Probing the secrets of hydrogen bonding in organic salt phase change materials: the origins of a high enthalpy of fusion

Abstract

The development of new phase change materials that can store large amounts of renewable thermal energy will aid the decarbonisation of the energy sector that is crucial for future generations. For substantial development of these materials, understanding the relationships that underpin the enthalpy of fusion (ΔH_f) is critical. In the present work, the role of hydrogen bonding in the energy storage mechanism is elucidated by single-crystal X-ray diffraction and Hirshfeld surface analyses on a range of novel and known protic organic salts based on the guanidinium, acetamidinium, and formamidinium cations. Bonding motifs and strengths are probed through the steric and electronic properties of the anions, specifically with the use of the methanesulfonate ([CH₃SO₃]⁻), trifluoromethanesulfonate ([CF₃SO₃]⁻), para-toluenesulfonate ([C₇H₇SO₃]⁻), trifluoroacetate ([CF₃COO]⁻) and chloride (Cl⁻) anions. We consider entropic contributions to melting thermodynamics, and the results highlight the importance of strong hydrogen bonds in the solid state, finding an increased density of hydrogen bonds can be detrimental to ΔH_f if the result is non-geometrically favoured or bifurcated interactions. The results propose a new rationale for the design of these systems through matching the number of hydrogen bond donor and acceptor sites. As well as increasing the linearity and strength of hydrogen bonds, such matching aids in the formation of predictable, energetically favourable supramolecular motifs, that contribute to ΔH_fvia the energetic cost of their dislocation upon melting. These insights should inform the design of future high-performance phase change materials that can efficiently store sustainable energy, helping to overcome the intermittency issues typically associated with renewable energy systems.