Giant-Thermopower Ionogels for Multifunctional Energy Harvesting through Molecularly Selective Ionic Pairing and Hydrogen Bonding

<p>Ionogels are emerging multifunctional materials for low-grade energy conversion and storage due to their high thermopowers, low costs, and facile productions. However, the thermoelectric synergy between ion-ion and polymer-ion interactions at the molecular level remains unexplored. Here, thermopower of ionogels composed of PVDF-HFP and EMIM:TFSI are enhanced from 5.3 to 21.2 mV/K by molecularly tailoring Coulombic and Lewis acid-base interactions. First, doping the ionogels with sodium bis(trifluoro-methylsulfonyl) imide (Na:TFSI) can greatly improve the thermopower. This improvement is ascribed to the stronger Coulombic interactions between the doped Na⁺ and TFSI^-, which selectively induces the formation of [Na:(TFSI)_n]^1–n contact ion pairs, blocks the migration of TFSI^-, and increases the difference in mobilities between EMIM⁺ and TFSI^–. Second, large amounts of hydrogen bonds are selectively formed between the terminal hydroxyl groups (-OH) in the added polyethylene glycol (PEG) and EMIM⁺. The Lewis acid-base interaction between the O˙ with lone-pair electrons in -OH of PEG and the acidic protons in EMIM⁺ is critical in promoting the heat of transport of EMIM⁺, which further increases the thermopower. In terms of applications, we demonstrate the potential of ionogels in multifunctional energy harvesting with high thermovoltages, including thermoelectric supercapacitors, highly sensitive thermal sensors, and thermoelectric ionogel-based triboelectric nanogenerators. Overall, this work offers molecular insights into Coulombic and Lewis acid-base interactions for enhancing thermopowers of ionogels, also broadening their applications in low-grade energy harvesting and self-powered electronic devices.</p>