Low‐Hysteresis Self‐Powered Flexible Humidity Sensor Based on Sulfonated Graphene Oxide for Breath Monitoring

ABSTRACT Self‐powered humidity sensors promise low power, long‐term deployment, and miniaturization for smart agriculture, industrial automation, and wearable health monitoring. Conventional graphene oxide (GO) devices suffer from strongly bound water and high through‐thickness diffusion resistance, leading to slow recovery and pronounced hysteresis. Here we report a 3D sulfonated GO (S‐GO) self‐powered flexible humidity sensor. Introducing sulfonic groups with higher hydrophilicity and polarity enables proton generation at low relative humidity and rapid recombination during drying. The electrostatic repulsion induced by sulfonation slightly enlarges and homogenizes the interlayer spacing, and, together with a 3D interconnected framework, forms low‐tortuosity pathways for water transport. These features jointly shorten response and recovery times and markedly reduce hysteresis. The device operates without external bias and produces a stable voltage under humidity perturbations, exhibiting excellent cycling stability. We further demonstrate that the sensor can distinguish different breathing patterns. The proposed S‐GO architecture offers a promising materials‐and‐structure route toward fast, low‐hysteresis, and stable humidity sensing for flexible wearables and human‐machine interaction.