Confinement-induced giant ionic thermovoltage at minimal temperature gradients via series-integrated micro-thermoelectric cells in hierarchical hydrogels

ABSTRACT Harvesting low-grade waste heat, particularly from ubiquitous small temperature fluctuations, requires materials that deliver high voltage outputs under minimal thermal gradients. Conventional ionic thermoelectric (i-TE) hydrogels rely on the global, continuous thermodiffusion of ions (i.e. the Soret effect) across the entire material, which typically yields limited voltages at small temperature differences (ΔT). Here, we report a hierarchically structured ionic hydrogel that fundamentally alters this paradigm via a ‘series-integrated micro-thermoelectric cell’ mechanism. By infiltrating a poly(acrylic acid)/sodium acetate (PAA/NaOAc) electrolyte into a directionally freeze-cast polyvinyl alcohol (PVA) scaffold, we construct an anisotropic architecture where dense crystalline PVA domains act as ionic blocking layers perpendicular to the thermal gradient. Under an applied temperature difference, ions undergo short-range thermodiffusion within the confined micro-domains, generating local thermovoltage that accumulates in series. Our hierarchical structured ionic hydrogel yields a record-high ionic Seebeck coefficient of 71.3 mV K−1 at minimal temperature gradients (ΔT ≤ 2.0 K). Crucially, the thermovoltage exhibits a non-linear saturation behavior at elevated ΔT, revealing a different interfacial charge effect in microstructure-confined ion separation. Combined with exceptional mechanical robustness, this work establishes a transformative paradigm shifting from global ion transport to micro-structural series integration, offering a robust material platform for ultra-sensitive thermal sensing and durable self-powered wearable electronics.