Improved hydrogen storage properties of MgH2 by the addition of TiCN and its catalytic mechanism
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The hydrogen storage properties of MgH2–x wt%TiCN (x = 5, 10, 15) composites were systematically investigated and the results show that the addition of TiCN can effectively improve the de/rehydrogenation kinetics of MgH2. Taken the onset dehydrogenation temperature and isothermal de/rehydrogenation kinetics into consideration, the MgH2–10 wt% TiCN composite was shown to have the best performance. It was found that the MgH2–10 wt% TiCN composite could release 4 wt% H2 in 17.3 min at 300 °C while the as-synthesized MgH2 could not release any hydrogen under the same condition. Besides, the MgH2–10 wt% TiCN composite could absorb 4.63 wt% H2 under 3.2 MPa hydrogen pressure at 300 °C within 20 s. Compared with as-synthesized MgH2, the activation energy of the MgH2–10 wt% TiCN composite was significantly decreased from 183.76 ± 10 to 106.82 ± 5 kJ/mol. X-ray diffraction analysis revealed that the TiCN remained stable during the ball milling and the following de/rehydrogenation cycle. The catalytic mechanism was also proposed that the TiCN particles absorbed on MgH2 not only served as charge transfer centers and accelerated the hydrogen incorporation and dissociation rate but also provided more diffusion channels for hydrogen, which contributed to the good de/rehydrogenation properties of MgH2.
KeywordsHydrogen storage MgH2–TiCN composites Kinetics Catalytic mechanism
Nowadays, it is urgent to find a cost-effective and renewable energy source to replace fossil fuels for a sustainable and clean world . Among various new energy carriers, hydrogen is one of the most promising candidates as it can be converted to electricity through fuel cell without emitting pollutants . However, the realization of the “hydrogen economy” requires a high energy density and safe hydrogen storage technology . So far, numerous efforts have been made by researchers worldwide to develop hydrogen storage materials to meet the requirements of effective capacity, safety, mild reaction temperature and operating pressure. Magnesium hydride is hopeful to be used as hydrogen storage materials due to its high hydrogen capacity (7.6 wt% and 110 kg/m3). However, the high thermodynamic stability and slow kinetic properties hinder the practical use of MgH2 [4, 5]. In recent years, it has been found that adding additives or catalysts to matrixes can reduce the energy of the metal-hydrogen bonds and lower the desorption energy of MgH2 [6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Sulaiman et al.  demonstrated that the onset dehydrogenation temperature of MgH2 doped with 10 wt% K2NiF6 and 5 wt% CNTs could be reduced to 245 °C. Antonio et al.  revealed that VNbO5 could lower the desorption temperature of MgH2 from 330 to 235 °C. Yahya et al.  reported that the MgH2–5 wt% K2NbF7 composite started to release hydrogen at 255 °C, which was 75 °C lower than the as-milled MgH2. Ali et al.  studied the effect of nanolayer-like-shaped MgFe2O4 on the performance of MgH2 for hydrogen storage, the isothermal desorption kinetic study showed that the doped sample could desorb approximately 4.8 wt% H2 in 10 min while the milled MgH2 desorbed less than 1.0 wt% H2 at 320 °C.
Compared with above additives and catalysts, Ti-based materials are superior because of their unique electrical and chemical properties [20, 21, 22, 23, 24, 25, 26]. For instance, Zhang et al.  found that the MgH2–10 wt% TiO2@C sample started releasing hydrogen at 205 °C, which was 95 °C lower than that of pristine MgH2. Ma et al. . found that the hydrogen absorption of MgH2 can be largely completed within 25 s at a moderate temperature range (40–100 °C) after adding 4 mol % TiF3. Pandey et al. . found that the desorption temperature of 50 nm TiO2 modified MgH2 was 310 °C, which is 96 °C lower compared to that of commercial MgH2. Cui et al. . coated a Ti nano-layer on the surface of Mg,which remarkably accelerated the hydrogen dissociation and recombination process. Recently, it has been demonstrated that TiC and TiN can greatly improve the hydrogen storage performances of MgH2 [31, 32, 33, 34]. For instance, Fan et al.  found that the MgH2–10 wt% TiC composite could release 6.3 wt% H2 at 300 °C and absorb 4.1 wt% H2 under 1 MPa at 100 °C. Wang et al.  successfully synthesized TiN@rGO nanocomposites through a simple “urea glass” technique. The MgH2–10 wt% TiN@rGO composite began to release hydrogen at 167 °C and could release 6.0 wt% H2 within 18 min at 309 °C.
In this work, the catalytic effect of TiCN on the hydrogen storage properties of MgH2 was investigated. So far as we know, the hydrogen storage properties of MgH2–TiCN composites have never been systematically studied before. Here, we use ball milling technology to prepare MgH2–TiCN composites. The hydrogen absorption and desorption properties of MgH2–TiCN composites were investigated and its catalytic mechanism was also discussed in detail.
2.1 Preparation of as-synthesized MgH2
The MgH2 used was synthesized by mechanical ball milling and hydrogenation heat treatment. In brief, 6 g Mg powder (Sinopharm Chemical Reagent Co., Ltd, 99%, 100–200 mesh) was hydrogenated at the hydrogen pressure of 65 bar and 380 °C for 2 h. Then the power was divided into two parts and milled at 450 rpm for 5 h. Repeat the above steps and MgH2 can be obtained after finally hydrogenate the power at the hydrogen pressure of 65 bar and 380 °C for 2 h.
2.2 Preparation of MgH2–TiCN composites
The TiCN (aladdin, 1–2 µm, 99%) and the as-synthesized MgH2 (0.5–2 µm, shown in Fig. S1) were mixed with a mass ratio of 5:95, 10:90, and 15:85, respectively. The mixtures were then ball milled at a speed of 450 rpm for 2 h under 0.1 MPa argon atmosphere. The ball to material ratio was 40:1. To avoid contamination, all the samples handling and transferring were carried out in an Ar-filled glove box (Mikrouna) with a water/oxygen content of less than 1 ppm.
XRD tests of the samples were performed on an X’Pert Pro X-ray diffractometer (PANalytical, the Netherlands) with Cu K alpha radiation at 40 kV, 40 mA. During transferring and scanning, a home-made argon-filled device was adopted to avoid oxygen and moisture contamination. The morphology and elemental distribution of the samples were further analyzed using a scanning electron microscope (SEM, Hitachi SU-70, 3.0 kV) equipped with an X-ray energy spectrometer (EDX, HORIBAX-MAX). The DSC curve was carried out on an analyzer model (Netzsch STA449F3) with flowing argon (99.999%, 50 mL/min) as protective gas. The quantitative de/rehydrogenation properties of the as-synthesized MgH2 and as-prepared MgH2–5 wt% TiCN, MgH2–10 wt% TiCN, MgH2–15 wt% TiCN composites were measured with a Sieverts apparatus. Dehydrogenation was carried out at a temperature of 275, 300 and 325 °C under a hydrogen pressure of 3 kpa, respectively. Rehydrogenation was performed at a temperature of 200, 250 and 300 °C under an initial hydrogen pressure of 3.2 MPa, respectively. It is worth noting that the hydrogen capacity is given in weight percent of the entire composite including the additives.
3 Results and discussion
The operating temperatures of different samples from DSC
Initial temperature (°C)
Peak temperature (°C)
MgH2–5 wt% TiCN
MgH2–10 wt% TiCN
MgH2–15 wt% TiCN
In summary, the MgH2–x wt%TiCN (x = 5, 10, 15) composites prepared by ball milling and their microstructure and hydrogen storage properties were systematically investigated. Studies found that the de/rehydrogenation kinetics of MgH2 can be significantly improved by the addition of TiCN. Considering the onset dehydrogenation temperature and isothermal de/rehydrogenation kinetics, the MgH2–10 wt% TiCN composite showed the best performance. The MgH2–10 wt% TiCN composite only needed 61.8, 17.3 and 8.5 min to release 4 wt% hydrogen at 275, 300 and 325 °C, respectively. However, as-synthesized MgH2 did not release any H2 at 275 °C and 300 °C. Besides, MgH2–10 wt% TiCN composite could absorb 4.63 wt% H2 under 3.2 MPa hydrogen pressure at 300 °C within 20 s. The activation energy of MgH2–10 wt% TiCN was reduced to 106.82 ± 5 kJ/mol. XRD analysis demonstrated that the TiCN remains stable and acts as active catalytic specie during the de/rehydrogenation process. Based on the experimental results, a mechanism was proposed to illustrate how the TiCN acted as charge transfer between Mg/MgH2 and H2, consequently enhancing the de/rehydrogenation properties of MgH2–TiCN composite.
This study was funded by National Natural Science Foundation of China (Grant Nos. 51801078 and 51702300) and the National Science Foundation of Jiangsu Province (Grant Nos. BK20180986, 17KJB480003 and SJCX18-0772).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.