Hydrogen Sorption Mechanisms in Magnesium-based Nanolayers

0932249章车载储氢被认为是实现氢经济最具挑战性的障碍之一。Mg具有较高的储氢容量（7.6wt%），是一种很有前途的储氢材料。不幸的是，在块体Mg基材料中的氢吸收和解吸仅在高于573 K的温度下才是可能的，在PEM燃料电池的期望操作范围之外。同时，氢加载和释放的动力学受到几个因素的限制，包括不可渗透屏障的形成、Mg内部H的低扩散速率、氢化期间金属/氢化物界面的流动性以及Mg表面上H2分子的有限解离速率。虽然已经有许多努力通过减少特征氢传输长度以及通过引入催化剂来改善氢吸附的动力学，但是这些方法中很少有能够降低氢解吸所需的温度，因为这必然需要载氢相的去稳定化。智力优势：PI建议研究纳米结构多层薄膜的储氢和释放/加载行为，其中可以调整应力/应变以及化学，以优化氢吸附的动力学并降低载氢相的稳定性。工具，如HRTEM，STEM和EELS将被用来了解氢吸附在纳米长度尺度的基本机制。同时，先进的计算方法的基础上密度泛函理论将被用来提供一个基本的理解的界面结构，应变和化学的热力学和动力学的储氢的影响。本项目的具体目标是：1）研究界面密度和结构对氢扩散动力学的影响; 2）探索氢化过程中合成薄膜的微观结构演变; 3）通过计算方法预测化学、应力和薄膜结构对氢扩散动力学的影响; 4）通过表征和第一性原理计算研究了亚稳中间氢化物相对吸氢动力学的影响：5）通过原位XPS研究了分子水平上的吸氢;以及6）通过微观结构表征、计算研究和原位XPS研究来研究应力和应力演化对载镁相的热力学稳定性的影响。该项目将在若干领域产生更广泛的影响，例如（a）通过实验和计算方法，在材料和能源相关研究方面培训研究生和本科生（NSF-REU）;（B）为学生提供在国家实验室工作的经验，（c）开发本科和研究生课程;（d）招收少数民族学生;（f）通过国家科学基金-可再生能源技术方案向更广泛的受众传播知识。

0932249ZhangOn-board hydrogen storage is considered to be one of the most challenging barriers to the realization of a hydrogen economy. Mg is one promising hydrogen storage candidate materials due to its relatively high hydrogen capacity (7.6 wt %). Unfortunately, hydrogen absorption and desorption in bulk Mg-based materials is only thermodynamically possible at temperatures above 573K, outside of desired operating range of PEM fuel cells. At the same time, the kinetics of hydrogen loading and release are limited by several factors, including the formation of impermeable barriers, the low diffusion rate of H inside Mg, the mobility of the metal/hydride interface during hydrogenation and the limited dissociation rate of H2 molecules on the Mg surface. While there have been numerous efforts to improve the kinetics of hydrogen sorption by reducing the characteristic hydrogen transport length as well as through the incorporation of catalysts, few of these approaches have been able to lower the temperature necessary for hydrogen desorption as this necessarily requires the destabilization of the hydrogen-carrying phase. Intellectual Merits: The PIs propose to investigate the hydrogen storage and release/loading behavior of nanostructured multi-layered thin films in which stress/strain as well as chemistry can be tuned to optimize the kinetics of hydrogen sorption and lower the stability of hydrogen-carrying phases. Tools such as HRTEM, STEM and EELS will be used to understand the fundamental mechanism of hydrogen sorption at nanometer length scale. Concurrently, advanced computational methods based on density functional theory will be used to provide a fundamental understanding of the effects of interface structure, strain and chemistry on the thermodynamics and kinetics of hydrogen storage. The specific objectives of this project are to: 1) investigate the influence of interface area density and structure on hydrogen desorption kinetics; 2) explore microstructural evolution of the synthesized thin films during hydrogenation; 3) predict the influence of chemistry, stress and film structure on the hydrogen diffusion kinetics through computational methods; 4) investigate the influence of metastable intermediate hydride phases on the kinetics of hydrogen sorption through characterization and first principles calculations; 5) explore H sorption at the molecular level through in situ XPS; and 6) investigate the influence of stress and stress evolution on the thermodynamic stability of Mg-carrying phases via microstructural characterization, computational investigation and in situ XPS studies.Broader impacts: The project will have broader impact in several areas, such as (a) training graduate and undergraduate students (NSF-REU) in materials and energy-related research through experimental and computational approaches; (b) providing students experience of working at national laboratories, (c) the development of courses at both the undergraduate and graduate levels; (d) recruiting minority students; and (f) disseminating the knowledge to broader audience through NSF-RET program.