Flexible capacitive humidity sensor based on potassium ion-doped PVA/CAB double-layer sensing film

Humidity measurement plays an essential role in industrial and agricultural production, meteorological monitoring, medical services, and people’s comfortable life. Polymer materials-based humidity sensors have gained much research focus due to the good dielectric properties and the compatibility with a variety of coating and pattern processes of polymer materials. In this paper, for the first time, a flexible capacitive humidity sensor is presented based on doped polyvinyl alcohol (PVA)/ cellulose acetate butyrate (CAB) double-layer sensing film and polyimide (PI) substrate. Potassium ions (K⁺) are introduced into PVA for high effective capacitance, and CAB is used as protective film for preventing the hydrolysis of the doped PVA layer in high humidity environment. The fabricated novel sensor with a size of 20 × 20mm² has a capacitance of 61.84pF at 10% relative humidity (% RH) and 188.42pF at 90% RH. The average sensitivity is 1.414 pF/% RH. The maximum humidity hysteresis is about 5.83% RH at 50% RH. The response and recovery times from room humidity to 75% RH were likewise measured to be 78.71s and 6.19s, respectively. In addition, the long-time measurements for 7 days and bending tests for 5000 cycles demonstrated a good long-time humidity detecting stability and high mechanical flexibility for the capacitive humidity sensor based on potassium ion-doped PVA/CAB double-layer sensing film.