Investigation of the Dependence of Electrocatalytic Activity of Copper and Palladium Nanoparticles on Morphology and Shape Formation

Abstract

A synthesis strategy for the manufacture of inexpensive highly efficient nanostructured catalysts has been developed. The developed unique nonplatinoid copper-based catalysts with different surface morphology were investigated as a functional layer with high activity in the ethanol oxidation in alkaline media. A modifying layer with controlled morphology, composition, and excellent electrocatalytic activity was synthesized by electrochemical deposition by varying such synthesis parameters as deposition temperature and time, concentration of structure-forming additives, and electrodeposition current. The dependence of the samples’ electrocatalytic activity on the shaping factors was established. According to the electrochemical study results, the highest current density peak of up to 33.01 mA cm⁻², and hence the highest catalytic activity in comparison to other samples, were possessed by a catalyst with a regular cubic particle shape. A catalyst consisting of plate-like nanoparticles with a certain percentage of disclinations had similar, but slightly less activity, with a current density peak of up to 31.59 mA cm⁻². The samples’ activity values are 8 times higher for cubic particles and 7.5 times higher for particles with a triangular plate shape than for an unmodified smooth copper film. The developed samples can be considered as quite competitive to platinoid catalysts, which significantly outperform copper analogues.

1. Introduction

Metal nanoparticles with certain facets and shapes differ from bulk materials in terms of thermal, optical, electrical, and magnetic aspects due to their unique morphology and size effects. This makes them promising materials in the field of catalysis [1,2], spectroscopy [3], nanomedicine [4,5], electronics [6,7], energy conversion [8], and hydrogen energy, including high-purity hydrogen evolution (99.999% purity) [9,10,11,12], etc. The branch of synthesis of functional nanostructured coatings with a greater specific active surface area and improved catalytic, protective, and other necessary properties is of great interest [13,14,15].

Catalysts with high activity, low cost, extended lifetime, and high stability are essential for the industrialization of many areas of life, including those mentioned above. Nowadays, catalysts based on noble metals such as platinum and palladium are widely used due to their high catalytic activity, selectivity, and milder reaction conditions [16,17]. However, the high cost and limited availability of these metals may prevent their use in industrial processes [18]. As a result, there are numerous studies of the possibility of using transition metal catalysts to replace noble metal catalysts in order to increase economic benefits. It has been shown that catalysts based on transition metals exhibit significant activity in various reactions [19,20,21].

Copper nanoparticles are particularly attractive for this purpose because they often enable reactions to be conducted under green or sustainable reaction conditions that would reduce the activity of conventional catalysts. This could be the Cu-nanoparticle-mediated catalysis of click chemistry, reduction and oxidation reactions, A3 coupling, cross coupling, tandem and multicomponent reactions, C–H functionalization, clock reactions, borylation, oxidative coupling, or other miscellaneous reactions [22]. For example, copper oxide nanocubes doped with gold nanoparticles with a controlled electronic structure were developed in a recent study by A. Jiao et al. [23]. An efficient electron transfer was demonstrated in the resulting Au/Cu₂O heterostructures from metal nanoparticles to Cu₂O nanocubes due to the difference in Fermi energy between the two components. The resulting particles had increased peroxidase catalytic activity and the possibility of further application of the developed catalyst in the fields of biomedicine and food safety.

In general, the search and investigation of the main factors determining the catalytic properties of nanoparticles is one of the main goals of this paper as well as the entire catalysis science. A decrease in particle size, and hence a decrease in the average lattice coordination number and free surface energy, lead to increase in catalytic activity [22].

Another significant activity factor is the structure of the catalyst nanoparticles. For example, the copper nanoparticles’ structure can influence the methanol and ethanol oxidation in direct alcohol fuel cells. D. Giziski et al. showed that nanostructured copper oxides formed by anodization had a highly developed surface area, and therefore exhibited unique adsorption properties with respect to the most important reaction intermediates [24].

The particles’ shape is also a substantial factor affecting the activity of Cu catalysts. There are many different strategies aimed at manufacturing catalytically active copper particles of various shapes. Recently, R. Rai and D.K. Chand have developed catalysts based on copper (I) oxide nanoparticles using glucose from starch as a reducing agent [25]. In turn, M.J. Siegfried and K.-S. Choi were varying the current–voltage conditions and the pH level to adjust the adsorption of a surfactant (sodium dodecyl sulfate) on the surface of growing copper oxide (Cu₂O) nanostructures in order to control the nanoparticles’ shape during electrodeposition. Thus, the relative deposition rate density of nanoparticles on different surface facets was controlled [26]. A similar strategy of manipulating the growth of certain facets was used to synthesize sharp-faced copper nanocubes, octahedrons, and rhombododecahedrons, limited exclusively to the (100), (111), and (110) facets. Y. Huang et al. used the reduction of copper chloride with hydroxylamine hydrochloride in the presence of sodium dodecyl sulfate as a stabilizer [27]. Cu₂O rhombic dodecahedrons bounded by (110) facets had the highest catalytic activity in the triazoles’ synthesis, probably due to completely exposed surface copper atoms.

Moreover, the origins of the substrate material can affect the morphology of the copper nanoparticles, which was recently shown in a study by A. Ashok et al. [28]. Copper nanoparticles were deposited on copper and zinc foils with pH control of the growth solution. As a result, cuboid structures with flat smooth facets were formed on the copper foil, while densely spread flower-like structures were formed on the Zn foil, in which each flower consisted of Cu microspikes growing perpendicularly from a common center.

In addition, a surfactant is an effective tool for tuning the morphology of copper nanoparticles in the synthesis processes, which was shown by A. Radi et al. [29] and T. Haba [30]. The scientists concluded that there was a dependence between the binding of surfactant molecules to copper nanoparticles on the applied potential, which led to a heterogeneity of the inhibitory effect on the surface and enhanced differences in the morphology and size of the deposited copper.

Thus, the synthetic method, experimental conditions, and the type of structure-forming agents play a decisive role in the result. The rational choice of synthetic technique is becoming increasingly significant in the manufacture of copper nanoparticles with the desired shape, size, and morphology for various applications, as well as to investigate structures dependent on their activity.

There is a fairly wide field of application of copper nanoparticles. For example, a significant area of Cu nanoparticle application is the reduction of CO₂ emissions. In a recent paper by D. Ren [31], it was shown that an optimized population of edges and steps on the surface of a copper catalyst is necessary to facilitate the dissociation of CO₂ and dimerization of the corresponding intermediate CH_xO compounds.

Furthermore, catalysts with nanostructured copper oxides can be considered as a promising replacement for catalysts based on platinoid metals in fuel cells. A.E. Attar and colleagues demonstrated that ethanol molecules are completely oxidized on Cu₂O nanodendrites and form CO₂ molecules [32]. This suggests that highly active surface copper atoms act as electron transport mediators in many important electrochemical reactions.

Of course, platinum is known for its unique catalytic properties: high catalytic activity, resistance, stability. However, the strong adsorption of reaction products on platinum (for example, CO) often poisons the catalyst based on it, and thus reduces its durability [33]. Nanostructured catalysts with the inclusion of copper nanoparticles of a certain morphology can be a solution to this problem. For example, a group of scientists from Morocco recently studied Cu₂O nanodendrites deposited on a polypyrrole film, which demonstrated high electronic conductivity and good ability to oxidize ethanol [32]. Another important characteristic of copper in electrocatalysis is its much lower affinity for carbon monoxide than that of other well-known metals such as Ru, Rh, Pd, and Pt [22]. The usage of copper nanoparticles instead of expensive platinoid metals in the field of alternative energy, combined with the improvement of synthesis methods, will reduce the cost of the final product.

Over the past few years, a wide variety of various synthesis methods have been proposed, including changing the shape and composition of copper nanoparticles with subsequent improvement of their catalytic characteristics by adjusting structure-forming factors [34,35]. However, a determination of the exact catalytic activity dependences on the shaping and structure-forming factors is a rather difficult task. In this regard, the purpose of this paper was to investigate the dependence of the electrocatalytic activity of copper particles in the alkaline ethanol oxidation on their morphology and shaping. Ultimately, this may be a step towards the manufactory of unique inexpensive copper catalysts with similar activity to platinum analogues.

2. Materials and Methods

2.1. Synthesis of Copper and Palladium Particles

The electrochemical cathodic deposition was used for the copper particles’ synthesis on the surface of Cu film with a thickness of 150 μm. The copper film was fixed in a holder, washed in 96% ethanol, then degreased with acetone and washed in distilled water. The electrolytic cell consisted of a working electrode—a copper film as a cathode, which is the substrate for the future nanoparticles′ growth—and a counter electrode, the same copper film as an anode. The electrodes were pretreated in 0.05 M H₂SO₄ for 7.5–15 min before each deposition at a current density of −10–−20 mA cm⁻² by a potentiostat-galvanostat P-40X. It was performed to avoid an oxide layer formation on the substrate surface, which could prevent the control of kinetics and the shaping additive effect. After that, the sample was washed and put in a copper-plating electrolyte prepared on a bidistilled water with the addition of 250 g L⁻¹ CuSO₄ 5H₂O (AR¹) and 90–100 g L⁻¹ 1 H H₂SO₄ (CP reagent ²) at room temperature. KBr (AR¹) was chosen as a structure-forming additive at a concentration of 0.15 to 1.6 g L⁻¹. The particle deposition was carried out in the galvanostatic mode by varying the current density from −15 to −3 A m⁻² and the deposition time from 30 s to 15 min. In order to control the change in the surface morphology during electrolytic deposition, a test sample was taken for electron microscopy by cutting off a part of the film with each change in the synthesis parameters. For this reason, the current density was tuned according to the electrode area for the further process.

The synthesis of classical palladium black was carried out according to the method described in [36,37]. Prior to deposition of a nanoparticles of Pd, metal films were washed and degreased. The clean foil on an inert support was placed into an electrolytic cell and subjected to anodic polarization in 0.1 M hydrochloric acid and cathodic polarization in 0.05 M sulfuric acid. Both treatments were performed at a constant current density of 10–20 mA cm⁻². Then, the cell was filled with a working solution of palladium chloride (2% H₂PdCl₄) and a highly dispersed palladium coating was deposited at a current density of −5–−6 mA cm⁻², after which the film was washed with doubly distilled water.

All experimental samples of copper films were modified on both sides.

Electron microscopy was carried out in the SE mode (secondary electrons) on a JEOL JSM-7500F scanning electron microscope (JEOL, Tokyo, Japan).

¹ 

AR—analytical reagent.

² 

CP reagent—chemically pure reagent.

2.2. Electrochemical Measurements

The electrocatalytic activity study in the reaction of alkaline ethanol oxidation was carried out by cyclic voltammetry (CV) at room temperature using a three-electrode cell on a potentiostat-galvanostat (“Elins” P-40X, Electrochemical Instruments, Chernogolovka, Russia). Samples of copper films modified with various types of coatings were used as working electrodes. The reference electrode was an Ag/AgCl electrode. A smooth, nonmodified copper foil was used as the counter electrode in each measurement. Scanning of electrodes in an alkaline solution of 0.2 M NaOH was carried out in the working potential range from −1.0 V to +1.0 V at a scanning rate of 50 mV s⁻¹. Multiscanning was carried out in the working potential range from −0.2 V to +1.0 V at a scanning rate of 10–15 mV s⁻¹ in 0.1 M NaOH aqueous solution with 0.5 M ethanol. Cyclic voltammetry of electrodes with classical palladium black was carried out in the potential range from −0.9 to +0.5 V at a scanning rate of 50 mV s⁻¹ in an alkaline solution of 1 M NaOH with 0.5 M C₂H₅OH. The currents were normalized to the geometric area of the electrodes, and all potentials were reported relative to the silver chloride electrode.

3. Results and Discussion

3.1. Electrochemical Study of Synthesized Particles in Ethanol Oxidation Processes

The influence of the copper particles’ morphology and shaping on the catalytic characteristics of the modified film samples was studied using cyclic voltammetry (CV) in the alkaline ethanol oxidation. The investigation of the developed catalysts in the ethanol oxidation reaction in the laboratory was an imitation of the processes occurring in industrial fuel cells with direct oxidation of alcohols (1)–(4) [32]:

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}+2{\mathrm{O}\mathrm{H}}^{-}\stackrel{}{\to }{\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+{2\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}$$

(1)

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+{\mathrm{O}\mathrm{H}}^{-}\stackrel{}{\to }{\mathrm{C}\mathrm{H}}_3\mathrm{C}{\mathrm{O}}_{\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}+{\mathrm{H}}_2\mathrm{O}$$

(2)

$$\mathrm{C}{\mathrm{H}}_3{\mathrm{C}\mathrm{O}}_{\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}\stackrel{}{\to }{\mathrm{C}\mathrm{H}}_3\stackrel{\mathrm{H}}{\to }{\mathrm{C}\mathrm{H}}_4$$

(3)

$$\mathrm{C}{\mathrm{H}}_3{\mathrm{C}\mathrm{O}}_{\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}\stackrel{}{\to }\mathrm{C}\mathrm{O}\stackrel{2{\mathrm{O}\mathrm{H}}^{-}}{\to }{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}$$

(4)

Reactions (1)–(4) describe the process of ethanol oxidation with the formation of intermediate reaction products. Acetaldehyde (CH₃CHO) was obtained as the anodic reaction yield (1). It should be noted that an important role in the ethanol oxidation mechanism is played by OH– hydroxide ions, which are required in larger amounts compared to acetaldehyde to produce CO₂. The integral products are acetate ions with the subsequent addition of the hydroxide ion (Reaction (2)). In this case, part of the CH₃CO_(ads) fragments decomposes by splitting the C–C bond into CH_x (Reaction (3)) and CO, which is oxidized to carbon dioxide (Reaction (4)). [38,39]. The mechanism of ethanol oxidation with regard to catalysts based on copper particles according to Reactions (5)–(8) looks like [40]:

$$2\mathrm{C}\mathrm{u}+2{\mathrm{O}\mathrm{H}}^{-}\stackrel{}{\to }{\mathrm{C}\mathrm{u}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}$$

(5)

$${\mathrm{C}\mathrm{u}}_2\mathrm{O}+{\mathrm{H}}_2+2{\mathrm{O}\mathrm{H}}^{-}\stackrel{}{\to }2\mathrm{C}\mathrm{u}{\left(\mathrm{O}\mathrm{H}\right)}_2+2{\mathrm{e}}^{-}$$

(6)

$${\mathrm{C}\mathrm{u}\mathrm{O}\mathrm{H}}_2+{\mathrm{O}\mathrm{H}}^{-}\stackrel{}{\to }2\mathrm{C}\mathrm{u}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$

(7)

$$\mathrm{C}\mathrm{u}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}\stackrel{}{\to }2\mathrm{C}\mathrm{u}{\left(\mathrm{O}\mathrm{H}\right)}_2+{2\mathrm{C}\mathrm{O}}_2+{8\mathrm{H}}_2\mathrm{O}+11{\mathrm{e}}^{-}$$

(8)

Figure 1 showed the cyclic voltammetry of the electrodes modified with the developed copper particles carried out in the potential range from −1.0 to +1.0 V at a scanning rate of 50 mV s⁻¹ in a solution without ethanol (0.2 M NaOH) and without one (1 M NaOH + 0.5 M C₂H₅OH) at a scanning rate of 15 mV s⁻¹. Cyclic voltammograms of the 15th scanning cycle are shown in Figure 1. A number of well-pronounced peaks were observed both in the anodic and cathodic half-cycles, which characterize redox transitions, and are designated I–III in Figure 1. As is known, peak Ia on the anodic branch of voltammograms indicates the Cu/Cu^(I). Under these operating conditions, copper (I) hydroxide is the main product, which turns into the corresponding oxide during Reactions (9) and (10):

$$\mathrm{C}\mathrm{u}+\mathrm{O}{\mathrm{H}}^{-}\stackrel{}{\to }\mathrm{C}\mathrm{u}\mathrm{O}\mathrm{H}+{\mathrm{e}}^{-}$$

(9)

$$2\mathrm{C}\mathrm{u}\mathrm{O}\mathrm{H}+\mathrm{O}{\mathrm{H}}^{-}\leftrightarrows {\mathrm{C}\mathrm{u}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(10)

These reactions reflect the process described in Reaction (5).

Peak II_a indicates Cu/Cu^(II) as well as Cu^(I)/Cu^(II) via electrode processes (11)–(13):

$$\mathrm{C}\mathrm{u}+2\mathrm{O}{\mathrm{H}}^{-}\stackrel{}{\to }\mathrm{C}\mathrm{u}{\left(\mathrm{O}\mathrm{H}\right)}_2+{2\mathrm{e}}^{-}$$

(11)

$${\mathrm{C}\mathrm{u}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+{2\mathrm{O}\mathrm{H}}^{-}\stackrel{}{\to }\mathrm{C}\mathrm{u}{\left(\mathrm{O}\mathrm{H}\right)}_2+{2\mathrm{e}}^{-}$$

(12)

$$\mathrm{C}\mathrm{u}{\left(\mathrm{O}\mathrm{H}\right)}_2\leftrightarrows \mathrm{C}\mathrm{u}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(13)

These reactions can be correlated with Reaction (6). The subsequent Reaction (7) describes the process of formation of soluble particles. A slight decrease in peaks Ia and IIa is observed in the cyclic voltammogram of the copper electrode in the presence of 0.5 M ethanol in the electrolyte. Peak IIIa indicates the Cu^(II)/Cu^(III) pair and always appears during the oxidation of copper and copper-containing modified electrodes [41,42]. The participation of Cu^(III) compounds in the redox process is confirmed by the fact that the formal reduction potential of Cu^(III)/Cu^(II) in 1 M NaOH solution is 560 mV relative to Ag, AgCl [43]. Reaction (8) describes the complete oxidation of ethanol with cleavage of the C–C bond in the ethanol molecule. Peaks IIIc, IIc, and Ic in the reverse cathodic half-cycle are related to the reduction of Cu^(III) to Cu^(II), Cu^(II) to Cu^(I), and Cu^(I) to Cu, respectively [44,45,46].

Alcohol is inactive at low potentials and is poorly capable of dissociative adsorption on copper electrodes. This means that ethanol is initially adsorbed on copper oxides with a lower valence state and further oxidized at the expense of Cu^(III), which is formed at more positive potentials and acts as a redox mediator in ethanol oxidation [22]. Based on this, a narrowed potential range from −0.2 to +1.0 V was chosen for subsequent research, in which special attention was paid to the anodic current density peak III_a as the main electrocatalytic activity indicator of the developed nanostructured copper catalysts.

It is also necessary to study the long-term stability to evaluate the electrochemical characteristics of the modified electrodes for the practical application of the developed particles as catalysts. Durability experiments were carried out to study the effect of copper regeneration on the stability of catalysts (Figure 2).

It can be seen in Figure 2 that the electrode material retained most of its activity over 100 (estimated) cycles. However, the activity of the catalyst based on copper particles had increased during this experiment, until it reached the limit value of 30.07 mA cm⁻² at the 100th cycle, which was 1.6 times higher than the value at the 1st cycle. The increase in activity can be explained by electrochemical activation of the material surface. The electrode with a copper catalyst was washed with bidistillated water after 100 cycles and its durability measurement was carried out in a fresh solution. It can be seen from the figure that the cyclic voltammogram recorded in an alkaline ethanol solution is similar to that obtained at 100 cycles, which indicates a recovery efficiency.

The morphological characterization of copper nanoparticles in the catalyst after 100 cycles was also studied to advance the approach further (Figure 3). On the regenerated surface, the modified electrode morphology did not change even after 100 cycles, which explains the good stability.

3.2. Investigation of the Influence of Synthesis Parameters of Particles in the Composition of Catalysts on Electrocatalytic Activity

3.2.1. Influence of Deposition Time

The morphology of copper particles in the first sample series was studied by changing the deposition time from 3 to 15 min at a KBr concentration of 1.0 g L⁻¹ and a current density of −5 A m⁻². Figure 4 shows microphotographs of lamellar particles with certain facets and identified disclinations. Copper-based particles of various lamellar shapes, including triangular formations with a thickness of 125 to 500 nm and a size of 600 nm to 1.5 μm, as well as areas with smaller triangular and cubic particles ranging in size from 200 to 250 nm, were observed at a deposition time of 3 min (Figure 4a). Due to an increase in the electrodeposition time to 7.5 min, there is an increase in the particle size from 800 nm to 1.5 µm, and the absence of small particles is fixed (Figure 4b). Particle growth is fixed with a further increase in the time to 15 min, their average sizes range from 900 nm to 2.5 µm (Figure 4c).

The catalytic activity study of a sample series obtained by electrodeposition at different times was carried out by CV in the reaction of alkaline oxidation of ethanol (Figure 5). It can be seen that the current density peak was 31.59 mA cm⁻² when the deposition time reached 7.5 min. It is explained by the fact that atoms had time to form stable nanoparticles of Platonic shapes, such as cubes and pyramids, and lamellar crystallites with clearly defined edges during the specified time. The catalyst based on copper particles with this organization probably had the largest number of active centers, which explains its high electrocatalytic activity (Figure 5, upper inset). Nanoparticles with a shorter deposition time did not have time to form up to certain nanocrystallites, representing spherical particles with no visible facets. In turn, the formed crystallites of various shapes began to agglomerate with an increase in the deposition time over 7.5 min.

The optimal electrodeposition time was chosen as 7.5 min based on the obtained data. Under this condition, samples with high catalytic activity were obtained.

3.2.2. Influence of Current–Voltage Parameters of Deposition

The study of the copper particle morphology of the second sample series was carried out by varying the deposition current from −15 to −3 A m⁻² for 7.5 min. The particles were self-organized into Platonic structures with well-defined facets—cubes ranging in size from 500 nm to 1 µm at a deposition current density of −5 A m⁻², as can be seen in Figure 6b. The particles incompletely formed particles (Figure 6a) at a lower value of −3 A m⁻²; their sizes ranged from 80 to 450 nm. Coalescence of particles was observed at a higher current of −15 A m⁻² (Figure 6c) with a large size distribution from 300 nm to 2 µm. The particles obtained at a current density of 3 A m⁻² contributed to a 17.9× increase in the real working surface area of the electrode with a roughness factor of −20.1. The working area increase was 18.6 times with a roughness factor of up to 21.6 for particles synthesized at 5 A m⁻², and particles obtained at 15 A m⁻² demonstrated a 15.1× increase with a roughness factor of up to 16.4.

The highest current density peak of 31.59 mA cm⁻² was observed for catalysts deposited at −5 A m⁻² (Figure 7). Peak values in the case of deposition current of −15 and −3 A m⁻² demonstrate a decrease in electrocatalytic activity, which may be due to a smaller number of active centers. Based on this, the current density value of −5 A m⁻² was chosen as optimal for obtaining a copper catalyst with the highest electrocatalytic characteristics.

3.2.3. Effect of KBr Concentration in Particle Growth Solution

The morphology investigation of the third sample series of copper particles was carried out for 7.5 min and at a constant current density of −5 A m⁻² by changing the structure-forming agent KBr concentration in the growth solution from 0 to 1.6 g L⁻¹. According to the SEM images shown in Figure 8, particle morphology changes from being cubic without the addition of KBr to being lamellar and pyramidal. The larger phase appeared with a KBr concentration increase. The percentage of large particles with a size of 0.8–1.1 µm increased at a concentration of 0.15 g L⁻¹ (Figure 8c), of 1.0–1.3 µm—at 0.5 g L⁻¹ (Figure 8d) and of 0.8–1.5 µm—at 1.0 g L⁻¹ (Figure 8e). As the structure-forming agent concentration increases, the particles change their shape from bulk particles of pyramidal and cubic shapes to lamellar polygonal ones. Particles’ coalescence and crystallinity decrease are noticeable (Figure 8f) at a concentration of 1.6 g L⁻¹; thus, a further potassium bromide concentration increase will lead to a simplification of the morphology, and consequently, a performance decrease.

The catalytic activity in ethanol alkaline oxidation of the obtained samples is shown in Figure 9.

According to the obtained results, the largest current density peaks (up to 33.01 mA cm⁻²) were exhibited by a catalyst with a regular cubic shape of particles ranging in size from 600 nm to 1.5 μm, obtained at potassium bromide concentration of 1.0 g L⁻¹ and current density of 5 A m⁻² for 7.5 min. A catalyst prepared from a solution containing no KBr has similar but slightly lower activity, with a peak current density of up to 31.59 mA cm⁻². The catalyst consisted of big lamellar and pyramidal particles ranging in size from 800 nm to 1.5 µm with many disclination defects. Relative to the nonmodified copper film, the increase in the catalytic activity of the synthesized catalysts was up to 8 times for cubic particles and up to 7.5 times for particles with lamellar triangular shape. The high catalytic activity in the ethanol oxidation reaction for a similar particle shape is perhaps due to the presence of the largest number of active centers compared to other formations.

The obtained experimental data allow us to say that the growth of the catalytic activity of the developed catalysts is due not only to an extensive way—an increase in the development and specific surface area—but also an intensive way—to create a need for structural organization of the catalytic layer. Let us conduct a “thought experiment” on the example of cubic particles to confirm the validity of the judgment. Imagine that a single “isolated” cube, meaning that it has not merged with another cubic particle, is located on a perfectly flat surface; therefore, the “electrode active area” has increased locally. In this case, we adhere to the assumption that the cube side facets exhibit the same electrochemical behavior as the surface of the working electrode. The cube has four additional surfaces (four cube side facets) on which electrochemical processes can take place. The “top” cube surface is expected to show the same reactivity as the substrate. In turn, the “bottom” surface of the cube will not be electrochemically active, as it is in electrical contact with the substrate, and this “thought experiment” does not assume that liquid is present. This would already lead to an 5× increase in the local surface area. Similarly, one can describe a surface with pyramidal (four side facets rising above the surface) and lamellar particles.

Furthermore, data on the electrochemically active surface area for the studied electrode samples (ECSA) were obtained to confirm the validity of the judgment and the results obtained. The calculated ECSA values for electrodes with cubic copper particles and mixed lamellar and pyramidal particles were 25.8 cm² and 19.25 cm², respectively. this value was 6.06 cm² for an unmodified smooth film. This increase in ECSA suggests that the electrodes modified with cubic and plate-like pyramidal particles had more active sites available for the catalytic reaction. This suggests that the high rates of catalytic activity in the ethanol oxidation reaction for a similar particle shape may be due to a combination of factors increasing the number of localizations of active centers and the specific surface area, compared with other formations.

Thus, the KBr concentrations of 0 and 1.0 g L⁻¹ are optimal to achieve high electrocatalytic activity in the alkaline oxidation of ethanol.

3.3. Morphology and Catalytic Characteristics of the Synthesized Palladium Catalyst

A classic palladium black catalyst was also synthesized in the course of the study to visually compare with the developed copper catalysts and determine the effectiveness of the latter ones. The Figure 10 shows SEM images of the synthesized particles’ morphology. The particles mostly have spherical shape with a characteristic size in the range of 500–800 nm.

Cyclic voltammograms of the obtained samples were taken in the potential range from −0.9 to +0.5 V at a scanning rate of 50 mV s⁻¹ at room temperature (25 °C) in an alkaline solution of 1 M NaOH with 0.5 M C₂H₅OH. According to Figure 11, the synthesized palladium catalyst showed a peak current density value of 56.51 mA cm⁻² during direct scanning on the anode side. The subsequent decrease in current density is due to the blocking of the catalyst surface by the products of electrochemical adsorption processes. The lower-intensity peak observed during the reverse sweep is due to the surface restoration due to the removal of chemisorbed carbonaceous particles, which makes further oxidation of ethanol possible [47,48].

The synthesized platinoid catalyst based on palladium particles is an analogue of a commercial catalyst used in many industrial processes, including catalytic processes. This experiment was carried out to reliably compare the synthesized copper particles with different morphology as a catalyst for the oxidation of alcohols with widely used platinum catalysts.

3.4. Comparison of Characteristics of Obtained Copper Catalysts with Analogues

Table 1. Comparative characteristics of the developed catalysts with literature analogues.

Catalyst	C, M	v, mV s−1	j, mA cm−2	Ref.
1	2	3	4	5
Cu lamellar nanoparticles Cu octahedra	0.25 CH3OH 0.1 NaOH	10	8.3 4.3	[35]
Cu-CeO2/Cu nanofibers decorated with copper particles	2.0 CH3OH 1.0 KOH	50	12.0	[49]
Pt@Au honeycomb particles Pt@Au50Cu50 nanodendrites	1.0 CH3OH 1.0 NaOH	50	0.82 0.4	[50]
Pd80Cu20 needle particles	1.0 CH3OH 1.0 KOH	25	11.81	[51]
Pd48Co52 lamellar particles Pd23Co77 lamellar particles	0.1 C2H5OH 2.0 KOH	10	15.0 22.5	[52]
Cu@Pd IV particles smaller than 10 nm	1.0 C2H5OH 2.0 KOH	20	3.0	[53]
PtCu/Cu2-xSe nanowires with a large number of defects	1.0 C2H5OH 1.0 KOH	50	5.03	[54]
Cu-Ni spherical particles doped with Cu atoms	0.05 C2H5OH 1.0 NaOH	10	30.0	[55]
PPy/Cu2O/CPE octahedra nanodendrites	0.2 C2H5OH 0.1 NaOH	10	2.25 4.0	[30]
Cu dendritic and spherical particles	1.0 C2H5OH 0.5 NaOH	10	6.0	[56]
Cu-Ni porous nanorods	0.05 C2H5OH 1.0 NaOH	20	30.0	[57]
Cu@Pd/SnO2-Gr-5 spherical particles on graphene sheets	1.0 C2H5OH 0.5 NaOH	10	90.0	[46]
Cu-BDC/CE octahedrons	0.01 C2H5OH 1.0 NaOH	50	0.7	[58]
Pd-black	0.5 C2H5OH 0.1 NaOH	50	56.51	This work
Cu cubic particles Cu lamellar and prismatic particles	0.5 C2H5OH 0.1 NaOH	10	33.01 31.59	This work

Note: C is the concentration of substances in solution; v is the scanning speed; j is the peak value of the current density.

presents analogues that have been reported in the literature in recent times for a comparative assessment of the parameters of the developed copper catalysts.

Comparative analysis with similar reactions of alcohol alkaline oxidation showed confirmation that the obtained catalysts are at a fairly high competitive level with platinoid ones and are an order of magnitude more active than copper analogues. For example, L.-S. Tsui et al. [52] demonstrated particles of similar lamellar morphology based on Pd₂₃Co₇₇, which had a peak current density slightly more than 20 mA cm⁻², while the peak current density for pure Pd was only 2 mA cm⁻². PPy/Cu₂O/CPE octahedra with a similar size around 1.2 μm showed a peak current density of approximately 9 mA cm⁻² in 0.1 M NaOH and 0.2 M ethanol solution alcohol, and with concentrations of up to 5 M, the maximum catalytic activity reached 18 mA cm⁻² [30]. The studies are noticeably inferior in their catalytic characteristics in comparison with the copper catalysts developed in this work, which demonstrated peak current densities up to 33.01 mA cm⁻².

The study of platinoid catalysts based on palladium nanoparticles made in this work, presented as analogs to commercial catalysts, makes it possible to visually verify the efficiency and high advantages of the developed copper catalysts. According to the results, the peak current density in the ethanol oxidation reaction for the developed copper catalysts turned out to be only 1.7 times lower than for the palladium catalyst synthesized in the work. This result is very promising, since it demonstrates the competitiveness of the developed copper catalysts with respect to the platinoid catalysts that are widely used in the industry. The adjustment of parameters for the synthesis of copper particles described in this work will make it possible to obtain highly active catalysts with a certain surface morphology that is most suitable for the selected processes. The introduction of such copper catalysts will significantly reduce the cost of industrial processes by refusing of catalysts based on platinum and palladium, while maintaining a level of process efficiency.

4. Conclusions

An integrated approach to the synthesis of unique cost-efficient nanostructured highly active copper catalysts was reported in this paper. Such catalysts have been obtained through fine-tuning of the synthesis parameters. The catalysts consisted of particles of cubic, lamellar, and pyramidal shapes. The formation of a nanostructured functional layer on the surface of a copper film was made possible by varying the deposition current density from 3 to 15 A m⁻², the deposition time from 3 to 15 min, and the structure-forming agent (KBr) concentration from 0 to 1.6 g L⁻¹. Such a synthesis parameter variation in the copper particles’ deposition allowed for the achievement of a certain morphology and shape with a larger number of active sites, which greatly enhanced the catalytic activity.

Investigation of catalysts synthesized on thin films with a deposition current of 5 A m⁻² for 7.5 min and a KBr concentration of 0 or 1 g L⁻¹ in the reaction of ethanol oxidation in an alkaline medium revealed increased activity of the catalyst of cubic and lamellar polygonal shapes. The catalysts demonstrated high current densities in the ethanol oxidation reaction of up to 31.59 and 33.01 mA cm⁻², respectively, which are 7.5 and 8 times higher than for an electrode based on a smooth film without a catalyst.

All catalytic activity data obtained during the study of the developed copper catalysts are closely correlated with each other. This fact validates the results, confirms the effectiveness of the developed catalyst of cubic and lamellar polygonal shapes for usage as promising multifunctional electrodes, and opens up opportunities for their wide application in direct alcohol fuel cells. The revealed patterns will make it possible to correctly vary the synthesis conditions to manufacture highly efficient copper-based catalysts with desired characteristics. Such catalysts based on copper particles of various morphologies with high activity characteristics will significantly reduce the cost of many industrial processes instead of platinoid analogues.