Conductivity Studies and Characterizations of PVA-Orthophosphoric Electrolytes

Solid polymer electrolyte (SPEs) has drawn great attention nowadays, particularly in the field of science and applied science, specifically, those used as separators in industries and researchers for the assemblage of some “classified batteries” and super capacitors. In this report, we present the grandeur of this SPE (pure type), particularly at room temperature, such that, it can be applied as an electrolyte as well as a separator in supercapacitor fabrication. The SPEs were produced from the composition of polyvinyl alcohol (PVA) and Phosphoric acid (H3PO4), such that, the PVA samples were kept constant while the phosphoric acid was varied at 0, 10, 20, 30, 40, 50, 60, and 70 wt. %. After the experimentation, the pure polymers show excellent results in term conductivity, in that, it has recorded a conductivity as high as 2.56 x 10 Scm at the compositions of 70 wt. %. Aside that, we also observed that, the bulk moduli (Rb) value decreases with the increase in concentration, with pure solid polymer electrolyte (PSPE) recording 2.5 Ω at the highest composition of 70 wt. %.

Research has shown that, there are three classifications of polymer electrolyte systems, namely (i) Polyelectrolytes (PEs): -which have an independent ion-generating groups chemically bond to the macromolecular chain and presence of counter-ion in order to maintain the electroneutrality of the salt.In dry conditions, they have low conductivity of about 10 -10 -10 -15 Scm -1 but in the presence of dielectric solvent such as water, there conductivities do improve (Ulaganathan et al., 2012).(ii) Solvent Swollen Polymer Electrolytes (SSPE): -In SSPE, solvents, both aqueous and non-aqueous swells the host polymer.Thus, the dopant ionic solutes such as H 3 PO 4 is hosted in the swollen lattice thereby permitting the movement of ions in the solvent swollen region of the polymer host.Their conductivity depends on the compactness of the solvent in the region that is being puffed up (Ulaganathan et al., 2012).And lastly (iii) the Solvent Free Polymer Electrolytes (SFPE): These are polymer salt complex, which is formed by complexes between salts of alkali metals and polymer containing solvating hetroatoms, example; O, S and N (Ulaganathan et al., 2012).The complexation between poly ethylene oxide (PEO) and alkali metal salts is a good example of SFPE.SFPE are sub-classed into three also.They are; (a) Solid Polymer Electrolytes (SPEs), (b) Gel Polymer Electrolytes (GPEs), and (c) Composite polymer electrolytes (CPEs) (Kuo et al., 2013;Tripath et al., 2012).
GPEs can be only considered as plasticizers incorporated polymer-salt complex (Ulaganathan et al., 2012).They are therefore consisted of a liquid electrolytes immobilized in a polymer matrix (Hashmi, (a), 2013) and exhibits high ionic conductivity (10 −3 to 10 −4 Scm −1 at ambient temperature) and can be obtained by either loading of a liquid electrolyte in a microporous matrix or by increasing the viscosity of a liquid electrolyte through the addition of a soluble polymer until gel consistency is achieved or by mixing a liquid electrolyte with monomers of relatively low molecular weight, then curing the mixture by other means, such as UV polymerization, thermal polymerization and electron beam radiation polymerization.They are criticized for having weak mechanical strength and poor interracial properties, (Qiu et al., 2004;Hashmi, (b), 2013).In CPEs, they are normally prepared by addition of high surface inorganic filters like SiO 2 , MgO, TiO 2 and alike so as to improve the mechanical strength and stiffness of the complex systems (Ulaganathan et al., 2012).SPE is a thin film that has ionic conductivity, especially, when an alkali, salt dissolves in polymer matrix.SPE have drawn great attention in the development of science and technology of lithium secondary batteries.SPEs have several advantages over liquid, gel and even composite electrolytes in that, it has process ability, flexibility, light weight, elasticity and transparency (Zhang et al., 2006), desirable shape moldability, being free from leakage, mechanical strength, better stability especially in high temperature, and high specific energy and power (Lim et al., 2012).Furthermore, use of use of solid polymer electrolyte can negate the need for a separator and be amendable to low cast manufacturing technologies (Qiu et al., 2004).Some of the applications of SPE are in Batteries, fuel cell, supercapacitors and other electrochemical devices.(Sawada et al., 2000) and other electrochemical devices (Sotta et al., 2010).
Although number of works have been carried out on the characterizations of SSPE like those of Prajapati et al., (2010) and Ahmad Khiar & Arof, (2010), this piece tries to offer a tone by step approach on the conductivity studies of this transparent polymer in order to ascertain the suitability composition for the usage in the supercapacitor fabrication which is the main target of our future work.Thus, careful measures have been put in place to ensure that every detail characteristic of the active materials, especially of the PVA -its molecular weight and its percentage hydrolysis -were taken into considerations.

Preparation of the Composite PVA Electrolytes
The pure solid polymer electrolyte (PSPE) which can also be served as separators in supercapacitors fabrications have their laboratory preparations discussed elsewhere by us, in Hashim et al., (2012), Hashim et al., (2014) and by Jiang et al., (2013).But for the purpose of elaboration; the active materials comprise of H 3 PO 4 and PVA.The H 3 PO 4 functions as an ionic liquid, while PVA acts as a polymeric matrix, allowing ionic transport while simultaneously functioning as a separator between the electrodes of the supercapacitor.H 3 PO 4 (>85 wt.% in water, molar mass of 98.00 g/mol, 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 is 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 100:0, 90:10, 80:20, 70:30, 60:40, 50:50, 40:60 and 30:70 wt.% for the PSPE to be obtained (this has been summarized in Table 1).
In order to vary the percentage of the H 3 PO 4 used, the following formula was used; Where x is the percentage of acid require (0, 10, 20, 30, 40, 50, 60, and 70 % wt), and n is the value of H 3 PO 4 in grams.
The mixing is done in a drop-wise manner with magnetic stirring at 60 °C for about one hour or thereabouts until it completely turns to homogeneous solution (transparent in this case).The mixture is then allowed to cool down to an ambient temperature.The resulting homogeneous solution of PVA/H 3 PO 4 is cast over a plastic Petri dish.Prior to this, the Petri dish was scraped off the dust using a tissue soaked in acetone.The PVA/H 3 PO 4 solution solidifies onto the Petri dish and upon curing for about 3 -4 weeks at room temperature.This is due to its high molecular weight and consequently more hydrolyzed.The solid layer was easily peeled off from the Petri dish after it dries as a freestanding layer.The thickness of the solid layers formed in this procedure can be controlled depending on the composition that have been enumerated above and as shown in Figure 1.The resulting polymer film was then put in the plastic bags for safe keeping, and to avoid contamination from surrounding, for further property analysis.

Conductivity and Chemical Composition Analysis
Conductivity measurements were carried out on both the solid and hybrid polymer electrolyte films which are also composed of all the different percentage ratio of PVA-H 3 PO 4 an impedance method which also was highlighted by Ahmad Khiar and Arof (2010).The solid polymer films were sandwiched between a stainless steel, ion-blocking www.ccsen liked meas holder.
The imped machine n device was   , 10, 20, 30, 40, 50, 60, and 70 wt.% ratio of the H 3 PO 4 at room temperatures respectively.For the sample with 0 and 10 wt.% of the H 3 PO 4, it can be observed that the point of the graph was dispersed, which clearly indicates the meager or absent of acid in this sample.
The semicircles found on the Figure of the samples containing 20, 30 and 40 wt.% could however be connected with the impact of the introduction of the percentage acid and consequently the immobile polymer chain begins to emerge.
As the acid concentration continues to increase, so the conductivity, thereby making the semicircle disappear, which is an indication that, ions that move around the polymer matrix made resistive component of the polymer electrolytes to exist.(Ahmad Khiar & Arof, 2010;Ulaganathan et al., 2012).
The calculated R b for all the eight samples above were obtained from the frequency intercepts on each and every plotted graph.We also observed that, the R b value decreases with the increase in concentration.The calculated values of the R b are 1.15 x 10 8 , 8.00 x 10 4 , 2.40 x 10 4 , 1.70 x 10 3 , 2.10 x 10 3 , 1.93 x 10 1 , 1.26 x 10 1 and 2.58 Ω for 0, 10, 20, 30, 40, 50, 60, and 70 wt.%ratio of the H 3 PO 4 respectively.The highest conductivity of PSPE sample was observed at 2.56 x 10 -3 Scm -1 when the H 3 PO 4 was increased to 70 wt.%while the lowest conductivity of the sample was noticed at 2.25 x 10 -11 Scm -1 where the H 3 PO 4 percentage was nil.Full details about the average values of the conductivity of PSPE is summarized in Table 2. On a general note, it can be said that, the sample of PSPE performed wonderfully well in terms of both bulk resistance calculation and the conductivity as a whole.
As mentioned earlier, that conductivity is dependent on the concentration of the H 3 PO 4 .Table 2 provided us with the information that, there is a relationship between the acid concentration and the polymer matrix.In addition to that, it also be deduce from the table that, the thickness on each sample, also had some impact on the conductivity of the samples.Hence, because of the thinner nature of the films in PSPE, the conductivity seems to be very promising.
It can be observed from the semi-crystalline peak of the PVA (θ=20 o ) which also affirmed the argument put across by (Hashmi, (c), 2013) about the semi-crystal behavior of the PVA.The addition of H 3 PO 4 causes the decrease in the degree of the crystallinity and consequently the increase in the amorphicity of the material and its conductivity. Figure

Table 2 .
Parameters that Determine the Conductivity in PSPE