T.M. Masikhwa

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.

High performance asymmetric supercapacitor based on CoAl-LDH/GF and activated carbon from expanded graphite

An asymmetric supercapacitor fabricated with a CoAl-layered double hydroxide/graphene foam (LDH/GF) composite as the positive electrode and activated carbon derived from expanded graphite (AEG) as the negative electrode in aqueous 6 M KOH electrolyte is reported. This CoAl-LDH/GF//AEG cell achieved a specific capacitance of 101.4 F g−1 at a current density of 0.5 A g−1 with a maximum energy density as high as 28 W h kg−1 and a power density of 1420 W kg−1. Furthermore, the supercapacitor also exhibited an excellent cycling stability with ∼100% capacitance retention after 5000 charging–discharging cycles at a current density of 2 A g−1. The results obtained show the potential use of the CoAl-LDH/GF//AEG material as a suitable electrode for enhanced energy storage in supercapacitors.

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.

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.

A dilute Cu(Ni) alloy for synthesis of large-area Bernal stacked bilayer graphene using atmospheric pressure chemical vapour deposition

A bilayer graphene film obtained on copper (Cu) foil is known to have a significant fraction of non-Bernal (AB) stacking and on copper/nickel (Cu/Ni) thin films is known to grow over a large-area with AB stacking. In this study, annealed Cu foils for graphene growth were doped with small concentrations of Ni to obtain dilute Cu(Ni) alloys in which the hydrocarbon decomposition rate of Cu will be enhanced by Ni during synthesis of large-area AB-stacked bilayer graphene using atmospheric pressure chemical vapour deposition. The Ni doped concentration and the Ni homogeneous distribution in Cu foil were confirmed with inductively coupled plasma optical emission spectrometry and proton-induced X-ray emission. An electron backscatter diffraction map showed that Cu foils have a single (001) surface orientation which leads to a uniform growth rate on Cu surface in early stages of graphene growth and also leads to a uniform Ni surface concentration distribution through segregation kinetics. The increase in Ni surf...

High electrochemical performance of hybrid cobalt oxyhydroxide/nickel foam graphene

In this study, we report the in-situ hydrothermal synthesis of mesoporous nanosheets of cobalt oxyhydroxide (CoOOH) on nickel foam graphene (Ni-FG) substrate, obtained via atmospheric pressure chemical vapour deposition (AP-CVD). The produced composite were closely interlinked with Ni-FG, which enhances the synergistic effect between graphene and the metal hydroxide, CoOOH. It is motivating that the synthesized CoOOH on the Ni-FG substrate showed a homogenous coating of well-ordered intersected nanosheets with porous structure. The electrochemical properties of the material as electrode showed a maximum specific capacity of 199mAhg-1 with a capacity retention of 98% after 1000 cycling in a three electrode measurements.

A wafer-scale Bernal-stacked bilayer graphene film obtained on a dilute Cu (0.61 at% Ni) foil using atmospheric pressure chemical vapour deposition

A bilayer graphene film was synthesized on a dilute Cu (0.61 at% Ni) foil using atmospheric pressure chemical vapour deposition (AP-CVD). Atomic force microscopy average step height analysis, scanning electron microscopy micrographs and the Raman optical microscopy images and spectroscopy data supported by selected area electron diffraction data showed that the bilayer graphene film obtained on a dilute Cu (0.61 at% Ni) foil is of high-quality, continuous over a wafer-scale (scale of an entire foil) and mainly Bernal stacked. These data clearly showed the capability of a dilute Cu (0.61 at% Ni) foil for growing a wafer-scale bilayer graphene film. This capability of a dilute Cu (0.61 at% Ni) foil was ascribed primarily to the metal surface catalytic activity of Cu and Ni catalyst. A wafer-scale bilayer graphene film obtained on a dilute Cu (0.61 at% Ni) foil has a sheet resistance of 284 Ω sq−1 (measured using a four-point probe station). Time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy showed a high surface concentration of Ni in the dilute Cu (0.61 at% Ni) foil which altered the surface catalytic activity of the Cu to grow a wafer-scale bilayer graphene film.

Microwave-assisted synthesis of cobalt sulphide nanoparticle clusters on activated graphene foam for electrochemical supercapacitors

This work is based upon research supported by the South African Research Chairs Initiative (SARChI) of the Department of Science and Technology and National Research Foundation (NRF) of South Africa (Grant No. 61056). T.M. Masikhwa and M.J. Madito acknowledge financial support from the University of Pretoria and the NRF for their Postdoctoral fellowship.