Converting low-grade heat into useful electricity requires a technology that is efficient and cost effective. Here, we demonstrate a cellulosic membrane that relies on sub-nanoscale confinement of ions in oxidized and aligned cellulose molecular chains to enhance selective diffusion under a thermal gradient. After infiltrating electrolyte into the cellulosic membrane and applying an axial temperature gradient, the ionic conductor exhibits a thermal gradient ratio (analogous to the Seebeck coefficient in thermoelectrics) of 24 mV K-1-more than twice the highest value reported until now. We attribute the enhanced thermally generated voltage to effective sodium ion insertion into the charged molecular chains of the cellulosic membrane, which consists of type II cellulose, while this process does not occur in natural wood or type I cellulose. With this material, we demonstrate a flexible and biocompatible heat-to-electricity conversion device via nanoscale engineering based on sustainable materials that can enable large-scale manufacture.