Shaping Electronic Properties of Metal-Organic Frameworks for Energy Storage Applications
Authors: Klokic Sumea , Marmiroli Benedetta , Birarda Giovanni , Dal Zilio Simone , Vaccari Lisa , Amenitsch Heinz 
Published: December 2024

Metal-Organic Frameworks (MOFs) form through self-assembly between metal ions or clusters with multifunctional organic ligands carrying functional groups, resulting in intricate connected structures. Each constituent, whether metal nodes, connecting functional groups, or bridging organic ligands, imparts specific physical and chemical properties to the MOF, including band energy, charge transport characteristics, and photophysical behaviour. In addition, the crystalline structure of MOF defines the exciton transport and dynamic properties, [1] which are critical to evaluate in order to enhance the system’s performance when employed as electrode material or in storage applications including photonic, mechanical or thermal energy conversion. Hence, understanding the dynamic transport properties in MOFs is vital in order to develop real-world industrial-scale applications, where MOF film structures are particularly desired mainly because the pulverous nature of MOFs presents challenges, such as the gradual loss of active material that can undermine performance and conversion efficiencies.

Our research aims to unravel the dynamic processes involved during photon- and mechanical energy storage in both, powdered MOFs and their film counterparts [2,3] for which an automated film-fabrication protocol has been developed (Figure 1). [4] We reveal that the efficiency and quantity of stored energy is intricately linked to factors including as the crystallite morphology and chemical composition. To elucidate these complex interdependencies and determine the timescales of dynamic processes during energy storage in crystalline MOFs, we utilize a diverse set of techniques available at facilities like the Elettra synchrotron and the free-electron laser FERMI in Italy. These techniques include time-resolved X-ray scattering, infrared and terahertz spectroscopy. Results show that the energy storage process of MOF powders can be accelerated from hours (>5 h) up to seconds (~6 sec) upon their fabrication as film structures, which additionally guarantee a fully reversible storage/release process initiated remotely. [2,3] Moreover, we show that the storage process rate can be tailored by adjusting the MOF's electronic properties through variations in chemical composition and thickness, [3] with implications for electrode materials and gas sequestration applications. [5]

These findings provided valuable insights into energy storage dynamics in MOF systems, paving the way for a deeper understanding of similar or more advanced film systems and offering new avenues for research in this field.

[1] Dincă M. and François Léonard, MRS Bulletin, 2016,
41, 854–857.

[2] Klokic, S., Naumenko, D., Marmiroli B., Carraro, F., Linares Moreau, M., Dal Zilio, S., Birarda, G., Kargl, R., Falcaro, P., Amenitsch, H., Sci., 2022, 13, 11869-11877.

[3] Klokic, S., Marmiroli B., Naumenko, D., Birarda, G., Dal Zilio, S., Velasquez-Hernandez, M.-J., Falcaro, P., Vaccari, L., Amenitsch, H., accepted in CrystEngComm, 2024.

[4] Linares‐Moreau, M.; Brandner, L.; Kamencek, T.; Klokic, S.; Carraro, F.; Okada, K.; Takahashi, M.; Zojer, E.; Doonan, C.; Falcaro, P. Mater. Interfaces 2021, 8 (21), 2101039.

[5] Klokic, S., Marmiroli B., Birarda, G., Holzer, P., Sartori, B., Asbaghi-NA, B., Dal Zilio, S., Vaccari, L., Amenitsch, H., submitted, April 2024.

Figure 1