Quasi-Solid-State Thermogalvanic Cells Based on TEMPO-Oxidized Cellulose Nanofibril/Poly(vinyl alcohol) Double-Network Hydrogels

Quasi-solid-state thermogalvanic cells (TGCs) have emerged as a highly promising technology for low-grade waste heat harvesting. In this work, we developed a robust double-network (DN) hydrogel electrolyte combining TEMPO-oxidized cellulose nanofibrils (t-CNFs) and poly(vinyl alcohol) (PVA) to maximize thermogalvanic efficiency. While the tailored DN architecture ensures excellent structural integrity and mechanical stability, the abundant negative surface charges of the t-CNF network play a crucial role in accelerating ionic migration within the hydrogel matrix. Benefiting from this synergistic design, the t-CNF/PVA DN hydrogel integrated with a [Fe(CN)₆]^3−/4− redox couple exhibits remarkably enhanced thermogalvanic properties. Consequently, the optimized TGC achieves a substantial increase in Seebeck coefficient, current density, and output power density compared to conventional pure PVA-based counterparts. This study demonstrates that leveraging the surface charge of nanocellulose within a double-network framework is a powerful and practical strategy for boosting the performance of flexible energy-harvesting systems.