Organometallic redox-interfaces for selective electrochemical separations

Abstract

Electrochemical separation methods are promising due to their modularity, fast kinetics and potential integration with renewable sources. However, they are still limited in application due to high energetic costs and lack of chemical selectivity. This work explores redox-electrodes as a platform for targeting aqueous and organic contaminants with high separation factors, in the contexts of environmental water remediation, chemical product purification in organic synthesis, metal-recovery and bio-separations. The design of selective stimuli-responsive interfaces is a crucial challenge for advanced electrochemical processes. Whereas redox-electrodes are well known in sensing, catalysis and energy storage, here we focus on their unique potential for selective ion removal - cases in which one dilute compound is targeted in the presence of large excess of competing electrolyte. In particular, organometallics and associated metalcomplexes offer an attractive material platform, due to their flexible metal-ligand design and as a consequence, extensive control allowed of their electronic properties. The first major thrust is the molecular design of various organometallic species for specific interactions with charged compounds in solution. We developed a series of heterogeneous, nano-structured metallocene interfaces to control the selective sorption and release of anions, cations, and even proteins, based on electrochemical potential. In parallel, through a combination of electronic structure calculations and spectroscopy, we unraveled the unique binding mechanism between ferrocenium and organic ions demonstrating an unusual redox-mediated hydrogen-bonding between cyclopentadienyl and carboxylates; and utilize this knowledge to further tune our redox-systems to enhance chemical selectivity. We expanded our organometallic set to various bi-pyridines and functionalized metallocenes, and studied various problems ranging from reactive separations to catalytic remediation of contaminants of emerging concern. A second major thrust consists in utilizing asymmetric pseudo capacitors as the next generation configuration for electrochemical separation devices. Asymmetric systems were shown to have much higher energy storage capabilities as well as separation efficiencies. We focused on counter-electrode design, in which the redox reaction at the cathode works in tandem with the anode, thus maintaining the water chemistry by suppressing parasitic reactions which otherwise lower current efficiency. From a fundamental perspective, the novel interaction mechanisms explored in this thesis were shown to have broader implications in deionization, sensing, catalysis and energy storage. For chemical engineering, this work demonstrated redox-based electrochemical methods as an energy-efficient and sustainable route to process intensification, and paved their way for practical implementation in industry.