Hydrogen generation properties and the hydrolysis mechanism of Zr(BH4)4·8NH3

Daifeng Wu a, Liuzhang Ouyang *abc, Jiangwen Liu ac, Hui Wang ac, Huaiyu Shao *d and Min Zhu ac
aSchool of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou, 510641, People's Republic of China. E-mail: meouyang@scut.edu.cn
bKey Laboratory for Fuel Cell Technology in Guangdong Province, Guangzhou, 510641, People's Republic of China
cSchool of Natural Sciences and China-Australia Joint Laboratory for Energy & Environmental Materials, Guangzhou, 510641, People's Republic of China
dInstitute of Applied Physics and Materials Engineering (IAPME), University of Macau, Macau SAR, China. E-mail: hshao@umac.mo

Received 17th May 2017 , Accepted 30th June 2017

First published on 30th June 2017


Zr(BH4)4·8NH3 is considered to be a promising solid-state hydrogen-storage material, due to its high hydrogen density and low dehydrogenation temperature. However, the release of ammonia hinders its practical applications. To further reduce the dehydrogenation temperature and suppress ammonia release, here we investigated its hydrolysis process to evaluate its hydrogen generation performance. The results showed that the hydrolysis of Zr(BH4)4·8NH3 in water can generate about 1067 mL g−1 pure hydrogen in 240 min at 298 K without the release of diborane or ammonia impurity gases. With heat-assistance, the hydrogen generation rate can be significantly enhanced, and its activation energy was calculated to be 29.38 kJ mol−1. The hydrolysis mechanism was clarified. The results demonstrate that Zr(BH4)4·8NH3 may work as one promising hydrogen generation material.


1. Introduction

Hydrogen is considered as a clean and renewable energy carrier owing to its high energy density, lightweight and environmentally benign products of oxidation, making it a potential candidate for the replacement of fossil fuels.1–4 However, for the wide utilization of hydrogen in automotive applications, one greatest challenge is to develop safe, inexpensive, lightweight and high hydrogen density storage materials that may operate at moderate temperatures. Metal hydrides,5,6 carbon materials,7 and activated charcoal8 have been tested to meet some of these requirements, but unfortunately none of them could show satisfactory performance for commercial vehicular application.

Recently, complex hydrides offered a possibility to design a potential hydrogen storage system.9–11 Borohydrides, alanates and amides are close to meeting the U.S. Department of Energy (DOE) targets in terms of storage capacity, reversibility and cost.12–14 Due to their high hydrogen densities, borohydrides have been widely investigated as promising hydrogen-storage materials.15–18 Nevertheless, borohydrides still suffer from the drawbacks of relatively high desorption temperature and the potential release of toxic boranes.

To decrease the desorption temperature and suppress the release of boranes, the introduction of NH3 is a useful way. The local combination of NHδ+⋯BHδ dihydrogen bonds in their structures may lead to a decrease in the temperature of hydrogen release and may restrict the formation of diborane on heating.19,20 Thus, metal borohydride ammoniates (MBAs, M(BH4)m·nNH3) are currently some of the most intensively investigated hydrogen storage materials.21–23

Recently, we reported a new metal borohydride ammoniate (MBA), Zr(BH4)4·8NH3, for solid-state hydrogen storage study.24 Zr(BH4)4·8NH3 has a high hydrogen capacity of 14.8 wt% and a low main dehydrogenation temperature of 403 K, and exhibits impressive air stability. However, 16.3 mol% of ammonia is released during the dehydrogenation process of Zr(BH4)4·8NH3, which greatly reduces the dehydrogenation purity and amount and brings an obstacle to its practical application for fuel cell vehicles. To decrease the dehydrogenation temperature and suppress the ammonia release, we adopted the combination strategy and investigated the dehydrogenation performance of Zr(BH4)4·8NH3 modified with NH3BH3, LiBH4 and Mg(BH4)2.25,26 However, the dehydrogenation temperature was still higher than the operational temperature of fuel cells,27 and the released hydrogen is still with impurity gases which will poison membranes in the fuel cell. Studies on metal borohydride ammoniates are usually focused on their thermal decomposition properties, but the hydrolysis performance of metal borohydride ammoniates (such as Zr(BH4)4·8NH3) has rarely been studied.28–32

Hydrolysis is an efficient and convenient hydrogen generation method with high purity, great yield and low operation temperature.33,34 Generally, studies on hydrolysis were focused on the reaction of metals, metal hydrides,35,36 borohydrides37 and ammonia borane38,39 with water. Based on its hydrogen storage properties, hydrolysis of Zr(BH4)4·8NH3 is expected to show a high hydrogen capacity at low temperature and high purity hydrogen considering the high solubility of ammonia in water.40 In this work, Zr(BH4)4·8NH3 was synthesized and its hydrolysis performance was evaluated. Through the study of its hydrolysis product, the hydrolysis mechanism of Zr(BH4)4·8NH3 was clarified.

2. Experimental

2.1. Preparation

The starting materials, LiBH4 (>98%) and anhydrous ZrCl4 (>99.5%) were purchased from Sigma-Aldrich and Alfa Aesar, respectively. Ammonia (>99.9%) was obtained from Guangdong Huate Gas Co. All the materials were used as-received without further purification with the exception of NH3, which was purified by soda lime prior to use.

Zr(BH4)4 was prepared by ball milling a mixture of LiBH4 and ZrCl4 using a planetary ball mill (QM-3SP4, Nanjing NanDa Instrument Plant). In a typical procedure, a mixture of LiBH4 and ZrCl4 (molar ratio of 4[thin space (1/6-em)]:[thin space (1/6-em)]1) was mechanically milled for 6 h at 300 rpm under an argon atmosphere at relative ambient temperature (<293 K). The ball-to-powder ratio was 30[thin space (1/6-em)]:[thin space (1/6-em)]1 and the milling process was carried out by alternating 6 min of milling and 6 min of rest. Due to the volatility of the Zr(BH4)4, sublimated Zr(BH4)4 would separate with LiCl and deposit on the lid of the ball mill pot directly during the ball milling process. Zr(BH4)4·8NH3 was prepared as a white powdery solid upon exposing the Zr(BH4)4 crystals to an atmosphere of anhydrous ammonia in an ice-water bath for at least 8 h. All handling was performed in a glove box equipped with a recirculation and regeneration system, which maintained the oxygen and water concentrations below 5 and 1 ppm, respectively.

2.2. Hydrolysis test

Hydrogen-generation measurements of Zr(BH4)4·8NH3 were performed using in-house developed equipment.41 After loading about 0.1 g of the sample in a 50 mL flask, 10 mL of deionized water was injected into the flask for the hydrolysis process. Then, the hydrogen was generated and pushed into a Monteggia washing bottle filled with water at room temperature, exhausting water into a beaker placed on an electronic scale. The mass of exhausting water against time was recorded by a computer connected with this electronic scale. Through these collected data, hydrogen evolution curves (hydrogen volume against time) can be drawn, and the hydrogen generation rate and yield can be determined. The Avrami–Erofeev equation42 was used to calculate the activation energy of the hydrolysis process.

2.3. Characterization

The composition phases of the synthesized Zr(BH4)4·8NH3 and the hydrolysis products dried with a freeze-drying machine (ALPHA 1-2, GHRIST) were determined by X-ray diffraction (XRD, PANalytical) with Cu-Kα radiation, performed with a step size of 0.013°. The samples for measurement were loaded on a glass sheet and wrapped with a 3 M film to protect them from exposure to air and moisture during the measurement process. The gas generated by hydrolysis was analyzed using a mass spectrometer (MS, HIDEN QIC-20). The chemical bonds of the species were identified via a Fourier transform infrared spectrometer (IS50 FT-IR, NICOLET) in the range of 500–4000 cm−1 with 32 scans. The tested samples were pressed with potassium bromide (KBr) powder. The elemental analysis of the Zr(BH4)4·8NH3 hydrolysis product was studied by field emission scanning electron microscopy (FE-SEM, Zeiss-Supra 40) using an energy dispersive spectrometer (EDS, Oxford) with an acceleration voltage of 15 kV.

3. Results and discussion

3.1. Synthesis of Zr(BH4)4·8NH3 and its hydrolysis properties

The Zr(BH4)4·8NH3 sample was synthesized by a ball milling method and the XRD pattern of the synthesized sample was compared with the one reported in our previous publication24 as shown in Fig. 1. As confirmed from the XRD patterns, Zr(BH4)4·8NH3 was synthesized successfully.
image file: c7ta04308b-f1.tif
Fig. 1 XRD patterns of the synthesized Zr(BH4)4·8NH3 sample and the one in our previous report.24

To test its hydrolysis properties, Zr(BH4)4·8NH3 was hydrolyzed in deionized water at 298 K. Fig. 2(a) shows the hydrogen evolution curve for the hydrolysis of Zr(BH4)4·8NH3. The hydrogen generation rate was relatively slow at room temperature. Only 222 mL g−1 H2 was generated in 10 min, and finally 1067 mL g−1 H2 was obtained in 240 min. To analyze the purity of the hydrogen generated by hydrolysis, a MS test was carried out. Fig. 2(b) shows the MS signals of H2, NH3 and B2H6 during the hydrolysis of Zr(BH4)4·8NH3. As Fig. 2(b) shows, neither B2H6 nor NH3 peaks occurred, indicating that the generated hydrogen was with high purity.


image file: c7ta04308b-f2.tif
Fig. 2 (a) Hydrogen evolution curves and (b) MS signals of H2, NH3 and B2H6 for the hydrolysis of Zr(BH4)4·8NH3 in deionized water at 298 K.

In order to test the kinetic properties of Zr(BH4)4·8NH3, hydrolysis tests at different temperatures were carried out. Fig. 3 shows the hydrogen evolution curves for the hydrolysis of Zr(BH4)4·8NH3 at different temperatures (298, 308, 318 and 328 K). As the temperature was raised, the hydrogen generation rate was significantly improved. The sample generated 408, 688, and 953 mL g−1 H2 in 10 min at 308, 318 and 328 K, respectively. Also, the hydrogen generation yields were enhanced. The total hydrogen generation yields of Zr(BH4)4·8NH3 at 308, 318 and 328 K were 1160, 1110 and 1083 mL g−1 H2, respectively. However, the highest hydrogen yield at 328 K of 1160 mL g−1 H2 was equal to 10.3 wt%, which is still much lower than the theoretical hydrogen capacity of 14.8 wt% for Zr(BH4)4·8NH3.


image file: c7ta04308b-f3.tif
Fig. 3 Hydrogen evolution curves for the hydrolysis of Zr(BH4)4·8NH3 in deionized water at different temperatures (298, 308, 318 and 328 K).

In addition, the hydrolysis activation energy (Ea) of Zr(BH4)4·8NH3 was analyzed by using the Avrami–Erofeev equation.43–45Fig. 4 shows the hydrogen evolution kinetic curves for the hydrolysis of Zr(BH4)4·8NH3 at different temperatures fitted by the Avrami–Erofeev equation. The inner picture of Fig. 4 presents an Arrhenius plot of ln(k) versus absolute temperature (1000/T). The R2 value was close to 0.99, which indicates that the Arrhenius equation is appropriate to describe the hydrolysis of Zr(BH4)4·8NH3. As shown in Fig. 4, the hydrolysis Ea of Zr(BH4)4·8NH3 in deionized water was calculated to be 29.38 kJ mol−1.


image file: c7ta04308b-f4.tif
Fig. 4 Hydrolysis kinetic curves for the hydrolysis of Zr(BH4)4·8NH3 at different temperatures (inner: an Arrhenius plot for the calculation of the activation energy).

3.2. Hydrolysis mechanism

To elucidate the hydrolysis mechanism of Zr(BH4)4·8NH3, the hydrolysis product was characterized by XRD after freeze-drying, as presented in Fig. 5(a). After the hydrolysis reaction, the hydrolysis product was composed of two parts: the solution (water-solute) and the white precipitate at the bottom of the solution (see Fig. 5(d)). In order to clarify the phase compositions of the hydrolysis product, the precipitate was separated from the hydrolysis solution simply after a couple of hours of standing. After freezing-drying, the powder samples were obtained from both the water-solute and the precipitate. From Fig. 5, we can see that the precipitate was amorphous, while the water-solute was crystallized. Interestingly, the water-solute just contributed to the diffraction peaks in Fig. 5(a), while the precipitate composed the baseline of the XRD pattern of the hydrolysis product. Through analysis by Highscore software, the reflection peaks of the water-solute were indexed to be Zr(BH4)4, NH4B5O8·4H2O, NH4B5O7(OH)2·H2O and (NH4)2B12(OH)12.
image file: c7ta04308b-f5.tif
Fig. 5 XRD patterns of (a) hydrolysis product, (b) water-solute and (c) precipitate of Zr(BH4)4·8NH3 and (d) photo of the hydrolysis product.

To clarify the elemental composition of the precipitate, EDS analysis was performed. Fig. 6(a) presents the EDS results of the precipitate. As indicated in Fig. 6(a), the precipitate mainly consisted of Zr, O and B elements. Furthermore, the atomic ratio of Zr[thin space (1/6-em)]:[thin space (1/6-em)]O was close to 1[thin space (1/6-em)]:[thin space (1/6-em)]4. After being heated to 1073 K, the precipitate was transformed into ZrO2 (tetragonal and monoclinic), as shown in Fig. 6(b). Based on the above-mentioned results and discussion, the precipitate is considered to be amorphous Zr(OH)4.


image file: c7ta04308b-f6.tif
Fig. 6 (a) SEM image and EDS mapping of the precipitate and (b) the XRD pattern after heating to 1073 K.

The FT-IR results (Fig. 7) revealed that the chemical bonds change before and after Zr(BH4)4·8NH3 hydrolysis. In Zr(BH4)4·8NH3, the stretching and bending bands of the B–H bonds were in the regions between 2180–2470 cm−1 and 1080 cm−1, while the stretching and bending bands of the N–H bonds were in a broad region ranging from 2950–3330 cm −1 and 1405 cm−1. After hydrolysis, the vibrations corresponding to the B–H absorptions almost disappeared, while those associated with the N–H absorptions remained the same, which implied that the hydrogen released in hydrolysis mainly originated from the breaking of the B–H bonds. As for the water-solute, the vibrations corresponding to the N–H absorptions and B–H absorptions were relatively strong in the water-solute, which implied that N–H and B–H bonds existed in the water-solute. In contrast to the water-solute, those vibrations were quite weak in the precipitate. Incidentally, vibrations corresponding to the O–H absorptions were stronger in the precipitate, which was consistent with the suggestion that the precipitate contained Zr(OH)4.


image file: c7ta04308b-f7.tif
Fig. 7 FT-IR results of (a) Zr(BH4)4·8NH3, (b) hydrolysis product, (c) water-solute and (d) precipitate.

As the FT-IR results presented, the N–H bonds existed in both of the samples before and after hydrolysis. Thus, the ligand NH3 of Zr(BH4)4·8NH3 might be firstly dissolved in water and did not take part in the hydrolysis reaction. To prove this, the pH value of the hydrolysis solution was tested. The pH value was measured to be 10.5 as shown in Fig. 8(a), indicating that the solution was alkaline. To further confirm the existence of NH4+, some NaOH solution was added to the hydrolysis solution, and then the moist pH test paper near the mouth of the test tube turned blue (Fig. 8(b)), suggesting that some NH3 gas was generated. So, it was concluded that once in contact with water, Zr(BH4)4·8NH3 was decomposed into Zr(BH4)4 and NH3, and then NH3 was dissolved in water.


image file: c7ta04308b-f8.tif
Fig. 8 The pH value tests of (a) the Zr(BH4)4·8NH3 hydrolysis solution and (b) the hydrolysis solution after NaOH solution was added.

Based on the above-mentioned discussion, the hydrolysis reaction pathways of Zr(BH4)4·8NH3 should be summarized as follows:

Once in contact with water, Zr(BH4)4·8NH3 was decomposed into Zr(BH4)4 and NH3, and then NH3 reacted with water to form NH4OH:

 
Zr(BH4)4·8NH3 + 8H2O = Zr(BH4)4 + 8NH4OH(1)

Then Zr(BH4)4 reacted with water to form Zr(OH)4 and B(OH)3, meanwhile H2 was released.

 
Zr(BH4)4 + 16H2O = Zr(OH)4 + 4B(OH)3 + 16H2(2)

Lastly, B(OH)3 reacted with NH4OH and became NH4B(OH)4. Because the Zr(BH4)4 was hydrolyzed under alkaline conditions, the reaction kinetics was slow.46 During the freeze-drying, some H2O in NH4B(OH)4 was evacuated, and so NH4B(OH)4 was decomposed into NH4B5O8·4H2O, NH4B5O7(OH)2·H2O or (NH4)2B12(OH)12.

 
B(OH)3 + NH4OH = NH4B(OH)4(3)

In summary, the hydrolysis reaction should be:

 
Zr(BH4)4·8NH3 + 24H2O = Zr(OH)4 + 4NH4B(OH)4 + 4NH4OH + 16H2(4)

Furthermore, according to this hydrolysis mechanism, the theoretical capacity of Zr(BH4)4·8NH3 should be 11.2 wt% H2, equivalent to 1250 mL g−1 H2, and this was close to the observed hydrogen yield, as shown in Fig. 3.

4. Conclusions

The Zr(BH4)4·8NH3 compound was synthesized by a ball milling method and the hydrolysis reaction in deionized water at 298 K can release about 1067 mL g−1 H2 in 240 min and the generated hydrogen was with high purity without toxic diborane and ammonia impurity gases. With heat-assistance, the hydrogen generation rate was significantly improved, and its activation energy was calculated to be 29.38 kJ mol−1. The hydrolysis reaction mechanism of Zr(BH4)4·8NH3 can be summarized as follows:
Zr(BH4)4·8NH3 + 24H2O = Zr(OH)4 + 4NH4B(OH)4 + 4NH4OH + 16H2

Acknowledgements

This work was supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. NSFC51621001), National Natural Science Foundation of China Projects (Nos. 51431001) and by the Project Supported by Natural Science Foundation of Guangdong Province of China (2016A030312011 and 2014A030311004). The authors also thank Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014).

Notes and references

  1. L. Schlapbach and A. Zuttel, Nature, 2001, 414, 353–358 CrossRef CAS PubMed .
  2. L. Z. Ouyang, X. S. Yang, M. Zhu, J. W. Liu, H. W. Dong, D. L. Sun, J. Zou and X. D. Yao, J. Phys. Chem. C, 2014, 118, 7808–7820 CAS .
  3. R. Mohtadi and S.-i. Orimo, Nat. Rev. Mater., 2016, 2, 16091 CrossRef .
  4. K. T. Møller, T. R. Jensen, E. Akiba and H.-W. Li, Prog. Nat. Sci.: Mater. Int., 2017, 27, 34–40 CrossRef .
  5. X. Yao, C. Wu, A. Du, G. Q. Lu, H. Cheng, S. C. Smith, J. Zou and Y. He, J. Phys. Chem. B, 2006, 110, 11697–11703 CrossRef CAS PubMed .
  6. Z. Cao, L. Ouyang, H. Wang, J. Liu, M. Felderhoff and M. Zhu, J. Mater. Chem. A, 2017, 5, 6042–6046 CAS .
  7. W. Cui, Q. Liu, N. Cheng, A. M. Asiri and X. Sun, Chem. Commun., 2014, 50, 9340–9342 RSC .
  8. M. Sevilla and R. Mokaya, Energy Environ. Sci., 2014, 7, 1250 CAS .
  9. T. K. Mandal and D. H. Gregory, Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem., 2009, 105, 21 RSC .
  10. S. S. Muir and X. Yao, Int. J. Hydrogen Energy, 2011, 36, 5983–5997 CrossRef CAS .
  11. B. R. S. Hansen, M. Paskevicius, H.-W. Li, E. Akiba and T. R. Jensen, Coord. Chem. Rev., 2016, 323, 60–70 CrossRef CAS .
  12. X. Kang, Z. Fang, L. Kong, H. Cheng, X. Yao, G. Lu and P. Wang, Adv. Mater., 2008, 20, 2756–2759 CrossRef CAS PubMed .
  13. L. Li, X. Yao, C. Sun, A. Du, L. Cheng, Z. Zhu, C. Yu, J. Zou, S. C. Smith, P. Wang, H.-M. Cheng, R. L. Frost and G. Q. Lu, Adv. Funct. Mater., 2009, 19, 265–271 CrossRef CAS .
  14. Z. Li, G. Zhu, G. Lu, S. Qiu and X. Yao, J. Am. Chem. Soc., 2010, 132, 1490–1491 CrossRef CAS PubMed .
  15. H.-W. Li, Y. Yan, S.-i. Orimo, A. Züttel and C. M. Jensen, Energies, 2011, 4, 185–214 CrossRef CAS .
  16. G. Xia, Q. Gu, Y. Guo and X. Yu, J. Mater. Chem., 2012, 22, 7300 RSC .
  17. J. Huang, Y. Yan, A. Remhof, Y. Zhang, D. Rentsch, Y. S. Au, P. E. de Jongh, F. Cuevas, L. Ouyang, M. Zhu and A. Zuttel, Dalton Trans., 2016, 45, 3687–3690 RSC .
  18. H. Pan, S. Shi, Y. Liu, B. Li, Y. Yang and M. Gao, Dalton Trans., 2013, 42, 3802–3811 RSC .
  19. Y. Guo, G. Xia, Y. Zhu, L. Gao and X. Yu, Chem. Commun., 2010, 46, 2599–2601 RSC .
  20. Y. Tan, Q. Gu, J. A. Kimpton, Q. Li, X. Chen, L. Ouyang, M. Zhu, D. Sun and X. Yu, J. Mater. Chem. A, 2013, 1, 10155 CAS .
  21. Y. Tan, Y. Guo, S. Li, W. Sun, Y. Zhu, Q. Li and X. Yu, J. Mater. Chem., 2011, 21, 14509 RSC .
  22. Z. Tang, Y. Tan, Q. Gu and X. Yu, J. Mater. Chem., 2012, 22, 5312 RSC .
  23. F. Yuan, Q. Gu, Y. Guo, W. Sun, X. Chen and X. Yu, J. Mater. Chem., 2012, 22, 1061–1068 RSC .
  24. J. Huang, Y. Tan, J. Su, Q. Gu, R. Cerny, L. Ouyang, D. Sun, X. Yu and M. Zhu, Chem. Commun., 2015, 51, 2794–2797 RSC .
  25. J. Huang, Y. Tan, Q. Gu, L. Ouyang, X. Yu and M. Zhu, J. Mater. Chem. A, 2015, 3, 5299–5304 CAS .
  26. J. Huang, L. Ouyang, Q. Gu, X. Yu and M. Zhu, Chemistry, 2015, 21, 14931–14936 CrossRef CAS PubMed .
  27. M. Winter and R. J. Brodd, Chem. Rev., 2004, 104, 4245–4269 CrossRef CAS PubMed .
  28. M. Mostajeran, D. J. Wolstenholme, C. Frazee, G. S. McGrady and R. T. Baker, Chem. Commun., 2016, 52, 2581–2584 RSC .
  29. M. Paskevicius, L. H. Jepsen, P. Schouwink, R. Cerny, D. B. Ravnsbaek, Y. Filinchuk, M. Dornheim, F. Besenbacher and T. R. Jensen, Chem. Soc. Rev., 2017, 46, 1565–1634 RSC .
  30. Z. Tang, F. Yuan, Q. Gu, Y. Tan, X. Chen, C. M. Jensen and X. Yu, Acta Mater., 2013, 61, 3110–3119 CrossRef CAS .
  31. E. Welchman and T. Thonhauser, J. Mater. Chem. A, 2017, 5, 4084–4092 CAS .
  32. H. Wu, X. Zhou, E. E. Rodriguez, W. Zhou, T. J. Udovic, T. Yildirim and J. J. Rush, J. Solid State Chem., 2016, 242, 186–192 CrossRef CAS .
  33. L. Z. Ouyang, J. M. Huang, C. J. Fang, H. Wang, J. W. Liu, Q. A. Zhang, D. L. Sun and M. Zhu, J. Alloys Compd., 2013, 580, S317–S319 CrossRef CAS .
  34. J. M. Huang, L. Z. Ouyang, Y. J. Wen, H. Wang, J. W. Liu, Z. L. Chen and M. Zhu, Int. J. Hydrogen Energy, 2014, 39, 6813–6818 CrossRef CAS .
  35. M. Huang, L. Ouyang, H. Wang, J. Liu and M. Zhu, Int. J. Hydrogen Energy, 2015, 40, 6145–6150 CrossRef CAS .
  36. H. Zhong, H. Wang, J. W. Liu, D. L. Sun, F. Fang, Q. A. Zhang, L. Z. Ouyang and M. Zhu, J. Alloys Compd., 2016, 680, 419–426 CrossRef CAS .
  37. D. Bhattacharjee and S. Dasgupta, J. Mater. Chem. A, 2015, 3, 24371–24378 CAS .
  38. M. Mahyari and A. Shaabani, J. Mater. Chem. A, 2014, 2, 16652–16659 CAS .
  39. L. Guo, X. Gu, K. Kang, Y. Wu, J. Cheng, P. Liu, T. Wang and H. Su, J. Mater. Chem. A, 2015, 3, 22807–22815 CAS .
  40. M. Huang, L. Ouyang, J. Ye, J. Liu, X. Yao, H. Wang, H. Shao and M. Zhu, J. Mater. Chem. A, 2017, 5, 8566–8575 CAS .
  41. L. Ouyang, M. Ma, M. Huang, R. Duan, H. Wang, L. Sun and M. Zhu, Energies, 2015, 8, 4237–4252 CrossRef CAS .
  42. L. Z. Ouyang, J. M. Huang, H. Wang, Y. J. Wen, Q. A. Zhang, D. L. Sun and M. Zhu, Int. J. Hydrogen Energy, 2013, 38, 2973–2978 CrossRef CAS .
  43. K. F. Kelton, J. Mater. Sci. Eng. A, 1997, 226, 142–150 CrossRef .
  44. L. Z. Ouyang, Z. J. Cao, H. Wang, J. W. Liu, D. L. Sun, Q. A. Zhang and M. Zhu, J. Alloys Compd., 2014, 586, 113–117 CrossRef CAS .
  45. Z. Cao, L. Ouyang, Y. Wu, H. Wang, J. Liu, F. Fang, D. Sun, Q. Zhang and M. Zhu, J. Alloys Compd., 2015, 623, 354–358 CrossRef CAS .
  46. B. Weng, F. Xu, Z. Wu and Z. Li, Int. J. Hydrogen Energy, 2014, 39, 14942–14948 CrossRef CAS .

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