Impedance study on humidity dependent conductivity of polymer composites with conductive nanofillers

In many previous studies on humidity sensors based on composites embedded with conductive nanofillers, opposite (positive or negative) dependence of their electrical resistance on humidity has been reported. In this study, we systematically investigate the origin of opposite humidity-dependence of the resistance of agarose/carbon nanotube composites (A-CNTs) as a model composite material. Experimental results and impedance analysis reveal that the composites with more conductive fillers present positive dependence of resistance on humidity as an electronic pathway through the well-connected fillers is preferred for electrical conduction. In contrast, the composites with low filler content show negative dependence as the ionic pathway is more dominant for electrical conduction. The corresponding equivalent circuits for the nanofiller-embedded composites are suggested and the expected Nyquist plots depending on the filler content and humidity conditions are discussed. The process based on the impedance analysis potentially allows for non-destructive analysis of compositions and electrical property of composites as well as highly sensitive humidity detection.