Electrolyte-Driven Advances in pH Sensor Development, CO2 Conversion, and Concentrated Electrolyte Modeling

Abstract

Electrolytes are critical yet underexplored factors in electrochemical systems. This dissertation advances mechanistic understanding of electrolyte behavior at or near electrochemical interfaces, for improved control and design of electrochemical processes. The dissertation is organized into three research chapters. Chapter 2 focuses on developing potentiometric solid-state pH sensors. Anodically grown platinum oxide ultramicroelectrodes fabricated under alkaline conditions exhibited a near-Nernstian response and excellent stability in aqueous electrolyte environments, with fast temporal resolution when fabricated on nanoelectrodes. These findings highlight the crucial roles of electrolyte synthesis conditions and geometric scaling in enabling fast and reliable pH sensing, especially in challenging aqueous environments. Chapter 3 investigates the mechanistic role of ion pair formation in the electroreduction of CO2 to formate in KHCO3/KCl electrolytes. Through cathodic linear sweep voltammetry, bulk electrolysis, and Tafel analysis, KHCO3 ion pairs, not free bicarbonate or dissolved CO2, serve as the dominant electroactive species toward formate production. Electrolyte optimization led to the identification of 1.75 M KHCO3/2 M KCl as the optimum electrolyte, exhibiting the highest formate current density on planar tin electrodes (~20 mA.cm-2) at reduced overpotentials. These findings establish a new framework for electrolyte design, showing that ion pair speciation directly impacts selectivity and activity in CO2 electroreduction. Chapter 4 presents a modified Debye-Hückel framework that quantitatively links microscopic ion structuring with macroscopic properties in symmetrical and asymmetrical concentrated electrolytes. The derived mean ionic activity coefficients and effective water activities enable accurate prediction of electromotive force and conductivity over a wide concentration range. This analytically tractable, non-empirical model overcomes century-old limitations of classical theory, providing a unified, predictive foundation for electrolyte design and optimization. These contributions advance both the mechanistic and applied understanding of electrolyte role in electrochemical systems. They establish critical principles that guide innovation in electrolyte-driven technologies, including electrochemical sensing, selective electrosynthesis, and electrolyte design and control across diverse scientific and engineering applications.