New Electrolytic Media and Methods for Energy Storage and Conversion

Citation

McNicholas, Brendon James (2020) New Electrolytic Media and Methods for Energy Storage and Conversion. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1sew-r360. https://resolver.caltech.edu/CaltechTHESIS:06082020-143301441

Abstract

New electrolytic media and methods for energy storage and conversion are needed to fully realize the sustained use of renewable energy and complete removal of dependence on fossil fuels. Motivated by this urgency, researchers today are heavily invested in developing new electrocatalytic systems for carbon dioxide sequestration to reduce greenhouse gas emissions and new battery architectures, such as non-aqueous redox flow batteries, to keep solar energy at our disposal during peak times of energy consumption. Chapter 1 provides an overview of the conceptual frameworks for these two alternative energy technologies and a literature review of relevant work and inspiration in these fields. Chapter 2 demonstrates one of the first examples of ionic liquid voltammetry of a molecular species, namely (tpfc)Mn, and its electron transfer reactivity in ionic liquids with varying solvent viscosity, effective electrolyte concentration, and donor strength. As well as showing the non-unity diffusional properties of molecular species in ionic liquids and the capability of ionic liquid anions to coordinate to molecular species, these studies suggest the viability of ionic liquids as conductive media for energy storage and conversion. Chapter 3 introduces methods for immobilization of molecular catalysts in polymeric ion gels, a general strategy that bridges the divide between homogeneous solution-state catalysis and heterogeneous solid-state catalysis. These results provide insight into how environment, catalyst concentration, catalyst mobility, substrate availability, and dielectric properties of a medium all affect the catalytic response and overpotential for CO2 reduction. Further implementation of these ion gel composites in solid-state devices, in aqueous environments, and in gas diffusion electrodes is also discussed. In Chapter 4, a brief overview of the use of boranes as capping ligands for cyanide is provided. The synthesis, electronic properties, and theoretical calculations of homoleptic, boronated Fe(II) hexacyanoferrates are reported. Addition of borane to cyanometallates dramatically alters electronic structures and is a novel method for permanent modification of formal potentials while simultaneously maintaining or improving electrochemical reversibility and ambient stability. These complexes are characterized and studied by cyclic and differential pulse voltammetry, UV-vis, IR, and Raman spectroscopy, and flash-quench photolysis. Chapter 5 extends the unique reactivity of borane adducts to the characterization of a full series of hexaisocyanoboratometallates (Cr, Mn, Fe, Ru, Os), compounds which demonstrate the concept of cyanide as a “variable-field” ligand, including magnetic circular dichroism spectroscopy, electron paramagnetic resonance spectroscopy, luminescence studies, excited-state lifetime studies, and electrochemistry. As electrolytes for non-aqueous redox flow batteries, these species exhibit excellent Coulombic and voltage efficiencies and fast electron transfer rates. The highly oxidizing species will also find use as reversible oxidants for chemical oxidations. Chapter 6 extends the concept of modifying formal potentials to heteroleptic cyanometallates (M = Fe, Ru) with diimine ligands (L = bipyridine, phenanthroline, 4,4’-trifluoromethylbipyridine). These species are shown to be potent excited-state reductants and oxidants, strong and long-lived phosphors, and promising electrolytes for symmetric, non-aqueous redox flow batteries. These data also demonstrate improved excited-state lifetimes for borane-appended species, likely due to inhibition of non-radiative decay pathways. Chapter 7 focuses on the generation of a solution stable, square pyramidal Co(II) species, which is studied by electrochemistry, UV-vis-NIR spectroscopy, X-band and Q-band CW EPR, and pulsed EPR techniques (HYSCORE, ENDOR). These studies demonstrate that boronation of cyanide differentially affects the energies of ligand field transitions based on π backbonding ability.

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