All-printed carbon nanofiber-encapsulated reduced graphene oxide functional ink for high-performance humidity sensors and energy storage device

The development of low-cost and flexible electronic devices has increased significantly due to their wide range of applications in electronic systems. Screen-printing is a cheap and scalable solution for fabricating devices, be it for Internet of Things (IoT) applications or even for real-time monitoring of agricultural produce. In the context of current requirements, we put forth a novel, multi-functional and all-carbon conductive ink with a nail polish-based binder system, designed for both humidity sensing and micro-supercapacitor applications, with the use of rGO (Reduced Graphene Oxide) and CNF (Carbon nanofiber) based ink formulation. Both CNF and rGO play a synergistic role, where CNF acts as a spacer that substantially reduces the aggregation and restacking of rGO, while rGO enhances capacitive and sensing properties with its residual oxygen-containing sites. The screen-printed humidity sensor performs notably with ranges spanning 11–97% relative humidity (RH), rapid response time (33 s), and recovery time (42 s). The printed sensors were utilised for real-time monitoring of a Solanum lycopersicum (tomato). The ink finds utility in the fabrication of micro-supercapacitor devices fabricated with the same ink as the humidity sensor, without any variation in composition, while yielding an areal capacitance of 38.7 mF cm −2 , energy density of 8.52 µWh cm −2 and a power density of 189.45 µW cm −2 . The successful incorporation of a single ink for the fabrication of both flexible humidity sensors and micro supercapacitors makes it a possible candidate for real-time monitoring and energy storage technologies.