Laser-induced graphene electrodes scribed onto novel carbon black-doped polyethersulfone membranes for flexible high-performance microsupercapacitors.

A facile and expandable methodology was successfully developed to fabricate laser-induced graphene from novel pristine aminated polyethersulfone (amPES) membranes. The as-prepared materials were applied as flexible electrodes for microsupercapacitors. The doping of amPES membranes with various weight percentages of carbon black (CB) microparticles was then performed to improve their energy storage performance. The lasing process allowed the formation of sulfur- and nitrogen-codoped graphene electrodes. The effect of electrolyte on the electrochemical performance of as-prepared electrodes was investigated and the specific capacitance was significantly enhanced in 0.5 M HClO4. Remarkably, the highest areal capacitance of 47.3 mF·cm-2 was achieved at a current density of 0.25 mA·cm-2. This capacitance is approximately 12.3 times higher than the average value for commonly used polyimide membranes. Furthermore, the energy and power densities were as high as 9.46 µWh·cm-2 and 0.3 mW·cm-2 at 0.25 mA·cm-2, respectively. The galvanostatic charge-discharge experiments confirmed the excellent performance and stability of amPES membranes during 5,000 cycles, where more than 100% of capacitance retention was achieved and the coulombic efficiency was improved up to 96.67%. Consequently, the fabricated CB-doped PES membranes offer several advantages including low carbon fingerprint, cost-effectiveness, high electrochemical performance and potential applications in wearable electronic systems.

Physicochemical characterizations of novel dicyanamide-based ionic liquids applied as electrolytes for supercapacitors.

Novel ionic liquids (ILs), containing a dicyanamide anion (DCA-), are synthesized and applied as suitable electrolytes for electrochemical double layer capacitors (EDLCs). The prepared ILs are either composed of triethyl-propargylammonium (N222pr +) or triethyl-butylammonium (N2224 +) cations paired with the DCA- anion. The structure of the cation influences its electrostatic interaction with the DCA- anion and highly impacts the physical and electrochemical properties of the as-prepared ILs. The geometry and the length of the alkyl chain of the propargyl group in N222pr + enhance the ionic conductivity of N222pr-DCA (11.68 mS cm-1) when compared to N2224-DCA (5.26 mS cm-1) at 298 K. It is demonstrated that the Vogel-Tammann-Fulcher model governs the variations of the transport properties investigated over the temperature range of 298-353 K. A maximum potential window of 3.29 V is obtained when N222pr-DCA is used as electrolyte in a graphene based symmetric EDLC system. Cyclic voltammetry and galvanostatic measurements confirm that both electrolytes exhibit an ideal capacitive behavior. The highest specific energy of 55 W h kg-1 is exhibited in the presence of N2224-DCA at a current density of 2.5 A g-1.

Structural in situ study of the thermal behavior of manganese dioxide materials: toward selected electrode materials for supercapacitors.

The thermal behavior of a series of MnO2 materials was investigated toward MnO2 microstructures under inert atmospheres. The byproduct formed during MnO2 heat treatments from the room temperature to 800 °C were characterized by in situ X-ray diffraction analyses. It was found that annealing spinel and ramsdellite phases caused the formation of MnO2 pyrolusite at 200 °C, Mn2O3, at 400 °C, and then Mn3O4 at higher temperatures. In the case of cryptomelane and birnessite phases, the heating process resulted in the formation of K0.51Mn0.93O2 at 600 °C, while Mn3O4 was also formed and still present up to 800 °C. Heat-treating Ni-todorokite and OMS-5 up to about 450 °C led to the formation of NiMn2O4 and NaxMnO2, respectively, and again Mn3O4 at higher temperatures. All of these structural transformations were correlated to resulting weight losses of MnO2 powders, measured by thermogravimetric analyses, during the heating process. Cyclic voltammetry measurements were performed in the presence of 0.5 M K2SO4 aqueous solution for annealed cryptomelane, K0.51Mn0.93O2, and Mn3O4-based electrodes. It was found that MnO2 cryptomelane is electrochemically stable upon heating. The long-term charge/discharge voltammetric cycling revealed that the specific capacitance of Mn3O4-based electrode is significantly improved from 14 F·g(-1) (after 20 cycles) to 123 F·g(-1) (after 500 cycles).

Microstructural effects on charge-storage properties in MnO2-based electrochemical supercapacitors.

The charge-storage mechanism in manganese dioxide (MnO2)-based electrochemical supercapacitors was investigated and discussed toward prepared MnO2 microstructures. The preparation of a series of MnO2 allotropic phases was performed by following dedicated synthetic routes. The resulting compounds are classified into three groups depending on their crystal structures based on 1D channels, 2D layers, or 3D interconnected tunnels. The 1D group includes pyrolusite, ramsdellite, cryptomelane, Ni-doped todorokite (Ni-todorokite), and OMS-5. The 2D and 3D groups are composed of birnessite and spinel, respectively. The prepared MnO2 powders were characterized using X-ray diffraction, scanning electron microscopy, the Brunauer-Emmett-Teller technique, cyclic voltammetry (CV), and electrochemical impedance spectroscopy. The influence of the MnO2 microstructure on the electrochemical performance of MnO2-based electrodes is commented on through the specific surface area and the electronic and ionic conductivities. It was demonstrated that the charge-storage mechanism in MnO2-based electrodes is mainly faradic rather than capacitive. The specific capacitance values are found to increase in the following order: pyrolusite (28 Fx g(-1)) < Ni-todorokite < ramsdellite < cryptomelane < OMS-5 < birnessite < spinel (241 Fx g(-1)). Thus, increasing the cavity size and connectivity results in the improvement of the electrochemical performance. In contrast with the usual assumption, the electrochemical performance of MnO2-based electrodes was not dependent on the specific surface area. The electronic conductivity was shown to have a limited impact as well. However, specific capacitances of MnO2 forms were strongly correlated with the corresponding ionic conductivities, which obviously rely on the microstructure. The CV experiments confirmed the good stability of all MnO2 phases during 500 charge/discharge cycles.