The electrochemical and physicochemical impacts of sodium chloride on the formation of a self-healing double network hydrogel using two methods of post-treatment and in-situ integration

Abstract

Physically cross-linked natural polymer-based hydrogels have found numerous applications in medical, pharmaceutical, and flexible energy storage fields due to their ease of preparation, high water content, nontoxicity, high flexibility, and self-healing properties. The physical, chemical, and electrochemical features of these hydrogels are greatly influenced by the level of intermolecular forces and the type of physical cross-linking agents. In this study, two approaches were applied to incorporate NaCl between the polymer chains of polyvinyl alcohol (PVA) and sodium alginate (SA) as a metal salt crosslinking agent; (1) the conventional post-treatment method based on immersing the as-prepared hydrogel in an aqueous solution of NaCl, and (2) in-situ integration of NaCl via adding the salt to the polymer casting solution before going through the repeating cycles of freezing-thawing. The influences of the NaCl concentration and applied crosslinking strategy on the structural, morphological, mechanical, electrochemical, flame-resistant, and self-healing properties of the resulting hydrogels as gel polymer electrolyte (GPE) in carbon-cloth-based flexible supercapacitors (FSCs) were investigated. The GPE prepared by in-situ integration of 1 M NaCl enjoying a higher degree of intermolecular forces showcased a lower degree of crystallinity and more porous pores, while providing a considerably higher elongation at break of (310%) and higher ionic conductivity of (14.12%) compared to the GPE prepared by immersing in a 1 M NaCl aqueous solution for a sufficient time. Furthermore, the in-situ NaCl integrated GPE exhibited a superior self-healing and flame-resistant properties than the other sample probably due to the formation of more reversible coordination bonds and higher ionic crosslinking. FSCs fabricated based on the in-situ NaCl integrated GPE demonstrated a higher specific capacitance (368.47 mF cm-2 at a current density of 0.5 mA cm-2), and power and energy densities than the FSC prepared based on GPE immersed in the NaCl solution.