Boosting Gel-Based Thermogalvanic Energy Harvesting via Metal Oxide Nanostructured Electrodes

Gel-based thermogalvanic cells (TGCs) have emerged as promising candidates for low-grade heat harvesting due to their intrinsically high Seebeck coefficients and simple device architectures. However, their power output remains limited by electrode kinetics and significant interfacial resistance. To address this problem, an effective strategy is developed herein by hydrothermally integrating transition metal oxide nanoparticles (TiO₂, WO₃, ZnO) onto carbon cloth (CC) electrodes, and combining this with a double-network poly(vinyl alcohol) (PVA)/gelatin hydrogel containing the ferro-/ferricyanide (Fe(CN)₆ ^3-/4-) redox couple. Compared to the pristine CC, all of the modified electrodes exhibit markedly enhanced current densities due to enlarged electroactive surface areas and abundant oxygen vacancies. In particular, the CC/TiO₂ electrode delivers the best performance due to unique coordination interactions between TiO₂ and Fe(CN)₆ ^4-, which facilitate interfacial charge transfer, as confirmed by spectroscopic and electrochemical analyses. The optimized device delivers a maximum power density of 579.8 mW m^-2 and a normalized maximum power density of 0.64 mW m^-2 K^-2. Furthermore, a nine-cell prototype generates ∼0.2 V under a modest temperature gradient of 15 K, thus highlighting the potential of hydrogel-based TGCs for low-grade heat recovery and flexible, wearable energy devices.