A Bioinspired Ionic Heterojunction Generator Enabling Stimulus‐Free, Scalable Energy Harvesting

The growing demand for sustainable power sources in distributed electronics and wearable devices requires stable, scalable, and maintenance‐free energy harvesters. However, most existing systems rely on mechanical deformation, environmental fluctuations, or engineered gradients, leading to unstable outputs and limited lifetimes. Here, we present a bioinspired ionic heterojunction energy harvester that generates direct current solely through spontaneous interfacial ion migration, without requiring repeated external inputs. The device is based on the asymmetric bilayer structure, formed by ionic liquids and charged polymers within a thermoplastic polyurethane matrix, establishes a built‐in potential that drives directional ion migration upon contact. A single 0.2‐mm‐thick unit delivers ∼0.71 V and a volumetric power density of 66.8 µW/cm 3 , with stable operation exceeding 60 h and robust tolerance to mechanical strain (up to 50%) and humidity (up to 90% RH). Modular stacking enables linear voltage scaling, directly powering practical devices such as a 6 W light bulb, calculator, and watch without rectification. This all‐solid‐state, stimulus‐free platform offers a scalable and sustainable route toward self‐powered wearable and distributed electronics.