| With the social development and the gradual implementation of China's "dual carbon" policy,hydrogen energy industry has received great attention.At present,the hydrogen energy industry chain includes the preparation,storage,transportation and use of hydrogen.The preparation of hydrogen plays an important role in the development of hydrogen energy.Recently,a new hydrogen generation method has attracted great attention.This method uses hydrolysis reaction between hydrogen generation material and water to produce hydrogen,which can realize fast supply of hydrogen and avoid the storage and transportation of hydrogen.This not only saves costs,but also reduces the risk of hydrogen leakage,enabling convenient,fast,safe and efficient hydrogen supply.Existing hydrogen generation materials include sodium borohydride,aluminum-based material and magnesium-based material.Magnesium-based materials have become a hot topic in hydrogen generation field due to their high hydrolysis activity,low material cost and environmental friendliness.However,in the process of hydrolysis of magnesium-based materials,the reaction product magnesium hydroxide deposit easily on the surface of the material,which will hinder the further reaction.Based on this issue,this paper proposes the preparation of nanoporous magnesium,and nanoporous lithium-and zinc-doped magnesium by using physical vapor deposition to weaken the passivation effects caused by magnesium hydroxide,to improve the hydrogen generation performance of magnesium-based materials.Firstly,the nanoporous magnesium was prepared by physical vapor deposition method,its microstructure in two-and three-dimensional space were characterized,and its hydrogen generation performance was tested.The results show that the pore size of nanoporous magnesium is 100 nm?500 nm,and the porosity is 20.4%.Its hydrogen generation performance is obviously improved compared with that of magnesium metal plate,and the amount and speed of hydrogen generation are increased by about 1.4 times.At 25?,5 wt.%NaCl solution,the hydrogen generation amount reaches 1016 mL·g-1,and the hydrogen generation rate reaches 85.5 mL·g-1·min-1.The hydrogen generation rate increases gradually with the increase of temperature,and it reaches 361 mL·g-1·min-1 when the reaction temperature is 50?.Secondly,based on the preparation of nanoporous magnesium,the gas phase doping of lithium was carried out.By adjusting the parameters of evaporationdeposition process,magnesium-lithium materials with nanoporous structure were successfully prepared.The results show that the pore sizes of Li-doped nanoporous magnesium are in the range of 200 nm to 400 nm,and the porosity is?42.4%.The hydrogen generation performance testing results show that the doping of lithium significantly improves the hydrogen generation performance of nanoporous magnesium.At 25?,in 5 wt.%NaCl solution,the hydrogen generation amount of Li-doped nanoporous magnesium is 826 mL·g-1,and the hydrogen generation rate is 34.4 mL·g-1·min-1.Compared with nanoporous magnesium under the same conditions,the hydrogen generation amount and hydrogen generation rate of Li-doped nanoporous magnesium are 1.2 times and 5.6 times higher.The hydrogen generation amount and rate of nanoporous Mg-Li material with substrate can reach 1051 mL·g-1 and 119.1 mL·g-1·min-1,and the hydrogen generation rate increases gradually with the increase of temperature,and it reaches 400.6 mL·g-1·min-1 when the temperature is 50?.This performance is considerably better than those of existing magnesium-based materials.Thirdly,based on the nanoporous magnesium,zinc was doped by gas phase co-deposition.By adjusting the evaporation-deposition parameters,nanoporous Mg-Zn with three types of structures were obtained,which were lotus root-like,coral-like,and dual-phase nanoporous Mg-Zn materials.The pore sizes of them are about 150 nm,100 nm and 75 nm,respectively,and their porosity are 52.0%,37.4%and 11.1%,respectively.The hydrogen generation performance testing of the three nanoporous Mg-Zn materials show that the lotus root-like nanoporous Mg-Zn does not hydrolyze in 5 wt.%NaCl solution at 25?,while the coral-like nanoporous Mg-Zn material and the dual-phase nanoporous Mg-Zn material show good hydrogen generation performance.The hydrogen generation amount is 300 mL·g-1 and 375 mL·g-1,respectively,and the hydrogen generation rate both are 10 mL·g-1·min-1,comparable to that of nanoporous Mg.The addition of 0.5 wt.%acetic acid solution can significantly improve the hydrogen generation performance of dual-phase nanoporous Mg-Zn.In 4.0 wt.%acetic acid solution,the dual-phase nanoporous Mg-Zn exhibits excellent hydrogen generation performance,which is comparable to that of Li-doped nanoporous magnesium.By investigating the effects of preparation parameters on the morphologies and structures of the nanoporous magnesium materials,the growth mechanism of the nanoporous magnesium material was proposed.Under the conditions of appropriate atomic energy and concentration,atoms can only migrate in a small range near the deposition point and will preferentially grow along the direction with the maximum interatomic attraction and the minimum atomic potential energy,that is,grow along the closest packed direction,which results in the formation of strip-shaped structure.Because there are three equivalent closet packed directions,the strips will bifurcate and cross each other,which eventually leads to the formation of nanoporous structure.Lithium doping and zinc doping increase the porosity,which is related to the saturated vapor pressure of the two kinds of metal atoms and their interatomic force with the magnesium atoms.Finally,the hydrogen generation mechanism of nanoporous Mg was analyzed.The nanoporous structure not only makes magnesium have a greater hydrolysis tendency,but also makes its hydrolysis product,magnesium hydroxide,become more easily to dissolve and diffuse,thus weakening its passivation effects.The addition of lithium metal can provide more active sites,which further accelerates the hydrolysis process of nanoporous magnesium material.Meanwhile,the hydrolysis product of lithium is water-soluble,which can destroy the formation of magnesium hydroxide passivation layer and make the hydrolysis reaction of nanoporous magnesium material more complete.The addition of zinc provides an additional galvanic effect,which speeds up the hydrolysis reaction of magnesium metal.However,the hydrolysis reaction is inhibited when the zinc content is too high. |
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