Room-temperature proton transport and its effect on thermopower in a solid ionic semiconductor, TTFCOONH4

Abstract

Ammonium proton in a solid ionic semiconductor, TTFCOONH 4 , is shown to be mobile under anhydrous conditions at room temperature by the hydrogen concentration cell method. Isotope substituted TTFCOOND 4 exhibits a 2.2 H/D isotope effect in ion carrier mobility with TTFCOONH 4 . First-principles calculations reveal that an efficient proton-transfer pathway via low-barrier N/H + /N hydrogen bonds reduces the activation energy to 0.12 eV, which is quite small and comparable to that reported in a bulk water system. The ac conductivity of TTFCOONH 4 and TTFCOOND 4 is similar at room temperature, reflecting similar hole carrier concentrations. In sharp contrast, the thermopower exhibits a large isotope effect: TTFCOONH 4 shows 260 mV K À1 , which is twice as large as that predicted by the hole carrier concentration and the value of TTFCOOND 4 , with 138 mV K À1 . The 1.9 H/D isotope effect in thermopower closely relates to the 2.2 H/D isotope effect in ion carrier mobility. Proton carriers in the temperature gradient enhance thermopower without cancelling out the effect of holes in the solid state owing to possession of the same positive charge.

Isotope Effect of Thermopower in the TTFCOONH3Ph Single Crystal

“…Evaluation of the curve‐fitting shows that the inclusion of protonic defects is 25 % for TTFCOONH 3 Ph and 20 % for TTFCOOND 3 Ph (see the Supporting Information), that is, one in every four or five molecular pairs includes a protonic defect, respectively. It was thus confirmed that isotope substitution of the ammonium protons has little effect on the doping level in single crystals as well as in powder crystals of TTFCOONH 4 3c. The doping levels determined by the XPS analysis support the results obtained with similar molecular structures containing the TTF moiety and the electrical conductivity of these crystals.…”

“…The number of protonic defects in the two salt‐bridge networks was evaluated to be almost the same by XPS analysis, and so the isotope effect of this ion transport likely originates in the mobility of ion carriers derived from the difference in mass. However, their absolute EMF values are very small, on the 0.1 mV scale, which is almost negligible compared with the theoretical value (inset of Figure 3b), and two orders of magnitude smaller than that of the TTFCOONH 4 system 3c…”

“…It is of interest to note that the absolute values differ by a factor of around two. The isotope effect of thermopower has also been observed for the TTFCOONH 4 system,3c but in this case the effect of grain boundary that arises when measuring pellet samples could not be excluded. We show clearly here the significant isotope effect of the thermopower in this system based on single crystals.…”

“…The proton transport abilities of TTFCOONH 3 Ph and TTFCOOND 3 Ph were evaluated by the hydrogen concentration cell method, because the isotope effect of thermopower in TTFCOONH 4 is strongly correlated with its proton‐transport ability 3c. A sample separated by different hydrogen gas concentrations produces a voltage between the edges of C/Pt electrodes, with the following reactions established at the electrodes [Equations (1), (2), and (3)]: …”

Rational Construction of Molecular Electron-Conducting Nanowires Encapsulated in a Proton-Conducting Matrix in a Charge Transfer Salt

“…However, there is no report on the single crystalline compound which allows the investigation of structure–property relationship to guide the rational design of such molecular mixed conductors. Although there are several reports on single-crystalline proton–electron mixed conducting ion radical salts, the radical cations in the system exhibit quasi one- or two-dimensional arrangements instead of insulated molecular wire systems.…”