Abstract
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 °C was indicative of in situ incorporation of nitrogen groups from the onion peels. The specific surface area and pore volume of the best OPAC sample was found to be 3150 m2 g−1 and 1.64 cm3 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 °C). Additionally, the OPAC-800 °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 °C//OPAC-800 °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.
Highlights
Carbon-based porous materials have widely been used for hydrogen storage as well as in electrochemical energy storage applications
Samples can be correlated to the onion peels sheet-like structures. This observation reinforces the postulation that the structure of the raw material as well as the activation conditions plays an important role in determining the morphology of the resulting activated carbons (ACs) materials
We can postulate that upon the pre-carbonization of the onion hydrochar, the carbon wall tends to expand and this process can be accelerated by the expulsion of carbon dioxide during the activation step to lead to the creation of the observed nanosheets that resembles graphene sheets
Summary
Carbon-based porous materials have widely been used for hydrogen storage as well as in electrochemical energy storage applications. Even though these materials can be derived from many different feedstocks,[1,2] the use of biomass is o en preferred because it is available, low cost and renewable.[2,3,4] Examples of biomass that have been used to generate activated carbons (ACs) are such as tree bark,[5] banana peel,[6] algae,[2] olive stones,[7] among many others. N-doped ACs have received great interest in electrochemical energy storage applications since N presence leads to enhancement of the speci c capacitance.[10,11] The carbonization of N-containing biomass is
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