Structural study of 2-hydroxyethyl cellulose (2HEC) solid biopolymer electrolyte doped with NH4Br: effect on ionic conductivity

In this work, solid biopolymer electrolyte (SBE) was developed into films by using 2-hydroxyethyl cellulose (2HEC) as a polymer host and doped with different weight percentages (wt.%) of ammonium bromide (NH₄Br). The 2HEC-NH₄Br SBE films were limited to only 30 wt.% salt composition before the SBE became physically unstable. The 2HEC-NH₄Br SBE films were analyzed through several characterizations: electrical impedance spectroscopy (EIS), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). EIS analysis shows that the highest conductivity achieved was 3.95 × 10⁻⁵ Scm⁻¹ for a sample with 20 wt.% of NH₄Br composition. The amorphous and semi-crystalline structures were determined through XRD, with the highest conducting sample exhibiting the semi-crystalline structure, consistent with changes in the sample’s optical appearance. The amorphous nature was further confirmed through analysis of the DSC curve. FTIR analysis confirmed the interaction between NH₄Br and 2HEC occurred at the -OH group of 2HEC. A comparative analysis of ionic conductivity across studies sheds light on several contributing factors such as glass transition temperature and lattice energy that affect SBE performance.