Ringed Electrode Configuration Enhances the Sensitivity of QCM Humidity Sensor Based on Lignin Through Fringing Field Effect

This paper presents a new, effective, and facile method to enhance the humidity sensitivity of lignin-based QCM sensor through fringing field effect. The biodegradable and renewable lignin was deposited on the surface of metal electrode as humidity-sensitive material. The microscopic morphology, hydrophilic groups, and water contact angle of the lignin sample were characterized. Then, the humidity-sensing performances of the lignin-based QCM sensors with symmetric and ringed electrode configurations were investigated through the oscillating circuit method. The experimental results showed that the humidity sensitivity of the ringed electrode QCM (61 Hz/%RH) is higher than that of the symmetric electrode QCM in the range of 11.3%-97.3% RH. Moreover, the ringed electrode QCM based on lignin exhibited low humidity hysteresis (similar to 6.2% RH), good logarithmic relationship, rapid response/recovery time (28/5 s), excellent repeatability, and long-term stability. In addition, the Langmuir adsorption isotherm model and the Gibbs free energy were used to analyze the adsorption of water molecules on the surface of the lignin film. Finally, the finite element method (FEM) was used to analyze the fringing field distributions of the QCMs with symmetric and ringed electrode. The sensitivity enhancement mechanism of the lignin-based QCM humidity sensor with ringed electrode configuration was discussed through the equivalent electronic circuit analysis and simulation methods. This work demonstrates that the lignin is a potential candidate material for humidity detection, and it is an excellent method to improve the humidity sensitivity of QCM sensors through optimizing the electrode structure configuration using fringing field effect.