Bio-Waste Argan Nut Shell-Derived Porous Activated Carbon for High Performance Supercapacitor Electrode Material
Authors: EL-Ouahabi Hajar , Khaddor Mohamed , Lillo-Rodenas Maria Angeles , Roman Martinez Maria DEL Carmen , Elmouwahidi Abdelhakim , Pérez-Cadenas Agustín F. , Ouzzine Mohamed 
Published: September 2023

The supercapacitors (SCs) have become a great candidate for high power applications in energy storage units which bridge the gap between conventional capacitors and batteries fuels cells. This is because of their relatively higher specific energy compared to normal parallel plate capacitors, and higher power density and longer cycling stability, fast charge–discharge time, low level of heating, appropriate dimension/weight and low cost. The Carbon materials are considered as ideal electrode materials for supercapacitors, they play an important role in energy storage devices due to their large specific surface area, tunable pore size, high electrical conductivity and strong mechanical property, nevertheless most of the commercial activated carbons are produced from fossil fuel-based precursor (petroleum and coal) which made them to be expensive and environmentally non-friendly. hence the increasing focus on biomass precursors which are cheaper, readily available, renewable, structurally porous and environmentally friendly. Agriculture production and agro-industrial processes generate a large amount of waste. That can potentially be used to obtain important functional materials. Thus, it is possible to prepare activated carbon with high specific surface from various type of biomass.

Taking all this into account, our work has focused on the use of Argan residues, very abundant in Morocco, for the production of activated carbons with developed porosity. The resulting material was evaluated as electrode material for the supercapacitor, Valuable strategies are the use of aqueous electrolytes and activated carbon that enable the valorization of wastes in a circular economy approach.

The Argan residues (ANS, Argan Nut Shell) were ground to obtain a particle size of 1-3 mm The ACs were prepared by chemical activation with KOH in two steps: carbonization of the precursor followed by activation of the biochar. In the two-stage process, carbonization was carried out at 500 °C and in all cases, KOH is used in lentils. The following process variables were studied: activation temperature (700, 800 and 850 °C) and KOH/AS ratio (3/1 and 4/1). After the heat treatment, the samples were washed with 5 M HCl solution until neutral pH and dried (110 °C, 12 h). Activated carbons were characterized by N2 -196 °C and CO2 adsorption at 0 °C, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).

The symmetric SC was assembled with the two identical electrodes (positive/negative) by sandwiching a filter paper as a separator (Whatman). Then, few drops of aqueous 1M H2SO4 dropped in between the electrodes. The electrochemical techniques used were Cyclic voltammetry and galvanostatic charge/discharge cycling. Cyclic voltammetry (CV) was scanned from 5 to 80 mV/s. Galvanostatic charge−discharge experiment was performed with a specific current from 0.125 to 12A.g-1.

The study showed that the new mechanism used to produce the activated carbon from Argan Nut shell lead to high surface area. The optimum activated carbon obtained by operating carbonization temperature as 500°C and activation temperature at 800°C with KOH activation, with these conditions the results were (SBET =3091 m2.g-1, VT=1.52 cm3.g-1), In contrast, the symmetric two electrode system exhibits outstanding charge storage capacity, reaching a high specific capacitance ranging between 512-320 F.g−1 at 0.125 A g−1. Remarkable electrochemical performance is further demonstrated using Argan Nut Shell, with an excellent rate performance after 2500 cycles, the capacitance still remained at 92 %, indicating great cycle stability.

This study demonstrated the interesting nature of Argan Nut shell to develop cost effective electrode materials for supercapacitors with high performance. To conclude, high surface area and porosity of the prepared ACs can serve as a flexible backbone for constructing wearable energy storage devices. Clearly, using biomasses are definitely the right track towards making renewable carbon materials and energy storage.

Figure 1