Bicyclic-ring base doping induces n-type conduction in carbon nanotubes with outstanding thermal stability in air.

The preparation of air and thermally stable n-type carbon nanotubes is desirable for their further implementation in electronic and energy devices that rely on both p- and n-type material. Here, a series of guanidine and amidine bases with bicyclic-ring structures are used as n-doping reagents. Aided by their rigid alkyl functionality and stable conjugate acid structure, these organic superbases can easily reduce carbon nanotubes. n-Type nanotubes doped with guanidine bases show excellent thermal stability in air, lasting for more than 6 months at 100 °C. As an example of energy device, a thermoelectric p/n junction module is constructed with a power output of ca. 4.7 µW from a temperature difference of 40 °C.

Outstanding Electrode-Dependent Seebeck Coefficients in Ionic Hydrogels for Thermally Chargeable Supercapacitor near Room Temperature.

Thermoelectric power generation from waste heat is an important component of future sustainable development. Ion-conducting materials are promising candidates because of their high Seebeck coefficients. This study demonstrates that ionic hydrogels based on imidazolium chloride salts exhibit outstanding Seebeck coefficients of up to 10 mV K-1. Along with their relatively high ionic conductivities (1.6 mS cm-1) and extremely low thermal conductivities (∼0.2 W m-1 K-1), these hydrogels have good potential for use in heat recovery systems. The voltage behavior in response to temperature difference (stable or transient) differs significantly depending on the metal electrode material. We evaluated the electrode-dependent temperature sensitivity of the double layer capacitance of these hydrogels, which revealed that the thermally induced polarization of ions at the interface is one of the main contributors to the thermovoltage. Our results demonstrate the potential capability for ion and metal interactions to be used as an effective baseline for exploring ionic thermoelectric materials and devices. The developed thermoelectric supercapacitor exhibits reversible charging-discharging behavior under repeated disconnecting-connecting of an external load with a constant temperature difference, which offers a novel strategy for heat-to-electricity energy conversion from steady-temperature heat sources.

Thermodynamics of ionic liquid evaporation under vacuum.

The low volatility of ionic liquids (ILs) is one of their most interesting physico-chemical properties; however, the general understanding of their evaporation dynamics under vacuum is still lagging. Here, we studied the thermodynamics of IL evaporation by employing thermogravimetry (TG) measurements under vacuum. The thermodynamic parameters of ILs, such as the evaporation onset temperatures, enthalpies, entropies, saturation vapor pressures, and boiling points were quantified by analyzing the TG data. The obtained evaporation enthalpies (110-140 kJ mol-1) were higher than those of typical molecular liquids, and the entropies (>88 J mol-1 K-1) suggested that they are exceptions of the Trouton's rule. The obtained Clausius-Clapeyron equations demonstrated that the saturation vapor pressures of ILs only depend on temperature. Further, we derived the empirical equation for estimating the upper limit temperature of the liquid phase of IL under given external pressures. Using the evaporation behaviors of referential normal alkanes and charge-transfer complex and the evaporation entropies of the ILs, the vaporized IL structure was thermodynamically modelled. The ILs were found to evaporate as ion pairs, instead of as individual ions or higher-ordered cluster structures. By comparing a series of ILs with various cations and a fixed anion, it was found that the IL evaporation dynamics under vacuum is strongly and systematically affected by their chemical structures, charge balances between the cations and the anions, molecular weights, and the higher-ordered structures including polar and non-polar regions. Our concept, measurement method, and equation can be extended to other ILs and low-volatile liquids under vacuum, and help with the design of ILs with higher thermal stabilities.

Current-voltage characteristics of organic photovoltaic cells following deposition of cathode electrode.

The current-voltage characteristics of benzoporphine-fullerene solar cells were measured subsequent to the deposition of Al as a cathode material. Even in vacuum, a shift in the open circuit voltage was observed at 20 min after Al deposition. Moreover, the displacement of inert gases (N(2)or Ar) in the evaporation chamber enhanced the photovoltaic parameters. The power conversion efficiency was increased by 24% over the initial characteristics (from 1.04% to 1.29%), which indicates that the structure of the organic-metal interface changed rapidly after Al deposition, even if the process was performed in an air-free glovebox.