Effect of thermo-alkali treatment on the morphological and electrochemical properties of biopolymer electrolytes based on corn starch–Al(OH)₃

A proton-conducting biopolymer electrolyte system based on corn starch doped with 0.0 to 4.0 wt% of Al(OH)₃ has been prepared through the solution casting method. Scanning electron microscopy and confocal micrographs showed a partial alteration of starch granule boundary and aluminum agglomerates formation as a function of the Al(OH)₃ concentration. X-ray diffraction and Fourier transform infrared spectra showed a decrease in the relative crystallinity and in the absorbance ratio 1047/1022, associated with the disruption of double-helix structures and the region of starch granule disorder, in concordance with the C₁ and C₄ deconvoluted curves obtained by CP/MAS ¹³C-NMR. Furthermore, corn starch films (CSF) mostly showed AlO₅ (pentahedral) and AlO₆ (octahedral) species as a function of the Al(OH)₃ concentration, associated with a suitable Al³⁺ distribution through the CSF arrangement obtained by CP/MAS ²⁷Al-NMR. Particularly, CSF with 2.0 wt% of Al(OH)₃ showed 27.5 F g⁻¹ of specific capacitance and 7.5 mS cm⁻¹ of electrical conductivity values, using voltammetry cyclic and the van der Pauw four-point test, respectively. The electrochemical behaviour of CSF was occasioned by saturation limit of chemical bonds between the functional groups into the starch molecule and the Al³⁺ ions was achieved, this disarrange and/or bankrupt the sequence of CSF microstructure, caused by the coexistence of free Al³⁺ ions and aluminum agglomerates. All these results implied that the present proton-conducting biopolymer electrolyte system based on corn starch–Al(OH)₃ has the potential to be applied in electrochemical devices.