Charge-engineered cellulose membranes from wood waste for ionic thermoelectric energy conversion

Approximately 60-67% of global energy production is lost as low-grade heat. Recovering even a small fraction of this waste heat could materially improve overall energy efficiency. Here, we present a sustainable ionic thermoelectric platform based on cellulose membranes derived from waste wood chips and surface-functionalised with N-[(3-trimethoxysilyl)propyl]ethylenediamine triacetic acid trisodium salt (TMSDA). Silanisation introduces tricarboxylate groups, increasing the carboxylate content from 0.16 to 0.86 mmol g^-1 (∼5.4×) and enhancing the zeta potential to -33.5 mV. Fitting the concentration-dependent ionic conductivity data across a wide range of KCl concentrations revealed a nearly 9-fold increase in surface conductivity and a crossover concentration of c* ≈1.7 × 10^-3 M, supporting surface-governed ion transport in the dilute regime. At 10^-4 M KCl, TMSDA-cellulose membranes exhibited an ionic conductivity of 1.51 mS cm^-1, ∼13× higher than pristine cellulose, driven by enhanced counter-ion mobility within charged porous pathways. When integrated into a stacked thermocell and subjected to a 10 K temperature gradient, the membranes generated an open-circuit thermovoltage (V _oc) of -128 mV, corresponding to an apparent ionic Seebeck coefficient (S _i = V _oc/ΔT) of -12.8 mV K^-1 (for ΔT = 10 K) and a power factor of 24.7 ± 6.8 µW m^-1 K^-2. These values represent an approximately 3.2-fold improvement over pristine membranes and rank among the highest reported for cellulose-based ionic thermoelectrics operating in dilute, neutral aqueous electrolytes. Unlike many state-of-the-art systems that require ionic liquids or harsh electrolytes, our approach combines biomass valorisation, scalable membrane assembly, and green functionalisation chemistry to establish charge-engineered, waste-derived cellulose as a versatile and environmentally responsible platform for ionic thermoelectrics.