Catalytic Hydrolysis of Borohydride for Fuel Cells

RESEARCH NOTES Chinese Journal of Chemical Engineering, 19(4) 693ü697 (2011)

Catalytic Hydrolysis of Borohydride for Fuel Cells* WANG Lianbang (ฆॕ͙)**, ZHAN Xingyue (Ⴓ໲ၥ), YANG Zhenzhen (ཷქქ) and MA Chun’an (৴ҡ̝) State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Material Science, Zhejiang University of Technology, Hangzhou 310014, China

Abstract Borohydrides present interesting options for the electrochemical power generation acting either as hydrogen source or anodic fuel for direct borohydride fuel cells (DBFC). In this work, Mg-Ni composite synthesized by mechanically alloying method, used as the catalyst for the hydrolysis of borohydride, has been investigated. Co-doping treatment has been carried out for the purpose of improving the hydrolysis rate further. The as-prepared and Co-doped Mg-Ni composites with low cost showed high catalytic activity to the hydrolysis of borohydride for hydrogen generation. After Co-doping, the hydrogen generation rate was around 280 ml·g1·min1. Borohydride would be a promising hydrogen source for fuel cells. Keywords borohydride, catalytic hydrolysis, Mg-Ni composite, fuel cell

1

INTRODUCTION

Fuel cells are attractive alternative energy conversion devices due to their higher energy conversion efficiency and low pollution. The increasing demand for high efficiency and clean power sources is hastening the development of fuel cells. Hydrogen is an ideal fuel for all fuel cells. However, the production, storage, transportation and utilization technologies of hydrogen are still on the way for the purpose of practical applications. Borohydrides, such as LiBH4, NaBH4 and KBH4, are considered as potential fuel for fuel cells. Borohydride could be directly oxidized as the anodic fuel for the direct borohydride fuel cell (DBFC) [15], or be hydrolyzed to generate hydrogen gas feed to fuel cells as the following reaction [68]: BH 4  2H 2 O  o BO 2  4H 2 n

their good catalytic activity and cost effectiveness. In our group, AB5-type hydrogen storage alloys used as the catalysts for the direct borohydride fuel cell (DBFC) have been investigated [1517]. AB5 hydrogen storage alloy showed high catalytic activity both to the oxidation and hydrolysis of borohydride. It is reported that Mg-based hydrogen storage alloys after fluorizing treatment showed high catalytic activity for the hydrolysis of borohydride [14]. In this work, Mg-based hydrogen storage alloy with low cost has been used as the catalyst for hydrolysis reaction of borohydride to generate hydrogen gas. The structure and catalytic activity of the Mg-based alloy used as catalyst have been investigated in detail. 2

EXPERIMENTAL

(1)

Borohydride used as the hydrogen source by hydrolysis reaction shows some advantages, such as high hydrogen density (10.6% for NaBH4), easy storage and transportation, high H2 purity etc. At room temperature, only a small percentage of the theoretical amount of H2 was liberated by the hydrolysis reaction of NaBH4 and H2O [9], but the hydrolysis could be enhanced and controlled by using catalysts. Kojima et al. [8] developed a 10 kW-scale hydrogen generator based on aqueous solution of sodium borohydride, NaBH4, catalyzed by a Pt-LiCoO2-coated honeycomb monolith. As in the previous reports, metal halides, colloidal platinum, Raney nickel [10], Ru [11], cobalt and nickel borides [12, 13], fluorinated Mg-based hydride [14] etc., could be used as the catalysts for the hydrolysis reaction of borohydride solution. Among all these catalysts, cobalt and nickel compounds were considered as alternative choices for the hydrolysis of borohydride due to

The alloy was synthesized by mechanically alloying (ball-milling) Mg and Ni powder (MgΉNi 2Ή1, mole ratio, all in 99.9% purity, 10 ȝm) with a planetary mill (Fritsch Pulverisette 5, Germany) using stainless steel vials (80 ml) and balls under Ar atmosphere at 350 r·min1 for 48 h. The mass ratio of ball to sample was 10Ή1. A surface treatment has been carried out to improve the catalytic activity of as-prepared alloy to the hydrolysis reaction of borohydride. The as-prepared alloy (0.5 g) was immersed into 10 ml of 1 mol·L1 CoCl2 aqueous solution. A spontaneous reaction proceeded due to the reduction of Co on the surface of sample. After 10 min, the sample was filtrated and washed with deionized water, and then dried at 80 °C under vacuum. The crystal structure of the sample was examined by means of a Thermo X’TRA X-ray diffractometer

Received 2010-07-05, accepted 2011-04-18. * Supported by the Natural Science Foundation of Zhejiang Province (Y405496) and the State Key Development Program for Basic Research of China (2007CB216409). Work originally presented on the 2nd Int. Symp. Sustainable Chemical Product and Process Engineering, held at Hangzhou, China from May 9 to 12, 2010. ** To whom correspondence should be addressed. E-mail: [email protected]

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with CuKĮ radiation source. The SEM (scanning electron microscopy) and EDS (energy dispersive using X-ray)were carried out to determine the surface morphology and element components of the sample. The catalytic activity of the sample to the hydrolysis reaction of borohydride may be expressed by the hydrogen generation rate, and was tested by dispersing 0.5 g of catalyst into 10 ml solution of 1 mol·L1 NaBH4-1 mol·L1 NaOH under magnetic stir at 200 r·min1. The generated hydrogen was collected by the water replacement method. (a) As-prepared ×500

3 3.1

RESULTS AND DISCUSSION Characterization

Figure 1 shows the XRD (X-ray diffraction) patterns of the as-prepared and Co-doped Mg-Ni composites. Experimental results showed that the as-prepared composite was composed of Mg2Ni and a small quantity of MgO and Ni. It revealed that part of Mg was oxidized during ball-milling, leaving metal Ni in excess. The broadened diffraction peaks of Mg2Ni phase in the XRD pattern suggested that the as-prepared Mg-Ni composite was made of nanocrystalline particles. It found that reaction occurred vigorously into 1 mol·L1 aqueous solution of CoCl2, with bubbles and a lot of heat generated. The XRD pattern of the Co-doped Mg-Ni composite shown in Fig. 1 (b) revealed that the composite was composed of Mg2Ni alloy, Ni metal and Mg(OH)2, which was transformed from MgO in the as-prepared composite, and no Co metal had been found in the composite because of its low concentration.

(b) As-prepared ×25000

(c) Co-doped ×500

Figure 1 XRD patterns of (a) as-prepared Mg-Ni composite, and (b) Co-doped Mg-Ni composite ƻ Ni; × MgO; + Mg2Ni; *Mg(OH)2

In order to determine the surface morphology and elements composition on the surface of composites, the SEM and EDS have been carried out. Figs. 2 (a) and 2 (b) showed that the size of particles of the as-prepared Mg-Ni composite was around 15 Pm. Some particles were congregated to form large particles. After treated in 1 mol·L1 CoCl2 solution, the surface morphology of the composite had been changed, an acicular crystal was formed on the surface of Co-doped

(d) Co-doped ×25000 Figure 2 The surface morphology of as-prepared and Co-doped composites

Mg-Ni composite particles shown in Figs. 2 (c) and (d). According to the results of XRD, the crystals on the surface of the particles could be attributed to the formed Mg(OH)2.

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Table 1 presents the elemental composition and content on the surface of as-prepared and Co-doped composites. It was clear that the as-prepared composite was composed of Mg, Ni and O elements. The result was consistent with the conclusion of XRD examination that the composite was composed of Mg2Ni, MgO and Ni. After treatment in Co-containing solution, elemental Co was found on the surface of the composite. The content of O element increased, and that of elements Mg and Ni decreased. According to the experimental phenomena, it suggested that a replacement reaction has occurred as the following:

Mg 2 Ni  2Co 2   4OH   o 2Mg(OH) 2  Ni  2Co

(2)

MgO  H 2 O  o Mg(OH) 2

(3)

Table 1

The elements composition of the Mg-Ni composites

Samples

wMg/%

wNi/%

wO/%

wCo/%

wtotal/%

as-prepared

22.68

73.20

4.13

0

100

Co-doped

18.44

43.04

15.36

23.16

100

In Co-contained solution, Co2+ ion was reduced by Mg, and adsorbed on the surface of the Mg-Ni composite particles. 3.2

Hydrolysis activity

The catalytic activity of as-prepared and Co-doped

Figure 3 The hydrolysis of 10 ml of 1 mol·L1 NaBH4-1 mol·L1 NaOH solution catalyzed by Mg-Ni composites ƶ as-prepared;ƽCo-doped

composites to the hydrolysis of borohydride for generation of hydrogen has been investigated. Fig. 3 shows the hydrogen generation rate by hydrolysis of 10 ml of 1 mol·L1 NaBH4-1 mol·L1 NaOH solution catalyzed by Mg-Ni composite. The as-prepared Mg-Ni composite showed relative poor catalytic activity to the hydrolysis of borohydride solution compared to the Co-doped composite because of the effect of compact magnesium oxide on the surface of the composite. After Co-doped treatment, MgO was transformed into Mg(OH)2, and at the same time metal Co was formed on the surface. It was reported that Co-containing catalyst showed high performance to the hydrolysis of borohydride [7]. In this work, the hydrogen contained in alkaline borohydride solution could be released within 5 min, the hydrogen generation rate was round

BH 4  o BH3OH   o BH 2 (OH)2  o BH(OH)3  o B(OH) 4  o BO 2 Figure 4 The sketch of the hydrolysis process of borohydride Table 2

EZ/Hartree

EZ: free energy.

Calculated total energy at the B3LYP/6-311G** level

BH 4

BH3OH

BH 2 (OH) 2

BH(OH)3

B(OH) 4

BO 2

27.235

102.497

177.778

253.067

328.364

175.486

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Figure 5

A schematic diagram of hydrogen system for PEMFC

280 ml˜g1˜min1. The as-prepared and Co-doped Mg-Ni composites with low cost showed much higher catalytic performance than 150 ml˜g1˜min1 of Co-B alloy in Ref. [7] because of the interaction of Co and Mg-Ni alloy, in which activated and fresh Co and Ni elements were the favorable catalytic sites. It looked like that there was an induction period for Co-doped catalyst, perhaps because the inactive oxide coating on the surface was removed gradually during the hydrolysis. Figure 4 shows the sketch map of the hydrolysis process of borohydride solution. The zero point energy of the possible intermediate product formed during the hydrolysis was presented in Table 2 as calculated by the Gossian software. The order of total energy of the intermediate products was BH 4 > BH 3OH  > BH 2 (OH) 2 > BH(OH)3 > B(OH)4 . The hydrolysis reaction of aqueous borohydride solution could occur spontaneously along the reaction route shown in Fig. 4. But the ultimate product was B(OH)4 , hydrated metaborate (BO 2 ˜ 2H 2 O) , since it was impossible to form BO 2 at normal conditions. The addition of alkali could depress the hydrolysis of borohydride due to the high OH concentration, while the activated Ni and Co elements could absorb H0 and promote the hydrolysis of borohydride.

The hydrogen generator could power a standard PEMFC below 35 W by controlling the flux of the borohydride solution. A H2 storage system for fuel cells with high gravimetric and volumetric H2 densities based on the chemical hydride has been realized without high H2 pressure. Borohydride would be a promising hydrogen source for fuel cells. REFERENCES 1 2

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3.3

Hydrogen supplier system

Based on the results above, we designed a hydrogen supplier system as shown in Fig. 5. The size of reactor was 1.0 L (10.0 ×10.0×10.0 cm3), and the mass of reactor was 1.2 kg, in which catalyst pasted and pressed onto Ni foam described in Ref. [15] was installed in the reactor. The stable hydrogen generation rate was 500 ml·min1. Assuming a standard proton exchange membrane fuel cell (PEMFC) operating at 0.7 V, the generated hydrogen was equivalent to 50 W.

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