Flexible humidity sensor based on crosslinked polyethyleneimine/tannic acid and porous carbonaceous interdigitated electrode

For their advantages of flexibility in nature, low cost, and ease of processing, polymers and polymer composites are considered to be ideal candidates for the development of wearable and flexible humidity sensors. Nevertheless, the commonly adopted polymer-based humidity sensing materials are usually subjected to long-term stability issue, due to a lack of chemically crosslinked network structure. Herein, we rely on the robust Michael addition reaction between polyethyleneimine (PEI) and tannic acid (TA) in aqueous solution for fabricating the chemically crosslinked TA-x-PEI based humidity sensors through a simple spray coating process. With assistance of direct laser writing carbonization (DLWc) technique, we demonstrate that the combination of TA-x-PEI with porous carbon interdigitated electrode (PC-IDE) allows for facile fabrication of high-performance flexible humidity sensors, which possess fast response and recovery performance (28 s/12 s), low hysteresis (2%) in the range of 35% RH - 90% RH, good long-term stability (impedance change < 2% over 30 days) and high sensitivity, as demonstrated by a comparison of key metrics with similar humidity sensors reported in the literature. SEM microscopy, Raman and FTIR spectroscopy, frequency-dependent impedance measurement, as well as equivalent circuit modeling have been performed to gain an in-depth understanding of the humidity sensing behavior and the underlying mechanism for the newly developed PC-IDE enabled TA-x-PEI based humidity sensors.