The exponential growth rates of population density and the worldwide economy has required a significant investment in energy storage devices, particularly those which are portable and can be used for future flexible electronics.
To meet the increasing energy demands of a growing population, not only are new ways of creating the energy being devised, but so are new ways of storing this energy, and a team of Researchers from India have developed a hybrid nanomaterial composed of graphene and flower-shaped MoS2 nanostructures to store energy in a prototype supercapacitor.
As a result of an ever-expanding population and its associated energy consumption, there is a projection that the demand for energy in 2050 will exceed 40 terawatts (TW). Because of the requirements for a high amount of energy, new ways of producing renewable energy are being researched and implemented, as current non-renewable fuels will eventually run out.
Due to both the energy increase and nature of the produced energy, new materials are also being developed that can store this energy efficiently.
At present, such storage capabilities are not close to meeting the energy demands set out in future predictions. Current devices can only store 1% of renewable energy that storage devices do for fossil fuels.
As such, there is a great need to not only create materials which can store renewable energy, but to also produce materials with a real-world function that can rival non-renewable storage options, potentially as a variant of Li-ion and Na-air batteries that can hold renewable-produced energy.
The team of Researchers have created a hybrid nanomaterial composed of flower-like MoS2 nanostructures and 3D graphene heterostructures to be used as an active material in energy storage and transfer devices. The Researchers also tested and employed the material in a solid-state supercapacitor, where the 3D graphene-MoS2 material was used with a graphite current collector.
To create the active material, the Researchers first created MoS2 nanospheres through a hydrothermal method using ammonium molybdate and thiourea. A modified hydrothermal method was then utilized to deposit 3D graphene oxide onto a graphite electrode using a series of wet synthetic steps.
The MoS2 nanostructures were then also deposited onto the graphene sheets. To create the supercapacitor, the Researchers, alongside the electrodes, used a polyvinyl acetate (PVA) gel and a gel-soaked whatman filter paper as part of the internal components. A drying time of 12 hours was required for the device to be fully fabricated.
To characterize the active material, the Researchers employed a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. Cyclic voltammetry (CHI electrochemical workstation with a three-electrode system) and electrochemical impedance spectroscopy (EIS) were used to calculate the electrochemical performance of the material.
The flake-like nanoflower MoS2 morphologies decorated onto the graphene/graphite electrodes were found to exhibit excellent supercapacitance properties. Most notably the specific capacitance (Csp) of 169.37 F/g, energy density of 28.43 Wh/Kg and a power density of 10.18 W/Kg. The prototype supercapacitor device exhibited a Csp of 58.0 F/g, energy density of 24.59Wh/Kg, and power density of 8.8W/Kg, with dimensions of 23.6 x 22.4 x 0.6 mm3.
The Researchers connected the supercapacitors in series to demonstrate a real application using a light emitting diode (LED) that remained lit up for 40 seconds, even though it was only charged for 25 seconds. This specific study yielded a high overpotential window of around 2.7 V (−1.5 to +1.2 V) and negated the need for any expensive ionic liquid medium. Although, the Researchers stated that the overpotential window could potentially be improved by using an ionic-liquid gel-based electrolyte.
The fabricated supercapacitor device was thin and flexible in nature and showed outstanding performance, particularly when the current collector was incorporated next to the MoS2 nanoflowers. The excellent performance has been attributed to a combined effect between using MoS2 layers and the 3D graphene materials, and the material and device as a whole was found to be highly stable.
This material has shown a great potential for future high-performance energy storage/transfer devices. The fabricating method is of low cost and is easily scalable, promoting its possibility to be used in commercially focused applications, such as in solar energy storage devices.
Other applications could also be as a replacement for batteries in flexible and thin electronics. The research also opens the doors to exploring the possibilities for using other transition metal dichalcogenides on top of a 3D graphene matrix for use in supercapacitor devices.
“Three-dimensional Graphene with MoS2 Nanohybrid as Potential Energy Storage/Transfer Device”- Singh K., et al, Scientific Reports,2017, DOI:10.1038/s41598-017-09266-2