pH and Swelling Study of Chitosan-Based Polymer Electrolytes with Various Salt Additives
Authors: Poosapati Aswani , Negrete Karla , Madan Deepa 
Published: May 2020

Electrolyte and its components play a vital role on battery performance, as it is the interface at which all redox reactions responsible for working of a battery happen. Our study focuses on the use of Chitosan, a naturally available polysaccharide to form free-standing, flexible films that act as gel electrolytes. Chitosan was chosen among various polysaccharides for testing due to the ease of preparation from the shells of crabs, shrimps and prawns, natural abundance, renewability, biodegradability, cost-effectiveness, eco-friendly nature and a high degree of functionality which is not available in most synthetic polymers. During initial testing, the ionic conductivity value obtained for pristine chitosan was approximately 10-6S/cm, which is at least two orders of magnitude higher than the conventional synthetic polymers (PEO, PVP, etc.). This inherent advantage encouraged us to explore the possibility of their use as an electrolyte layer in batteries. It was noted that a semi-stable gel is formed when these biopolymers were vigorously mixed with minute amounts of acetic acid and dissolved in deionized water and allowed to sit for a while. It was also noted from the literature that chitosan films have an enormous surface area per unit volume (high aspect ratio), which makes us anticipate higher efficiencies. In our effort to make stable yet flexible organic films, various amounts of PVA (x=0.2, 0.4, 0.6, 0.8, 1) were added individually to Chitosan (CPx, CP=Chitosan: PVA), vortex mixed, cast into films, dried and then tested against SS blocking electrodes. Ionic conductivity studies on the various films provided a best average ionic conductivity value of 0.0068S/cm for CP0.2. This sample was then chosen and used for future experimentations due to enhanced conductivity from solvent retention of PVA and no rigidity induction in comparison to others.
The CP0.2 samples were then soaked in varied concentrations (1, 2, 3, 4, 5, 6, 7 and 8M) of KOH, ZnCl2, and ZnSO4 for varied time intervals for swelling and pH studies. These studies were performed since previous literature indicated that the concentration of salt in which a polymer electrolyte is soaked affects its physical properties along with its cycling performance (reversibility and reactivity) in batteries. The CP0.2 samples were soaked in respective salt solutions (1-8M KOH/ ZnCl2/ ZnSO4) and measured for their weights for every 15 minutes for 1 hour and every hour for 6 hours. pH values of these varied electrolytes in salt solutions were measured simultaneously during experimentation. The pH values varied from 12-14, 6-2.5 and 5.5-3.5 for increasing concentration (1-8M) for samples in KOH, ZnCl2 and ZnSO4 respectively. The swelling coefficients (final weight/initial weight) were then calculated based on the values obtained. For all samples it was observed that the highest swelling coefficients were observed within 45 minutes of soaking, after which the swelling coefficients remained constant. The highest swelling coefficient of 2.1 was observed for CP0.2 soaked in 6M KOH for 45 minutes. Similarly, 1.97 for 1M ZnCl2 and 2 for 1M ZnSO4. These samples were also measured for ionic conductivities to assess their performance. The ionic conductivities of samples increased with soaking in higher concentrations of salt until 5M and then reduced beyond for KOH salt. A highest ionic conductivity of 0.916S/cm was recorded. The increased conductivity until 5M salt concentration may be due to increase in number of mobile ions (OH-) in the system generated from dissociation of additional KOH salt available. Beyond 5M, the decrease in ionic conductivities might be due to aggregation of excess ions causing reduction in mobility. It also may be due to salt crystallization observed, that greatly reduces the migration ability. For ZnCl2 and ZnSO4 samples the phenomenon of reduction in ionic conductivities were observed from the very beginning. The highest recorded ionic conductivities were 0.004913S/cm and 0.019183S/cm for samples in 1M ZnCl2 and ZnSO4 samples respectively. During experimentation it was observed that, the samples soaked in ZnCl2 were significantly turning brittle and non-flexible compared to other samples. We think this is due to salt crystallization in the sample and is confirmed by the lowest ionic conductivity values. When analyzing the pH and swelling studies in conjunction it was noted that maximum swellability of samples occurred at lower acidities for ZnCl2 (pH-6) and ZnSO4 (pH-5.5) solutions and at higher basicities for KOH (pH-13.5) solution. Also analyzing ionic conductivities together indicated that, higher swelling implied higher ionic conductivities. The best samples obtained with this study will further be tested in Zn-based batteries to access the effect and performance of the prepared varied pH alkaline and Zn ion salt-based electrolytes.