Electrochemical charge-carrier modulation of carbon nanotubes using ionic liquids derived from organic superbases for stable thermoelectric materials

Abstract

Carbon nanotubes (CNTs) are attractive candidates for thermoelectric (TE) device components. However, a doping strategy for precise charge-carrier modulation and stabilization of the doped states of the CNTs is essential for thermoelectric applications. In particular, existing n-type CNTs tend to have inadequate stability in air and at elevated temperatures. In this study, we used newly synthesized ionic liquids (ILs) derived from organic superbases and bis(trifluoromethanesulfonyl)imide (TFSI) for electrochemical doping, expecting these ions to function as stabilizers of the n- and p-doped states of CNTs, respectively. The thermoelectric power factor was optimized by modulating the Seebeck coefficient and electrical conductivity over a wide range of values by varying the polarity and magnitude of the applied potential. Furthermore, n- and p-type doping could be simultaneously realized for the working and counter CNT electrodes by the one-time application of a potential. The doped states were retained as an electric double layer on the CNT electrodes even after their removal from the electrolyte solution. In particular, the use of ILs derived from bicyclic ring superbases and TFSI were shown to stabilize both n- and p-type CNTs at high temperatures for extended periods of time. Our approach is useful for preparing TE materials with an optimized power factor, controlled polarity, and long-term thermal stability.