Visualizing energy-relevant phase transitions within individual palladium hydride nanoparticles
- Alternative energy technologies such as wind and solar power offer a promising avenue towards reducing our reliance on fossil fuels, but suffer from intermittency. Methods to store energy produced during peak generation periods are among the most crucial advances necessary to enable growth of the renewable energy industry. Many storage systems, including electrical energy storage in lithium ion batteries and chemical energy storage in metal hydrides, rely on ion intercalation into nano-structured materials, inducing a phase change. However, the design rules governing champion nanomaterials for energy storage remain unknown, because it is challenging to directly visualize the phase transformation in-situ. In this dissertation, we examine the thermodynamics and reaction dynamics of nanoscale phase transitions induced by ion intercalation at the single particle level. As a model system, we focus on the palladium/hydrogen system, as it is among the oldest and most well-studied bulk hydrogen-storage systems. Pd nanoparticles undergo a phase change from hydrogen-poor alpha phase to a hydrogen-rich alpha phase. Using in-situ studies in an environmental transmission electron microscope (TEM), we examine the thermodynamics of single-crystalline and multiply-twinned nanoparticles and also follow the reaction in real time to gain insights into the transformation mechanism. First, we use electron energy loss spectroscopy (EELS) to monitor the thermodynamics of the hydrogenation process in single-crystalline particles, including nanocubes and prisms. We find that single nanocrystals absorb hydrogen over a very narrow pressure range and do not show phase coexistence at equilibrium, in striking contrast to studies performed on ensembles of particles. Next, we study multiply-twinned palladium nanoparticles to understand the influence of crystalline irregularities, such as defects and strain. In contrast to single crystals, these particles absorb hydrogen over a relatively wide pressure window and do not hydrogenate completely. EELS mapping, dark field imaging, and electron diffraction allow us to look inside individual nanoparticles, visualizing the local phase with sub-2 nm spatial resolution. We find that highly compressively strained regions exclude hydrogen and that individual crystallites uptake hydrogen at distinct pressures. Finally, we investigate the reaction dynamics of the phase transition, probing the reaction intermediates. Intriguingly, we find that nanoparticles self heal during the absorption process. We show how our work helps to unravel the mechanism of Li-ion insertion in battery electrodes or hydrogen absorption in state-of the-art metal hydride catalysts, and provides a platform to screen for champion nanomaterials in future energy storage systems.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Stanford University, Department of Materials Science and Engineering.
|Dionne, Jennifer Anne
|Dionne, Jennifer Anne
|Statement of responsibility
|Submitted to the Department of Materials Science and Engineering.
|Thesis (Ph.D.)--Stanford University, 2016.
- © 2016 by Tarun Chandru Narayan
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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