Tetra-<i>n</i>-butylammonium Tricarboxylate Semiclathrate Hydrates as Phase Change Materials: Phase Equilibrium Relations and Memory Effect in Reformation

Semiclathrate hydrates (SCHs), one of the phase change materials for cold energy storage, are crystalline compounds consisting of host water molecules and appropriate guest substances. The typical guest substances of SCHs are tetra-n-butylammonium (TBA) salts. In the present study, we investigated the phase equilibrium relations of (TBA)3 tricarboxylate SCHs, where citrate (−Cit) and 1,2,3-propanetricarboxylate (−123Pro) were used as environmentally friendly tricarboxylate anions. The maximum equilibrium temperatures of (TBA)3-Cit and (TBA)3-123Pro SCHs, whose phase diagrams exhibited a congruent-type phase behavior, were 286.02 ± 0.05 and 286.34 ± 0.05 K, respectively. Their dissociation enthalpies were 186 ± 3 and 193 ± 3 kJ/kg, respectively, which are as large as that of TBA-bromide SCH that has been put to practical use in cold energy storage applications. As an index for suppressing the degree of supercooling in SCH reformation, we have focused on the memory effect. The ability to retain the memory effect in the (TBA)3-Cit and (TBA)3-123Pro SCHs was relatively large. The (TBA)3-Cit and (TBA)3-123Pro SCHs would be superior phase change materials for cold energy storage.