HIGHLIGHTS
• Ionocaloric materials can provide refrigeration through using ions to generate solid-to-liquid phase transitions.
• Ethylene carbonate was used as the solid, and sodium iodide was the source of ions in initial work that demonstrated the ionocaloric effect. 
• Electrodialysis was found to be the best current option for facilitating ion separation to enable the liquid to transition back to a solid so that the cycle can continue. 

The transfer of heat from a low temperature to a high temperature environment has been the basis for refrigeration, which assumes great importance in applications such as air conditioning. The vapor compression method utilized in refrigeration has been very effective but has relied on liquid-based refrigerants that exhibit various environmental concerns. 

Since the onset of the 1987 Montreal Protocol, a transition has taken place in employing liquid refrigerants from chlorofluorcarbons to the currently used hydrofluorocarbons. Over the past 36 years, this has reduced stress on the atmospheric ozone layer. Still, the current refrigerants exhibit detrimental environmental characteristics including high global warming potentials. 

Research has been underway to identify materials that can facilitate refrigeration through an alternative process known as the caloric effect. Drew Lilley, a graduate research assistant at the Lawrence Berkeley National Laboratory and a doctoral candidate at the University of California, Berkeley, in Berkeley, Calif., says, “The caloric effect involves triggering a phase transition through the use of an external stimulus. Solid-state caloric materials have been under evaluation through the use of external electric and magnetic fields. The former is known as the electrocaloric effect while the latter is known as the magnetocaloric effect.” 

While refrigeration lubricants have been an important component in systems with liquid refrigerants, they are not required for use with caloric materials. 

In a previous TLT article,¹ a new caloric material based on nickel and titanium was produced through a three-dimensional printing process. This metal alloy exhibits a solid-to-solid phase transition characteristic of shape memory alloys that can be used in refrigeration. This phenomenon is known as elastocaloric effect. Two phases with different stoichiometric ratios of nickel to titanium were identified in the metal alloy. The cooling efficiency of this material increased by a factor of four to seven over previously developed elastocaloric species and the nickel, titanium alloy was found to be highly durable. 

Solid-to-liquid phase transitions can be induced through application of an electrochemical field to a specific solid material. Lilley says, “This phenomenon changes the atomic structure of the system with the caloric response based on a changing ionic environment.” 

Lilley and his colleague Ravi Prasher, associate lab director of the Energy Technologies Area and senior scientist at Lawrence Berkeley National Laboratory and adjunct professor in the department of mechanical engineering at the University of California, Berkeley, have identified a new caloric effect through the use of ions that trigger solid-to-liquid phase transitions in a technique known as the ionocaloric effect. 

Ethylene carbonate/sodium iodide 
The researchers take advantage of the ability of a salt to depress the melting point of a solid followed by melting the solid at the lower melting point, separating the salt from the newly formed liquid, and then crystalizing the solid at the higher melting point. Lilley provides details: “We start by identifying a system where addition of a salt to a solid will induce a reduction in melting point. This is similar to what a salt such as sodium chloride does to reduce the melting point of ice. The presence of salt changes the electrochemical potential of the system and leads to a transition from a low energy to a high energy state by absorbing heat from the environment to melt the solid to a liquid. At this stage, the system is acting to cool a specific environment in an identical manner to a refrigerator.” 

Now that the cooling step has taken place, the next step is to complete the cycle by removing the salt so that the liquid can undergo the reverse phase transition back to a solid. Lilley continues, “By removing the salt, the system moves from a high to a low energy state leading to a release of heat to the environment, which will prompt the liquid to become a solid. The cycle is then ready to start again.” 

Figure 1 shows the back-and-forth nature of the solid-to-liquid phase transition with ions present outside of the solid material on the left and the ions present in the melted solid on the right. 


Figure 1. The ionocaloric refrigeration cycle starts with addition of ions to a solid (on the left) enabling a phase transition to a liquid (on the right) through heat absorption, which cools the environment. To complete the cycle, the ions are separated from the liquid allowing for a transition back to the solid (on the left) with the release of heat. Figure courtesy of Lawrence Berkeley National Laboratory.

The ideal ionocaloric material should have a high latent heat of melting and demonstrate durability in absorbing energy and releasing energy for multiple cycles, according to Lilley. He adds, “The material must have a high enthalpy of fusion, low specific heat capacity, a melting point above room temperature and good compatibility with a salt.” 

The researchers selected ethylene carbonate for these reasons and decided to utilize sodium iodide as the salt. Ethylene carbonate has a melting point of approximately 35 C. When mixed with 23% sodium iodide by weight, the melting point declines to approximately 6 C. 

The challenge for the researchers was to find an efficient manner to separate sodium iodide from ethylene carbonate. Lilley says, “We evaluated a number of techniques including thermal separations and reverse osmosis. Thermal separations are inefficient and, in the case of reverse osmosis, the pumps used are not readily compatible with organic solvents.” 

Electrodialysis, or the separation of ions through the application of an electric field, was found to be the best option. Lilley says, “One of the reasons sodium iodide is used as the salt to facilitate electrodialysis is to take advantage of the iodide-triodide redox couple.” 

Future work will involve identifying a better separation technique and a better ionocaloric material than ethylene carbonate. Additional information can be found in a recent article² or by contacting Prasher at rsprasher@lbl.gov or Lilley at dililley@lbl.gov. 

REFERENCES
1. Canter, N. (2020), “3D Printing of elastocaloric materials for refrigeration,” TLT 76 (4), pp. 16-17. Available here. 
2. Lilley, D. and Prasher, R. (2022), “Ionocaloric refrigeration cycle,” Science, 378 (6626), pp. 1344-1347.