Crystal-facet-dependent catalysis of anatase TiO2 on hydrogen storage of MgH2

Journal of Alloys and Compounds 822 (2020) 153553

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Crystal-facet-dependent catalysis of anatase TiO2 on hydrogen storage of MgH2 Zhongliang Ma a, b, c, Jiangchuan Liu a, b, Yunfeng Zhu a, b, *, Yingyan Zhao a, b, Huaijun Lin d, **, Yao Zhang b, e, ***, Haiwen Li f, Jiguang Zhang a, b, Yana Liu a, b, Wentian Gao a, b, Shanshan Li a, b, Liquan Li a, b a

College of Materials Science and Engineering, Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816, PR China Jiangsu Collaborative Innovation Centre for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing, 211816, PR China Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, Fukuoka, 819-0395, Japan d Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan University, Guangzhou, 510632, PR China e School of Materials Science and Engineering, Southeast University, Nanjing, 211189, PR China f Platform of Inter/Transdisciplinary Energy Research, International Research Center for Hydrogen Energy, International Institute for Carbon-Neutral Energy Research, Kyushu University, Fukuoka, 819-0395, Japan b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 November 2019 Received in revised form 25 December 2019 Accepted 26 December 2019 Available online 27 December 2019

Various catalysts are developed to enhance the hydrogen storage performances of magnesium based hydride. In particular, the morphology of catalysts governs their surface energy and chemical interactions with Mg and hydrogen, which makes it possible to adjust the hydrogen reaction kinetics of MgH2 through morphological design of the catalysts. Here, we report a novel crystal-facet-dependent catalysis of TiO2 on hydrogen storage of MgH2. Different facets dominant anatase TiO2 (named TFx, x ¼ 0, 10, 30, 50, 70 and 80) are prepared via a hydrothermal synthesis approach and then doped into MgH2 via ball milling. Results show anatase TiO2 with high percentage {001} facets has much better catalytic effect than that with low percentage {001} facets. The MgH2-TF70 exhibits the best hydrogen sorption kinetics, showing apparent activation energy for dehydriding of only 76.1 ± 1.6 kJ mol1. The present study constructs a new bridge between the catalytic effect of catalysts with different crystal facets and the hydrogen storage property of MgH2. © 2019 Elsevier B.V. All rights reserved.

Keywords: Mg hydride TiO2 Crystal-facet-dependent catalysis Hydrogen storage materials

1. Introduction Hydrogen, an environment friendly resource with high energy density and abundance in nature, is considered as one of the most promising renewable energy resources to replace fossil fuels. However, the realization of hydrogen economy is a challenging project limited by hydrogen storage technology, which not only demands high hydrogen storage density, but also requires safety during application [1,2]. Magnesium hydride, a traditional metal hydride, is believed to be an ideal hydrogen storage medium in

* Corresponding author. College of Materials Science and Engineering, Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816, PR China. ** Corresponding author. *** Corresponding author. Jiangsu Collaborative Innovation Centre for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing, 211816, PR China. E-mail addresses: [email protected] (Y. Zhu), [email protected] (H. Lin), [email protected] (Y. Zhang). https://doi.org/10.1016/j.jallcom.2019.153553 0925-8388/© 2019 Elsevier B.V. All rights reserved.

vehicle energy and nickel/metal hydride (Ni/MH) rechargeable batteries due to its high hydrogen content, cheap in price, and excellent reversibility. Fortunately, magnesium is an element abundant in earth and is friendly with environment [3e9]. However, slow hydrogen absorption/desorption kinetics and stable thermodynamics owing to the high reaction energy barrier and large reaction enthalpy still hinder the practical applications, which require long hydrogen sorption period and high operation temperature. Until now, how to promote the application of Mg-based hydride remains an attractive research area, and many efforts have been devoted recently [10e13]. There are several possible strategies to promote the gaseous and electrochemical hydrogen storage performance of Mg-based hydride [3,13e27]. Alloying of Mg with transition metals to form stoichiometric and nonstoichiometric hydrides can reduce the hydrogen desorption temperature and improve the high-rate discharge ability [20,28e31]. For example, hydride of Mg2Ni has lower hydrogen desorption enthalpy with the preferential formation of NieH bonds, which

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subsequently combine with Mg atoms [18,21]. However, the addition of transition metals leads to the decrease in hydrogen storage capacity. Alternatively, adding small amount of catalyst (metal, metal oxide, metal halide, etc.) can cut down the hydrogen desorption period prominently and maintain high hydrogen storage capacity [25e27]. TiO2 has been deemed as an effective catalyst to enhance the performances of energy storage materials in energy storage and conversion applications due to its easy availability, low cost and better catalysis among the various kinds of metal oxides [32,33]. It has been demonstrated that introducing TiO2 can reduce the onset desorption temperature and speed up the reaction kinetics of Mg-based hydride effectively. In addition, hybrid catalysts, such as carbon supported TiO2, Ni@TiO2 coreshell structure and multi-valance Ti-based catalysts have been designed to expound the synergistic catalysis on MgH2 [4,34,35]. Very recently, aside from catalyst category and particle size [26,27,34e39], the shape of catalyst has been also discovered significant to materials chemistry on account of the strong correlation between their morphologies and the intrinsic properties [10,27,40]. Many catalysts with attractive shape have been synthesized to study the mechanism of catalytic chemistry. Therefore, it is important to synthesize and select high efficiency catalyst with much catalytic sites. As known, TiO2 has two crystal structures of rutile and anatase, which show different effect in catalysis on photocatalytic water splitting and hydrogen sorption kinetics of metal hydrides [41e43]. Moreover, the as-prepared anatase TiO2 is usually exposed with {001} and {101} facets [44]. Compared with the anatase TiO2 exposed with {101} facet, the anatase TiO2 exposed with {001} facet shows better photocatalytic activity [45]. Therefore, it is necessary and attractive to introduce the crystalfacet-dependent TiO2 catalyst into MgH2. Up to date, given the importance of the facet-dependent properties of TiO2, to the best of our knowledge, the effect of anatase TiO2 with different morphologies on MgH2 is rarely presented. Herein, based on the controllable preparation of metal oxides, the anatase TiO2 catalysts exposed with different amounts of {001} and {101} facets were synthesized by a hydrothermal synthesis approach, and doped into MgH2 via ball milling. The crystal-facetdependent catalysis of anatase TiO2 on hydrogen storage of MgH2 has been reported in this work. 2. Experimental section 2.1. Syntheses of TiO2 nanosheets TiO2 crystals exposed with different amounts of {001} and {101} facets have been prepared by hydrothermal synthesis method [44]. Hydrofluoric acid aqueous solutions with volume concentrations of x % (x ¼ 10, 30, 50, 70 and 80) were prepared, respectively. Then, Ti(OBu)4 (25 mL) was dropwise mixed with hydrofluoric acid aqueous solution (15 mL) with different volume concentrations. The above solution was placed into a dried Teflon-lined stainlesssteel autoclave. The sealed autoclave was heated to 180  C and kept for 24 h. Afterwards, deionized water and ethanol were used to wash the resultant product three times to remove the residue. Finally, the white product was dried at 60  C in a vacuum oven overnight. As-prepared samples were labeled as TFx (x ¼ 10, 30, 50, 70 and 80), based on different volume concentrations of HF. 2.2. Syntheses of MgH2 and MgH2e TiO2 composite MgH2 was prepared by hydriding combustion synthesis (HCS) method as reported in our previous work [26]. The as-prepared TiO2 catalysts (5 wt%) exposed with various amounts of {001} and {101} facets were added respectively into MgH2 via ball milling

under Ar atmosphere. The ball milling conditions (QM-3SP2 planetary ball mill) include milling time (10 h), milling speed (400 rpm) and ball to powder ration (30:1). MgH2 was also milled under the same conditions for comparison. To prevent the samples from being poisoned, all handling of the samples was operated in an Arfilled glove box with low concentration of O2 (<1 ppm) and H2O (<1 ppm). 3. Results and discussion 3.1. Structural characterizations of TiO2 X-ray diffraction (XRD) patterns of the TiO2 samples modified by HF with different volume concentrations are shown in Fig. 1 (a). Compared with the PDF JCPDS 21-1272 (anatase TiO2), all the diffraction peaks can be search-matched. With the volume concentration of HF increased from 10% to 70%, the diffraction peaks become sharper, along with better crystallinity of TiO2. However, when the volume concentration of HF continued to increase to 80%, it is unconducive to the crystallinity of TiO2. The transmission electron microscope (TEM) images of TiO2 (TF10, TF30, T50, TF70 and TF80) samples are presented in Fig. 1(bef), and the inset images are the SAED (Selected area electron diffraction) patterns. All the samples are sheet-shaped, and the rectangular outline of the particles gradually becomes obvious from TF10 to TF70. Fig. 1 (f) shows that the TF80 sample is extremely papery. Moreover, the TiO2 particles grow up obviously when the volume concentration of HF increases from 10% to 70%. This phenomenon can be ascribed to the regulation of HF on crystal surface of the anatase TiO2 [44,46]. The result suggests that F is more easily adsorbed on the surface of {001} facets. After the decomposition of Ti(OBu)4, anatase TiO2 tends to grow along (100) and (010) axis and exposes much {001} facets during the process of recrystallization [45]. In addition, the red region of Fig. 1(d) shows that the particle consists of three layers of the TiO2 nanosheets. Each layer reveals the {101} and {001} facets of TiO2 distinctly. In particular, the {001} facet is easy to cohere with each other. Obviously, the high resolution transmission electron microscope (HRTEM) images also show that the particle size of TiO2 increases with the increase of volume concentration of HF (Fig. S1). Additionally, superfluous HF will hinder the exposing of specific facets due to the corrosion of F on the TiO2 nanosheet, which was supported by the ultra-thin irregular TF80 sample (Fig. S2) and the unsuccessful preparation of TF90 sample under the same condition. Moreover, based on the Wulff structure model (Fig. S3) [46], the percentages of {001} facets for TF10, TF30, T50 and TF70 samples were calculated to be 20%, 34%, 59% and 88%, respectively (Table S1). Noting that the TF80 sample is so irregular, and it is hard to calculate the percentages of {001} facets. Therefore, we did not study its effect in the following experiments. 3.2. Hydrogen storage performances of MgH2eTiO2 composite The temperature-programmed-desorption (TPD) profiles and differential scanning calorimetry (DSC) curves of the MgH2 doped with 5 wt% TiO2 (TF10, TF30, TF50 and TF70) and the as-milled MgH2 are presented in Fig. 2. The onset dehydrogenation temperature is decreased obviously, and the desorption kinetics is improved for MgH2 upon the addition of TiO2 catalysts. The onset dehydrogenation of as-milled MgH2 occurs at around 300  C with a total release of 7.28 wt% hydrogen. The MgH2-5 wt.% TF70 shows the lowest onset dehydrogenation temperature of around 220  C. From the DSC curves, the dehydrogenation peak temperature of the as-milled MgH2 is 373.9  C, and those of MgH2-5 wt.% TF10 and MgH2-5 wt.% TF70 shift to 355.1 and 315.3  C, respectively. The TPD and DSC experiments suggest that MgH2 doped with the {001}

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Fig. 1. Structure characterizations of TiO2 samples. (a) XRD patterns of TiO2 samples; TEM images of the TiO2 samples: (b) TF10, (c) TF30, (d) TF50, (e) TF70 and (f) TF80.

Fig. 2. TPD and DSC curves of the MgH2 doped with 5 wt% TiO2 (TF10, TF30, TF50 and TF70) and the as-milled MgH2.

facets dominated anatase TiO2 presents better dehydriding properties, especially for the sample doped with TF70. Moreover, the total dehydrogenation capacities of MgH2 doped with 5 wt% TiO2 (TF10, TF30, T50 and TF70) are 6.63, 6.66, 6.70 and 6.89%, respectively. Compared with the as-milled MgH2, the slight capacity loss is due to the addition of catalysts. The isothermal hydrogenation curves of the samples were measured at 100 and 200  C (Fig. 3 (a, b)). The MgH2-5 wt.% TF70 sample shows much better kinetics than that of the as-milled MgH2 and MgH2-5 wt.% TF10. MgH2-5wt.% TF70 absorbs hydrogen as much as 2.70 wt% in 500 s even at 100  C. On the contrary, the asmilled MgH2 can hardly absorb hydrogen, and MgH2 doped with 5 wt% TF10 absorbs only 0.84 wt% hydrogen in 500 s. When the hydrogen absorption time increases to 120 min, the MgH2-5wt.% TF70 can uptake 4.5 wt% hydrogen at 100  C (the inset of Fig. 3 (a)). At 200  C, all samples show improved kinetics and the MgH2-5wt.% TF70 exhibits the best performance, with 5.3 wt% hydrogen absorbed in 44 s. We can thus conclude that TF70 plays a significant role in facilitating the hydrogen absorption performance of MgH2. Fig. 3 (c, d) show the isothermal dehydrogenation curves of the samples measured at 275 and 300  C. Dehydrogenation performance of MgH2 is improved obviously after the addition of TiO2 catalyst, and the MgH2-5 wt.% TF70 still shows the optimal

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Fig. 3. Isothermal hydrogenation curves of as-milled MgH2, MgH2 doped with 5 wt% TF10 and TF70 at (a) 100  C, (b) 200  C under 3.0 MPa H2 and isothermal dehydrogenation curves of as-milled MgH2, MgH2 doped with 5 wt% TF10 and TF70 at (c) 275  C, (d) 300  C under 0.005 MPa H2.

hydrogen desorption performance, which can release 6.0 wt% hydrogen in 60 min at 275  C. At 300  C, all samples show faster desorption kinetics, and the MgH2-5 wt.% TF70 can release 6.4 wt% hydrogen within 700 s. From above, it can be concluded that TF70 with predominant {001} facets has a significant positive influence on facilitating both the hydrogenation and dehydrogenation kinetics of MgH2. It is suggested that the {001} facets provide much more active sites than that of {101} facets. Compared with the {101} facets exposing both six coordinated and five coordinated Ti, the {001} facets expose five coordinated Ti completely, leading to a better chemical reaction activity [46]. In addition, compared with the surface energy of {101} facets (0.44 J m2), {001} facets has higher surface energy of 0.90 J m2, which is believed to be another reason for its better reaction activity [46,47]. Isothermal de/hydrogenation cycling kinetics of the MgH2-TF70 sample was measured at 300  C for 10 cycles (Fig. 4). The de/hydrogenation dynamics remain stable during the 1st to 10th cycle. Fig. S4 and Table S2 show that the de/hydrogenation capacities are higher than 6.4 wt% in each cycle, and the capacity retentions of hydrogenation/dehydrogenation are 97.0% and 97.6% after 10 cycles, respectively. Thus, it proves that MgH2 shows excellent cyclic performances by doping with TF70. Additionally, DSC test was further conducted to study the hydrogen desorption kinetics of the samples, and the corresponding Ea (dehydrogenation activation energy) of the samples are calculated in Fig. S5. The Ea of MgH2-5 wt.% TF70 is 76.1 kJ mol1, which is much lower than those of as-milled MgH2 (142.3 kJ mol1) and MgH2-5wt.% TF10 (101.9 kJ mol1). The decreased Ea implies that doping TF70 could decrease the energy barrier for the conversion of MgH2 into Mg. Compared with other MgH2-catalyst systems, such as MgH2eFeB [48], MgH2eNiCu [49], MgH2eFe and

Fig. 4. De/hydrogenation cycling curves of the MgH2-TF70 composite at 300  C. The initial hydrogen pressures for absorption and desorption are 3.0 and 0.005 MPa, respectively.

its oxides [6], MgH2eNiHCl [7], MgH2eV2O3 [15], MgH2eNi@C [26,27], the TF70 catalyst in the current work shows an improved performance. 3.3. Catalytic mechanism of MgH2eTiO2 composite It is essential to study the microstructure of the doped/undoped samples by electron microscope, which is helpful to understand the catalytic mechanism of TF70 on modifying MgH2. The scanning electron microscopy (SEM) images of MgH2 with and without

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doping TF70 are compared in Fig. S6. The particle size of the undoped MgH2 ranged from 1 to 5 mm with inhomogeneous distribution. After introducing TF70 by ball milling, the particle size of MgH2 becomes more ultrafine and uniform, and most of the particle size is smaller than 2 mm. The TF70 might play a role of milling agent to refine the particle size of MgH2, leading to the shortened diffusion distance for hydrogen. Moreover, TiO2 is successfully detected in the MgH2 matrix before and after cycling, and any other Ti-based substances are not found (Fig. 5). Both MgH2 and TiO2 remain stable in the composite during cycling, accounting for the excellent cycling performance. Understanding the mechanism of MgeTiO2 system is indispensable. The poor dehydrogenation kinetics and high operation temperature of MgH2 are limited by the stabilized chemical bonding between magnesium and hydrogen, and the inherent formation enthalpy of MgH2. The low hydrogen diffusion coefficient in Mg/MgH2 is also a significant factor for the slow reaction kinetics. An interesting point to consider is that the particle size of MgH2eTiO2 system is decreased via ball milling, shortening the hydrogen diffusion distance. Another more important and attractive factor is for sure, large numbers of active sites and defects were introduced on the surface of the {001} facets dominated anatase TiO2, which provides more options for electron transfer and interaction [47]. As a result, the reaction kinetics would be enhanced obviously. Meanwhile, the {001} facets with higher surface energy can accelerate hydrogen dissociation and recombination. A hydrogen concentration gradient is formed near the catalyst to facilitate the de/hydrogenation (Fig. 6). Furthermore, the activation energy of MgH2 could be influenced actively by the addition of TiO2. Above all, the hydrogen storage performances of MgH2 are promoted obviously via the addition of {001} facets dominated TiO2. 4. Conclusions In summary, TiO2 crystals exposed with various amounts of {001} and {101} facets (TF10, TF30, TF50 and TF70) have been prepared by hydrothermal synthesis approach. The percentages of {001} facets are 20, 34, 59 and 88% for TF10, TF30, TF50 and TF70, respectively. TF70 presents a prominent catalytic effect on de/hydrogenation of MgH2. The MgH2-TF70 has the lowest onset dehydrogenation temperature of around 220  C. It also exhibits the

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Fig. 6. Schematic illustration of hydrogen dissociation and recombination on the interface of TiO2.

fastest de/hydrogenation kinetics, which can uptake 5.3 wt% hydrogen in 44 s at 200  C, and release 6.4 wt% hydrogen in 700 s at 300  C. The hydrogen absorption and desorption capacity retentions are 97.0% and 97.6% after 10 cycles at 300  C, respectively. The apparent dehydrogenation activation energy of the MgH2-TF70 is 76.1 ± 1.6 kJ mol1, lower than 101.9 ± 4.2 kJ mol1 for MgH2TF10. Anatase TiO2 with dominated {001} facets exhibits remarkable catalysis on MgH2, accounting for the prominent de/hydrogenation kinetics and cycling stability of MgH2. Our results build a relationship between crystal facets difference of catalyst and de/ hydrogenation kinetics of metal hydride, and expand the way to modify the hydrogen storage performances of metal hydride via doping catalyst with high active crystal facets.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 5. TEM micrographs, SAED patterns, and HRTEM micrographs of the MgH2-5 wt.% TF70 composite before (aec) and after (def) 10 de/hydrogenation cycles, respectively. XRD patterns of the MgH2-5 wt. % TF70 composite before and after 10 de/hydrogenation cycles are inset in (a) and (d), respectively.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (51771092, 51601090 and 21975125), Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX19_0834), Six Talent Peaks Project in Jiangsu Province (2018, XNY-020), Guangdong Basic and Applied Basic Research Foundation (No. 2019A1515011985), the Program of China Scholarships Council (CSC, No. 201908320406) and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.153553.

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Author contributions section

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