Onion-derived activated carbons with enhanced surface area for improved hydrogen storage and electrochemical energy application

High surface area activated carbons (ACs) were prepared from a hydrochar derived from waste onion peels. The resulting ACs had a unique graphene-like nanosheet morphology. The presence of N (0.7%) and O content (8.1%) in the OPAC-800 degrees C was indicative ofin situincorporation of nitrogen groups from the onion peels. The specific surface area and pore volume of the best OPAC sample was found to be 3150 m(2)g(-1)and 1.64 cm(3)g(-1), respectively. The hydrogen uptake of all the OPAC samples was established to be above 3 wt% (at 77 K and 1 bar) with the highest being 3.67 wt% (800 degrees C). Additionally, the OPAC-800 degrees C achieved a specific capacitance of 169 F g(-1)at a specific current of 0.5 A g(-1)and retained a capacitance of 149 F g(-1)at 5 A g(-1)in a three electrode system using 3 M KNO3. A symmetric supercapacitor based on the OPAC-800 degrees C//OPAC-800 degrees C electrode provided a capacitance of 158 F g(-1)at 0.5 A g(-1). The maximum specific energy and power was found to be 14 W h kg(-1)and 400 W kg(-1), respectively. Moreover, the device exhibited a high coulombic efficiency of 99.85% at 5 A g(-1)after 10 000 cycles. The results suggested that the high surface area graphene-like carbon nanostructures are excellent materials for enhanced hydrogen storage and supercapacitor applications.