Humidity sensors are of great significance in the domains of wearable electronic products, environmental and food quality monitoring, and human healthcare. In general, they necessitate an external power source in the form of a battery. Despite considerable efforts, developing self-powered sensing systems without reliance on an external power supply remains a major challenge. Herein, an electrochemical humidity sensor with primary battery structure based on redox reaction was designed, in which polydopamine (PDA)-modified MXene/ TEMPO-oxidized cellulose nanofibers/LiCl (PDMM/TOCNFs/LiCl) composite film serves as electrolyte layer. The introduction of TOCNFs confers outstanding structural stability and superior tensile strength upon the composite film. Importantly, the hygroscopic and ionic conductivity properties of the PDMM/TOCNFs/LiCl electrolyte allow the sensor to generate spontaneous voltage over a wide relative humidity (RH) range of 11 – 91%. In addition, the developed sensor exhibits excellent humidity-sensing performances, including high voltage response, fast response/recovery time, and superior humidity-sensing stability. Moreover, mussel-inspired PDA improves the ambient stability of MXene by engineering interfacial interactions, which imparts the hygroscopic PDMM/TOCNFs/LiCl composite film with desirable stability for practical applications. Finally, the potential applications of the sensor and the composite film in human respiration, non-contact sensing, and humidity actuating are demonstrated. This work paves the way for the development of an innovative self-powered hu­midity sensing system.