N-type thermoelectric composite hydrogels enabling dual-channel ionic/electronic transport

Thermoelectric hydrogels have drawn increasing research interest owing to their high ionic thermopower (α) values and superior mechanical stretchability. However, they are critically constrained by rapid dehydration and the poor interactions between polymeric frameworks and electrolytes, which fail to differentiate the diffusion rates of cations and anions under a certain temperature gradient (ΔT). This study reports a dual-channel ionic/electronic composite hydrogel implemented by directional freezing a cross-linked network of PEDOT:PSS-coated poly(vinyl alcohol) (PVA) and then immersion into an ionic electrolyte of CuCl2. Introducing 5 vol% conductive polymer PEDOT:PSS aqueous solution into the PVA hydrogel strengthened chemical coordination between the polymer matrix and Cu2+ ions to facilitate mobile Cl‒ ions, resulting in an ultrahigh α of ‒22.8 mV K−1, further elevated to − 33.8 mV K–1 via freezing–thawing cycles. Driven by ΔT, holes accumulate at the cold end of the composite hydrogel, in part neutralizing Cl‒ migration and mitigating open-circuit voltages to an equilibrium value, whereas holes drifting toward electrodes allow charge extraction to the external circuit. Notably, the composite hydrogel2 retained 90% of the peak voltage in a steady state at an optimum 0.5 M CuCl2. Furthermore, introducing CuCl2 doubles the water retention capacity of the hydrogel and simultaneously endows it with excellent re-usability and long-term stability. A hybrid thermoelectric generator delivers a stable output of ‒4.0 mV K−1 with a 100 kΩ external load. Such a device permits generating electricity continuously under not only temperature fluctuations but also a stable ΔT.