Frequency-tunable and absorption/transmission-switchable microwave absorber based on a chitin-nanofiber-derived elastic carbon aerogel

•Smart microwave absorption is realized by a chitin-derived elastic carbon aerogel.•Compressing the aerogel tunes its thickness, impedance match, and dielectric loss.•Absorption frequency is tuned by varying the compression strain from 0 to 60%.•Absorption/transmission switching is achieved by further compression to 80% strain.•This smart microwave absorption is reversible and repeatable over 60,000 cycles.