Improvement of Hydrogen Storage Properties of MgH2 Catalyzed by

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Improvement of Hydrogen Storage Properties of MgH Catalysed by KNbF and Multiwall Carbon Nanotube 2

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Muhammad Syarifuddin Yahya, and Mohammad Ismail J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02162 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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The Journal of Physical Chemistry

Improvement of Hydrogen Storage Properties of MgH2 Catalysed by K2NbF7 and Multiwall Carbon Nanotube M. S. Yahya, M. Ismail* School of Ocean Engineering, University Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia *Corresponding author. Tel.: +60 9 6683487; fax: +60 9 6683991. Email address: [email protected]

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Abstract A study has been conducted on the enhancement of the MgH2 hydrogen storage properties by the 10 wt% of K2NbF7 and 5 wt% of MWCNT. The composites are prepared using ball milling method. The dopants have significantly reduced the initial decomposition temperature of the MgH2 – 10 wt% K2NbF7 – 5wt% MWCNT composite to 248 oC with a capacity of 6.2 wt%. The composite is able to absorb 5.2 wt% of hydrogen at 150 oC in 60 minutes. In addition, 6 wt% of hydrogen is released at 320 oC and 1 atm. The cycle performance study has shown that the absorption capacity of the co-doped composite is reduced by 0.4 wt% while the milled MgH2 is reduced by 4.6 wt% after 5 cycles. The co-doped composite decomposition activation energy is 75.9 kJ/mol, 59.4 kJ/mol lower than the milled MgH2. XRD analyses have found the formation of MgF2, KH and Nb after the heating process. The formed elements are believed to be the active species that helped to improve the hydrogen storage properties of the composite. In addition, the incorporation of MWCNT are believed to play a significant role on the improvement of the modified system.


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1. Introduction Hydrogen is seen as a benign and viable alternative for energy storage. Attractive properties like high energy density, which is three times higher than petroleum1 and environmental friendly that releases water vapour as the waste product are hard to be missed. In general, hydrogen can be stored in three forms; gas, liquid and solid. Unlike compressed and liquid hydrogen storages, solid-state hydrogen storage technology is still not commercially available. Albeit of being relatively new technology, the solid-state hydrogen storage offers several advantages over the compressed and liquid hydrogen storages. The storage method provides higher energy capacity with moderate temperature and pressure requirements.2 On the other hand, high working pressure (70 MPa) is required by the compressed hydrogen storage to achieve a storage capacity of 4.8 wt%. As for the liquid hydrogen storage, the uncompromising temperature precondition has limited its application. Since hydrogen has to be stored at -253 oC, it causes a significant amount of energy losses (up to 40%) which unsuitable for a long-term hydrogen storage.3 Magnesium hydride (MgH2) is regarded as one of the most propitious candidates for the solid-state hydrogen storage material. This lightweight hydride has a high theoretical storage capacity, up to 7.6 wt% of hydrogen and a volumetric density of 110 g/L.4-5 Regretfully, unfavourable properties like poor sorption kinetics and high decomposition temperature have limited its commercialization.6 Thus, numerous efforts have been done by researchers to unravel the issues such as particles’ surface and size modifications by ball milling method7-9, plasma milling method10-11 and catalyst dopings.12-18 One of the favourable catalysts used for the solid-state hydrogen storage materials is potassium (K). This catalyst has been substantially studied by researchers and for instance, Wang et al.19 reported that the decomposition peak temperature of Mg(NH2)2/2.0LiH composite was reduced by 50 oC with the addition of 3 mol% of KH. Meanwhile, Dong et Page 3 of 33 ACS Paragon Plus Environment


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al.20 studied four potential potassium compound; KBr, KCl, KF and KOH on LiH-NH3 system. The studied catalysts were found to be beneficial in improving the kinetic properties of the LiH-NH3 system. On the other hand, for Liu et al.21 reported the synergistic catalytic effects of K, Ti and F. Interestingly, the 0.025 mol of K2TiF6 was able to reduce the decomposition temperature of NaAlH4 to 100 oC. As for the transition metal compounds, NbF5 has been acknowledged as one of the catalysts that is beneficial on the enhancement of hydrogen storage properties of MgH2. A number of studies have corroborated the catalytic effects of the NbF5. For example, Floriano et al.4 reported that 2 mol% of NbF5 had a better catalytic effect as compared to 2 mol% of FeF3 in the context of MgH2 desorption kinetic performance. A study by Mao et al.22 found that at 400 oC, the addition of 0.1 mol of NbF5 was able to ameliorate the hydrogen absorption capacity of LiBH4 – MgH2 composite by 1.6 wt%. Meanwhile, Pighin et al.23 showed that 7 mol% of NbF5 that was milled for 80 hours with MgH2 was able to absorb 2 wt% of hydrogen at 250 oC in 200s as compared to half of the capacity for the MgH2 without any catalyst under similar condition. In addition, Recham et al.24 learnt that a composite made of MgH2 and 2 mol% of NbF5 that was milled for 25 hours was able to dehydrogenate for about 3.2 wt% of hydrogen at 200 oC in 50 minutes. Furthermore, for the milled MgH2, no dehydrogenation activity was reported for any temperature below 250 oC. As for Luo et al.25, they asserted that 2 mol% of NbF5 enhanced the sorption kinetic property of MgH2. At 300 o

C, the composite was reported to be able to absorb 5wt% of hydrogen in 12 s. Other than K and NbF5, other reactive catalysts were previously reported for the

enhancement of the hydrogen storage properties of the Mg-based material. For instance, Ma et al.26 found that the CaMg1.9Ni0.1 alloy was able to absorb 5.1 wt% of hydrogen at 25 oC in 16 hours. Meanwhile, Ouyang et al.27 reported the catalytic properties of In and Ni on the performance enhancements of the Mg-based material. The studied Mg2In0.1Ni solid solution Page 4 of 33 ACS Paragon Plus Environment

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was found to have a decomposition activation energy (Ea) of 28.9 kJ/mol and enthalpy value of 38.4 kJ/mol H2. In the other study, Ouyang et al.28 reported an investigation on Mg(In)MgF2 that was synthesized via plasma milling method. The plasma milling method was found the be efficiently synthesized the Mg(In) solid solution. Furthermore, the MgF2 catalyst was formed in-situ during the milling process and the composite was able to release about 4.5 wt% of hydrogen in 15 minutes at 350 oC. The decomposition Ea of the composite was found to be 127.7 kJ/mol. While Cao et al.29 studied the effect of In, Al and Ti on the Mg-based material. The Mg86In5Al5Ti5 alloy was prepared in one step method by means of plasma milling. MgF2 catalyst that was formed in situ during the milling process was introduced into the alloy. The alloy was able to release about 6.4 wt% of hydrogen in 30 minutes at 346 oC and the decomposition Ea was found to be 125.2 kJ/mol. As for Xia et al.30, they had reported a study on MgH2 nanoparticles that were self-assembled on graphene. With the help of Ni as the active catalyst, the composite was able to release about 5.4 wt% of hydrogen in 30 minutes at 250 oC. In addition, the decomposition Ea of the composite was reported to be 64.7 kJ/mol. The effects of Ag and Al on the Mg-based material were studied by Lu et al.31 through the Mg80Ag15Al15 composite. The composite was able to absorb about 2.5 wt% of hydrogen at 200 oC in 60 minutes. In addition, at 320 oC, the composite was able to release about 3.3 wt% of hydrogen in 120 minutes. As for the decomposition Ea of the composite, the calculated value was 124.8 kJ/mol. Despite reactive catalysts, non-reactive and inert additives that provide good catalytic characteristics are worth mentioning. In this regard, additives like carbon nanotubes (CNT), silicon carbide (SiC), titanium carbide (TiC) and strontium titanate (SrTiO3) act as a cracking agent to reduce the size of particles. Interestingly, a study by Huang and Chuang32 stated that 5 wt% of single-wall CNT provided a notable catalytic effect to an activated composite of MgH2 – 5wt% ZrO2. The rehydrogenation capacity of MgH2 – ZrO2 - CNT was claimed to be Page 5 of 33 ACS Paragon Plus Environment


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4.0 wt% of hydrogen at 25 oC. Meanwhile, Luo et al.33 discovered that the incorporation of 5 wt% of single-wall CNT into MgH2 – 5wt% NbF5 composite was able to enhance the rehydrogenation capacity by 0.5 wt% and 0.8 wt% at 200 oC and 100 oC, respectively. As for the ceramics like SiC and TiC, they are well known due to their hardness which is in the range of 8 to 9 Mohs.34-35 Referring to Imamura et al.,36 the initial dehydrogenation temperature of the MgH2 – SiC composite was lowered to 207 oC and 164 oC with the addition of 10 mol% and 75 mol% of SiC. Then, Fan et al.37 substantiated that the incorporation of 10 wt% of TiC into MgH2 had reduced the decomposition temperature and activation energy by 95 oC and 46.7 kJ/mol respectively as compared to ball milled MgH2. Meanwhile, Yahya and Ismail38 have recently published a study on the synergistic catalytic properties of SrTiO3 and Ni on MgH2 system. The introduction of the catalyst-additive dopant was vindicated to be able to increase the rehydrogenation capacity from 1.1 wt% of hydrogen (milled MgH2) to 4.3 wt% of hydrogen (milled MgH2 with SrTiO3 and Ni) at 150 oC. To the best of our knowledge, no study has been done on the catalytic effects of potassium, a transitional metal compound that contains Nb and F as well as carbon nanotube. Thus it is riveting to study the catalytic effects of these materials. In this study, the synergistic catalytic effects of K2NbF7 and carbon nanotube on MgH2 system potassium are presented. The current work is an extension of our previous study on MgH2 - K2NbF7 which has been submitted for publication. Catalytic effects of the catalysts were assessed and analyzed based on Sieverts-type pressure-composition-temperature (PCT), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscope (SEM).


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2. Methodology 2.1 Preparation of samples Pristine hydrogen storage grade MgH2 (98% purity), K2NbF7 (98% purity) and multi-walled carbon nanotubes (MWCNTs) (>90% carbon basis) were supplied by Sigma-Aldrich. All the starting materials were used without further purification. In order to avoid contamination from oxygen and water moisture, all sample preparations were done in a glovebox with an argon atmosphere (MBraun Unilab). The compositions of the prepared samples are shown as in Table 1. Each composite was ball-milled in a sealed stainless steel jar with four stainless steel ball. The ratio of the weight of the composite to the weight of the steel balls was about 100 to 1. Then each composite was ball-milled in a planetary ball mill (NQM-0.4) for an hour with a rotation speed of 400 rpm. Table 1: Composition of the prepared samples Composites

Catalyst/Additive

Pristine MgH2 (reference) Milled MgH2 MgH2 - 10 wt% K2NbF7 MgH2 - 10 wt% K2NbF7 – 5 wt% MWCNT MgH2 - 10 wt% K2NbF7 – 10 wt% MWCNT

K2NbF7

MWCNT

No No 10 wt% 10 wt% 10 wt%

No No No 5 wt% 10 wt%

2.2 Characterization of samples A set of characterizations were done to each composite to study their performance of hydrogen storage properties. A Sievert-type pressure-composition-temperature (PCT) equipment from Advanced Materials Corporation was used to study the onset dehydrogenation temperature and the sorption kinetics of the composites. The heating rate



used for the characterization was 5

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o

C/min. Meanwhile, for the sorption kinetics

characterization, the hydrogen pressure was set to 27 atm for the absorption kinetics and 1 atm for the rehydrogenation kinetics. A differential scanning calorimetry, DSC (Mettler Toledo TGA/DSC 1) equipment was used to obtain the decomposition peak temperature for four heating rates; 15 oC/min, 20 o

C/min, 25 oC/min and 30 oC/min. Each test was done from room temperature to 500 oC with

an argon flow of 50 ml/min. As for the microstructure observations, scanning electron microscopy (SEM; JEOL JSM-6360LA) was used. EDS mode was applied to observed the elements distribution. Meanwhile, an X-ray diffraction (Rigaku MiniFlex II diffractometer with Cu Kα radiation) equipment was used to investigate the phase composition in the composites.

3. Results and discussions 3.1 Decomposition temperature profiles The incorporation of K2NbF7 and MWCNT have shown favourable catalytic effects on the improvement of hydrogen storage properties of MgH2. The ball milling and the synergistic catalytic effect of the dopants on the decomposition temperature of MgH2 can be perceived from Fig. 1. Referring to the figure, the initial decomposition temperature of the pristine MgH2 is 414 oC. After one hour of ball milling process, the initial decomposition temperature of the milled MgH2 is reduced by 84 oC. The milled MgH2 initial decomposition temperature reduction substantiates the positive effect of the ball milling which in agreement with others work.8, 23, 39 In this regard, the ball milling method is able to induce the formation of surface defects and to diminish the size of particles that will significantly provide larger surface areas for the hydrogen sorptions.7, 40 Page 8 of 33 ACS Paragon Plus Environment

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Fig. 1: Decomposition temperature profiles of the pristine MgH2, milled MgH2, MgH2 – 10 wt% K2NbF7, MgH2 – 10 wt% K2NbF7 – 5 wt% of MWCNT and MgH2 – 10 wt% K2NbF7 – 10 wt% of MWCNT.

An introduction of 10 wt% of K2NbF7 into the MgH2 system has reduced the initial decomposition temperature to 270 oC. Meanwhile, for the composite with the incorporation of the K2NbF7 and CNT, further temperature reduction can be attained as in Fig. 1. As for the MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite, the initial decomposition has been reduced by 22 oC. Interestingly, the increment of MWCNT, from 5 wt% to 10 wt% as in MgH2 – 10 wt% K2NbF7 - 10 wt% MWCNT composite has slightly increased the initial decomposition to 260 oC. The outcome has suggested that the excessive amount of CNT is not beneficial for the MgH2 system. As reported by Cai et al.41, MgH2 particles contact are important for the nucleation in the dehydrogenation process. In this particular issue, they found that CNT may provide a negative effect on the dehydrogenation process by buffering the nucleation of the MgH2 particles. Therefore, in this study, the excessive CNT has slightly increased the decomposition temperature of the composite which is speculated due to the buffering effect from the CNT. Similar observations were reported by other researchers.42-43



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On the other hand, from the perspective of desorption capacity, the addition of K2NbF7 and MWCNT has significantly reduced the amount of hydrogen released from the system. For instance, the milled MgH2 was able to release about 6.8 wt% of hydrogen. Meanwhile, the MgH2 – 10 wt% K2NbF7, MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT and MgH2 – 10 wt% K2NbF7 - 10 wt% MWCNT composites, the hydrogen released is amounted to 6.3 wt%, 6.2 wt% and 6.0 wt%, respectively. This is happening because of the dead weight of the dopants which does not have hydrogen.21, 44 As for the milled MgH2, the total amount of released hydrogen is 0.2 wt% lower than the pristine MgH2. The possible reason for this matter is that some hydrogen may have been released during the ball milling process.45 3.2 Sorption kinetics The effects of the addition of K2NbF7 and MWCNT on the sorption kinetics of the MgH2 system were conducted. Fig. 2 collates the absorption kinetic profiles of the MgH2 system and its composites that were conducted at 320 oC under 27 atm of hydrogen. Acceding to the figure, the MgH2 system and its composites are asserted to have pronounced absorption kinetics. The milled MgH2 is observed to have an absorption capacity of 7.3 wt% of hydrogen while the composites are able to absorb more than 6wt% of hydrogen except for the MgH2 – 10 wt% K2NbF7 with the capacity of 5.9 wt% of hydrogen. This outcome is predicted since MgH2 and its composites are known to be able to absorb about 6wt% of hydrogen and more at a temperature higher than 300 oC. A number of published reports can be found in the literature to substantiate the notable absorption kinetics of MgH2 and its composites at the high temperature.15, 33, 46-48


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Fig. 2: Absorption kinetics profiles of the milled MgH2 and its composites at 320 oC and 27 atm. The absorption kinetics study was conducted at a temperature of 150 oC and 27 atm of hydrogen pressure (Fig. 3) to further observe the effect of K2NbF7 and MWCNT on the absorption kinetic properties of MgH2 at a lower temperature. As for a reference, the absorption kinetics of the milled MgH2 was done. The MgH2 system is only able to absorb 1.1 wt% of hydrogen in this operating condition. Meanwhile for the MgH2 – 10 wt% K2NbF7 composite, the absorption capacity is about 4.7 wt% of hydrogen. Meanwhile, for the MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite, it has the highest absorption capacity with an amount of 5.1 wt% of hydrogen. As for the MgH2 – 10 wt% K2NbF7 - 10 wt% MWCNT composite, it has the lowest hydrogen absorption capacity in 60 minutes among the studied composites with a value of 4.6 wt% of hydrogen. However, albeit of being the composite with the lowest total absorption capacity, the MgH2 – 10 wt% K2NbF7 - 10 wt% MWCNT composite has the highest absorption kinetics for the first 6 minutes. The composite is able to absorb 1.5 wt% and 1.7 wt% of hydrogen for the first 0.7 and 2.4 minutes, respectively. Meanwhile, for the MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite that shows the highest absorption capacity in 60 minutes, the composite is only able to absorb 1.0 wt% and 1.4 wt% of hydrogen in 0.7 and 2.4 minutes, respectively. The outcomes have suggested that



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the excessive MWCNT content may not beneficial on the total hydrogen absorption capacity but it may be useful to enhance the absorption kinetics of the MgH2 system at the lower operating temperature.

Fig. 3: Absorption kinetics profiles of the milled MgH2 and its composites at 150 oC and under 27 atm H2 pressure. Investigations on the effect of K2NbF7 and MWCNT on the desorption kinetics were done to understand the impact of the dopants on the isothermal dehydrogenation of the composites. Fig. 4 compares the desorption kinetics profiles of the milled MgH2 (reference) and the studied composites at 320 oC and 27 atm. According to the figure, the milled MgH2 has the lowest hydrogen desorption capacity with a value of 4.6 wt% and followed by the MgH2 – 10 wt% K2NbF7 composite with a value of 5.6 wt%. As for the composites with the co-doping, albeit of having a comparable desorption capacity (about 6 wt%), the MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite shows a favourable improvement on the aspect of desorption kinetics. The MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite is able to desorb 3.6 wt%, 4.7 wt% and 5.2 wt% of hydrogen at 2.4 min, 3.4 min and 4.4 min, respectively. In comparison, the MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite is only able to desorb 3.0 wt%, 4.0wt% and 4.6 wt% of hydrogen under the similar time frame.


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The MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite shows promising catalytic performances as compared to previously studied composites. For example, Song et al.

49

reported that the Mg – 5Ni – 2.5Fe – 2.5Ti is able to release about 3.8 wt% of hydrogen in 10 minutes at 320 oC while our studied composite is able to release 2.2 wt% of hydrogen at the similar test temperature and time period. Meanwhile, for the Mg(In) – MgF2 composite that studied by Ouyang et al.28, the composite is able to release about 3.5 wt% of hydrogen at 306 o

C in 2 hours and 4.5 wt% of hydrogen at 327 oC in 75 minutes. Meanwhile, the

Mg85In15Al5Ti5 composite is able to release about 5.4 wt% of hydrogen in 60 minutes at 327 o

C.29 As for the Mg85Ag5Al1031, the composite is able to release about 3.3 wt% of hydrogen in

2 hours.

Fig. 4: Desorption kinetics profiles of the milled MgH2 and its composites at 320 oC and under 1.0 atm. Investigations on the decomposition temperature and the sorption kinetics show the significant improvement done by K2NbF7 and MWCNT especially for the composite with the 10 wt% K2NbF7 and 5 wt% MWCNT. In this regard, additional investigation on the decomposition activation energy and phase compositions as well as microstructure morphology will be focused on the MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite. In



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addition, investigation on the milled MgH2 and MgH2 – 10 wt% K2NbF7 composite will be conducted for comparison purposes. 3.3 Cycling performance Cycling performance of the system was carried out for the MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite. The composite has absorbed and desorbed hydrogen repeatedly for 5 consecutive cycles at 320 oC with a pressure of 27 atm for the absorption and 1.0 atm for the desorption. In addition, the cycling performance of the milled MgH2 is included as a comparison. Fig. 5 (a) and (b) depict the absorption and desorption kinetics profiles for the milled MgH2 and the MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composite.

Fig. 5: Absorption and desorption kinetics profiles of the milled MgH2 and its composite at 320 oC for five consecutive cycles. The cycle study has shown significant improvements by the doped composite as compared to the milled MgH2. Although the milled MgH2 has a superior hydrogen absorption capacity for the first cycle with a value of 7.1 wt%, after the 5th cycle, the absorption capacity is greatly reduced to about 2.5 wt%. In contrast, the doped composite is able to absorb about 6.1 wt% of hydrogen for the 1st cycle and 5.7 wt% for the 5th cycle.


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The significant reduction of the milled MgH2 absorption capacity is believed due to its poor desorption kinetics as shown in Fig. 5 (b). The milled MgH2 is only able to release about 2.7 wt% (1st cycle) and 2.1 wt% (5th cycle) of hydrogen. Meanwhile, for the doped composite, it is able to release about 5.9 wt% (1st cycle) and 5.7 wt% (5th cycle) of hydrogen. Thus this study has corroborated the synergistic catalytic effect of the K2NbF7 and MWCNT as compared to the milled MgH2. 3.4 Decomposition activation energy A critical minimum energy that is needed by a system to initiate a chemical reaction is known as Ea.50 As for this study, a decomposition Ea can be defined as a minimum energy that is essential for the MgH2 and the doped systems to undergo a decomposition process. The minimum amount of energy involved in the process is possible to be calculated by means of a method proposed by Homer E. Kissinger in 1957.

Kissinger equation that utilizes

temperature data from DSC measurements was used. The equation is given as follows: ln[β/T2p] = -Ea / RTp + A

(1)

In this equation, β is assigned to the value of heating rate used in the DSC measurement. Meanwhile, Tp is assigned for the value of peak temperature from the DSC profile. R is the gas constant value while A is a linear constant. Then the Ea is calculated based on the downslope value of the Kissinger’s plot that is given by [β/T2p] versus 1000/Tp.



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Fig. 6: DSC decomposition temperature profiles for the (a) milled MgH2, MgH2 – 10 wt% K2NbF7 and MgH2 – 10 wt% K2NbF7 – 5%MWCNT composites at the heating rate of 15 o

C/min, (b) the milled MgH2, (c) MgH2 – 10 wt% K2NbF7 composite, (d) MgH2 – 10 wt% K2NbF7 – 5%MWCNT composite and (e) Kissinger’s plot for the studied composites.

Fig. 6 (a) to (d) exhibit the decomposition temperatures profiles of the studied composites based on the DSC measurement. Fig. 6 (a) compares the temperature profiles of the MgH2 and its composites for the heating rate of 15 oC/min. The comparison shows that Page 16 of 33 ACS Paragon Plus Environment

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the introduction of 10 wt% K2NbF7 into the system has reduced the peak temperature by 61.8 o

C. Then, the incorporation of 5wt% MWCNT as in the co-doping composite has further

reduced the decomposition temperature by 15.7 oC. Since the peak temperature values are used in the Kissinger equation for the quantification of the Ea value, therefore the lower peak temperature clearly indicates the reduction in the Ea value. It is worth to note that the onset decomposition temperatures measured by the DSC are higher as compared to the values measured by the PCT. The differences may due to the dissimilarity in the measurement conditions between the PCT and the DSC which had been previously experienced by other researchers.51-53 The decomposition temperature profiles of the milled MgH2, MgH2 – 10 wt% K2NbF7 and MgH2 – 10 wt% K2NbF7 - 5 wt% MWCNT composites are shown as in Fig. 6 (b) to (d). The figures show that the peak temperature of the co-doping composite is lower as compared to the single doping composite and the MgH2. Meanwhile, the Kissinger plot is depicted as in Fig. 6 (e). Referring to the figure, the Ea value of the milled MgH2 is calculated to be 135.3 kJ/mol. The addition of 10 wt% K2NbF7 has successfully reduced the Ea value by 32.2 kJ/mol. Meanwhile, the addition of 5 wt% of MWCNT has further reduced the Ea value to 94.0 kJ/mol. The reduction of Ea value shows the favourable synergistic catalytic effect of K2NbF7 and MWCNT in reducing the decomposition temperature of the MgH2 system. Table 2 compares several Ea of decompositions for the doped MgH2 composites. Referring to the table, several co-doped composites have shown lower decomposition Ea values as compared to single doped composites. It shows that multi catalysts are able to work synergistically and effectively reducing the activation energy of a composite. As for the single doped MgH2 – nano flake Ta2O5 composite (Table 2), the significant activation energy reduction is speculated due to the introduction of the small particles catalyst (nanosize) that is known to work effectively due to larger surface areas for a better reaction54. In this table, our Page 17 of 33 ACS Paragon Plus Environment


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composite does not have the best Ea value as compared to other composites. The lowest Ea value is shown by the MgH2 – C - Si composite with a value of 62.2 kJ/mol while the highest Ea is the MgH2 – TiB2 – SiC with a value of 289 kJ/mol. However, considering the amount of energy and time used for the ball milling in preparing the composites, the MgH2 composite doped with K2NbF7 – MWCNT catalysts (current work) shows a promising synergistic catalytic effect in reducing the Ea value as compared to other dopants. Table 2: Decomposition activation energy, Ea of several doped MgH2 systems (n.a – not available). Composites

Activation energy, Ea (kJ/mol) 62.2

Total milling time (h) 20

63

n.a

MgH2 - C - Ni30

64.7

n.a

MgH2 – K2NiF6 – MWCNT43

70.0

1

MgH2 – nano flake Ta2O557

74.0

2

MgH2 – K2NbF7 – MWCNT(current work)

75.9

1

84

20

MgH2 – TiC – Fe - Cr59

97.7

50

MgH2 – SrTiO3 – Ni38

98.6

1

MgH2 – Nb2O5 – SWCNT60

102.7

1

104

n.a

MgH2 – FeCl3 – MWCNT61

112.0

1

MgH2 – Nb2O5 – MWCNT60

141.5

1

Mg - Ba62

173.9

n.a

MgH2 – TiB2 – SiC63

289.0

10

MgH2 - C - Si55 MgH2 –CeH - Ni56

MgH2 – Li2TiO358

MgH2 –CeH56


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3.4 Morphology structure Morphology structures of the MgH2 and its composites were observed using SEM apparatus. The purpose of this observation is to perceive the effect of the ball milling and the introduction of dopant(s) to the physical structure of the MgH2 system. The Fig. 7 (a) to (h) show the SEM images of the pristine MgH2, pristine dopants, milled MgH2 and the composites.



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Fig. 7: SEM images of the (a) pristine MgH2, (b) pristine K2NbF7, (c) pristine MWCNT, (d) milled MgH2, (e) MgH2 – 10 wt% K2NbF7 (f) MgH2 – 10 wt% K2NbF7 – 5% MWCNT (g) MgH2 – 10 wt% K2NbF7 – 5% MWCNT after 5th rehydrogenation and (h) MgH2 – 10 wt% K2NbF7 – 5% MWCNT after 5th dehydrogenation. Based on the figure, the microstructure of the pristine MgH2 (Fig. 7 (a)) is shown to have solid and thin flake-like particles. The particles have smooth surfaces and irregular sizes that are in the range of 100 µm or more. Meanwhile, the pristine K2NbF7 particles have an angular branch shape in the range of 50 µm (Fig. 7 (b)). As for Fig. 7 (c), the image exhibits the microstructure of the MWCNT particles. The MWCNT particles are made of tube-like shape that varies in diameter and length. As for the milled sample, after 1 hour of the ball milling process, the microstructure of the MgH2 particles has been significantly modified. The originally solid, large and flakelike particles have been modified to a clump of particles that are agglomerated and inconsistent in sizes (Fig. 7 (d)). On the other hand, for the MgH2 – 10 wt% K2NbF7 composite, the size of the particles are smaller which is in the range of 1 µm. Unlike the milled MgH2, the composite particles are freed from the agglomeration (Fig. 7 (e)). Meanwhile, for the co-doped composite particles, the MgH2 – 10 wt% K2NbF7 – 5wt% MWCNT, the size of the particles are in the range of 1 µm and free from agglomeration (Fig. 7 (f)). In addition, the red arrows in the figure show tube-like particles that belong to the MWCNTs. The appearance of MWCNT in Fig. 7(f) has corroborated that the carbon is still Page 20 of 33 ACS Paragon Plus Environment

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able to retain its shape and is not destroyed after 1 hour of the milling process, which is in line with the findings by Kukovecz et al.64 As for the Fig. 7 (g) and (h), they are representing the co-doped composite at 5th re/dehydrogenation cycles. The figures show that the particles have agglomerated to some extent after the 5th cycle of the re/dehydrogenation. Despite being agglomerated, the surfaces are rough and full of asperities. The rough surface structures are important in providing large surface areas for the hydrogen transfer.7 In order to clarify the presence of the elements and the distribution of the particles, EDS characterization was carried out for the MgH2 – 10 wt% K2NbF7 – 5wt% MWCNT composite. The magnification was set to 1000 in order to record the element distribution in a wide area. The SEM images in the EDS mode are shown as in Fig. 8 (a) to (f). These images corroborated the presence of the elements, which are Mg, K, Nb, F and C. In addition, all the elements are distributed uniformly inside the composite.

Fig. 8: EDS images of the (a) actual MgH2 – 10 wt% K2NbF7 - 5 wt%MWCNT, (b) Mg, (c) K, (d) Nb, (e) F and (f) C.



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3.5 XRD XRD characterizations are done for the MgH2 – 10 wt% K2NbF7 – 5wt% MWCNT composite (Fig. 9 (a)) to understand the phase compositions at three different stages; (a) after ball milling, (b) after dehydrogenation at 450 oC and (c) after rehydrogenation at 320 oC. Based on the figure, only MgH2 peaks are detected after the ball milling process. No K2NbF7 peak can be detected and it may happen due to the small amount of K2NbF7. However, the presence of the elements has been corroborated with the EDS images as in Fig. 8 (a) to (f). Several published reports can be found in the literature regarding this issue.43,

65-66

Meanwhile, for the dehydrogenation stage at 450 oC (Fig. 9 (b)), the presence of Mg peaks has shown the dehydrogenation of the MgH2. On the other hand, the MgH2 peaks that are detected in Fig. 9 (c) have shown the reversibility of the system. As for the MgO peaks in stage (b) and (c), the peaks are speculated to form due to the oxygen contamination between Mg and O2 during the sample preparation and transfer process prior the XRD measurements.14, 67


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Fig. 9: XRD profiles of the MgH2 – 10 wt% K2NbF7 - 5 wt%MWCNT composite after (a) ball milling, (b) after dehydrogenation at 450 oC and (c) after rehydrogenation at 320 oC.

Additional XRD measurements were carried out for the re/dehydrogenated samples after 5 consecutive cycles. The XRD profiles of the composite after the cycles are shown as in Fig. 10 (a) and (b). Like the XRD profiles without the cycling study (Fig. 9 (b) and (c)), only MgH2 and Mg peaks that are clearly visible. No carbon and catalyst peaks are able to be detected by the XRD. As for the MgO peaks, the peaks are speculated to form due to the oxygen contamination between Mg and O2 during the sample preparation prior the XRD measurements. 14, 67



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Fig. 10: XRD profiles of the MgH2 – 10 wt% K2NbF7 - 5 wt%MWCNT composite after 5 cycles: (a) after rehydrogenation at 320 oC and (b) after dehydrogenation at 320 oC.

Similar XRD measurements were repeated for the MgH2 – 50 wt% K2NbF7 – 30wt% MWCNT to further understand the phase compositions of the composite. Fig. 11 exhibits the XRD profiles for the composite at three different stages. For this composite, the K2NbF7 peaks are finally visible (Fig. 11 (a)). As for the after dehydrogenation stage, new peaks that are composed of KH, Nb, MgF2 and C are detected (Fig. 11 (b)). On the other hand, similar composition peaks are detected after the rehydrogenation peaks at 320 oC except for the Mg peaks. The replacement of Mg peaks with the MgH2 peaks has suggested the reversibility of the system. Meanwhile, the occurrence of the MgO peaks is due to the oxygen contamination between Mg and O2 during the sample preparation and transfer process prior the XRD measurements.

Fig. 11: XRD profiles of the MgH2 – 50 wt% K2NbF7 - 30 wt%MWCNT composite after (a) ball milling, (b) after dehydrogenation at 450 oC and (c) after rehydrogenation at 320 oC.


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Based on the phase composition analyses, the chemical reaction that has occurred during the heating process can be written as: 7MgH2 + 2K2NbF7 + C → 7MgF2 + 4KH + 2Nb + C + 5H2

(2)

Referring to the chemical reaction, the improvement on the hydrogen storage properties of the MgH2 – 10 wt% K2NbF7 – 5wt% MWCNT composite are speculated to happen due to several circumstances. The MgF2 compound that has formed after the heating process has increased the hydrogen sorption properties of the composite.68-69 In this context, the F-anion will weaken the Mg–H bonding which results in the enhancement of the hydrogen sorption.46 Yin et al.70 reported that the replacement of H- to F- in the NaAlH4 – TiF3 composite has led to a reduction in the decomposition enthalpy. Additionally, the formation of KH has contributed to the favourable hydrogen storage properties of the studied composite. This is based on the study by Wang et al.19 that found that the introduction of 3 mol% of KH into the Mg(NH2)2/2.0LiH composite has additionally reduced the decomposition temperature by 50 oC. In addition, Luo et al.71 reported that the incorporation of KH (

Improvement of Hydrogen Storage Properties of MgH2 Catalyzed by

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