Shape-shifting MOF works wonders for heating and cooling across climates

A metal–organic framework (MOF) is capable of cooling and heating without external electrical inputs. Additionally, the material works well across a wide range of temperatures, which could contribute to greener thermal solutions across climates.¹

In 2022, researchers at the University of A Coruña in Spain, discovered the shape shifting properties of a particular MOF during adsorption and desorption of carbon dioxide triggered temperature changes. Now, subsequent studies on the same material have unveiled exciting new applications. ‘While previously a swinging pressure produced the effect, our new approach maintains a static pressure of carbon dioxide,’ explains lead author Juan Manuel Bermúdez-García. In this ‘reversed’ system, the external temperature triggers adsorption and desorption of carbon dioxide, making the MOF a self-regulating solution to store heat. ‘During the day, as temperature rises, carbon dioxide is desorbed, which counterintuitively cools down the MOF, and could cool down our buildings,’ says Bermúdez-García. At night, the opposite process pushes the pores to readsorb carbon dioxide, heating the house back up.

‘This is a novel way to think about using these materials,’ says Claire Hobday, an expert in solid state refrigeration at the University of Edinburgh, UK, who wasn’t involved in the study. At a fixed pressure, heating up the MOF ‘causes a cooling effect’ because it experiences ‘an endothermic phase transition’, she explains. Interestingly, researchers can change ‘the charge and discharge temperatures’ by fluctuating the fixed gas pressure. Therefore, ‘the same material [could] adapt to different heating and cooling needs’, says Hobday.

Traditionally, the transition temperature of thermal materials is predetermined by the energy requirements of the phase change. However, these shape shifting MOFs ‘are extremely pressure sensitive’, says Bermúdez-García. ‘Small shifts in the static pressure of CO₂ – between 5 and 25 bar – already alter the transition temperature dramatically, from –30ºC to 120ºC,’ he adds. In contrast, current technologies only achieved ranges of 40ºC, with even bigger boosts of pressurisation up to 1000 bar – the pressure at the deepest parts of our oceans, explains Bermúdez-García. ‘This solves a significant challenge in thermal storage,² adaptability across climates, which previously required a myriad of materials.’

The MOF would work ‘in sunny Spain or rainy Scotland’, says Hobday. ‘It’s exciting for the field.’ The material also allows for interesting scenarios, playing with pressure and phase changes. For example, this MOF can absorb heat at low temperatures and releases it in hot weather. This is ‘not breaking any thermodynamic laws, [but demonstrates] a very thorough understanding’ of the different phase transitions, explains Hobday. Once the carbon dioxide is inside the MOF, the shape-shifting process at low pressure results in the absorption of heat. Then, pressurising the system, the transition temperature increases, making the MOF release heat at higher temperatures, a process potentially interesting for energy efficient industrial applications. ‘Absorbing and storing heat at low temperatures is easier and cheaper,’ says Bermúdez-García. ‘We could compress carbon dioxide in our MOF to release heat at high temperatures, which is usually costly.’

Finally, the team from A Coruña also adapted a mathematical model to screen published papers and databases for cooling applications. This theoretical thermodynamic framework predicts properties such as pressure, the transition temperatures and the magnitude of the MOF capacity for heat exchange. In the US, the National Institute of Standards and Technology has created a curated catalogue of MOF adsorption isotherms, explains Hobday. Together with this simulation model, these resources could become ‘essential to [the] discovery of novel shape-shifting MOFs with refrigerant potential’, she notes.