Enclathration of 2,3,3,3-tetrafluoropropene (HFO-1234yf) within structure II hydrates in the presence of CO2: Implications for cold energy storage

Abstract

Refrigerant hydrates are promising phase change materials for cold energy storage systems due to their high heat of fusion and suitable temperature ranges. This study was conducted to examine the enclathration of 2,3,3,3-tetrafluoropropene (HFO-1234yf), an alternative to the conventional refrigerant 1,1,1,2-tetrafluoroethane (HFC134a), in the presence of CO2. The investigation focused on the phase equilibria, structural, and compositional features of HFO-1234yf + CO2 hydrates. The findings showed that the phase equilibrium curves of HFO-1234yf + CO2 hydrates occurred at higher temperatures and lower pressures than those of pure CO2 hydrate. These differences in the equilibrium curves became more pronounced as the HFO-1234yf concentration increased (from 10 % to 30 %). Powder X-ray diffraction (PXRD) analysis revealed that the HFO-1234yf + CO2 + water mixtures formed structure II (sII) hydrates with the Fd3m space group. Rietveld refinement of the PXRD patterns and Raman spectra demonstrated that HFO-1234yf molecules were encapsulated exclusively in the large (51264) cages of the sII hydrates, while CO2 molecules were predominantly captured in the small (512) cages. Furthermore, the dissociation enthalpy of the HFO-1234yf (10 %) + CO2 (90 %) hydrate was determined using a highpressure micro-differential scanning calorimeter and found to be 473.4 +/- 7.4 J/g-water (60.9 +/- 0.7 kJ/molguest). These findings provide in-depth insights into the potential use of HFO-1234yf + CO2 hydrates as cold energy storage media for air-conditioning systems.