# Influence of Acetonitrile on Cu Electrochemical Nucleation and Growth: Preliminary Test of Catalytic Activity for eCO 2 RR

Akbayan Bekey, Florence Vacandio, Khaisa Avchukir

► To cite this version:

Akbayan Bekey, Florence Vacandio, Khaisa Avchukir. Influence of Acetonitrile on Cu Electrochemical Nucleation and Growth: Preliminary Test of Catalytic Activity for eCO 2 RR. Electrocatalysis, 2025, 16 (3), pp.513-525. 10.1007/s12678-025-00940-y. http://dx.doi.org/10.1007/s12678-025-00940-y

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# Influence of Acetonitrile on Cu Electrochemical Nucleation and Growth: Preliminary Test of Catalytic Activity for eCO₂RR

Akbayan Bekey<fnref n="1" /> · Florence Vacandio<fnref n="2" /> · Khaisa Avchukir<fnref n="1" />

## Abstract

In this study, the nucleation and growth mechanisms of copper on a glassy carbon (GC) electrode from mixed acetonitrile-water (AN/H₂O) solutions were investigated using cyclic voltammetry (CV), chronoamperometry (CA) and scanning electron microscopy (SEM). The mechanism of copper nucleation in the AN and 70:30 AN/H₂O solutions is progressive, whereas for the 50:50, 60:40 and 80:20 AN/H₂O solutions, it is close to the mixed mechanism. The nucleation parameters, including the nucleation rate, nucleation density and average radius of active nuclei centres, were calculated using a three-dimensional electrochemical nucleation model developed by Scharifker-Hills, in accordance with the identified mechanism. The experimental results demonstrated that the average radius of electrodeposited particles could be reduced from 10.8 to 3.2 µm by shifting the applied potential to the negative region, from -0.43 to -0.48 V in a solution of 0.01 M CuCl₂ + 0.5 NaClO₄ + 70:30 AN/ H₂O. SEM images of the obtained coatings demonstrated the formation of uniformly distributed “monanthes-like” structures of copper particles. The catalytic activity of the Cu electrocatalysts were determined by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) methods in a 0.5 M NaHCO₃ solution saturated with CO₂ under bulk electrolysis conditions. LSV shows that the Cu catalyst electrodeposited from 50:50 AN/H₂O mixture exhibited the best performance for electrochemical CO₂ reduction reaction (eCO₂RR) with a Tafel slope of 168 mV dec⁻<fnref n="1" />, exchange current density of 6.81 × 10⁻⁴ A cm⁻<fnref n="2" /> and charge transfer resistance of 9.4 Ω cm<fnref n="2" />. This study may provide an economical approach for developing low-cost and efficient copper-based electrocatalysts for CO₂ electroreduction.

**Keywords** Electrodeposition · Nucleation and growth · Copper catalyst · Acetonitrile · Carbon dioxide electroreduction · Electrocatalysis · Electrochemical active surface area · EQCM

## Introduction

Copper is a well-known and efficient material that has attracted much attention for sustainably converting CO₂ into multi-carbon products. It has been shown that the cop- per nanostructures have their high surface area and appro- priate binding properties [1–5]. Cu catalysts show high catalytic activity and product selectivity for the electro- chemical reduction of CO₂ to C2–C3 at low overpotentials [6, 7]. Much of the work present in the literature is related to the mechanism in aqueous electrolytes. For instance, the electrodeposition of copper has been investigated using various solution used as electrolytes, including sulphate [8], perchlorate [9], ammonium media [10] and pyrophosphate [11]. Nevertheless, aqueous solutions exhibit limitations, such as low thermal stability, narrow electrochemical windows and the evolution of hydrogen molecules during electrolysis. Non-aqueous or mixed solvents have attracted growing interest because of their good heat capacity and wide electrochemical window [12]. In addition, solvent “indirectly” affects the morphology of deposits and has a strong impact on the mobility of the electroactive species [13].

Acetonitrile is one of the most important organic solvent and serves as a key material for electrodeposition, improving the size and morphology of deposits. Zhu and co-workers have reported the electrodeposition of nickel in the presence of acetonitrile from 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ionic liquid and showed that fine-grained and smoother deposit was obtained in Ni(TFSA)₂/BMPTFSA with AN [14]. In another work, Bechelany and co-workers discovered the possibility of depositing stable and very dense Si in the case of CH₃CN [15]. Acetonitrile and water are very effective electrolytes for producing amorphous hydrogenated carbon films on SnO₂ coated glass substrates via electrodeposition [16]. Cao et al. obtained small and compact balls and smooth carbon films on ITO-coated glass substrates in acetonitrile [17]. Similar to carbon films, Mele et al. obtained smaller nuclei and compact silver deposits with increasing

<footnote n="1">¹ Center of Physical Chemical Methods of Research and Analysis, Al-Farabi Kazakh National University, 96a Tole Bi Str., 050012 Almaty, Kazakhstan</footnote>

<footnote n="2">² Aix-Marseille University, CNRS, MADIREL UMR 7246 Cedex 20, 13397 Marseille, France</footnote>
acetonitrile content [18]. Magneli-phase titanium oxide films from a saturated peroxo-titanium solution in acetonitrile clearly indicate a porous structure [19]. Moreover, the electrochemical behaviour and kinetic parameters of copper were investigated in an AN-H₂O solution [20]. Vaskevich et al. concluded that the Cu-upd in acetonitrile solution on Au proceeds in two stages and kinetics of formation of the upd layer are faster on Au{111} [21]. Also, Rudnev et al. observed that adsorbed acetonitrile (([AcN] ≤ 4 mM) molecules accelerated the formation of a co-adsorption lattice of copper adatoms on platinum. However, the Cu-upd process was hampered at higher acetonitrile concentrations and at [AcN] = 0.4–40 mM accelerate the growth of the bulk copper deposits [22]. In another study, the authors demonstrated the influence of acetonitrile on Cu-upd on Pt (110), Pt (100) and Pt (111) [23]. Nevertheless, the electrochemical nucleation of copper (II) in AN-H₂O mixed solvents has not been studied yet. Nucleation and growth kinetics are pivotal for understanding and controlling the size and morphology of electrodeposits in order to obtain electrocatalysts with various morphologies. In the present work, we have demonstrated the nucleation and growth of In(III)/In in the presence of DTPA in aqueous solutions [24]. Moreover, it is essential to develop the activity and selectivity of Cu-based catalysts for CO₂RR by controlling the size and morphology of the copper nanoparticles [25–27]. Thus, in this paper, we present the nucleation and growth mechanism of copper in acetonitrile solution in the presence of various ratios of AN/H₂O (50:50, 60:40, 70:30, 80:20 and 100). Cyclic voltammetry and chronoamperometry were used to investigate the kinetic parameters and nucleation mechanisms. Morphology studies were performed using SEM analysis. The electrocatalytic activities and electrochemical active surface area of the electrocatalysts were measured using linear sweep voltammetry, electrochemical impedance spectroscopy and cyclic voltammetry methods. This study highlights the role of electrolytes in controlling the morphology and size of copper catalysts for electrochemical CO₂ reduction reactions.

## Experimental

### Nucleation and Growth

Copper deposits on a glassy carbon electrode were carried out from an organic solution containing 0.01 M CuCl₂ (98%) and different ratios of acetonitrile (AN, ≥ 99.8%, Sigma Aldrich) and water (100, 50:50, 60:40, 70:30 and 80:20) with 0.5 M sodium perchlorate monohydrate (NaClO₄, ≥ 98%, Sigma Aldrich) at pH 1.5 (adjusted with HCl). The solutions were prepared in deionized water. Electrochemical experiments were performed in a three-electrode cell with a glassy carbon working electrode (area of 0.07 cm²), Ag|AgCl (3 M KCl) as the reference electrode and platinum as the counter electrode. An Autolab PGSTAT 302N potentiostat/galvanostat was used for electrodeposition and electrochemical characterization tests. Cyclic voltammetry was performed in the potential range of 1.5 to –1.0 V at 10 mV s⁻¹ to understand the electrochemical behaviour of copper in AN/H₂O solution and to identify the electrodeposition process. The nucleation mechanism and kinetic parameters were determined by chronoamperometry method which is the simplest and most often used method for investigating nucleation and growth mechanisms.

In addition, EQCM-CV studies were conducted using quartz crystal element coated with thin gold electrode with the geometric surface area of 0.361 cm². EQCM measurements were conducted in a small three-electrode electrochemical cell produced by Metrohm Autolab, which was equipped with a temperature sensor. Prior to experimentation, the piezoelectric element was calibrated in accordance with the standard procedure recommended by the manufacturer. The principal characteristics and methodology for utilizing this module are presented in the work [28]. The auxiliary electrode employed in these measurements was a platinum spiral, while the reference electrode was a silver chloride electrode. All experiments were performed at 25 °C, and each measurement was performed three times to ensure reproducibility.

### Methods of Characterization

Scanning electron microscopy (SEM) experiments were conducted using a Quanta 3D 200i dual system. The copper samples were electrodeposited on a glassy carbon electrode using a three-electrode electrochemical cell. The working surfaces of the glassy carbon electrodes were polished with emery paper (1000, 1500 and 2500 grit) and then mechanically polished with aluminium oxide paste. All copper nanoparticles were electrodeposited potentiostatically on GC at potential of –0.45 V and 1800s from a 70% acetonitrile and 30% water electrolyte solution containing 0.01 M CuCl₂ + 0.5 M NaClO₄ at a pH = 1.5 and T = 25 °C. After electrodeposition, all copper electrocatalysts were washed with an ethanol solution and then with deionized water.
Electrochemical CO₂ Reduction Reaction at Bulk Electrolysis Condition

Catalytic measurements were performed in a three-electrode cell, where the working electrode was glassy carbon covered with copper (at E = -0.45 V, 30 min and T = 25 °C), and each experiment was performed on a fresh sample. Ag|AgCl was used as the reference electrode, and platinum was used as the counter electrode. The electrochemical CO₂ reduction reaction under bulk electrolysis conditions was conducted in 0.5 M NaHCO₃ (≥ 99.5%, Sigma Aldrich) saturated with CO₂ gas for 30 min at a flow rate of 4 ml min⁻¹ until the pH was 7.5. Subsequently, the solution was continuously purged with CO₂ during the analysis.

Carbon dioxide electroreduction was performed using linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry methods. Electrochemical active surface areas of electrocatalysts were determined by cyclic voltammetry method. LSV curves were used to obtain Tafel plots and were recorded at a scan rate of 10 mV s⁻¹. The Tafel slope was determined from the LSV curves to understand the reaction kinetics of the CO₂ electroreduction at the copper electrodes. The potential values measured against Ag|AgCl were converted into a reversible hydrogen electrode (RHE). The equation [29]:

$$E(\text{vs.RHE}) = E(\text{vs.Ag}|\text{AgCl}) + 0.207V + 0.0591V \times pH.$$

The electrochemical impedance spectroscopy measurements were performed in the frequency range from 0.1 Hz to 100 kHz and E = -1.00, -1.10, -1.20, -1.30 and -1.40 V (vs. Ag|AgCl) using a signal amplitude of 0.01 V.

Results and Discussion

Copper Discharge-Ionization Behaviour

The electrodeposition kinetics of copper on glassy carbon from aqueous and acetonitrile/water mixtures were investigated using cyclic voltammetry. Figure 1(A) shows the cyclic voltammogram obtained in an aqueous acidic solution containing 0.01 M CuCl₂ and 0.5 M NaClO₄ at pH = 1.5. It is clear from the voltammogram that two cathodic peaks are observed. First peak in the potential range 0.20–0.00 V relating with Cu²⁺/Cu⁺ (Reaction 1), but deposition of metallic copper does not occur. The second peak Cu⁺/Cu⁰ in the potential range of from -0.20 to -0.25 V related with the deposition of copper (Reaction 2).

$$Cu^{2+}+e^{-} \rightarrow Cu^{+} \quad (1)$$

$$Cu^{+}+e^{-} \rightarrow Cu^{0} \quad (2)$$

$$Cu^{0} \leftrightarrow Cu^{+}+e^{-} \quad (3)$$

$$Cu^{+} \leftrightarrow Cu^{2+}+e^{-} \quad (4)$$
The first reduction step of copper is much slower than that of the second step [9]. The mechanism of copper nucleation is related to the crossover between the cathodic and anodic currents appearing in the reverse sweep [30]. In the anodic zone, it was observed two peaks at around 0.20 V (Reaction 3) and 0.40 V (Reaction 4), which is associated with the oxidation of copper:

Acetonitrile is a very weak hydrogen bond donor ($α$ = 0.19) and a weak hydrogen bond acceptor ($β$ = 0.40). This characteristic indicates that acetonitrile possesses the ability to specifically solvate cations while demonstrating poor efficacy in the specific solvation of anions [31]. In the case of acetonitrile, two noteworthy effects were observed in the voltammograms. Firstly, a significant shift of the reduction peaks in pure acetonitrile solution towards more negative potentials was evident when compared to the peak obtained in the aqueous solution (Fig. 1(A) purple curve).

In addition, cyclic voltammogram of pure AN solution shows a plateau after the cathodic peak related to a limiting current owing to the limiting diffusion of the electroactive species of copper. Compared with the aqueous solution in acetonitrile, the limiting current is increased. This can be related to the formation of copper-acetonitrile complexes with different coordination numbers $[Cu(AN)₄]^{2+}$, $[Cu(AN)₅]^{2+}$, and $[Cu(AN)₆]^{2+}$ [32]. The donor properties of acetonitrile are higher than those of water, and the thermodynamic stability of $[Cu(AN)₆]^{2+}$ is higher than that of $[Cu(H₂O)₆]^{2+}$.

Secondly, an analysis of Fig. 1(A) demonstrates that the discharge-ionization voltammogram of copper on glassy carbon in acetonitrile solution differs substantially from its aqueous counterpart, exhibiting two distinct oxidation-reduction pairs. The initial redox pair, observed at a potential of 0.8 V, likely corresponds to the reduction of Cu²⁺ to Cu⁺ ions. This interpretation is corroborated by findings reported in [21], which show that voltammograms of copper reduction from monovalent salt solutions lack a peak, whereas the electroreduction of copper from divalent salt solutions in acetonitrile displays a well-defined peak in the 1.0 V potential region. Furthermore, increasing acetonitrile concentration induces a positive shift in the cathodic peak C1 (Fig. 1(B)) and enhances the peak current density. Collectively, these observations suggest that acetonitrile facilitates the reduction of copper ions. It is noteworthy that, in contrast to the mixed solution, in the

<figure><img src="image_2.png" /><figcaption>**Fig. 1 (A)** Cyclic voltammograms of 0.5 M $NaClO_4$ dissolved in acetonitrile (red curve), 0.01 M $CuCl_2$ + 0.5 M $NaClO_4$ aqueous and acetonitrile solution obtained onto glassy carbon electrode, (B) at different ratios of acetonitrile-water mixture (50:50, 60:40, 70:30, 80:20 and 100:0), pH=1.5, T=25 °C and scan rate 10 mV s⁻¹</figcaption></figure>

<figure><img src="image_3.png" /><figcaption></figcaption></figure>

pure acetonitrile solution at the cathode peak bifurcates.

Copper reduction to metal presumably corresponds to peak C2. However, the third cathode peak C3 should not be attributed to copper reduction, as the voltammograms demonstrate that the intensity of this peak exhibits a clear dependence on the acetonitrile content in the solution and increases markedly in the pure acetonitrile solution. To elucidate this phenomenon, voltammograms of the background electrolyte 0.5 M $NaClO_4$ in acetonitrile on the copper electrode were obtained and are presented in Figure S1. The voltammograms of the background solution, as well as the voltammogram in Fig. 1(B), exhibit a cathodic peak at a potential of -0.6 V, corresponding to peak C3. This observation indicates the demonstration of the catalytic activity of copper for the electroreduction of acetonitrile. However, it is noteworthy that significant decomposition of the acetonitrile solvent occurs at more negative potentials of approximately -0.95 V, corresponding to peak C1 in Figure S1(B). The reduction of acetonitrile appears to result in the formation of products that impede further transfer of the charge (because, with an increase in the content of acetonitrile in the solution, there is a decrease in the intensity of the peak of anodic oxidation of copper, peak A2 in Fig. 1), and the complex nature of the anodic branches of the voltammograms (peaks A1 and A2) shown in Figure S1 suggests a diversity of reduction products. It is evident that the anodic dissolution of copper
occurs at potentials more positive than -0.2 V, which corresponds to peaks A3 in Figure C1(B) and peak A2 in Fig. 1(B). Subsequent oxidation of copper to Cu<sup>2+</sup> occurs in the potential region corresponding to peak A1 in Fig. 1.

The electrochemical behaviour of copper in a mixed and acetonitrile solution exhibits considerable complexity. It is noteworthy that, in the region of the limiting current, between the peaks C1 and C2, not only the reduction of copper to Cu<sup>+</sup> occurs (a simple one-electron reaction without the formation of a new phase). This is evidenced by the voltammograms of the gold electrode covered to the surface of the piezoelectric quartz element shown in Figure S2. The results of electrochemical quartz crystal microbalance (EQCM) measurements in aqueous solution (Figure S2 (A)) demonstrate voltammograms similar to glassy carbon shown in Fig. 1(A). The change in the frequency of the piezoelectric quartz electrode (decrease in its frequency) starts from a potential of -0.1 V, which indicates a change in the mass of the electrode and the initiation of copper electrodeposition on the surface of the gold electrode. Anodic oxidation, as well as a rapid increase in the frequency of the piezoelectric element, begins at a potential of 0.3 V. The increase in frequency to its initial value (to the initial mass of the electrode) during the reverse, anodic scanning of the potential indicates the reversibility of the copper discharge-ionization process. However, the EQCM curves of the copper discharge-ionization on the gold electrode exhibit a highly complex nature (Figure S2(B)). In an acetonitrile solution, the reduction of copper(II) to copper(I) occurs at the peak C1, as confirmed by an insignificant change in frequency associated with the adsorption of electroactive particles (Figure S2(C)). Unlike an aqueous solution, in an acetonitrile solution, the increase in the mass of the electrode, i.e. the decrease in the frequency of the piezoelectric element occurs at significantly more positive potentials, starting from the peak C2 to the potential of -0.2 V, compared to an aqueous solution. During this period, the frequency decreases to -4 kHz (Figure S3(A)), which is slightly higher than that of the aqueous electrolyte for the full cycle of cathodic deposition (-3.2 kHz). This phenomenon can be attributed to the electrodeposition of copper under underpotential deposition (Cu<sub>UPD</sub>) condition; otherwise, such a significant increase in the electrode mass would be inexplicable. A further increase in the electrode mass, i.e. a decrease in the frequency with an even steeper slope is associated with the overpotential deposition (Cu<sub>OPD</sub>) of copper (peak C3 Figure S3(A)). The reverse potential scan reveals a stable electrode mass, with peak A5 primarily attributed to acetonitrile reduction rather than copper electroreduction, as indicated by peak C4 (Figure S2(B)). Minimal oxidation of electrodeposited copper, corresponding to peaks A4 and A3 (Figure S2(B)), has negligible impact on the electrode mass. The increase in frequency and decrease in electrode mass to its initial value are solely attributable to peaks A2 and A1, demonstrating copper's bulk dissolution. Notably, the piezoelectrode frequency post-measurement remains approximately -1 kHz below its initial value, suggesting either partial process irreversibility or surface contamination by solvent decomposition products.

The experimental findings suggest that acetonitrile molecules, characterised by a smaller diffusion coefficient, can displace [Cu(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> hydrated copper ions. Chronoamperometric measurements revealed diffusion coefficients of copper ions in 50:50 acetonitrile/water solution and pure acetonitrile solution to be 6.24 × 10<sup>-6</sup> cm<sup>2</sup> s<sup>-1</sup> and 1.54 × 10<sup>-7</sup> cm<sup>2</sup> s<sup>-1</sup>, respectively. Therefore, organic solvents preferentially fill the surface of the electrode, whereas the reactant includes water in its solvation shell [33].

Moreover, the higher polarization change in the acetonitrile + perchlorate electrolyte indicates a strong tendency for ion pair formation. Theoretical studies have shown that perchlorate ions in acetonitrile have a synergistic effect on the copper redox couple. The stabilization of Cu(I) by acetonitrile is related to its strong π acceptor nature and orientation polarization, leading to the formation of an ion pair. Reorganization of the solvent by perchlorate ions promotes rapid electron transfer. The theoretically calculated Gibbs free energy and redox potential of the copper (II)-copper (I) couple increased in the order of water < acetonitrile < acetonitrile + perchlorate [34]. In addition, acetonitrile solvates monovalent metal ions well, followed by divalent metal ions. This can be attributed to the very small K<sub>d</sub> value [35], which is corroborated by the disproportionation constants of the reaction in water and acetonitrile [34].

$$2Cu^{+} = Cu^{2+} + Cu(s)$$

$$(\log K_{d(aqua)} = 6.26 \text{ and } \log K_{d(AN)} = -20.8)$$
Electrochemical Nucleation and Growth of Cu

The kinetics and nucleation mechanisms of Cu on the GC electrode were determined in the potentiostatic regime. Potentiostatic current transients of the copper electrodeposition are obtained in different ratios of acetonitrile/water solution at a more negative potential of -0.45 V for 2 s, as shown in Fig. 2(A).

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2 (A) Potentiostatic current transients of Cu electrodeposition from perchlorate-chloride solution at different ratio of AN/H₂O, E = -0.45 V, pH = 1.5 and 25 °C; (B) dimensionless current transients for copper nucleation, theoretical plots for three-dimensional “progressive,” and “instantaneous” nucleation mechanisms (solid lines)</figcaption></figure>

A decrease in the current (absolute value of the current) was observed which was related to difficult to identify the nucleation mechanism. The kinetic double-layer charging (initial part) and a slight increase up parameters of nucleation and growth are determined as a to the current maxima related to the formation of the first progressive mechanism for all Cu-AN/H₂O mixtures and nuclei on the active sites. The current density increases are presented in Table 1.

to a maximum, indicating an increase in the electroactive where Ns is the saturated number of active nuclei centres; area which is related to the growth of previously formed AN∞ is the nucleation rate; and Dcu2+ is the diffusion coefficient of Cu2+ ions. diffusion-controlled electrochemical processes [36]. The diffusion coefficient of Cu2+ ions calculated from Figure 2(B) shows the experimental dimensionless cur- chonoamperograms (Fig. 2) according to jmax and tmax was rent transients compared with the theoretical curves calcu- taken to be 1.38 × 10⁻⁵ cm² s⁻¹ in aqueous solution and lated according to the Scharifker-Hills model. This model is 15.4 × 10⁻⁶ cm² s⁻¹ in acetonitrile solution, respectively. simple for classifying experimental transients into “inanta- The diffusion coefficient of copper ions is dependent on the neous” and “progressive” nucleation mechanisms (Eqs. (5) AN/H₂O ratio and is higher in the 80:20 and 50:50 AN/H₂O and (6), respectively) [37].

$$ \frac{j^2}{j_{\max}^2} = \frac{1.9542}{\frac{t}{t_{\max}}} \left\{ 1 - \exp\left[-1.2564\left(\frac{t}{t_{\max}}\right)\right] \right\}^2 \quad (5) $$

$$ \frac{j^2}{j_{\max}^2} = \frac{1.2254}{\frac{t}{t_{\max}}} \left\{ 1 - \exp\left[-2.3367\left(\frac{t}{t_{\max}}\right)^2\right] \right\}^2 \quad (6) $$

where jmax and tmax are the maximum values of current den- deposition mechanism changed from instantaneous to porosity and time in the chronoamperogram, respectively.

As shown in Fig. 2(B), the mechanism of nucleation from -0.43 V to more negative potentials, the nucleation of Cu deposition on the GC electrode in the pure AN mechanism tended to mix nature. and 70:30 AN/H₂O mixture solutions is “progressive”, while for 50:50, 60:40, and 80:20 AN/H₂O solutions, it is difficult to identify the nucleation mechanism. The kinetic parameters of nucleation and growth are determined as a progressive mechanism for all Cu-AN/H₂O mixtures and are presented in Table 1.
where Ns is the saturated number of active nuclei centres; AN∞ is the nucleation rate; and DCu2+ is the diffusion coefficient of solvated Cu2+ ions.

The diffusion coefficient of Cu2+ ions calculated from chronoamperograms (Fig. 2) according to jmax and tmax was taken to be 1.38 × 10⁻⁵ cm² s⁻¹ in aqueous solution and 15.4.10⁻⁶ cm² s⁻¹ in acetonitrile solution, respectively.

The diffusion coefficient of copper ions is dependent on the AN/H₂O ratio and is higher in the 80:20 and 50:50 AN/H₂O electrolytes. It shows that the transport of ions was faster in the aqueous solution. The diffusion coefficients of copper (II) ions in various non-aqueous electrolytes are listed in Table 2.

A series of current transients for Cu electrochemical deposition at different applied potentials for aqueous and AN/H₂O solutions are shown in Fig. 3. The mechanism of Cu nucleation in aqueous solutions is instantaneous. For the system Cu-70:30 AN/H₂O, it was found that the electrodeposition mechanism changed from instantaneous to progressive. However, as the applied potential was decreased from -0.43 V to more negative potentials, the nucleation mechanism tended to mix nature.

From the experimental current transients, the parameters of electrochemical nucleation were calculated to be progressive for various applied potentials (Table 3). As the potential became more negative, it was associated with an increase in the nucleation rate AN and nuclei density Ns. The diffusion coefficient of copper ions calculated for 70–30 AN/H₂O solution is 7.75 × 10⁻⁶ cm² s⁻¹, which differs from the values in aqueous solution 1.38 × 10⁻⁵ cm² s⁻¹. In this instance, the mobility of copper ions is lower in non-aqueous solutions than in aqueous solutions.

where ravg is the average radius of the growing centres, which was calculated from Ns values using Eq. (5) shown in [45].

<table><caption>**Table 1** Kinetic parameters of Cu nucleation on GC electrode from 0.01 M CuCl₂ + 0.5 M NaClO₄ in different ratios of AN/H₂O solutions at -0.45 V, calculated according to the equations given in [24]</caption><thead><tr><th>AN/H₂O ratio</th><th>j<sub>max</sub>, mA cm<sup>-2</sup></th><th>t<sub>max</sub>, s</th><th>N<sub>s</sub>, 10<sup>6</sup> cm<sup>-2</sup></th><th>AN<sub>∞</sub>, 10<sup>7</sup> cm<sup>-2</sup> s<sup>-1</sup></th><th>D<sub>Cu2+</sub>, 10<sup>-6</sup> cm<sup>2</sup> s<sup>-1</sup></th></tr></thead><tbody><tr><td>50:50</td><td>6.70</td><td>0.15</td><td>15.41</td><td>16.78</td><td>6.24</td></tr><tr><td>60:40</td><td>9.02</td><td>0.12</td><td>14.47</td><td>20.54</td><td>8.67</td></tr><tr><td>70:30</td><td>7.21</td><td>0.20</td><td>7.49</td><td>6.11</td><td>9.63</td></tr><tr><td>80:20</td><td>6.09</td><td>0.18</td><td>13.71</td><td>12.79</td><td>6.01</td></tr><tr><td>100:0</td><td>3.40</td><td>1.44</td><td>0.65</td><td>0.07</td><td>15.4</td></tr></tbody></table>

<table><caption>**Table 2** Diffusion coefficient values of Cu(II) species in various nonaqueous solutions</caption><thead><tr><th>D<sub>Cu2+</sub>, 10<sup>-6</sup> cm<sup>2</sup> s<sup>-1</sup></th><th>T, K</th><th>Electrode</th><th>Solution</th><th>Ref</th></tr></thead><tbody><tr><td>6.29</td><td>298</td><td>Cu</td><td>CuSO<sub>4</sub> in tetraethylammonium hydrogen sulfate NEt<sub>4</sub>HSO<sub>4</sub></td><td>[38]</td></tr><tr><td>5.06</td><td></td><td></td><td>Tetrabutylammonium hydrogen sulfate NBu<sub>4</sub>HSO<sub>4</sub></td><td></td></tr><tr><td>2.80</td><td>293</td><td>GC</td><td>CuSO<sub>4</sub> in 10% H<sub>2</sub>O–10% methanesulfonic acid–80% methanol solution</td><td>[39]</td></tr><tr><td>1.0</td><td>298</td><td>GC</td><td>Cu(NO<sub>3</sub>)<sub>2</sub> in protic ionic liquid ethylammonium nitrate</td><td>[40]</td></tr><tr><td>0.68</td><td>375</td><td>GC</td><td>CuCl<sub>2</sub> in 1-hexyl-3-methylimidazolium</td><td>[41]</td></tr><tr><td>0.15</td><td>303</td><td>W</td><td>Cu(I)/Cu(II) in EMI-C-BF<sub>4</sub></td><td>[42]</td></tr><tr><td>0.01</td><td>293</td><td>Pt</td><td>CuCl<sub>2</sub> in the urea-based ionic liquid</td><td>[43]</td></tr><tr><td>0.007</td><td>298</td><td>GC</td><td>CuCl<sub>2</sub> in TOMAC/CH<sub>3</sub>Cl</td><td>[44]</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 3** Chronoamperograms of Cu electrodeposition from aqueous solution: 0.01 M CuCl₂ + 0.5 M NaClO₄ (A) and from perchlorate-chloride AN/H₂O 70:30 solution (B) for different applied potentials; Corresponding dimensionless current transient (A') and (B'), respectively</figcaption></figure>

<table><caption>**Table 3** Kinetic parameters of Cu nucleation on GC electrode from aqueous: 0.01 M CuCl₂ + 0.5 M NaClO₄ and AN/H₂O solution: 0.01 M CuCl₂ + 0.5 M NaClO₄ in 70–30 AN/H₂O at various applied potentials</caption><thead><tr><th colspan="3">Aqueous solution</th><th colspan="4">70:30 AN/H<sub>2</sub>O solution</th></tr><tr><th>E, V</th><th>N<sub>0</sub>, 10<sup>6</sup> cm<sup>-2</sup></th><th>r<sub>avg</sub>, µm</th><th>E, V</th><th>N<sub>s</sub>, 10<sup>6</sup> cm<sup>-2</sup></th><th>AN<sub>∞</sub>, 10<sup>6</sup> cm<sup>-2</sup> s<sup>-1</sup></th><th>r<sub>avg</sub>, µm</th></tr></thead><tbody><tr><td>-0.05</td><td>0.99</td><td>17.8</td><td>-0.43</td><td>2.70</td><td>0.86</td><td>10.8</td></tr><tr><td>-0.06</td><td>1.81</td><td>13.2</td><td>-0.44</td><td>4.94</td><td>2.69</td><td>7.9</td></tr><tr><td>-0.07</td><td>2.74</td><td>10.7</td><td>-0.45</td><td>7.48</td><td>6.11</td><td>6.5</td></tr><tr><td>-0.08</td><td>4.55</td><td>8.3</td><td>-0.46</td><td>12.42</td><td>15.61</td><td>5.0</td></tr><tr><td>-0.09</td><td>6.11</td><td>7.2</td><td>-0.47</td><td>16.67</td><td>28.66</td><td>4.3</td></tr><tr><td>-0.10</td><td>11.35</td><td>5.3</td><td>-0.48</td><td>31.00</td><td>84.37</td><td>3.2</td></tr></tbody></table>

In addition, the grain size of electrodeposited copper particles ($r_{avg}$) was calculated using the nucleation density (Fig. 4). As shown in Table 3, the average radius of the grains decreases as the overpotential and saturated number of active nuclei centres increases.

Metal nanoparticles exhibit unusual chemical and physical properties for catalysis and electronic devices. Copper nanoparticles can be synthesized using various methods, such as chemical reduction and electrochemical, photochemical,
and thermal decomposition [46]. Copper nanoparticles were synthesized in an acetonitrile/water solvent using the borohydride reduction of copper nitrate under inert argon purged conditions [47].

<figure><img src="image_7.png" /><figcaption>**Fig. 4** Dependence of the saturated number of active nuclei centres ($N_s$) and the average nucleus radius ($r_{avg}$) on the applied potential (E), obtained by Scharifker-Hills analysis of the potentiostatic current transient, for aqueous and AN/H₂O 70:30 solution</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 5** SEM images of copper particles electrodeposited on GC electrode at -0.45 V for 1800s from 0.01 M CuCl₂ + 0.5 M NaClO₄ in AN/H₂O 70:30 solution</figcaption></figure>

In this study, Cu particles were synthesized using an electrochemical method. As shown in Fig. 5, SEM images of the Cu particles electrodeposited on GC from 70:30 AN/H₂O solution are obtained under the optimal conditions (T = 25 °C, E = -0.45 V and deposition time 1800s). The deposit obtained from 70:30 AN/H₂O exhibited a “monanthes-like” morphology, which is consistent with the previously indicated progressive nucleation mechanism. Furthermore, the size of copper nanoparticles in non-aqueous solution was 6.5 µm (E = -0.45 V), which is smaller compared to the aqueous solution at 10.7 µm (E = -0.07 V). The reduction in particle size can be attributed to the blocking and stabilizing effects of acetonitrile through adsorption on the GC electrode surface. Acetonitrile creates a barrier to the subsequent aggregation of Cu particles and reduces the surface energy.
The particle size distribution curve given in Figure S3(B) reveals the formation of copper particles with a mean grain size of 4.25 µm, which aligns with the findings from the electrochemical nucleation investigation (Fig. 4). X-ray diffraction analysis of the electrodeposited copper deposits onto glassy carbon surface demonstrates the presence of polycrystalline copper (Cu(111) and Cu(200)). Owing to the reactivity of the obtained copper particles, a part of the surface undergoes oxidation, as indicated by the peaks corresponding to CuO(110), Cu<sub>2</sub>O(111) and Cu<sub>2</sub>O(311) (Figure S3(C)).

Scanning electron microscopy images of copper electrodeposited from aqueous perchlorate-chloride solution exhibit the deposition of irregularly distributed copper particles of diverse dimensions (Figure S4(A)). The glassy carbon surface is predominantly covered by large copper particles (highlighted in yellow), while the presence of small cubic particles (highlighted in purple) is presumably related to secondary nucleation. The absence of small copper particles in the vicinity of large growth centres indirectly supports this postulation, as a semi-spherical diffusion front develops around the expanding larger growth centre, resulting in the formation of a depleted region. Moreover, the intersection of hemispherical zones between large growth centres diminishes the probability of new growth centre formation. As elucidated by the electrochemical nucleation study (Fig. 3(A')), copper deposition in an aqueous medium proceeds via a 3D instantaneous nucleation mechanism, characterized by the rapid expansion of initially active growth centres, while the rate of new growth centre formation remains minimal. The introduction of acetonitrile to the solution markedly alters this process by influencing the nucleation mechanism and deposit's morphology, as evidenced by the SEM images of mix solutions (Figure S4(B–E)). It was determined that acetonitrile promotes the production of uniformly distributed copper particles.

Smaller copper particles with lower size show higher activity for CO₂ electroreduction reactions and increase reaction selectivity [48]. The smaller nanoparticles provide much larger active surface areas [49]. The roughened surface of the sample can provide active sites and improve the Faradaic selectivity of the CO₂ electroreduction products [50]. Therefore, it is important to synthesize Cu particles with a high surface area and “monanthes-like” structures.

## Determination of the Electrochemical Active Surface Area

To investigate the inherent properties influencing the catalytic efficacy of electrodeposited electrodes, cyclic voltammetry was employed to characterize preliminary activity of the modified GC electrodes. Scanning electron microscopy demonstrated negligible morphological variations among electrodes deposited from acetonitrile/water mixed solutions. Electrochemical active surface area determinations were performed within the non-Faradaic potential region, as shown in the work [51]. Figure 6(A) shows the cyclic voltammograms of the glassy carbon electrode modified with the copper catalyst. The double layer capacitances, obtained from the slope of the linear relationship shown in Fig. 6(B), are presented in Table S1 (Supplementary information).

An analysis of the data presented in Table S1 reveals that the double layer capacity values of the investigated catalysts exhibit minimal variation, suggesting comparable electrochemical active surface areas amongst the samples.
<figure><img src="image_9.png" /><figcaption>**Fig. 6** (A) CVs of the Cu-50:50 AN/H₂O catalysts obtained in deaerated by N₂ 0.5 M NaHCO₃ solution at various scan rates; (B) the relationship between charge-discharge current density difference (Δj) and scanning rate</figcaption></figure>

Electrocatalysis Test of Electrodeposited Cu for eCO₂RR

The electrochemical performance of the electrocatalysts was investigated using a three-electrode electrochemical cell under bulk electrolysis conditions. The Cu-AN/H₂O electrocatalysts were obtained by the electrodeposition of copper from an acetonitrile/water solution at a potential of -0.45 V for 1800s.

To estimate the activity of Cu-AN/H₂O electrocatalysts toward CO₂ electroreduction the LSV measurements are conducted in CO₂ saturated 0.5 M NaHCO₃ (pH = 7.5) solution in the potential range of -0.6 to -1.4 V (vs. Ag/AgCl) at a scan rate of 10 mV s⁻¹. As shown in Fig. 7(A), all electrodes exhibited excellent catalytic activity compared to the bare GC electrode. Among them, the Cu-50:50 AN/H₂O electrocatalyst displayed the best performance with a higher current density of 24 mA cm⁻² at -1.4 V.

It is well established that nearly all developed and investigated catalysts for the electrochemical conversion of carbon dioxide are affected by parasitic reactions. To elucidate the contribution of the hydrogen evolution reaction to the eCO₂RR rate, linear voltammograms of 0.5 M NaHCO₃ solution saturated with N₂ and CO₂ were obtained (Fig. 8(A)). The LVS curve demonstrates the catalytic activity of the copper catalysts obtained by electrochemical deposition at eCO₂RR, particularly in the region of low polarization. It is evident that at a potential more negative than -1.2 V, the contribution of the hydrogen evolution reaction becomes significant. A more comprehensive representation in this regard is provided by the Nyquist curves shown in Fig. 8(B). It is apparent that the electrodeposited copper catalyst exhibits a 3.5 times smaller semicircle in the presence of CO₂, which unequivocally indicates its superior catalytic activity. The charge transfer resistance (Rct) of the Cu-50:50 AN/H₂O electrode at -1.2 V saturated with N₂ and CO₂ were 32.8 Ω cm² and 9.3 Ω cm², respectively. The small value of Rct in CO₂ saturated solution implies that the main products are organic compounds which formed during CO₂ electroreduction. In conclusion, the electrochemical results demonstrated that the copper electrocatalysts derived from the AN/H₂O solutions exhibited electrochemical activity for CO₂ conversion, and the determination of its selectivity remains the subject of further investigation.
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 7 (A)** LSV of eCO₂RR obtained in 0.5 M NaHCO₃ saturated with CO₂ on bare GC electrode, and Cu electrodes electrodeposited from pure acetonitrile and AN/H₂O solutions at a scan rate of 10 mV/s; (B) the corresponding Tafel plots; (C) experimental (solid</figcaption></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

line) and calculated (dash line) Nyquist plots of Cu-50:50 AN/H₂O electrocatalysts at various applied potentials (insert: electrical equivalent circuit of eCO₂RR); (D) experimental Nyquist plots of the obtained Cu-AN/H₂O electrocatalysts at -1.2 V potential

<table><caption>**Table 4** Kinetic parameters of eCO₂RR on Cu-AN/H₂O electrocatalysts obtained from LSV and EIS measurements in a 0.5 M NaHCO₃ solution saturated with CO₂ at 25 °C</caption><thead><tr><th>Electrode</th><th>R<sub>cp</sub>, Ω cm<sup>2</sup></th><th>Tafel slope – b, mV dec<sup>-1</sup></th><th>Transfer coefficient, α<sub>c</sub></th><th>Exchange current density – j<sub>0</sub>, mA cm<sup>-2</sup></th></tr></thead><tbody><tr><td>Cu-50:50 AN/H<sub>2</sub>O</td><td>9.4</td><td>168</td><td>0.322</td><td>0.681</td></tr><tr><td>Cu-60:40 AN/H<sub>2</sub>O</td><td>12.1</td><td>188</td><td>0.288</td><td>0.591</td></tr><tr><td>Cu-70:30 AN/H<sub>2</sub>O</td><td>11.6</td><td>181</td><td>0.299</td><td>0.633</td></tr><tr><td>Cu-80:20 AN/H<sub>2</sub>O</td><td>11.2</td><td>180</td><td>0.301</td><td>0.643</td></tr><tr><td>Cu-100 AN</td><td>9.5</td><td>175</td><td>0.309</td><td>0.662</td></tr></tbody></table>
<figure><img src="image_14.png" /><figcaption>**Fig. 8 (A)** LSV of eCO₂RR obtained in 0.5 M NaHCO₃ saturated with CO₂ (pH=7.5) and N₂ (pH=9.0) on Cu electrode electrodeposited from 50:50 AN/H₂O solution at a scan rate of 10 mV/s; **(B)**</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Nyquist plots of Cu-50:50 AN/H₂O electrode at E = -1.2 V (insert: electrical equivalent circuit of eCO₂RR)</figcaption></figure>

**Conclusions**

In summary, the nucleation mechanisms of Cu electrodeposition from aqueous acetonitrile solutions were investigated using electrochemical methods and SEM. EQCM investigations of copper discharge-ionization in pure acetonitrile electrolyte demonstrated the complex nature of the process in comparison to an aqueous solution. The results of the chronoamperometric transients analysed with the Scharifker Hills nucleation model indicate that the mechanism of copper nucleation in the AN and 70:30 AN/H₂O solutions was progressive, whereas for 50:50, 60:40 and 80:20 AN/H₂O solutions, it was approximately mixed. The diffusion coefficient of copper ions in acetonitrile solution was estimated to be 15.4 × 10⁻⁶ cm² s⁻¹ using chronoamperograms. SEM images revealed well-dispersed “monanthes-like” copper nanoparticles, which were attributed to the stabilization effect of acetonitrile.

Carbon dioxide electroreduction was examined at Cu-AN/ H₂O based catalysts through linear sweep voltammetry, electrochemical impedance spectroscopy and cyclic voltammetry methods. The Cu-50:50 AN/H₂O catalyst exhibited a high current density of 24 mA cm⁻² at -1.4 V, Tafel slope of 168 mV dec⁻¹ and exchange current density of 0.681 mA cm⁻² for eCO₂RR. Electrochemical impedance spectroscopic measurements demonstrated that the Cu-50:50 AN/H₂O catalyst possessed a significantly lower charge transfer resistance of 9.4 Ω cm² compared to other catalysts. This study suggests that acetonitrile may be beneficial for obtaining copper catalysts with enhanced morphology for the electrochemical CO₂ reduction reaction.

**Acknowledgements** The authors thank Dr. Mukhit Muratov and Ms. Dinara Akhmetsadyk from the National Nanotechnological Laboratory of Open Type, Al-Farabi Kazakh National University, and Dr. Amantay Dalbanbay from Satbayev University for providing SEM analysis of the electrodeposited copper samples.

**Author Contribution** Akbayan Bekey Writing-original draft, Investigation, Formal analysis. Florence Vacandio Conceptualization, Writing -
References

1. Y. Hori, I. Takahashi, O. Koga, N. Hoshi, J. Phys. Chem. B **106**, 15 (2002)

2. D. Raciti, C. Wang, ACS Energy Lett. **3**, 1545 (2018)

3. Z. Gu, H. Shen, L. Shang, X. Lv, L. Qian, G. Zheng, Small Methods **2**, 1 (2018)

4. K.P. Kuhl, E.R. Cave, D.N. Abram, T.F. Jaramillo, Energy Environ. Sci. **5**, 7050 (2012)

5. Y. Li, F. Cui, M.B. Ross, D. Kim, Y. Sun, P. Yang, Nano Lett. **17**, 1312 (2017)

6. D. Kim, C.S. Kley, Y. Li, P. Yang, Proc. Natl. Acad. Sci. U. S. A. **114**, 10560 (2017)

7. B. Liu, C. Cai, B. Yang, K. Chen, Y. Long, Q. Wang, S. Wang, G. Chen, H. Li, J. Hu, J. Fu, M. Liu, Electrochim. Acta **388**, 138552 (2021)

8. A. Vicenzo, P.L. Cavallotti, J. Appl. Electrochem. **32**, 743 (2002)

9. D.E. García-Rodríguez, L.H. Mendoza-Huizar, C.H. Rios-Reyes, M.A. Alatorre-Ordaz, Quim. Nova **35**, 699 (2012)

10. A. Ramos, M. Miranda-Hernández, I. González, J. Electrochem. Soc. **148**, C315 (2001)

11. A. Radisic, J.G. Long, P.M. Hoffmann, P.C. Searson, J. Electrochem. Soc. **148**, C41 (2001)

12. W. Simka, D. Puszczyk, G. Nawrat, Electrochim. Acta **54**, 5307 (2009)

13. R. Al-Salman, H. Sommer, T. Brezesinski, J. Janek, Electrochim. Acta **246**, 1016 (2017)

14. Y.L. Zhu, Y. Katayama, T. Miura, Electrochim. Acta **55**, 9019 (2010)

15. M. Bechelany, J. Elias, P. Brodard, J. Michler, L. Philippe, Thin Solid Films **520**, 1895 (2012)

16. S. Gupta, R.L. Rahini, Electrochim. Acta **258**, 1 (2017)

17. C. Cao, H. Zhu, H. Wang, Thin Solid Films **368**, 203 (2000) 18. C. Mele, S. Rondinini, L. D'Urzo, V. Romanello, E. Tondo, A. Minguzzi, A. Vertova, B. Bozzini, J. Solid State Electrochem. **13**, 1577 (2009)

19. Z. Ertekin, U. Tamer, K. Pekmez, Electrochim. Acta **163**, 77 (2015)

20. I.D. Macleod, A.J. Parker, P. Singh, J. Solution Chem. **10**, 757 (1981)

21. A. Vaskevich, I. Rubinstein, J. Electroanal. Chem. **491**, 87 (2000)

22. A.V. Rudnev, E.B. Molodkina, A.I. Danilov, Y.M. Polukarov, J.M. Feliu, Russ. J. Electrochem. **42**, 381 (2006)

23. A.V. Rudnev, E.B. Molodkina, A.I. Danilov, Y.M. Polukarov, J.M. Feliu, Electrochem. Commun. **10**, 502 (2008)

24. A. Bekey, G.L. Badavamova, F. Vacandio, K. Avchukir, J. Solid State Electrochem. **27**, 3439 (2023)

25. N.T. Suen, Z.R. Kong, C.S. Hsu, H.C. Chen, C.W. Tung, Y.R. Lu, C.L. Dong, C.C. Shen, J.C. Chung, H.M. Chen, ACS Catal. **9**, 5217 (2019)

26. K. Rossi, R. Buonsanti, Acc. Chem. Res. **55**, 629 (2022)

27. C. Xiao, J. Zhang, ACS Nano **15**, 7975 (2021) 28. Z. Mansurov, Z. Supiyeva, K. Avchukir, A. Taurbekov, M. Yeleuov, G. Smagulova, M. Mansurova, M. Biisenbayev, V. Pavlenko, Eurasian Chem. J. **21**, 283 (2019)

29. J.J. Lv, M. Jouny, W. Luc, W. Zhu, J.J. Zhu, F. Jiao, Adv. Mater. **30**, 1 (2018)

30. M. Eyraud, S. Kologo, L. Bonou, Y. Massiani, J. Electroceramics **16**, 55 (2006)

31. K. Kaupmees, I. Kaljurand, I. Leito, J. Solution Chem. **43**, 1270 (2014)

32. Y.W. Hsiao, U. Ryde, Inorganica Chim. Acta **359**, 1081 (2006)

33. L.M. Doubova, S. Daolio, C. Pagura, A. De Battisti, S. Trasatti, Russ. J. Electrochem. **39**, 164 (2003)

34. M.A. Rizvi, S.A. Akhoon, S.R. Maqsood, G.M. Peerzada, J. Anal. Chem. **70**, 633 (2015)

35. I. Persson, J.E. Penner-Hahn, K.O. Hodgson, Inorg. Chem. **32**, 2497 (1993)

36. J. Vazquez-Arenas, G. Vázquez, A.M. Meléndez, I. González, J. Electrochem. Soc. **154**, D473 (2007)

37. B. Scharifker, G. Hills, Electrochim. Acta **28**, 879 (1983)

38. Q. Zhang, X. Yu, Y. Hua, W. Xue, J. Appl. Electrochem. **45**, 79 (2015)

39. S.J. MacHardy, L.J.J. Janssen, J. Appl. Electrochem. **34**, 169 (2004)

40. C.A. Gunawan, B.H.R. Suryanto, C. Zhao, J. Electrochem. Soc. **159**, D611 (2012)

41. K. Shakeela, A.S. Dithya, C.J. Rao, G.R. Rao, J. Chem. Sci. **127**, 133 (2015)

42. P. Chen, I. Sun, Electrochim. Acta **45**, 441 (1999)

43. A.P. Abbott, K. El Ttaib, G. Frisch, K.J. McKenzie, K.S. Ryder, Phys. Chem. Chem. Phys. **11**, 4269 (2009)

44. I. Ben Assaker, M. Dhahbi, J. Mol. Liq. **161**, 13 (2011)

45. K. Avchukir, N.N. Yessaly, B.D. Burkitbayeva, Eurasian Chem. J. **21**, 157 (2019)

46. H.R. Ghorbani, Orient. J. Chem. **30**, 803 (2014)

47. Y. Kusumoto, M. Muruganandham, Mater. Lett. **63**, 2007 (2009)

48. I. Merino-Garcia, J. Albo, A. Irabien, Nanotechnology **29**(2018)

49. G.E. Johnson, R. Colby, J. Laskin, Nanoscale **7**, 3491 (2015)

50. W. Tang, A.A. Peterson, A.S. Varela, Z.P. Jovanov, L. Bech, W.J. Durand, S. Dahl, J.K. Nørskov, I. Chorkendorff, Phys. Chem. Chem. Phys. **14**, 76 (2012)

51. Y. Zhang, L. Chen, F. Li, C.D. Easton, J. Li, A.M. Bond, J. Zhang, ACS Catal. **7**, 4846 (2017)