A giant enhancement of capacitance in graphene supercapacitors by introducing on their electrodes recycled hydroxyapatite from bovine bone

Abstract

This study investigates the use of hydroxyapatite (HAp) from bovine bone waste (HAp-N) to produce supercapacitors (SC). For this purpose, graphene (G) electrodes were coated with a layer of the extracted HAp-N powder. With these G + HAp-N electrodes, the fabricated SC device, referred to as G + HAp-N-SC, exhibited a capacitance of 652.65 F g⁻¹ and an energy density of 90.64 Wh kg⁻¹. To see the effect of using the natural HAp, a second SC device was made with synthetic HAp (HAp-S), which was deposited on the SC electrodes. This device was named G + HAp-S-SC and achieved maximum capacitance of 526.37 F g⁻¹ and energy density of 73.10 Wh kg⁻¹. In addition, SCs made with HAp-N had 23 % higher capacitance than these made with HAp-S. In addition, HAp-N was combined with Vanadium Oxide (V₂O₅, VO) in a physical mixture of 50 wt%. The HAp-N/VO mixture was deposited on the G electrodes and the SC labeled as G + HAp-N-VO-SC was fabricated. This third device showed an exceptional capacitance of 1698.02 F g⁻¹ and an excellent energy density of 235.83 Wh kg⁻¹. The G + HAp-N-VO-SC device made with VO had 160 % higher capacitance compared to the G + HAp-N-SC device. In addition, both HAp-N and VO contained abundant oxygen vacancy defects, which acted as electron traps according to UV–Vis and Raman measurements. In addition, XPS analysis revealed that the VO with different oxidation states of 5+, 4+ and 3+ contributed to the storage of charge through redox reactions. Interestingly, the thermochromic property of the VO allowed the operation of G + HAp-N-VO-SC device at a high temperature of 80 °C. In fact, under temperature of 80 °C, the device retained 91 % of its initial capacitance after 1000 charge/discharge cycles, while the capacitance retention at room temperature (RT), was 95 %. Meanwhile, 92 % of the primary capacitance of the G + HAp-N-VO-SC device was retained after 1000 bending cycles at RT. Overall, the direct use of HAp from bovine bone waste for the fabrication of SCs is promising for the development of low-cost energy storage devices. In fact, the high energy density obtained from the HAp based SCs (which are compact and flexible) suggests that they could be used as energy source for wearable electronics.