Fully flexible, in-plane stretchable n-p type ionic thermoelectric gel devices for low grade heat harvesting

Ionic thermoelectric (i-TE) gel devices can directly convert low-grade heat into electricity, especially for use in self-powered wearable electronics. However, i-TE gel devices exhibit weak contact with curved human skin during frequent joint movements, resulting in reduced i-TE performance. Here, we demonstrate a fully flexible and in-plane stretchable gel device that integrates n-type and p-type i-TE gel cells connected in series, using Au-coated silver nanowire electrodes AgNWs@Au/PDMS. The p-type i-TE gel, PAA-{Mo 132 }-Ti 3 C 2 T x , employs high-nuclearity molybdenum-oxo clusters {Mo 132 } as new redox couple. This p-type i-TE gel cell demonstrates an ionic thermopower of 30.3 mV K −1 , a normalized maximum power density of 3.1 mW m −2 K −2 , and 13% relative resistance retention after 1000 stretching cycles at 50% strain, enabling continuous folding/twisting deformations inaccessible to conventional ones. In addition, a gel device assembled from eight pairs of n-type (MAA-I − / -KCl) and p-type i-TE gel cells maintains 92% power density at 25% strain, delivers an open-circuit voltage of 3.9 V, and achieves a maximum power of 98 μW under 2500 W m −2 illumination. This work advances ionic thermoelectrics toward truly wearable, bendable devices by establishing a co-design of flexible electrodes and i-TE gels, providing an effective guideline for other fully flexible energy gel devices. • Aqueous S i of 1.7 mV K −1 was shown for a new p-type redox couple {Mo 132 } α−1/α . • A concept of fully flexible, in-plane stretchable i-TE gel cell/device was proposed using AgNWs@Au/PDMS electrodes. • It delivered a S i of 30 mV K −1 and a P max /(Δ T ) 2 of 3.1 mW m −2 K −2 at 50% strain. • A n-p type i-TE gel device showed voltage of 3.9 V and P max of 98 μW at 2500 W m −2 .