The electrochemical and physicochemical impacts of sodium chloride on the formation of a self-healing double network hydrogel using two methods – 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 a 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 sufficient time. Furthermore, the in situ NaCl integrated GPE exhibited superior self-healing and flame-resistant properties compared to 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 of 368.47 mF cm⁻² at a current density of 0.5 mA cm⁻² and higher power and energy densities than the FSC prepared based on the GPE immersed in the NaCl solution.