Synthesis of NiCu–Polymeric Membranes for Electro-Oxidizing Ethylene Glycol Molecules in Alkaline Medium

Abstract

Binary metallic nickel–copper nanocatalysts were anchored onto a polyvinylidene fluoride-co-hexafluoropropylene membrane [NiCu/PVdF–HFP] using the electrospinning technique, followed by the chemical reduction of the relevant precursor salts by introducing sodium borohydride to the synthesis mixture. A series of varied Ni:Cu weight % proportions was developed in order to optimize the electroactivity of this binary nanocomposite towards the investigated oxidation process. A number of physicochemical tools were used to ascertain the morphology and chemical structure of the formed metallic species on polymeric films. Cyclic voltammetric studies revealed a satisfactory performance of altered NiCu/PVdF–HFP membranes in alkaline solution. Ethylene glycol molecules were successfully electro-oxidized at their surfaces, showing the highest current intensity [564.88 μA cm⁻²] at the one with Ni:Cu weight ratios of 5:5. The dependence of these metallic membranes’ behavior on the added alcohol concentration to the reaction electrolyte and the adjusted scan rate during the electrochemical measurement was carefully investigated. One hundred repeated scans did not significantly deteriorate the NiCu/PVdF–HFP nanostructures’ durability. Decay percentages of 76.90–87.95% were monitored at their surfaces, supporting the stabilized performance for prolonged periods. A much-decreased R_ct value was estimated at Ni₅Cu₅/PVdF–HFP [392.6 Ohm cm²] as a consequence of the feasibility of the electron transfer step for the electro-catalyzing oxidation process of alcohol molecules. These enhanced study results will hopefully motivate the interested workers to explore the behavior of many binary and ternary combinations of metallic nanomaterials after their deposition onto convenient polymeric films for vital electrochemical reactions.

1. Introduction

The continuous use of fossil fuels has led to many environmental problems arising. This has directed many research scientists to seek out suitable energy sources that have good stability and increased yield [1,2,3,4]. Direct alcohol fuel cells have many advantages such as being simple to handle, promising efficiency for energy transformation, increased power density and good safety [5,6,7,8,9]. Among different alcohols, ethylene glycol appears as a promising fuel with its encouraging features of reduced toxicity, nonvolatile nature, increased energy density and boiling constant [10,11,12,13,14]. Platinum-containing nanomaterials are considered the most popular anodes for direct alcohol fuel cells. However, their increased cost and reduced reaction kinetics might significantly retard their widespread application. These problems account for the need to explore inexpensive nanocatalysts with satisfactory electrocatalytic performance [15,16].

Numerous preparation schemes have been proposed to design nickel and nickel oxide-containing nanostructures as a result of their enhanced activity and stability towards many catalytic processes [17]. Active surfaces of nickel species were sufficient to improve antibacterial efficiency [18], alcohol oxidation [19,20], hydrogen evolution [21], oxygen evolution [22] reactions and energy storage applications [23,24]. For example, Nagajyothi et al. [25] have examined the electroactivity of hydrothermally prepared NiFe₂O₄ nanoparticles for electro-oxidizing a number of alcohols such as ethanol, methanol and propanol in a basic solution. Significant stability was shown during the operation of this nickel ferrite nanocatalyst within 50,000 s. A remarkable reduction in the measured E_onset value was also demonstrated, especially when methanol molecules were introduced to the investigated medium. Loading nickel–molybdenum oxide onto nickel foam after its doping with nitrogen displayed a promoted alcohol oxidation current density of 100 mA cm⁻² at a decreased potential of 1.338 V (RHE). An increased faradic efficiency for electro-oxidizing benzyl alcohol species was also measured [98.7%] [26]. Incorporating tungsten into the chemical structure of electrodeposited nickel nanoparticles onto a carbon steel surface appreciably facilitated the surpassing of the accompanying energy barrier with the cleavage step of C–C bonds. An ethanol oxidation current density of 160 mA cm⁻² was monitored at 1.7 V (RHE) with a superior durability performance within 2000 cycles [27]. Electrodeposited Ni_0.8Co_0.2(OH)₂ electrodes exhibited an increased conversion rate of glycerol molecules as 99.8% after 4 h through an energy consumption of 36.38 W mol⁻¹ [28]. Ag/NiO/CNTs nanocomposites in different proportions showed encouraging activities towards the ethanol conversion process—the presence of 25 wt.% Ag was sufficient to increase the oxidation current density at the formed nanomaterial by about five times [29]. NiMn/CNFs nanocatalysts were synthesized by the electrospinning process for the relevant acetates of nickel and manganese in different weight percentages at 20 kV with further heating for 300 min at 850 °C. Fruitful activity towards oxidizing ethylene glycol molecules was measured while increasing the incorporated Mn content in the fabricated NiMn/CNFs nanomaterials. The electrode that had 60Ni:40Mn composition showed an increased oxidation current density and good stability behavior [30]. Mixing spinel NiCo₂O₄ nanopowders with nitrogen-doped carbon nanotubes and graphene effectively motivated their oxidation activity for ethylene glycol molecules with E_onset value of 1.08 V (RHE) [31].

Conducting polymers have been recently employed in numerous fields such as sensors [32], energy storage [33,34], photocatalysis [35] and medical purposes [36]. Their specific mechanical, optical, electronic and magnetic features elect polymer-based nanostructures as promising catalysts with increased surface areas and surprising reactions [37,38]. Polyaniline [39,40], polypyrrole [41], poly(3,4-ethylene dioxythiophene) [42], polydiphenylamine [43] and poly(vinyl alcohol) [44] significantly improve the electrocatalytic behavior of carbonaceous supports [45,46]. Polyvinylidene fluoride-co-hexafluoropropylene is a linear polymer chain that contains H, C and F atoms [47]. Its satisfactory chemical and heat resistances, as well as increased piezoelectricity and flexibility, account for the ability to exploit PVDF–HFP films in energy harvesting, sensors and antibacterial media [48,49]. Supporting PVdF–HFP membrane with montmorillonite effectively decreases its porosity while improving the thermal stability of the fabricated composite when applied for direct alcohol fuel cells. The dissociation degree of sulfonic acid was extensively promoted by the action of adsorbed water molecules that, in turn, increased the ionic conductivity of the formed film [50]. Activated PVdF–HFP surface with p-toluene sulfonic acid/phosphoric acid/sulfuric acid displayed an ionic conductivity of 27.27 mS cm⁻¹ and decreased permeability of methanol molecules as 9.7 × 10⁻⁸ cm² s⁻¹ [51]. Alloyed Ni-Co nanoparticles were deposited onto PVdF–HFP membrane and examined for glucose electro-oxidation reaction in a basic medium. A surprising detection performance was monitored towards the sugar molecules within a linear concentration region as 1 μM–7 mM [52]. Anchoring copper nanoparticles onto this film also motivated its activity to determine glucose concentration down to 11 nM with an excellent sensitivity of 506.62 μA mM⁻¹ cm⁻² [53]. Ren et al. [54] have built a nanocomposite of nitrogen-doped carbon quantum dots with Bi₂WO₆ and loaded it onto PVDF–HFP film surface. About 85.3% of oxytetracycline was adsorbed by this modified membrane with water flow of 34.4% and visible light of 63.1%. An acceptable reusability was also observed for this studied film when operated for numerous cycles.

Modifying the morphological structures of nickel-dependent nanocatalysts by alloying with numerous metals provides increased catalytic active sites for improved reaction rates. Copper is a low-cost transition metal that sufficiently participates in valuable purposes such as oxygen evolution [55], sensors [56], water splitting [57] and electro-oxidation processes [58,59,60]. The introduction of copper into NiFeCu oxohydroxides structure was beneficial in modifying the d-band center of nickel atoms and reducing the strength of adsorbed byproducts onto the fabricated electrode surface [61]. Promoted mechanical stability and electrical conductivity were also developed after doping NiO membranes with copper species [62]. Cao et al. [63] have measured an increased electrochemical active surface area at electrodeposited NiCuO nanoflowers as 85.9 m²/g. It accounted for their surprising activity towards glucose molecules oxidation reaction with a mass activity of 376.80 A/g and facile charge transfer characteristics. Hierarchical NiCu arrays were effectively employed to produce adipic acid and hydrogen with valuable stability within 200 h and increased yield. In situ spectroscopic investigation indicated that Cu_0.81Ni_0.19 nanoalloy motivated the conversion reaction kinetics with enhanced free energy during the intermediate adsorption step [64]. Ni₄Cu₁ deposits onto polyphenylenediamine activated ethanol oxidation process with reduced onset potential of 390 mV and effective current density of 70.5 mA/cm² [65]. By considering these fruitful results, a series of Ni_xCu_1–x nanodeposits onto polyvinylidene fluoride-co-hexafluoropropylene membrane was evaluated for electro-catalyzing ethylene glycol oxidation reaction in basic solution. These polymeric films were first synthesized using the electrospinning method, followed by the formation of binary nickel–copper coatings via the chemical reduction of their precursor salts by sodium borohydride. These metallic polymeric films were characterized through scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and Fourier Transform Infrared spectroscopy (FTIR). Various electrochemical tools involving cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy measurements were conducted to measure the electroactivity of these polymeric nanostructures and optimize their performance.

2. Results and Discussion

Numerous advantageous characteristics were gained when the electrospinning technique was employed to fabricate nanofibrous membranes. Promoted porosity, increased interconnectivity, satisfied flexibility and outperformed surface-to-volume ratios were provided by electrospun films [66]. PVdF–HFP, as one of these promising polymeric surfaces, displays suitable features including increased dielectric constant, good piezoelectric and pyroelectric properties, enhanced thermal stability and encouraging hydrophobicity. This might rationalize the motivated studies to create metallic nanofibers using this polymeric membrane [50,67]. SEM image in Figure 1a revealed the formation of homogeneous and bead-free fibrous mats of PVdF–HFP. The fast evaporation of acetone as the solvent, along with the jet movement from the positively charged syringe needle to the negatively charged collector, participated in creating nanoporous architecture during the electrospinning process. This might help to provide a suitable microenvironment for metal ions to settle. Incorporating hydrated precursor salts (nickel and copper acetates) also contributed to the phase separation process (liquid–liquid demixing) inside the hydrophobic polymeric matrix. This might increase the pores’ density and provide a chance for the analyte molecules to trap easily with much-reduced diffusion barriers [68]. The rate of the hydrogen evolution reaction was consistently enhanced, and the formed membrane conductivity was promoted as a result of inserting varied metallic species. The possibility of developing a gel state also facilitated the elongation of the axial gel as much as possible [53]. SEM pictures of different NiCu/PVdF–HFP membranes were presented in Figure 1b–e. The sample with Ni:Cu ratio of 3:7 [see Figure 1e] displayed rough and interwoven surfaces. The porous nature of these fibers enabled the distribution of some metallic deposits onto their external sides. The lowered reduction potential of copper ions relative to those of nickel favored their chemical reduction using sodium borohydride, especially when they were present in increased amounts [3Ni:7Cu]. Therefore, a rapid chemical reduction process was conducted, resulting in uncontrolled deposition of localized copper nuclei on the polymeric film surface. This irregular growth of metallic nanoparticles resulted in the formation of rough clusters. The EDX chart of Ni₃Cu₇/PVdF–HFP film was constructed at certain surface points as indicated in Figure S1. Definite peaks of nickel and copper were distributed along with the investigated EDX chart. Additionally, carbon and fluorine peaks appeared to characterize the fabricated polymeric surface. The XRD pattern of Ni₃Cu₇/PVdF–HFP membrane in Figure S2 demonstrated a number of defined peaks at 2θ values of 18.20°, 20.00°, 26.60°, 36.15°, 43.40° and 50.40°. These peaks match well those of Ni and Cu crystallites with the respective diffraction planes of (100), (020), (110), (021), (111) and (200) based on JCPDS cards of Ni(04-0850) and Cu(04-0836) [51,69]. Accordingly, a face-centered cubic structure was expected with comparable lattice parameters as 3.523 Å for nickel and 3.615 Å for copper. FTIR charts of PVdF–HFP membrane and its relevant metallic forms displayed some defined peaks as shown in Figure 2. The α and β crystalline phases were revealed through their corresponding peaks at 749 and 837 cm⁻¹ [38]. Another two bands at 672 and 872 cm⁻¹ were created due to CF and CH₂ wiggling vibrations in the amorphous polymeric surface. On the other hand, symmetric C–F stretching, CF₂ stretching and bending vibrations were clarified via characteristic bands at 1071, 1175 and 1400 cm⁻¹ [70]. An extra band at 1561 cm⁻¹ indicated that NiO and CuO species were formed in the studied nanofibrous network [71].

The cyclic voltammetric behavior of varied NiCu/PVdF–HFP electrodes in 0.5 M NaOH solution was investigated in Figure 3 at 5 mV s⁻¹. All studied nanocomposites exhibited a defined redox couple for the electrochemical transformation of nickel ions between (2+) and (3+) oxidation states [73,74]. Ni₅Cu₅/PVdF–HFP membrane in Figure 3a displayed this Ni(OH)₂/NiOOH conversion at potential values of 541 and 425 mV in the forward and backward directions, respectively. A more positive potential shift of 89 and 91 mV was monitored for the film that contained 90 wt.% Ni-10 wt.% Cu [see section (b)] for the corresponding oxidation and reduction processes of this redox transfer. By calculating ΔE of Ni(II)/Ni(III) redox reaction for different NiCu/PVdF–HFP films, values of 121, 116, 114 and 95 mV were estimated for those with Ni:Cu weight ratios of 3:7, 5:5, 9:1 and 7:3, respectively [see sections (c, d)]. These ΔE values were much lower than those reported at NiC/CNFs [231 mV], NiC-MnC/CNFs (95:5) [155 mV] and NiC-MnC/CNFs (90:10) [139 mV] nanomaterials to reveal the facile electron transfer reaction at NiCu-polymeric films [75]. Moreover, promoted current densities of 47.70 and −33.71 μA cm⁻² were shown by Ni₅Cu₅/PVdF–HFP nanostructure in the forward and backward routes of the examined transformation to clarify the availability of an increased number of active surface sites with an acceptable reaction rate. However, the other samples with Ni:Cu weight ratios of 7:3, 3:7 and 9:1 had much lower oxidation current densities of 36.03, 32.28 and 29.56 μA cm⁻², correspondingly, compared to values of −12.84, −12.49 and −9.31 μA cm⁻² at these electrodes surfaces during the reduction step of Ni(OH)₂/NiOOH conversion. This study result illustrates the preferential performance of Ni₅Cu₅/PVdF–HFP membrane during the formation of multiple layers of nickel hydroxide/oxyhydroxide species.

The electrochemical response of Ni₅Cu₅/PVdF–HFP nanomaterial was investigated in an alkaline electrolyte at varied scan rates in Figure 4a. The Ni(II)/Ni(III) transformation peaks were clearly shown at all chosen scan rates. Adjusting the scanned measurement at higher sweep rates fairly shifted the oxidation and reduction peaks of this redox couple towards more positive and negative potentials, respectively. The charge transfer step was greatly affected by the contact between the film surface and its surrounding medium [76,77,78,79]. This might account for this potential change in Ni(OH)₂/NiOOH transition with altering the sweep rate, as inferred from Figure 4b. Additionally, a continuous current intensity enhancement was noticed for runs at higher sweep rates when the forward and backward directions of Ni(OH)₂/NiOOH conversion were considered. These current densities were plotted against υ in Figure 4c. Straight lines were attained on both sides of Ni(II)/Ni(III) transformation. The coverage of film surface with the deposited metallic species (Γ*) was derived from the slopes of (I–υ) relations [80] as 1.928 × 10⁻¹² mol cm⁻². Another linear relation was also extracted when these currents were varied against υ^0.5 in Figure 4d. This result supported the contribution of diffusion rules during nickel hydroxide/oxyhydroxide transition. Moreover, the Randles–Sevick equation was employed to calculate the diffusion coefficient of alcohol molecules within the Ni₅Cu₅/PVdF–HFP membrane as 1.954 × 10⁻⁴ cm² s⁻¹ [81,82]. This value greatly exceeded the reported one at deposited nickel oxide species onto activated carbon (3.79 × 10⁻¹⁰ cm² s⁻¹) and multi-walled carbon nanotubes (7.69 × 10⁻¹⁰ cm² s⁻¹) [83] and calcined NiCo₂O₄ (20 wt.%) nanoparticles onto graphite (1.2558 × 10⁻⁸ cm² s⁻¹) [84].

The cyclic voltammogram of Ni₉Cu₁/PVdF–HFP nanocatalyst in (1.0 M EG + 0.5 M NaOH) solution was presented in Figure 5a at 10 mV s⁻¹. The alcohol oxidation process was started at 532 mV and a rapid current density increase was noticed to display an oxidation peak at 730 mV with a current density of 67.81 μA cm⁻². The reduction step of NiOOH species was further detected in the reverse scan at 500 mV. The relevant activities of Ni₇Cu₃/PVdF–HFP, Ni₅Cu₅/PVdF–HFP and Ni₃Cu₇/PVdF–HFP films were demonstrated in Figure 5b–d. Their respective E_onset values were gradually shifted in the negative direction by 44, 24 and 72 mV when compared to the measured one at Ni₉Cu₁/PVdF–HFP electrode to clarify the feasibility of NiCu/PVdF–HFP film surfaces in different Ni:Cu proportions to electro-oxidize ethylene glycol molecules. The alcohol oxidation peak also revealed promising current densities for the membranes with Ni:Cu wt.% ratios of 7:3 [323.15 μA cm⁻²] and 5:5 [564.88 μA cm⁻²] showing 4.77 and 8.33 folds activity promotion when related to that at Ni₉Cu₁/PVdF–HFP. However, the insertion of 30%Ni:70%Cu in the fabricated polymeric membrane resulted in a decreased oxidation current density [50.17 μA cm⁻²] to suggest the suitability of moderate loadings of nickel and copper for better performance towards ethylene glycol molecules. A volcano-shaped plot was drawn when representing the alcohol oxidation current density at each chosen Cu wt.% value inside the formed membranes as indicated in Figure 5e. The loading of varied metallic ingredients into the examined nanostructures significantly controlled their activity. For example, Rodriguez et al. [85] have fabricated a series of Pt_10−xFe_x/CNTs [x = 1, 2, 3, 4 and 5] nanocatalysts through the Brust–Schiffrin protocol and investigated their behavior for electro-oxidizing methanol molecules. These nanomaterials observed a volcano-shaped performance while altering the incorporated Fe:Pt ratio during the synthesis step. The presence of 30% Fe or less was sufficient for achieving enhanced alcohol electro-oxidation current density due to the favored rearrangement of Pt atoms by iron species with their promoted electronic environment. On the contrary, higher contents of Fe atoms [40% or more] might delay the rate of the alcohol dehydrogenation reaction, leading to decreased nanocatalyst activity. The mass activity of Pd₄Au₃ aerogels onto carbon support for oxygen reduction reaction was 11.6 and 17.4 folds higher than those at Pt/C and Pd/C, respectively [86]. Pd₆Bi₁ nanowires exhibited mass activities of 1231, 2066 and 3047 mA mg_Pd⁻¹ for the corresponding electro-oxidation reactions of glycerol, ethanol and ethylene glycol molecules. Density functional theory studies clarified that altered d-band positions were created based on the introduced Pd:Bi stoichiometric ratio in the synthesized nanocomposite. This might vary the formed coordination environment of palladium species in order to get a balanced ratio between OH* and CO* radicals [87].

The influence of varying the alcohol concentration during the evaluation of the metallic membranes’ activity was studied using the cyclic voltammetry technique. The obtained cyclic voltammograms at Ni₅Cu₅/PVdF–HFP electrode in 0.5 M NaOH solution are demonstrated in Figure 6a at 10 mV s⁻¹ after inserting 0.1, 0.3, 0.4, 0.5, 0.7 and 1.0 M EG molecules. A subsequent activity enhancement of this investigated nanocatalyst was noticed with increasing the added alcohol concentration in the supporting electrolyte. Recording this oxidation peak current density as a function of the selected alcohol concentration in Figure 6b confirmed this strong dependence in a linear tendency. The order of the oxidation process with respect to ethylene glycol concentration was derived from elucidating the slope of log (I)–log (C) plot in Figure 6c. An order value of 0.809 was deduced.

The durability of varied NiCu/PVdF–HFP nanocatalysts for prolonged working to electro-oxidize EG molecules in 0.5 M NaOH solution was evaluated using chronoamperometry and repeated cyclization techniques. Chronoamperograms of these membranes were operated at 600 mV for 1800 s in Figure 7a. The presence of appreciable contents of alcohol molecules at the electrode surfaces resulted in increased oxidation current densities at the first few moments of this study. They rapidly decreased, reaching a steady response after about 100–400 s, with a decaying oxidation rate by collecting its byproducts at the available reaction sites [88,89]. By considering the steady state oxidation current densities of NiCu/PVdF–HFP electrodes after 1800 s, introducing equal ratios of nickel and copper species into the polymeric film composition [50:50] demonstrated the highest stability within the prepared electrodes [126.47 μA cm⁻²]. This current intensity value outperformed the obtained one at Ni₇Cu₃/PVdF–HFP, Ni₉Cu₁/PVdF–HFP and Ni₃Cu₇/PVdF–HFP by 1.52, 4.56 and 6.60 folds, respectively. The superiority of Ni₅Cu₅/PVdF–HFP nanomaterial was also supported by estimating the decay percentages of altered electrodes at the end of the chronoamperometry test. The related oxidation current density at Ni₃Cu₇/PVdF–HFP, Ni₉Cu₁/PVdF–HFP, Ni₇Cu₃/PVdF–HFP and Ni₅Cu₅/PVdF–HFP decreased by 71.36%, 77.73%, 82.98% and 87.42% when compared to the recorded magnitudes at the start of the stability region. The cyclic voltammograms of varied NiCu/PVdF–HFP nanocomposites were also scanned for repeated 100 cycles at 10 mV s⁻¹ in (1.0 M EG + 0.5 M NaOH) solution. The obtained result was demonstrated in Figure S3 to reveal the consecutive activity decline of all NiCu/PVdF–HFP electrodes upon cyclization for increased runs. The oxidation current densities of alcohol molecules were followed at different cycles and drawn in Figure 7b. After 100 cycles, satisfactory stability performances were achieved by the fabricated NiCu/PVdF–HFP films with percentages ranging between 76.90% and 87.95%. The incorporation of 50%Ni:50%Cu into the prepared polymeric film was recommended for its encouraging activity and durability to be optimized for further catalytic processes. The contribution of variable wt.% values of copper oxide species to the chemical composition of the investigated nanocatalyst appreciably affected its durability during the alcohol oxidation process. Our earlier studies demonstrated that the insertion of moderate proportions of copper oxide into the nanocomposite skeleton efficiently promoted its stability behavior. Adding 5, 7.5, 10, 12.5 and 15 wt.% copper oxide to nickel oxide nanostructure onto graphite substrate yielded respective steady state current densities of 1.51, 2.56, 2.80, 3.26 and 4.29 mA cm⁻² when ethanol molecules were added to basic solution. However, higher metallic oxide contents might exhibit an adverse effect on the durability test, as concluded from the chronoamperometric studies [90]. Here, in this present work, a gradual improvement in the stability performance of NiCu/PVdF–HFP film was shown by introducing an increased copper oxide wt.% in the prepared polymeric surface up to 50%. The irregular distribution of catalytic nanoparticles for the nanostructures with higher oxide content (70 wt.% copper oxide), as revealed from the relevant SEM image, might reduce the available number of active sites for operating the oxidation reaction and impede the electron transfer process, resulting in decayed activity and stability performances.

Varying the scan rate while conducting the linear sweep voltammetric runs at Ni₅Cu₅/PVdF–HFP film in (1.0 M EG + 0.5 M NaOH) solution is shown in Figure 8a. This measurement parameter significantly affects the obtained oxidation peak current density and potential values. Increased membrane activities were monitored for the related voltammetric scans at higher rates. Sketching the obtained current intensities as a function of υ^0.5 in Figure 8b resulted in a linear region as a consequence of the prevalence of diffusion rules when the alcohol molecules were electro-oxidized at the membrane surface. A continuous potential shift of the oxidation peak towards positive values was also observed at Ni₅Cu₅/PVdF–HFP film as its linear sweep voltammetric studies were working at faster rates. Drawing this oxidation peak potential against log υ in Figure 8c reflected this observation via a straight-line plot. The electron transfer coefficient (α) magnitude was extracted from this line slope that equals [2.303RT/(1–α)nF] [91]. It was found to be 0.410, as a comparable value to that reported when ethanol molecules were electro-oxidized at Ni/PVdF–HFP membrane [0.468] [92]. The linear sweep voltammogram at 1 mV s⁻¹ was exploited to calculate some kinetic parameters, such as Tafel slope and the exchange current density magnitudes. The varied potentials against log I were followed for this curve in Figure 8d. A linear trend was observed with a Tafel slope value of 132.35 mV dec⁻¹. Additionally, Ni₅Cu₅/PVdF–HFP film exhibited an exchange current density of 4.232 × 10⁻⁹ A cm⁻². This result demonstrated an enhanced alcohol oxidation reaction rate at NiCu-polymeric surface when contrasting its Tafel slope to that reported at the relevant Ni/PVdF–HFP membrane [369 mV dec⁻¹] [92].

The phase shift plots of varied NiCu/PVdF–HFP nanostructures were studied in basic solution after inserting subsequent alcohol concentrations in Figure 9a–d for the films with Ni:Cu weight ratios of 9:1, 7:3, 5:5 and 3:7, respectively. They were scanned within the frequency region of 10,000–0.1 Hz at 575 mV. Steady phase angle values were monitored irrespective of ethylene glycol concentration within the high-frequency region for all investigated NiCu/PVdF–HFP nanomaterials. This resistive performance was recorded at the outer electrode layer. A gradual phase angle increase was shown as the scanned frequency decreased. This phase shift might be linear when reduced ethylene glycol contents were present in the chosen medium to reflect the controlled electrode behavior by diffusion-limited kinetics. However, increased alcohol concentrations in NaOH solution were responsible for lowering the obtained phase angles, displaying a characteristic maximum within the intermediate and decreased frequency sections. This phase angle maximum was formed at increased frequency values as the amount of alcohol introduced to the basic electrolyte increased. This was clearly apparent for the metallic polymer surfaces with Ni:Cu weight ratios of 7:3 [see Figure 9b] and 5:5 [see Figure 9c]. This phase angle peak is a transition to a capacitive attitude within the electrode double layer. The influence of altering the metallic Ni:Cu proportions during the membrane’s fabrication onto their reported phase angles within the scanned frequency region was concluded from the corresponding diagrams of NiCu/PVdF–HFP electrodes in (0.3 M EG + 0.5 M NaOH) solution in Figure 9e. The increased copper content in the formed film significantly improved its activity as revealed from continuous phase angle decrease in the lowered frequency section for electrodes with Ni:Cu weight ratios of 9:1, 7:3 and 5:5. The highest catalytic behavior was assigned for the Ni₅Cu₅/PVdF–HFP membrane. At the same time, a linear trend was then shown for Ni₃Cu₇/PVdF–HFP electrode with increased phase angles to illustrate its lowered activity.

On the other hand, bode plots of Ni₉Cu₁/PVdF–HFP, Ni₇Cu₃/PVdF–HFP, Ni₅Cu₅/PVdF–HFP and Ni₃Cu₇/PVdF–HFP nanocatalysts were examined in an alkaline medium adding ethylene glycol molecules in increased proportions till 1.0 M; the obtained results are presented in Figure 10a–d. A silent electrochemical response was recorded for all prepared metallic membranes when higher frequencies were scanned. However, a fast increase in the measured impedance values was significantly observed when medium and low frequencies were operating. Moreover, log Z values were consecutively decreased for electrolytes with higher alcohol concentrations to monitor the lowest resistance magnitudes when 1.0 M ethylene glycol was inserted. This result significantly supported the promising activity of the fabricated nanocatalysts for electro-oxidizing ethylene glycol species. The Ni:Cu weight ratio in the evaluated NiCu/PVdF–HFP electrodes appreciably controlled the charge transfer rate of the oxidation process across their surfaces. This was derived from contrasting the related bode curves of altered NiCu/PVdF–HFP nanocomposites in (1.0 M alcohol + 0.5 M NaOH) in Figure 10e. A remarkable decrease in the noticed impedance values was shown as the copper content in the synthesized membrane increased to exhibit the lowest log Z magnitudes for the one with Ni:Cu weight ratio of 0.5:0.5. On contrary, the metallic film with a higher Cu content (70 wt.%) demonstrated much increased resistance when compared to the recorded values for those with reduced copper contribution. This situation was similar to that reported by our research group when studying the response of deposited NiCu oxide [90] and NiCo₂O₄ [84] nanoparticles onto graphite surface during ethanol oxidation reaction in an alkaline solution using EIS experiments. Including different proportions of transition metal oxides in these nanocomposites appreciably affected their measured resistance values. As concluded from the related results for copper- and cobalt-containing binary oxides, a gradual reduction in the obtained resistance value was monitored as the content of copper and cobalt in the synthesized nanostructures increased up to 15 and 20 wt.%, respectively. Above these proportions, the activities of formed nanomaterials declined, showing increased resistance.

The respective Nyquist curves of different NiCu/PVdF–HFP films were drawn using a basic solution with a gradual addition of increased volumes of ethylene glycol. The achieved curves are clarified in Figure 11 for the membranes with metallic weight % magnitudes of Ni:Cu as 9:1 1 [section (a)], 7:3 [section (b)], 5:5 [section (c)] and 3:7 [section (d)]. They all exhibited two depressed semicircles in the high and low frequency regions in the presence of altered alcohol concentrations; however, the measured resistances subsequently decayed as more ethylene glycol molecules were incorporated in the alkaline electrolyte. The diameter of the formed semicircle in the decreased frequency zone clearly clarified this conclusion. It continuously exhibited a reduced curvature reflecting lowered charge transfer resistances at the studied electrode surface. This Nyquist diagram was contrasted for the films with different Ni:Cu weight % ratios in Figure 11e when 1.0 M ethylene glycol molecules were present in NaOH solution. The Ni₉Cu₁/PVdF–HFP electrode had the largest curvature of recorded semicircles in the low-frequency section among the examined nanomaterials. This result supported its increased impedance for a facile charge transfer step during the alcohol oxidation reaction. Ni₇Cu₃/PVdF–HFP and Ni₅Cu₅/PVdF–HFP electrodes had much lower resistance values. Favored reaction kinetics were expected at the Ni₅Cu₅/PVdF–HFP film surface. Further increase in the copper amount in the fabricated membrane was not suggested for a valuable electro-oxidation process as a result of its increased resistance to an efficient electron transfer mechanism. This result was further confirmed by proposing a convenient electric circuit structure with specific components in appropriate connection to simulate the theoretical results with the obtained experimental data. The presented model in Figure 11f has two (R–CPE) sections: They are in a serial arrangement with a solution resistance component (R_s). Here, R₁ and R₂ reflect a numerical representation of the measured impedance values in the outer and inner regions of the electrode surface, respectively. Moreover, CPE expresses the related constant phase elements in these two sections with a good replacement for the capacitive role in the constructed circuit. The unfair distribution of the electrolyte impedance and double-layer capacitance was handled through inserting this CPE component instead of a capacitor [93]. By employing this electric circuit skeleton to the measured curves of varied NiCu/PVdF–HFP electrodes in NaOH solution with altered alcohol concentrations, the simulated values of these model constituents are presented in

Table 1. The magnitudes of altered electric circuit components after the fitting process of the experimental results to the theoretical ones at different NiCu/PVdF–HFP membranes [(a) Ni₉Cu₁/PVdF–HFP, (b) Ni₇Cu₃/PVdF–HFP, (c) Ni₅Cu₅/PVdF–HFP and (d) Ni₃Cu₇/PVdF–HFP] in NaOH solution with inserting increments of ethylene glycol molecules. These measurements were conducted at 575 mV in the frequency range of 10,000–0.1 Hz.

Ethylene Glycol Concentration/M	Rs/ Ohm	Yo1/ Ohm−1 sa cm−2 × 10−3	a1	R1/ Ohm cm2	Yo2/ Ohm−1 sa cm−2 × 10−4	a2	R2/ Ohm cm2
(a)
0.1	649.0	1.000	0.940	370.0	1.082	0.593	8478.0
0.2	597.0	1.519	0.992	384.4	1.106	0.579	8028.0
0.3	653.0	1.990	0.822	380.0	1.115	0.702	7661.0
0.4	624.8	2.553	0.820	374.6	1.125	0.701	7220.0
0.6	646.8	3.495	0.899	363.4	1.138	0.684	5774.0
0.8	623.1	4.584	0.947	366.2	1.160	0.692	4752.0
1.0	618.4	5.389	0.974	368.8	1.200	0.625	4230.0
(b)
0.1	1463.1	1.432	0.825	134.3	1.526	0.470	4487.0
0.2	1461.7	2.120	0.852	137.7	1.815	0.340	3490.0
0.3	1478.5	2.991	0.852	158.5	2.366	0.457	2890.0
0.4	1460.1	3.851	0.866	141.4	3.115	0.593	1926.0
0.6	1438.2	5.793	0.875	143.4	4.128	0.644	1206.0
0.8	1420.3	7.455	0.882	157.7	5.190	0.564	1062.0
1.0	1406.0	9.512	0.883	160.4	6.107	0.500	1056.0
(c)
0.1	412.7	3.794	0.940	79.0	2.594	0.485	1900.0
0.2	409.5	4.536	0.918	72.0	2.905	0.489	1543.0
0.3	407.1	5.146	0.915	67.4	3.282	0.520	1180.0
0.4	404.7	5.882	0.895	74.5	3.854	0.510	894.5
0.6	404.0	7.171	0.896	69.7	4.818	0.568	522.2
0.8	405.4	8.526	0.836	59.8	5.828	0.514	408.1
1.0	410.9	10.366	0.853	52.6	6.600	0.489	392.6
(d)
0.1	3906.0	1.208	0.918	400.7	1.309	0.672	5158.0
0.2	3912.0	1.758	0.946	373.5	1.338	0.544	4598.0
0.3	3960.0	2.410	0.937	357.4	1.381	0.633	4062.0
0.4	3960.0	2.998	0.999	337.6	1.486	0.685	3680.0
0.6	3916.0	4.468	0.999	384.8	1.538	0.621	2970.0
0.8	3872.0	5.543	1.000	331.6	1.725	0.589	2262.0
1.0	3878.0	6.834	1.000	364.2	1.875	0.665	1954.0

. A consecutive decay of R₂ magnitudes was reported for each investigated metallic polymeric membrane as the content of alcohol molecules in the selected basic electrolyte increased. Varied electrode surfaces displayed the lowest resistance values in the presence of 1.0 M ethylene glycol. Under such experimental conditions, R₂ value gradually decreased when Ni:Cu weight % ratio in the synthesized film was in the order: 9:1 [4230.0 Ohm cm²] > 3:7 [1954.0 Ohm cm²] > 7:3 [1056.0 Ohm cm²] > 5:5 [392.6 Ohm cm²] [see the listed results in

Table 2. A comparison of the simulated values of electric circuit constituents during the fitting step of obtained curves at NiCu/PVdF–HFP membranes, using varied Ni:Cu weight % ratios, in (1.0 M ethylene glycol + 0.5 M NaOH) solution.

Nanomaterial	Rs/ Ohm	Yo1/ Ohm−1 sa cm−2 × 10−3	a1	R1/ Ohm cm2	Yo2/ Ohm−1 sa cm−2 × 10−4	a2	R2/ Ohm cm2
Ni9Cu1/PVdF–HFP	618.4	5.389	0.974	368.8	1.200	0.625	4230.0
Ni7Cu3/PVdF–HFP	1406.0	9.512	0.883	160.4	6.107	0.500	1056.0
Ni5Cu5/PVdF–HFP	410.9	10.366	0.853	52.6	6.600	0.489	392.6
Ni3Cu7/PVdF–HFP	3878.0	6.834	1.000	364.2	1.875	0.665	1954.0

].

The potential value at which the EIS experiment operated significantly affected the obtained charge transfer rate. This was clarified by conducting a series of EIS measurements for Ni₅Cu₅/PVdF–HFP film in (0.3 M ethylene glycol + 0.5 M NaOH) solution at a number of potentials (350–700 mV) in Figure 12. The phase shift of this membrane showed a gradual decrease in its phase angles in the middle- and low-frequency sections as the examined potential was shifted towards more positive values. A lowered phase maximum was also detected at increased potentials, reaching its most reduced value at 700 mV [see section (a)]. Moreover, the bode diagrams of Ni₅Cu₅/PVdF–HFP film at altered potentials presented in Figure 12b demonstrate decreased impedance values for the working measurements at more positive potentials. Within the detected potentials, the lowest resistance was attained at 700 mV. Nyquist curves in Figure 12c displayed subsequent reduction in the semicircles’ diameters as the swept potential was positively moved. The smallest resistance value was monitored for the metallic membrane surface at 700 mV. The constructed electric circuit in Figure 11f was applied for fitting these experimental EIS results with their corresponding theoretical data, and the derived elements’ magnitudes were shown in

Table 3. The fitting results of the experimental data at Ni₅Cu₅/PVdF–HFP membrane with those of the theoretically predicted values in (0.3 M ethylene glycol + 0.5 M NaOH) solution at different potentials.

E/mV	Rs/ Ohm	Yo1/ Ohm−1 sa cm−2 × 10−3	a1	R1/ Ohm cm2	Yo2/ Ohm−1 sa cm−2 × 10−4	a2	R2/ Ohm cm2
350	388.4	3.199	0.835	59.3	1.603	0.570	1976.0
400	377.0	4.396	0.879	71.1	2.054	0.508	1644.0
450	378.8	4.756	0.906	56.5	2.340	0.565	1536.0
500	379.6	5.002	0.932	58.0	2.654	0.534	1318.0
550	382.4	5.076	0.932	73.5	3.130	0.511	1249.0
600	418.4	5.277	0.937	58.3	3.438	0.574	1158.0
650	423.2	5.557	0.922	74.0	3.554	0.517	1088.0
700	424.2	5.664	0.924	60.8	3.627	0.533	1048.0

. A consecutive decrease in the charge transfer resistance was observed by positively adjusting the potential value during the EIS test. It exhibited 1.885 folds reduction for the operated experiment at 700 mV in relation to that at 350 mV.

Ni–Cu alloys were selected in this study instead of Co–Cu or Fe–Cu analogues. This choice was motivated by the well-established synergistic interaction between Ni and Cu, where Ni provides abundant Ni(II)/Ni(III) redox sites for electro-catalyzing the alcohol oxidation process. At the same time, Cu enhances the electrical conductivity of fabricated nanostructures and modulates the electronic structure of Ni. This in turn weakens the bonding interactions of the metallic sites with accumulated poisoning species [CO-like intermediates]. Accordingly, nickel–copper-containing nanocatalysts significantly display greater resistance to surface deactivation and maintain increased activity retention when applied for long-term electro-oxidation processes [94,95,96,97]. Moreover, reduced onset potentials and promoted current densities were measured at this binary system during alcohol oxidation reaction when contrasted to the reported results at Co- or Fe-incorporated nanomaterials. Integrating nickel–copper nanoparticles into the synthesized PVdF–HFP films significantly combines these enhanced features of metallic counterparts with the feasible ionic transport characteristics of polymeric supports to have a highly efficient electrocatalyst platform with a satisfactory durability. The synergy between nickel and copper oxides in the formed nanostructures originates from the availability of redox active sites of nickel hydroxide/oxyhydroxide species for promoting the alcohol electro-oxidation process rate, while the electronic conductivity of these nanocomposites was greatly enhanced by introducing copper oxide nanoparticles in appreciable contents, showing the optimum performance for the electrode surfaces with 50%Ni:50%Cu. A satisfactory durability behavior was also demonstrated by NiCu/PVdF–HFP membranes as a result of modifying the electronic environment of nickel sites via their d-band centers to reduce the accumulation of CO-like poisoning intermediates. The shortage of copper oxide content in the Ni₉Cu₁/PVdF–HFP electrode might account for its decreased steady state oxidation current density. In contrast, the inhomogeneous distribution of deposited copper oxide sites for membranes with excessive copper oxide wt.% values [Ni₃Cu₇/PVdF–HFP] [as demonstrated by the morphological studies] dissipated the active reaction centers. This explained the decayed activity and durability of electrodes with higher copper oxide loadings [above 50 wt.%]. EIS measurements of the alcohol oxidation reaction at varied NiCu/PVdF–HFP films supported the obtained results by cyclic voltammetry and chronoamperometry. Among all examined membranes, Ni₅Cu₅/PVdF–HFP displayed the lowest R_ct magnitudes to illustrate the pronounced electron transfer rate at its surface with rapid reaction kinetics. The encouraging performance of deposited metallic species onto polymeric films might direct numerous interested research groups to fabricate comparable nanomaterials for different electrocatalytic fields.

3. Experimental Section

3.1. Chemicals

Poly(vinylidene fluoride-co-hexafluoropropene) with M.wt. of 65,000 g mol⁻¹, Ni(II) acetate tetrahydrate, Cu(II) acetate tetrahydrate and sodium borohydride were delivered from Sigma-Aldrich (St. Louis, MO, USA). N,N-dimethylformamide (DMF) and acetone were purchased from Fluka. All chemicals were employed as received without additional treatment.

3.2. The Formation of Metallic PVdF–HFP Membrane Surfaces in Altered Ni:Cu Proportions

The polymeric film (15 wt.%) was firstly formed by inserting 1.5 g of PVdF–HFP powder inside a mixed solution of acetone and DMF in a volume proportion of 1:4. The metallic acetate precursors in varied weight percentage ratios [Ni_xCu_10−x, x = 3, 5, 7 and 9] were subsequently dissolved into DMF, followed by sufficient stirring to finally have a transparent mixture. The metallic salt solutions and polymeric sol gel were homogenized together by stirring overnight. A plastic capillary syringe was then filled with this mixture and fixed inside a lab-scale electrospinner. One section of this syringe contained a copper rod, while the other counterpart was connected to a power supply acting as the positive pole under an increased voltage. The negative electrode was joined to a ground iron drum with an Al sheet to collect the formed fibers. The electrospinning process was conducted at 20 kV with consecutive drying under vacuum at room temperature. The reduction step of incorporating metallic ions inside PVdF–HFP films was carried out using sodium borohydride solution with an increased molar ratio of five times compared to that of mixed metallic precursor salts. The polymeric films were immersed in this reduction solution until hydrogen molecules were completely evolved without further deposition of black spots onto the film’s surfaces. They were finally cleaned with double-distilled water and ethanol, and dried in an air oven within 300 min at 60 °C.

3.3. The Physical Characterization of NiCu/PVdF–HFP Membranes Using Convenient Analysis Tools

The crystallinity and the size of the formed metallic particles were investigated using an X-ray diffractometer (Rigaku Co., Tokyo, Japan). Cu K_α radiation with a wavelength of 1.54056 Å was employed. ATR-FTIR model “Nicolet iS 10” (Thermo Fisher Scientific, Waltham, MA, USA) was also connected to a specular reflectance for evaluating the incorporation of binary metallic species into the selected PVdF–HFP membrane. A wavenumber range extending from 500 to 4000 cm⁻¹ was scanned. The morphology of fabricated nanomaterials was studied through scanning electron microscopy (SEM, Hitachi S-7400, Tokyo, Japan) in conjunction with energy-dispersive X-ray (EDX) equipment for the determination of numerous nanocatalyst elements distribution and their relative weight percentages.

3.4. The Electrochemical Investigation of Varied NiCu/PVdF–HFP Membranes Activity for Electro-Oxidizing Ethylene Glycol Molecules

The electrocatalytic activity of altered NiCu/PVdF–HFP films towards ethylene glycol electro-oxidation reaction was tested in an alkaline medium. Different electrochemical tools, including cyclic voltammetry, chronoamperometry, linear sweep voltammetry and electrochemical impedance spectroscopy were operated via Gamry potentiostat. A three-electrodes cell was constituted using NiCu/PVdF–HFP membrane [2.0 × 1.0 cm²] as the working electrode. It was fixed to a suitable copper wire to be connected to the potentiostat workstation. On the other hand, a Pt wire and a silver/silver chloride electrode were inserted in the electrochemical system as the counter and reference electrodes, respectively. A stabilization of the electrochemical performance of examined nanocatalysts was achieved after their continuous cyclization in 0.5 M NaOH solution at 50 mV s⁻¹ for 40 cycles using a potential region from 0 up to 900 mV (Ag/AgCl). The optimum alcohol concentration was predicted by investigating the influence of changing ethylene glycol content in the supporting electrolyte on the nanomaterials’ activity. Cyclic voltammetric studies were also conducted at numerous scan rates during the oxidation process. The durability of NiCu/PVdF–HFP electrodes for efficient activity after more extended working periods was checked using the respective chronoamperograms at 575 mV for 30 min. A number of electrochemical impedance spectroscopy experiments were conducted at the fabricated NiCu/PVdF–HFP membranes’ surfaces to explore their impedance behavior towards differing alcohol concentrations and potential magnitudes during the electro-oxidation process. This whole study was carried out at 25 °C using an aerated electrochemical cell.

4. Conclusions

This work demonstrated the possibility of applying anchored NiCu nanoparticles onto electrospun PVdF–HFP membranes for a highly efficient and durable platform during the alkaline electro-oxidation process of ethylene glycol molecules. Among the investigated compositions, the Ni₅Cu₅/PVdF–HFP membrane achieved the best balance of activity and stability performances. It delivered an oxidation current density of 564.88 μA cm⁻² and a reaction order of 0.809 with respect to ethylene glycol concentration. After 100 repeated oxidation cycles, this Ni₅Cu₅/PVdF–HFP film retained 87.95% of its initial activity. A decreased R_ct value [392.6 Ohm cm²] was also measured to ascertain the rapid charge transfer kinetics at these metallic polymeric surfaces. This surprising activity of 50 wt.%Ni:50 wt.% Cu composition was explained by the synergistic interplay between the distribution of sufficient Ni(II)/Ni(III) redox sites within the membrane surface and the improved conducting environment by providing copper-based species in appreciable contents. They efficiently weakened the adsorption tendency of poisoning intermediates to the catalytic surface leaving highly active areas for prolonged oxidation reaction. On the other hand, the insufficient and irregular arrangements of these metallic sites for films with Ni:Cu weight ratios of 9:1 and 3:7 might result in lowered steady state oxidation current densities and increased R_ct values during their electrochemical activity investigations. This preparation strategy of modified polymeric membranes significantly presents a cost-effective alternative to noble metals containing nanocatalysts, extending their applicability to other valuable electrocatalytic reactions in biosensing and energy storage fields.