Simple pyrolysis of alginate-based hydrogel cross-linked by bivalent ions into highly porous carbons for energy storage.

Abstract Searching for a sustainable precursor that is capable of absorbing microwave energy is crucial for its rapid conversion into porous carbon via microwave heating. Here, alginate-based hydrogel beads cross-linked by bivalent ions (Cu2+, Zn2+, and Ca2+) are converted into porous carbons (SPCs) in 10 min by applying simple microwave-assisted pyrolysis. Water wrapped in hydrogel beads and bivalent ions serve as initial microwave absorbers to convert alginate into char which acts as a good microwave absorber in the following stages. Additionally, bivalent ions also act as a gelling agent to generate hydrogel beads, as a porogen to obtain a highly porous structure, and as a metal donor to form the SPC/Cu composite. The resultant SPC has a high specific surface area of 1336 m2 g−1, a large total pore volume of 0.56 cm3 g−1, interconnected macropores, as well as a high oxygen content of up to 13.2%. These attractive characteristics give SPC a remarkable rate capability of 72% at 50 A g−1. Interestingly, the interconnected macropores in SPC offer sufficient space for Cu growth via the redox reactions of Cu2+; thus, the SPC/Cu composite without acid wash shows a specific capacitance as high as 804 F g−1 at 0.5 A g−1.