Hydrogen production via hydrolysis of Mg-oxide composites

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Hydrogen production via hydrolysis of Mg-oxide composites Minghong Huang, Liuzhang Ouyang*, Zhiling Chen, Chenghong Peng, Xiaoke Zhu, Min Zhu School of Materials Science and Engineering and Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou, 510641, PR China

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abstract

Article history:

Mg is an attractive candidate for hydrogen generation due to its low cost and high avail-

Received 18 October 2016

ability as well as its high theoretical H2 yield and the formation of environmentally friendly

Received in revised form

byproducts during hydrolysis. On the other hand, the hydrolysis reaction of Mg is rapidly

29 November 2016

interrupted by the formation of a passive magnesium hydroxide layer. Hydrogen genera-

Accepted 10 December 2016

tion via the reaction of ball milled Mg-oxide composites with 3.5% NaCl solution at room

Available online xxx

temperature was investigated in this paper. Several cheap metal oxides (Fe2O3, CaO, MoO3,

Keywords:

powder. The results show that Mg-5 wt% MoO3 and Fe2O3 demonstrate the best hydrolysis

Hydrolysis

performance (above 888 mL/g and 95.2% of theoretical hydrogen generation yield in 10 min)

Hydrogen generation

in comparison to MgeFe3O4, MgeTiO2, MgeNb2O5 and MgeCaO composites. In addition,

Magnesium

the effects of different Mg contents and milling times on the hydrolysis property of the Mg

Ball milling

powder were also studied and it was concluded that the addition of 5 wt% oxide and a

Transition metal oxides

milling time of 1 h are the optimal parameters for the production of the MgeMoO3 com-

Fe3O4, Nb2O5 and TiO2) were used to assess the effects of hydrolysis on the magnesium

posite. Moreover, the valence state of metal ions was found to have an important influence on the hydrolysis reaction for the first time. The effect of valence state has been studied for Mg-Fexþ and Mg-Moxþ composites and the results show that a higher valence value of the transition metal ions leads to a better hydrolysis property of Mg. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen as an energy carrier is an attractive solution to the problems of energy supply security and greenhouse gas reduction due to its high energy density, cleanness, and abundance of resources [1e3]; however, hydrogen storage and highly efficient hydrogen generation/production are still critical issues preventing its commercial application. With respect to hydrogen generation, most industrial hydrogen has

been produced in the past decades from water electrolysis [4] or fossil resources via gas reformation [5]. Unfortunately, these methods are inconvenient and external energy introduction is necessary, making them unsuitable for on-board hydrogen generation. Recently, hydrolysis has been recognized as an optimal method for the production of hydrogen in proximity of the area in which it is needed [6]. Many kinds of materials, such as borohydrides [7,8], metals [9], metal hydrides [10,11] and alloys [12] [13,14] have been studied in the literature. Among these materials, magnesium metal has

* Corresponding author. Fax: þ86 20 87112762. E-mail address: [email protected] (L. Ouyang). http://dx.doi.org/10.1016/j.ijhydene.2016.12.099 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Huang M, et al., Hydrogen production via hydrolysis of Mg-oxide composites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.099

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7

attracted much attention for hydrogen production due to its electrochemical activity, low cost, abundance and production of environmentally friendly products. The reaction between magnesium and water can be described as: Mg þ 2H2O / Mg(OH)2 þ H2 DH ¼ 354 kJ/mol

(1)

The theoretical hydrogen yield of this reaction is as high as 8.2 wt% (no water included in the calculation); however, the hydrolysis reaction is quickly interrupted by the formation of a passive magnesium hydroxide layer that covers the unreacted Mg particle. Many attempts have been made to improve the material's hydrolysis performance, in order to qualify magnesium for this application. An efficient way to destroy the passive magnesium hydroxide layer is using acid. Kushch et al. [15] have demonstrated that hydrogen generation is improved via the interaction of magnesium with organic acids. However, the use of the acid solution causes the corrosion and environmental pollution. At the same time, various efficient methods, e.g. using nano Mg materials [16,17], saline solution [18e20], catalyst [21], ultrasonic irradiation [22], and ball milling with salts [23e27], hydrides [28], metals [29,30], carbon materials [31] were attempted. Among these approaches, ball milling and alloying are two effective methods to improve the chemical activity of Mg-based materials [32e35]. Grosjean et al. [23] showed that ball milling of Mg in presence of a transition metal could improve the hydrolysis performance. The Mg-10 at% Ni composite milled for 30 min displayed excellent hydrolysis kinetics and achieved 100% conversion in a 1 M KCl solution. Obviously, the microgalvanic cell formed between Mg and Ni is very beneficial for the hydrolysis, but the results confirmed that the addition of transition metals has no effect on the hydrolysis properties in pure water and a salt solution is necessary for the establishment of these micro-galvanic cells [31]. So far, many metal oxides have been tried as catalysts for Mg and its hydride as hydrogen storage materials [36,37], but only a few metal oxides (V2O5 and Nb2O5) have been studied, by Awad et al., for the hydrolysis reaction [31,38]. The highenergy milled Mg, with 10 wt% V2O5 or Nb2O5, displayed excellent hydrolysis kinetics and achieved 100% conversion in a 3.5% NaCl solution at room temperature. In the present work, we investigated the effects of the introduction of cheap metal oxides (Fe3O4, Fe2O3, CaO, Nb2O5, TiO2 and MoO3) on the hydrolysis behaviour of Mg-based materials. The hydrogen generation kinetics and hydrolysis yield of Mg-metal oxide composites shows that their application in portable power generation for fuel cells in a 3.5% NaCl solution (similar to sea water) at room temperature is promising.

Experiment Mg powder (50 mm) was purchased from Goodfellow, with 99.8% purity and was ball milled with different metal oxides, indicated with X (X ¼ Fe3O4, Fe2O3, CaO, Nb2O5, TiO2 and MoO3). The milling process was carried out in a planetary ball miller (QM-3SP4, Nanjing University Instrument Plant, China) with a ball-to-powder weight ratio of 20:1 and was performed by alternately repeating milling (for 15 min) and cooling (for

5 min). During the ball milling process, the rotation speed was set to 250 rpm and the milling time varied from 6 min to 5 h. After milling, the hydrogen generation reactions of the activated Mg-5 wt% X composites with 3.5% NaCl solution were performed in a 50-mL flask reactor with two openings. The equipment used was similar to the one described in our previous study [39]. After the hydrolysis reaction was completed, the products were centrifuged in a TG16-WS ultracentrifuge at 8000 r/min for 10 min and then dried in a drying cabinet at 60  C. The phase compositions of samples and hydrolysis products were analysed by X-ray diffraction (XRD, PANalytical EMPYREAN) using a Cu Ka radiation. XRD analyses were performed over a range of 10 <2q < 90 with a step size of 0.013 and a time per step of 20 s.

Results and discussions Hydrogen production of Mg-oxide composites via hydrolysis Fig. 1 shows the XRD patterns of Mg milled with 5 wt% of Fe3O4, Fe2O3, CaO, Nb2O5, TiO2 and MoO3 for 1 h. The results only show the peaks relative to Mg and Fe3O4, Fe2O3, CaO, Nb2O5, TiO2 and MoO3. No characteristic peaks indicating the presence of a new phase were detected, meaning that no reaction occurred between Mg and oxides during milling. Theoretically, the Mg could react with Fe3O4, Fe2O3, Nb2O5, and MoO3, but the relatively low rotation speed and ball-to-powder weight ratio applied in this work could not provide sufficient energy to break the oxide bonds and form a new phase. Hence, relatively stable Mg-oxide composites for hydrogen production via hydrolysis were fabricated using ball milling. Fig. 2 shows the hydrolysis characteristics of Mg-oxide (Fe3O4, Fe2O3, CaO, Nb2O5, TiO2 and MoO3) composites in 3.5% NaCl solution. Judging from Fig. 2, pure Mg powder milled for 1 h has low reactivity in 3.5% NaCl solution and produces only 257 mL/g of hydrogen (27.5% of theoretical amount of hydrogen) in 10 min. In contrast, Mg-oxide (Fe3O4, Fe2O3, CaO, Nb2O5, TiO2 and MoO3) composites with transition metal oxides as catalyst show better hydrolysis properties, especially in the case of MoO3, Fe3O4 and Fe2O3. The hydrogen yields of the Mg-oxide (Fe3O4, TiO2, Nb2O5 and CaO) composites are 826, 651, 435, and 137 mL/g in 10 min, respectively. While the hydrogen yields of MgeMoO3 and MgeFe2O3 are, respectively, 888 and 869 mL/g in 10 min, higher than those obtained with of Mg-oxides (Fe3O4, CaO, Nb2O5 and TiO2) composites. It should be noted that MgeCaO composites show the lowest hydrogen yield, even lower than that of Mg, which means that CaO had a negative effect on the hydrolysis properties of Mg. In particular, Mg-5 wt% CaO composites produced only 137 mL/g of hydrogen, equivalent to a yield of 14.7%, compared to the 27.5% yield obtained with pure Mg. The different roles of transition metal oxides and alkaline earth oxides may be due to their atomic affinity with Mg.

Effect of oxide quantity and milling time on the hydrolysis properties To study the effect of different amounts of catalyst on hydrolysis properties, the MgeMoO3 composites, which had

Please cite this article in press as: Huang M, et al., Hydrogen production via hydrolysis of Mg-oxide composites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.099

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Fig. 1 e XRD patterns of Mg-5 wt% oxides composites milled for 1 h.

demonstrated best hydrogen generation kinetics properties, were studied in detail. Fig. 3(a) shows the hydrolysis behaviour of MgeMoO3 composites with different amounts of MoO3 introduced and ball milled for 1 h at room temperature. As shown in Fig. 3(a), the MgeMoO3 composite with 0.5 wt% MoO3

shows excellent hydrolysis properties and releases 85.1% hydrogen in 1 min, which indicates that the trace amounts of MoO3 added could significantly accelerate and enhance the hydrolysis reaction rate of Mg. Moreover, the initial hydrogen generation rate increases with increasing amounts of MoO3.

Fig. 2 e (a) Hydrogen generation curves and (b) conversion yield of Mg-5 wt% oxides (Fe3O4, Fe2O3, CaO, Nb2O5, TiO2 and MoO3) composites milled for 1 h. Please cite this article in press as: Huang M, et al., Hydrogen production via hydrolysis of Mg-oxide composites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.099

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Fig. 3 e Hydrogen generation curves of (a) Mg-x wt% MoO3 milled for 1 h; (b) Mg-5 wt% MoO3 milled for different hours. The reaction rate of the MgeMoO3 composite with 0.5 wt% MoO3 is 794 mL/g/min and increases to 810 mL/g/min, 856 mL/ g/min and 857 mL/g/min as the amount of MoO3 increases to 2 wt%, 5 wt%, 10 wt%, respectively. It is reasonable to assume that MoO3 can act as a cutter during the milling process and create large amounts of fresh surfaces and defects on the magnesium particles, favouring the kinetics of the hydrolysis reaction. It is interesting to note that the hydrogen yield

reaches 85.1% with only 0.5 wt% MoO3 and the hydrogen yield could be further improved to 91.7% with 5 wt% MoO3. The hydrolysis properties are affected by the quantity of MoO3 as well as by the milling time. Fig. 3(b) shows the hydrogen generation curves of Mg-5 wt% MoO3 obtained with different milling times. As shown in Fig. 3(b), it is observed that the ball-milling time can effectively change the hydrolysis properties of the composite. It was observed that when

Fig. 4 e XRD patterns and hydrogen generation curves of (a and b) Mg-0.5 wt% iron oxides composites; (c and d) Mg-0.5 wt% molybdenum oxides composites. Please cite this article in press as: Huang M, et al., Hydrogen production via hydrolysis of Mg-oxide composites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.099

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7

milling time increases, the hydrogen yield initially increases and then decreases, in agreement with the findings of previous studies [31,40]. The hydrogen yield of the MgeMoO3 composite reaches 72.7% and 91.6% when the milling time increases from 0.1 to 0.5 h. The maximum hydrogen yield of the MgeMoO3 composite reaches 94.4% after milling for 1 h; additional increases, to 3 and 5 h, cause a decrease in hydrogen yield, to 91.3% and 79.3%, respectively. In a word, the enhanced hydrogen generation properties can be attributed to the decreased particle size and the enlarged specific area and defects, which can increase the reactivity of Mg powder. The decrease in hydrogen yield after prolonged milling can be due to the agglomeration of Mg particles caused by cold welding during the ball milling process.

Effect of valence state of metals ions on the hydrolysis properties The effects of oxides with transition metals having different valence states on the hydrolysis properties was also studied. For general consideration, the catalytic efficiency of both iron

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oxides and molybdenum oxides were investigated and compared. Mg-0.5 wt% iron oxide (Fe, Fe2O3 and Fe3O4 (Fe3O4 can be written as FeO*Fe2O3)) composites and Mg-0.5 wt% molybdenum oxides (Mo, MoO2 and MoO3) composites were prepared under identical conditions and tested for hydrolysis. Fig. 4(a) and (c) show the X-ray diffraction patterns of Mg0.5 wt% iron oxides (Fe, Fe3O4 and Fe2O3) composites and Mg0.5 wt% molybdenum oxides (Mo, MoO2 and MoO3) composites obtained after ball milling for 1 h. Obviously, no new phase was formed. Fig. 4(b) and (d) show hydrogen generation curves of Mg milled with 0.5 wt% iron oxides and molybdenum oxides. As expected, both oxides show noticeable catalytic effects on the hydrolysis properties of Mg. Judging from Fig. 4(b), the hydrogen yield of Mg catalysed by Fe, Fe3O4 (FeO*Fe2O3) or Fe2O3 increases to above 80%. A small difference in catalytic activity could also be identified for different valence states of iron; as it increased from Fe(0) to Fe(II*III) and Fe(III), the hydrogen yield increased from 80.3% to 80.6% and 82%, respectively. The effect of the valence state of Mo on the catalytic activity is even more evident, with the hydrogen yield increasing from 67.8% to 88.5%

Fig. 5 e XRD patterns of hydrolysis product of Mg-5 wt% oxides composites milled for 1 h. Please cite this article in press as: Huang M, et al., Hydrogen production via hydrolysis of Mg-oxide composites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.099

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and 90.1% as the valence state of molybdenum increased from Mo(0) to Mo(IV) and Mo(VI). Similarly, it has been found that the catalytic activity of hydrogen desorption on Mg increases with the valence state of the transition-metal ion [41]. In our study, the higher valence states of Fe or Mo led to a higher hydrolysis rate and hydrogen yield, indicating that it plays an important role in the hydrolysis of Mg.

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, 2014A030311004) and 2014GKXM011. The Project Supported by Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014) is also acknowledged.

Hydrolysis by-products In order to investigate the hydrolysis reaction mechanisms, the hydrolysis products of Mg-5 wt% oxide (Fe3O4, Fe2O3, TiO2, Nb2O5, MoO3 and CaO) composites were characterized, as shown in Fig. 5. The by-product is mainly composed of Mg(OH)2, and remaining Fe3O4, Fe2O3, TiO2, Nb2O5 and NaCl, while MoO3 and CaO can hardly be detected after the hydrolysis process due to their poor crystallinity. As no new phase had been detected either in the ball milled Mg-5 wt% oxides composites or in the by-product, the hydrolysis mechanism of Mg-oxides composite is considered to be the same as that of Mg catalysed by the oxides during the hydrolysis process. The catalytic effects of the addition of oxides can be generalized from two perspectives. Firstly, and similarly to other ball milling additives, oxides can serve as ball milling intermediates during the milling process and create large fresh surfaces and defects, which are beneficial to promote the hydrolysis reaction. Secondly, mechanochemical treatment of Mg during the ball milling process with transition metals or oxides led to a higher rate of Mg oxidation in aqueous salt solutions by forming micro-galvanic cells [42,43]. Therefore, we speculate that the promotion of hydrolysis properties of Mg-5 wt% oxides (Fe3O4, Fe2O3, TiO2, Nb2O5 and MoO3) is mainly due to the micro-galvanic cells formed between Mg and transition metals or their oxides. What is more, the catalytic activity increased with the rise of the valence state of the transition-metal ions, further confirming the catalytic effect of oxides on Mg.

Conclusions The enhancement effect on the hydrolysis properties of Mg could be ordered as MoO3 > Fe2O3 > Fe3O4 > TiO2 > Nb2O5 > CaO. MgeMoO3, MgeFe3O4 and MgeFe2O3 composites lead to a remarkable improvement on the hydrolysis rate and the hydrogen yield reaches 85.1% with only 0.5 wt% MoO3. The hydrogen yield was further increased to 95.2% when using 5 wt % MoO3 and 888 mL/g of hydrogen were produced in 10 min. Moreover, the valence state of transition metal ions had an important role in the hydrolysis of Mg and a higher valence state of transition metal led to a better catalytic effect. With the valence state of iron increasing from Fe(0) to Fe(II*III) and Fe(III), the hydrogen yield Mg-0.5 wt% iron oxides composite increased from 80.3% to 80.6% and 82%, respectively.

Acknowledgements This work was supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation

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Please cite this article in press as: Huang M, et al., Hydrogen production via hydrolysis of Mg-oxide composites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.099