Date of Award
Doctor of Philosophy (PhD)
Materials Science & Engineering
The ever-increasing presence of catastrophic climate events around the globe signify the effects of the last century’s climb from below 300 ppm to over 400 ppm of atmospheric carbon. Materials Science and Engineering offers us an exit strategy. In this dissertation, we aim to connect three of the primary aspects of Materials Science and Engineering – synthesis, processing, structure, and performance – with fundamental X-ray scattering studies on dynamic processes in nanoporous gold, the model system for a class of materials called nanoporous metals, which have applications in fossil-fuel curbing technologies such as catalysis, energy storage and conversion, and mechanical-to-electrical energy harvesting. In the first part of the dissertation, we connect materials synthesis and underlying structure by (i) developing a novel, eco-friendly free corrosion dealloying synthesis route to nanoporous gold and comparing its structure to that of nanoporous gold created by conventional free corrosion in concentrated nitric acid, and (ii) using real-time X-ray scattering techniques to study the microstructural evolution and kinetic behavior of nanoporous gold during synthesis by electrolytic dealloying. In (i), we use a chelating agent to sequester copper ions as they are removed from a parent Au-Cu alloy in a neutral electrolyte. Without a chelating agent, these Cu ions would react to form a passive surface film that would prevent dealloying from proceeding in neutral electrolyte. In (ii), we study the microstructure and crystal structure of nanoporous gold simultaneously during electrolytic dealloying in perchloric and nitric acids; here, we discover that a passive silver chloride (AgCl) film grows on its surface in perchloric acid, whereas no film growth was observed in nitric acid. This passive film slows down dealloying kinetics and ultimately results in a smaller characteristic size as AgCl blocks the surface diffusion of gold adatoms. In the second part of the dissertation, we use real-time X-ray scattering techniques to study the coarsening behavior of nanoporous gold during thermal processing, thereby connecting materials processing and underlying structure. We find that during heat treatment, the characteristic size of nanoporous gold departs from the conventional t1/4 power law for surface diffusion coarsening, slowing down to t1/20 within 2 hours of heating at 200 oC. Using new insight gleaned from this study on nanoporous gold’s behavior during thermal treatment, we connect material structure and material performance in the last part of this dissertation. Here, we fabricate and electrochemically test novel aperiodic 3D energy storage devices (3D nanocapacitors and 3D batteries) that use nanoporous gold as a current-collecting scaffold. Our aperiodic 3D nanocapacitor outperforms its 2D counterpart by 4x in terms of capacitance, and our aperiodic 3D battery is the first demonstration of its kind. These devices have the potential to outpace the energy and power density of conventional 2D energy storage technologies.
Welborn, Samuel Scott, "X-Ray Scattering Investigations Into Nanoporous Gold's Kinetic Behavior During Dealloying And Coarsening For Applications In 3d Energy Storage" (2022). Publicly Accessible Penn Dissertations. 4706.