nanoparticles; polyoxometalate; TiO2;

Concentrated NaClO4 aqueous solutions have been proposed as the electrolytes in electric double-layer capacitors. The advantages of this kind of electrolytes have been addressed in the terms of enlarged specific capacitance, enhanced rate capability and elevated low-temperature performance of porous carbon electrodes. Thermal analysis, ionic conductivity measurement and Raman spectroscopic investigations have been performed on the NaClO4 aqueous solutions in conjunction with the electrochemical study of porous carbon electrodes in this kind of electrolytes. The correlation between the hydration number of ions in the solutions and capacitive behavior of porous carbon has been clarified. The rate performance improvement in porous carbon electrode has also been connected to the increase in ionic conductivity of the electrolytes. The enhanced capacitance retention of porous carbon electrode at low temperatures in concentrated solutions has been ascribed to a fall of freezing point.Graphical abstract.Research highlights▶ Concentrated NaClO4 aqueous solutions as electrolytes for electric double-layer capacitors (EDLCs). ▶ High ionic conductivity leads to the elevation of rate capability. ▶ Low hydration numbers cause the increase in specific capacitance. ▶ Low freezing points guarantee EDLCs work well in cold environments.

The electrolyte salts composed of tetramethylammonium (TMA+) cation and difluoro(oxalato)borate (DFOB−) or bis(oxalato)borate (BOB−) anions have been proposed for the application in activated carbon (AC)/graphite capacitors. The electrochemical performance of AC/graphite capacitors has been studied using these electrolyte salts dissolved in propylene carbonate (PC). The intercalation behaviors of anions (BF4−, DFOB−, and BOB−) at the graphite positive electrodes have been investigated by in situ XRD measurements. The bigger the anion is, the higher the cell voltage is where the intercalation happens. Accordingly, the bigger the anion is, the smaller discharge capacity delivered by an AC/graphite capacitor. The charge mechanism of TMA+ at the AC negative side has also been addressed. Compared with other bigger quaternary alkyl ammonium cations, the specific capacitance of the AC negative electrode towards TMA+ adsorption is somehow smaller as estimated.Highlights► (oxalato)borate-based electrolyte salts for activated carbon/graphite capacitors. ► Bigger anions are difficult to be intercalated into graphite electrodes smoothly. ► PC-solvated tetramethylammonium cation is adsorbed at activated carbon electrodes.

Ethylene carbonate molecules in electrolyte solutions can bind tightly with PF6− anions, and prevent the intercalation of these anions into the interlayer spaces of graphite positive electrodes in activated carbon/graphite capacitors.

KPF6 dissolved in propylene carbonate (PC) has been proposed as an electrolyte for activated carbon (AC)/graphite capacitors. The electrochemical performance of AC/graphite capacitor has been tested in XPF6-PC or XBF4-PC electrolytes (X stands for alkali or quaternary alkyl ammonium cations). The AC/graphite capacitor using KPF6-PC electrolyte shows an excellent cycle-ability compared with other electrolytes containing alkali ions. The big decomposition of the PC solvent at the AC negative electrode is considerably suppressed in the case of KPF6-PC, which fact has been correlated with the mild solvation of K+ by PC solvent. The relationship between the ionic radius of cation and the corresponding specific capacitance of AC negative electrode also proves that PC-solvated K+ ions are adsorbed on AC electrode instead of naked K+ ions.

The capacitive characteristics of micro- and meso-porous carbon materials have been compared in cyclic voltammetric studies and galvanostatic charge–discharge tests. Meso-porous carbon can keep certain high capacitance values at high scan rates, whereas micro-porous carbon possesses very high capacitance values at low scan rates but fades quickly as the scan rate rises up. For better performance of electric double-layer capacitors (EDLCs), the cooperative application of both kinds of carbon materials has been proposed in the following two ways: mixing both kinds of carbons in the same electrode or using the asymmetric configuration of carbon electrodes in the same EDLC. The cooperative effect on the electrochemical performance has also been addressed.

Ethylene carbonate molecules in electrolyte solutions can bind tightly with PF6− anions, and prevent the intercalation of these anions into the interlayer spaces of graphite positive electrodes in activated carbon/graphite capacitors.