Sustainable Energy Harvesting by Employing Biocompatible Ferroelectric Molecular Cu(II) Complexes

To address the escalating global energy consumption, resulting in swift depletion of non-renewable fossil fuels, it is imperative to explore alternative approaches. A smart way to tackel this problem is harnessing abundant, renewable but otherwise wasted environmental energies (vibrations, pressure, heat, solar, tidal) and converting them into useful electrical energy, with the help of the self-powered nanogenerators (SPNGs). Researchers have employed pure inorganic materials like bulk oxides (perovskites and ceramics), polymers, organic molecules, even peptides and organic-inorganic hybrid structures for this purpose. [1] Among these, the oxide systems show excellent piezo and ferroelectric as well as energy harvesting properties but they suffer from huge disadvantages like costly high temperature synthesis methods, almost zero mechanical flexibility, tunability of properties in the long-range and of course bio-incompatibility due to the presence of toxic heavy metals. In case of polymers or peptides, stabilizing the polar ferroelectric state which is required for the application purpose (for example β-state of PVDF) is an extremely daunting task. Alternatively, hybrid organic-inorganic perovskite systems offer low temperature and easy processing method, as well as great output performances but these systems also suffer from a number of problems, including the stability of reactive metal halide bonds under aerobic conditions and moisture and the presence of toxic heavy metal ions like lead (Pb). [2] By careful molecular engineering and controlled design strategy, we have overcome almost all of the above-mentioned caveats by employing bio-compatible discrete molecular complexes, which is an active area of research in the current scenario. They are light weight, mechanically flexible and easily polarizable in presence of an electric field, making them perfectly suitable to use in various energy harvesting nanogenerators. The Cu(II) metal based molecular complexes synthesized by us not only provide high values of piezoelectric co-efficients (d₃₃= 10-30 pm/V), comparable to the popular bulk materials like LiNBO₃, ZnO and polymers like PVDF, PVDF-TrFE etc., but also has appreciable spontaneous polarization values as well, leading to appreciable energy harvesting capabilities in both single-crystal and composite forms. We have shown to obtain a high value of output voltage of 8 V, power density of 0.85 μW/cm² and an output current of 5 μA, with a discrete molecular ferroelectric complex which is used in the form of a flexible composite film with a non-polar polymer matrix polyvinylalcohol (PVA), and . [3] Our efforts successfully afford an efficient route of energy harvesting which can be modified for futuristic applications. On the other hand, an open circuit output voltage of (7.4 V/cm²) and an appreciable pyroelectric coefficient of 29 µC/m²K was obtained by a single-crystal based device made by us using another biocompatible molecular complex. In contrast to the single crystals of conventional bulk oxides, single crystals of the discrete molecular complexes are easier to synthesize and can be envisaged for various other applications such as anisotropic sensors, ultrasound transducers, and acoustic devices so as to avoid multidirectional mechanical and heat flux fluctuation aided artifacts. This opens up a lot of possibilities for molecular complexes in real-world applications.<br/><br/>References:<br/>[1] C. R. Bowen, H. A. Kim, P. M. Weaver and S. Dunn, <i>Energy Environ. Sci.</i>, 2014, <b>7</b>, 25–44.<br/>[2] T. Vijayakanth, D. J. Liptrot, E. Gazit, R. Boomishankar and C. R. Bowen, <i>Adv. Funct. Mater.</i>, 2022, <b>32</b>, 2109492<br/>[3] R. Haldar, A. Kumar, B. Mallick, S. Ganguly, D. Mandal and M. Shanmugam 2023, <b>62</b>, 202216680