Electrochemically synthesized graphene/TEMPO-oxidized cellulose nanofibrils hydrogels: Highly conductive green inks for 3D printing of robust structured EMI shielding aerogels

We report on the design and synthesis of bio-based, electrically conductive green inks for direct ink writing (DIW) of lightweight electronics and electromagnetic interference (EMI) shields. The inks incorporate fibrillated cellulose and electrochemically synthesized graphene oxide (EGO), with no production and/or consumption of hazardous chemicals. The cellulosic component, TOCNF ((2,2,6,6-tetrame-thylpiperidin-1-yl) oxidanyl (TEMPO)-oxidized cellulose nanofibrils), improves the colloidal dispersion and the rheological properties of EGO-based inks for high-resolution 3D printing via DIW. The printing fidelity and shape retention significantly rely on the EGO/TOCNF loading and ratio in the precursor hydrogel inks. Aerogels result from freeze drying, allowing the production of 3D ultra-lightweight materials with prescribed macro-scale design featuring excellent stability and ease of handling. It is shown that the nano-and micro-scale design of the aerogels can be readily tuned by the solid content and EGO/TOCNF ratio in the inks. This multi-scale materials design provides a unique opportunity to control the mechanical and electrical properties of the printed structures. For instance, aerogels with compression modulus in the range of 250-1096 kPa are obtained based on the composition of the inks. For the optimized ink, an excellent EMI shielding effectiveness, as high as 55.6 dB, is achieved.