With traditional fossil energy sources and energy storage technologies gradually unable to meet the needs of future portable flexible electronic devices, there is a growing interest in research of power sources with small sizes, high energy density, and durability. Here, inspired by the cell membrane potential in living species, we demonstrate a two-dimensional nanofluidic gradient structure (TNGS) by integrating a series of graphene oxide (GO) and reduced GO (rGO) in the order of the size of interlayer spacing along the horizontal direction. Through asymmetric adsorption of hygroscopic CaCl2 salt, a sustained built-in hygroscopic gradient is constructed in the TNGS. Driven by the built-in hygroscopic gradient, the TNGS not only offers a proton concentration gradient but promotes selective migration of Ca2+. Under the synergistic effect of the TNGS and redox reaction, the directional Ca2+ and proton transport is converted into electrical energy on electrodes. Under ambient humidity, such a solid-state power source (0.9 cm2 × 10 μm) can output a voltage of ∼1.35 V for over 100 h and a volumetric power density of ∼515 μW cm–3. This finding opens a universal avenue for harvesting hygro-ionic electricity and offers a constructive insight into designing portable, easy-preparation, and cost-effective self-powered devices.