Flexible parallel-type capacitive humidity sensors based on the composite of polyimide aerogel and graphene oxide with capability of detecting water content both in air and liquid

High performance flexible capacitive humidity sensors (FCHS) are desirable due to application potentials in wearable electronics. Herein, we report a self-standing FCHS consisting of a composite of polyimide (PI) aerogel and graphene oxide (GO). Specifically, PI aerogel was modified with NaOH and GO, and treated with laser direct writing to reduce GO to construct electrodes on the surfaces of PI aerogel to fabricate a parallel-type FCHS. Resulting FCHS with 3D interconnected porous structure exhibited capacitance change from -10 pF to -1000 pF between relative humidity of 11-98 %, showing high sensitivity. Moreover, it displayed good stability against large deformations. The FCHS could monitor respiration and differentiate non-contact finger movement. Of note is that it could detect water content in both ethanol and hydraulic oil with high sensitivity, indicating dual function of measuring water content in both air and liquids. The work provides a facile approach for developing advanced flexible water sensors with broader application prospects.