Structural morphology and electronic conductivity of blended Nafion®-polyacrylonitrile/zirconium phosphate nanofibres
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This paper aimed to study the influence of zirconium phosphate (ZrP) nanoparticles on reducing the diameter of nanofibres during electrospinning. Addition of metal oxide such as zirconium phosphate decreases the diameter and smooths on the polyacrylonitrile (PAN) nanofibres as observed by the SEM techniques. Furthermore, this work investigated the effect of zirconium phosphate on the morphology and conductivity of modified PAN nanofibres under SEM, XRD and electrochemical cells. The PAN/zirconium phosphate nanofibres were obtained with the diameter ranges between 100 and 200 nm, which mean that the nanofibres morphology significantly changed with the addition of the zirconium phosphate nanoparticles. The conductivity of PAN and PAN-Nafion zirconium phosphate nanofibres was more improved when compared to that of the plain PAN nanofibres as observed under electrochemical measurements. The plain PAN nanofibres show the total degradation on thermal gravimetric analysis results when compared to the modified PAN with zirconium phosphate nanoparticles. The thermal properties and proton conductivity make the PAN/ZrP nanofibres as promising nanofillers for fuel cell electrolytes.
KeywordsZirconium phosphate Nanofibres Polyacrylonitrile Electrospinning Electrochemical Conductivity
Atomic force microscopy
Electrochemical impedance spectroscopy
Fourier transform infrared
Scanning electron microscopy
Thermal gravimetric analysis
Polymer nanofibres are prepared by electrospinning a polymer solution, with a high-voltage electric field applied to a polymer solution ejected from a metal syringe needle. Electrospinning is when the electric forces are utilised within the polymer solution to produce the varied morphology. Moreover, by varying viscosity, surface tension, molecular structure, molecular weight, solution concentration, solvent structure, additive and operational conditions such as rotating speed, spinning head diameter, nozzle diameter and nozzle-collector distance of the same solution may produce a nanofibre web with the various fineness, orientation and surface morphology (Zhang and Lu 2014). Many researchers focus on how to synthesise high surface area nanofibres (less than 1000 nm), which make them useful in fuel cell membranes, tissue engineering, catalysis, sensors, separations, electrochemical cells, drug delivery and chemical filtration (Choi et al. 2008). Some of the researchers also focus on the proton conductivity of electrospun nanofibrous mats (Choi et al. 2008). Nafion® membrane is a perfluorinated state-of-the-art polymer developed by DuPont in the 1970s. Nafion® at low temperature maintained a high proton conductivity and chemical resistance due to its hydrophobic tetrafluoroethylene backbone and sulfonate groups. Electrospinning Nafion® solution increases the proton conductivity of nanofibres with the reduced nanometre scale (Dong et al. 2010). Electrospinning the plain Nafion® solution is impossible due to the low shear viscosity that makes Nafion® aggregates in the solution (Tran and Kalra 2013; Zhou et al. 2010). Nafion® solution was blended with other polymers such as poly(ethylene oxide) (PEO) (Ballengee and Pintauro 2011), poly(acrylic) (PAA) (Mauritz and Moore 2004), or poly(vinylalcohol) (PVA) (Chraska et al. 2000) and polyacrylonitrile (PAN) (Tran and Kalra 2013; Sharma et al. 2014) in order to enhance the mechanical properties and electrospinning. Furthermore, blending Nafion® solution with electron-conducting polymers may enhance their protons’ conductivity and their electrons. PAN is mostly the chosen copolymer for the preparation of fibrous filter media as it can be fabricated easily into nanofibres by electrospinning due to their superior mechanical properties, excellent weatherability and chemical stability (Nie et al. 2013). Moreover, PAN also maintains a good thermal stability at a higher temperature of 130 °C and has a good resistance to many organic solvents. PAN has been studied as a separator material, and PAN-based separators show promising properties, including high ionic conductivity, good thermal stability, high electrolyte uptake and good compatibility, with lithium metal (Gopalan et al. 2008). Electrospinning of composited PAN nanofibres has been found to have multifaceted applications (Sahay et al. 2012), such as electrode materials in supercapacitors and fuel cells (Kim et al. 2004). Zirconium oxide (ZrO2) has several properties that make it a useful material. These properties include high density, hardness, electrical conductivity, wear resistance, high fracture toughness, low thermal conductivity and relatively high dielectric constant. Because of its high refractive index and high oxygen-ion conduction, ZrO2 has been applied as resistive heating elements, oxygen sensors, catalysts and fuel cells (Chraska et al. 2000). The use of a totally stabilised zirconia in fuel-cell technology obtains a good ionic conductivity of cubic zirconia at medium and high temperatures (Jones and Rozière 2001). Zirconium phosphates (ZrPs) are inorganic cation-exchange material with high thermal stability. Zirconium phosphate has the features of increasing conductivity due to high proton mobility on the surface of its particles and good water retention. The reduced methanol permeability of the polymer membrane, while maintaining a high power density, is obtained by impregnating it with zirconium phosphate (Jones and Rozière 2001; Carriere et al. 2003). In this work, we study the effect of ZrP nanofillers on the improvement of the morphology and conductivity of blended Nafion®-PAN nanofibres compared with pure PAN nanofibres, which can be fabricated by electrospinning into nanofibre mats.
Nafion® solution D521 (Ion Power) was purchased from Ion Power. Polyacrylonitrile (PAN), average Mw 150,000 g/mol, was purchased from Sigma-Aldrich. N, N-dimethylformamide (DMF) (99.8%) (Merck), sodium hydroxide (Merck), phosphoric acid (Merck), sulphuric acid (Merck), zirconium oxychloride hydrate (Merck) and potassium chloride (KCl) were obtained and used as received.
Preparation of electrospinning solutions
The XRD analysis was performed using a Philips X-ray automated diffractometer, with a Cu K radiation source. Samples were scanned in continuous mode, from 5° to 90° (2θ). The thermal properties of the samples and their characteristics were studied by thermal gravimetric analysis (TGA) under nitrogen flow. TGA data was obtained with the PerkinElmer instrument, over nitrogen and at a heating rate of 10 °C/min from 50 to 1000 °C. Fourier transform infrared (FTIR) spectroscopy was used to determine the quality and composition of the sample. FTIR spectra were obtained with a (Vertex 70 Bruker) FTIR instrument over a range of 4000–400 cm−1 and a resolution of 4 cm−1. The surface morphology of the nanofibres was analysed by atomic force microscopy (AFM) and scanning microscopy (SEM).
Electrochemical measurements were observed under three electrodes. While a silver-silver chloride (Ag/AgCl) electrode was used as the reference electrode (Anand et al. 2014), zirconium nanoparticles coated on glassy carbon were used as a working electrode, and Pt wire was used as a counter electrode. 0.03 g of PAN, PAN/ZrP and Nafion®-PAN/ZrP nanofibres were ultra-onicated in 0.5 mL of 1 wt% Nafion® in absolute ethanol for 30 min, pipetted a 0.05 mL suspension onto the glassy carbon electrode, and dried at ambient temperature (Dong et al. 2009). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were observed under 2 M of KCl electrolyte. The scan rates used were 10 mV s−1, 20 mV s−1, 30 mV s−1, 50 mV s−1 and 100 mV s−1 with the CV test ranging from − 0.15 to 0.55 V vs. EIS measurements were obtained under the frequency range of 100 kHz to 0.01 Hz.
Results and discussion
Morphologies and structures
The X-ray diffraction (XRD) analysis
Fourier transform infrared spectroscopy (FTIR)
Thermo-gravimetric analysis (TGA) and derivative thermo-gravimetric
The SEM images show that the electrospun PAN-Nafion nanofibres modified by ZrP nanoparticles obtained a reduced diameter and roughness without the formation of beads. In addition, PAN shows a more reduced diameter of 100 nm due to the ZrP nanoparticles being well distributed within the nanofibres. The XRD results also show well-crystallised zirconium phosphates within the modified PAN nanofibres. Moreover, the obtained results show that the thermal degradation properties of PAN-Nafion/ZrP nanofibres improve at a high temperature of 500 °C and with a high conductivity under CV and Nyquist plots. It can be concluded that the addition of ZrP nanoparticles within PAN and PAN/Nafion nanofibres can reduce the diameter while improving the conductivity. When PAN is blended with Nafion solution, it stabilised the high decomposition temperature of Nafion. Furthermore, PAN-Nafion/ZrP nanofibres show a high stability in Nyquist plots due to the incorporation of zirconium phosphate nanoparticles that allow improved electrode surface accessibility. The plots obtained rectangular curves at high electronic conductivity and good charge dissemination. This makes composited nanofibres with ZrP nanoparticles suitable to be used as the electrolyte of a promising fuel cell application.
The authors are thankful to thank UNISA for the SEM results. We also acknowledged National Research Funding (NRF) and University of South Africa (Academic Qualification Improvement Programme (AQIP)) for their financial support.
National Research Funding (NRF) (Grant UID:95333) and University of South Africa (Academic Qualification Improvement Programme (AQIP)).
Availability of data and materials
All data analysed during this study are available from the corresponding author on request.
The authors read and correct the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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