Polymeric and metal oxide structured nanofibrous composites fabricated by electrospinning as highly efficient hydrogen evolution catalyst

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Abstract

Polyacrylonitrile polymeric (PAN/CoAc) and metal oxide (Co3O4/Nfs) nanofibrous structured composite catalysts for the hydrogen production are successfully fabricated by simple, low cost, high yield, and effective technique: electrospinning. Two selected process parameters, as applied voltage (15–25 kV) and distance (5.0–7.5 cm), are adjusted at two different values and interaction plots on the fiber diameters are analyzed statistically. Morphology, crystalline, chemical and thermal properties of composites are characterized by several techniques (SEM, TEM, BET, XRD, FT-IR, TG/DTG, DSC, DMA) and activity of composites as catalysts are tested for ammonia borane (NH3BH3) hydrolysis for hydrogen evolution as a green fuel for future energy applications. It is concluded that nanofibrous metal oxide structured Co3O4/Nfs-1 composite provides the best catalytic activity in terms of hydrogen production rate with 2.54 l H2 min−1 gcat−1 and good repeatability.

Introduction

Electrospinning offers unique capabilities for fabrication of novel nanofibers with diameters ranging from nanometers to micrometers, controllable surface morphology and, a superior mechanical performance by applying electrical forces to natural or synthetic polymers solutions [1], [2]. Electrospinning process is based on fundamental principle calls “electrostatic interaction”, under high voltage, a polymer solution is controlled via syringe pump, and needle behaves like an electrode to charge the solution and spun it to conductive metal collector surface [3]. Many parameters in terms of solution properties (surface tension, viscosity, conductivity, etc.), electrospinning set up parameters (intensity of electric field, distance between nozzle and collector surface, polymer flow rate, etc.) and ambient conditions (temperature, humidity, etc.) affects morphology, tunable diameter and chemical composition of obtaining nanofibers [4].

Compared with another known form of the materials; nanofibers- ultra-fine, flexible, high strength/toughness, good thermal and electrical conductivity, bio/compliant properties- are candidates for many critical applications such as energy engineering (fuel cell, catalyst, etc.), tissue engineering-scaffolding (membrane for skin, blood vessel, nerve generations, etc.), life sciences applications (drug delivery, haemostatic devices, etc.), military protective clothing, nano-sensors (piezoelectric sensor, thermal sensor, etc.) [2].

The global environmental pollution and energy demand problems force to research and commercial fields to effort developing the many new types of clean energy materials like catalyst, storage mediums, electrode membranes, separators, collectors, super-capacitors, etc. and storage devices such as solar and fuel cells, lithium and sodium ion batteries, etc. [5]. To provide pollution- and carbon-free power for buildings, transport, and industries, hydrogen as an energy carrier plays a critical role in future energy options. In order to knock-down hydrogen price for practical applications, storage and releasing problems must be solved for the utilization of safe and efficient methods [6].

Last decades, ammonia borane (NH3BH3) identified as hydrogen (H2) reservoir with high energy carrier capacity, especially for jets and rockets applications. For power consumption, hydrolysis is the best methodology to release hydrogen, which chemically deposited in NH3BH3 in the presence of noble and non-noble metals/metal oxides, acids, bases, nano-alloys, and nanocomposites as catalysts as shown in Eq. (1) [7], [8], [9]:NH3BH3(s)+2H2O(l)catalystNH4+(aq)+BO2-(aq)+3H2(g)

In particular, to increase the interaction of NH3BH3 with catalysts’s active sites –, a different type of organic and inorganic support materials such as Al2O3, SiO2, carbon, polymers, clays, etc. have been evaluated - [7], [8], [9]. Among of them, polymers (PAN, PVA, PMMA, PANi, etc.) have been gain a special interest in the field fabrication of nanofibers, nanorods, and mats types prepared by low cost and efficient electrospinning technique [2], [10], [11], [12], [13], [14]. The catalysts in the nanofiber structure, especially fabricated by electrospinning, have highly active sites to provide active morphological properties that provide active interaction with the reactant molecules due to their small and interrelated pores. The position of the catalytic active sites in the polymer skeleton and the polymer coating affects the catalyst reactivity [15], [3], [16]. Based on the increase in performance of the catalyst, investigation on regarding different polymer has been carried out in order to determine which is more effective as a matrix, and photocatalytic activity was tested against the toxicant simulated 2-chloroethylphynilesulfate under ultraviolet light irradiation [17]. Yu and Liu (2007) reported that when Pd/PAN-AA nanofibers were evaluated for their hexane productivity, they exhibited 4.7 times higher activity than Pd/γ-Al2O3, as well as their – reusability properties [18]. Electrospun NiCu nanorods@carbon nanofibers for highly efficient dehydrogenation of NH3BH3 had been synthesized based on electrospinning of polyvinyl alcohol (PVA) solutions, and introduced fibers showed superficial activity [19]. Pd doped Co nanofibers were prepared by using PVA and tested for hydrolysis of NH3BH3 under sun and daylight. It was concluded that nanofibers could also be explored as a photocatalyst for waste water treatment [20]. Electrospun CdS–TiO2 doped carbon nanofibers were introduced for visible-light-induced photocatalytic hydrolysis of NH3BH3 [21]. Electrospun polyacrylonitrile (PAN) nanofibers supported alloyed Pd–Pt nanoparticles as recyclable catalysts tested for hydrogen generation from the hydrolysis of NH3BH3 and catalysts showed high potential to find applications for the development of hydrogen generation for clean energy [22]. Li et al. (2014) prepared composite nanofibers using cobalt (II) chloride and PAN and found that they exhibited catalytic activity in hydrogen production from sodium borohydride (NaBH4) solutions [23]. Demir et al. (2004) found that Pd/PAN-AA nanofibers were 4.5 times more active in dehydrolinalol hydrogenation than Pd/Al2O3 catalysts [24]. Coşkuner Filiz and Kantürk Figen (2016) prepared Co, Ni, Cu metal oxide nanofibers from PVA (5 wt%)/metal acetate composites and they demonstrated that the Co-NF catalyst exhibits higher catalytic efficiency for hydrogen release from NH3BH3 than the Ni- and Cu-based metal oxide-NF catalysts [25].

Parametric investigation of electrospinning parameters is beneficial for improving the electrospinability of polymer solution and quality of the fibers by eliminating of beads and formation of uniform diameter. Based on the above consideration, herein we reported the fabrication and physicochemical characterization of polyacrylonitrile polymeric (PAN/CoAc) and metal oxide (Co3O4/Nfs) nanofibrous structured composite catalysts prepared by electrospinning technique. To best of our knowledge, statistically analyzed spinning parameters of Co-containing PAN solution to obtain smooth, beadles, homogeneous, and porous nanofibrous structures and especially their application in hydrogen production has not been reported before. The obtained fibrous - in the form of nanofibers were catalytically active as a catalyst used for NH3BH3 dehydrogenation with good repeatability during the fast reaction of hydrogen release.

Section snippets

Materials

Polyacrylonitrile (PAN; (C3H3N)n, % 99.995, Sigma-Aldrich, MW = 150000 g/mol), cobalt(II)acetate tetrahydrate (CoAc; (CH3CO2)2Co·4H2O, % 99.8, Sigma-Aldrich), dimethylformamide (DMF; HCON(CH3)2, Merck), ammonia borane (AB; NH3BH3, Sigma-Aldrich, >97%), were purchased and used without any further purification.

Preparation and characterization of nanofibrous composites

CoAc (5 wt%) and PAN (10 wt%) solutions firstly prepared by dissolving in DMF and stirred at 60 °C for 2 h to form a homogeneous solution. The prepared solution was placed in a plastic

Physicochemical properties of polymeric nanofibrous composites

Processing variables were tested to determine their effect on fiber diameter determined from Fig. 1a-d, for the purpose of this system involving two significant independent variables as applied voltage, and nozzle-target in two alternative levels as 15–25 kV and 5.0–7.5 cm. Analysis of variance (ANOVA) test was carried out to conclude the significant levels of the factors and p-values was set to <0.05. Analysis results from two Factor ANOVA with replication which shows its degree of freedom

Conclusion

We purposed a facile technique as electrospinning for fabrication of Co based PAN polymeric and oxide nanofibrous composites, provided high catalytic activity and the repeatability in the hydrolysis of NH3BH3 for hydrogen evolution. In present study, electrospinning process parameters were statistically optimized for the preparation of beadles, smooth and mesoporous surfaced with nano-scale fibers in order to integrate as a catalyst, which had not been addressed in literature about spinning of

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