Giant Thermoelectric Performance of N-Type Ionogels by Synergistic Dopings of Cations and Anions

It is of great significance to develop high-performance n-type ionic thermoelectric materials. Herein, the turnover from p- to n-type and great improvement in the n-type ionic Seebeck coefficient of ionogels through synergistic dopings of cations and anions, is reported. Doping with silver trifluoromethanesulfonate (AgOTf) can turn the ionic Seebeck coefficient of the ionogels consisting of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM:BF4) from 5.52 to -26.4 mV K-1 under a relative humidity (RH) of 75%. The TE performances can be further improved through the combination with the anti-solvent method, and the ZTi value can be up to 1.7 under 91% RH, the highest for n-type ionogels. Such p- to n-type turnover and great improvement in the n-type ionic Seebeck coefficient can be attributed to synergistic doping by Ag+ and OTf-. Ag+ can form a complex with BMIM+, and OTf- can have strong Coulomb interaction to BMIM+. Both can retard the migration of the BMIM+ cations and thus increase the difference in the mobilities of BMIM+ and BF4-. This is the first time that synergistic dopings by both cations and anions for ionic TE materials has been observed.