A self-powered PVA-based flexible humidity sensor with humidity-related voltage output for multifunctional applications

In the context of the industry that supports green energy and electronics, the development of a humidity sensor with self-powered characteristics and the ability to be worn by humans is crucial for expanding its practical applications. Here, we report a self-powered flexible humidity sensor based on a sandwich structure of polyvinyl alcohol (PVA)/nanocarbon powder (NCP)/magnesium chloride (MgCl2) moisture-sensitive composite film (PCMF) and Cu and Al conductive adhesive tape. This unique sandwich structure allows it to readily adsorb and diffuse water molecules in humid environments, greatly enhancing its responsiveness to humidity changes. The Cu/PCMF/Al sensor generates voltage corresponding to different relative humidity (RH) levels and has a unique RH-Voltage linear response characteristic (R2 = 0.9978), notable for its fast response/recovery (6/11 s), wide detection range (RH 11%−98%), long-term stability (over 30 days), and high voltage (∼0.6 V) and current (∼2.3 µA) output at RH 98%. More importantly, due to the good water solubility of PVA and MgCl2 salts, the designed sensor exhibits recyclability and reusability properties (90.31% of the original response voltage value), which can significantly minimize material waste and potentially reduce manufacturing costs. In addition, this sensor can be successfully applied to domains involving human wearables, such as being integrated into a mask for monitoring human respiration, and has excellent identification capabilities even for simple coughs and pronunciations. The demonstration of non-contact humidity sensing validates the prospective application of this sensor in non-contact switching devices. This work presents a novel design idea for the development of wearable humidity sensors with low cost, simple preparation, and high responsiveness, as well as self-powered characteristics.