Two-Dimensional Nanofluidic Gradient Graphene Oxide Membranes as Portable Power Sources

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.