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.

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1. Introduction

Electronic waste (e-waste) is the fastest-growing type of solid waste globally due to the increased usage and shortened lifetimes of electronic devices. The generation of electronic garbage increased by over 54.8 million tonnes (MT) globally in 2021, and it is predicted to reach 74.7 MT by 2030.¹ Out of which 9.3 MT of officially recorded electronic garbage were collected and recycled in 2019, accounting for 17.4% of all e-waste produced. Since 2014, this amount has increased by 1.8 MT, or about 0.4 MT each year. Nevertheless, at the same time, the total amount of e-waste generated increased by 9.2 MT.² Copper accounts for approximately 20% of the composition of electronic equipment due to its exceptional thermal conductivity, superb electrical conductivity, impressive ductility and malleability, utility as a corrosion-resistant material, and cost-effectiveness compared to alternative conductive materials.³ Considering the present circumstances, the demand for Cu is projected to surge by 275% to 350% by the year 2050,⁴ a level that cannot be met solely by primary sources in the long term. Consequently, it is imperative to investigate alternative sources for Cu production. Because of its higher Cu content, e-waste represents a notable secondary source for its extraction.⁵ Utilizing e-waste as a secondary source for Cu recycling offers environmental and energy-related benefits. Within the realm of e-waste, WPCBs represent the most intricate and potentially hazardous component, constituting roughly 4% of the total e-waste.⁶ These boards come in different varieties, including single-sided (with a single copper layer), double-sided (featuring two copper layers), and even multi-sided configurations.⁷ These WPCBs comprise a mixture of metals, organic polymers, and glass fibres.⁸

Cu recovery from its secondary source like e-waste was achieved by various methods, among which hydrometallurgy was identified to have numerous advantages over other methods. Hydrometallurgical recovery involves selective extraction of Cu from acidic leachate solutions⁹ or selective leaching using liquids or solid materials. Cu leaching from e-waste was performed using mineral acids like H₂SO₄,¹⁰ HNO₃,¹¹ and HCl¹² usually at higher concentrations. Mineral acid leaching is quantitative, but it suffers from poor selectivity. Alternatively, methods such as ammonia leaching,¹³ bioleaching,¹⁴ and organic acids such as citric acid,¹⁵ acetic acid,¹⁶etc., have been employed for copper leaching. However, these processes typically operate within a high pH range and yield lower copper recovery of an average of 57–75%. Amino acid glycine was also used to recover Cu by leaching from WPCB board powder.¹⁷ Glycine could leach Cu selectively at pH 10 and demonstrated that leaching with coordinating molecules could be a good strategy to recover metals from waste PCBs. Leaching of metal with all these small molecules was performed in an aqueous medium in a dissolved state, which resulted in a metal-dissolved solution as a product. However, leaching using solid materials has obvious advantages in terms of ease of separation, simultaneous leaching–sorption, and multiple-cycle usage of these materials.¹⁸ Our group has recently reported selective leaching of Cu from WPCBs using chitosan and phosphorylated cellulose under mild conditions,¹⁸ however, only 6–8% of Cu leaching was achieved using these biopolymers. Hydrogen peroxide is often used in combination with the chemical agents to improve Cu leaching yield from e-waste,¹⁹ however, it could not be used in combination with these biopolymers since it cleaves the polymer linkages. On the other hand, reduced graphene oxide (rGO) is a carbon material with abundant functional groups (epoxy, hydroxyl, and carboxylic acid), and has high thermal and chemical stability.²⁰ The oxygen groups present on rGO layers can bind the metal ions through coordination²¹ and enable their recovery, and due to the high chemical stability of rGO, this can be used in combination with oxidizing agents like H₂O₂. For the first time, we explored rGO for selective leaching of Cu from PCB waste utilizing its exotic properties. Furthermore, the recovered metal sorbed rGO was studied as the negative electrode material for the supercapacitor application.

There has been a surge in interest in energy storage devices as fossil fuel reserves continue to deplete rapidly. The exceptional performance of supercapacitors over the other devices is demonstrated by their high-power density, extended cycle life, high-rate capabilities, swift charging and discharging, wide working temperature range, safety, and lightweight design.²² Researchers are actively exploring electrode materials to advance the development of supercapacitors.²³

A collection of materials including metal sulphides like CoS₂, NiS,²⁴ various transition metal oxides such as MnO₂, NiO,²⁵ RuO₂,²⁶ Fe₂O₃,²⁷ Co₃O₄,²⁸ SnO₂,²⁹ MnO₂,³⁰ and CuO,³¹ and metal hydroxides like Co(OH)₂, Ni(OH)₂ (ref. 32 and 33) etc. have been investigated as electrode materials for high-power electrochemical pseudo capacitors.³⁴ Among the metal oxides, CuO is one of the most promising pseudo-capacitor electrode materials concerning its environmental compatibility, and cost-effectiveness.³⁵ Because of its low activation energy, CuO with a monoclinic structure is unique among transition metal oxides as it can promote electron transport between cations.³⁶ Most flexible fibre supercapacitors have predominantly utilized carbon-based materials such as activated carbon, carbon nanotubes, or carbon onions as electrode materials. There is an advantage of composites of transition metal oxide and carbon since carbon can help increase the electrical conductivity of metal oxides. Activated carbon (AC)^37,38 metal oxide composites,^39,40 and rGO composites^41,42 possessing specific capacitances in the range of 100–600 F g⁻¹ were explored as negative electrode materials in supercapacitor applications. Developing such promising composite materials from e-waste is advantageous from the perspective of both the environment and the circular economy.

To this end, rGO is employed for the recovery of Cu from printed circuit board e-waste. This extraction process involves simultaneous leaching and sorption within an aqueous medium at pH 7. The Cu(I)/Cu(II)@rGO obtained from the sorption experiment was subsequently treated with a 2 M H₂SO₄ solution to extract Cu as copper sulfate (CuSO₄). Furthermore, Cu(I)/Cu(II)@rGO was also used to prepare Cu(0) and CuO nanoparticle supported carbon (Cu/CuO@C) by calcination at 500 °C. The supercapacitor behaviour of Cu/CuO@C is investigated by electrochemical studies such as CV, and GCD in 1 M KOH as an electrolyte. Interestingly, a specific capacitance of 432.5 F g⁻¹ is achieved for Cu/CuO@C when cycled at 1 A g⁻¹ in 1.0 M KOH electrolyte with 86% capacitance retention after 4000 cycles.

2. Materials and methods

2.1. Materials

Graphite with 99% purity was procured from Research-Lab Fine Chem Industries, Mumbai, India. H₂SO₄ 98% extra pure, was purchased from RANKEM Avantor Performance Materials India Pvt Ltd, Maharashtra, India. KMnO₄ 99% pure, and HCl 35% extra pure were procured from LOBA Chemie Pvt Ltd, Mumbai, India. NaOH pellets, extra pure AR, 98%, were purchased from SRL, Chennai, India. Printed circuit board waste was collected from local mobile and laptop service centres. Poly(vinylidene fluoride) (PVDF), super-p carbon, and KOH pellets (extra pure AR, 98%) were purchased from SRL, Chennai, India. All the chemicals were used as received.

2.2. Experimental section

2.2.1 Synthesis of reduced graphene oxide. Reduced graphene oxide (rGO) was synthesized through a modified Hummers' method using natural graphite powder as the precursor.⁴³ Initially, 4 g of graphite was added to a round bottom flask placed in an ice bath, followed by the addition of 184 mL of concentrated H₂SO₄. Subsequently, 12 g of KMnO₄ was slowly introduced to the flask to initiate the oxidation process, ensuring the temperature remained below 10 °C. After an hour, the reaction temperature was allowed to rise to 45 °C and stirring continued for 2 hours. To the resulting mixture, 100 mL of distilled water was added, and the reaction was stirred at 95 °C for 12 hours. Upon completion, the reaction was cooled to room temperature, and 500 mL of distilled water was gradually added, along with 15 mL of 30% H₂O₂, causing the solution's color to change from dark brown to yellow. The mixture was then subjected to centrifugation to separate the solid materials, yielding reduced graphene oxide (rGO). To remove any residual metal ions, the obtained rGO was washed with 2 M HCl, followed by additional washing with distilled water until reaching a neutral pH of 7.

2.2.2 Simultaneous leaching and sorption of Cu(II) onto rGO. The discarded WPCBs from the used mobile phones were first separated and then treated with 10 M NaOH to remove the epoxy coverings. To leach out copper, 332 mg of epoxy coating removed waste PCB flakes and 1 g of rGO are added to a mixture of 38.8 mL of water at pH 7 and 1.2 mL (3 volume percentage) of 30% H₂O₂ at 60 °C. The reaction was continued for 48 hours. Following the reaction, the residue was separated by centrifugation and dried for 12 h at 80 °C in a hot air oven. The obtained solid material (Cu(I)/Cu(II)@rGO) was used for the stripping of Cu(II) and to prepare the anode material for the supercapacitor. The metals present in the solution and solids were analyzed by AAS, ICP-OES and UV-Vis spectrophotometry.

2.2.3 Stripping and quantification of recovered Cu(II). Cu(II) was stripped from Cu(I)/Cu(II)@rGO and recovered as CuSO₄. For stripping, 50 mg of Cu(I)/Cu(II)@rGO was added to 5 mL of 2 M H₂SO₄ and shaken for 2 h. Following the shaking, the solution was separated and quantified using UV-Vis spectrophotometry. The calibration curve was made by preparing a series of known concentrations of CuSO₄·5H₂O.

2.2.4 Synthesis of Cu/CuO@C from Cu(I)/Cu(II)@rGO. Cu(I)/Cu(II)@rGO was calcined at 500 °C in a muffle furnace for 1 h under ambient air conditions to obtain Cu/CuO@C. The obtained material was characterized using XRD, Raman spectroscopy, and SEM.

2.2.5 Electrode preparation. To investigate the efficiency of the synthesized Cu/CuO@C as an electrode material in a supercapacitor, 75 wt% Cu/CuO@C was mixed with 15 wt% of conductive super p carbon and 10 wt% of binder PVDF and ground with a mortar and pestle; a few drops of NMP solvent were also added to obtain a homogeneous slurry. The slurry was coated on a nickel foam (current collector) by the repeated brush coating method. The coated Ni foam electrodes were dried in a hot air oven overnight at 100 °C. The area and active mass of the coated electrodes were 1 cm² and 1.2 mg, respectively. The electrochemical performance of Cu/CuO@C was tested by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) cycles using a three-electrode system with Cu/CuO@C as the working electrode, Pt foil as the counter electrode, and Hg/HgO as the reference electrode. All the electrochemical studies were carried out using an aqueous solution of 1 M KOH as an electrolyte. An asymmetric supercapacitor of Cu/CuO@C‖AC was assembled in a coin cell (CR2032) with Cu/CuO@C as a negative electrode and activated carbon (AC) as a positive electrode with Whatman glass fiber filters as a separator soaked with an aqueous electrolyte of 1 M KOH.

2.3. Characterization

X-ray diffraction (XRD) analysis was conducted using a Malvern PANalytical X'pert instrument within the 2θ range of 5 to 90°. Fourier-transform infrared (FT-IR) spectra were acquired employing an attenuated total reflection (ATR) technique with a SHIMADZU IRTRACER 100 spectrometer, spanning the wavenumber range from 4000 to 400 cm⁻¹. The quantification of Cu was carried out utilizing an Agilent Cary 60 UV-Vis spectrophotometer. The morphology and electron diffraction patterns of the synthesized material were investigated using a Apreo S model high-resolution scanning electron microscope (SEM) from Thermo Scientific. A Physical Electronics X-ray photoelectron spectroscopy (XPS) instrument was employed to determine the metals present in the material and their oxidation states. The binding energy (E_B) was calibrated using the C 1s peak at 284.6 eV, and peak separation was facilitated using CasaXPS software (version 2.3.25 PR 1.0). The quantitative analysis utilized Scofield relative sensitivity factors (RSFs) for correcting the peak areas in CasaXPS software. Thermal stability was assessed via thermogravimetric analysis (TGA) using a Netzsch-STA 2500 Regulus instrument. The electrochemical performance evaluation of Cu/CuO@C was conducted through cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) tests in a three-electrode cell setup. The working electrode consisted of the active material coated on nickel foam, with Hg/HgO serving as the reference electrode and Pt foil as the counter electrode, employing the ZIVE-SP1 instrument. Aqueous solution of 1 M KOH was utilized as the electrolyte. Raman spectra were recorded using a HORIBA (Japan), LABRAM instrument, employing 633 nm wavelength of light.

3. Results and discussion

3.1. Simultaneous leaching and sorption of Cu(II) from waste PCB flakes

The simultaneous leaching and sorption of Cu(II) from printed circuit board flakes was conducted by adding WPCB flakes and rGO in water. Various parameters like rGO : WPCB ratio (1 : 1, 2 : 1, and 3 : 1), pH (4, 7, and 10), H₂O₂ concentration (1%, 3%, and 5%), temperature (25, 60, and 80 °C), and time are thoroughly studied. The results of parameter studies are presented in Fig. S1a–e.† A good amount of Cu(II) recovery was found under all the conditions, and the suitable set of conditions for 100% Cu recovery are identified as 3 : 1 rGO : WPCB w/w ratio, 60 °C, pH 7, and 48 h. A detailed discussion of the parameter study is provided in the ESI.† The schematic representation of the overall procedure is provided in Fig. 1. The solid materials recovered from the leaching–sorption test were named Cu(I)/Cu(II)@rGO. The Cu(II) leaching was confirmed and quantified by characterizing Cu(I)/Cu(II)@rGO.

Fig. 1 Schematic representation of the recovery of Cu from the printed circuit board flakes using rGO in an aqueous medium and upcycling Cu/CuO@C for use in electrode material supercapacitor application.

The X-ray diffraction (XRD) patterns were collected for graphite and rGO, Cu(I)/Cu(II)@rGO, and Cu/CuO@C and presented in Fig. S2a and b.† The XRD patterns of the pure graphite exhibit a distinct, high-intense (002) reflection at a 2θ of 25.9°, along with three minor peaks at 2θ values of 42°, 44.3°, and 54.1°, which correspond to the (100), (101), and (004) reflections, respectively⁴⁴ and matches well with JCPDS number 00-012-0212. rGO exhibits reflection at 2θ of 23.6° indicating the distance between graphene layers, and 2θ at 42.8° indicating a short-range order in stacked graphene layers.⁴⁵

X-ray photoelectron spectroscopy (XPS) spectra were collected for rGO, Cu(I)/Cu(II)@rGO, and Cu/CuO@C and presented in Fig. 2. The C 1s spectra for rGO, Cu(I)/Cu(II)@rGO, and Cu/CuO@C are given as Fig. 2a, d and g. The rGO C 1s spectrum (Fig. 2a) deconvoluted into four peaks with binding energies of E_B [C 1s] = 284.6 eV, 285.5 eV, 286.5 eV, and 288.5 eV corresponding to C–C, C–OH (hydroxy), C=O (carbonyl), and OH–C=O, respectively.⁴⁶ Similarly, the C 1s spectrum of Cu(I)/Cu(II)@rGO (Fig. 2d) is also deconvoluted into four peaks with binding energies of E_B [C 1s] = 284.6 eV, 285.2 eV, 286.4 eV, 288.5 eV which were assigned as C–C, C–OH (hydroxy), C=O (carbonyl), and OH–C=O, respectively. The XPS spectra of the O 1s orbital for rGO, Cu(I)/Cu(II)@rGO, and Cu/CuO@C, are shown in Fig. 2b, e and h. There were two distinct components identified in the rGO O 1s spectrum (Fig. 2b). The peak observed at E_B [O 1s] = 531.0 eV is assigned to C=O, and the peak at E_B [O 1s] = 533.0 eV is assigned to C–OH.⁴⁷ However, in Cu(I)/Cu(II)@rGO, the O 1s spectrum (Fig. 2e) is separated into three peaks. The peaks at E_B [O 1s] = 531.6 eV, and 533.0 eV were assigned to C=O, and C–OH, respectively, and a new peak observed at 530.7 eV, is assigned to the oxygen of COOH bonded to Cu(II).⁴⁸

Fig. 2 XPS spectra of C 1s (a, d and g), O 1s (b, e and h), Cu 2p (c, f and i) for rGO, copper sorbed rGO (Cu(I)/Cu(II)@rGO), material obtained after calcination at 500 °C (Cu/CuO@C).

The XPS spectra of Cu 2p orbital for rGO, Cu(I)/Cu(II)@rGO, and Cu/CuO@C XPS are shown in Fig. 2c, f and i. A comparison of Cu 2p spectra revealed that rGO has no evidence for the presence of Cu (Fig. 2c), while Cu(I)/Cu(II)@rGO (Fig. 2f) and Cu/CuO@C (Fig. 2i) both showed the peaks for Cu 2p orbitals indicating successful leaching of Cu(II) by rGO. Cu 2p spectra for Cu(I)/Cu(II)@rGO (Fig. 2f) showed two major peaks which are deconvoluted to two peaks each. The peaks at E_B [Cu 2p_1/2] = 952.6 eV, and E_B [Cu 2p_3/2] = 933.0 eV are assigned to Cu¹⁺, and peaks at E_B [Cu 2p_1/2] = 954.6 eV, and E_B [Cu 2p_3/2] = 934.9 eV are assigned to Cu²⁺ coordinated to the carboxylic acid functional groups of rGO.⁴⁹ The peaks at E_B [Cu 2p] = 962.8, 944.0, and 939.0 eV are assigned as satellite peaks. The “shakeup” process, which occurs when surplus electrons are stimulated to higher energy levels, is typically the cause of these satellite peaks.^50,51 Overall, XPS spectra indicated the successful leaching and simultaneous sorption of Cu from e-waste WPCB flakes onto rGO, and Cu is adsorbed to rGO in the form of both Cu²⁺ and Cu¹⁺ by coordinating with the available oxygen-containing functional groups.⁵²

The leaching and sorption of copper ions on reduced graphene oxide (rGO) are primarily driven by coordination with the carboxylic acid functional groups, with additional contributions from hydroxy functional groups. The coordination of copper (Cu) with rGO is detailed in eqn (S1) to (S4) found in the ESI.† The probable chemical structure of the Cu(I)/Cu(II)@rGO complex is illustrated in Fig. 3. It is important to note that during XPS analysis of the material recovered after copper leaching by rGO, the interaction of X-rays with Cu²⁺ ions could possibly reduce some Cu²⁺ to Cu⁺.

Fig. 3 Chemical structure of Cu coordinated rGO (Cu(I)/Cu(II)@rGO).

Thermogravimetric analysis (TGA) was performed to understand the thermal profile of Cu(I)/Cu(II)@rGO and the result is presented in Fig. S3.† The decrease in weight observed below 100 °C is due to loss of physically adsorbed water. The range between 100 °C and 300 °C shows a loss of 25.1% and a loss of 41.3% between 300 °C and 500 °C. Above 500 °C, the weight of the material remains stable.

3.1.1 Recovery of Cu(II) from Cu(I)/Cu(II)@rGO as CuSO₄ and its quantification. The Cu(I)/Cu(II)@rGO recovered after the leaching–sorption test was treated with 2 M H₂SO₄ solution. The initially transparent 2 M H₂SO₄ aqueous solution transformed into a blue colour due to Cu(II) stripping into solution from the sorbed material. The obtained solution was tested with UV-Vis to confirm the quantity of Cu(II) recovered. The absorption maxima obtained at 815 nm confirmed the recovery of Cu(II) as CuSO₄.^53,54 The recovered Cu(II) was quantified using UV-Visible spectrophotometry and ICP analysis. The UV-Vis spectra of the stripped solutions are provided in Fig. 4a. Cu was stripped entirely from rGO in two steps, as confirmed by no absorption in the UV-Vis spectra for the third stripping solution. The amount of recovered Cu(II) was quantified as 82.9 mg g⁻¹ of rGO corresponding to 100% of Cu present in WPCB.

Fig. 4 (a) UV-Vis spectra of CuSO₄ recovered from the stripping of Cu(I)/Cu(II)@rGO. (b) XRD of recovered CuSO₄. (c) FTIR of recovered CuSO₄.

The stripped solution was concentrated, and the solids were calcined at 400 °C for 1 hour, to obtain CuSO₄ anhydrous salt, and confirmed by XRD, and FTIR presented in Fig. 4b and c. The data obtained agrees with copper sulfate's standard JCPDS file (JCPDS 77-1900).⁵⁵ Furthermore, SEM-EDAX analysis shown in Fig. S4† suggested high purity of the recovered CuSO₄. The stretching vibration of the O–H group was responsible for the band seen at 3055 cm⁻¹ in the recovered copper sulfate sample. We identified the O–H group's bending vibration mode at 1616 cm⁻¹. The S–O group's bending vibration mode was found to be at 675 cm⁻¹, while its stretching vibration mode was found to be at 1041 cm⁻¹. Along with the Cu–O–H vibration, the vibration mode corresponding to the metal ion Cu²⁺ can be detected and designated as 856 cm⁻¹.⁵⁵

3.1.2 Recyclability of rGO. Cu-sorbed rGO was treated with a 2 M H₂SO₄ aqueous solution to recover Cu and reuse rGO for further sorption cycles. Cu was recovered by treating 200 mg of Cu(I)/Cu(II)@rGO with 20 mL of the 2 M H₂SO₄ solution. After the stripping adsorbed Cu(II) with H₂SO₄, rGO was washed three times with distilled water to regenerate it and used for further cycles to recover Cu(II) from WPCB flakes. This sorption and desorption of Cu(II) was performed for three cycles following the same procedure. In all three cycles, complete recovery of Cu(II) from the WPCB was observed as presented in Fig. 5. These results indicate that rGO is an excellent material for the recovery of Cu(II) from WPCB e-waste.

Fig. 5 Regeneration of rGO and its reusability to recover Cu(II) from WPCB flakes.

3.2. Synthesis of Cu/CuO@C from recovered Cu(I)/Cu(II)@rGO

The Cu(I)/Cu(II)@rGO, recovered from the leaching–sorption step was calcined at 500 °C, resulting in Cu(0) and CuO nanoparticle-supported carbon, and was named Cu/CuO@C. The obtained materials were characterized by XPS, XRD, Raman spectroscopy, and SEM. The XRD patterns of the materials are presented in Fig. S2b.† Cu/CuO@C has reflections at 43.0°, 50.1°, and 73.7° corresponding to the Miller indices of (111), (200), and (220), indicating the presence of Cu(0) nanoparticles, which is in good accordance with JCPDS card no.: 85–1326, and a reflection at 25.6° is due to carbon.⁵⁶ The XRD profile also has two reflections at 36.2°, and 38.3° indicating the presence of CuO. It also showed reflections at 35.2°, 48.5°, 53.1°, 61.1°, 65.8°, 67.7°, and 72.2°. These reflections correspond to the crystallographic planes (110), (−111), (111), (−202), (020), (202), (−113), (−311), (220), and (311) due to the monoclinic structure of CuO. This matches well with the JCPDS no. 5-661.⁵⁷ XRD suggests that the material is a mixture of Cu(0) and CuO nanoparticles supported on carbon (Cu/CuO@C).

The observations in XRD are well complemented by XPS spectra presented in Fig. 2g–i. The C 1s spectrum of Cu/CuO@C (Fig. 2g) was deconvoluted into two peaks with binding energies at E_B [C 1s] = 284.6 eV and 283.7 eV which are assigned to C–C and C=C respectively.⁵⁸ It is worth noting that the C 1s corresponding to the COOH group present in rGO and Cu(I)/Cu(II)@rGO disappears after calcination. The O 1s spectra of Cu/CuO@C were also separated into three peaks (Fig. 2h). The peaks with the binding energy at E_B [O 1s] = 533.2 eV and E_B [O 1s] = 532.0 eV correspond to C–OH and C=O like rGO. The third peak at E_B [O 1s] = 530.7 was assigned for Cu–O.⁵⁹ Cu 2p spectra of Cu/CuO@rGO (Fig. 2i) also showed two major peaks which are deconvoluted into two peaks each. The peaks at E_B [Cu 2p_1/2] = 954.2 eV and E_B [Cu 2p_3/2] = 934.5 eV, are assigned to the presence of CuO. Additionally, two peaks at E_B [Cu 2p_1/2] = 952.6 eV and E_B [Cu 2p_3/2] = 932.9 eV are assigned to Cu(0), these assignments are in correlation with the earlier reports.⁶⁰ The other peaks observed at E_B of 962.0 eV, 943.5 eV, and 940.8 eV are identified as satellite peaks. The formation of CuO and Cu(0) nanoparticles is further complemented by the observed XRD patterns as presented in Fig. S2b.†

The relative concentration of Cu(0) and CuO at the surface of the material was estimated using CasaXPS software using area and a relative sensitive factor (RSF) which was obtained as 57.7% (Cu(0)) and 42.3% (CuO). XPS revealed that the obtained material is a composite comprising Cu(0) and CuO nanoparticles supported on carbon. In Raman spectra, a D band at 1342 cm⁻¹, 1340 cm⁻¹, 1338 cm⁻¹, 1345 cm⁻¹, and a G band 1598 cm⁻¹, 1595 cm⁻¹, 1614 cm⁻¹, 1605 cm⁻¹ were observed for graphite, rGO, Cu(I)/Cu(II)@rGO and Cu/CuO@C (Fig. S5†) showing lattice distortions from the graphene structure. The Raman bands of Cu(I)/Cu(II)@rGO and Cu/CuO@C reveal three more phonon modes at 296, 363, and 670 cm⁻¹ corresponding to Cu–O or CuO^61,62 respectively, suggesting the obtained material as a composite of Cu, CuO nanoparticles and carbon. SEM images along with elemental mapping were captured to understand the surface properties of the composite nanoparticles. The SEM images in Fig. 6. 1a, and 2a exhibit the surface morphology of Cu(I)/Cu(II)@rGO, and Cu/CuO@C. The pictures revealed that Cu(0) and CuO spherical nanoparticles are distributed on carbon sheets.

Fig. 6 HRSEM images and elemental mapping for (1a–4a) Cu(I)/Cu(II)@rGO, (1b–4b) Cu/CuO@C.

3.2.1 Band gap analysis. The absorption measurements were used to estimate the optical band gap by the Tauc method using , where the relationship between absorption coefficient (α) and incident photon energy (hν) can be expressed as follows:

αhν² = A(hν − E_g) (1)

E_g is the optical band gap of the film and A is a constant. The band gap of the material is calculated by using the relationship of absorption coefficient (α) and incident photon energy (hν) given by eqn (2),where A is the absorption and d is the thickness inside the cuvette (1 mm). The band gap values of direct transition are extracted from the linear portion of (αhν)²versus photon energy (eV) plots, as shown in Fig. S6.†^63,64 The direct bands of the Cu(I)/Cu(II)@rGO and Cu/CuO@C are shown in ESI Fig. S6.† Cu(I)/Cu(II)@rGO and Cu/CuO@C have band gaps of 3.8 eV and 3.5 eV, respectively. The band gap will decrease during the annealing process because the particle size will increase during the calcination process. Increasing the particle size is responsible for decreasing the material's band gap.^65,66 The band gap of the copper oxide nanoparticles was comparable to that of earlier reports.^67–69

3.2.2 Electrochemical studies of Cu/CuO@C. While carbon materials are usually used in commercial supercapacitors, their specific capacitance is limited in the range of 100–250 F g⁻¹. Transition metal oxides are preferable because of their higher specific capacitance than EDLC of carbon materials. There are several reports on Cu-based oxides for energy storage applications.⁷⁰ CuO and Cu₂O synthesized by various methods have already been reported for supercapacitor applications.^70–72 Generally, Cu-based oxides are used as a positive electrode material in aqueous KOH electrolytes, where they exhibit specific capacitances in the range of 300–600 F g⁻¹.^73–76 M. Manickasundaram et al. leached CuO/Cu@C from the e-waste and studied its electrochemical performance as an anode material for Li-ion batteries, where it exhibits a reversible discharge capacity of about 433 mA h g⁻¹ at 1C.¹⁸ Wang et al. developed a CuO/CoO core/shell material and studied its performance as an anode for Li-ion batteries. Interestingly, it displayed a specific capacity of about 1364 mA h g⁻¹ at a current density of 100 mA g⁻¹.⁷⁷ Although CuO has been extensively investigated as the positive electrode material, there is no report yet on CuO-based oxides as a negative electrode for supercapacitor applications. This is why we have investigated the performance of extracted Cu/CuO@C for the 1st time as a negative electrode material for supercapacitors in an aqueous electrolyte of 1 M KOH.

3.2.3 Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) cycling. As there are two types of supercapacitors, e.g., electrical double-layer capacitors and pseudo-capacitors, it is essential to perform cyclic voltammetry, which can distinguish between them. To analyze the supercapacitor behavior of Cu/CuO@C, CV was performed at various sweep rates in the potential range of 0 V to (−0.8) V in an aqueous solution of 1 M KOH (Fig. 7a). The CV at 10 mV s⁻¹ of Cu/CuO@C exhibits two pairs of oxidation and reduction peaks at (−0.35 V), (−0.1 V) and (−0.66 V), (−0.33 V), respectively, indicating its pseudocapacitive properties. With an increase in the sweep rate, the CV changes towards rectangular shapes without any distinct redox peaks, indicating a change from a diffusion-controlled to a surface capacitive-controlled process. The current response is found to increase with an increase in the sweep rate.

Fig. 7 Electrochemical performances of Cu/CuO@C (half-cell) in the potential range of (−0.8 V) to 0 V in 1 M KOH (a) CV at different scan rates, (b) GCD cycles at a specific current of 1 A g⁻¹, (c) GCD at different specific currents, (d) rate capability, (e) charge–discharge profiles of different cycles at a specific current of 3 A g⁻¹ and (f) plot of specific capacitance vs. cycle number during extensive cycling at a specific current of 3 A g⁻¹.

The specific capacitance of the Cu/CuO@C electrode was measured from the GCD cycles. Fig. 7b shows the GCD cycles of Cu/CuO@C at a specific current of 1 A g⁻¹ in the potential range of (−0.8 V) to 0 V. The specific capacitance was calculated using eqn (3)

SC = (I × t)/(m × ΔV) (3)

where I/m = specific current, t = charge–discharge time, and ΔV = potential window.

Cu/CuO@C exhibits a specific capacitance of 432.5 F g⁻¹ at a specific current of 1 A g⁻¹. It should be noted that activated carbon can provide 100–200 F g⁻¹ when used as a negative electrode material in supercapacitors.^22,78 Zhao et al. had previously synthesized leaf-like CuO on graphite sheets by a hydrothermal method, which exhibited a specific capacitance of 331.9 F g⁻¹ at a specific current of 0.6 A g⁻¹ when used as a positive electrode in 6 M KOH in the potential range of 0 to 0.4 V and displayed a capacitance retention of about 95% after 1000 cycles.⁷⁹

To examine the rate performance of Cu/CuO@C, GCD was performed at different specific currents varying from 1 A g⁻¹ to 20 A g⁻¹ in the potential range of (−0.8 V) to 0 V in 1 M KOH, which is shown in Fig. 7c. The specific capacitance of Cu/CuO@C decreases with an increase in specific currents due to the decreased utilization of active mass at higher currents. The rate capability of Cu/CuO@C evaluated from GCD is shown in Fig. 7d. Cu/CuO@C exhibits a higher specific capacitance of 432.5 F g⁻¹ at a specific current of 1 A g⁻¹, which decreased to 150 F g⁻¹ at a specific current of 20 A g⁻¹, thus the capacitance retention is about 34.6% of its initial high specific capacitance. The long-term cycling stability of Cu/CuO@C was evaluated by performing galvanostatic cycling at a specific current of 3 A g⁻¹ in the potential range of (−0.8 V) to 0 V for 4000 cycles in 1 M KOH electrolyte. Fig. 7e shows the charge–discharge profile for different cycle numbers of Cu/CuO@C when tested at a specific current of 3 A g⁻¹. An initial C_s of 305 F g⁻¹ is achieved, which gradually decreases to a value of 264 F g⁻¹ after 1000 cycles. Thereafter, the charge discharge profiles merge with each other showing the high cycling stability up to 4000 cycles. Fig. 7f displays the cycling performance of Cu/CuO@C in the potential range of (−0.8 V) to 0 V at a specific current of 3 A g⁻¹ in 1 M KOH. As explained, an initial C_s of 305 F g⁻¹ decreased to 280 F g⁻¹ after 500 cycles, which was then stabilized to a value of 262.8 F g⁻¹ after 4000 cycles. Thus, Cu/CuO@C exhibits excellent cycling stability with a capacitance retention of 86% after 4000 cycles.

For the comparison, the CV and GCD profiles of bare Ni foam are measured under similar experimental conditions and provided in ESI Fig. S7.† Fig S7a† shows the CVs where the current response of bare Ni foam is found to be negligible compared to that of Cu/CuO@C. For further confirmation, we have performed GCD of bare Ni foam (Fig. S7b†) at a specific current of 1 A g⁻¹, where it exhibits a very low specific capacitance (0.7 F g⁻¹).

To observe the morphological changes of Cu/CuO@C electrodes upon cycling, SEM images of both pristine and cycled electrodes have been recorded and compared in Fig. S8.† From the SEM images, no significant changes in the morphology were observed even after cycling for 4000 cycles, indicating the highly stable nature of the prepared Cu/CuO@C electrodes.

3.2.4 Performance of the Cu/CuO@C‖AC asymmetric supercapacitor. Because of its high specific capacitance, it was combined with AC for assembling a hybrid CuO‖AC supercapacitor. There are several reports on Cu-based oxides as positive electrodes and AC as negative electrodes. The performance of hybrid supercapacitors AC‖CuO, AC‖CuCo₂O₄/CuO, etc. was already reported in KOH electrolyte, with energy density in the range of 18–43 W h kg⁻¹.^22,79–82 Before assembling the full hybrid supercapacitor, the capacitive properties of AC were evaluated in 1 M KOH by CV and GCD measurements in the potential domain of 0 to 0.4 V vs. Hg/HgO. The CV exhibits a rectangular shape at 10 mV s⁻¹, indicating the ideal EDLC behavior, however, it distorted with higher scan rates (Fig. S9a and b†). The specific capacitance of AC was determined to be 144.7 F g⁻¹ at a specific current of 1 A g⁻¹ in the potential range of 0–0.4 V in 1 M KOH electrolyte. Considering the specific capacitance and potential window of Cu/CuO@C and AC, the mass ratio was determined using eqn (4).

m₊/m₋ = (C_s−ΔE₋)/(C_s+ΔE₊) (4)

where ‘m’ stands for the mass of electrodes, ΔE the operational potential window, ‘C_s’ the specific capacitance, and the subscripts ‘+’ and ‘−’ for the positive and negative electrodes, respectively. The mass ratio of positive and negative electrodes was calculated to be 5.9 : 1. The cyclic voltammogram of the Cu/CuO@C‖AC asymmetric supercapacitor was analyzed at different scan rates in the voltage of 0–1.2 V in 1 M KOH electrolyte. The CV revealed a pair of oxidation and reduction peaks at 0.48 V and 0.09 V, respectively, indicating the pseudo-capacitive properties of the device. The peak intensity is found to increase with an increase in the scan rates (Fig. 8a).

Fig. 8 Electrochemical performances of Cu/CuO@‖AC in the voltage domain of 0 V to 1.2 V (a) CV at different scan rates, (b) GCD at different specific currents varying from 1 A g⁻¹ to 8 A g⁻¹, (c) rate capability and (d) galvanostatic charge–discharge profile for different cycles at a specific current of 2 A g⁻¹ and (e) cycling stability of the Cu/CuO@C‖AC asymmetric supercapacitor in 1 M KOH electrolyte.

For determining the specific capacitance and rate capability of the Cu/CuO@C‖AC asymmetric supercapacitor, GCD was performed at different specific currents varying from 1 A g⁻¹ to 8 A g⁻¹ in the voltage range of 0 to 1.2 V in 1 M KOH solution (Fig. 8b). The Cu/CuO@C‖AC asymmetric supercapacitor exhibits a specific capacitance of 88.8 F g_cell⁻¹ at a specific current of 1 A g⁻¹ in 1 M KOH. Although the specific capacitance of the supercapacitor decreased with an increase in specific current, the Cu/CuO@C‖AC asymmetric supercapacitor can provide a specific capacitance of 42 F g_cell⁻¹ at a higher specific current of 8 A g⁻¹, showing its suitability even at high currents. These results clearly signify the high-power characteristics of the Cu/CuO@C‖AC asymmetric supercapacitor in 1 M KOH electrolyte (Fig. 8c).

For evaluating the long-term cycling stability of the Cu/CuO@C‖AC asymmetric supercapacitor, GCD was performed at a specific current of 2 A g⁻¹ in the voltage range of 0–1.2 V; the voltage profile and variation in specific capacitance with cycling are shown in Fig. 8d and e. A specific capacitance of 79 F g_cell⁻¹ was obtained during the initial cycles, which decreased to a value of 70 F g_cell⁻¹ after continuous 8000 cycles, with a capacitance retention of about 88.6%. The above results signify the superior cycling stability of the Cu/CuO@C‖AC asymmetric supercapacitor in 1 M KOH electrolyte.

3.2.5 Evaluation of energy and power densities. Energy density and power density are important characteristic properties of supercapacitors. The energy density and power density of supercapacitors can be calculated using eqn (5) and (6), respectively.where ‘E’ is the energy density, ‘C_s’ is the specific capacitance, ‘V’ is the operating voltage, and ‘I’ is the specific current applied in GCD.

The Ragone plot for the hybrid supercapacitor is plotted using the values of energy density and power density obtained at different specific currents (Fig. 9). The hybrid Cu/CuO@C‖AC supercapacitor can deliver an energy density of 17.8 W h kg⁻¹ at a specific current of 1 A g⁻¹ in 1 M KOH and possesses a higher power density of 8 kW kg⁻¹. For comparison, we have assembled an AC‖AC symmetric supercapacitor under the same conditions in a coin cell and evaluated its electrochemical performances in a 1 M KOH electrolyte. Fig. S10a.† displays the CV of the AC‖AC symmetric supercapacitor in the voltage range of 0–1.2 V, which is in a rectangular shape. Fig. S10b.† exhibits the GCD of the AC‖AC symmetric supercapacitor, where it was cycled in the voltage range of 0–1.2 V at different specific currents varying from 1 A g⁻¹ to 3 A g⁻¹. The symmetric supercapacitor exhibits a specific capacitance of 11 F g_cell⁻¹ at a specific current of 1 A g⁻¹. Based on the rate capability investigations (Fig. S10c†), it is evident that the specific capacitance decreased from 11 F g_cell⁻¹ at a specific current of 1 A g⁻¹ to 6.2 F g_cell⁻¹ at 3 A g⁻¹. Fig. S10d.† depicts the Ragone plot of the AC‖AC symmetric supercapacitor, where it provides an energy density of only 2.2 W h kg⁻¹ with a power density of 0.6 kW kg⁻¹. These results indicate the superior capacitive performance of Cu/CuO@C‖AC compared to that of the symmetric AC‖AC supercapacitor and that the Cu/CuO@C extracted from e-waste is a useful negative electrode material for hybrid supercapacitor applications.

Fig. 9 Ragone plots of the asymmetric Cu/CuO@C‖AC supercapacitor in an aqueous solution of 1 M KOH.

4. Conclusions

This study highlighted the environmentally conscious process of reclaiming copper from discarded waste printed circuit boards (WPCBs). Reduced graphene oxide material was synthesized and used for the simultaneous leaching and sorption of Cu from WPCB flakes in an aqueous medium. The highest leaching–sorption was achieved at a pH of 7 and a temperature of 60 °C, i.e., 82.9 mg g⁻¹. A specific capacitance of 432.5 F g⁻¹ was obtained for Cu/CuO@C as a negative electrode when cycled at a specific current of 1 A g⁻¹ in the potential range of −(0.8) to 0 V in 1 M KOH electrolyte. Cu/CuO@C exhibits excellent cycling stability with a capacitance retention of 86% after 4000 cycles when cycled at a specific current of 3 A g⁻¹ in 1 M KOH. Interestingly, the Cu/CuO@C‖AC asymmetric supercapacitor exhibits a specific capacitance of 88.8 F g⁻¹ at a specific current of 1 A g⁻¹. Cu/CuO@C‖AC was stable up to 8000 cycles with a capacitance retention of 88.6% upon cycling at a specific current of 2 A g⁻¹ in the voltage range of 0–1.2 V in 1 M KOH, which indicates the remarkable cycling stability of the supercapacitor device. Thus, this study enlightens the application of recovered e-waste for fabricating useful capacitive electrode materials, which will find application in energy storage devices and can be important towards environmental remediation and sustainability.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank the Science and Engineering Research Board (SERB), India, for providing financial support through the Start-up Research Grant to S. Kancharla (SRG/2021/001457) and P. K. Nayak (SRG/2022/001178). S. Kancharla would like to thank SRM for providing financial support through Selective Excellence Research Initiative (SERI). RM, AAN and SBMK would like to thank SRM for providing a research scholarship.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta04107k

‡ Both the authors have contributed equally.

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