3.1. Physiochemical Characterization
Electrospinning is a powerful technique in the fabrication of inorganic NFs either from solutions or melts. The inorganic NFs fabrication process used a solution that consists of metal acetates or metal alkoxides as precursors. Polymers have been shown to have more simple polycondensation characteristics with metal acetates than those of alkoxides [
2]. Furthermore, the electrospun solution that consisted of metal acetate showed an excellent morphology in the final product. The polycondensation process can be explained, as shown in
Scheme 1.
The decomposition process of the precursor salts consist of metal acetates at high temperature under vacuum, and in the presence of an inert gas led to the generation of reducing gases (e.g., Co and H
2), which result from the decomposition of acetate anions. The formed gases reduced the salt and metal oxides to form pure Ni and Cr and/or their alloys rather than their oxides [
2]. The formation of Ni and Cr can be defined by the following reactions [
44,
45]:
Various metal acetates have been applied to produce inorganic NFs with good morphology. Herein, the NiAc, CrAc, and PVA appear to have a good electrospun NF mats morphology, as shown in the SEM image (
Figure 1a,b) after vacuum drying. The nanofibrous morphology of the sintered electrospun NF mats was not affected by the high calcination temperature. This may be attributed to the polycondensation of the NiAc and CrAc with PVA [
3,
20,
44,
46].
Figure 1c,d shows the sintered NFs obtained after the abnormal calcination of electrospun NF mats composed of NiAc/CrAc/PVA. It was observed in the figure that the NFs are cross-linked, and a net-like structure with some NPs is distributed on the rough surface. The formed fiber was decorated within the well-dispersed NPs.
3.2. Crystal Structure
The analytical analysis of the chemical composition of the produced NFs was achieved using an XRD technique (
Figure 2). Nickel and chromium are in the same row and neighbors in the periodic table. They can form a solid solution until 30 wt% at room temperature, while above this value, an eutectic solution is formed. Accordingly, the identification peaks in the XRD database are overlapped. Moreover, the formed NiCr alloys have dissimilar crystal structures compared to the pure metals. For instance, nickel can be identified in the XRD pattern if peaks are detected at 2è values of 44.5°, 51.8°, and 76.4° corresponding to (111), (200), and (220) crystal planes, respectively (JCDPS #04-0850). However, chromium can be identified in the XRD pattern if peaks are detected at 2è values of 44.4°, 64.6°, and 81.72° matching to (110), (200), and (211) crystal planes, respectively (JCDPS #06-0694). Interestingly, the obtained 2è values at 43.9°, 51.2°, and 75.7° match well with (111), (200), and (220) of Cr
2Ni
3 (JCPDS # 65-629; Cr
2Ni
3) [
47]. The same results were obtained for all formulations. According to the higher melting point of the two metals, 1455 °C and 1907 °C for Ni and Cr, respectively, than the used calcination temperature, vaporization of the produced bimetals is not predictable. Moreover, the nickel precursor represented a high amount in the initial electrospun solution compared to chromium; thus, the detected peaks in the XRD pattern (
Figure 2) can be consigned to free Ni and a Cr
2Ni
3 alloy. It is worth noting that pure nickel and the bimetallic alloy can combine in the same nanoparticle, which was verified by the TEM EDX study below. Furthermore, a peak can be observed at 2θ ~26°, representing an experimental d spacing of 3.37, which confirmed the presence of graphite-like carbon (d (002), PDF#; 41-1487). According to our previous studies, Ni NPs and its alloys have vital roles in the catalytic transformation of polymer NFs into graphite-like carbon during the calcination process [
2,
46,
48,
49]. These results suggested a successful formation of Cr
2Ni
3 alloy
[email protected] 3.3. Internal Structure
A typical TEM image of nanofibers (NFs) is presented in
Figure 3a. It is clear that numerous dark spots from NPs are consistently anchored and distributed on the rough surface of NFs. HRTEM images are used to determine the crystalline of the produced NFs (
Figure 3b,c). The clear appearance of the distance between the parallel crystal planes in
Figure 3b,c affirmed that the NPs are crystalline, while the carbon is amorphous. The distance between two parallel crystals planes is mostly the same in the NPs in
Figure 3b; this confirmed the presence of one phase only, as mentioned in the XRD analysis. Accordingly, the crystalline NPs and NFs matrix can be assigned to the detected Cr
2Ni
3 NPs and graphite, respectively.
To confirm the aforementioned result, the TEM-EDX was performed to check the elements and chemical composition of the obtained NFs. The STEM results (
Figure 4a) show that the NPs are encapsulated inside the NFs matrix. TEM-EDX analysis corresponding to the line shown in
Figure 4a, as shown in
Figure 4b–d, indicates that the Ni and Cr nanoparticles (NPs) are uniformly distributed along the chosen line in the specified area, yielding the same distribution of Ni and Cr, which indicates the alloy formation. This verifies the XRD result regarding the formation of the Cr
2Ni
3 phase. Accordingly, the formed NPs and NFs were assigned to the Cr
2Ni
3 alloy and CNFs, respectively. Moreover, Cr
2Ni
3 NPs were packed well into a thin crystalline CNF layer. The layer was present at all points along the chosen line. Carbon could enhance the catalytic activity, material conductivity, and stability. Moreover, CNFs have high adsorptive capacity, which increases the attachment of the target with the catalytic material and easily creates an outlet product due to the porosity.
3.4. Hydrogen Release from Ammonia Borane Complex
The synthesized Ni
xCr
1-x NPs–CNFs were tested as catalysts for the AB dehydrogenation.
Figure 5 shows plots of the time versus equivalent H
2 (n
H2 mol/n
AB mol) for the AB dehydrogenation in the presence of all the formulations and Ni–CNFs at the same molar ratio from the active catalyst and same concentration of AB at room temperature. As shown in the figure, the introduced NFs showed a higher catalytic performance for H
2 generation compared to the pure metallic NPs (Ni–CNFs). This may be ascribed to the synergistic effect of the introduced alloy NPs. The pure metallic NPs took a relatively longer experimental time of 27 min to release 2 mol of H
2/1 mol of AB. The hydrogen release rate increased and the duration time decreased as the amount of Cr increased, as the most of the reaction’s stoichiometric H
2 was released. The sample composed of 15 wt% Cr took a shorter time to release stoichiometric H
2 compared to the other formulations. In addition, it was observed in the figure that the H
2 generation is directly proportional to the AB dehydrogenation duration time. This good performance can be attributed to the CNFs. The CNFs have a high absorptive capacity, a high surface area, and interact with a good dispersion of active catalysts, which creates a large number of active sites. It is well known that the electrospun CNFs have the well nanoporous structure, which facilitates the contact between the reactants and the active metals as well as H
2 evolution. Accordingly, nanofibrous morphology introduced a high surface area and active sites for AB dehydrogenation. The performance can also be associated with the addition of Cr to Ni, which increases the surface area and works as an atomic barrier between Ni atoms for preventing agglomeration. Furthermore, the interaction between Ni and Cr species modified the electronic structure of the alloy [
39].
Accordingly, the migration electrons from Cr to Ni enhance the active sites with the electron-enriched Ni NPs. These electrons enhance the detachment of H2 atoms from the catalyst surface. However, the increase of the Cr concentration and decrease of the H2 release could be due to the screening of the Ni active sites, which reduces the electron transfer. Furthermore, the decrease in the H2 generation at a Cr concentration lower than that of 15 wt% CrAc may be due to the surface oxidation.
Figure 6 demonstrates the graphs of (n H
2/1 mole AB) versus the duration time of aqueous AB hydrolysis (31.5 mg and room temperature) in the presence of different concentrations of the introduced NFs (15 wt% sample). As shown in the figure, with an increase in the catalyst and the AB ratio, the time required for H
2 generation decreases. From the linear portion of the different concentrations, a graph between the log H
2 generation rates versus log catalyst concentration was plotted. From this graph, a straight line within a slope of 0.997 was obtained, indicating that the AB hydrolysis followed a pseudo-first-order reaction with respect to the concentration of the introduced NFs (
Figure 6b).
The effect of AB concentration (100–500 mM) on the rate of H
2 generation was also investigated. As shown in
Figure 7, most of the stoichiometric hydrogen was evolved for each concentration. The logarithmic rate constant obtained from the linear portion of concentration (
Figure 7b) was plotted versus logarithmic AB concentrations, and the slope was to be found to be −0.0979, demonstrating that the hydrolysis reaction followed the zero-order reaction with respect to the AB concentration.
To determine the reaction activation energy (E
a), activation entropy (ΔS), and activation enthalpy (ΔH), the hydrolytic reaction was carried out at different temperatures ranging from 298 K to 323 K. As shown in
Figure 8, as expected, the duration time for H
2 generation was reduced from 12.5 min to 3.5 min with an increasing chemical reaction temperature from 298 K to 323 K, respectively. The constant rate (k) value was determined from the linear portion from
Figure 8a. The E
a, ΔS, and ΔH of the reaction were determined from the Arrhenius and Eyring plots (
Figure 8a,b), and were found to be 37.6, 0.094, and 35.03 kJ/mol, respectively. Here, the obtained E
a value is comparable to the Ni-based catalyst values reported in the literature (
Table 1), indicating the superior catalytic performance of the introduced NFs (15 wt%, sample). In addition, the TOF was determined to evaluate the catalytic activity, and was found to be 5.78 mol H
2 min
−1 (mol metal
−1). This value is also comparable to values reported in the literature for Ni-based catalysts (
Table 1), indicating the high activities of the introduced NFs (15 wt%, sample).
The reusability test is very significant for the practical applications of catalysts. In the present work, the reusability test of a 15 wt% CrAc sample was conducted for the hydrolytic reactions of AB. After the complete hydrolysis of AB (31.5 mg, 3n
H2/1n
AB), the catalyst was decanted from the solution via centrifugation, washed with deionized (DI) water, and then, the reactivate catalyst was again dispersed in the same solution with the addition of the same amount of AB for the next cycle. As shown in
Figure 9, the catalytic activities of the catalyst indicate stability for two runs. However, a gradual decline was observed from the third cycle, reaching about 65% at the fifth cycle, demonstrating that the catalyst had a good reusability in the AB dehydrogenation at room temperature. The decline of catalytic activity may be attributed to the following: (1) the partial clustering of the metal NPs, (2) the increase of the solution viscosity, and (3) the passivation of the NFs’ surface, which was attributed to the accumulate boron products (e.g., metaborate) on the reaction solution and catalyst surface that inhibited the metal active sites [
39,
49].