Abdulhakeem Bello

High-performance symmetric electrochemical capacitor based on graphene foam and nanostructured manganese oxide

We have fabricated a symmetric electrochemical capacitor with high energy and power densities based on a composite of graphene foam (GF) with ∼80 wt% of manganese oxide (MnO2) deposited by hydrothermal synthesis. Raman spectroscopy and X-ray diffraction measurements showed the presence of nanocrystalline MnO2 on the GF, while scanning and transmission electron microscopies showed needle-like manganese oxide coated and anchored onto the surface of graphene. Electrochemical measurements of the composite electrode gave a specific capacitance of 240 Fg−1 at a current density of 0.1 Ag−1 for symmetric supercapacitors using a two-electrode configuration. A maximum energy density of 8.3 Whkg−1 was obtained, with power density of 20 kWkg−1 and no capacitance loss after 1000 cycles. GF is an excellent support for pseudo-capacitive oxide materials such as MnO2, and the composite electrode provided a high energy density due to a combination of double-layer and redox capacitance mechanisms.

Renewable pine cone biomass derived carbon materials for supercapacitor application

The environmental degradation and hazard to human life caused by the depletion of fossils fuels and the urgent need for sustainable energy sources to meet the rising demand in energy has led to the exploration of novel materials that are environmentally friendly, low cost and less hazardous to human life for energy storage application using the green chemistry approach. Herein, we report on the transformation of the readily abundant pine cone biomass into porous carbon via KOH activation and carbonization at 800 °C as electrode materials for supercapacitors. The porous carbon material exhibited a mesoporous framework with a specific surface area of 1515 m2 g−1, a high voltage window of 2.0 V, a gravimetric capacitance of 137 F g−1, energy density of 19 W h kg−1 and excellent cyclability in neutral 1 M Na2SO4 electrolyte for a symmetric carbon/carbon electrode cell. The result shows that the material is robust and shows great promise with neutral electrolytes in high-performance energy-storage devices.

Asymmetric supercapacitor based on VS2 nanosheets and activated carbon materials

An asymmetric supercapacitor was fabricated using VS2 nanosheets as the positive electrode and activated carbon (AC) as the negative electrode, with a 6 M KOH solution as electrolyte. These materials were combined to maximize the specific capacitance and enlarge the potential window, therefore improving the energy density of the device. A specific capacitance of 155 F g−1 at 1 A g−1 with a maximum energy density as high as 42 W h kg−1 and a power density of 700 W kg−1 was obtained for the asymmetric supercapacitor within the voltage range of 0–1.4 V. The supercapacitor also exhibited good stability, with ∼99% capacitance retention and no capacitance loss after 5000 cycles at a current density of 2 A g−1.

High performance asymmetric supercapacitor based on molybdenum disulphide/graphene foam and activated carbon from expanded graphite.

Molybdenum disulphide which has a graphene-like single layer structure has excellent mechanical and electrical properties and unique morphology, which might be used with graphene foam as composite in supercapacitor applications. In this work, Molybdenum disulphide (MoS2)/graphene foam (GF) composites with different graphene foam loading were synthesized by the hydrothermal process to improve on specific capacitance of the composites. Asymmetric supercapacitor device was fabricated using the best performing MoS2/GF composite and activated carbon derived from expanded graphite (AEG) as positive and negative electrodes, respectively, in 6M KOH electrolyte. The asymmetric MoS2/GF//AEG device exhibited a maximum specific capacitance of 59Fg-1 at a current density of 1Ag-1 with maximum energy and power densities of 16Whkg-1 and 758Wkg-1, respectively. The supercapacitor also exhibited a good cyclic stability with 95% capacitance retention over 2000 constant charge-discharge cycles. The results obtained demonstrate the potential of MoS2/GF//AEG as a promising material for electrochemical energy storage application.

Asymmetric supercapacitor based on an α-MoO3 cathode and porous activated carbon anode materials

Low cost porous carbon materials were produced from cheap polymer materials and graphene foam materials which were tested as a negative electrode material in an asymmetric cell configuration with α-MoO3 as a positive electrode. These materials were paired to maximize the specific capacitance and to extend the potential window, hence improving the energy density of the device. The asymmetrical device exhibits significantly higher energy density of 16.75 W h kg−1 and a power density of 325 W kg−1.

Hydrothermal synthesis of manganese phosphate/graphene foam composite for electrochemical supercapacitor applications

Manganese phosphate (Mn3(PO4)2 hexagonal micro-rods and (Mn3(PO4)2 with different graphene foam (GF) mass loading up to 150mg were prepared by facile hydrothermal method. The characterization of the as-prepared samples proved the successful synthesis of Mn3(PO4)2 hexagonal micro-rods and Mn3(PO4)2/GF composites. It was observed that the specific capacitance of Mn3(PO4)2/GF composites with different GF mass loading increases with mass loading up to 100mg, and then decreases with increasing mass loading up to 150mg. The specific capacitance of Mn3(PO4)2/100mg GF electrode was calculated to be 270Fg-1 as compared to 41Fg-1 of the pristine sample at a current density of 0.5Ag-1 in a three-electrode cell configuration using 6M KOH. Furthermore, the electrochemical performance of the Mn3(PO4)2/100mg GF electrode was evaluated in a two-electrode asymmetric cell device where Mn3(PO4)2/100mg GF electrode was used as a positive electrode and activated carbon (AC) from coconut shell as a negative electrode. AC//Mn3(PO4)2/100mg GF asymmetric cell device was tested within the potential window of 0.0-1.4V, and showed excellent cycling stability with 96% capacitance retention over 10,000 galvanostatic charge-discharge cycles at a current density of 2Ag-1.

Investigation of different aqueous electrolytes on the electrochemical performance of activated carbon-based supercapacitors

In this study, porous activated carbons (AC) were synthesized by an environmentally friendly technique involving chemical activation and carbonization, with an in-depth experimental study carried out to understand the electrochemical behaviour in different aqueous electrolytes (KOH, LiCl, and Na2SO4). The electrochemical performance of the AC electrode was evaluated by different techniques such as cyclic voltammetry, galvanostatic charge/discharge and impedance spectroscopy. The results obtained demonstrate that the AC materials in different electrolytes exhibit unique double layer properties. In particular, the AC electrode tested in 6 M KOH showed the best electrochemical performance in terms of specific capacitance and efficiency. A specific capacitance of 129 F g−1 was obtained at 0.5 A g−1 with a corresponding solution resistance of 0.66 Ω in an operating voltage window of 0.8 V, with an efficiency of ∼100% at different current densities.

Effect of conductive additives to gel electrolytes on activated carbon-based supercapacitors

This article is focused on polymer based gel electrolyte due to the fact that polymers are cheap and can be used to achieve extended potential window for improved energy density of the supercapacitor devices when compared to aqueous electrolytes. Electrochemical characterization of a symmetric supercapacitor devices based on activated carbon in different polyvinyl alcohol (PVA) based gel electrolytes was carried out. The device exhibited a maximum energy density of 24 Wh kg−1 when carbon black was added to the gel electrolyte as conductive additive. The good energy density was correlated with the improved conductivity of the electrolyte medium which is favorable for fast ion transport in this relatively viscous environment. Most importantly, the device remained stable with no capacitance lost after 10,000 cycles.

Electrochemical performance of polypyrrole derived porous activated carbon-based symmetric supercapacitors in various electrolytes

The electrochemical performance of porous carbon prepared from the polymerization and carbonization of pyrrole is presented in this work. The produced carbon exhibited a high specific surface area and high mesopore volume that are desirable and beneficial for high capacitive performance. Symmetric supercapacitor devices fabricated from this carbon were tested in three different electrolytes (6 M KOH, 1 M NaNO3, and 1 M Na2SO4). Higher capacitive performance (specific capacitance of 131 F g−1) in the 1 M Na2SO4 medium was obtained compared to the other two electrolytes a with specific capacitance of 108 F g−1 in 6 M KOH and 94 F g−1 in 1 M NaNO3 respectively. The difference observed in capacitance in the three electrolytes is linked to the individual properties of the electrolytes which include the conductivity and different ion solvation sizes. A potentiostatic floating test at the maximum voltage for 140 h was used to study the stability of the devices and from the experimental data, a 7% capacitance decrease was observed in the 6 M KOH electrolyte which is related to the corrosive atmosphere and oxidation of the positive electrode. A decrease of 18% in capacitance was observed in 1 M NaNO3 with an increase in resistance and 1% capacitance decay was observed in 1 M Na2SO4 with no change in resistance value at the end of the floating test. These results suggest the good performance of the polypyrrole based activated carbon for symmetric supercapacitors in aqueous electrolytes in general with 1 M Na2SO4, in particular, showing excellent stability after floating.

Cycling and floating performance of symmetric supercapacitor derived from coconut shell biomass

The South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation of South Africa (Grant No 97994). F. Barzegar and D. Y. Momodu acknowledge the University of Pretoria for Postdoctoral fellowship support, while A. Bello acknowledges financial support from NRF through SARChI in Carbon Technology and Materials.