Preparation of supercapacitor electrode materials from e-waste: eco-friendly Cu recovery from printed circuit board waste using reduced graphene oxide and upcycling to Cu/CuO@C

Abstract

Copper finds its application in nearly all electronic devices owing to its exceptional electrical and thermal characteristics essential for transmitting signals and dispersing heat. As renewable energy technologies like wind turbines and electric vehicles continue gaining prominence, the demand for copper steadily grows yearly. However, as these devices become obsolete after a few years of use, recovering copper from this discarded equipment is imperative, driven by environmental concerns and energy considerations. This research work investigates the utilization of reduced graphene oxide (rGO) in an aqueous medium for the concurrent leaching and sorption processes aimed at recovering copper from printed circuit board flakes. Copper recovery was conducted at a temperature of 25 to 80 °C and a pH of 4 to 10. The optimal conditions for leaching–sorption were identified as a pH of 7 and temperature of 60 °C, yielding a maximum of 82.9 mg g⁻¹ of rGO corresponding to 100% of Cu present in waste printed circuit boards (WPCBs). The recovered solid material serves a dual purpose, (i) for the recovery of copper as copper sulfate using 2 M H₂SO₄ and (ii) to transform into the Cu/CuO@C material through calcination at 500 °C. The prepared Cu/CuO@C offers a new perspective as a high-performance negative electrode material for supercapacitor applications. Cu/CuO@C exhibits superior performance with a high coulombic efficiency, validated through cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) profiles, and extended cycle testing at 3 A g⁻¹. Cu/CuO@C exhibited a specific capacitance of 432.5 F g⁻¹ at a specific current of 1 A g⁻¹ in the potential range of (−0.8) to 0 V in 1 M KOH. Furthermore, an asymmetric supercapacitor (ASC) is fabricated with activated carbon (AC) as a positive electrode and Cu/CuO@C as a negative electrode, which exhibits a specific capacitance of 88.8 F g_cell⁻¹ at a specific current of 1 A g⁻¹ with an operating voltage of 1.2 V and can provide an energy density of 17.8 W h kg⁻¹ in 1 M KOH electrolyte. This work paves the way for the preparation of capacitive electrode materials using e-waste under eco-friendly conditions.