Carboxymethyl cellulose-MgCl₂ composite based humidity sensor: self-powered, flexible, and multifunctional sensing applications

Humidity sensors have been widely used in fields of agriculture, industrial production, precision instruments and human health monitoring. To address the problem that humidity sensors need to be driven by an external power source, a self-powered flexible humidity sensor based on the principle of primary battery was proposed in this paper. The carboxymethyl cellulose with moisture-sensitive properties and MgCl₂ with moisture-absorbent properties were applied as the sensitive layer. And commercialized conductive copper and nickel adhesive tapes were used as the positive and negative electrodes. The micro-morphology and surface elements of the sensitive layer of the sensor were characterized by SEM and EDS, the moisture sensitivity mechanism was analyzed based on complex impedance spectroscopy, and the multifunctional applications of the sensor was demonstrated. The sensor sensitive layer has good hydrophilicity, which can ionize the MgCl₂ in it to produce carriers such as Mg²⁺ and Cl⁻ after contacting the water molecules in the environment. The directional movement of these carriers can generate output voltage. The response value can reach 177% when the relative humidity changes from 11% to 95%. The sensor can be used for human respiratory frequency and characteristic detection, soil moisture and urinary moisture detection, finger distance detection, and providing electrical energy. Experimental results show its potential for applications in health monitoring, environmental humidity monitoring, non-contact switching, and power-supply areas.