Ion-selective transport in surface-modified cellulose membranes for aqueous ionic thermoelectrics

Efficient recovery of low-grade heat (≤100 °C) remains a significant challenge in sustainable energy conversion. Here, we report a strategy to enhance ionic thermoelectric performance in biocompatible regenerated cellulose (RC) membranes by tailoring their surface charge. Surface functionalisation was achieved using two oppositely charged organic moieties: 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) to introduce carboxyl groups, and 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHMAC) to graft quaternary ammonium functionalities. This work presents the first direct, side-by-side comparison of oppositely charged surface functional groups, carboxyl (TEMPO) and quaternary ammonium (CHMAC), on ionic thermoelectric behaviour in cellulose membranes. These modifications significantly increased the fixed surface charge density, promoting ion selectivity and enabling efficient ion transport under a thermal gradient. CHMAC-functionalised RC membranes exhibited the highest performance, with a Seebeck coefficient of +6.1 mV K⁻¹ in a stacked membrane configuration using 0.1 mM HCl electrolyte, representing a tenfold enhancement compared to unmodified RC membranes. Correspondingly, ionic conductivity increased by up to 950-fold, with an ionic thermoelectric power factor of 1.38 μW m⁻¹ K⁻². This study establishes a clear link between surface charge engineering and thermodiffusion-enhanced ionic transport in RC membranes, offering a scalable and sustainable route for harvesting low-grade thermal energy using green, aqueous systems.