Synergistic Regulation of Heat and Ion Transport in Horizontal Covalent Organic Framework Membranes for Efficient Low‑Grade Heat Harvesting.

Selective ion thermodiffusion within nanofluidic membranes offers a promising route for low-grade heat harvesting. However, their thermoelectric performance often suffers from parasitic heat conduction across the thin membrane, which rapidly dissipates the thermal driving force. This calls for an integrated strategy that simultaneously regulates ion transport and heat conduction. Here, we report a nanofluidic energy conversion platform based on horizontally stacked covalent organic framework (COF) membranes that enables synergistic regulation of both heat and ion transport. By extending the lateral heat conduction pathway, this architecture effectively suppresses parasitic heat loss, preserving a robust thermal driving force. In addition, surface‑charge‑governed two‑dimensional nanochannels promote highly selective cation thermodiffusion, which amplifies ionic charge separation. As a result, the system delivers stable power generation with an ionic Seebeck coefficient as high as 2.4 mV/K in the absence of any concentration gradient or redox reaction. Our findings demonstrate that geometrical engineering of the confined "channel-ion" interaction is pivotal for optimizing energy harvesting performance. This work provides a generalizable strategy for designing high‑performance ionic thermoelectric materials and devices.