Semiclathrate hydrate (SCH) is a crystalline inclusion compound promising for use as an electroconductive and energy-storage material. In this study, the electrochemical properties of three single-crystalline tetra-n-butylammonium (TBA) carboxylate SCHs, namely, TBA-formate, TBA-acetate, and (TBA)2-oxalate SCHs, were investigated. All three SCHs exhibit Arrhenius-type conductivity, with TBA-formate SCH demonstrating the highest electrical conductivity. Electrochemical impedance analyses correlated the differences in carrier concentration among the SCHs with variations in electrical conductivity. These findings suggest that defect generation within the hydrogen-bonding network plays a significant role in enhancing the electrical conductivity, potentially serving as a key conduction mechanism in these materials. These results contribute to a deeper understanding of the potential uses of SCHs as advanced materials. © 2025 American Chemical Society

Semiclathrate hydrates (SCHs) have been expected to be one of the phase change materials for cold energy storage. The large degree of supercooling in SCH formation, however, is a major obstacle to the practical applications of SCHs. As a way to suppress the degree of supercooling, we focused on the memory effect (a kind of temperature hysteresis) in the tetra-n-butylammonium dicarboxylate SCHs. The tartarate, malate, and succinate anions, which have similar chemical structures, were used as anions. As a result, the ability to retain the memory effect in the TBA-tartarate SCH was the largest of the three SCHs. The interaction of the substituted hydroxy group(s) of anions with surrounding water molecules in the aqueous solution might improve the thermal stability of the solution structure that causes the memory effect, resulting in promotion of the SCH reformation.

Semiclathrate hydrates are electroconductive materials for possible solid electrolytes and are also useful for monitoring their formation and dissociation processes. In the present study, the electrical conductivities and electrical relaxation times in the single-crystalline tetra-n-butylammonium (TBA) chloride and fluoride semiclathrate hydrates were measured and compared with those of the TBA-bromide semiclathrate hydrate. In the descending order of the electrical conductivity, the largest was the TBA-bromide semiclathrate hydrate, followed by TBA-chloride and TBA-fluoride semiclathrate hydrates. On the other hand, H-2 NMR spin-lattice relaxation times in their deuterates were similar. Although the reorientation motion of water molecules should be a significant factor to govern the electrical conductivity in these semiclathrate hydrates, the present results reveal that the difference between the electrical conductivities in three TBA-halide semiclathrate hydrates would be caused by the concentration of the proton, a conduction carrier, rather than the diffusion processes. Additionally, electrical conductivity in the single-crystalline TBA-hydroxide semiclathrate hydrate was measured. The electrical conductivity even in single crystals was much higher than those in the TBA-halide semiclathrate hydrates.

The dynamics of the water molecules in tetra-n-butyl-d36-ammonium bromide semiclathrate hydrate was investigated by quasi-elastic neutron scattering (QENS). The QENS results clearly revealed a fast reorientation motion of water molecules in the temperature range of 212-278 K. The mean jump distance of hydrogen atoms was within 1.5-2.0 Å. The relaxation time of water reorientation was estimated to be 100-410 ps with an activation energy of 10.2 ± 5.8 kJ·mol−1. The activation energy was in good agreement with the cleavage energy of hydrogen bonds. Such a short relaxation time of water reorientation is possibly due to strong interaction between a bromide anion and its surrounding water molecules (similar to so-called negative hydration), which suggests a unique strategy for designing efficient, safe, and inexpensive proton conductors having the framework of semiclathrate hydrates. © 2023 Author(s).

Semiclathrate hydrate (SCH) has been expected to be one of the phase change materials suitable for cold energy storage. In the present study, the equilibrium temperature, dissociation enthalpy, and crystal structure of tetra-n-butylammonium(TBA)-Tar, tetra-n-butylphosphonium(TBP)-Tar, and TBA-Mala SCHs were investigated, where the hydroxycarboxylate anions were malate (-Mala) and tartarate (-Tar). The maximum equilibrium temperature of each SCH system at the stoichiometric composition was (279.54 ± 0.05) K for TBA-Tar SCH, (275.44 ± 0.05) K for TBP-Tar SCH, and (278.69 ± 0.05) K for TBP-Mala SCH at atmospheric pressure. Each dissociation enthalpy was (172 ± 3) kJ·kg-1 for TBA-Tar SCH, (161 ± 3) kJ·kg-1 for TBP-Tar SCH, and (172 ± 3) kJ·kg-1 for TBP-Mala SCH. The SCHs with hydroxy group(s) in the anion of onium salts are more environmentally friendly than the SCHs with a halide anion. The environmentally friendly SCHs measured in this study could be used as refrigerants in the logistics of vegetables and fruits because the equilibrium temperature is suitable to preserve their freshness. © 2023 American Chemical Society.

Ionic clathrate hydrates (ICHs) have been applied to thermal storage materials, which can be utilized as unusual thermal energy sources. In the present study, we developed a novel ICH including tri-n-butyl(cyclobutylmethyl)phosphonium bromide (P444(1c4)-Br) as an unprecedented guest species. The highest equilibrium temperature and the dissociation enthalpy of P444(1c4)-Br ICH were 279.5 +/- 0.1 K and 202 +/- 2 J g(-1), respectively, at the mole fraction of 0.0255 +/- 0.0008. The highest equilibrium temperature of P444(1c4)-Br ICH was slightly lower than that of the tetra-n-butylphosphonium bromide (P4444-Br) ICH. The dissociation enthalpy of P444(1c4)-Br ICH was larger than that of other ICHs with similar highest equilibrium temperatures. Cyclic hydrocarbon groups, together with normal, branched, saturated, and unsaturated hydrocarbon chains, can be one of the options for tuning the equilibrium temperature of ICHs.

Semiclathrate hydrates (SCHs) have received attention as thermal storage materials because of their equilibrium temperature and relatively large dissociation enthalpy. The alkyl length of quaternary onium cations significantly affects the equilibrium temperature and dissociation enthalpy of quaternary onium salt SCHs. To investigate the effects of the double bond introduced in onium cations, in the present study, the thermodynamic stability of tri-n-butylalkenylphosphonium bromide (P444(alkenyl)-Br: alkenyl = 3-butenyl (3.1) or 4-pentenyl (4.1)) SCH was investigated. The equilibrium temperatures and the dissociation enthalpies were determined to be (277.5 +/- 0.1) K and (199 +/- 2) J/g for P444(3.1)-Br SCH, and (274.8 +/- 0.1) K and (188 +/- 2) J/g for P444(4.1)-Br SCH. The equilibrium temperatures of P444(3.1)-Br SCH and P444(4. 1)-Br SCH were lower than those of the corresponding normal alkyl SCHs (P4444-Br and P4445-Br), although the equilibrium temperature of tri-n-butylallylphosphonium bromide SCH was higher than that of the P4443-Br SCH.