Biopolymer-based electrolytes: conductivity and structural insights into methylcellulose-magnesium triflate-DES electrolyte

Polymer electrolytes are solid-state materials and flexible substances that conduct ions that have the combined benefits of mechanical flexibility and ionic conductivity, making them attractive for use in energy storage devices such as batteries, capacitors, and fuel cells. Methylcellulose (MC) was chosen as host biopolymer since it is a biodegradable and can be processed into films. However, the biggest limitation of MC is that it is semi-crystalline in nature, which restricts polymer chain mobility and results in poor conductivity. To overcome, this study performs modification of MC using deep eutectic solvent (DES) composed of choline chloride and 1,3-propanediol. The DES serves as a plasticizer enhancing segmental mobility and reduces crystallinity to facilitate improved ion transport. Further enhancement was achieved by doping the system using magnesium triflate (MgTf) in 10, 30 and 50 wt.% which serves as a reservoir of mobile Mg2+ ions to facilitate conductivity. MC-DES sample itself had a conductivity value of 6.46 x 10-6 S/cm, while the addition of 30 wt.% inclusion of MgTf generated a higher conductivity value 3.68 x 10-7 S/cm. The results both validate that the combination of DES and MgTf significantly increases ionic transport in MC matrix. FTIR shows broadening and redshift O-H stretching peak, validating enhanced hydrogen bonding and amorphous. These structural changes are in direct correlation with EIS results which showed reduced bulk resistance and enhanced ionic mobility.