The simplicity of the electrospinning process, the diversity of the electrospinnable materials, and the unique features of the obtained electrospun nanofibers provide especial interest for both of the technique and the resultant products. Various polymers have been successfully electrospun into ultrafine fibers in recent years mostly in solvent solution and some in melt form. Moreover, functional inorganic nanofibers can be produced by using sol–gel composed of metal(s) precursor(s) and proper polymer(s). In the field of metallic nanofibers, electrospinning process has a good contribution as it has been invoked to produce several pristine metallic nanofibers [18
]. Beside the metal alkoxides, metal acetates have been widely utilized as metal precursors, as these promising salts have a good tendency for polycondensation to form electrospinable sol-gels with the proper polymers [22
]. The polycondensation reaction can be explained as follows [22
where M is Ni. Accordingly, the prepared NiAc/PVP solution produced good morphology, smooth and beads-free electrospun nanofibers, as shown in Figure A. Due to the polycondensation characteristic, the calcination of the prepared electrospun nanofibers did not affect the nanofibrous morphology as shown in Figure B. Figure C represents the SEM image for the synthesized NiO NPs. From Figures B and C, it can be concluded that the average diameters of the synthesized NFs and NPs are approximately 70 nm.
Figure 1 SEM images of electrospun PVP/NiAc electrospun nanofibers (A), synthesized NiO nanofibers (B), and NPs (C). SEM images of the electrospun PVP/NiAc nanofiber mats (A) and after calcination at 700°C (B). SEM image of the synthesized NiO NPs (C) (more ...)
It was expected that the calcination of the prepared NiAc/PVA nanostructures in air will lead to eliminate the polymer and decompose the metallic precursor to the oxide form; this hypothesis was affirmed by using the XRD analysis. As shown in Figure , the XRD spectra of the synthesized NiO NPs and NFs are similar and match the standard spectra of NiO (JCPDS number 44–1159). From the obtained XRD spectra, the grain size could be estimated using Scherrer equation [23
]. The determined sizes were 36 and 37 nm for the NPs and NFs, respectively.
XRD analyses for the prepared NiO nanofibers and nanoparticles.
Due to its surface oxidation properties, nickel reveals good performance as electrocatalyst. Many materials involving nickel as a component in their manufacture could be used as catalysts in fuel cells. Nickel is commonly used as an electrocatalyst for both anodic and cathodic reactions in organic synthesis, water electrolysis, and electrooxidation of alcohols [24
]. Surface activation of the nickel-based materials is an important step to create NiOOH compound on the surface and initiate the electrochemical activity. For instance, NiOOH compound has to be originated on the surface to initiate the electrochemical activity. Similarly, the investigated NiO nanostructures in this study were activated by applying cyclic voltages for 50 times in 1 M KOH electrolytes (the utilized scan rate was 100 mV/s). The cyclic voltammetric behaviors of NiO NPs and NFs are shown in Figure . In the voltammograms of the nickel oxide nanoparticles and nanofibers, the cathodic and anodic peaks corresponding to Ni(II)/Ni(III) couple are observed at about 0.35 and 0.42 V (vs. Ag/AgCl), respectively. As the chemical composition and the grain size are similar in both nanostructures, the same behavior was obtained as shown in the figure. Typically, these peaks refer to the formation of NiOOH in accordance with these reactions [27
Consecutive cyclic voltammogram of the synthesized NiO NPs and NFs in 1 M KOH at scan rate of 50 mVs−1.
Increasing the number of potential sweeps results in a progressive increase of the current density values of the cathodic peak because of the entry of OH−
into the surface layer, which leads to the progressive formation of a thicker NiOOH layer corresponding to the NiO/NiOOH transition [24
]. It is noteworthy mentioning that the formed NiOOH layer is responsible for the electrocatalytic activity of nickel-based electrocatalysts [17
The linear scan voltammograms for the methanol oxidation on the NiO NPs and NFs surfaces in different methanol concentrations are shown in Figure . The methanol-containing electrolyte was previously purged with argon. The onset potential is an important indicator among the invoked parameters to demonstrate the electrocatalytic activity. The onset potential indicates the electrode overpotential. In other words, the onset potential can be utilized to evaluate the efficacy of the electrocatalyst. In methanol electrooxidation, more negative onset potential indicates high activity and less overpotential. Generally, the main reason behind increasing the onset potential is the OH−
and CO adsorbed layer on the surface of the electrodes, this gas layer leads to overpotential [30
]. Sometimes, carbon monoxide is an intermediate compound in the methanol electrooxidation; it accumulates on the surface of the electrode until further oxidation step to carbon dioxide occurs. Usually, adsorption of CO appears to take place with the formation of islands of adsorbate [31
], and electroactivity appears to be restricted to the outsides of these islands. Accordingly, good catalytic activity is related with the rate of CO removal and/or skipping formation of CO intermediate. From the obtained results, the onset potentials are 0.37 and 0.39 V (vs. Ag/AgCl) for the NiO NPs and NFs, respectively; these values are good compared with many reported materials.
Linear scan voltammograms for the methanol oxidation on the NiO NPs and NFs surfaces. Cyclic voltammograms at a scan rate of 50 mVs−1 and 25°C for NiO NPs and NFs in 1 M KOH and different methanol concentrations.
Two important findings can be observed in Figure : First, the nanofibrous morphology strongly enhances the electrocatalytic activity as the maximum current density significantly increased from 6 (in case of NPs) to 25 mA/cm2 (in case of NFs). Second, the optimum methanol concentration increased from 0.1 M in case of nanoparticulate morphology to 1 M in case of the nanofibers. Actually, concentrated methanol solution is a target fuel in the DMFCs to reduce the volume. However, increasing of methanol concentration can have a negative influence on the current density, so each electrocatalyst corresponds to a certain methanol concentration. The obtained good performance of the nanofibrous morphology can be assigned to the influence of the one-dimensional feature which facilitates the electron transfer through the electrocatalyst. It is expected that the electron paths through the nanoparticles will be corrugated; however, as the nanofibers have very high axial ratio, almost straight paths are expected. Moreover, within the nanoparticles, the electrons pass through several contact points as they have to move through many nanoparticles; this adds more constraints for the electrons transfer which distinctly affects the catalyst performance. Figure shows a conceptual illustration for the electrons paths through the nanofibrous and nanoparticulate electrocatalysts.
Schematic diagram showing the electron paths in case of NiO nanofibers and nanoparticles.