Polysaccharide-based hydrogel electrolytes enriched with poly(norepinephrine) for sustainable aqueous electrochemical capacitors

One of the main drawbacks of conventional energy storage systems is the possibility of electrolyte leakage due to faulty encapsulation or accidental overcharge. In this work, novel environmentally benign all-biopolymer-based hydrogels have been designed to be used as electrolytes in aqueous electrochemical capacitors. A primary component of gels, i.e. microcrystalline cellulose dissolved in NaOH/urea mixture and cross-linked with epichlorohydrin, was used in combination with agarose, or subsequently introduced agarose and poly(norepinephrine) to form the three-dimensional interpenetrated polymer network of enhanced mechanical stability. The effect of introduced components has been studied independently and investigated in terms of morphology, conductivity, swelling degree, and mechanical/structural properties. The electrochemical diagnostic tests of electrochemical capacitors utilizing acidic (H2SO4, H4SiW12O40) and neutral (Na2SO4) electrolyte-saturated hydrogels of the best overall characteristics have revealed the performance approaching the one characteristic of the liquid electrolyte-based counterparts but with lower self-discharge. The systems were characterized by the capacitance of 97–127 F g−1 at 0.1 A g−1 with retention of 33–89% at 20 A g−1, and cycling life of 10 000 cycles with a maximum of 10% of capacitance loss. The effect of intentionally improved hydrophilic properties of activated carbon electrodes, on the interfacial compatibility between electrode/hydrogel electrolyte is discussed as well. The results are in line with the global trend of green and sustainable energy storage designed for specific applications, i.e. in-vivo systems and wearable electronics.