Management of Charge Carriers in Aqueous-Nonaqueous Phases and Interfaces in Electrochemical Systems for Energy Storage and Conversion

Abstract

The escalating global impact of climate change, driven in large part by fossil fuel combustion and resultant CO₂ emissions, demands urgent innovation in sustainable energy systems. The rapid expansion of renewable energy sources — such as wind and solar — has intensified the need for energy technologies such as grid scale storage systems that can accommodate their inherent intermittency. Concurrently, breakthroughs in electrosynthesis have highlighted the potential of electrochemical systems to convert renewable electricity into valuable chemicals and fuels. In many of these applications, the efficient management of charge carriers, particularly protons and hydroxide ions, is essential for achieving high selectivity, energy efficiency, and operational longevity. The transport and control of ions and electrons across aqueous-nonaqueous and solid-liquid interfaces can further open up the opportunity of using electrochemistry in nonaqueous systems. This dissertation presents a comprehensive investigation into the control and optimization of transport of protons, hydroxide ions, and ion-coupled electrons across aqueous and nonaqueous solutions and their interfaces to advance electrochemical technologies for sustainable energy storage and conversion. In Chapter Two, a mild pH‐decoupling aqueous redox flow battery is developed with a practical pH recovery system. By separating the pH conditions of the negolyte and posolyte, the design achieves cell voltages well above 1.23 V. The crossover of proton/hydroxide ions is investigated to assure high Coulombic efficiency and long-term stability, which are essential for scalable grid energy storage. Chapter Three introduces an electrochemical acid-base generator designed for decoupled carbon management. By minimizing the acid-base crossover, the system produces concentrated acid and base streams at high current efficiency and low energy cost. This device is applied to carbon capture scenarios — from simulated flue gas to direct air capture — demonstrating its potential for integrated decarbonization strategies. In Chapter Four, we reveal that redox mediators that operate via proton-coupled electron transfer (PCET) can be explored to mediate electrochemical hydrogen storage or ion exchange. Chapter Five extends the approach to electrosynthesis by leveraging interfacial PCET at aqueous-nonaqueous interfaces, bridging the aqueous electrochemistry and nonaqueous chemistry to electrifying nonaqueous synthesis. This work demonstrates how aqueous redox mediators can transfer proton-electron pairs as a charge-neutral fragment into/onto nonaqueous phases to drive selective chemical synthesis, exemplified by the electrification of industrial hydrogen peroxide production and nonaqueous organic hydrogenation, solving the long-existent challenge of achieving high current density and high efficiency during nonaqueous electrosynthesis. Collectively, the studies establish a unified framework for managing charge carriers in electrochemical systems, offering key insights into interfacial transport phenomena and laying the groundwork for next‐generation, sustainable energy technologies. The innovations presented here promise to improve both the efficiency and economic viability of energy storage, carbon management, and electrosynthetic processes.