Novel biopolymer pectin-based hydrogel electrolytes for sustainable energy storage

Abstract

Low degree of esterification (DE) pectin, a biopolymer found in plant waste, has been extracted from tomato pomace and soybean hull and compared to a commercially available citrus variety. Raising the pH during extraction increased pectin yield by 23% for soy and 116% for tomato sources while decreasing the DE. A hydrogel forms upon mixing with low concentrations of aqueous CaCl₂, while monovalent salts were also added to improve results. Physical characteristics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). Electrochemical stability windows of 1.6–2.0 V are shown via linear sweep and cyclic voltammetry (CV), while transference numbers of 0.73–0.96 were measured by Wagner's DC Polarization method. The effect of salt concentration on ionic conductivity was determined and the highest conductive hydrogels showed values at room temperature of 1–250 mS cm⁻¹ from electrochemical impedance spectroscopy (EIS), which is directly compared and on the same order of magnitude as the corresponding aqueous electrolytes. Each source of pectin performed similarly, while the change in salt had a much more prominent impact. CaCl₂ was only incorporated in small amounts for its ability to structurally impact the polymer matrix, while monovalents salts at high concentration provided orders of magnitude better conductivity and ion mobility. Application of the best performing hydrogel in a coin cell supercapacitor was evaluated by EIS, galvanostatic charge–discharge cycling, and CVs, showing minimal specific capacitance loss over a long cycling period (5000 cycles). This work highlights the properties of novel biopolymer hydrogel electrolytes that deserve more investigation with the growing importance of sustainable energy storage mechanisms.