In-situ recomposition of polyethyleneimine additive enables a multiprocess long-lifetime thermocell

Ionic thermocells offer a compelling route for converting low-grade heat into electricity, yet their real-world deployment is hindered by performance decay under fluctuating conditions and limited synergistic pathways. Herein, we introduce polyethyleneimine as a self-limiting sacrificial additive in a K3Fe(CN)6/K4Fe(CN)6 thermocell. During initial operation, only a fraction of amine groups in polyethyleneimine engages in redox with Fe(CN)63-/Fe(CN)64-, while the remaining amine groups mediate thermally induced adsorption-desorption, selective condensation, and homogeneous catalysis. These cascaded effects boost the thermopower from 1.4 mV K-1 to 7.76 mV K-1. Crucially, the self-limiting nature and temperature-dependent reactivity of polyethyleneimine-Fe(CN)63- reaction not only generates solvation-perturbing species but also ensures long-term functional stability (>1000 hours). A proof-of-concept panel delivers over 5 V and 7.5 mW under a 50 K temperature difference, demonstrating system scalability. This work highlights the potential of sacrificial additive engineering to enable durable and high-performance thermocells for sustainable heat-to-electricity conversion. The study introduces polyethyleneimine as an additive in a thermocell, enhancing thermopower from 1.4 to 7.76 mV/K. It achieves long-term stability and scalability by enabling thermosensitive adsorption-desorption, selective reactions, and Manning condensation, converting low-grade heat to electricity efficiently.