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)₆^3-/Fe(CN)₆^4-, 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)₆^3- 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.