The electrical, structural and electrochemical studies of magnesium‒ agarose based gel polymer electrolytes incorporated with deep eutectic solvent / Nurul Izzati Ali

Three systems of agarose-based gel polymer electrolyte, which are agarose‒Mg(NO3)2, agarose‒Mg(ClO4)2, agarose‒Mg(ClO4)2‒DES are prepared by using the solution casting method and characterized to determine their electrical, structural and electrochemical properties. Effects of lattice energy of the additive salts (Mg(NO3)2 and Mg(ClO4)2) on ionic species and conductivity characteristics in the gel polymer electrolyte is studied because the value of lattice energy will affect the degree of salt dissociation in polymer matrix. However, the combination of agarose and Mg salt alone are retained in high crystallinity which limited the performance of agarose-based polymer electrolyte. Thus, an additive known as DES is added to the complexed polymer electrolyte to enhance the electrical, structural dan electrochemical behaviour of agarose-based polymer electrolyte. The ionic conductivity can be calculated where the system with agarose‒30 wt% Mg(ClO4)2 exhibit a better ionic conductivity about 6.247 × 10-4 S∙cm-1 while the agarose‒30 wt% Mg(NO3)2 defined at 1.48 × 10-5 S∙cm-1, indicate the system with low lattice energy contribute the effectiveness dissociation of salt that promotes the production of free ions. The agarose-based polymer electrolyte is further prepared by addition of DES into the agarose‒30 wt% Mg(ClO4)2 and recorded the Rb value at 23.559 Ω and results in enhancement of ionic conductivity from 6.247 × 10-4 S∙cm-1 (0 wt% addition of DES) to 1.525 × 10-3 S∙cm-1 after involving 30 wt% of DES. The presence of DES in polymer electrolyte not only increased the dissociation of salt but also introduced new free ion, Cl- as alternative pathway during the transportation of ions. The ionic conductivity for all three system was also studied at various temperature (323 K, 343 K and 363 K), where are represented in graph of log σ against 1000/T to prove the Arrhenius rule are obeyed in all system. By using the same plot, the activation energy is calculated and recorded lowest at high conducting sample of all system. The structural properties of agarose-based polymer electrolyte is examined by using XRD and reinforced by using FTIR. From XRD pattern, the semicrystalline properties of agarose are observed at 2θ=18.8°, where these peaks are deconvoluted to determine the crystallite size in all three system. The system with the presence of DES recorded the smallest crystallite size about 0.91 nm compared to the system of agarose‒Mg(NO3)2 and agarose‒Mg(ClO4)2 that having crystallite size of 3.02 nm and 1.54 nm, respectively. The behaviour is supported by the FTIR study to observe the molecular interaction between the constituent occur in the systems. The percentage of free and contact ions are also determined by using FTIR deconvolution and result in higher percentage of free ion appear in the high conducting sample of all system. The electrochemical properties are done by using LSV and CV. The LSV curve shows the widest range of potential window obtained in agarose‒Mg(ClO4)2‒DES polymer electrolyte, which reveal at 3.75 V and recorded the voltage breakdown at 3.85 V, thus proves the suitability of this electrolyte to be applied in electrochemical application as they are having excellent range (>3 V). While CV is operated to observe the presence of anodic and cathodic peak appear in electrolyte system which prove the presence of Mg2+ conduction in the agarose-based polymer electrolyte systems.