纳米镁镍氢化物表面超薄MgO膜的可控制备及其透氢抗氧机制

Storing hydrogen in light metals such as Mg-based alloys is a promising method for hydrogen carrier. Nanosizing has been believed to be an important approach to simultaneously improve the kinetic and thermodynamic properties of Mg-based hydrides. However, this leads to two key issues: poor surface oxidation resistance due to the increased active surface area and the structural stability and sustainability of high-activity upon hydrogenation/dehydrogenation cycles. To solve this, we have proposed a surface modification strategy derived from microencapsulated nanoconfinement, in which ultrathin MgO layer were homogenously coated onto Mg2NiH4 surface through a gas-solid reaction method under specific atmosphere, realizing gas-selective permeability and structural stabilization. Based on the previous work, this project will systematically study the gas-selective permeability mechanism of the ultrathin MgO coating layer. Firstly, we will tune the thickness of the surface MgO layer in a controllable way and establish the correlative mechanism between the thickness and the hydrogen sorption kinetics of the underlying hydride. Secondly, well-characterized microstructural and interfacial features will be given, revealing the fundamental structure basis for the gas-selectivity of the ultrathin MgO surface layer. At last, based on the particular MgO@Mg2NiH4 core-shell structure，in situ hydrogen desorption mechanism at the single-particle-scale for single-crystal Mg2NiH4 will be clarified. This research may provide new insights into preparing high reactive nanosized materials with desirable structural stability even in the complete absence of foreign species and will shed light on the beneficial effects of the surface passivating films of the nanostructured metal-based hydrogen storage materials.

借助轻质金属如镁基合金实现氢气的致密存储是重要的储氢方式，材料纳米化被认为是实现动力学和热力学双调控的重要手段。纳米镁基储氢材料存在两个关键科学问题：活性表面增加导致的氧化和吸放氢循环过程中结构、高活性保持。为此我们提出基于微胶囊化纳米限域的表面改性，通过特定气氛下气-固合成技术成功将超薄MgO膜均匀包覆于单分散镁基合金氢化物表面，实现透氢抗氧和结构稳定化。本项目将在此前期工作基础上系统开展超薄MgO膜透氢抗氧机制研究。首先实现表面MgO膜厚度的可控制备并建立厚度与活性物质储氢动力学的关联机制；其次研究MgO膜微观结构特征和界面特性，揭示其透氢抗氧的结构基础；最后，基于特殊的MgO@Mg2NiH4核壳结构，阐明单个颗粒尺度上Mg2NiH4单晶的原位脱氢机制。通过本项目的开展，为制备具有优异结构稳定性的纳米高活性镁基储氢材料提供新思路，有助于进一步理解纳米金属基储氢材料表面钝化膜的有益效果。

镁基储氢材料作为典型的高容量轻质金属固态储氢材料，以其资源丰富、价格低廉和储氢容量高等优势吸引了广大研究者的关注。由于金属镁的高活性，导致镁基储氢材料暴露于空气中会被迅速钝化失活，这在实验室条件下是不可避免的。因此深入理解金属氢化物的表面钝化，并以此平衡储氢材料的高活性和高空气稳定性是其实际应用的关键挑战。.本项目依次剖析了镁基氢化物表面的氧化层、氢氧化层对材料吸放氢性能的影响，并讨论了极端条件（水解制氢）下，产生的钝化层对体系制氢性能的影响。首先，基于微胶囊化纳米限域的表面改性，通过特定气氛下气-固合成技术成功将超薄MgO膜均匀包覆于单分散镁基合金氢化物表面，实现透氢抗氧和结构稳定化，研究了MgO膜微观结构特征，揭示其透氢抗氧的结构基础。随后，讨论了表面水解产生的Mg(OH)2对镁镍氢化物吸放氢性能的影响，结合表面分析技术与DFT计算，发现含Ni体系水解后表面生成的Ni@Mg(OH)2层利用空位介导氢溢流机制对氢原子的扩散、解吸有很大的促进作用，从而大大提高材料的吸放氢性能，使其脱氢峰值温度降低了108.2 °C；同时由于Mg(OH)2层的结构稳定性，材料展现出优异的空气稳定性。最后，研究了钝化层Mg(OH)2对Mg-Mg2NiH4体系水解反应的影响，发现Mg2NiH4快速水解原位生成的超细Ni颗粒与Mg形成大量微原电池，加速Mg的电化学腐蚀，从而促进了整个体系的制氢效率。.综上，本项目发现了镁镍氢化物的表面MgO和Mg(OH)2这两种钝化因素的有益效果。一方面超薄MgO层的结构稳定性可以阻止活性较高的金属氢化物的进一步钝化，使其同时具有高反应活性和空气稳定性；另一方面，通过特定的表面功能化技术可以提高金属氢化物的反应活性，从而大大提升材料的储、制氢性能。研究结果为空气稳定型金属氢化物的设计和制备提供了有益见解，使其大规模使用和长期存储成为可能。