A Bioinspired Metal-Free Bilayer Hydrogel Battery

Bioelectricity generation in electric eels is enabled by the precise arrangement of electrocytes that drive ion gradients to produce electrical output. Inspired by this mechanism, we present a scalable, metal-free hydrogel battery that generates electricity through spontaneous ion codiffusion. The system comprises stacked bilayers of polyacid (PAMPSA) and polybase (PEI) hydrogels, which establish intrinsic proton (H⁺) and hydroxide (OH^-) ion concentration gradients. This ion separation drives directional diffusion and produces a maximum output of 0.55 V per unit with a current density of 9 μA/cm². By serially connecting 31 units, the device achieves an open-circuit voltage of 10 V, and a stacked configuration equivalent in size to a standard AA battery yields a 5-fold voltage enhancement over commercial counterparts. The batteries demonstrate stable discharge performance and power small electronic devices, including capacitors and miniature calculators. The underlying transport mechanism is validated through both experimental studies and numerical simulations. This work offers a simple, sustainable, and scalable strategy for soft energy devices, providing a promising platform for portable and wearable electronics that bypasses reliance on metal-based charge carriers.