Interfacial reactions in ionic thermoelectrics: Unlocking charge transfer vs. triggering poisoning traps

Ionic thermoelectrics (i-TEs) hold great promise for low-grade heat harvesting, yet their practical application is hindered by an incomplete understanding of the electrode/electrolyte interface — often tacitly assumed to be an inert current collector. Herein, we challenge this assumption by systematically dissecting the multifaceted role of interfacial reactions across three progressively interconnected levels. First, using a decisive Cu versus carbon cloth electrode comparison paired with reactive (Cl - -based) versus inert (F - -based) electrolytes, we demonstrate that a mild interfacial reaction is not a parasitic side effect but the essential key that unlocks charge transfer, without which no measurable output emerges. Second, we uncover the dark side of this key: for weakly hydrated ions like K + , the same interfacial reactivity, when coupled with strong electrostatic locking and facile dehydration, triggers catastrophic precipitation crystallization — a “poisoning trap” that physically blocks ion transport and extinguishes output. Third, exploiting imidazolium ionic liquids with tunable alkyl chains (DMIM + to BMIM + ), we reveal a fundamental molecular trade-off: longer chains enhance interfacial ordering and maximize energy density (550 J m -2 ) but, by creating hydrophobic microdomains and covering hydrophilic sites, they compromise water retention (89.2% loss) and shorten operational lifetime; shorter chains invert this balance, prioritizing stability over peak output. Collectively, these findings establish a unified model that reconciles the dual role of the interface, as both indispensable initiator and potential poison, and provides a molecular blueprint for designing i-TE materials where performance and longevity can be rationally balanced for targeted applications. Interfacial effects dictate the fate of ionic thermoelectrics: a mild reaction unlocks output (key-lock), while uncontrolled reactivity for weakly hydrated ions triggers catastrophic crystallization (poisoning trap). Chain-length engineering further reveals a molecular trade-off between interfacial ordering and water retention, providing a blueprint for balancing performance and stability.