Robust interfacial network and ionic conduction of carbon-doped hexagonal boron nitride for dual-mode strain and humidity sensor

Ionic conducting films garner significant interest as a substitute for traditional rigid electronic conductors due to their inherent flexibility and low power consumption, but simultaneously achieving high toughness, mechanical strength, ionic conductivity, and multifunctionality remains a critical challenge. In this study, we present a novel multifunctional sensor based on ionic conduction in composite films with surface-functionalized, carbon-doped boron nitride (f-BCN) as fillers in polyvinyl alcohol (PVA) matrix. The optimized film exhibits ionic conductivity up to 46 mS/cm under hydrated conditions. The successful formation of proton-conducting channels, attributed to interfacial hydrogen bonding between the functional groups of PVA and f-BCN, is identified as a key factor contributing to the high proton conductivity. Additionally, the f-BCN/PVA composite demonstrates high tensile strength (∼44 MPa) and toughness (∼73 MJ/m³), while maintaining stable piezoresistive behavior under cyclic strains. As a flexible strain sensor, the composite film achieved a gauge factor of 2.39 for low strains, with remarkable response and recovery of ∼0.8 s and ∼ 0.6 s, respectively. Furthermore, the composite film is cast on interdigitated silver (Ag) electrodes to prepare a humidity sensor. The sensor demonstrates exceptional sensitivity (718.7 kΩ/%RH and 1268.4 pF/%RH) over a wide humidity range while maintaining fast response and recovery of ∼1.5 s and ∼ 3 s, respectively. We also studied the cross-interference between strain and humidity sensing and found that strain and humidity can be decoupled via GF calibration. These results demonstrate the potential of f-BCN/PVA composite film, offering high ionic conductivity, dual-sensing functionality, flexibility, high mechanical strength, and low power consumption.