Observing Mixed Chemical Reactions at the Positive Electrode in the High-Performance Self-Powered Electrochemical Humidity Sensor.

Electrochemical humidity (ECH) sensors that integrate power generation and humidity sensing have attracted great research attention in recent years. However, the design of high-performance ECH sensors faces many challenges. Namely, the working mechanism of the ECH sensors is still controversial, and related self-powered applications have not been well implemented. To overcome these limitations, this study constructs an ECH sensor with high power-generation and humidity-sensing performance using the KCl/carbon black/halloysite nanotubes (KCl/CB/HNTs) as a humidity-sensing electrolyte. The proposed ECH sensor has a wide humidity-sensing response of 10.9%-91.5% relative humidity (RH), and a single ECH sensor can output 1.46 V with a maximum power of 133.2 μW at 91.5% RH. Particularly, the unprecedented mixed chemical reactions at the positive electrode, including the hydrogen evolution reaction and oxygen reduction reaction, are analyzed using multiple characterization and testing techniques. The analysis results provide solid experimental evidence for the current controversial working mechanism of ECH sensors. Due to the advantage of high-power generation, the proposed ECH sensor can be used for self-powered humidity detection. This study provides a valuable reference for improving the power generation of ECH sensors and solid evidence for clarifying their working mechanism, which could be beneficial for guiding the future development of ECH sensors.