The researchers from University of South Africa together with their colleagues from UNESCO African Chair for nano sciences and nano technologies developed a new type of hybrid super capacitor using waste – activated carbon for the electrode was received from effluent sludge. Creating cheap energy storage systems simultaneously with minimizing the amount of household waste difficult to dispose of are critical tasks for the regions, where the load on electricity grids is growing and reliable infrastructure is needed for RES and backup power supply.
The researchers used nickel-manganese phosphate NiMn(PO₄)₂ as a positive electrode. This material is capable of accumulating the charge due to reversible chemical processes with nickel and manganese; hence, it is interesting for the energy industry. However, it has a serious drawback – it badly conducts electrical current, due to which this material loses significant portion of its potential. To solve this problem, scientists combined phosphate with electrochemically split graphene. Such graphene conducts the current very well and closely contacts with the active substance; hence electrons and ions in the electrode move faster, and the material demonstrates better performance.
The experiments showed: this solution provides for a tangible benefit. The optimal electrode with added 50 mg of graphene reached the specific capacity of 822 coulomb per gram give 1 ampere per gram current density. To compare: phosphate without graphene has the capacity of 389 coulomb per gram, i.e., twice as little. Even given harder modes of operation, when charging and discharging are faster, the electrode preserves a significant part of its parameters. After 5,000 charge-discharge cycles given the current of 5 amperes per gram it maintains 71% of its initial capacity, while as the same material without graphene – only about 52%. The measurements also showed that the inherent losses were reduced: charge transfer resistance decreased down to approximately 1.5 ohm, which proves effective movement of current inside the electrode.
The scientists made the negative electrode of activated carbon received from effluents sludge of treatment facilities of Johannesburg and Pretoria. Usually, such sludge is considered to be a problematic waste requiring significant expenses for disposal. In this case it was treated with phosphoric acids and exposed to thermal treatment turning it into porous carbon material. Its specific surface area makes circa 134 m²/g, and pores size 2-5 nanometers assure good penetration of electrolyte fluid. Such electrode quickly accumulates and releases the charge, and maintains about 91% of its capacity after 5,000 cycles even given high current density.
When the scientists combined both electrodes in one appliance, they received asymmetric hybrid super capacitor capable of operating under 1.4 volts. In such configuration the appliance demonstrated the specific capacity of 220 coulomb per gram, maximum energy density of circa 40 W·h/kg and power density of up to 6.5 KW/kg. This is apparently higher than classic super capacitors, bringing such systems closer to batteries with maintaining their main advantage – quick charge/discharge and high resource.
Eventually, similar developments may result in more affordable and eco-friendly energy storage systems for transport, power grids and urban infrastructure simultaneously decreasing the environmental impact due to waste processing.
