Electrochemical energy storage and conversion using redox-active organic materials
Keywords: Engineering.
Chemical Engineering.

With increasing global demand for energy and continuing climate change, energy storage and conversion technologies are critical for the support of renewable electricity generation and for the sustainable production of fuels and chemicals. To meet these challenges, significant research has gone into developing battery materials to store energy generated from intermittent sources and power our increasingly electrified economy, and electrocatalysts, to produce green hydrogen and other fuels. Inorganic materials fill the majority these roles today: transition metal oxides containing metals such as nickel and cobalt are ubiquitous battery cathodes, state-of the art flow battery electrolytes employ dissolved vanadium species, and commercial electrocatalysts require precious metals, especially platinum and iridium, which face high costs and potential supply challenges. Recently, there has been increased interest in using organic materials for these applications due to several benefits. Organic materials are composed of earth-abundant elements and can potentially be produced from sustainable resources at low costs. Importantly, their structures are highly tunable via chemical synthesis, resulting in properties which rival those of their inorganic counterparts and can be tailored to suit particular applications. Due to these benefits, organic materials have seen increased exploration in fields including battery engineering and organic electronics. In this dissertation I investigate novel applications of organic materials in energy storage and conversion. Part one focuses on organic liquids, and we discuss the development of a high-energy density, solvent-free redox flow battery electrolyte, designed using eutectic mixing of small organic molecules. While reaching high concentrations of redox active species is generally challenging for flow battery chemistries due to poor solubility of redox active species, we show that, by depressing the melting point of a mixture of chemically similar redox-active species, we can engineer highly concentrated, low viscosity liquids composed almost entirely of redox active molecules. While the strategy of entropy-driven eutectic mixing is quite general, here we use quinones as a model system. We discover a ternary benzoquinone eutectic mixture and a binary naphthoquinone eutectic mixture which have theoretical redox-active electron concentrations of 16.8 and 8.8 M e-, respectively. We investigate compatibility with protic supporting electrolytes and quantify relevant flow battery properties, including ionic conductivity and viscosity, for quinone eutectic electrolytes across multiple states of charge, and find that we can simultaneously increase redox-active concentration and decrease viscosity through careful mixture design. We demonstrate proof-of-concept by cycling a binary naphthoquinone eutectic with a protic ionic liquid supporting electrolyte (7.1 M e-, theoretical volumetric capacity 188 Ah/L and achieve a volumetric capacity of 49 Ah/L in symmetric static cell cycling. These preliminary results suggest that entropy-driven eutectic mixing is a promising strategy for developing high energy density flow battery electrolytes. In part two, we focus on organic mixed ionic electronic conducting (OMIEC) polymers, and investigate the function of these materials as electrocatalysts, without binder or conductive additives. This work focuses on electrocatalytic transformations involving oxygen, which are at the core of many key electrochemical technologies including metal-air batteries, fuel cells, and electrolyzers. First we investigate p-type materials as potential catalysts for the oxygen evolution reaction, which is an important anode reaction in electrolysis processes. Here we uncover stability challenges for OMIECs under anodic potentials and investigate structure-property differences that influence reactivity. For a series of n- and p-type polymers we identify the regimes of applied potential which result in either stable charging or degradation over a range of electrolyte pH conditions. We find that OMIECs have improved stability at cathodic potentials and investigate the activity of n-type OMIEC materials for electrochemical reduction reactions. Here our focus is the oxygen reduction reaction, a key reaction for fuel cells and hydrogen peroxide production. While many OMIECs and other carbonaceous materials selectively reduce oxygen to hydrogen peroxide via a non-catalytic outer-sphere pathway, we find that the n-type ladder polymer BBL (poly(benzimidazobenzophenanthroline)) undergoes a series mechanism to reduce oxygen to the four-electron product in alkaline electrolytes. We investigate the pH dependent activity and selectivity for ORR on this material through electrochemical measurements in buffered electrolytes. Finally, we compare the performance with chemically similar materials in an effort to develop design rules for next generation organic mixed conducting electrocatalysts. We end by summarizing the key takeaways and outlook for redox-active organic materials in the previously described applications.