Abstract
Magnesium has bright market prospects such as generating thrust for under water engines and hydrogen production. However, the passive oxide film on the surface of magnesium powder prevents the further reaction of magnesium with water at room temperature. In this paper, highly active magnesium-based materials were prepared via ball milling pure Mg with different chlorides (NiCl2, CoCl2, CuCl2, FeCl3). The activity of the as obtained powder was analyzed through Scanning Electron Microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), synchrotron X-ray tomography, Extended X-ray Absorption Fine Structure (EXAFS), etc. Among the various compositions, the Mg-6%CoCl2 composite exhibited the best hydrogen production performance with a hydrogen generation volume of 423 mL/(0.5 g) and a conversion yield of 96.6%. The related activation mechanism was thoroughly studied, showing that the addition of chloride during ball milling can effectively break the continuity of oxide films on Mg surfaces and introduces a large number of micro defects. In addition, the EXAFS and tomography data verified that metallic cobalt was generated during the ball milling process, subsequently forming a Mg-Co micro glance cell, and the Cl− in the system accelerates the corrosion of Mg. The active mechanism can be verified as synergistic effects of micro glance cell and as-generated surface microcracks.
Highlights
As a promising new energy source [1,2,3], magnesium-based materials with unique physical and chemical properties, exhibit high mass and volume energy density [4,5].The abundant resources of magnesium in the earth and low cost make it a promising candidate material for energy storage and energy conversion [6,7]
We studied the inner morphology of ball milling Mg-6%CoCl2 composite using synchrotron X-ray tomography and the valence state of cobalt through Extended X-ray Absorption Fine Structure (EXAFS)
The Mg peaks of pure Mg/Mg-chloride structures after ball milling and the Mg standard card are almost constant, only showing a difference in peak intensity. It shows that the crystal forms of Mg in the raw materials after ball milling are all the same, and the crystallinity of different crystal forms of Mg has changed after adding different chlorides
Summary
As a promising new energy source [1,2,3], magnesium-based materials with unique physical and chemical properties, exhibit high mass and volume energy density [4,5]. Once the hydrolysis reaction begins, a dense oxide film will generate on the surface of the magnesium, which would highly reduce the activity of magnesium powder [13,14,15,16]. To solve this problem, the development of novel material systems and multiple modification processes have been reported. The Mg could form a galvanic in the seawater medium, which promotes the corrosion of Mg. Huang et al [22] investigated the activity of Mg powder with Co3 O4 addition during ball milling, which can produce Mg powders with smaller size and higher specific surface area. The synergistic effects of ball milling, micro-galvanic corrosion, and modification effect of Cl- are attributed to the activation of Mg
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