Exploring How Nanoparticle Shape Improves Energy Storage
A team of Stanford University engineers has obtained a first look inside phase-changing nanoparticles, elucidating how their shape and crystallinity – the arrangement of atoms within the crystal – can have dramatic effects on their performance. The work has immediate applications in the design of energy storage materials, but could eventually find its way into data storage, electronic switches and any device in which the phase transformation of a material regulates its performance. For instance, in a lithium ion battery, the ability of the battery to store and release energy repeatedly relies on the electrode’s ability to sustain large deformations over several charge and discharge cycles without degrading. Recently, scientists have improved the efficiency of this process by nanosizing the electrodes. The nanoparticles allow for faster charging, increased energy storage and an extended lifetime, but it is unknown which nanoparticle shapes, sizes and crystallinities produce the best performance. Jennifer Dionne, an assistant professor of materials science and engineering, and her group have been studying the behavior of individual particles to establish a stronger link between structure and function that can direct the design of next-generation energy storage materials. Dionne’s group examined how varying the shapes and crystallinity of palladium nanoparticles affected their ability to absorb and release hydrogen atoms – an analog to a lithium-ion battery discharging and charging. They prepared cubic, pyramidal and icosahedral nanoparticles and developed novel imaging techniques to look inside nanoparticles at various hydrogen pressures, determining where the hydrogen was located. The researchers found that nanoparticle structure significantly influences performance. The icosahedral structures, for instance, show reduced energy storage capacity and more gradual hydrogen absorption than the single crystalline cubes and pyramids. High-resolution maps of the particles demonstrate that hydrogen is excluded from the center of the particle, thus lowering the overall capacity to incorporate hydrogen. Structural characterization shows that the gradual absorption of hydrogen occurs because different regions of the particle absorb hydrogen at different pressures, unlike what is observed in single crystals.