Combining Organic and Inorganic Redox-Activity: Porphyrin-Based Metal-Organic Frameworks As Electrode Material

Abstract

Active cathode materials in lithium ion batteries are often based on transition metals like cobalt or nickel. High cost and toxicity as well as the mining and fabrication under ethically questionable conditions are serious drawbacks of these metals used in state-of-the-art materials. On the route towards a “greener” and more sustainable lithium ion technology, alternatives such as organic and hybrid inorganic/organic active materials obtained increasing attraction in the last years. [1] One promising hybrid material class are metal-organic frameworks (MOFs). Due to their high surface area and well-defined porosity as well as their flexible and designable structure MOFs have received increasing attention over the last decades for different potential applications like gas storage or catalysis. More recently, this promising class of materials has also been investigated for energy storage devices as electrode active material. [2] MOFs consist of inorganic metal-oxo clusters (called secondary building unit, SBU in short) coordinated to multivalent rigid organic molecules (often referred as “linker”), which can be modified with a variety of functional groups forming a crystalline porous structure. The well-defined pore size is tailorable by the length of the linker molecules leading to a high flexibility accessible for reversible insertion and removal of guest molecules. The highly porous crystalline structure, especially their large pore size, make them interesting for reversible cation or anion storage, e.g. in the dual-ion battery concept. [3] Furthermore, multivalent metal ions in the SBU as well as organic linker molecules can act as redox-active sites, leading to a promising active material. [4] Porphyrin-based organic derivates, which occur. e.g. in human blood and vitamin B12, are well known for their catalytic- and redox-activity. In the present work, we synthesize various porphyrin-based MOFs with different coordinated metals, which are successfully applied as an energy storage material in a lithium metal cell and characterized with respect to the structural and surface properties. Combining the redox-active porphyrin derivate, Tetrakis(4-carboxyphenyl)porphyrin (TCPP), and a redox-active metal(oxo-)cluster, a non-toxic and environmentally friendly cathode material was achieved. Constant current cycling and cyclic voltammetry studies reveal a high and reversible redox activity. Using suitable methods such as X-ray diffraction (XRD), the redox reaction behavior of the metal-organic framework and the structural properties of the MOFs were investigated upon charge/discharge operation. Furthermore, the influence of different conductive salts and solvents on the electrochemical performance were analyzed.