Measuring temperature and heat flux is important for regulating any physical, chemical, and biological processes. Traditional thermopiles can provide accurate and stable temperature reading but they are based on brittle inorganic materials with low Seebeck coefficient, and are difficult to manufacture over large areas. Recently, polymer electrolytes have been proposed for thermoelectric applications because of their giant ionic Seebeck coefficient, high flexibility and ease of manufacturing. However, the materials reported to date have positive Seebeck coefficients, hampering the design of ultra-sensitive ionic thermopiles. Here we report an “ambipolar” ionic polymer gel with giant negative ionic Seebeck coefficient. The latter can be tuned from negative to positive by adjusting the gel composition. We show that the ion-polymer matrix interaction is crucial to control the sign and magnitude of the ionic Seebeck coefficient. The ambipolar gel can be easily screen printed, enabling large-area device manufacturing at low cost.

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We here report on the conformational evolution of the bis(trifluoromethanesulfonyl)imide anion (TFSI − ) in protic and aprotic TFSI − -based ionic liquids as a function of temperature. The investigation is performed by Raman spectroscopy in the spectral ranges 240-380 cm −1 and 715-765 cm −1 , where the interference from bands due to the cations is negligible. The contribution from each TFSI − conformation, i.e. the cisoid (C 1 ) and the transoid (C 2 ), is quantified in order to estimate the enthalpy of conformational change, H, which is found to be in the range 3.4-7.3 kJ/mol in the liquid state. Conformational information is for the first time determined from the 740 cm −1 band, which previously mainly has been used as an indicator of ion-ion interactions. The similarity in H values obtained from the two spectral ranges demonstrates the validity of using also the 740 cm −1 band for the quantification of the TFSI conformational evolution.

As a probe of local structure, the vibrational properties of the 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF 4 ] ionic liquid were studied by infrared (IR), Raman spectroscopy, and ab initio calculations. The coexistence of at least four [bmim] + conformers (GG, GA, TA, and AA) at room temperature was established through unique spectral responses. The Raman modes characteristic of the two most stable [bmim] + conformers, GA and AA, according to the ab initio calculations, increase in intensity with decreasing temperature. To assess the total spectral behavior of the ionic liquid both the contributions of different [bmim] + conformers and the [bmim] + − [BF 4 ] − interactions to the vibrational spectra are discussed.

We report on how the local structure and the diffusional motion change upon increasing the alkyl chain length in 1-alkyl-3-methylimidazolium cation ionic liquids. This study has been performed by combining pulse field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and small angle X-ray scattering (SAXS) experiments. The cationic side chain length varies from ethyl (n = 2) to hexadodecyl (n = 16), while the anion is always bis(trifluoromethanesulfonyl)imide (TFSI). We find that the self-diffusivity of the individual ionic species is correlated to the local structure in the corresponding ionic liquid, namely the nano-segregation into polar and non-polar domains. In agreement with previous results, we observe that for relatively short alkyl chains the cations diffuse faster than the anions; however we also note that this difference becomes less evident for longer alkyl chains and a cross-over is identified at n ≈ 8 with the anions diffusing faster than the cations. Our results indicate that this controversial behavior can be rationalized in terms of different types of cation-cation and anion-anion orderings, as revealed by a detailed analysis of the correlation lengths and their dispersion curves obtained from SAXS data. We also discuss the validity of the Stokes-Einstein relation for these ionic liquids and the evolution of the extrapolated cationic radius that was found to depend non-strictly linearly on n, in agreement with the cation-cation correlation lengths.