Influence of the nanofibrous morphology on the catalytic activity of NiO nanostructures: an effective impact toward methanol electrooxidation
© Barakat et al.; licensee Springer. 2013
Received: 7 September 2013
Accepted: 23 September 2013
Published: 28 September 2013
In this study, the influence of the morphology on the electrocatalytic activity of nickel oxide nanostructures toward methanol oxidation is investigated. Two nanostructures were utilized: nanoparticles and nanofibers. NiO nanofibers have been synthesized by using the electrospinning technique. Briefly, electrospun nanofiber mats composed of polyvinylpyrolidine and nickel acetate were calcined at 700°C for 1 h. Interestingly, compared to nanoparticles, the nanofibrous morphology strongly enhanced the electrocatalytic performance. The corresponding current densities for the NiO nanofibers and nanoparticles were 25 and 6 mA/cm2, respectively. Moreover, the optimum methanol concentration increased to 1 M in case of the nanofibrous morphology while it was 0.1 M for the NiO nanoparticles. Actually, the one-dimensional feature of the nanofibrous morphology facilitates electrons' motion which enhances the electrocatalytic activity. Overall, this study emphasizes the distinct positive impact of the nanofibrous morphology on the electrocatalytic activity which will open a new avenue for modification of the electrocatalysts.
KeywordsMethanol electrooxidation NiO nanofibers Electrospinning Direct methanol fuel cells
In the last decades, nanostructural materials have been intensively investigated because of their high surface area which strongly affects their physicochemical characteristics. Of the reported nanostructures shapes, special attention has been paid to the one-dimensional forms such as nanorods, nanowires, and nanofibers. This is due to their potential applications in nanodevices [1–3]. Nanofibers (NFs) received special consideration due to their high axial ratio, good mechanical properties, and easy manageability. Compared to nanoparticles (NPs), nanofibers have small surface area which might have a negative impact upon using them as catalyst in the chemical reactions. However, it was reported that the axial ratio distinctly enhances the catalytic performance, especially in case of electrons' transfer-based processes. For instance, in the photocatalysis, the nanofibrous morphology strongly modifies the performance [3–5].
Direct methanol fuel cells (DMFCs) received much attention during the last decade because methanol is an inexpensive, readily available, and easily stored and transported liquid fuel . DMFCs do not have the fuel storage problem because methanol has a higher energy density than hydrogen - though less than gasoline or diesel fuel. Methanol is also easier to supply to the public using our current infrastructure. In the DMFCs, methanol is directly oxidized to carbon dioxide and water, providing a new way to store and convey the energy [7–9]. The electrocatalysts are the backbone not only of the DMFCs but also of any kind of fuel cells. The successful commercialization is quite dependent on the cost, activity, and durability of the electrocatalysts [9, 10]. At present, almost all pre-commercial low-temperature fuel cells use Pt-based electrocatalysts [11–14]. Accordingly, the manufacturing cost is relatively high which constrains wide applications. Moreover, the catalyst poisoning by CO or CHO species is another real problem facing most of the Pt-based electrocatalysts [9, 15, 16].
To develop new non-precious electrocatalysts, most of the researchers focus only on modifying the composition and ignore the morphology impact. Therefore, many transition metal NPs were introduced as alternative non-precious electrocatalysts to replace the Pt-based materials. However, those NPs suffer from low chemical stability which keeps non-stop research activities to improve the performance as well as the stability.
Compared to metals, it is known that metal oxides have higher chemical stabilities in various media. Accordingly, metal oxides are good candidates as electrocatalysts if the performance could be improved. Recently, NiO nanoparticles deposited on carbon nanotubes showed good behavior toward methanol electrooxidation . In this study, the electrocatalytic activity of NiO toward methanol oxidation could be improved by modification of its nanomorphology. Interestingly, compared to NiO NPs, NiO NFs which were synthesized by the electrospinning process revealed higher performance.
To prepare NiO NFs, a sol–gel composed of nickel acetate tetra-hydrate (NiAc, 1 g, 98% assay Junsei Chemical Co., Ltd, Japan), polyvinylpyrolidine (PVP 1 g, molecular weight = 1,300,000 g/mol, Sigma-Aldrich Corporation, St. Louis, MO, USA) and ethanol (10 g) was electrospun at 10 kV and feeding rate of 0.05 ml/min. The electrospun mat was first vacuously dried and then sintered in air at 700°C. The utilized NiO NPs were synthesized from the same mixture; however, instead of spinning, the solution was dried, grinded and sintered at the same conditions. The electrochemical measurements were performed in a 1 M KOH solution at room temperature. Preparation of the working electrode was carried out by mixing 2 mg of the functional material, 20 μL Nafion solution (5 wt.%) and 400 μL isopropanol. The slurry was sonicated for 30 min at room temperature. Fifteen microliters from the prepared slurry was poured on the active area of the glassy carbon electrode which was then subjected to drying process at 80°C for 20 min. The measurements were performed on VersaSTAT 4 (Oak Ridge, TN, USA) electrochemical analyzer and a conventional three-electrode electrochemical cell. A Pt wire and an Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively. Surface morphology was studied by scanning electron microscope (SEM; JEOL JSM-5900, JEOL Ltd., Tokyo, Japan) and field-emission scanning electron microscope equipped with EDX analysis tool (FESEM; Hitachi S-7400, Hitachi Ltd., Chiyoda, Tokyo, Japan). Information about the phase and crystallinity was obtained by using Rigaku X-ray diffractometer (XRD, Rigaku Corporation, Tokyo, Japan) with Cu Kα (λ = 1.540 Å) radiation over Bragg angle ranging from 10° to 90°.
Results and discussion
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 [18–21]. 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 . The polycondensation reaction can be explained as follows :
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 . It is noteworthy mentioning that the formed NiOOH layer is responsible for the electrocatalytic activity of nickel-based electrocatalysts [17, 24].
Electrospinning technique can be utilized to fabricate NiO nanofibers from PVP and nickel acetates sol–gel. The morphology has a distinct influence on the electrocatalytic activity of the nickel oxide nanostructures toward methanol oxidation. Compared to the nanoparticles, the nanofibrous morphology facilitates the electrons' motion which positively affects the performance. It is expected that the good impact of the nanofibrous morphology is a common feature, so it can be utilized with other electrocatalytic materials.
This research was supported by NPST program by King Saud University project number 11-ENE1721-02. Also, this work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012R1A2A2A01046086).
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