ZIF-67 derived [email protected] nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism

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ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism Meijia Liu a,b, Xuezhang Xiao a,b,*, Shuchun Zhao b, Sina Saremi-Yarahmadi c, Man Chen a,b, Jiaguang Zheng a,b, Shouquan Li a,b, Lixin Chen a,b,d a

State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, 310027, PR China School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China c Department of Materials, Loughborough University, Loughborough, LE11 3TU, UK d Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou, 310013, PR China b

article info

abstract

Article history:

Transition-metal nanoparticles (NPs) can catalytically improve the hydrogen desorption/

Received 12 September 2018

absorption kinetics of MgH2, yet this catalysis could be enhanced further by supporting NPs

Received in revised form

on carbon-based matrix materials. In this work, Co NPs with a uniform size of 10 nm

24 October 2018

loaded on carbon nanotubes (Co@CNTs) were synthesized in situ by carbonizing zeolitic

Accepted 9 November 2018

imidazolate framework-67 (ZIF-67). The novel Co@CNTs nanocatalyst was subsequently

Available online xxx

doped into MgH2 to remarkably improve its hydrogen storage properties. The MgH2Co@CNTs starts to obviously release hydrogen at 267.8
C, displaying complete release of

Keywords:

hydrogen at the capacity of 6.89 wt% at 300
C within 15 min. For absorption, the MgH2-

MgH2

Co@CNTs uptakes 6.15 wt% H2 at 250
C within 2 min. Moreover, both improved hydrogen

Co@CNTs

capacity and enhanced reaction kinetics of MgH2-Co@CNTs can be well preserved during

Hydrogen storage properties

the 10 cycles, which confirms the excellent cycling hydrogen storage performances. Based

Synergetic catalysis

on XRD, TEM and EDS results, the catalytic mechanism of MgH2-Co@CNTs can be ascribed to the synergetic effects of reversible phase transformation of Mg2Co to Mg2CoH5, and physical transformation of CNTs to carbon pieces. It is demonstrated that phase transformation of Mg2Co/Mg2CoH5 can act as “hydrogen gateway” to catalytically accelerate the de/rehydrogenation kinetics of MgH2. Meanwhile, the carbon pieces coated on the surfaces of MgH2 particles not only offer diffusion channels for hydrogen atoms but also prevent aggregation of MgH2 NPs, resulting in the fast reaction rate and excellent cycling hydrogen storage properties of MgH2-Co@CNTs system. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, 310027, PR China. E-mail address: [email protected] (X. Xiao). https://doi.org/10.1016/j.ijhydene.2018.11.078 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078

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Introduction Faced with severe energy crisis and environmental issues, many measures have been taken to explore renewable and clean alternative to fossil fuels. Among the various kinds of new energy vectors, hydrogen is considered as one of the most promising energy due to nearly no emission of pollutant [1e5]. Compared with the liquid and gaseous hydrogen storage, storing hydrogen in a solid state has advantages in security and high volumetric capacity. MgH2, a representative metalbased hydrogen storage materials, is a promising material because of its high hydrogen storage capacity (7.6 wt%), relatively low price and reversible hydrogen storage performance. However, high thermodynamic stability and slow kinetics have restricted its extensive practical applications. At present, the proposed solutions to these limitations are centered on alloying [6e8], nanosizing [9,10], nanoconfinement [11e13] and catalyst doping [14e17]. According to the present reported methods, doping catalyst into MgH2 plays a significant role in enhancing the hydrogen absorption and desorption performance, as a result of maintaining high capacity and obvious improvement on hydrogen storage properties. Particularly, transition metals (TMs) have proven to be effective on improving the absorption and desorption properties of MgH2. For instance, Liang et al. [18] used 3d-transition metal elements (Ti, V, Mn, Fe, Ni) to improve the sorption kinetics of MgH2, and found that composites containing Ti or V exhibited the most rapid absorption or desorption kinetics, respectively. Recently, Cui et al. [19] coated multi valence Ti-based materials (TiH2, TiO2 and TiCl3) on the surface of Mg, and the multiple valence Ti acted as the intermediate and catalytic active sites for the electron transfers between Mg2þ and H, which led to high catalytic efficiency for the hydrogen desorption. It has been confirmed that nano-sized catalysts are more effective on hydrogen absorption/desorption processes of MgH2. Moreover, In, Al, Ti and other element show dual-tuning effects on the thermodynamics and kinetics of Mg-based hydrogen storage materials [20e23]. It has been reported that In dissolved in Mg2Ni and formed Mg2In0.1Ni solid solution, reducing dehydrogenation activation energy and enthalpy change from 80 kJ/mol and 64.5 kJ/mol H2 to 28.9 kJ/ mol and 38.4 kJ/mol H2 [22], respectively. However, NPs are prone to aggregate into microparticles, which can severely deteriorate their catalytic activity. It is well known that CNTs possess big specific surface area and high electronic conductivity, which makes them as favorable candidates for catalyst carrier to uniformly disperse NPs. Large size (diameter > 100 nm) graphene tubes were used to disperse 20 wt% Pt NPs, and they demonstrated significantly enhanced oxygen reduction reaction (ORR) activity and excellent stability [24]. Recently, Co/CNTs and CoB/CNTs nanocomposites were proved to effectively facilitate the hydrogen storage properties of MgH2, because of the synergetic effect of Co, MgCo-H phase and CNTs [25,26]. However, the amount of catalytic additives of Co/CNTs and CoB/CNTs were up to 10 wt%, resulted in the obvious loss of hydrogen storage capacity for catalyzed MgH2 system. Particularly, the specific mechanism concerning the synergetic effect of CoB/CNTs was not explained [21]. Moreover, the intrinsic hydrophobicity of the

CNTs prevents the direct loading of most metal or metallic oxide NPs on their surfaces. Hence the pretreatment with strong acid for CNTs is necessary before loading NPs, modifying the edges of the tubes with carboxyl and hydroxyl groups through the oxidation reaction [27,28], which is adverse to the hydrogen storage property when used as catalyst for MgH2. Herein, we propose a novel and facile method for in situ synthesis of Co@CNTs, in which CNTs serve as carrier to disperse Co NPs. It is worth noting that the formation of CNTs is almost simultaneous with the loading of Co NPs during the in situ synthetic process. The CNTs are free of oxygen impurities without any strong acid treatment. To the best of our knowledge, no studies have been reported on the use of in situ synthesized Co@CNTs as catalyst for MgH2. In this work, the remarkably enhanced hydrogen absorption and desorption properties of MgH2 catalyzed by Co@CNTs are investigated, and the catalytic mechanism is explored and discussed in details.

Experimental methods 2-Methylimidazole (MeIM, 98% in purity) and cobalt nitrate hexahydrate (Co(NO3)2.6H2O, AR, 99% in purity) were purchased from Aladdin Industrial Corporation. Mg powders (99% in purity) were purchased from Sinopharm Chemical Reagent Co., Ltd. Active carbon (AC, 99.9% in purity) was purchased from Alfa Aesar. In this paper, the method of preparing ZIF-67 precursors was drew on the experience of which mentioned in the report [29] with some modifications, in order to explore the effects of dosage ratio on the formation of ZIF-67. ZIF-67 precursors with molar compositions of 1 Co2þ/4 MeIM/1100 H2O, 1 Co2þ/8 MeIM/1100 H2O, 1 Co2þ/58 MeIM/1100 H2O and 1 Co2þ/80 MeIM/1100 H2O were synthesized and collected, respectively. Then 2 g of the obtained ZIF-67 precursors by different dosage ratios was heated at 600
C with heating rate of 1
C/min under Ar atmosphere in tube furnace, followed by a continuous treatment for 6 h. Finally, the black powders of Co NPs loaded on carbon nanotubes or carbon sheets (Co@CNTs, Co@CSs) were obtained. In order to acquire higher purity MgH2, commercial Mg powders were treated by commutative ball milling and hydrogenation to prepare the MgH2. The specific experimental steps were referred and described in our previous paper [30]. The self-prepared MgH2 (97.6% in purity), MgH2-5 wt.% Co@CSs and MgH2-5 wt.% Co@CNTs samples were ball milled for 2 h at a speed of 400 rpm with the ball-to-powder weight ratio of 40:1 under a H2 pressure of 10 bar, which are denoted as as-milled MgH2, MgH2-Co@CSs and MgH2-Co@CNTs, respectively. In order to illuminate the catalytic effect of the carbon materials on the hydrogen storage properties of MgH2, 5 wt% AC was also doped into MgH2 by ball milling in the same conditions, which is denoted as MgH2-AC. All of the materials handing was carried out in a glove box with O2 and H2O concentrations<1 ppm. X-ray diffraction (XRD) analyses of all samples were carried on an XPert Pro X-ray diffractometer (PANalytical, the Netherlands) using Cu Ka radiation at 40 kV and 40 mA to

Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078

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detect the phase compositions. The morphology, microstructures and element distribution were characterized by scanning electron microscopy (SEM, Hitachi SU-70) and transmission electron microscopy (TEM, Tecnai G2 F20 STWIN) with an energy dispersive spectroscopy (EDS). The dehydrogenation properties at different heating rates (2, 5, 8 and 10
C/min) were investigated by differential scanning calorimetry (DSC, Netzch STA 449F3). The hydrogen absorption and desorption performances were conducted by a Sieverts type volumetric apparatus. Temperature programmed desorption (TPD) tests were heated from room temperature to 450
C with a heating rate of 5
C/min. The isothermal desorption properties were tested at various temperature (250
C, 280
C, 300
C and 325
C) under hydrogen pressure below 0.02 bar, and the isothermal absorption properties were measured at various temperature (200
C, 250
C, 280
C and 300
C) under hydrogen pressure of 30 bar. Cycling stability was determined under the conditions of isothermal dehydrogenation (0.02 bar of H2) and hydrogenation (30 bar of H2) at 280
C.

Results and discussion In a typical bottom-up synthesis process, the Co@CNTs were in situ synthesized by combining hydrothermal method with pyrolytic technique. Fig. 1 exhibits the SEM and TEM images of the as-synthesized ZIF-67 precursors and carbonized products. SEM images in Fig. 1(a)e(d) capture the morphology evolution of the ZIF-67 particles while increasing the

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proportion of MeIM. At a molar composition of 1 Co2þ/4 MeIM/ 1100 H2O, the obtained particles show poor crystallinity and uniformity without rule-governed structures, which is far away from the typical ZIF-67 structure. Some edges of the particles can be observed when the proportion of MeIM is doubled, but the crystallinity and uniformity are still dissatisfactory. When sharply increasing the molar ratio of Co2þ: MeIM to 1:80, regular dodecahedrons with smooth surfaces are synthesized, and the particle sizes have a uniform distribution of 800e1000 nm. Additionally, XRD results in Fig. S1 agree well with the morphology evolution of ZIF-67 particles. Higher proportion of MeIM leads to more defined diffraction peaks, indicating the better crystallinity of ZIF-67 precursors. It has been reported that MeIM is closely relevant to the construction of the ZIF-67, and the Co NPs serve as centre for the framework [31,32]. However, it seems that ratios of Co2þ: MeIM ¼ 1:4 or 1:8 are not sufficient to drive the formation of dodecahedrons. The obtained ZIF-67 precursors with various morphologies were carbonized at 600
C under Ar atmosphere. As shown in Fig. 1(e)e(h), there are mainly two types of carbon-based materials generated after carbonization. Combining with XRD (Fig. S2) and TEM (Fig. 1(i)e(l)) results, Co NPs with different particle sizes are found loading on the carbon based materials during the in situ reduction reaction. It is postulated that the ligands of ZIF-67 precursors are decomposed into carbon chains when pyrolyzing the 2-MeIM, and Co element is reduced simultaneously. It is worth noting that ZIF-67 precursors with irregular morphology (Fig. 1(a)e(b)) tend to form CNTs loaded with Co NPs after carbonization, and the main difference lies in the tube

Fig. 1 e Typical SEM and TEM images of the as-synthesized ZIF-67 precursors with different dosage ratios, and the products after carbonizing. (a, e, i, g) 1 Co2þ/4 MeIM/1100 H2O, (b, f) 1 Co2þ/8 MeIM/1100 H2O, (c, g, k, l) 1 Co2þ/58 MeIM/1100 H2O, (d, h) 1 Co2þ/80 MeIM/1100 H2O. Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078

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Fig. 2 e XRD patterns of the samples after ball milling: (a) as-milled MgH2, (b) MgH2-AC, (c, f) MgH2-Co@CSs, (d, e) MgH2-Co@CNTs.

diameter. Evidently, along with doubling the proportion of MeIM, the mean diameter of tubes is increased by 2e3 times than those in Fig. 1(e). After carbonizing the regular dodecahedrons (Fig. 1(c)e(d)), Co NPs are embedded or loaded on the carbon sheets (CSs) instead of CNTs. It is worth noting that Co NPs loaded on the CNTs are much finer than those on the CSs, with mean particle size of about 10 nm. Therefore, it is concluded that the initial morphology of ZIF-67 precursors determines the types of carbon-based carrier and the particle size of Co NPs after carbonization. In particular, CNTs loaded with fine Co NPs are more likely to generate from ZIF-67 precursors with irregular morphology rather than regular dodecahedrons. Importantly, Co NPs uniformly disperse on the surfaces of CNTs and CSs carriers, which are named as Co@CNTs and Co@CSs, are successfully synthesized by a novel in situ hydrothermal and pyrolytic method. The phase compositions of MgH2 and MgH2 doped with various carbon based materials by ball milling were determined by XRD in Fig. 2. It can be seen that the diffraction peaks of MgH2 doped with carbon based materials in Fig. 2(b)e(d) become much wider than those of the as-milled MgH2 (Fig. 2(a)), which confirms the grain refinement by introducing the AC (Fig. S3), Co@CSs, and Co@CNTs. Additionally, the diffraction peaks of Co are detected in the MgH2eCo@CSs and MgH2eCo@CNTs composites, verifying that there is no obvious chemical reaction occurring between MgH2 and carbon based materials during the process of ball milling. However, CNTs are not detected by the XRD after ball milling, even when studied in the Co@CNTs state (Fig. S2(a)). Similarly, CNTs were not seen in the XRD patterns after introducing to the LiBH4eMgH2 composites after ball milling [33]. Weak peaks of MgO are detected in all the XRD patterns, possibly due to the little impurities originating from raw Mg powder.

In order to examine the catalytic effect of the carbon based additives on the hydrogen desorption property of MgH2, TPD and DSC analyses were performed. Fig. 3 depicts the hydrogen desorption kinetics of carbon-based additives doped and undoped MgH2 samples, revealing a single step of releasing hydrogen. It is observed that the introducing of carbon based materials can reduce the onset and peak temperatures in various degree, and in particular, Co@CNTs seems to be the most effective one. Based on the DSC results in Fig. S4, the asmilled MgH2 begins to release hydrogen at 330.2
C, and finally reached a hydrogen capacity of 7.42 wt%. This hydrogen desorption capacity is very close to the theoretical hydrogen storage capacity of MgH2 (7.6 wt%), indicating that the purity

Fig. 3 e TPD curves of the as-milled MgH2, MgH2-AC, MgH2-Co@CSs and MgH2-Co@CNTs.

Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078

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of such self-prepared MgH2 is very high. Importantly, the hydrogen desorption initial temperatures could be reduced to 291.5
C, 281.3
C and 268.7
C for the MgH2-AC, MgH2Co@CSs and MgH2-Co@CNTs samples, respectively. At the same time, the hydrogen desorption peak temperatures of the MgH2-AC, MgH2-Co@CSs and MgH2-Co@CNTs samples are reduced to 333.4
C, 317.5
C and 303.3
C, which are considerably lower than that of the as-milled MgH2 (368.9
C). Finally, high desorption capacity exceeding 7.12 wt% is reached by all the MgH2 samples doped with carbon-based materials. It can be seen that the respective introduction of AC, Co@CSs and Co@CNTs enhances the hydrogen desorption kinetics and reduces the onset and peak temperatures, but the catalytic effects of different carbon-based additives on the dehydrogenation differ from each other depending on composition and morphology. Notably, carbon-based materials loaded with Co NPs (Co@CSs and Co@CNTs) act more effectively than AC. Moreover, different types of carbon-based materials loaded with Co NPs show distinct catalytic effect, and Co NPs loaded on CNTs seem more effective than those on CSs in terms of reducing the onset and peak temperature and enhancing the hydrogen desorption kinetics, which may result from the small particle sizes. Consequently, the catalytic effect of Co@CNTs on the dehydrogenation and rehydrogenation of MgH2 will be further studied in details. Considering the positive effect of Co@CNTs on the hydrogen desorption property of MgH2, it is relevant to further explore the isothermal hydrogen absorption and desorption properties of MgH2-Co@CNTs at various temperature, and the results are shown in Fig. 4. For isothermal hydrogen desorption performance, MgH2-Co@CNTs can release 3.92 wt% H2 within 100 min even at temperature as low as 250
C. Moreover, MgH2-Co@CNTs shows complete hydrogen desorption process with faster kinetics even at relatively low temperature of 280
C, which can release 6.26 wt% H2 within 50 min. As the hydrogen desorption temperature increases to 300
C, not only

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the desorption capacity is raised, but also the dehydrogenation kinetics are improved, the MgH2-Co@CNTs can rapidly obtain the maximum hydrogen capacity of 6.48 wt% in less than 16 min. At 325
C, 6.89 wt% of H2 is released within 10 min, corresponding to 95.43% of the theoretical capacity value (7.22 wt%) of MgH2-Co@CNTs. By contrast, the as-milled MgH2 sample shows poor hydrogen desorption kinetics, as shown in Fig. S5. At a low temperature of 280
C, extremely little H2 (0.67 wt%) can be released within 100 min. Unless a relative high temperature of 325
C is applied for the as-milled MgH2, 5.38 wt% H2 is released within 100 min, only equal to 70.79% of the theoretical capacity value (7.6 wt%) of MgH2. Moreover, similar trends can be observed from the isothermal hydrogen absorption performance. With the temperature increasing, the hydrogenation kinetics and hydrogen absorption capacity are enhanced. At 200
C, MgH2-Co@CNTs obtains a hydrogen capacity of 6.02 wt% within 4 min. In particular, the hydrogen absorption time is reduced to 1.5 min when the temperature increases to 300
C, reaching a hydrogen capacity of 6.89 wt%. Whereas, as shown in Fig. S6, the as-milled MgH2 requires 10 min to achieve a capacity of 5.96 wt%, and even at 300
C, 5 min is needed to achieve the hydrogen absorption balance. As a result, it is concluded that Co@CNTs act as an excellent catalyst to improve desorption and absorption properties of MgH2. It has been experimentally confirmed that Co@CNTs shows excellent catalytic effect on reducing the hydrogen desorption temperature of MgH2, whilst their effect on the hydrogen desorption kinetics is not clear. To further understand the enhanced kinetics of MgH2-Co@CNTs, the activation energy (Ea) of the hydrogen desorption is calculated based on the Kissinger method [34], which can be described as follows: b ln 2 Tp

!

Ea AR ¼ þ ln Ea RTp

Fig. 4 e (a) Isothermal desorption curves under 0.02 bar and (b) isothermal absorption curves under 30 bar for the MgH2-Co@CNTs composites at different temperature. Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078

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Fig. 5 e DSC curves of (a) as-milled MgH2 and (b) MgH2-Co@CNTs at various heating rates (2, 5, 8 and 10
C/min) and (c) estimation of the apparent active energies using the Kissinger method.

where b, Tp, A and R are heating rate, absolute temperature related to the maximum desorption rate, the pre-exponential factor and gas constant, respectively. The values of Tp for the as-milled MgH2 and MgH2-Co@CNTs samples are obtained from the DSC curves with various heating rates (2, 5, 8 and 10
C/min), as shown in Fig. 5. Subsequently, based on the   good linear relationship between ln Tb2 and T1m , the values of Ea m

for the as-milled MgH2 and MgH2-Co@CNTs are respectively calculated to be 178.00 and 130.36 kJ/mol from the slope of the two nearly straight lines in Fig. 5(c). This implies that the doping of Co@CNTs catalyst contributes to the distinct decreasing Ea, as well improving the dehydrogenation kinetics. Compared with other Mg-RE system ever published [35e38], such as Mg17Ba2 (173.92 kJ/mol) [38], MgH2-Co@CNTs show relatively low activation energy, but higher than that of CaMg2-based alloys (41.74 kJ/mol) [35]. What is more, preserving long-term kinetic is a great challenge in the practical hydrogen storage applications, so the cycling performance of MgH2-Co@CNTs will be further studied. Based on the superior hydrogen absorption and desorption properties of MgH2 doped with Co@CNTs, more detailed investigation is focused on its performance during the process of several desorption and absorption processes. As shown in Fig. 6, the MgH2-Co@CNTs was further subjected to the cycling test under the conditions of isothermal dehydrogenation (0.02 bar of H2) and hydrogenation (30 bar of H2) at 280
C. It is evident that both the capacity and kinetics are well preserved from the first to the tenth cycle, which suggests the excellent cycling performance of the MgH2-Co@CNTs composite. Under these experimental conditions, the catalyzed MgH2 can steadily release 6.26 wt% H2, whilst the hydrogenation is quickly achieved within 3 min. In general, MgH2 particles tend to grow and aggregate during the thermolysis, which leads to the degenerating cycling properties [39,40]. The above excellent cycling hydrogen absorption and desorption properties indicate that doping Co@CNTs into MgH2 could possibly

prevent the growth and aggregation of the particles, and this speculation was examined by SEM and TEM in the next section. Moreover, the remarkable cycling performances of MgH2-Co@CNTs are believed to make them promising solid hydrogen storage system for future development. As presented above, Co@CNTs shows excellent catalytic effect on the reversible hydrogen storage properties of MgH2. Therefore, the microstructure analyses of MgH2-Co@CNTs in de/rehydrogenation states were further conducted by the combination of XRD, SEM and TEM-EDS measurements to shed light on the possible morphological and phase transformations during the catalytic mechanism of Co@CNTs. As shown in Fig. 7, Mg and MgH2 are the main phases after dehydrogenation and rehydrogenation processes, suggesting a complete hydrogen desorption and absorption during the cycling. To clearly find out the evolution of Co@CNTs in the process of hydrogen storage, the yellow and green rectangles parts are enlarged. Importantly, weak diffraction peak of

Fig. 6 e Cycling profiles of MgH2-Co@CNTs with dehydrogenation conditions of 280
C under 0.02 bar of H2 and rehydrogenation conditions of 280
C under 30 bar of H2.

Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078

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Fig. 7 e XRD patterns of MgH2-Co@CNTs in different states, (a, c) dehydrogenated state, (b, d) rehydrogenated state.

Mg2Co is detected in the dehydrogenation state, which makes great difference from the phase structure of Co in the MgH2Co@CNTs mixture before dehydrogenation in Fig. 2(d). During the rehydrogenation, Mg can reabsorb H2 and generate MgH2, while Mg2Co transforms into Mg2CoH5, as shown in Fig. 7(b). It is believed that the transformation between Mg2Co and Mg2CoH5 during cycling can be one of the reasons behind Co@CNTs exhibiting excellent catalytic effect on hydrogen storage properties of MgH2. To further address this viewpoint, SEM measurements were conducted to observe morphologies of these samples. It can be learned from Fig. 8(a) and (d) that the sizes of the most particles in ball milling state are reduced with the doping of Co@CNTs. It is well known that smaller particle sizes can improve the de/rehydrogenation kinetics because of abundant defects and increased specific surface area [41,42], which accounts for the enhanced hydrogen absorption and desorption properties MgH2-Co@CNTs. Whereas, there is no determined shapes of CNTs exhibiting after ball milling, suggesting the original structure of CNTs is broken by the mechanical force. After dehydrogenation, the particles of the as-milled MgH2 present severe agglomeration state with increased sizes. In sharp contrast, MgH2 doped with Co@CNTs shows relatively more dispersed and smaller particles. Additionally, both as-milled MgH2 and MgH2-Co@CNTs particles tend to grow and aggregate in varying degree after reabsorbed hydrogen, but doping Co@CNTs can restrain this trend to some extent, which contributes to the faster hydrogen desorption kinetics. As mentioned above, catalytic effect of Co@CNTs lies in the transformation between Mg2Co and Mg2CoH5 during cycling, but the distribution of Co@CNTs on the MgH2 particles, which plays an another important role in determining the catalytic efficiency, keeps unknown to us. Taking this into

consideration, TEM, HRTEM and EDS were used to investigate the morphology, microstructure and element distribution of the MgH2-Co@CNTs after ball milling and cycling. Particles free of agglomeration are clearly observed with an average size of about 300 nm in Fig. 9. Based on the EDS data, it is concluded that the elemental carbon is distributed homogeneously on the surfaces of MgH2 particles. It should be noted that no obvious tube structures can be seen, probably the initial morphology is destroyed by the mechanical force, which coincides well with the SEM result in Fig. 8(d). The broken CNTs, which may exist in the form of carbon pieces, are easily to be separated on the surfaces of MgH2 particles, effectively preventing the aggregation of the MgH2/Mg particles during hydrogen storage process. Different from the uniform distribution of carbon, elemental Co distributes with some enrichment areas. Moreover, the lattice spacing of 0.217 nm confirms that Co exists in its metallic phase, which is in good accordance with the result that there is no chemical reaction between Co and MgH2 during the ball milling in Fig. 2(d) and (e). Hence, Co NPs loaded on the carbon pieces can effectively restrict the aggregation of the MgH2 particles during the ball milling, which is in favour of improving the cycling hydrogen absorption and desorption kinetics. Based on the remarkable cycling stability of MgH2Co@CNTs after 10th dehydrogenation, the microstructure was investigated by TEM to further elucidate the catalytic mechanism of Co@CNTs. It can be seen that the particle sizes after 10th dehydrogenation (Fig. 10) are close to those in ball milling state with average particle size of about 250 nm (Fig. 9), suggesting that there is no obvious particle growth during the cycling process. By contrast, as-milled MgH2 particle sizes after cycle increase to over 400 nm, as shown in Fig. S7. It is well known that degenerating cyclic properties are usually caused by growth and aggregation of the particles [3,43,44], so

Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078

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Fig. 8 e SEM images of the as-milled MgH2 and MgH2-Co@CNTs in different state, (a, d) ball milling state, (b, e) dehydrogenated state, (c, f) rehydrogenated state.

the well preserved absorption and desorption kinetics of MgH2-Co@CNTs can be partially ascribed to the preservation of particles size without obvious growth. From the perspective of darkness, the particles after cycling show lighter color due to the release of hydrogen [39]. However, dark outline can be observed in these particles, which may be caused by the carbon pieces coated on their surfaces. Hence, it demonstrates that Co@CNTs indeed plays an important role in restricting the growth and aggregation of the particles, which contributes to the excellent cycling performance of MgH2. According to the HRTEM patterns, Mg2Co (lattice spacing of 0.219 nm) is found on the surface of MgH2 particles. Additionally, XRD results in Fig. S8 confirmed that reversible chemical transformation between Mg2Co and Mg2CoH5, which contributes to excellent cycling performances of MgH2-Co@CNTs. It has been reported that the formation of Mg2Co contributes to the hydrogen diffusion within the grains of Mg, being a key factor influencing the hydrogen sorption kinetics [15,26,45]. What is more, due to the direct contact of Co within the Mg matrix, Mg2Co becomes predominant phase in which Co exists after dehydrogenation, which may be the attributing reason why

MgH2-Co@CNTs displays outstanding hydrogen absorption and desorption kinetics. As mentioned above, Co@CNTs shows satisfactory catalytic effect on improving hydrogen absorption and desorption properties of MgH2, and the mechanism has been deeply explored by XRD, SEM, TEM, HRTEM and EDS. For intuitionistic exhibition, Fig. 11 describes two main processes, involving in the formation of the Co@CNTs and the mix process of the MgH2 and Co@CNTs. During the heat treatment, 2-MeIM is decomposed and amounts of carbon gases are released. Meanwhile, Co NPs are generated by in situ reduction reaction accompanying with the formation of CNTs, which is usually referred to tip-growth mechanism [46]. An excellent balance among the kinetics of the pyrolysis and reduction contributes to the in situ synthesis of Co@CNTs with uniform tube diameter and particle size. As mentioned above, Co@CNTs shows outstanding catalytic effect on the hydrogen desorption and absorption of MgH2, and the catalytic mechanism mainly lies in two factors. On the one hand, Co NPs loaded on the carbon carriers can react with MgH2 and generate Mg2Co in the hydrogen desorption process, then

Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078

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Fig. 9 e TEM images, HRTEM patterns and EDS results of MgH2-Co@CNTs after ball milling.

Mg2Co transforms into Mg2CoH5 reversibly during the subsequent hydrogen absorption process. The reversible phase transformation between Mg2Co and Mg2CoH5 acting as “hydrogen gateway” to catalytically accelerate the de/rehydrogenation kinetics of MgH2 [47]. On the other hand, the physical transformation of the CNTs should be responsible for the excellent hydrogen absorption and desorption cycling properties. Particularly, CNTs are ball milled into carbon pieces by the mechanical force and covered on the surfaces of the MgH2 particles. The carbon pieces coated on the surfaces of MgH2 particles offer diffusion channels for the hydrogen atoms, resulting in the faster reaction rate. Moreover, the

high specific surface area and thin layers of CNTs contribute to the homogeneous distribution of the carbon pieces, preventing the growth and aggregation of MgH2 particles during the cycling process. The synergetic effect of elemental Co and carbon materials in our work, which combines with chemical phase transition with physical morphology transition and achieves superior catalytic effect, distinctly differs from the conventional synergetic modification ever reported [48e50]. In summary, the synergetic catalysis of the reversible Mg2Co/Mg2CoH5 “hydrogen gateway” catalytic active sites and carbon pieces coated around MgH2 particles are the main mechanisms responsible for the excellent cycling hydrogen

Fig. 10 e TEM images and HRTEM patterns of MgH2-Co@CNTs after 10th dehydrogenation. Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078

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international journal of hydrogen energy xxx (xxxx) xxx

Research Team in University of Ministry of Education of China (IRT13037).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.078.

references

Fig. 11 e Schematic illustration for the Co@CNTs generation and the synergetic catalytic effect on MgH2.

desorption and absorption properties of MgH2-Co@CNTs system.

Conclusions In summary, Co@CNTs is in situ synthesized by carbonizing ZIF-67 precursors, combining hydrothermal method with pyrolytic technique. The as-synthesized Co@CNTs are doped into MgH2 by ball milling, showing superior catalytic performance on the hydrogen desorption of MgH2 with full release of hydrogen at 300
C within 15 min. For absorption kinetics, MgH2-Co@CNTs absorb 6.15 wt% H2 within 2 min. Moreover, MgH2-Co@CNTs show excellent cycling performance, preserving the capacity and kinetics well from the first to the tenth cycle. The apparent activation energy for the MgH2-Co@CNTs composites is estimated to be 130.36 kJ/mol by the Kissinger method. The catalytic effect of Co@CNTs mainly lies in two factors. Co NPs contribute to the reversible transformation between Mg2Co and Mg2CoH5 during the cycling processes, which can act as “hydrogen gateway” to catalytically accelerate the hydrogen desorption and absorption kinetics of MgH2. CNTs are laniated into carbon pieces, which homogeneously covered on the surfaces of MgH2 particles, preventing the aggregation of MgH2 particles and providing large numbers of diffusion channels for hydrogen atoms. This synergetic effect of the reversible Mg2Co/Mg2CoH5 transformation and carbon pieces coated on the MgH2 particles is in favour of reducing the hydrogen desorption/absorption temperatures and further improves the reaction kinetics of MgH2. Furthermore, these findings offer an effective and promising method for catalytically improving the reversible hydrogen storage performances of Mg-based hydrogen storage materials.

Acknowledgements The authors gratefully acknowledge the financial supports for this research from the National Natural Science Foundation of China (51571179 and 51671173), the Program for Innovative

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Please cite this article as: Liu M et al., ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.078