Wide-detection-range, highly-sensitive, environmental-friendly and flexible cellulose-based capacitive humidity sensor.

Humidity sensors are widely used in industries, agriculture, climate control, and health monitoring, especially in flexible, environmentally friendly wearable electronics. However, existing sensors often suffer from limited detection ranges and low sensitivity. This work reports a flexible, environmentally friendly, highly sensitive humidity sensor with a wide detection range, which is made of cellulose, MXene, and sodium polyacrylate via vacuum filtration and impregnation method. The abundant polar functional groups of the materials enable humidity sensing across a broad range. This study reveals the significant enhancement in sensitivity of capacitive humidity sensors through the construction of multiple water layers that create proton and ion transport channels. The sensor shows a wide detection range (10 % RH to 90 % RH), low hysteresis (1.42 %), sensitivity of 2.11 pF/%RH (10 % RH to 41 % RH), and high sensitivity of 60.97 pF/%RH (41 % RH to 84 % RH). Additionally, it can distinguish breathing patterns and rates, enabling both non-contact and contact sensing, with potential applications in health monitoring. This work offers new insights into the development of flexible, environmentally friendly capacitive humidity sensors with high sensitivity and wide detection ranges.