PROTON CONDUCTING POLYMER ELECTROLYTES OF CARBOXYL METHYLCELLULOSE DOPED OLEIC ACID: CONDUCTIVITY AND IONIC TRANSPORT STUDIES M.N. Chaia and M.I.N. Isab Department of Physical Sciences, Faculty of Science & Technology, University Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia a email@example.com, firstname.lastname@example.org.Keywords: Solid polymer electrolyte, carboxyl methylcellulose, oleic acid.Abstract. Solid polymer electrolytes (SPE) of carboxyl methylcellulose (CMC) as the polymer hostand oleic acid (OA) as a dopant were prepared by the solution casting technique. The films obtainedwere transparent and no phase separation. The highest ionic conductivity, σ, was found to be 2.11 x10-5 S cm-1 at room temperature (303 K) for sample CMC-OA 20 wt. %. The ionic mobility anddiffusion coefficient that was calculated in this work is in good agreement with the increment ofweight percent (wt. %) of acid concentration. The value of cation of diffusion coefficient and ionicmobility was higher than value of anion. Thus, the results proven that the present samples wereproton conductor.INTRODUCTIONOver the past year, the electrochemical power was obtained by using liquid electrolyte due to itsconductivity. Unfortunately, this liquid electrolyte gives a lot of problem such as leakage, reactionwith electrode, and poor electrochemical stability, which makes it unsuitable for use in electro-chemical devices . In the world of modern technologies, commercial batteries represent a largenumber of toxic and hazardous materials which brings harm to the environment and human health.In this study, a proton-conducting solid polymer electrolyte or SPEs is presented to overcome thisproblem. Electrolyte from cellulose or cellulose derivative is chosen. CMC is a naturally occurringpolysaccharide and the most abundant organic substance on the earth. Due to the abundance, lowcost and easier process ability, so CMC is chosen in this research .EXPERIMENTAL METHOD2.1 Sample Preparation1 g of CMC powder was then dissolved in 33 ml distilled water. This solution was stirred for a fewhours until the CMC powder was completely dissolved. Then different weight percentages (wt. %) ofOA was dissolved in 66 ml ethanol then added to the CMC solution and stirred until they dissolved.The mixtures were then cast into Petri dishes and left to dry at 60 ºC. The films were then kept indesiccators (with silica gel) for further drying. The compositions of the CMC and OA used areshown in Table 1.
Table 1 Composition of the electrolyte with difference of wt. % Sample CMC (g) OA (wt. %) OA (g) OA-0 0 0 OA-5 5 0.0527 OA-10 10 0.1111 OA-15 1 15 0.1765 OA-20 20 0.2500 OA-25 25 0.3333 OA-30 30 0.42862.2 Electrochemical impedance spectroscopy (EIS)Conductivity of the CMC-OA biopolymer electrolytes were measured using the EIS by using theHIOKI 3532-02 LCR Hi-Tester that was interfaced to a computer in frequency range 50 Hz to 1MHz. The software controlling the measurement also calculate the real and imaginary impedance.The films were cut into a suitable size of 2 cm diameter and placed between the blocking stainlesssteel electrodes of a conductivity cell which are connected by leads to a computer. The bulkimpedance (Rb) value was obtained from the plot of negative imaginary impedance (Zi) versus realpart (Zr) of impedance and the conductivity of the sample was calculated from the equation 1.where A = Area of film–electrode contact and t =Thickness of the film (in cm)2.4 Transference Number Measurement (TNM)Transference number measurements (TNM) were performed to correlate the diffusion phenomena tothe conductivity behaviour of CMC-OA biopolymer electrolytes. The cation transference numbers, t+in the electrolytes were determined by monitoring the current as a function of time on application ofa fixed dc voltage (1.5 V) across the sample sandwiched between two stainless steel electrodes.2.4 Fourier transform-infrared (FT-IR) spectroscopyA Thermo Nicolet Avatar 380 FT-IR spectrometer was used to analyze the samples. Thespectrometer was equipped with an attenuated total reflection (ATR) accessory with a germaniumcrystal. The sample was put on a germanium crystal and infrared light was passed through the samplewith a frequency ranging from 4000 to 675 cm-1 with spectra resolution of 4 cm-1.RESULTS AND DISCUSSION3.1 Conductivity studyThe σr.t of CMC-OA is depicted in Table 2. The highest σr.t is 2.11 x 10-5 S cm-1 for OA-20. Theincrease in the ionic conductivity with increasing OA concentration can be related to the increase inthe number of mobile charge carriers. As the amount of salt added increases, the host matrix becamemore crowded with the dopant ions, thus, overcrowding reduces the number of charge carriers due tolimitation of ionic mobility. Hence, the conductivity decreases after 20 wt.% . Figure 1 depicts theplot of σ versus 1000/T for samples OA-0 to OA-30 from room temperature to 393 K. The linearvariations of the plot suggest an Arrhenius behaviour which implies that the conductivity is influenceby the temperature . The activation energy, was calculated from the slope of the logconductivity, σ versus 1000/T graph. It can be observed that the value of Ea is inversed to theconductivity as shown in Figure 2.
-3.5 OA-0 -4.0 OA-5 OA-10 Log conductivity, σ -4.5 OA-15 OA-20 -5.0 OA-25 OA-30 -5.5 -6.0 -6.5 2.4 2.6 2.8 3.0 3.2 3.4 1000/T (K-1) Figure 1 The temperature dependence for conductivity of CMC-OA electrolyte 2.50E-05 0.45 Conductivity (Scm-1) 2.00E-05 0.40 1.50E-05 Ea (eV) 0.35 1.00E-05 0.30 5.00E-06 0.00E+00 0.25 0 10 20 30 0 10 20 30 Concentration of OA (wt. %) Concentration of OA (wt. %) (a) (b) Figure 2 (a) Variation of conductivity as a function of salt content at room temperature and (b) Variation of activation energy as a function of salt content.The Rice and Roth model hypothesized that in an ionic conductor there is energy gap, whichconducting ions of mass, could be thermally excited from localized ionic states to free ion likestates in which the ion propagates throughout the solid with velocity, υ. The velocity is given byEquation 3.According to Shuhaimi et al. , can be considered as the distance between two coordinating sitesor two atoms with the lone pair electrons across which the ions may hop. From this result, the lengthof one chain segment was 1.5 nm.The number density of the mobile ions, , can be expressed by Equation 4.The ionic mobility, μ, can be calculated as Equation 5.The diffusion coefficient, D, is given by Nernst- Eistein equation.
The calculated parameters is tabulated in Table 3. The conductivity is dependent on ionic mobilityand diffusion coefficient. Further proved of the effect by ionic mobility and diffusion coefficient hadbe done by performing TNM. Table 3 The transport parameters of the CMC-OA biopolymer electrolytes at room temperature σ x 10-5 x 1022 μ x 10-9 D x 10-11 Sample τ x 10-13 (s) (S cm-1) (cm-3) (cm2 V-1 s-1) (cm2 s-1) OA-0 0.04 15.90 1.66 0.01 0.04 OA-5 0.19 9.63 1.79 0.13 0.33 OA-10 0.29 7.55 1.83 0.24 0.62 OA-15 1.19 4.88 1.98 1.53 4.00 OA-20 2.11 2.18 2.12 6.04 15.80 OA-25 1.59 4.09 2.03 2.43 6.40 OA-30 0.50 6.71 1.87 0.39 1.003.2 Ionic transport studyWhen a voltage V, which is below the decomposition potential of the electrolyte is applied to thecell, ionic migration will occur until steady state is achieved. At the steady state, the cell is polarizedand any residual current flows because of electron migration across the electrolyte and interfaces.The values obtained was then used to plot the graphs of normalised polarisation current versus timeas shown in Figure 3. The diffusion coefficients of cations and anions in each of the samples werecalculated according to the following equations :Besides, the ionic mobility can be defined according to the following equation:where, and is the ionic mobility of cation and anion. The calculated value of , , and are listed in Table 4. 1.2 Polarisation current, I (A) 1 0.8 0.6 Iion = 0.76 0.4 0.2 0 0 1000 2000 3000 4000 5000 Time, t (s) Figure 3 Polarization current versus time for samples OA-20
Table 4 Ionic mobility and diffusion coefficient of cations and anions Sample tion μ+ x 10-10 μ- x 10-10 D+ x 10-11 D- x 10-11 2 -1 -1 2 -1 -1 2 -1 (cm V s ) (cm V s ) (cm s ) (cm2s-1) OA-5 0.65 0.82 0.44 0.21 0.12 OA-20 0.76 45.90 14.50 12.00 3.79 OA-30 0.74 3.40 1.20 0.89 0.31From the Table 4, it can be observed that the value of the and the is found to be higher thanthe value of the and the . These properties of mobility concreted that CMC–OA electrolyte wasa proton conductor.3.3 FTIR studyAll samples prepared are transparent films with no phase separation. The FTIR spectrum of CMC isquite similar to that given by Abou Taleb et al. & Zaidi et al. [7,8]. The FTIR spectrum of OA issame as described by Kong et al. . Upon addition of OA, the intensity of the peak increasegradually with the addition of OA until 20 wt. % indicating that the deprotonation of the OAincreases. Further addition of OA caused the decrease of the peak intensity. This is accommodatingto the values of the conductivity obtained for the samples. The peak at 1710 cm-1 belongs to C=Ostretching of OA. The sharp band around 2920 cm-1 and 2850 cm-1 were assigned to C-H stretchingin asymmetric and symmetric, respectively. The band at 1597 cm-1 confirmed the presence of COO¯was assigned to stretching of the carboxyl group. The IR spectrum of CMC showed the band at 1040cm-1 was characteristic of the C-O stretching on polysaccharide skeleton. The intensity of the peakincreases with the addition of OA as shown in Figure 4. OA- 30 2850 2920 OA- 25 OA- 20 % Transmitance OA- 15 OA- 10 OA- 5 OA- 0 1597 3100 2400 1700 1000 Wavenumber (cm-1) Figure 4 FTIR spectrum of the sample in the region between 1000 and 3100 cm-1
Based on Figure 4, it can be shown that peak intensityof H+ (2920 cm-1 and 2850 cm-1) of OA isincreases whereas the peak of COO- (1597 cm-1) of CMC was less obvious with the addition of OA.It is suggested that protonation occurred in the present samples and the interactions between CMCand OA existed.CONCLUSIONThe CMC-OA biopolymer electrolyte obtained the highest conductivity of 2.11 x 10-5 S cm-1 at roomtemperature for sample OA-20 with OA concentration of 20 wt. %. By using Rice and Roth model,conductivity of CMC–OA biopolymer electrolyte is not only caused by the increase in theconcentration but also by the increase in ionic mobility and diffusion coefficient. From the TNM, itis proven that the sample is a proton conductor where the value of μ+ and D+ is found to be higherthan the value of μ- and D-. Thus, this prove that the present samples were proton conductor. Theconductivity is still low compared to the current conductivity based on polymer, it can be enhancedwith the addition of plasticizer.ACKNOWLEDGEMENTThe authors would like to thank the Department of Physical Sciences under the Faculty of Scienceand Technology, University Malaysia Terengganu, for the help and support given for this work.REFERENCES Idris, N.K., Nik Aziz, N.A., Zambri, M.S.M., Zakaria, N.A. & Isa, M.I.N. 2009. Ionic conductivity studies of chitosan- based polymer electrolytes doped with adipic acid. Ionics 15: 643-646. Fonseca, C.P., Rosa, D.S., Gaboardi, F. & Neves, S. 2006. Development of a biodegradable polymer electrolyte for rechargeable batteries. Journal of power source 155: 381-384. Siddhanta, A.K., Prasad, K., Meena, R., Prasad, G., Mehta, G.K., Chhatbar, M.U., et al. 2009. Profiling of cellulose content in Indian seaweed species. Bioresource Technology 100: 6669-6673. Khiar, A.S.A., Puteh, R., & Arof, A.K. 2006. Conductivity studies of a chitosan-based polymer electrolyte. Physica B 373: 23-27. Shuhaimi, N.E.A., Teo, L.P., Majid, S.R. & Arof, A.K. 2010. Transport studies of NH4NO3 doped methyl cellulose electrolyte. Synthetic Metals 160: 1040–1044. Tan, W. & Arof, A.K. 2005. FT-IR studies on interactions among components in hexanoyl chitosan- based polymer electrolytes. Spectrochimica Acta Part A 63: 677-684. Abou Taleb, M.F., Abd El-Mohdy, H.L. & Abd El-Rehim, H.A. 2009. Radiation preparation of PVA/ CMC copolymers and their application in removal of dyes. Journal of harzadous materials 168: 68-75. Zaidi, W., Oumellal, Y., Bonnet, J.P., Zhang, J., Cuevas, F., Latroche, M., et al. 2011. Carboxymethylcellulose and carboxymethylcellulose-formate as binders in MgH2–carbon composites negative electrode for lithium-ion batteries. Journal of Power Sources 196: 2854–2857. Kong, S., Zhang, P., Wen, X., Pi, P., Cheng, J., Yang, Z., et al.. 2008. Influence of surface modification of SrFe12O19 particles with oleic acid on magnetic microsphere preparation. Particuology 6: 185-190.