Thermal decomposition kinetics of light-weight composite NaNH2–NaBH4 hydrogen storage materials for fuel cells

https://doi.org/10.1016/j.ijhydene.2012.05.069Get rights and content

Abstract

Pristine and Co–B doped NaNH2–NaBH4 (2/1 molar rate) hydrogen storage materials were synthesized via a ball milling method. The kinetic parameters, especially the activation energies were calculated by the Achar differential and the Coats–Redfern integral methods, based on the thermogravimetry curves of the decomposition processes of the hydrogen storage materials. The as-obtained activation energy for the pristine NaNH2–NaBH4 (2/1) is 159.6 kJ/mol, while that for the Co–B doped NaNH2–NaBH4 (2/1) is 70 kJ/mol, only 43.9% of the former, which indicates the Co–B catalyst can dramatically promote the hydrogen generation reaction. The structural evolution and the hydrogen releasing performance of the Co–B doped NaNH2–NaBH4 (2/1) were investigated by X-ray diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA). Then, the catalytic mechanism of NaNH2–NaBH4 (2/1) decomposition promoted by Co–B was discussed and illustrated. It implies that the existence of Co–B catalyst can promote the loss of electron from H, thus improve the kinetics and accelerate the formation of H2 upon heating.

Highlights

► NaNH2–NaBH4 hydrogen storage materials are synthesized via ball milling in Argon. ► Active energies of light-weight NaNH2–NaBH4 materials are computed and compared. ► The kinetics of NaNH2–NaBH4 is greatly improved by Co–B catalyst. ► Catalytic mechanism of NaNH2–NaBH4 decomposition is put forward and illustrated.

Introduction

Hydrogen is the best fuel for fuel cells [1]. And the development of hydrogen storage and hydrogen supply techniques is of importance for the industrialization of fuel cells, especially for proton exchange membrane fuel cells (PEMFC), which depend heavily on high-purity hydrogen. Therefore, seeking for new hydrogen storage materials and systems with satisfactory hydrogen storage capacity is attracting more and more attention in this field. Recently, complex hydrides, including alanates, borohydrides, amides, imides, alane etc., offered a possibility to design a potential hydrogen storage system [2]. These metal–hydrogen complexes are especially interesting because of their light weight and the number of hydrogen atoms per metal atom, which are two in many cases [3]. Therefore, some light-weight complex hydrides for hydrogen storage, such as LiBH4 [4], [5], [6], NaBH4 [7], NaAlH4 [6], [8], LiAlH4 and LiNH2 [6], have been studied extensively. Furthermore, mixtures of these hydrides remain attractive and draw considerable interest of researchers [9], [10], [11]. According to Chen et al. [12], some amide–hydride or amide-coordination hydride composite hydrogen storage system can achieve high hydrogen-generation amount, because different components in these systems can reduce stability of each other.

Among the attractive amide–borohydride systems, LiNH2 and NaNH2 are the common candidates for the amides, while LiBH4, NaBH4, Mg(BH4)2 and Ca(BH4)2 are the common candidates for the borohydrides. However, because of the lack of high efficient synthesis methods, there are scarcely any commercial products for Mg(BH4)2 and Ca(BH4)2. Differently, LiNH2 and LiBH4 have commercial products, but are rather expensive. For NaNH2 and NaBH4, their commercial products are much cheaper than LiNH2 and LiBH4, therefore are the ones most closed to the practice. So far, there are a few researches focusing on composite NaNH2–NaBH4 systems. Chater et al. [13] and Somer et al. [14] synthesized composite NaNH2–NaBH4 systems by annealing the raw materials in sealed quartz tube or glass ampoule, while we achieved similar NaNH2–NaBH4 systems via ball milling [15]. All these studies indicate the composite NaNH2–NaBH4 is promising for developing new hydrogen storage materials.

To improve the kinetics of the composite NaNH2–NaBH4, amorphous alloy catalyst may be a good choice. The unique structural character of amorphous alloyed materials can result in excellent catalytic properties. Therefore these materials are widely used in many chemical reactions, such as hydrogenation, oxidation, cracking reaction and isomerization [16]. Additionally, due to the non-porosity property of amorphous alloys, the effect of internal diffusion on the surface reaction, which is serious for traditional heterogeneous catalyst, can be resolved. In some recent reports, amorphous Co–B alloys were used as catalysts for hydrogen generation from NaBH4 hydrolysis, and showed excellent catalytic performances [17], [18]. Therefore, in this paper, amorphous Co–B alloy is also used to improve the kinetics of composite NaNH2–NaBH4 (2/1), and shows good catalytic activity. The kinetics of the pristine and the Co–B doped NaNH2–NaBH4 (2/1) are investigated and compared, and the effects of the Co–B catalysts on the decomposition characteristics of NaNH2–NaBH4 (2/1) are discussed.

Section snippets

Preparation of composite NaNH2–NaBH4 (2/1)

The composite NaNH2–NaBH4 (2/1) was prepared with a molar ratio NaNH2:NaBH4 = 2:1 via a ball milling method, as described in detail in our previous work [15]. The only difference is that in this study, an optimized ball milling time, namely, 16 h, is selected. And the as-prepared sample is named as S16.

Preparation of Co–B catalyst

In order to improve the kinetics of the composite NaNH2–NaBH4 (2/1), a Co–B catalyst is used herein, which was prepared by a chemical reduction method, as described in previous studies [17], [19]

Selecting suitable calculating methods for the activation energy of NaNH2–NaBH4 decomposition

Activation energy is the minimum energy required for activating the reactant molecules to activated molecules in the chemical reaction. The chemical rate is closely related to the activation energy. The lower the activation energy is, the faster the chemical rate will be. Therefore, it is an effective way to promote the chemical reaction by reducing the activation energy.

So far, some methods have been developed to calculate the activation energies of the chemical reactions, including Ozawa

The activation energy of the main decomposition stage of NaNH2–NaBH4

According to our previous study, the decomposition of NaNH2–NaBH4 (2/1) mainly occurred in the second stage [15], namely, 266–420 °C. Therefore, here we are focusing on the activation energy of this temperature range. On the TG curve of the second stage, as shown in Fig. 1, the temperatures corresponding to the conversion rate α (from 0.1 to 0.9, respectively) are selected as the basic data. The corresponding activation energies corresponding with 32 different mechanisms are calculated and

Conclusions

In summary, pristine and Co–B doped NaNH2–NaBH4 (2/1) hydrogen storage materials can be efficiently synthesized via the ball milling method. The Achar differential and the Coats–Redfern integral methods are selected to calculate the activation energies of the hydrogen storage systems, as they are all based on the thermogravimetry curves of the decomposition processes, which are well match the experimental conditions herein. The as-obtained activation energy for the Co–B doped NaNH2–NaBH4 (2/1)

Acknowledgments

The present work is financially supported by National Natural Science Foundation of China (Grant No. 20806010), National “973” program of China (Grant No. 2009CB220100), Beijing Nova Program (Grant No. 200923) and Open fund of Beijing Higher Institution Engineering Research Center of Power Battery and Chemical Energy Materials.

References (27)

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