CuNi Alloy NPs Anchored on Electrospun PVDF-HFP NFs Catalyst for H2 Production from Sodium Borohydride

Non-noble CuxNi1−x (x = 0, 0.1, 0,2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1) alloy nanoparticles supported on poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) nanofibers (NFs) are successfully fabricated. The fabrication process is executed through an electrospinning technique and in situ reduction in Cu2+ and Ni2+ salts. The as-synthesized catalysts are characterized using standard physiochemical techniques. They demonstrate the formation of bimetallic NiCu alloy supported on PVDF-HFP. The introduced bimetals show better catalytic activity for sodium borohydride (SBH) hydrolysis to produce H2, as compared to monometallic counterparts. The Cu0.7 Ni0.3/PVDF-HFP catalyst possesses the best catalytic performance in SBH hydrolysis as compared to the others bimetallic formulations. The kinetics studies indicate that the reaction is zero order and first order with respect to SBH concentration and catalyst amount, respectively. Furthermore, low activation energy (Ea = 27.81 kJ/mol) for the hydrolysis process of SBH solution is obtained. The excellent catalytic activity is regarded as the synergistic effects between Ni and Cu resulting from geometric effects over electronic effects and uniform distribution of bimetallic NPs. Furthermore, the catalyst displays a satisfying stability for five cycles for SBH hydrolysis. The activity has retained 93% from the initial activity. The introduced catalyst has broad prospects for commercial applications because of easy fabrication and lability.


Introduction
The fast growth in the world population is leading to an increase in global energy consumption that is primarily formed from fossil fuels (e.g., petroleum oil, coal, and natural gas). Combustion of fossil fuels produces toxic gases (e.g., carbon dioxide, nitrous oxide, etc.) that deteriorate the ecological environment [1][2][3]. Thus, the search for alternative and sustainable energy has become urgent worldwide. Hydrogen (H 2 ) is considered one of the most hopeful energy carrier sources because of its high energy content and eco-friendly characteristics such as zero-pollutant emissions [4]. However, the practical application of H 2 in large quantities (e.g., fuel cell vehicles) suffers from unsafe and inefficient H 2 storage methods (e.g., compressed gasses or cryogenic liquids) at mild conditions, which is attributed to the low H 2 density > 5 wt% [5][6][7][8][9]. In recent years, metal hydrides (e.g., sodium borohydride (NaBH 4 , SBH), lithium hydride (LiH), sodium aluminum hydride (LiAlH 4 ), lithium aluminum hydride (LiAlH 4 ), and lithium borohydride (LiBH 4 )) have been recognized as promising hydrogen reservoirs because of their extraordinary H 2 storage density [10][11][12]. Amongst them, SBH is considered one of the ideal candidates for an H 2 reservoir owing to its low density and low molecular weight, low cost, non-toxicity, and high hydrogen content (~10.8 wt.%). H 2 can be generated through hydrolysis of SBH in the presence of appropriate catalysts like noble (e.g., Pt, Ru, and Pd) and non-noble metallic (e.g., Ni, Co, and Cu) nanoparticles (NPs) and their alloys [13][14][15]. Recently, bimetallic Cu-M (M=Fe, Co, Ni) with different geometries and compositions has become very attractive electrospinning technique, and finally (iii) reduction in the catalytic acetate precursor into their metallic forms.
To prepare the electrospinning solutions, first, polymer solutions, 15 wt.% PVDF-HFP, were prepared by dissolving 1.5 g of PVDF-HFP powder in a solvent mixture of DMF and acetone (4:1). The polymeric solutions were mixed in a magnetic stirrer for at least 6 h to make very homogenous sol-gel solutions. Second, different composition from the catalytic precursors, NiAc and CuAc, were added to 20 g of the prepared homogenous PVDF-HFP solutions, separately. The amount of NiAc and CuAc were adjusted to fabricate NFs with Ni 1−x Cu x (x = 0-0.9). To ensure perfect dispersion of the catalytic precursors, the electrospinning sol-gel solutions were stirred, after adding the catalytic precursors, for 5 h at 50 • C. The NiAc-CuAc/PVDF-HFP solutions were finally cooled at room temperature to become ready to be electrospun.
The prepared electrospinning solutions were then electrospun using a lab-scale electrospinner to fabricate polymeric NFs with different catalytic precursors compositions. The electrospinning set-up consists of three major components: solution reservoir to hold the electrospinning solution, power supply to generate an electrical field, and a collector to collect the fabricated NFs. The fabricated polymeric solutions were placed in a reservoir, a 5 mL plastic syringe. The electrical field was generated by inserting a copper wire inside the plastic syringe from one end (positive electrode) while the other side, the collector, was grounded. The rotating drum collector was covered with aluminum foil to collect the fabricated NFs. The electrospinning operating conditions were fixed at room temperature and pressure, 20 kV and 18 cm distance between the syringe and the collector. Generally, most of the solvents evaporate while the NFs travel from the syringe to the collector; however, some residual solvent stays in the fabricated NFs. Hence, the fabricated NF mats were dried at 40 • C under vacuum overnight to ensure full evaporation of the solvent mixture.

Chemical Reduction in CuAc and NiAc Supported on PVDF-HFP NFs
To reduce catalytic precursors and convert NiAc and CuAc into their pure metallic forms, SBH was used as a reducing agent. Typically, to start the reduction process, first, 100 mL of methanol was placed in a 500 mL beaker. Then, the fabricated electrospun nanofibrous mats were immersed in the methanol solution. After that, SBH was added in the beaker and stirred at 1000 rpm. The molar ratios between metals precursor and SBH were adjusted at 1:5 to obtain a complete reduction reaction. To prevent vigorous reduction reactions, the reduction reaction was operated at low temperature. It was noticed that as soon as the fibrous mat was immersed in the solution, a gas bubble (H 2 ) was produced with a green-to-black color change, indicating that NiAc and CuAc converted into Ni and Cu metals, respectively. The fibrous mat was kept in the SBH solution until no H 2 bubbles were seen. The produced mat was washed by ethanol and DI water to eliminate impurities. Last, the reduced nanofibrous mat was dried overnight at 40 • C under vacuum.

NaB(OCH 3 ) 4 + 2H 2 O → NaBO 2 + 4CH 3 OH
It is worth mentioning that researchers usually use aqueous SBH to reduce metal salts [39]. However, SBH here was dissolved in methanol and used to execute the reduction step. In fact, aqueous SBH was initially applied as a reducing agent, and it was seen that the reduction reaction was much slower than the SBH/methanol reducing agent.
Using methanol instead of water provides important advantages. For example, the by-product, NaB(OCH 3 ) 4 , does not rapidly polymerize into various types of polyborates. Preventing the precipitation of NaB(OH) 4 is necessary to stop the poisoning of the cata- lyst [58,59]. Hongming et al. [60] used hydrothermal and reduction procedures to prepare ultrafine Co NPs@carbon nanospheres as an effective catalyst for the generation of H 2 from SBH hydrolysis. Due to the insolubility of NaBO 2 in ethanol, partial NaBO 2 was precipitated out together with the Co NPs in the water media. This could separate the Co nanoparticles and further prevent them from agglomerating into larger clusters. They found that the reduction in Co ions in the ethanol solution media was more effective than in the water media. In the end, the NaBO 2 was removed by washing it with DI water. The surfaces of the membranes that were electrospun using Ni 2+ Cu 2+ /PVDF-HFP were green in hue. The chemical reduction in these metallic ions has a tendency to modify the color of their linked polymeric membranes into a black-look-alike black-dyed piece of fabric. This change in color is what indicated the formation of ultrasmall NiCu NPs on the surface of PVDF-HFP membranes.

Characterization
The fabricated catalytic NFs morphology was characterized by a scanning electron microscope (SEM, Hitachi S-7400, Japan) equipped with an energy-dispersive X-ray (EDX). A Fourier transform infrared (FTIR), using the smart ATR-FTIR model "Nicolet iS 10" (Thermo Fisher Scientific, Waltham, MA, USA) equipped with the specular reflectance, was also used to study the interaction between Ni Cu NPs and PVDF-HFP NFs. The fabricated NFs were positioned at the top of the spectrophotometer and the scanning range was 400-3500 cm −1 . X-ray diffraction (Rigaku Co., Tokyo, Japan) with Cu Kα (λ = 1.54056 Å) was utilized to determine the catalysts' crystalline structure and crystal size. An X-ray photoelectron spectroscopy analysis (XPS, AXIS-NOVA, Kratos Analytical, Manchester, UK) was conducted with the following conditions: base pressure of 6.5 × 10 −9 Torr, resolution (pass energy) of 20 eV, and scan step of 0.05 eV/step.

SHB Catalytic Hydrolysis Reaction
To run the reaction experiments, a two-neck flask was used as a reactor. The reactor was placed on a magnetic stirrer. The magnetic stirrer provided both mixing and heating. The reaction temperature was measured by a thermometer. The flask was connected by a plastic tube to a gradually cylinder filled with water to measure the generated H 2 gas. Water displacement from the gradually cylinder was reflected in the volume of H 2 generation gas. To start the reaction, first, 100 mg of the prepared catalytic membranes was placed inside the flask. Then, 50 mL of 1 mmol of SBH was fed to the reactor and mixed at 100 rpm and the temperature was kept at 25 • C.
The catalytic hydrolysis reaction was started rapidly without induction time when catalytic NFs were conducted in the reactor. The kinetics of the hydrolysis reaction was studied by changing the amount of the catalysts, concentration of SBH, and temperature. To study the effect of a catalyst on the H 2 production, different amounts of catalysts (100, 150, 200, and 250 mg) have been used. The effect of SBH concentration (1, 2, 3, and 4 mmol) on H 2 production was also studied. To calculate the reaction activation energy, different reaction temperatures (298, 308, 318, and 328 K) have been studied. The durability of the introduced NFs was also studied through a recycling process. The reusability test has been conducted at 100 mg, 1 mmol, and 298 K of catalyst, SBH, and temperature, respectively. The catalyst has been used at all cycles without any washing. At each cycle, 1 mmol SBH was added and other parameters were kept constant.

Results and Discussion
Increased interconnections, flexibility, excellent porosity, and remarkable surface-tovolume ratios are just a few of the potential benefits of a polymeric nanofibrous membrane produced using the electrospinning approach [61,62]. The semi-crystalline nature, high thermal stability, improved dielectric constant, hydrophobicity, and piezo-and pyroelectric properties of PVDF-HFP make it one of the most suitable polymeric materials for producing these films [63,64]. SEM images of dried electrospun PVDF-HFP nanofibrous mats, at both low and high magnifications, proved the fabrication of an excellent nanofibrous structure devoid of beads ( Figure 1A,B). It is well-known that the electrospinning technique produced nanoporous-structured NFs. These nanopores are generated in the electrospun NFs because of the high evaporation rate of solvents, especially acetone. Solvent evaporation happens when the NFs travel from the positive-charge electrode, syringe needle, to the negative electrode, rotating drum. This nanoporous structure is ideal for the nucleation of Ni and Cu crystals. In addition, since the CuAc and NiAc precursor salts have high water content, the produced hydrophobic PVDF-HFP membranes exhibit an increased demixing rate within the liquid-liquid phases. This demixing results in the formation of a high number of pores on their structure [65]. As a result, the analyte molecules would be able to get trapped in the pores with the minimal possibility of diffusion resistance, so making H 2 evolution more straightforward. High electrical conductivity and gelation of the PVDF-HFP solution, together with the formation of maximal elongation of a jet along its axis, are two additional benefits of the presence of metal salts in the process of producing nanoscale polymeric NFs [44].
Increased interconnections, flexibility, excellent porosity, and remarkable surface-tovolume ratios are just a few of the potential benefits of a polymeric nanofibrous membrane produced using the electrospinning approach [61,62]. The semi-crystalline nature, high thermal stability, improved dielectric constant, hydrophobicity, and piezo-and pyroelectric properties of PVDF-HFP make it one of the most suitable polymeric materials for producing these films [63,64]. SEM images of dried electrospun PVDF-HFP nanofibrous mats, at both low and high magnifications, proved the fabrication of an excellent nanofibrous structure devoid of beads ( Figure 1A,B). It is well-known that the electrospinning technique produced nanoporous-structured NFs. These nanopores are generated in the electrospun NFs because of the high evaporation rate of solvents, especially acetone. Solvent evaporation happens when the NFs travel from the positive-charge electrode, syringe needle, to the negative electrode, rotating drum. This nanoporous structure is ideal for the nucleation of Ni and Cu crystals. In addition, since the CuAc and NiAc precursor salts have high water content, the produced hydrophobic PVDF-HFP membranes exhibit an increased demixing rate within the liquid-liquid phases. This demixing results in the formation of a high number of pores on their structure [65]. As a result, the analyte molecules would be able to get trapped in the pores with the minimal possibility of diffusion resistance, so making H2 evolution more straightforward. High electrical conductivity and gelation of the PVDF-HFP solution, together with the formation of maximal elongation of a jet along its axis, are two additional benefits of the presence of metal salts in the process of producing nanoscale polymeric NFs [44].   In addition, the nanoporous nature of PVDF-HFP NFs means that reduced Ni and Cu ions may cover their surface after the mats are formed.
On display in Figure 2A-D is an elemental mapping image of the Ni 0.7 Cu 0.3 /PVDF-HFP membrane NFs. It is clear that there is a large distribution of Ni and Cu NPs throughout the membrane NFs, and this is supported by the pictures obtained from the SEM. The figure displays the EDX curve for the Ni 0.3 Cu 0.7 -PVDF-HFP membrane NFs ( Figure 3A,B). The detected chemical composition of the product are the elements carbon, copper, nickel, and fluorine. The weight % of Ni and Cu that are developed at different points is shown in the inset of Figure 3B. It is obvious that NPs are distributed greatly throughout the NFs, and their composition is perfectly similar to that of the precursors from which they were originated.    Using SBH as an effective reducing agent in a CH 3 OH medium, metallic Ni and Cu NPs were produced on the PVDF-HFP NFs surface. Because of their disparate reduction potentials (E 0 Cu ++ /Cu 0 = +0.34 V vs. SHE and E 0 Ni ++ /Ni 0 = −0.25 V versus SHE), SBH is more suited to reducing Cu ions to Cu 0 than Ni ions to Ni 0 . First, the generation of Cu 0 NPs stimulated SBH hydrolysis. Then, the production of an active intermediate, Cu-H species, becomes the key to facilitate Ni ++ reduction. As a result, Cu 0 plays a substantial role in improving the formation of the desired catalytic structure.
The FTIR spectra of PVDF-HFP and NiCu/PVDF-HFP fibrous mats are shown and explained in Figure 5. It is obvious that both fibrous mats had recognizable vibrational bands that were similar to one another. The phases of α and β were identified thanks to the corresponding peaks that occurred at frequency values of 749 and 837 cm −1 [67]. Two other important vibrational bands, centered at 672 and 872 cm −1 , were designated for the CF and CH 2 wagging of vinylidene units in the amorphous phase of the PVDF-HFP film. These bands were found in the amorphous phase of the film. Additionally, the symmetric C-F stretching, the CF 2 stretching, and the deformed vibrations in this fibrous mat were also revealed via their respective bands at 1071, 1175, and 1400 cm −1 [68]. During the process of fabricating the polymeric membrane, the addition of nickel and copper precursor salts caused the appearance of two more peaks. The stretching vibration maxima at 1561 cm −1 proved the production of NiO and CuO species inside this nanomaterial [69].

Hydrogen Release from SBH
In a two-neck flask connected to a graduated burette containing water, the catalytic activity of the fabricated catalysts (Ni/PVDF-HFP and Ni x Cu 1−x /PVDF-HFP) toward H 2 production from SBH was determined. Figure 6 shows the catalytic activity of various Ni x Cu 1−x /PVDF-HFP formulations compared to that of the mono-metallic Ni/PVDF-HFP; the bimetallic alloys display a high hydrogen-production volume. The H 2 production rate initially grew and subsequently fell over the course of a given time period. The catalytic reaction in this case seems to take place on the metal catalyst's surface [19]. These findings provide strong evidence for a significant role for the synergistic interaction between Cu and Ni in the hydrolysis of SBH. The synergistic effects of NPs alloy can be attributed mostly to the electronic effects, which become significant when the metal atoms have considerable variances in electronegativity and geometric effects [20]. The electronegativities of Cu and Ni are so similar: 1.90 and 1.91, respectively; therefore, it is likely that the obtained synergetic effects of the fabricated Ni x Cu 1−x /PVDF-HFP catalysts are mostly attributable to the geometric effects rather than the electronic effects. Since both Cu and Ni have an A1-type crystal structure, the formation of CuNi alloy NPs allows Ni to replace part of the Cu in the Cu lattice while still allowing the Cu crystal system to maintain its A1-type crystal structure, resulting in strong synergistic effects between Cu and Ni. The bimetallic catalyst Ni 0.3 Cu 0.7 showed the maximum catalytic activity, releasing 96 mL of H 2 in 36 min. The other four bimetal catalysts (Ni 0.9 Cu 0.1 , Ni 0.7 Cu 0.3 , Ni 0.5 Cu 0.5 , and Ni 0.1 Cu 0.9 ) catalyzed H 2 release equivalents of 71, 79, 87, and 83 mL, respectively, in the same time period. In terms of catalytic activity, the order of the engaged bimetal catalysts is as follows: Ni 0.3 Cu 0.7 > Ni 0.5 Cu 0.5 > Ni 0.1 Cu 0.9 > Ni 0.7 Cu 0.3 > Ni 0.9 Cu 0.1 . According to the findings, the NiCu's catalytic activity changes when the Cu/Ni ratio is altered. When comparing the catalytic activity of all five bimetal catalysts, it is found that as the percentage of Cu increases, the catalysts become more effective. It is theorized that an increase in the Cu content of NiCu alloy facilitates the passage of more electric charge from Cu to Ni, resulting in the generation of more active sites on the catalytic surface. The greater catalytic activity of bimetallic alloy in comparison to mono metals have been observed in other metal catalysts [21][22][23][24][25][26]. For example, Li [27] and Yamada [28] have independently discovered that the Cu concentration of a Cu-based bimetal catalyst considerably influences its catalytic activity. Furthermore, the low catalytic activity and nearly nonexistent catalytic activity of pure PVDF-HFP toward hydrolysis of SBH are readily apparent. These findings give conclusive evidence that the bi-functional impacts of CuNi-alloy NPs and PVDF-HFP considerably improved the reaction activity of the Ni 0.3 Cu 0.7 /PVDF-HFP catalyst, where the catalytic support PVDF-HFP NFs may serve as a suitable matrix for CuNi NPs despite their lack of involvement in SBH hydrolysis. The mechanism involved two main steps: at first, SBH is adsorbed on the CuNi nanoparticles' surface to produce the activated intermediate metal-H, a condition precedent to the hydrolysis process. Then, when water was thrown at the metal-H species, H 2 was generated [29,30].
The performance of the fabricated nanofibrous catalysis is a function of several factors including SBH concentration, reaction temperature, catalyst amount, and catalyst reusability. Therefore, the effect of operating conditions on the performance of the best fabricated catalyst, Ni 0.3 Cu 0.7/ PVDF-HFP, was studied. First, the effect of the amount of the catalyst on H 2 generation, at 25 • C and 1 mmol of SBH, was studied. Increases in the catalyst amount, as shown in Figure 7a, resulted in an increase in the generated H 2 volume. In fact, increasing the catalyst dose from 100 mg to 250 mg shortened the time it took for H 2 evolution from 36 minutes to 12 minutes. So, for all reactions carried out under similar conditions, the time required to produce H 2 reduces practically linearly as the amount of catalyst used increases. While this relationship is only somewhat linear, it does show that the hydrolysis process is first-order with regard to the amount of catalyst within a slope of 1.1 (Figure 7b). As a result, 100 mg of Ni 0.3 Cu 0.7 /PVDF-HFP was selected for use in the following studies.   Figure 8a shows the results of an experiment that investigated the impact of SBH concentration on reaction rate using 100 mg of Ni 0.3 Cu 0.7 /PVDF-HFP catalyst and a temperature of 25 • C. The rate of produced H 2 at the outset increases linearly with the concentration of SBH, as shown in the figure. This demonstrates that the volume of H 2 generation does not rely on the amount of SBH present. Since the start, SBH concentration has no effect on the hydrolysis process, and the slope of 0.06 indicates that the hydrolysis reaction is in zero order. The catalytic performance of the Ni 0.3 Cu 0.7 /PVDF-HFP catalyst may be affected by the reaction temperature, which in turn affects the chemical kinetic characteristic of the H 2 release process. To further understand this, 100 mg of Ni 0.3 Cu 0.7 /PVDF-HFP catalyst and 1 mmol of SBH were used to conduct experiments at four different reaction temperatures (298 K, 303 K, 308 K, and 328 K). Figure 9a shows that when the reaction temperature is raised, the reaction time for H 2 generation decreases. The values of the rate constant, k, can be determined by examining the slope of the linear portion of the plots at various reaction temperatures. Figure 9b displays the ln k vs. 1/T plot that was constructed using the activation parameters for the hydrolysis processes of SBH utilizing Ni 0.3 Cu 0.7 /PVDF-HFP catalyst, which were determined using the well-known Arrhenius and Eyring equations. For the Ni 0.3 Cu 0.7 /PVDF-HFP catalyst system, the activation energy for the hydrolysis process of SBH solution was determined to be Ea = 27.81 kJ/mol. The activation energy of the non-noble metal catalyst Ni 0.3 Cu 0.7 /PVDF-HFP is low when compared to that of Ni-based catalysts and Cu-based catalysts as shown in Table 1. It seems that the fabricated Ni 0.3 Cu 0.7 /PVDF-HFP catalyst achieves excellent catalytic performance in SBH hydrolysis.  To evaluate the catalytic reusability, the same catalyst (100 mg of Ni 0.3 Cu 0.7 /PVDF-HFP) was utilized up to five times in the reduction processes of 1 mmol SBH at 25 • C with a mixing rate of 1000 rpm ( Figure 10). The percentage of conversion remained the same as indicated in Figure 10, but the activity of the catalysts decreased to 93% after four cycles. The remarkable durability should be ascribed to the crystalline structure of the catalyst, which has remained mostly constant, as well as the ultrafine CuNi NPs, which have continued to be well-stabilized at PVDF-HFB throughout the catalytic cycles. It was important to point out that the SBH was introduced to the reactor without separating the catalytic NFs. At the end of each cycle, the hydrolysis process of SBH will have been finished completely. The decrease in catalytic activity, which began after four runs, may be attributed to the following: (1) an increase in the solution viscosity and (2) the passivation of the NFs' surface, which may be attributed to the accumulation of boron products (such as metaborate) on the reaction solution and polymer surface, which inhibited the metal active sites [12].

Conclusions
CuNi nanoparticles supported on PVDF-HFP nanofibers catalysts have been effectively fabricated by the electrospinning technique and in situ reduction in metal ions. The fabricated bimetal catalysts have presented a superior catalytic performance in H 2 generation from SBH as compared to monometallic counterparts. The alloy composed of Ni 0.3 Cu 0.7 /PVDF-HFP NFs has shown good catalytic activity as compared to other formulations. The excellent catalytic activity is in regard to the synergistic effects between Ni and Cu resulting from their geometric effects and uniform distribution of bimetallic NPs. According to the findings of the kinetics investigations, the reaction follows a zero-order and a first-order mechanism with regard to the SBH concentration and the catalyst amount, respectively. In addition, a low activation energy, evaluated as Ea = 27.81 kJ/mol, was obtained for the hydrolysis process that was performed on the SBH solution.
The catalyst has been reused for five cycles. The great catalytic performance and simple separation of the introduced catalytic NFs make them a promising candidate to be a highly efficient catalyst for a hydrogen storage system that enhances commercial applications.