Claire Hobday1
University of Edinburgh1
Heating and cooling processes are responsible for 78% of the UK’s environmentally damaging fluorinated gas emissions, primarily due to the employment of hydrofluorocarbon (HFC)-based refrigerant gases. [1] Such compounds are being phased out due to their devastating environmental impact, notably very high global warming potentials (GWP) (~2000 times that of CO<sub>2</sub>). This has created a major technological and scientific challenge to find new types of refrigeration materials which are made from sustainable resources, have increased efficiency and are environmentally friendly throughout their entire lifecycle. There is now a strong focus on developing solid-state materials, which are easier to recycle, that demonstrate caloric effects. These have been shown to out-perform vapour compression technology in domestic refrigeration, additionally resulting in lower energy costs. [2] Caloric effects rely on the reversible thermal response of solids to an externally applied field (magnetic or electric field, or hydrostatic pressure) to give rise to magnetocaloric (MC), electrocaloric (EC) or barocaloric (BC) materials.<br/><br/>This talk focusses on understanding the barocaloric effect via study of order-disorder transitions in organic ionic plastic crystals (OIPCs). OIPCs provide a rich parameter space in which to explore the tunability of the BC effect experimentally. This class of materials are comprised completely of ions and at room temperature are solids with significant disorder in the crystal lattice. [3] The disorder comes from the rotational, translational and conformational motions which allows them to flow under stress and increases their conductivity, which has rendered them desirable solid-state electrolytes that can be used in batteries, fuel cells and solar cells. [4] OIPC’s inherent order-disorder phase change properties are ideal as potential candidates for BC refrigerants; however, little is known about how to maximise their ionic conductivity, ability to flow and change phase.<br/><br/>In this work combined we explore the expansive chemical parameter space of ionic plastic crystals via a combined simulation and experimental approach. By employing pressure- and temperature-dependent single crystal/powder diffraction and differential scanning calorimetry experiments, we can understand structurally and thermally how these materials behave and by ion substitution, we demonstrate the ability to tune the transition temperature, isothermal entropy change, Δ<i>S,</i> and the barocaloric coefficient, <i>d</i>T/<i>d</i>P. Molecular dynamics simulations provide complementary information into the atomistic details of the complex landscape of the phase behaviour of these materials. Together, the results of this work can be used to inform the design of future solid-state refrigerants.<br/><br/><br/>[1] https://www.theccc.org.uk/publication/assessment-of-potential-to-reduce-uk-f-gas-emissions- ricardo-and-gluckman-consulting, (accessed 11th November 2022).<br/>[2] C. Aprea A. Greco, A. Maiorino, C.Masselli, <i>Energy</i>, <b>2020</b>, 190, 116404.<br/>[3] D. R. MacFarlane J. Huang, M. Forsyth, <i>Nature</i>, <b>1999</b>, 402, 792.<br/>[4] D. R. MacFarlane, M. Forsyth, P. C. Howlett, M. Kar, S. Passerini, J. M. Pringle, H. Ohno, M. Watanabe, F. Yan, W. Zheng, S. Zhang, J. Zhang, <i>Nat. Rev. Mater.</i>, <b>2016</b>, 1, 15005.