2.1 Materials

The chalcopyrite particles employed in this study was 6 µm in mean size and came from the Kastamonu Hanönü region. Its chemical composition made by X-ray fluorescence (XRF) are given in Table 1. In the experiments, deionized water, sodium chloride (NaCl, 99.5% pure, EMPROVE^®), magnesium chloride hekza hydrate (MgCl₂.6H₂O, 99% pure, EMSURE^®) and urea (CH₄N₂O, %98 pure, Rokkim) were used for copper and iron extraction from chalcopyrite (CuFeS₂). Nickel foam used for electrode substrate was purchased from Nanografi. Deionized water with a conductivity of 18.25 Ω was used for extraction.

2.2 Copper and iron extraction from chalcopyrite

The leaching of chalcopyrite for obtaining Cu and Fe was applied by hydrometallurgical process [27]. A 250 mL beaker was initially used, and 50 mL of deionized water was added to it. The mixture in the beaker was then heated and stirred magnetically until it reached a temperature of 60 ℃. Once the temperature reached this point, 3 g of chalcopyrite were carefully weighed and added to the beaker. Stirring was maintained at the same temperature and a rate of 360 r/min for one hour. Following this step, the mixture was removed from the heater and filtered through Watman filter paper using a funnel. The filtrate was subsequently centrifuged at 3000 r/min for 15 min to eliminate any remaining particles, completing the extraction process. Additionally, NaCl and MgCl₂ were individually used as secondary reactants to assess their impact on the concentrations of Cu and Fe elements in the extraction solution. These reactants were introduced into the solution alongside the chalcopyrite, following the quantities listed in Table 2. The quantities of NaCl and MgCl₂ that yielded. The highest Cu and Fe concentrations were then determined. Afterward, urea was added in accordance with the amounts specified in this table, along with the highest concentration of MgCl₂. All of these procedures were carried out using the setups illustrated in Figure 1.

Figure 1全屏查看

Schematic explanation of extraction and hydrothermal processes

2.3 Preparation of electrodes

This section delineates the construction of electrodes for supercapacitors utilizing the solution with the highest concentrations of Cu and Fe, as specified in the characterization section. The fabrication procedure occurred on the surface of a nickel foam substrate as conducted our previous studies [8, 28], functioning as a direct current collector, employing the hydrothermal method. Initially, 10 mL of M7U1 solution was placed in a 40 mL Teflon-coated stainless-steel autoclave. Two pieces of nickel foam, cut to dimensions of approximately 1 cm², were then inserted into the same autoclave. The Teflon lid and stainless-steel autoclave lid were tightly sealed. The autoclave was positioned in a preheated oven at 150 ℃ with a heating rate set to 10 ℃/min, allowing it to sit for 6 h. Afterward, the autoclaves were taken out of the oven and left to cool to room temperature. The nickel foams were detached from the lids, rinsed with deionized water to eliminate any remaining residues, and subsequently dried in an oven at 60 ℃ for one day. Furthermore, the autoclave solutions were utilized in characterization processes. These solutions were transferred to glass tubes, then centrifuged at 3000 r/min for 15 min to separate particulate matter. The remaining liquid was discarded, and the precipitate-like particles were dried in an oven for one day. Thus, the electrode fabrication processes were successfully concluded. This detailed representation of the narratives is schematically presented in Figure 1.

2.4 Materials characterization

In the initial phase of characterizations, the copper and iron concentrations in the post-extraction solutions were quantified using THERMO SCIENTIFIC ICE 3400 atomic absorption spectrometers (AAS). Subsequently, the characterizations delved into the crystallographic analysis of metal oxides synthesized through the hydrothermal method. These investigations were conducted using Rigaku Ultima IV X-ray diffraction (XRD) equipment, employing fixed monochromator Cu-based Kα radiation at 40 kV and 40 mA. Furthermore, Fourier transform infrared spectroscopy (FTIR) analyses were performed on these metal oxide compounds, covering a wave number range of 400 to 4000 cm^-1, with a resolution of 2 cm^-1, to elucidate their chemical bonds. Additionally, the microstructure and morphological images of these components were determined using the Carl Zeiss ULTRA PLUS scanning electron microscope (SEM).

2.5 Materials characterization

The electrochemical performance of hydrothermally synthesized electrodes was assessed using an Ametek Parstat 4000 device through cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD). All electrochemical experiments were conducted using a three-electrode setup. In this configuration, a sample prepared via the hydrothermal process was employed as the working electrode, while a graphite rod and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The electrolyte solution consisted of a 6 mol/L KOH solution prepared with deionized water (18.25 Ω). Portions of these nickel foams, measuring 1 cm², were immersed in the electrolyte after being compressed at approximately 10 MPa. The specific capacitance (C_s), energy (E) and power (P) densities of the electrodes were determined by Eqs. (1), (2), (3) [29]. In these equations, I, Δt, ΔV and S were current (mA), discharge duration (s), window (V) and surface area (cm²) of electrodes, respectively.

(1)

(2)

(3)

3.1 Extraction results

The copper and iron extraction results obtained from different leach solutions of chalcopyrite have been examined in this section. The concentrations of the leach solutions using the NaCl reactant are detailed in Figures 2(a) and (b). The samples referred to as “reference” in these results were generated using only deionized water, without the addition of any reactants. Based on these results, the Cu and Fe concentrations in the reference sample were determined to be 225 and 203 mg/L, respectively. In the following experiments, 1 g of NaCl was added as a reactant (N1) to the extraction solution, which led to Cu and Fe concentrations of 295 and 199 mg/L, respectively. Subsequently, another leach solutions containing 3 g (N3), 5 g (N5), 7 g (N7) and 9 g (N9) of NaCl were prepared to assess changes in the extraction rate with varying reactant quantities. Cu concentrations in these solutions were measured as 306, 333, 380 and 400 mg/L, respectively, while Fe concentrations were found to be 205, 221, 242 and 261 mg/L. These findings indicate that Cu concentrations increased by 28% to 76%, and Fe concentrations similarly increased by 28% to 76%, depending on the amount of NaCl added. This increase is likely related to the deformation of the sulphur layers in chalcopyrite by Na⁺ ions. However, the point of saturation in terms of Cu and Fe concentrations, depending on the amount of NaCl, could not be determined. Instead, the increasing trends of Cu and Fe concentrations were identified, with slopes of 0.86 and 0.34, respectively. These results have also been compared with the literature in the discussion section.

Figure 2全屏查看

Cu and Fe concentration after extraction with (a, b) NaCl, (c, d) MgCl₂, and (e, f) MgCl₂+urea

The effect of MgCl₂ as a second reactant has been investigated. For this purpose, leach solutions containing 1, 3, 5, 7 and 9 g of MgCl₂ were similarly prepared and labelled as M1, M3, M5, M7 and M9, respectively. The Cu and Fe concentrations of each solution were provided in Figures 2(c) and (d). According to findings, the Cu concentrations for M1, M3, M5, M7 and M9 were determined to be 318, 329, 336, 345 and 322 mg/L, respectively, while the Fe concentrations was measured at 220, 223, 299, 311 and 286 mg/L. The concentrations of Cu and Fe showed an increase of approximately 52%-53% until the M7 leach solution, after which they exhibited a decreasing trend. This M7 leaching solution can be considered as the point where Cu and Fe reach saturation. On the other hand, the effect of another reactant, urea (CH₄N₂O), an organic compound along with these salts, was also examined but, since no effect of urea was observed in combination with NaCl, it was not considered for evaluation. However, when used with MgCl₂, the changes observed were presented in Figures 2(e) and (f). The urea component was added to the M7 leach solution, which yielded the highest Cu and Fe concentrations, in quantities of 1, 3, 5, 7 and 9 g, respectively. These leach solutions prepared with these components were labelled as M7U1, M7U3, M7U5, M7U7 and M7U9, as shown in Table 2. The Cu concentrations for the M7U1, M7U3, M7U5, M7U7, and M7U9 samples were determined as 345, 373, 354, 324, 314 and 303 mg/L, respectively. Similarly, the Fe concentrations were found to be 311, 258, 239, 208, 203 and 202 mg/L. When compared to the M7 sample, these results showed an initial 8% increase for Cu, followed by a subsequent decrease of up to 12% in the M7U9 sample. Concurrently, Fe concentrations exhibited a direct decrease with the introduction of urea, reaching up to 35%. Therefore, the urea compound might be considered a suitable candidate for obtaining copper without iron, especially when MgCl₂ is used instead of NaCl.

3.2 Electrode characterization

The synthesis of CuFe₂O₄ metal oxide for supercapacitors was planned using copper and iron ions present in the leach solution obtained after extraction. For this purpose, the M7U1 solution was selected, considering that the thiourea formed after leaching might advantageously contribute to the electrode’s performance. The productions were carried out directly on a nickel foam substrate via hydrothermal method, and the obtained products were initially subjected to XRD analyses, as shown in Figure 3(a). According to the results, distinct crystal peaks were observed at 2θ values of 35°, 39°, 43°, 55°, 60° and 64°, which are likely attributed to the (311), (222), (400), (511), (440) and (300) planes of the spinel-structured CuFe₂O₄ [30, 31]. The majority of these peaks are also consistent with JCPDS data (Card No. 22—1086) of CuFe₂O₄. Simultaneously, additional peaks were detected at 11°, 17°, 22°, 26° and 32°, which are likely attributed to the (110), (211), (112), (301) and (312) planes of urea, as well as peaks at 16° and 17°, 19°, corresponding to the (200) and (211) planes of thiourea, respectively [32]. Furthermore, the FT-IR results, presented in Figure 3(b), revealed two distinct peaks at wave numbers 456 and 598 cm^-1, indicative of Cu—O and Fe—O bonds, respectively [7, 33]. Additionally, supplementary peaks were observed at 1082, 1409, 1558, 1627 and 598 cm^-1, suggesting the presence of carbon-like and NH₂ compounds. Likewise, the broad peak observed at 3095 cm^-1 is associated with O—H bonding, likely attributed to moisture within the structure [14].

Figure 3全屏查看

(a) XRD (b) FTIR results of hydrothermally produced electrode material

The microstructure of metal oxide nanoparticles based on Cu and Fe, synthesized on the surface of nickel foam, was examined using SEM, and the results are presented in the visuals within Figure 4. An overview of the untreated and treated nickel foam is provided in Figures 4(a) and (b), respectively. These images clearly show the formation that has taken place on the surface of the nickel foam. In the higher magnification images shown in Figures 4(c) and (d), it is evident that these formations exhibit a plate-like structure depicted in Figure 4(c). It can be reasonably approximately 45-50 nm wall thickness as inferred that these structures, characterized by their remarkably thin walls and nano-scale dimensions, primarily consist of CuFe₂O₄ crystal structures, as corroborated by the XRD and FT-IR analyses. This interpretation is further supported by relevant literature data.

Figure 4全屏查看

SEM images of the electrode: (a) Pure Ni foam; (b-d) Synthesized CuFe₂O₄ on Ni foam substrate image with different magnification

3.3 Electrode characterization

The synthesis of Cu-Fe metal oxide nanoparticles directly on the nickel foam surface via the hydrothermal method [2, 34], using the extraction solution, was confirmed through XRD, FT-IR, and SEM analyses. The resulting samples were intended for use as anode electrodes in energy storage applications, particularly in supercapacitors. In this context, we conducted investigations into the electrochemical performance of nickel foam electrodes coated with metal oxide nanoparticles on their surface. For this purpose, galvanostatic charge-discharge measurements were performed at various current densities, specifically 2, 3, 4, 5, 8 and 10 mA/cm², and the outcomes are presented in Figure 5(a). The corresponding discharge durations at these current densities were recorded as 251 s, 163 s, 107 s, 82 s, 46 s and 35 s, respectively. Furthermore, based on these discharge time, the specific areal capacitance values (C_a) were computed at each current level, resulting in values of 725, 702, 605, 571, 525 and 485 mF/cm², as illustrated in Figure 5(b). Notably, the highest energy storage performance was achieved at a current density of 2 mA/cm². Additionally, we calculated the energy and power densities of the electrode, and these results are depicted in Figure 5(c). According to our findings, the energy densities for each current density, ranging from 2 to 10 mA/cm², were determined to be 12.30, 11.90, 10.30, 9.70, 8.90 and 8.26 mW·h/cm², respectively. Simultaneously, the power density of the electrode, within the same current range, was computed as 175, 262, 350, 437, 700 and 875 mW/cm², as demonstrated in Figure 5(c). Consistent with the trend observed in specific capacitance, the highest energy and power density values were obtained at a current density of 2 mA/cm².

Figure 5全屏查看

Electrochemical measurement results of (a) galvanostatic charge-discharge, (b) specific capacitance and (c) energy-power density

3.4 Discussion

Copper and iron are two elements widely used in industry. Mineralogists conduct a series of studies to separately extract these elements from the chalcopyrite ore, where they are found together. These studies generally focus on various parameters such as the type of reactants, ambient temperature, duration, solid-liquid ratio, and pH. A brief summary of the studies conducted on chalcopyrite ore can be found in Table 3 in the literature. In order to compare the findings of our study with those in the literature, extraction rates were calculated according to the formula in the reference study [21]. According to findings from literature, it was observed that increasing the solution temperature from 70 to 90 ℃ resulted in a 50% increase in copper extraction in one study focusing on temperature [10]. Regarding duration, it was stated that a leaching process of 180 min is sufficient to reach copper saturation [38]. However, the simultaneous increase in iron extraction along with copper during this period is a significant disadvantage. When examined in terms of reactant type, it was observed that the highest copper extraction occurred with NaCl (150 g/L) and APS (250 g/L), reaching 80% [38]. At the same time, copper extraction with H₂SO₄ (1 mol/L) and NaCl (2 mol/L) reactants was 70% [10], while with NaCl (1 mol/L) and H₂SO₄ (100 mL) it was 60% [37]. However, the absence of comparison with pure water and the lack of iron extractions in these studies are considered significant shortcomings. In our study, this deficiency was addressed by initially conducting the leaching process with pure water, where copper and iron extractions were determined to be 33% and 18% respectively. When 3 mol/L NaCl was added to the leach solution, copper and iron extractions reached 60% and 23% respectively, while with MgCl₂ they were 52% and 27%. In terms of quantity, it was determined that copper and iron extractions continued to increase with NaCl addition, with slopes of increase being 0.86 and 0.34, respectively. However, for MgCl₂, it was understood that copper and iron extractions decreased after a concentration of 140 g/L. Likely, this is because Na⁺ ions exhibit higher reactivity in forming sulphated compounds with sulphur compared to Mg⁺² ions [39]. Additionally, it was observed that MgCl₂ reduced copper extraction while increasing iron extraction, indicating a disadvantage for obtaining copper-free iron. Apart from these, it was considered that a secondary component such as urea reacting with the sulphur around the chalcopyrite could allow for greater solubility of copper and iron. However, no change was observed when added with NaCl, and thus it was not included in the study. When added with MgCl₂, it was observed that only at a concentration of 20 g/L did it increase copper extraction while consistently reducing iron. The peak in copper extraction was determined to be 56%, which, although lower than NaCl, was relatively higher compared to pure MgCl₂. Additionally, when urea was added with MgCl₂, it likely formed a complex compound [40], reducing iron extraction, which could be suitable for obtaining copper-free iron. However, it is evident that this hypothesis requires further investigation.

In the second part of the study, the counterparts of CuFe₂O₄-based electrodes produced for supercapacitors were investigated in the literature, and the findings were summarized in Table 4. According to these results, the specific capacitance value of the electrode in this study was observed to be approximately 725 mF/cm² or (~504 F/g) at a current density of 2 mA/cm². When comparing among CuFe₂O₄-based electrodes developed so far, it is evident that those with a 2D geometric shape outperform spherical ones. Furthermore, the addition of graphene-like components with high conductivity to CuFe₂O₄ leads to a noticeable improvement in performance [22]. A similar study conducted by incorporating graphene and graphitic carbon nitride in a 2D structure achieved high performance, approximately 989 mF/cm² (~657 F/g) [7]. Now in this study, despite obtaining CuFe₂O₄ with only thiourea residues on its surface, remarkably high performance was still achieved. In fact, the results of this study were observed to surpass those obtained from a similar study using acidic leaching reagent solutions [8]. Therefore, the procedure in this study is considered promising and open to further development.