Overcoming Hysteresis in Barocaloric Materials

From air conditioning to refrigerators to heat humps, cooling technologies have played critical roles in enabling access to improved living conditions. Yet, ironically, the very technologies that helped us beat the heat are now making the planet warmer. Current cooling systems rely on volatile hydrofluorocarbon refrigerants, the majority of which are potent greenhouse gases. As demand for cooling continues to increase, the direct emission of volatile refrigerants is predicted to dramatically increase—well above 10% of greenhouse gas emissions by 2050. In the face of an increasingly warming world, new technologies are urgently needed to provide sustainable cooling. Solid-state cooling through barocaloric effects—pressure-induced thermal changes in a material—offers an environmentally-friendly alternative to conventional cooling technologies. Unlike gaseous refrigerants, solid refrigerants have zero global warming potential as well as a variety of potential advantages, including increased efficiency, reduced system size, and greater recyclability. Importantly, barocaloric materials are particularly well suited to providing cooling at scale. This is, in part, because pressure is easy to generate, can be applied to materials regardless of their form factor, and, due to the dominance of vapor-compression technology, extremely amenable to scaling.<br/><br/>To produce useful barocaloric effects at low pressures, phase transitions that feature large entropy changes, high sensitivity to pressure, and minimal hysteresis are needed. In particular, hysteresis—which represents the dissipated energy during a pressure-driven phase change—plays a central role in determining the cooling performance. In addition to increasing the operating pressure, large hysteresis compromises efficiency, cooling power, and cyclability. Despite the range of barocaloric materials that have been studied to date, it has proven challenging to manipulate—and understand—hysteresis in barocaloric materials. Because existing classes of barocaloric materials are often difficult to manipulate synthetically in a systematic fashion, it is challenging to establish structure–property relationships needed to inform efforts to overcome hysteresis. As such, strategies to reduce—and, ideally, eliminate—hysteresis in barocaloric materials have yet to be developed.<br/><br/>Here, we present systematic studies on hysteresis in barocaloric materials. We leverage the new classes of tunable barocaloric materials recently discovered in our laboratory—including two-dimensional hybrid perovskites [1] and molecular spin-crossover complexes [2]—to elucidate key molecular factors that govern the hysteresis of pressure-driven transitions. In particular, we characterize the compositionally diverse library of barocaloric materials using advanced calorimetry and operando X-ray diffraction techniques, as well as direct temperature measurements under variable-pressure conditions. These efforts are complemented by molecular modeling and microstructural analyses. Finally, we describe our efforts to understand the impact of hysteresis at the device level. By evaluating the performance of select barocaloric materials in our recently developed cooling prototype, we directly probe materials losses—especially those that originate from hysteresis—in a wide range of driving conditions. Bridging the gap between materials discovery and prototype development, this work represents a crucial step toward the development of barocaloric materials for practical solid-state cooling. We envision that our approach to understand barocaloric effects across length scales—ranging from molecular to device levels—will provide new insights and opportunities for studying phase transitions and thermal behaviors in materials.<br/><br/>[1] <i>Nature Communications</i> <b>2022</b>, <i>13</i>, 2536.<br/>[2] <i>J. Am. Chem. Soc. </i><b>2022</b>,<i> 144</i>, 6493.