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In this work, Mn₂P₂O₇ nanoparticles were synthesized via hydrothermal route without any templates or surfactants followed by heat treatment at 700${}^{\circ }$C. The as-prepared samples were characterized and described using thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FT-IR), Scanning electron microscopy (SEM) analysis. The resultant product was evaluated for electrochemical properties in organic electrolyte between −1.4 and 1.6 V using cyclic voltammetry in ambient condition. It revealed the specific capacitance of 565 F/g at scan rate of 5 mV/s. The outstanding pseudocapacitive performance was absorbed due to the faradaic oxidation and reduction reactions related to the intercalation/de-intercalation of the tetrabutylammonium cation (TBA${}^{+})$ electrolyte and inorganic pyrophosphate lattice. It was believed that the cost effective Mn₂P₂O₇ nanoparticles may be promising electrode materials for electrochemical capacitors.

A capacitive negative electrode material of Fe₃N has been synthesized via a simple precursor ammoniation treatment. Electrochemical measure shows that the electrode exhibits a specific capacitance of 270F$\cdot$g${}^{-1}$ at a current density of 0.5A$\cdot$g${}^{-1}$ in 6M KOH, 66% of the initial value remains at a specific current density of 5A$\cdot$g${}^{-1}$. Capacitance retention after 5000 cycles is about 61.5% of the initial capacitance. The electrochemical performance is relatively great in comparison with the literature reports of some Fe-based compounds. Because of the low cost and simple preparation method of Fe₃N, the material is promising in electrochemical capacitive energy storage applications.

Multi-element doped porous carbon materials are considered as one of the most promising electrode materials for supercapacitors due to their large specific surface area, abundant mesoporous structure, heteroatom doping and good conductivity. Herein, we propose a very simple and effective strategy to prepare nitrogen, sulfur co-doped hierarchical porous carbons (N-S-HPC) by one-step pyrolysis strategy. The effect of sole dopants as a precursor was a major factor in the transformation process. The optimized N-S-HPC-2 possesses a typical hierarchically porous framework (micropores, mesopores and macropores) with a large specific surface area (1284.87m² g${}^{-1})$ and N (4.63 atomic %), S (0.53 atomic %) doping. As a result, the N-S-HPC-2 exhibits excellent charge storage capacity with a high gravimetric capacitance of 360F g${}^{-1}$ (1 A g${}^{-1})$ in three-electrode systems and 178F g${}^{-1}$ in two-electrode system and long-term cycling life with 87% retention after 10,000 cycles in KOH electrolyte.

The biggest challenge for today’s supercapacitor systems readily possessing high power density is their low energy density. Their electrode materials with controllable structure, specific surface area, electronic conductivity, and oxidation state, have long been highlighted. Architecture engineering of functional electrode materials toward powerful supercapacitor systems is becoming a big fashion in the community. The construction of ion-accessible tunnel structures can microscopically increase the specific capacitance and materials utilization; stiff 3D structures with high specific surface area can macroscopically assure high specific capacitance. Many exciting findings in electrode materials mainly focus on the construction of ice-folded graphene paper, in situ functionalized graphene, in situ crystallizing colloidal ionic particles and polymorphic metal oxides. This feature paper highlights some recent architecture engineering strategies toward high-energy supercapacitor electrode systems, including electric double-layer capacitance (EDLC) and pseudocapacitance.

A porous organic framework as electrode material for supercapacitor was prepared successfully. It exhibited the remarkable specific capacitance of $170.66\mathrm{\text{F}}\cdot {\mathrm{\text{g}}}^{-1}$ at current density of $1.0\mathrm{\text{A}}\cdot {\mathrm{\text{g}}}^{-1}$, and 100% of capacitance retention after successive charge–discharge 10,000 cycles in an alkaline aqueous electrolyte solution. At the same time, we explored the electrochemical performance of porous N-doped carbon derived from the template of porous organic framework as supercapacitor electrode material. Impressively, it possessed the higher specific capacitance and better cycling stability, which showed specific capacitance of $180.88\mathrm{\text{F}}\cdot {\mathrm{\text{g}}}^{-1}$ at the current density of $0.4\mathrm{\text{A}}\cdot {\mathrm{\text{g}}}^{-1}$ and retained 100% of initial capacitance after 30,000 cycles.

Currently, the use of redox-active additives is an effective way to enhance the electrochemical performance of supercapacitors. Despite a lot of publications in the literature, the choice of redox-active electrolyte composition is still unclear. This work focuses on the study of the correlation between the K₄Fe(CN)₆ fraction (0–0.05 M) in 1 M Na₂SO₄ and electrochemical characteristics of rice-husk-derived activated carbon (AC). The effect of electrolyte composition was clarified by cyclic voltammetry, galvanostatic charge/discharge and impedance spectroscopy. The increase in the K₄Fe(CN)₆ fraction in the electrolyte significantly improved the specific capacitance of AC from 59 up to 288 F/g at a current density of 1 A/g. Nevertheless, despite the fact that the concentrated 0.05 M K₄Fe(CN)₆–1 M Na₂SO₄ electrolyte provided excellent capacitance, it led to a high self-discharge and charge transfer resistance, not beneficial for electrochemical applications.

In this paper, we developed an intriguing ion-exchange reaction to synthesize Ni-doped titanate nanotubes (Ni-Ti-NTs), and utilized the sample as an appealing pseudocapacitive electrode for electrochemical capacitors (ECs). The energy storage mechanism of the resulting Ni-Ti-NTs electrode in alkaline KOH aqueous electrolyte was further tentatively proposed. Compared with the pristine titanate nanotubes (Ti-NTs), Ni-Ti-NTs with rich porous channels possessed much better electronic conductivity and larger specific surface area, rendering the electrons and OH⁻ ions easily contact the Ni species uniformly distributed on their surfaces for efficient energy storage at high rates. Remarkably, the SC of the Ni-Ti-NTs electrode increased with further cycling, and even reached a SC of 278 F g⁻¹ at a current density of 2 A g⁻¹ after 7500 charge–discharge cycles without relaxation. The prominent electrochemical performance demonstrated that the Ni-Ti-NTs would be a promising candidate and/or even excellent support for other electroactive materials for advanced ECs application.