Thermodynamic and Electrochemical Models for Predicting Aqueous Materials Formation

Walters, Lauren Nichole

doi:https://doi.org/10.21985/n2-nyde-m466

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Thermodynamic and Electrochemical Models for Predicting Aqueous Materials Formation

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A fundamental materials science question is “why and how will this material form?” The experimental,computation, and time resources necessary to answer this question consume significant resources due to the predominantly trial-and-error based approaches common in materials research. This dissertation reintroduces a number of fundamental thermodynamics-based tools for the study of contemporary materials stability and formation. Primary modeling types include (multi-element) Pourbaix diagrams, stability diagrams, yield diagrams, convex hulls, ensemble (probability diagrams), and driving force parameters. First, we create (nonstandard state) Pourbaix and driving force diagrams sourced from density functional theory (DFT) calculations to model copper speciation in aqueous, electrochemical environments. We determine hybrid functionals as necessary for free energy of formation modelling to create accurate Pourbaix diagrams, and further develop methods for the efficient calculation of phase diagrams from less computationally-intensive functionals. At elevated temperatures, we report decreased passivation regions, particularly for CuO(s), and pressure dependent aqueous ion predominance. We also establish boundary conditions for thermodynamically driven lead corrosion for nonstandard states, phosophate solutions, and carbonate solutions. Multiple element Pourbaix and stability diagrams, created from DFT-sourced free energies of formation with spin-orbit coupling and van der Waals corrections included, demonstrate that the stability of hydroxylpyromorphite has been overestimated. This work suggests there may be alternative formation routes for lead orthophosphates, such as from early calcium apatite precursors. Our model underscores the need for future research on lead scale identity and stability. Next, we apply predominance diagrams to rationalize the hydrothermal synthesis design of a family of thermoelectric, layered, heteroanionic bismuth oxychalcogenide (BiMOQ) compounds. The stability regions of the targeted oxychalcogenides in initial Pourbaix diagrams indicate hydrothermal synthesizability. Analysis of optimized reaction conditions and potential byproduct creation is completed through extending thermodynamic tools such as probability diagrams, calculated from the canonical ensemble of chemical potentials, and stability-yield diagrams. We further explore chemical trends for successful synthesis of products, and find copper target phase domains to be larger than silver ones, selenium target domains are larger than sulfur domains, and heteroanionic material domains typically stabilized at moderate pHs. We interpret the second, seeded reaction step necessary to synthesize high yield BiAgOSe with classic nucleation theory and by exploring driving forces for byproduct generation. Finally, we introduce two new, quantitative aqueous materials formation descriptors for the development of future corrosion resistant systems: (i) the maximum driving force (MDF) to characterize initial surface formation of scales and (ii) the effective chemical potential to predict subsurface, compositionally constrained phase formation. We apply these thermodynamic analysis tools to understand nickel thin film evolution in variable pH and potentials. Moreover, we leverage the MDF to intuitively describe the aqueous stability of single element speciation including noble 4d metals (e.g., silver, gold) and soluble solid oxides. Finally, we report initial work in developing these parameters for future comparative studies on quantitative elemental- and composition-based corrosion. We conclude by discussing future work, and the important note that compound formation verified by experimentation is imperative for model validation. Successive projects to develop rational synthesis design strategies and corrosion resistance will accelerate materials discovery, enabling new energy, health, and electronics solutions.

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