Facile and scalable preparation of ultralight cobalt@graphene aerogel microspheres with strong and wide bandwidth microwave absorption

Cobalt@graphene aerogel microspheres (GCoAMs) with tunable porous structures are fabricated by using a unique spraying and crosslinking coagulation method. The graphene oxide (GO) and sodium alginate (SA) mixture aqueous dispersions were used for spraying. The cross-linking reaction occurs quickly between the carboxylic group of SA in the sprayed microdroplets and metal Co2+ in the collector bath, which fixes the microsphere shape and in-situ immobilizes the metal ion. After freeze-drying and the subsequent high-temperature thermal reduction, the ultralight GCoAMs are obtained. The pore structures of GCoAMs can be controlled by varying the GO/SA ratio. The GCoAMs exhibit strong and wide broadband microwave absorption due to its hierarchical porous structures and graphene/metal Co hybrid composition. At a GCoAMs loading content of 1.5 wt%, the minimal reflection loss reaches as high as −70.4 dB with a thin specimen thickness of 2.2 mm at 13.18 GHz and its effective absorption band achieves 5.65 GHz with a specimen thickness of 1.98 mm. At 2 wt% GCoAMs, the effective absorption band achieves 6.09 GHz with a specimen thickness of 2 mm, covering the whole Ku band. This work provides a novel, low-cost and easily scale-up method for preparing the lightweight and efficient microwave absorbers, which could be used in microwave telecommunication fields.