Damilola Y. Momodu

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.

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.

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.

Solvothermal synthesis of NiAl double hydroxide microspheres on a nickel foam-graphene as an electrode material for pseudo-capacitors

In this paper, we demonstrate excellent pseudo-capacitance behavior of nickel-aluminum double hydroxide microspheres (NiAl DHM) synthesized by a facile solvothermal technique using tertbutanol as a structure-directing agent on nickel foam-graphene (NF-G) current collector as compared to use of nickel foam current collector alone. The structure and surface morphology were studied by X-ray diffraction analysis, Raman spectroscopy and scanning and transmission electron microscopies respectively. NF-G current collector was fabricated by chemical vapor deposition followed by an ex situ coating method of NiAl DHM active material which forms a composite electrode. The pseudocapacitive performance of the composite electrode was investigated by cyclic voltammetry, constant charge–discharge and electrochemical impedance spectroscopy measurements. The composite electrode with the NF-G current collector exhibits an enhanced electrochemical performance due to the presence of the conductive graphene layer on the nickel foam and gives a specific capacitance of 1252 F g−1 at a current density of 1 A g−1 and a capacitive retention of about 97% after 1000 charge–discharge cycles. This shows that these composites are promising electrode materials for energy storage devices.

Growth of graphene underlayers by chemical vapor deposition

We present a simple and very convincing approach to visualizing that subsequent layers of graphene grow between the existing monolayer graphene and the copper catalyst in chemical vapor deposition (CVD). Graphene samples were grown by CVD and then transferred onto glass substrates by the bubbling method in two ways, either direct-transfer (DT) to yield poly (methyl methacrylate) (PMMA)/graphene/glass or (2) inverted transfer (IT) to yield graphene/PMMA/glass. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) were used to reveal surface features for both the DT and IT samples. The results from FE-SEM and AFM topographic analyses of the surfaces revealed the underlayer growth of subsequent layers. The subsequent layers in the IT samples are visualized as 3D structures, where the smaller graphene layers lie above the larger layers stacked in a concentric manner. The results support the formation of the so-called “inverted wedding cake” stacking in multilayer graphene growth.