Rapid communicationMgCl2 promoted hydrolysis of MgH2 nanoparticles for highly efficient H2 generation
Graphical abstract
Introduction
Hydrogen storage is one of the key technologies for the hydrogen economy [1]. So far, it remains highly challenging to develop reversible hydrogen storage systems meeting all the requirements for on broad applications [2]. An alternative approach is using disposable hydrogen carriers to generate hydrogen and recover the hydrogen carriers from the spent fuel ex situ [3], [4]. This approach is technically much less demanding and may have impact in the near term, which is particularly attractive for portable, onsite hydrogen generation when hydrogen refilling infrastructures are not immediately available.
The hydrolysis reaction of hydrides or reactive metals is able to produce hydrogen in mild conditions, which is very attractive for onsite hydrogen generation [5], [6]. The theoretical capacity of many hydrolysis systems easily surpass the 2015 target set by the U.S. Department of Energy for on-board application (5.5 wt%), even including the stoichiometric amount of water. However, to ensure complete utilization of the hydrogen carrier, a large excess of water has to be used to disperse the reactants, which significantly reduces the hydrogen capacity of the whole system. For instance, the theoretical capacity of the NaBH4 hydrolysis system is 10.8 wt% based on the reaction stoichiometry (NaBH4+2H2O). However, in most practical applications, NaBH4 is used in the form of 15–25 wt% solution stabilized by NaOH, which significantly reduces the hydrogen capacity to 3.2–5.3 wt% [7], [8]. Some researchers have carried out the hydrolysis of hydrides at elevated temperature by using water vapor or steam to reduce the water amount [9], [10]. Decent conversion of the hydrides is achieved with continuous feeding of water vapor or steam. The amount of water consumed, however, has not been taken into account. Therefore, a key challenge for the hydrolysis hydrogen generation systems (HHGS) to be competitive is to minimize the excessive water.
An alternative approach is to recycle the excessive water for repeated using. In this case, the hydrolysis of hydrides will regain their merit of high capacity. The recycling will involve separation of the excessive water from the reaction by-products. For easy separation, it will be advantageous if the reaction byproducts are insoluble solids. MgH2 is among the very few systems fulfilling this criterion. Hydrolysis of MgH2 delivers a high capacity of 6.5 wt% (based on MgH2+2H2O). The byproduct Mg(OH)2 shows low solubility in water and can be separated from the solution by simple filtration. However, Mg(OH)2 will passivate the MgH2 surface due to its low solubility, usually resulting in low hydrogen generation rate and incomplete hydrolysis. Previous study shows that the maximum conversion of MgH2 does not exceed 30% in 1 h in direct hydrolysis, which is inadequate for practical application [11], [12], [13], [14]. In fact, the same issue is the major kinetic barrier for HHGS with insoluble byproducts, such as the hydrolysis of Mg [12], [15] and Al [16], [17].
To overcome this kinetic barrier, the Mg based materials (MgH2 or Mg) have to be activated to facilitate the hydrolysis. Ball milling is most frequently used. Various addictive is also introduced during the ball milling stage, which will further enhance the hydrolysis kinetics [12], [13], [14], [18], [19], [20]. However, activation by high energy ball milling imposes extra energy penalty and is time consuming. The reactivity of the ball milled samples may also decline after long time storage. Moreover, the introduced dopants will cause an inevitable hydrogen capacity loss. For this reason, it is highly desirable if the MgH2 sample can be directly used in the hydrolysis reaction without any pretreatment.
Another approach is to carry out the hydrolysis reaction in salt or acid solution instead of pure water. Acid can effectively dissolve Mg(OH)2 and promote the hydrolysis reaction. However, using acid is generally not considered as a practical solution, as stoichiometric amount acid has to be consumed, which significantly lowers the hydrogen capacity. Inorganic chlorides, such as KCl [12], NaCl [21], [22] and MgCl2 [14], [23] also exhibit certain promotion effect. However, satisfactory H2 generation rate and utilization remain very challenging without ball milling activation of MgH2 [12], [14].
In this paper, we demonstrate that by reducing the particle size of MgH2 to sub-micrometer region together with the promotion effect of MgCl2, hydrolysis of MgH2 can proceed efficiently without any pretreatment of MgH2. Near theoretical amount of H2 can be released within 20 min. Moreover, the MgCl2 is not consumed during the reaction. Based on this reaction, we propose a hydrogen generation scheme including a recycling unit for the MgCl2 solution. Material based capacity approaching the theoretical limit of MgH2 hydrolysis (6.5 wt%) can be achieved with continuous MgH2 and water feeding.
Section snippets
Preparation and characterization of Mg and MgH2 nanoparticles
Mg nanoparticles are prepared by the hydrogen plasma metal reaction (HPMR) as described in our previous publications [24], [25]. MgH2 nanoparticles are prepared by direct hydrogenation of Mg nanoparticles. About 0.3 g Mg nanoparticles are loaded in a stainless steel chamber and are activated by heating the chamber to 400 °C, holding at 400 °C for 30 min and cooling to room temperature under vacuum. After heating the system to 400 °C under vacuum again, high purity H2 (99.999%) was introduced into
Characterization of the Mg and MgH2 nanoparticles
As shown by the XRD pattern, the Mg nanoparticles prepared by HPMR exhibit high purity corresponding to the hexagonal Mg phase (JCPDS no. 35-0821), without detectable crystalline oxide phase (Figure 1a). As shown in the SEM image (Figure 2a), the Mg nanoparticles exhibit polyhedral shape with average size around 500 nm, while with rather broad size distribution. The hydrogenated phase is well matched to the tetragonal β-MgH2 phase (JCPDS no. 12-0697) without any impurity related diffraction
Conclusion
In this paper, we demonstrate that in MgCl2 solution, MgH2 nanoparticles with average size of 800 nm can efficiently generate H2 through the hydrolysis reaction. Near theoretical amount of H2 (1820 mL g−1) can be released within 20 min in 1 M MgCl2 solution without any pretreatment of the MgH2 nanoparticles. The presence of Mg2+ in the solution will compete the OH− with the MgH2 surface. As a result, the Mg(OH)2 byproduct forms dispersed suspension instead of a passivation layer on the MgH2
Acknowledgments
The authors acknowledge financial support from the National Natural Science Foundation of China (NSFC, Nos. U1201241, 11375020, 51431001 and 21321001).
Jun Chen received B.S. degree in Chemistry from Wuhan University, China in 2013. He is currently a direct promoted Ph.D. student in Prof. Xingguo Li׳s group. His research is focused on materials for hydrogen storage and hydrogen generation.
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Jun Chen received B.S. degree in Chemistry from Wuhan University, China in 2013. He is currently a direct promoted Ph.D. student in Prof. Xingguo Li׳s group. His research is focused on materials for hydrogen storage and hydrogen generation.
He Fu received B.S. degree in Chemistry from Peking University, China in 2010. He is currently a direct promoted Ph.D. student in Prof. Xingguo Li׳s group. His research is focused on materials for hydrogen storage and hydrogen generation.
YiFu Xiong received his Master׳s degree from China Academy of Engineering Physics (CAEP) in 2001. He is currently an associate professor in CAEP. His research is focused on hydrogen and its isotopes in metals.
Jinrong Xu received her Master׳s degree from Peking University, China in 2005. She is now a senior engineer in College of Chemistry and Molecular Engineering, Peking University. Her research is focused on the device engineering for hydrogen generation and hydrogen storage.
Jie Zheng received his Ph.D. degree from Peking University, China in 2009 and from Eindhoven University of Technology, the Netherlands in 2010. He is now an associate professor in College of Chemistry and Molecular Engineering, Peking University, China. His research is focused on inorganic materials for hydrogen storage/generation, batteries and fuel cells.
Xingguo Li received his Ph.D. degree from Tohoku University, Japan in 1990. He became a professor in College of Chemistry and Molecular Engineering, Peking University, China since 2000. His research covers hydrogen storage/generation materials, lithium ion batteries and plasma processing of nanomaterials.