Article pubs.acs.org/JPCC
Positive and Negative Effects of Carbon Nanotubes on the Hydrogen Sorption Kinetics of Magnesium Wupeng Cai,*,† Xiaosong Zhou,† Lidong Xia,† Kaili Jiang,‡ Shuming Peng,*,† Xinggui Long,† and Jianhua Liang† †
Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, China
‡
S Supporting Information *
ABSTRACT: Magnesium is a promising hydrogen storage material with high capacity and low cost, but suffers from sluggish kinetics of hydrogen sorption. Carbon nanotubes (CNTs) are efficient in improving the sorption performance of Mg; however, proper understanding of the roles of CNTs is still required. Here, a simple and fast codeposition method is applied to use superaligned CNTs as a three-dimensional framework to prevent the baseline Mg particles from sintering. The effect of tubular structure of CNTs on the absorption kinetics is quantitatively determined. It is observed that dehydrogenation contains an incubation stage and a following accelerated stage. The desorption rate depends on the number of metal nuclei formed during the incubation stage rather than the diffusion of hydrogen. Addition of CNTs increases the grain-boundary areas and facilitates the nucleation. However, nucleation is inhibited when adjacent MgH2 particles are separated by CNTs, indicating that the dehydrogenation of MgH2 powder likely leads to a “chain” nucleation at elevated temperatures. In the literature, nanoconfinement in carbon frameworks is widely considered as a potential method to thermodynamically destabilize MgH2, whereas our work reveals that the separation of MgH2 particles by carbon addition will suppress the enhancement of desorption kinetics resulting from nanoconfinement.
1. INTRODUCTION The most difficult hurdle in the application of hydrogen energy is the storage of hydrogen.1,2 Owing to the high gravimetric (7.6 wt % of hydrogen) and volumetric (110 kg m−3) densities and low cost, magnesium shows great potential as a hydrogen storage material.3 However, fast absorption and desorption of hydrogen requires extremely high temperatures (>300 °C), hindering the application of Mg in the hydrogen economy. Numerous efforts have been made to improve the hydrogen sorption kinetics of Mg, including the mechanical milling of Mg powder with various additives, which can be basically divided into two groups: the reactive and inert ones. Transition metals reduce the energy barrier for the dissociation and recombination of hydrogen,4,5 remarkably degrade the activation energy,6−11 and promote the nucleation for hydrogenation and dehydrogenation.12 For transition-metal oxides, such as Nb2O5, the acceleration of sorption kinetics is due to metallic Nb or its oxides with oxygen deficiency, like MgNb2O3.67, which are formed during the thermal treatment.13,14 Carbon materials, as inert additives, have attracted much attention because of their efficiency in the improvement of hydrogen sorption kinetics of Mg.5,7−11,15−18 Among various carbon allotropes, carbon nanotubes (CNTs) exhibit the most prominent effect.16 After milling, the amorphous carbon, graphitic layers, and residual tubes derived from the destruction © 2015 American Chemical Society
of CNTs aggregate along the grain boundaries and on the surface of particles, facilitating the transportation of hydrogen atoms and preventing the growth of particles and grains.9−11,17,18 Therefore, addition of CNTs induces particular enhancement in the sorption kinetics, storage capacity, and cycling stability. In the reported work, the improved kinetics of hydriding and dehydriding for Mg−CNT composites were usually both attributed to the accelerated diffusion of hydrogen. Such speculation may be controversial, considering the distinct mechanisms of hydrogenation and dehydrogenation of Mg powder: a contracting-volume (CV) behavior is characteristic of absorption, whereas desorption follows the Johnson−Mehl− Avrami (JMA) model.19 In other words, hydrogen transportation is crucial to absorption, while nucleation of the metal phase plays an important role in desorption. Hence, it is possible that CNTs act in different ways for absorption and desorption, respectively. In this study, Mg−CNT composites with a series of combination states between Mg powder and multiwalled carbon nanotubes (MWCNTs) were prepared, the sorption performance of which was compared with that of the baseline sample to discuss the roles of CNTs. Some structureReceived: September 7, 2015 Published: October 22, 2015 25282
DOI: 10.1021/acs.jpcc.5b08740 J. Phys. Chem. C 2015, 119, 25282−25290
Article
The Journal of Physical Chemistry C
Figure 1. Hydrogen absorption profiles of MC0S, MC1, MC2, MC4, and MC8 at 150 °C (a), 200 °C (b), and 250 °C (c). α is the transformed fraction. α = 100% indicates that the composite is fully hydrided. Panel d shows the activation energy for hydrogenation (Eab) versus the comilling time.
2. EXPERIMENTAL SECTION Magnesium powder (purity >99.5 wt %, 300 mesh, GRINM, China) and highly purified MWCNTs (CNT content >95 wt %, the content of residual metallic catalysts
