Boosting the energy density of graphene supercapacitors by using sustainable/natural black-iron-sand

Supercapacitor (SCs) were fabricated using black iron-sand (BS) as redox-powder (composed mainly of SiO 2 /magnetite). In addition, it was studied the effect of two electrolytes (seawater (SW) and PVA/H 3 PO 4 (AD) mixture) on the SC performance. Electrochemical characterizations revealed that the BS(AD)-SC device (made with acidic electrolyte and BS) reached a capacitance of 668.66 F/g and energy density of 92.7 Wh/kg, while the BS(SW)-SC device (made with seawater (SW) electrolyte and BS), produced capacitance and energy density of 282.24 F/g and 39.2 Wh/kg, respectively. Reference SCs were fabricated without iron-sand powder, but contained SW or AD electrolytes. Those devices produced much lower capacitances of 54–94.1 F/g. Thus, adding BS to SCs increased the capacitance by 6-11.3 times. According to XPS/Raman analyses, BS powder contained oxygen vacancy defects, Si 4+ and Fe²⁺/Fe³⁺ species, which worked as redox-centers for charge storage. Consequently, devices made with BS had CV with intense redox peaks, while devices made without BS had a quasi-rectangular shape (stored charge only by electronic-double layer). Moreover, diffusion coefficient was calculated for devices made with acidic and seawater electrolyte and obtained values of 9.51 × 10⁻⁸ cm²/s, and 4.57 × 10⁻⁸ cm²/s, respectively. Thus, the ion-transport/charge-storage in SCs electrodes was more efficient by using the acidic electrolyte. Also, electrochemical performance of the BS(AD)-SC device was evaluated at 70 °C. At this temperature, the capacitance was slightly higher (684.2 F/g) and this occurred because heating increased the mobility of ions through the BS-based electrodes. This research demonstrated that using black sand (natural/abundant resource) is promising to build low-cost supercapacitors.