Supercapacitor Based On a Commercially Prepared Hydroxyl Multiwalled Carbon Nanotubes With Hybrid Polymer Electrolyte

This paper reports on the development of three pieces of supercapacitor cells for portable applications such as mobile and wearable energy storage. In a primary embodiment, the three developed supercapacitors cells, each, consist of two flexible electrodes fabricated on thin metal base substrates. The electrode, mainly a commercially prepared multiwalled hydroxyl carbon nanotubes (CPHMWCNTs), sandwich a hybrid solid polymeric separator doped with an appropriate ionic material acting as an electrolyte. The integrated separator and electrolyte layer was made of filter paper, a polyvinyl alcohol (PVA) doped with phosphoric acid at three different concentrations. The Three cells were then assembled and leveled as cell-A (C90PVdF-HFP10 |H50| C90PVdF-HFP10), cell-B (C90PVdF-HFP10 |H60| C90PVdF-HFP10) and cell-C (C90PVdF-HFP10 |H70| C90PVdF-HFP10). The evaluations of these three different electrodes and their substrate materials allowed for selection of a combination of active material and suitable percentage concentrations that yielded optimal supercapacitor performance. From the overall results of the electrochemical analysis of cyclic voltammetry (CV), cell-B delivered higher capacitance of 86.60.10 Fg which was higher than the capacitance obtained by cell-C (65 Fg) and even doubling the capacitance obtained by cell-A (42.1 Fg). Whereas the charge-discharge (CD) tests carried out in the cells reveals that, even at the lower voltage window of 1.5 V, cell-B delivered better than cells A and C with a balanced and better discharge capacitance of 119.0 Fg and higher energy/power densities of 597.0 Jg/12.6 Jgsand very low internal resistance.


Introduction
In every country across the globe, energy has become an issue of national security, especially with the current rise in the global economy (Dubal et al., 2013), the exhaustion of fossil fuels and increasing environmental pollution.Meeting the energy demands in an environmentally sustainable manner is currently the most important technological challenge facing the society.Energy storage devices can efficiently store electricity generated from renewable sources, such as solar, water, wind, thermoelectric, fuel cells, for reuse at many different scales.Therefore, energy storage devices, such as batteries and supercapacitors, have become a key to society (Shi et al., 2013).
Supercapacitor also known as Electrochemical Capacitor (EC) or ultracapacitor (Hashim et al., 2014;Inagaki et al.(a), 2014;Hashim & Khiar, 2011;Burke, 2009) affords new, eco-friendly and low-cost energy storage device for progressive civilization and growing ecological agencies (Dubal et al., 2014), their characteristics comprising a combination of high power density (Wu et al., 2013) and a balanced or reasonable energy density (Dubal et al., 2013) with fast response time and almost infinite life cycleability (Zhao et al., 2011) can complement other storage devices such as batteries, conventional capacitors and fuel cells.Hence, the above properties make them suitable for use in many applications ranging from applications related to portable electronic devices (Perera et al., 2013) UPS, to a hybrid motor vehicles (Inagaki et al.(b), 2014;Qian et al., 2013).
In Electrochemical Double Layer Capacitors (EDLCs), the charge is stored electrostatically at the electrodeelectrolyte interface, so, as a result of this storage mechanism, these devices can be charged and discharged within a blink of seconds (Bockenfeld et al., 2013;Gund et al., 2013;Jiang et al., 2013).Electrodes based on carbon nanotubes (CNTs) deliver an impressive power and energy due to their high surface area, high conductivity and consequently, when functionalized, these electrodes, can deliver up to an optimum level of power and energy densities (Jiang et al., 2013).Hence, this study focused on the fabrication of supercapacitor based on commercially prepared hydroxyl multiwalled carbon nanotubes (CPHMWCNTs).Functionalized supercapacitors are believed to have some key features due to the functionalization processes that occurs in the CNTs chain.The widening of the pore sizes of the CNTs (when functionalized) will result in their improvement for the electrolyte infiltration and ion diffusion throughout the electrode.Other features include the wettability, excellent capacitance, and good charge-storage properties (Van Hooijdonk et al., 2013).Stability for long term storage is one of the merits of the functionalized CNTs (Capek, 2009).

Electrolytic Materials
Three different polymer electrolytes which also served as the separators were prepared as follows; The H 3 PO 4 (>85 wt.% in water, molar mass of 98.00 gmol -1 , product, number of 1502-80) was obtained in aqueous form from R & M marketing, Essex, UK brand, while the PVA (molecular weight; 89,000-98,000, 99 + % hydrolyzed) was obtained from Sigma Aldrich.Both H 3 PO 4 and PVA were used as-received without further treatment or purification.An aqueous solution of PVA was then prepared by combining PVA with distilled water in the ratio of 1:10 by volume.This solution is mechanically agitated by magnetic stirring at 60 °C for five hours to thoroughly dissolve the PVA in the distilled water.H 3 PO 4 was then mixed with the PVA aqueous solution in the ratio of 70:30, 60:40 and 50:50 wt.%.These percentage weights were shown to have promising conductivities even at an ambient temperature.We have reported this development in (Hashim et al., 2012) for a hybrid electrolyte at a combination of 70:30 wt.%.After the mixture cools to a room temperature, the resulting homogenous solution of PVA/H 3 PO 4 was cast over a plastic Petri dish.This was done after a cellulose filter paper (Whatman brand) was cut into a 6 cm x 5.5 cm (sizes that fits into the Petri dish) and soaked into each of the aforementioned solutions.The above mixture took roughly four weeks before it dries.Thereafter, the resulting flexible hybrid solid polymer electrolyte (HSPE) that is peeled off from the Petri dish was used as the separator.

Electrode Materials and Cell Assembly
The Hydroxyl MWCNTs with the following specifications; -OH content of 0.71 wt.%, an outer diameter of > 50 nm, length of 10-20 μm, purity and Ash are both >95 wt.%, and <1.5 wt.%, respectively, Surface area of >40 m 2 g -1 and Conductivity of >10 2 Scm -1 was purchased locally in Malaysia (with material code MH8 11/202).The binder used was P(VdF-HFP) (average molecular weight of ~400,000; Mn of ~130,000 pellets; product number of 427160) was purchased from Sigma Aldrich.P(VdF-HFP) copolymer was found to meet all the requirements for use as a binder or even a polymer host in supercapacitor fabrication due to its electrochemical stability and performance, processability, and safety.Again, this co-polymer contains amorphous domains (HFP) which is capable of trapping large amounts of liquid electrolytes, and crystalline regions (VdF) which provide chemical stability and sufficient mechanical integrity for the processing (Lim et al., 2012).Moreover, P(VdF-HFP) is proven to have strong electron withdrawing, unique arrangement and high dielectric constant (Kim et al., 2013).
Therefore, the double layer capacitor was made with a mixture of 90 wt.% of the CPHMWCNTs and 10 wt.% of P(VdF-HFP), mixed inside a 20 ml of the acetone.The slurry was then cast onto to the Aluminum foil and allowed to dry for about two hours at room temperature.Prior to that, an applicator was used to polish the slurry with the view to leveling it and obtaining a desired thickness which was around 0.127 mm.The dried sample was then further dried in an oven for about 12 hours at a temperature of 100 0 C. Afterward, the solid films were obtained and were further cut into 2 cm 2 x 3 cm 2 each.The weights of the films were measured by means of a micro-balance (Santorius, Ax 224) with an accuracy of 0.001 mg.The average weights of two electrode films that make a cell was around 0.224 g.Using a Perspex of about 5 cm x 4 cm, the cell was set up by sandwiching two electrodes with the electrolyte and assembled in an innovative supercapacitor tester.

Microstructure Characterizations
In order to examine the microstructure of the samples in this work, XRD, and FESEM were used.The spectra of the X-ray in these samples were obtained from an XRD; a Philip X'Pert XR) model with Cu K α radiation of wavelength λ=1.54056 Å for 2θ angles between 10° and 80° that used Cu Kα radiation (λ = 1.5406Å) operating at 40 kV and 30 mA.The XRD analysis of the H90PVdF-HFP10 electrode is depicted in Figure 1.As seen from this figure, all diffraction peaks can be observed and the major diffraction peaks of the CPHMWCNTs can also be seen clearly.The first two peaks that appeared at in 2θ= 26 0 and θ= 43 0 which matched with ( 002) and ( 101) respectively indicate the presence of the embedded graphite CNT, while the last peak on the right side at 2θ= 56.1 0 might also be considered as part of the CNT.Thus, the intensity of the peak observed, could correspond to the high concentration of the as-prepared carbon electrode conductivity (Dong et al., 2007).Furthermore, at the left hand side of the trace, there appeared a diffraction peak at 2θ=20. 1 0 which correspond to the crystalline peaks of the PVDF (Stolarska et al., 2007).Similar XRD results were obtained by (Li et al., 2010).
Figure 1.XRD Analysis of CPHMWCNTs FESEM images for the as-prepared electrode and the HSPE are shown in the Figures 2-7.The morphological structure is necessary for the required electrochemical performance of the cell in order to facilitate the electrolyte penetration and ion diffusion in the electrode.Thus, Figure 2, depicts the FESEM images of sample electrode with 90 % of the CPHMWCNTs.This sample exhibits similar regular, entangled and smooth surface morphology with the outer diameter of ∼28 nm and lengths of several micrometers.Although, few larger tubes are cited in the FESEM structure, this indicates that, the original MWCNT has been functionalized, since the normal sizes does not exceed 50 nm.Detail structures can be seen more clearly in Figure 3, where the FESEM images of CPHMWCNTs were taken at different magnifications of (a); 3.6 mm x 50 k (1 μm) (b); 3.6 mm x 100 k (500 nm) and (c); 3.6 mm x 150 k (300 nm).However, in Figure 4 the FESEM images of HSPE containing 50 wt.% of both PVA and H 3 PO 4 at a magnification of (a); 3.5 mm x 35 LM (1.00 mm) (b); 3.5 mm x 150 LM (300 μm) and (c) 3.5mm x 300 LM (100 μm) were all shown.Displayed in the images is the entangling nature of the surface of the sample which was due to the incorporation of the filter paper.Except in a few spots, all over the surface of the polymer looks unique as the Watman filter paper was coated with composites of both PVA and H 3 PO 4 .
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