Molecular Design for Tunable Ionic Thermopower with High Stretchability

Wearable and stretchable electronics have attracted tremendous attention due to their wide applications in various areas. However, the lack of efficient, light-weighted and environmental friendly power sources has been the bottleneck that hinders the wide applications of wearable technologies. One of the attractive solutions is the thermoelectric generators (TEGs), which can directly convert waste heat from human body or environment into electricity. Though great development has been achieved with conventional TEGs, present inorganic thermoelectric materials still suffer from rigid properties and low Seebeck coefficient (several hundred uV/K scale), which requires an integration of hundreds of elements. To overcome these challenges, ionic thermoelectric materials utilizing ionic transport open up new possibilities for the recuperation of waste heat due to their enormous Seebeck coefficient, which can operate supercapacitors by energy harvesting.<br/>Herein, we introduce intrinsically stretchable acrylamide based polymers (PAM) as the ion conductive materials comprising graphene derivatives, which possess high stretchability and excellent thermoelectric performance. Interestingly, ionic conductivity and thermopower could be tunable according to the molecular structure of graphene-based additives. Chemical structure of graphene-based additives was successfully modified by covalent functionalization with phenylsulfonic acid (SGO) and amine species (NGO), resulting in strong positive and negative charged graphene nanosheets, respectively. Induced charge could significantly affect to Coulombic interaction between ion and polymer, or ion and solvent, rendering in large difference in ion concentration under temperature gradient. As a result, the proposed materials exhibit excellent and tunable Seebeck coefficient (up to 39.7 mV/K) as well as remarkable stretchability. The ionic thermoelectric performance was sustained during multiple stretch-release cycles (50% strain, 1000 cycles), which demonstrates high durability under severe deformation. Furthermore, we fabricate ionic thermoelectric supercapacitors (ITESC) with ionogels, demonstrating the heat-to-electricity conversion was successfully established. These findings show a promising strategy to obtain multifunctional ionic thermoelectric materials by engineering the molecular structures, and demonstrate the great potential of ionogels for harvesting low-grade heat in human-comfortable humidity environments.