A novel method to fabricate ion-doped microporous polyimide structures for ultra-high sensitive humidity sensing

This research develops a fast, simple and reliable fabrication process for fabricating an ion-doped microporous polyimide (PI) membrane for ultra-high sensitive humidity sensing. The humidity sensing performance can be enhanced both by the high surface area of microporous PI film and the deliquescence property of the doped salt of potassium acetate. The porosity of polyimide membrane can be controlled by immersing the coated PI films in different concentrations of ethanol–water solution. SEM images confirm that the micropores interconnect with their neighboring pores and also open to the outside air. The EDS mapping results show that potassium acetate is successfully introduced into the microporous structure. The measured humidity sensitivity of the ion-doped microporous PI film (1.23 wt% of potassium acetate doped) is over 100,000 times greater than the same PI film without pore and salt doping. Moreover, the proposed humidity sensor shows good linear capacitance response in the humidity range of 20–90% RH (R2 = 0.9525). A 72-h stability test also confirms the proposed humidity sensors have good long-term stability in 20% RH, 50% RH and 80% RH environments.