Understanding the molecular dynamics of water sorption into barocaloric MIL-53(Cr) using quasielastic neutron scattering

Barocaloric (BC) materials offer the opportunity to enable solid-state heating and cooling technologies, replacing harmful and high global warming potential vapor refrigerants. These materials undergo significant temperature changes under applied hydrostatic pressure without the risk of leakage into the atmosphere. Molecular organic frameworks (MOFs) have recently demonstrated so called ‘colossal’ BC effects due to the extreme volume changes upon sorption of CO21,2. However, liquid sorption would be preferred to improve thermal conductivity and reduce the volume of material required. We are currently building on these results by exploring the BC effects related to liquid water sorption in the MOF MIL-53(Cr). We have recently measured large BC effects in MIL-53(Cr) due to water sorption for the first time using synchrotron powder x-ray diffraction. Moreover, our variable temperature and pressure x-ray diffraction experiments have shown that MIL-53(Cr) exhibits structural changes that have not been observed previously for gaseous sorption processes. Since the molecular dynamics of this complex system aren’t yet well understood we propose to use quasielastic neutron scattering (QENS) to study the rotation and diffusion processes of the water molecules as a function of temperature as the MOF undergoes structural changes. This will provide crucial insights into the molecular dynamics of the sorption driven breathing transitions in MIL-53(Cr) that govern its BC performance.