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Article

Electrochemical Properties of Soluble CuCl·3TU Coordination Compound and Application in Electrolysis for Copper Foils

by
Wancheng Zhao
†,
Fangquan Xia
† and
Dong Tian
*
School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Chemistry 2025, 7(4), 114; https://doi.org/10.3390/chemistry7040114
Submission received: 30 May 2025 / Revised: 5 July 2025 / Accepted: 14 July 2025 / Published: 18 July 2025
(This article belongs to the Section Electrochemistry and Photoredox Processes)
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Abstract

As the crucial current collector for lithium-ion batteries (LIBs), electrolytic copper foils are generally manufactured by electrodeposition in acidic copper sulfate solution. However, there are many disadvantages for traditional electrolytic copper foils, such as coarse grains, insufficient mechanical properties, and high energy consumption. In order to improve the performances of electrolytic copper foil, a novel cuprous electrodeposition system was developed in this study. A soluble cuprous coordination compound was synthesized. In addition, XPS, FT-IR spectrum, as well as single-crystal X-ray diffraction illustrated that thiourea coordinated with Cu(I) through S atom and therefore stabilized Cu(I) by the formation of CuCl·3TU. Importantly, the corresponding electrochemical behaviors were investigated. In aqueous solution, two distinct reduction processes were demonstrated by linear sweep voltammetry (LSV) at rather negative potentials, including the reduction of adsorbed state and non-adsorbed state. Moreover, the observed inductive loops in electrochemical impedance spectroscopy further confirmed the adsorption phenomenon. More significantly, the designed cuprous electrodeposition system could contribute to low energy consumptions during electrolysis. and produce ultrathin nanocrystalline copper foils with appropriate roughness. Consequently, the electrolysis method based on CuCl·3TU could provide an improved approach for copper foils manufacturing in advanced LIBs fabrication.

1. Introduction

Attributed to the excellent electrical conductivity, metallic copper has become a significant material for advanced manufacturing technology. Based on the superiority, metallic copper has been employed as crucial components in many large-scale industrial applications, such as conductive copper wires in printed circuit boards, current collectors for lithium-ion batteries (LIBs) to support anodic materials, and electromagnetic shielding materials [1,2,3]. In modern industrial production, the synthesis of metallic copper for advanced components manufacturing primarily depends on electrolysis in aqueous solution. For example, in the production of electrolytic copper foil for LIBs fabrication, copper foils could be generated by electro-reduction of Cu(II) on smooth Ti substrate [4,5]. Generally, electrodeposition in copper sulfate aqueous solution has been extensively used for traditional manufacturing of electrolytic copper foils. However, the prepared electrolytic copper foils always exhibit coarse grains because of insufficient cathodic overpotential [6,7]. The overpotential of monovalent copper is relatively low. Multiple research teams made significant contributions to monovalent copper electrodeposition technology through electrolyte system optimization and process innovation. Myung Jun Kim et al. focused on alloy electrodeposition, successfully preparing copper-silver alloys in cyanide-based electrolyte systems while systematically optimizing copper deposition conditions [8]. Ana–Maria Popescu et al. employed ionic liquids to dissolve CuCl, achieving not only successful electrodeposition of copper layers but also conducting systematic investigations into the electrochemical behavior of this novel solution [9]. Concurrently, C. L. Aravinda et al. dedicated efforts to developing environmentally friendly copper plating processes. By utilizing trisodium citrate and triethanolamine to formulate an alkaline copper complex solution, they accomplished monovalent copper complexation and electrodeposition under cyanide-free conditions [10]. Furthermore, Dr. Neil R. Brooks et al. developed the novel ionic liquid [Cu(CH3CN)n][Tf2N], establishing a fundamentally new electrolyte system for copper deposition [11] Collectively, these studies advanced copper electrodeposition technology from diverse perspectives, expanding possibilities for related industrial applications. As the current collector for anode of LIBs, electrolytic copper foil will be repeatedly stretched by the anodic materials because the anodic materials repeatedly expand and shrink along with the intercalation and de-intercalation of Li(I) during cycling process. Therefore, the mechanical stability of electrolytic copper foil is desperately needed [12,13,14]. However, coarse grains will lead to poor mechanical properties, and consequently provide unsatisfying strength. Ultimately, electrolytic copper foils with coarse grains may be destroyed by the cycling process and lead to the degradation of LIBs [14,15,16,17].
Importantly, formation of fine grain in the electrolysis process would not only improve the mechanical properties of electrolytic copper foil, but also enhance the chemical stability [14,18,19]. Generally, the size of copper grain is related to the cathodic electrochemical polarization, higher cathodic overpotential will generate more crystal nuclei and lead to fine grains [20,21]. Consequently, several strategies have been employed to control the growth of copper grain to generate fine grains, such as the addition of coordination agents to increase the cathodic overpotential [13,18,22,23]. It is well known that the oxidation states of Cu element include Cu(I) and Cu(II). However, Cu(I) ion is regarded as an unstable species in aqueous solution due to the disproportionation during hydration process [24]. In order to improve the stabilization of Cu(I) in aqueous solution, it is necessary to supply strong coordination agents, such as cyanide [25,26,27]. Due to the presence of strong coordination agents in aqueous cuprous solution, the cathodic overpotential will be sufficient to generate metallic copper with fine grains [28,29]. In consideration of environmental protection, the substitution of nontoxic strong coordination agent for cyanide could greatly promote the development of cuprous electrolysis system [30]. Moreover, the electro-reduction of Cu(II) ion to metallic copper is a two-electron process in aqueous solution, while the formation of Cu(0) from Cu(I) ion only needs one electron. From the perspective of electrical energy consumption, electrolysis in aqueous cuprous solution will save half of the electricity.
On account of the advantages of cuprous electrodeposition system, the synthesis of soluble cuprous coordination compound is of great significance and is desperately needed for electrolytic copper foils manufacturing. Especially, thiourea (TU) is a strong and specific coordination agent for Cu(I) [31,32], Ag(I) [33], and Au(I) [34]. Based on the special coordination effect of TU, several applications have been developed, such as the electroplating of Ag alloy [33,35]. However, stable and soluble cuprous coordination compound with TU as the coordination agent has not been proposed, and the corresponding electrochemical properties have not been investigated. Additionally, the cuprous electrolysis system for electrolytic copper foil production has not been established and developed.
In this paper, a soluble cuprous coordination compound was synthesized. The coordination effect and composition were analyzed by fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and single-crystal X-ray diffraction (SC-XRD). It was confirmed that thiourea molecules coordinated with Cu(I) through S atoms and could consequently stabilize Cu(I) by the formation of CuCl·3TU. Furthermore, the electrochemical properties were investigated by linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS). It was revealed that the reduction processes of Cu(I) took place at rather negative potentials in aqueous solution, including the reduction of adsorbed state. The designed electrochemical system based on soluble CuCl·3TU was studied and employed for manufacturing of electrolytic copper foils. Importantly, ultrathin nanocrystalline copper foils with appropriate roughness could be produced with low energy consumption. This novel electrolysis approach would facilitate the development of copper foils, promote the performance improvement of LIBs, and even contribute to sustainable development.

2. Materials and Methods

2.1. Preparation of Cuprous Coordination Compound

Cuprous chloride (CuCl, 97%), thiourea (CH4N2S, 99%), and hydroquinone (C6H6O2, 99%) were bought from Aladdin Reagent. Hydrochloric acid (HCl, 36~38%) was bought from Sinopharm Chemical Reagent. Ultrapure water with a resistivity of 18.2 MΩ was used in all the pre-treatments, electrolytic synthesis and electrochemical measurements.
The preparation process of cuprous coordination compound was carried out as the following procedure. Firstly, pH of the aqueous solution was adjusted to 0.6 by hydro-chloric acid. Hydroquinone (16 g/L) was then added as a stabilizing agent. Subsequently, excess TU (140 g/L) was dissolved at high temperature in order to dissolve CuCl (44 g/L) precipitate by coordination reaction. Afterwards, a transparent solution was formed at high temperature after CuCl was thoroughly coordinated, which was marked as precursor solution. Ultimately, a typical crystal gradually crystallized from the precursor solution along with its cooling down. The generated crystal was the targeted cuprous coordination compound, and could be redissolved in aqueous solution.

2.2. Electrochemical Measurements

All electrochemical measurements for electrolysis process were performed on a CHI 760D (CH Instrument, Shanghai, China) electrochemical workstation in a three-electrode electrochemical cell. Aqueous solutions of cuprous coordination compound were employed as the electrolyte, and the corresponding temperature was controlled at 25 °C. Au electrode (L-type, 0.0707 cm2) was used as the working electrode, and a Cu electrode was used as the counter electrode. The saturated calomel electrode (SCE) was selected as the reference electrode, and all the potentials used were calibrated against SCE. The electrochemical reduction processes of Cu(I) in aqueous solution were demonstrated by LSV (1 mV/s). Electrochemical impedance spectroscopy (EIS) measurements were performed with an AC voltage amplitude of 5 mV across a frequency range of 0.1 to 100,000 Hz to systematically investigate the sub-processes occurring at the interface under various applied potentials. The electrolysis potentials were determined using chronopotentiometry in a modified electrolyte as the electrodeposition bath. Both the anode potentials and cathode potentials were respectively recorded during the measurement.

2.3. Characterizations

The coordination relationship between Cu(I) and TU in the coordination compound as well as the composition were investigated using FT-IR (VERTEX 70, Berlin, Germany), The original data of XPS (ESCALAB Xi+, Basingstoke, Britain) is peak-separated by using advantage 5.948, and SC-XRD (Gimi E, Tokyo, Japan). Scanning electron microscopy (SEM, Gemini300, Baden–Württem, Germany) and laser spectral confocal microscope (KC-X1000, Nanjing, China) were employed to analyze the microstructural of electrolytic copper foils prepared in the cuprous electrolysis system.

3. Results

3.1. Preparation of Coordination Compound and Analysis of Coordination Relationships

The prepared precursor solution was illustrated in Figure 1a. Obviously, the precursor solution was colorless and transparent. However, the dissolved cuprous coordination compound became supersaturated as the temperature decreased. As a result, the generated cuprous coordination compound gradually precipitated from the precursor solution and formed colorless crystal as shown in Figure 1b. After purification and drying treatment, the synthesized crystal was analyzed by FT-IR to investigate molecular structural features of the cuprous coordination compound. Comparison with the FT-IR spectrum of the thiourea reference sample [36,37] (Figure 1c) revealed a new characteristic shoulder peak observed at 3467 cm−1 [38,39]. This feature was assigned to the vibrational band of the –NH2 group, indicating that the group was influenced. Structural analysis suggested that this peak likely arises from the interaction of Cl− with the –NH2 groups in the lattice, leading to the emergence of this spectral feature [40]. The strong signal and broad region should be attributed to the N–H stretching vibrations of TU. Additionally, notable peak shifts were observed in characteristic vibrational modes. The peaks at 1462 cm−1 and 1083 m−1 (C–N vibrations) for TU molecule shifted to 1480 cm−1 and 1087−1 respectively for the coordination compound. While the sharp C = S stretching vibration peak at 734 cm−1 shifted to 713 cm−1 [41,42,43,44]. An enlarged view of the feature described above is shown in Figure 1d. Possible functional groups were labeled in Figure S1. These systematic shifts indicated that Cu(I) coordination through sulfur atoms. Consequently, it changed intramolecular bond vibrations, and induced characteristic spectral displacements.
High-resolution XPS analyses of the cuprous coordination compound (Figure 1e,f) further determined the coordination configuration. Comparison of the XPS spectra of thiourea (Figure S2) and the coordination compound revealed that the peak position for the S2p orbitals shifted to a higher binding energy in the coordination compound. This phenomenon was characteristic of coordinate bond formation [45]. It indicated that thiourea coordinated with CuCl via sulfur in the coordination compound. The S 2p spectrum exhibited characteristic double peaks at 162.8 eV and 163.6 eV, corresponding to bidentate bridging coordination (sulfur bonding to two Cu(I) ions) and monodentate coordination (sulfur bonding to single Cu(I) ion) respectively [36]. Notably, the N 1s orbital showed no significant shift in binding energy [46]. It confirmed that the coordination between TU molecules and Cu(I) ions took place through S atoms, and N atoms did not participated. Quantitative peak deconvolution of S 2p signals revealed a hierarchical chain-like coordination structure. Peripheral thiourea molecules adopted monodentate coordination, while central molecules engaged in bidentate bridging through S–Cu–S linkages. This structure enabled ordered assembly of Cu(I) ions through the different coordination of TU molecules. SC-XRD analysis (Figure 1g) further illustrated the coordination mechanism. The crystal lattice demonstrated that thiourea molecules coordinated to Cu(I) ions through sulfur atoms, forming a three-dimensionally extended network structure. Furthermore, analysis results and simulation explicitly that each Cu(I) ion was coordinated by four thiourea molecules. Two thiourea molecules coordinated to single Cu(I) ion via sulfur atoms, while the other two bidentate bridging thiourea molecules were mutually possessed by adjacent Cu(I) ions [44,47,48]. Therefore, it revealed the molecular formula as CuCl·3TU. The results of FT-IR, XPS interpretations, and crystallographic evidence were consistent with each others, and comprehensively validated sulfur as the primary coordination site in the cuprous coordination compound.

3.2. Electrochemical Behaviors of CuCl·3TU in Aqueous Solution

The physical characterizations revealed that Cu(I) ion might form a stable coordination compound with TU, which could effectively suppressed its disproportionation reaction (2Cu(I) ⇌ Cu(II) + Cu(0)) and the generation of free Cu(I) ions [31,32,49]. In order to comprehensively investigate the electrochemical properties of synthesized CuCl·3TU, it was redissolved to prepare aqueous solutions, and LSV was employed to determine the corresponding cathodic behaviors. Figure 2a showed two distinct reduction stages in the cathodic polarization region for solutions of different concentrations. As pH of the prepared CuCl·3TU solution was 6.2, a KCl solution (pH 6.2) was compared by LSV as exhibited in Figure 2b. A comparative analysis of the linear sweep voltammetry curves for 0.1 M and 0.01 M thiourea solutions revealed that no dual reduction processes were observed under identical voltage conditions. This behavior presented a distinct contrast to the voltammetric characteristics exhibited by coordination compounds. It implied hydrogen evolution reaction occurred below −1.4 V at this pH value and would not interfere the reduction of Cu(I) ion. Figure 2a indicated a stepwise kinetic mechanism for reduction of Cu(I) ion. Comparative analysis of 0.01 M and 0.1 M solutions revealed two significant observations. Analysis was initiated with the distinct second stage. Experimental data revealed that when the solution concentration differed by a factor of ten, the current response corresponding to the second stage in the LSV recorded at the same voltage also exhibited an approximately tenfold difference. This phenomenon clearly indicated that the process was diffusion-controlled: the concentration of the species in solution directly influenced the diffusion flux, which determined the diffusion rate and ultimately led to a proportional change in the reaction rate. In contrast, while the reaction rate for an adsorption process also increased with the analyte concentration, the change did not exhibit a 1:1 linear proportional relationship. This was because the amount of adsorbed species was also constrained by factors such as the electrode surface area. These results demonstrated that Cu(I) ions in the solution existed in two distinct states—adsorbed state and non-adsorbed state, which were reduced at different potentials. The first stage involved rapid reduction of adsorbed Cu(I) ions (onset potential at −0.46 V). In the cuprous coordination compound, the sulfur atoms preferentially coordinated with Cu(I) ion, leading to the free state of amino groups. This structural configuration would contribute to the chemical adsorption of free amino groups onto the surface of metallic copper substrate [50,51]. As a result, the adsorbed state of cuprous coordination compound could be realized. This reduction stage could generate densely packed metallic copper deposits with low surface-energy and high stability. The second stage involved the reduction of non-adsorbed copper (onset potential at −0.60 V). After the adsorption sites were saturated, this process occurred and led to loosely packed metallic copper deposits with enhanced electrochemical activity. Figure S6 shows a comparison of the microstructures observed for copper foil samples prepared at three different current densities (a: −0.15 mA/cm2, b: −0.30 mA/cm2, c: −0.45 mA/cm2). Sample a, representing the initial stage of electrodeposition, exhibited relatively fine-grained structure and a smooth surface. As the current density increased to −0.30 mA/cm2 (sample b), which was in a state transitioning between the first and second stages, the grain packing began to show signs of loosening. When the current density was further increased to −0.45 mA/cm2 (sample c, characteristic of the second stage), a more pronounced loosening of the surface grains was observed, resulting in a microstructure that appeared notably less compact.
Moreover, EIS plots at different potentials in 0.1 M CuCl·3TU solution further determined the interfacial mechanism. Generally. the presence of inductive loop implied the adsorbed state. At −0.5 V and −0.55 V (Figure 2c,d), inductive loops were prominent, suggesting the existence of adsorbed Cu(I) state [52,53]. However, the inductive loop gradually diminished with increased cathodic polarization, and completely disappeared when the potential located at the second reduction stage. The EIS plots measured at −0.75 V and −0.85 V in Figure 2e,f confirmed exclusive reduction of non-adsorbed Cu(I) state. The analyses of adsorption layer aligned with LSV curves, which synergistically elucidated the interfacial reaction dynamics of the cuprous coordination compound in aqueous solution. As the potential gradually shifted in the negative direction, a corresponding increase in the electron density of the copper substrate was induced. Due to the inherently high electron density of the amino groups themselves, repulsive interaction between the amino groups and the copper substrate occurred [54]. When the potential reached a critical value, this repulsion counterbalanced the chemical adsorption force between the amino groups and the copper substrate. Consequently, the chemical adsorption phenomenon would gradually diminished and ultimately vanished. From the perspective of electrochemical properties, the electrochemical reduction process would convert to the reduction of non-adsorbed state, and the inductive loop progressively disappeared. By systematically varying the scan rate, peak currents of the first stage were obtained as shown in Figure 3a–c. The relationship between current peak and scan rate was fitted, which revealed a linear relationship as shown in Figure 3d,f. It was in accord with the characteristic behavior of an adsorption-controlled process. Importanly, it was further confirmed that the reduction of coordination compound exhibited two distinct stages, including the reduction of adsorbed state and the reaction of diffused coordination compound.
The detailed electrochemical reduction mechanism of cuprous coordination compound was schematically illustrated in Figure 4. At the first stage (−0.40 V~−0.60 V), the cuprous coordination compound in aqueous solution could form adsorbed state on the surface of copper substrate, and sequentially be reduced by the supplied electrons. During the reduction, Cu(I) ions obtained electrons to form copper grains, and TU molecules were simultaneously released Meanwhile, adsorption sites were also released. As the reduction proceeded, non-adsorbed Cu(I) ions would transfer to the surface and adsorbed onto regenerated adsorption sites. However, as the potential shifted negatively, the adsorbed state gradually diminished attributed to the weakened chemical adsorption. Consequently. the reduction of non-adsorbed state gradually dominated. which only diffused to the cathode without adsorption step.

3.3. Applications in Electrolytic Copper Foil

On account of the specific electrochemical properties of CuCl·3TU aqueous solution, CuCl·3TU was used as the main salt in a improved electrolyte for the preparation of electrolytic copper foils. Besides, additional TU (35 g/L), auxiliary coordination agents (total concentration (64 g/L), hydrochloric acid (28 mL/L), potassium chloride (30 g/L), and hydroquinone (16 g/L) were added to improve the performance of the electrolyte. Of which, Additional Thiourea (TU) and Auxiliary Complexing Agents: These additives served to promote normal anode dissolution and reduce the anodic overpotential, resulting in finer crystallization and more uniform thickness distribution of the electrodeposited copper foil. Hydrochloric Acid: This was primarily utilized to regulate and control the pH of the electrolyte. A suitable acidic environment was essential to ensure the stability of thiourea complexes, maintain the normal progress of the deposition reaction, and prevent premature hydrolysis and precipitation of metal ions. Potassium Chloride:‌ This compound was added mainly as a ‌conductive salt‌. Its function was to enhance the electrolytic conductivity of the solution, thereby reducing the ohmic voltage drop. This led to a more uniform current distribution, facilitating the production of copper foil with more consistent thickness, and additionally contributed to lower energy consumption. Hydroquinone:‌ This was introduced primarily as an ‌antioxidant/stabilizer‌. It was readily oxidized in the solution, thereby preferentially consuming dissolved oxygen and preventing the oxidation of thiourea. In the improved electrolyte, copper foils could be manufactured by electrolysis on smooth Ti substrate. The fabricated ultrathin copper foil exhibited a thickness of 6 μm as shown in Figure 5a. Through physical measurement, a 2 × 2 cm standard square sample was weighed using an analytical balance (Figure S3), and the thickness was calculated via the volume formula. Considering minor cutting deviations, repeated measurements yielded a thickness range of 5.8–5.9 μm. As clearly observed in Figure S4, the copper foil thickness measured by SEM was 5.986 μm. This finding was consistent with the gravimetric results. Collectively, these data consistently support a nominal copper foil thickness of 6 μm. The copper foil displayed a characteristic pink appearance and a relative rough surface. More importantly, there was no obvious edge curl for the copper foil, indicating minimal residual internal-stress. LSV analysis in Figure 5b revealed that two reduction processes persisted in the improved electrolyte. Noteworthy, the potential regions of reduction processes showed a marked negative shift compared to the pure CuCl·3TU aqueous solution. It demonstrated a more negative cathodic polarization in the improved electrolyte, which would form fine grain and reduce large grain boundaries [20,21,55]. Therefore, the generated structural modification simultaneously enhanced the mechanical properties and electrical conductivity of the copper foils. Furthermore, the energy consumption analysis using chronopotentiometry was presented in Figure 5c. The experiment evaluated energy consumption during the electrolysis process by measuring the potential differences (cell voltages) between cathode and anode under different current densities. The measurements demonstrated that the cell voltage exhibited an upward trend with increasing current density when current densities were below 2 A/dm2. Importantly, the cell voltage could be controlled below 0.3 V, which indicated a considerable reduced energy consumption. However, when the current density reached 4 A/dm2, a distinct stepwise increase in cell voltage was observed. This abrupt change was attributed to the formation of a white passivation film on the anode surface under high current density conditions, which significantly increased the potential of the anodic reaction. This phenomenon could be effectively mitigated by enlarging the anode surface area. Subsequently, energy consumptions were determined in detail. Firstly, the theoretical thickness of copper foil was calculated as 6 μm based on Faraday’s law of electrolysis. And the actual thickness was 5.8–5.9 μm based on weight measurement as shown in Figure S3. And the actual thickness was further confirmed by SEM image as shown in Figure S4. It indicated that the current efficiency was 99%. Secondly, the voltage differences were calculated, and the energy consumptions were further determined by integration. In consideration of the current efficiency, the energy consumption in electrolysis for a certain quality of of copper was determined. The results indicated that, the energy consumptions were 236.07 J/g at 0.5 A/dm2, 291.78 J/g at 1 A/dm2, and 425.51 J/g at 2 A/dm2, respectively. Importantly, the energy consumption always reached 5.5–5.8 kW h/kg for traditional electrolysis using copper sulfate. It implied that the energy consumption was obviously reduced [56].
Microstructural characterizations of the prepared copper foil in Figure 5d,e revealed enhanced grain growth characteristics, including reduced grain sizes and grain boundaries. Absolutely, there were no large grain boundaries in the SEM images, and the size of formed nanocrystalline copper grain was about 30 nm. This structural modification not only improved mechanical properties, but also minimized electrical resistance and enhanced conductivity. The characterizations confirmed the synergistic effect of auxiliary complexing agents in the proved electrolyte, which could further provide increased cathodic polarization and promote nucleation [57,58]. Ultimately, nanocrystalline copper foils could be fabricated. The nanocrystalline structure might make it an ideal current collector material for anode in LIBs. Moreover, laser spectral confocal microscope demonstrated that the copper foil displayed a uniform surface with the roughness of 2.45 μm, as illustrated in Figure 5f. Although the roughness of Figure 5f is 2.45 μm, as shown in the copper foil SEM of Figure S5 at high magnification, the copper foil surface exhibited a fine-grained structure with uniform distribution. Conversely, at lower magnifications, no discernible surface features such as protrusions or depressions were observed across the examined area. These microscopic observations collectively confirm the homogeneous surface characteristics of the copper foil. The copper foil was further characterized by XRD. Figure 6a analysis revealed a significant correlation between grain orientation and current density in the copper foil. The (200) crystallographic plane exhibited a pronounced preferential growth tendency. The preferential growth of the (200) plane contributed to smaller grain sizes as well as reduced residual stress [59]. XPS measurements after Ar+ ion sputtering in Figure 6b showed no detectable satellite peak at 945 eV. It confirmed the exclusive presence of metallic copper [60]. Because relatively rough surface could supply a greater binding force with slurry and effectively support active materials, the specific surface should be beneficial to its application as a current collector. Briefly, this ultrathin nanocrystalline copper foil demonstrated multiple advantages in LIBs applications [61,62,63]. Firstly, the ultrathin thickness significantly enhanced the mass specific energy of LIBs [64]. Secondly, the rough surface structure facilitated the adhesion of active materials [61,62,63]. Thirdly, the nanocrystalline structure could improve the mechanical and electrical properties [65]. The manufactured electrolytic copper foil would establish a crucial material foundation for advanced energy storage devices.

4. Conclusions

In this study, a cuprous coordination compound (CuCl·3TU) was successfully synthesized. Characterizations demonstrated that Cu(I) ions formed coordination bonds with sulfur atoms of thiourea molecules. Moreover, CuCl·3TU was soluble and the corresponding electrochemical behaviors in aqueous solution were determined. Electrochemical measurements revealed two characteristic stages during the cathodic polarization process. EIS analysis further confirmed the adsorbed Cu(I) state in the cathodic polarization process by the presence of inductive loops. In addition, the detailed electrochemical reduction mechanism was proposed, including the reduction of adsorbed and non-adsorbed states. More significantly, an improved electrolysis system for the preparation of electrolytic copper foils was established based on the synthesized CuCl·3TU. The energy consumption of the designed electrolysis system was investigated, indicating a considerable reduced energy consumption. Furthermore, the performances of electrolytic copper foil could be obviously improved. The manufactured ultrathin copper foil showed nanocrystal and moderate roughness. Consequently, the copper foils would provide enhanced mechanical and electrical properties, high mass specific energy, as well as excellent adhesion of active materials for LIBs fabrication.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemistry7040114/s1, Figure S1. FT-IR functional group labeling diagram of compound crystals, Figure S2. Energy spectrum analysis of (a) S2p and (b) N1s in the XPS spectrum of thiourea, Figure S3. A 2 × 2 diagram characterizing the weight of copper foil, Figure S4. Cross-sectional view of the improved electroplated copper foil, Figure S5. SEM of the improved electroplated copper foil: (a) 200 nm and (b) 20 μm, Figure S6. The variations of the copper foil structure under three different current density conditions are respectively: (a) −0.15, (b) −0.30 and (c) −0.45 mA/cm2.

Author Contributions

W.Z.: Conceive the research topic, design the experimental process, complete the collection and analysis of the main data, and write the first draft of the paper. F.X.: Participate in the experimental design, be responsible for specific data processing, and revise the thesis. D.T.: Provide key technical guidance, review experimental ethics issues, and participate in the final draft proofreading. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data that support the findings of this study are available within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Preparation process and characterizations of the cuprous coordination compound. (a) Precursor solution at high temperature, (b) Crystal of the cuprous coordination compound from the cooled precursor solution, (c) FT-IR spectra of TU and the cuprous coordination compound, (d) FT-IR peak position shift magnification of compound crystals (the red lines in the figure indicate a red shift and the blue lines indicate a blue shift), (e,f) high-resolution S 2p and N 1s XPS spectra of the cuprous coordination compound, (g) SC-XRD pattern of the cuprous coordination compound.
Figure 1. Preparation process and characterizations of the cuprous coordination compound. (a) Precursor solution at high temperature, (b) Crystal of the cuprous coordination compound from the cooled precursor solution, (c) FT-IR spectra of TU and the cuprous coordination compound, (d) FT-IR peak position shift magnification of compound crystals (the red lines in the figure indicate a red shift and the blue lines indicate a blue shift), (e,f) high-resolution S 2p and N 1s XPS spectra of the cuprous coordination compound, (g) SC-XRD pattern of the cuprous coordination compound.
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Figure 2. Electrochemical characteristics of CuCl·3TU aqueous solution. (a) LSV of CuCl·3TU aqueous solution, (b) LSV of KCl solution with pH 6.2, (c–f) EIS plots in 0.1 M CuCl·3TU aqueous solution measured at (c) −0.50 V, (d) −0.55 V, (e) −0.75 V, and (f) −0.85 V.
Figure 2. Electrochemical characteristics of CuCl·3TU aqueous solution. (a) LSV of CuCl·3TU aqueous solution, (b) LSV of KCl solution with pH 6.2, (c–f) EIS plots in 0.1 M CuCl·3TU aqueous solution measured at (c) −0.50 V, (d) −0.55 V, (e) −0.75 V, and (f) −0.85 V.
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Figure 3. The current in the coordination compound solution varies linearly with the sweep rate: (a) 0.01 M, (b) 0.1 M and (c) Improve the plating solution. Linear fitting (d) 0.01 M, (e) 0.1 M and (f) Improved plating solution.
Figure 3. The current in the coordination compound solution varies linearly with the sweep rate: (a) 0.01 M, (b) 0.1 M and (c) Improve the plating solution. Linear fitting (d) 0.01 M, (e) 0.1 M and (f) Improved plating solution.
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Figure 4. The electrochemical reduction mechanism of cuprous coordination compound in aqueous solution (red text represented the reduction processes of adsorbed species, while blue text indicated the reduction processes of non-adsorbed species).
Figure 4. The electrochemical reduction mechanism of cuprous coordination compound in aqueous solution (red text represented the reduction processes of adsorbed species, while blue text indicated the reduction processes of non-adsorbed species).
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Figure 5. The application of the improved electrolyte and characterizations of the prepared electrolytic copper foils. (a) Appearance of the electrolytic copper foil, (b) LSV curve in improved electrolyte, (c) chronopotentiometry curves in improved electrolyte to determine the potential differences between anode and cathode during electrolysis, (d,e) SEM images of the prepared electrolytic copper foils, (f) laser spectral confocal microscope characterization of the prepared electrolytic copper foils.
Figure 5. The application of the improved electrolyte and characterizations of the prepared electrolytic copper foils. (a) Appearance of the electrolytic copper foil, (b) LSV curve in improved electrolyte, (c) chronopotentiometry curves in improved electrolyte to determine the potential differences between anode and cathode during electrolysis, (d,e) SEM images of the prepared electrolytic copper foils, (f) laser spectral confocal microscope characterization of the prepared electrolytic copper foils.
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Figure 6. Characterization of copper foil prepared by improved coordination compounds (a) XRD and (b) XPS.
Figure 6. Characterization of copper foil prepared by improved coordination compounds (a) XRD and (b) XPS.
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Zhao, W.; Xia, F.; Tian, D. Electrochemical Properties of Soluble CuCl·3TU Coordination Compound and Application in Electrolysis for Copper Foils. Chemistry 2025, 7, 114. https://doi.org/10.3390/chemistry7040114

AMA Style

Zhao W, Xia F, Tian D. Electrochemical Properties of Soluble CuCl·3TU Coordination Compound and Application in Electrolysis for Copper Foils. Chemistry. 2025; 7(4):114. https://doi.org/10.3390/chemistry7040114

Chicago/Turabian Style

Zhao, Wancheng, Fangquan Xia, and Dong Tian. 2025. "Electrochemical Properties of Soluble CuCl·3TU Coordination Compound and Application in Electrolysis for Copper Foils" Chemistry 7, no. 4: 114. https://doi.org/10.3390/chemistry7040114

APA Style

Zhao, W., Xia, F., & Tian, D. (2025). Electrochemical Properties of Soluble CuCl·3TU Coordination Compound and Application in Electrolysis for Copper Foils. Chemistry, 7(4), 114. https://doi.org/10.3390/chemistry7040114

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