Artificial asymmetric ionic membranes have attracted great interests in harvesting electricity from ubiquitous water activities, while mostly based on delicately-designed nanopores/nanochannels, either to harness saline water in mimic of cytomembranes or to harness moisture with carbon nanomaterials. Herein, fully biological asymmetric ionic aerogels were fabricated from biological oppositely-charged nanofibrils through a facile freeze-casting method. When exposing to moisture, these nanofibrils may be hydrated by capturing moisture and thus simulate the charged nanochannels for ion transport. Ion dissociation and diffusion ions would induce directional movement of charges, thereby leading to a potential up to 115 mV. With sustainability, biocompatibility and biodegradability, these biological nanogenerators may promise a low-cost and high-efficiency electricity harvest strategy from moist air, being capable of serving as self-powered wearable, biomedical and miniaturized electronic devices.