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r-GO catalyst
Journal of Power Sources 293 (2015) 343e350
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Hydrogen generation from deliquescence of ammonia borane using NieCo/r-GO catalyst Chang-Chen Chou, Bing-Hung Chen* Department of Chemical Engineering, National Cheng Kung University, 1 University Road, Tainan, 70101, Taiwan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Hydrogen produced from the solidstate ammonia borane composite with limited water. The solid-state ammonia borane composite gives 6.5 wt% as the H2 storage capacity. The NieCo/r-GO catalyst was synthesized with the electroless deposition technology. The hydrolysates are mainly metaborate and borate by 11B NMR analysis.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2015 Received in revised form 20 May 2015 Accepted 21 May 2015 Available online
Hydrogen generation from the catalyzed deliquescence/hydrolysis of ammonia borane (AB) using the Ni eCo catalyst supported on the graphene oxide (NieCo/r-GO catalyst) under the conditions of limited water supply was studied with the molar feed ratio of water to ammonia borane (denoted as H2O/AB) at 2.02, 3.97 and 5.93, respectively. The conversion efficiency of ammonia borane to hydrogen was estimated both from the cumulative volume of the hydrogen gas generated and the conversion of boron chemistry in the hydrolysates analyzed by the solid-state 11B NMR. The conversion efficiency of ammonia borane could reach nearly 100% under excess water dosage, that is, H2O/AB ¼ 3.97 and 5.93. Notably, the hydrogen storage capacity could reach as high as 6.5 wt.% in the case with H2O/AB ¼ 2.02. The hydrolysates of ammonia borane in the presence of NieCo/r-GO catalyst were mainly the mixture of boric acid and metaborate according to XRD, FT-IR and solid-state 11B NMR analyses. © 2015 Elsevier B.V. All rights reserved.
Keywords: Hydrogen Ammonia borane Graphene oxide NieCo catalyst Hydrolysis reaction Deliquescence
1. Introduction Energy utilization has become one of the most critical topics in recent decades because it profoundly affects the social development of human beings. Hence, to develop the renewable, clean, and almost pollution-free energy resources has become a goal of a relentless pursuit for many scientists and engineers in the
* Corresponding author. E-mail address: [email protected] (B.-H. Chen). http://dx.doi.org/10.1016/j.jpowsour.2015.05.091 0378-7753/© 2015 Elsevier B.V. All rights reserved.
worldwide [1e3]. Out of these renewable energy sources, hydrogen is one of the promising energy sources since it is not only the lightest and the most abundant element on earth but also carries a high energy density by weight and is environmentally benign [4e6]. Commonly, the utilization of hydrogen energy is often made possible through devices, such as proton exchange membrane fuel cells (PEMFCs), which deliver the electrical energy and emit water as the exhaust [2]. However, the safe and effective hydrogen storage materials, as well as the reliable generation and supplement of hydrogen to PEMFCs are still of grave concern and under extensive
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development to the realization of the hydrogen economy [7,8]. Recently, ammonia borane (NH3BH3, AB), one of chemical hydrides having drawn considerable research interests, is regarded as a promising candidate for the on-board hydrogen storage materials owing to its high hydrogen content (19.6 wt.%), non-toxicity, and high stability in air and water at room temperature [9e12]. Hydrogen gas can be released from ammonia borane compounds through its hydrolysis reaction in the presence of suitable catalysts. In general, the catalyzed hydrolysis reaction of ammonia borane in solution state [9,13,14] can be expressed as follows: catalyst
NH3 BH3 þ 2H2 O ! 3H2 þ NHþ 4 þ BO2
DH
1
¼ 156 kJ mol
(1)
However, the reacting system could be heated up significantly owing to the large exothermic heat of the hydrolysis reaction. Instead, ammonia could be evolved [9]. Therefore, the heat management of the reacting system has to be properly undertaken [10]. Furthermore, Liu et al. have shown that boric acid and ammonia gas are the main product from the hydrolysis reaction of AB in excess water, so the hydrolysis reaction can be also shown as follows [9]: catalyst
NH3 BH3 þ 3H2 O ! 3H2 þ NH3 þ H3 BO3 H2 Oexcess
(2)
In general, precious metal based catalysts, such as Pt, Pd, Ru, Au and Rh, show good catalysis in the hydrolysis of ammonia borane (Eq. (2)). However, their costs are usually too high to jeopardize the prevalence of the ammonia borane in the hydrogen storage applications [13,15e17]. Therefore, the exploration and synthesis of economic but efficacious catalysts, e.g. non-noble metal based catalysts, in catalyzed hydrolysis reaction of ammonia borane for hydrogen production are desired. Notably, to increase the hydrogen storage capacity to achieve the set-goal of the onboard hydrogen storage system by the U.S. Department of Energy (DOE), namely 5.5 wt.% or 40 g H2$L1 in system-level and a lifetime of 1500 cycles, reducing the mass of water involved in the hydrolysis reaction and the use of light-weighted catalyst was attempted and proposed in this work. Previously, the studies on the hydrogen production from ammonia borane fuel system in the presence of precious and nonnoble metal based catalysts have been attempted [9,10]. However, the ion-exchange resin beads, Amberlite IR-120, were used as the catalysts support, on which the specific loadings and dispersion of the active catalysts could still be improved. The good dispersion of the active catalysts in the ammonia borane fuel system is decisive to the complete hydrolysis of ammonia borane with a limited water supply [10]. Consequently, the graphene oxide with a high surface area and a lighter weight was tentatively chosen as the catalysts support in this work. The sparsely and scatteredly distributed charges on graphene oxide provide a way for a better dispersion of the active catalysts, such as Co, to be deposited on the surface of graphene oxide. Graphene oxide (GO), a carbon allotrope with sp2-carbon atoms bonded and honeycomb lattice arrangement, have attracted more attention as a promising support due to its larger specific surface area (~2630 m2 g1), superior electrical conductivity (105e106 S m1), excellent mechanical strength and thermal stability [18e21]. GO can be conveniently synthesized by the Hummers method [22]. Therefore, GO was used as the catalyst support in this work. The electroless deposition process (EDP) was utilized to plate nickel and cobalt metals on the GO support (denoted as NieCo/r-GO catalyst). The EDP is an autocatalytic chemical plating technology that can deposit the metals on the surface of preexisting metal and the front of as-deposited metals in the plating
bath without the use of the external voltage or current [23e25]. In other words, reducing agents such as sodium hypophosphite and hydrazine undergo chemical oxidation and liberate electrons to reduce the metal ions, e.g. Ni2þ and Co2þ, in the EDP [23e25]. As the limited amount of water is used, the effective contact between water molecules and ammonia borane compounds on the catalysts become pivotal in the complete hydrolysis reaction to liberate the hydrogen gas. Hence, ammonia borane powder and NieCo/r-GO catalyst were pulverized as uniformly as possible by high-energy ball milling. The obtained mixture of ammonia borane powder and NieCo/r-GO catalysts is denoted as the solid-state AB composite through the text of this report. Various instruments such as scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) were utilized to analyze the solid-state AB composite. The characterization of the catalysts and hydrolysates as well as the characteristics of the hydrogen production are reported and discussed in this report. 2. Experimental 2.1. Chemicals and reagents Sulfuric acid (SigmaeAldrich), sodium nitrate (SigmaeAldrich), potassium permanganate (J. T. Baker), hydrogen peroxide solution (SigmaeAldrich), stannous chloride, dehydrate (Mallinckrodt Chemicals), palladium (II) chloride (U.R. Chemicals), hydrochloric acid (SigmaeAldrich), sodium citrate, dehydrate (J. T. Baker), ammonium sulfate (J. T. Baker), maleic acid (SigmaeAldrich), cobalt (II) sulfate heptahydrate (Showa), nickel (II) sulfate hexahydrate (J. T. Baker), sodium hypophosphite monohydrate (Alfa Aesar), ammonium hydroxide solution (SigmaeAldrich) and ammonium borane (SigmaeAldrich) were of ACS reagent grade and utilized as received. Graphite was purchased from Bay Carbon Inc. (Bay City, MI). Deionized water from the Millipore Milli-Q ultra-purification system having resistivity greater than 18.2 MU cm was used in the sample preparation. 2.2. Hydrogen generation from the hydrolysis reaction of the solidstate NH3BH3 composite The experimental setup for hydrogen generation were given in details in our previous work [10]. In this work, the enhancement of the hydrogen storage capacity of the chemical hydride fuel was attempted both by increasing as more extents of the hydrolysis reaction as possible and by reducing the overall weights of the reactants. Therefore, limited water near stoichiometric ratio was supplied to the reacting system consisting of the ammonia borane and the catalysts (i.e. the molar ratio of H2O to AB ¼ 2.02, 3.97 and 5.93). Uniform dispersion of the active catalysts in the chemical fuel to increase the water contact with ammonia borane on catalysts also played a pivotal role in achieving this goal. The active catalysts used in this work were synthesized by electrolessly-depositing Ni and Co onto the surface of the r-GO supports, which was prepared in-house from graphite. Prior to the EDP, the proper sensitization and activation procedures were conducted on the nonconductive r-GO nanosheets [23e25]. In two different EDP procedures were adopted to plate Ni and Co nanoparticles on r-GO substrates to give as more uniform dispersion of the active catalysts on the supports as possible (Table 1): Procedure (1): Ni and Co were electrolessly reduced conjointly and simultaneously on activated r-GO substrates to give the catalyst denoted as NieCo/r-GO(j), and Procedure (2): First, Ni was electrolessly deposited on Pd/r-GO substrates and, sequentially, Co on the as-deposited Ni fronts to
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Table 1 The reaction condition and the compositions of the electroless plating baths for preparation of the r-GO support and the NieCo/r-GO catalysts. r-GO
NiSO4$6H2O CoSO4$7H2O (NH4)2SO4 Sodium citrate Maleic acid NaH2PO2$H2O
e e 30.0 59.0 8.0 19.0
NieCo/r-GO(j) (electrolessly-deposited simultaneously)
13.1 14.0 30.0 59.0 8.0 19.0
NieCo/r-GO(s) (electrolesslydeposited in succession) Step I
Step II
26.2 e 30.0 59.0 8.0 19.0
e 28.0 30.0 59.0 8.0 19.0
Reaction condition pH Temperature Reaction time
give NieCo/r-GO(s). Additionally, the graphene oxide was undergone with the same EDP in the plating baths without the prior sensitization-activation treatment as well as nickel and cobalt ions present to give the r-GO catalyst support (Table 1). In an EDP, chemical reducing agents, instead of external current, are used to provide electrons for the metal ions to be cathodically reduced to metal on the surface. Common reducing agents for electroless plating of nickel and cobalt include sodium hypophosphite in alkaline baths and dimethylamine borane in an acid bath [23e25]. The composites of the NH3BH3 powder and the NieCo/r-GO catalysts, denoted as the solid-state AB composite, were obtained by pulverizing the predetermined mixtures of NH3BH3 and NieCo/ r-GO catalyst in a high energy ball mill (Pulverisette, Fritsch GmbH, Germany) at 150 rpm for 10 min. Stainless steel balls of 5 mm in diameter were utilized as the milling media. The solid-state AB composite was placed in a three-necked round bottom flask in a fume hood at 30 C. Deionized water was introduced by an prompt injection with a syringe pump into the reactant piles in the flask to start the hydrolysis reaction. A thermocouple was inserted into the reactant pile for monitoring the instantaneous reaction temperature. The evolved gas was directed to pass through a bottle of 0.1 N H2SO4 solution to remove the produced ammonia. Subsequently, the gas went through the other bottle filled with silica gel particles to capture any residual moisture and the elapsed ammonia gas. Finally, the purified hydrogen gas was recorded with a Fujikin mass flowmeter/controller (MFC) to obtain the instantaneous flow rate and the cumulative volume of the produced hydrogen. After termination of the hydrolysis reaction, the hydrolysates and other residues were harvested and dried by a freeze dryer. Subsequently, the solid hydrolysates were analyzed with XRD, Raman, FT-IR and the solid-state 11B NMR spectroscopy. The conversion efficiency of ammonia borane to hydrogen could be calculated with the accumulation volume of the produced hydrogen gas. Additionally, the extent of the catalyzed hydrolysis reaction of the ammonia borane was measured with the quantitative analysis on the hydrolysates by the solid-state 11B NMR. Consequently, the conversion yield of the ammonia borane was obtained, accordingly. 2.3. Instrumental analyses Surface morphology and compositions of the solid-state AB composite after the high energy ball milling was observed by using the Hitachi SU-8000. The XRD patterns of the NieCo/r-GO catalysts were examined with the Rigaku Ultima IV X-ray diffractometer (Tokyo, Japan) over a range of diffraction angle (q) from 2q ¼ 10 to
9.5 90 C 2h
2q ¼ 80 at a 2q-rate of 4 min1 with Cu Ka radiation (40 kV and 20 mA) filtered by a monochrometer. Fourier-transform infrared spectroscopy (FT-IR) was performed on a Nicolet 6700 FT-IR. The DXR Raman microscope equipped with a 10 objective was employed to obtain the Raman spectra excited by 532 nm laser radiation at 5 mW (Thermo Fisher Scientific). An ICP-OES (JY Ultima 2000, Horiba, Kyoto, Japan) was used to quantity metallic loadings on NieCo/r-GO catalysts. The boron chemistry in the hydrolysates were examined with the solid-state 11B NMR spectroscopy using the Bruker Avance 400 NMR spectrometer (Rheinstetten, Germany). Sodium borohydride was used as the external reference in this work. The Micromeritics ASAP 2020 porosimeter using N2 at 77 K as the adsorbate was employed to obtain the BrunauereEmmetteTeller (BET) surface area of the catalysts and supports. 3. Results and discussion According to the ICP-OES measurements, the loadings of cobalt on NieCo/r-GO(j) and NieCo/r-GO(s) catalysts were 14.6 wt.% and 26.1 wt.%, respectively, based on the catalyst weights. The nickel contents were 17.6 wt.% and 39.5 wt.%, correspondingly, on NieCo/ r-GO(j) and NieCo/r-GO(s) catalysts. This indicates that the sequential EDP of Ni and Co would lead to more Co and Ni nanoparticles deposited on r-GO supports. Hence, 5.37 wt.% NieCo/rGO(j) catalyst and 3 wt.% NieCo/r-GO(s) catalyst based on the mass of ammonia borane were used in the preparation of the solid-state AB composites to maintain the same dosage of the active catalysts in the reacting system for the subsequent studies on the hydrogen production from the catalyzed hydrolysis of ammonia borane. The BET surface area of NieCO/r-GO(j) and NieCO/r-GO(s) catalysts are 1576 m2 g1 and 1805 m2 g1, respectively, less than that of the asprepared r-GO support (2285 m2 g1). 3.1. Characterization of the solid-state AB composite Surface morphology of the solid-state AB composite after highenergy ball milling were examined with SEM. Fig. 1 displayed the SEM images of ball-milled particles containing ammonia borane granules and 5.37 wt.% NieCo/r-GO(j) catalyst and 3 wt.% NieCo/rGO(s) catalyst, respectively. The granule size of the solid-state AB composites range from 60 to 95 mm. Moreover, catalysts were uniformly dispersed within the AB composites (Fig. S1). Fig. 2 revealed the FT-IR spectra of the ball-milled AB composites containing 5.37 wt.% NieCo/r-GO(j) catalyst and 3 wt.% NieCo/ r-GO(s) catalyst. In general, IR peaks associated with BeN, BeH and NeH bonds could be noticed on both AB-catalyst composites. For example, IR peaks were observed at 727 cm1 for BeN stretching, at
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797 cm1 for BeN stretching, at 1063 cm1 for BeH torsion, at 1160 cm1 for BeH torsion, at 1605 cm1 for NeH bending and at 2355 cm1 for BeH stretching, all arising from the ammonia borane. The broaden peaks appearing at 3200 to 3400 cm1 might originate from the OeH stretching resulted from the moisture and, possibly, from the NeH stretching of ammonia borane molecules. 3.2. Hydrogen generation from solid-state AB composite
Fig. 1. SEM images of the AB composites with (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst after high energy ball milling at 150 rpm for 100 min.
Fig. 2. FT-IR spectra of the AB composites with (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst.
According to the Le Chatelier's principle, more water present in the reacting system would drive the reaction equilibrium to the favor of hydrogen production and, therefore, accelerate the evolution of the hydrogen gas. However, the increase in water supply also adds up more weight to the system mass and, thus, would decrease the gravimetric capacity of the hydrogen storage system. Consequently, the effect of limited water supply to the hydrolysis reaction of ammonia borane for hydrogen production was studied in this work. A limited amount of water near the stoichiometry was supplied to the reacting system to initiate the hydrolysis reaction for hydrogen generation (Table 2). The molar feed ratios of water to ammonia borane, denoted as H2O/AB, were set at 2.02, 3.97 and 5.93, respectively (i.e., H2O/AB ¼ 2.02, 3.97 and 5.93). Water was injected quickly at once to the reaction flask to moderate the heat emitted from the hydrolysis reaction of ammonia borane. Fig. 3 showed the H2-generation profiles and the temperature profiles of the reaction pile. The temperature of the reaction system dropped very rapidly at first due to the introduction of water droplet to the reaction pile and, then, increased progressively owing to the exothermic heat emitted from the hydrolysis reaction of ammonia borane. With less water injected, the temperature of the reaction system could increase more rapidly and even reached almost 60 C. One possible explanation was that water could adsorb the heat and insufficient water could not effectively suppress the temperature. Afterwards, the generation rate of hydrogen slowed down as AB was consumed up. Notably, it has to mention the danger if the slow injection of water to the reacting mixtures was adopted. It often resulted in the blow-up of the experimental setup, as the reaction temperature could be elevated quickly above the boiling point of water. Consequently, great care and attention had to be paid in the experimental operation. Coincidently, similar trends in the profiles of hydrogen flowrate and the temperature of the reacting system could be observed (Fig. 3). That is, the flowrate of the hydrogen gas produced ascended to the maximum, as the temperature of the reacting system almost reached the utmost. Notably, the hydrogen release profiles from the reacting system containing 3 wt.% NieCo/r-GO(s) catalyst were relatively smoother than those with 5.37 wt.% NieCo/ r-GO(j) catalyst, which could benefit the hydrogen supply to PEMFCs [26]. Another important issue needed to be considered in the chemical hydride fuel system is the hydrogen storage capacity. For example, the US Department of Energy has revised the 2017-target for onboard hydrogen storage system for light-duty vehicles as 5.5 wt.% of the maximum system mass. Consequently, the hydrogen storage capacity of the AB composites were estimated from the accumulation of the hydrogen produced during the hydrolysis reaction of ammonia borane recorded by MFC (Table 2). In brief, the calculation on the hydrogen storage capacity was based on (1) one mole of ammonia borane could produce 3 mol of hydrogen, as shown in Eq. (1) and Eq. (2), and (2) the molar volume of hydrogen was approximated at about 22.892 L at 30 C. For comparison, the solid-state 11B NMR was utilized to analyze the hydrolysates to estimate the conversion efficiency of AB, calculated from the areaunder-curve of the NMR spectra (Table 2).
C.-C. Chou, B.-H. Chen / Journal of Power Sources 293 (2015) 343e350 Table 2 Effect of water dosage to the conversion efficiency of ammonia borane (AB) to hydrogen estimated from MFC measurement and H2O/AB (mol mol
1
)
AB composites with 5.37 wt.% NieCo/r-GO(j) catalyst (a) 2.02 (b) 3.97 (c) 5.93 AB composites with 3 wt.% NieCo/r-GO(s) catalyst (d) 2.02 (e) 3.97 (f) 5.93
347
11
B NMR analysis.
Deionized water (g)
Conversion by MFC/NMR (%)
Hydrogen storage capacity calculated from MFC/NMR (wt.%)
0.236 0.463 0.691
73.2/95.6 100/100 100/100
6.4/8.4 5.8/5.8 4.3/4.3
0.236 0.463 0.691
72.8/96.8 100/100 100/100
6.5/8.6 5.9/5.9 4.4/4.4
In general, the hydrogen storage capacity estimated from NMR analyses was greater than those based on the cumulative volume of the produced hydrogen recorded by MFC. The 11B NMR analysis may overestimate the conversion of ammonia borane. With restricted water supply, the temperature of the reacting system could increase significantly (Fig. 3(a) and (d)). Under the conditions of elevated temperature, the ammonia borane could form
polymeric aminoborane compounds and other borane complexes. That is, not all ammonia borane was converted to metaborate and borate to produce hydrogen. The discrepancy on the results calculated from MFC records and 11 B NMR analysis became obvious when water under stoichiometry, e.g. H2O/AB ¼ 2.02 by mol, was supplied to the reacting system. For example, the conversion of ammonia borane were 73.2% and 72.8
Fig. 3. Effect of water supply on the hydrogen generation profiles and the reaction temperature profiles from the hydrolysis of the solid-state AB composites having 5.37 wt.% NieCo/r-GO(j) catalyst fed with (a) H2O/AB ¼ 2.02, (b) H2O/AB ¼ 3.97, and (c) H2O/AB ¼ 5.93; the solid-state AB composites having 3 wt.% NieCo/r-GO(s) catalyst fed with (d) H2O/ AB ¼ 2.02; (e) H2O/AB ¼ 3.97; (f) H2O/AB ¼ 5.93.
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Fig. 4. XRD patterns of the hydrolysates and the solid residues harvested after the hydrolysis reaction of the solid-state AB composite containing (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst with different water dosages (H2O/AB ¼ 2.02, 3.97 and 5.93 by mol).
with 5.37 wt.% NieCo/r-GO(j) catalyst and 3 wt.% NieCo/r-GO(s) catalyst, respectively, based on the accumulation of the evolved hydrogen gas recorded by MFC. However, the conversion were, instead, 95.6% and 96.8%, if estimated from 11B NMR analysis. The conversion efficiency of AB could reach 100% for both MFC measurement and NMR analysis with the water dosage increased to H2O/AB ¼ 3.97 and 5.93 by mol. This implied that AB could be completely hydrolyzed in the presence of 5.37 wt.% NieCo/r-GO(j) catalyst and 3 wt.% NieCo/r-GO(s) catalyst under excess water. Nevertheless, the most hydrogen storage capacity was only about 5.9 wt.% of the solid-state AB composite under excess water, owing to a higher system mass contributed from the excess water. Instead, the hydrogen storage capacity could reach as high as 6.5 wt.% with under-stoichiometric water supply at H2O/AB ¼ 2.02 by mol, even though not all AB was completely hydrolyzed. 3.3. Analyses of the hydrolysates The residue of the solid-state AB composites after the hydrolysis reaction was harvested and sealed in desiccator to prevent any further hydrolysis reaction caused by the moisture. The chemistry
of the hydrolysate and the solid residue was probed by various instrumental analyses, such as XRD, FT-IR, Raman and the solidstate 11B NMR. Fig. 4 showed the XRD patterns of the harvested hydrolysates and the solid residues. The main XRD peaks at 2q ¼ 15.0 and 30.2 were consistent with those peaks of metaboric acid or boric acid, which illustrated the hydrolysates would be probably the mixture of metaboric acid and boric acid. Interestingly, no peak at 2q ¼ 24 representing the main characteristic of the ammonia borane compound could be observed in Fig. 4, even at the reacting system fed with H2O/AB at 2.02. This would certainly suggest that ammonia borane was nearly converted even with H2O/AB at 2.02. FT-IR analyses on the harvested hydrolysates and the solid residues after the hydrolysis reaction of the solid-state AB composites were performed and shown in Fig. 5. Three main IR peaks were observed: 780 cm1 for BeO bending, 1200 cm1 for BeO stretching, and 1330e1420 cm1 for OeH bending, which were coincient with the FT-IR characteristics of boric acid and metaboric acid. Furthermore, there existed other peaks such as 2300 to 2400 cm1 and 3200 to 3400 cm1, which could be ascribed to CO2 and water in the air, respectively.
Fig. 5. FT-IR spectra of the hydrolysates and the solid residues harvested after the hydrolysis reaction of the solid-state AB composite containing (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst with different water dosages (H2O/AB ¼ 2.02, 3.97 and 5.93 by mol).
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Fig. 6. Raman spectra of the hydrolysates and the solid residues harvested after the hydrolysis reaction of the solid-state AB composite containing (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst with different water dosages (H2O/AB ¼ 2.02, 3.97 and 5.93 by mol).
Fig. 6 demonstrated the Raman spectra of the harvested hydrolysates and the solid residues. On all Raman spectra of all samples, there existed two broad peaks centered near 1350 and 1580 cm1, commonly known as D-band and G-band of carbon compounds, respectively. The D-band is originated from the lattice defect of disordered carbon structures, while the G-band arises from well-ordered sp2-hybridized carbon structures. Moreover, the graphitic characters could be obtained from the relative intensity ratio of D-band to G-band (ID/IG) [27]. The calculated ID/IG value for hydrolysates from the AB composites with 5.37 wt.% NieCo/r-GO(j) catalyst fed with H2O/AB ¼ 2.02, 3.97 and 5.93 by mol were 2.230, 1.903 and 1.603, respectively. It seems that the ID/IG value would decrease with increasing water dosage to the reacting system. It is possible that more water introduced to the reacting system could buffer the variation of the temperature and, consequently, decreased the formation of the disordered carbon structures on the r-GO supports. Similarly, the ID/IG values found in the hydrolysates from the AB composites containing 3 wt.% NieCo/r-GO(s) catalyst with H2O/AB ¼ 2.02, 3.97 and 5.93 by mol were 2.018, 1.971 and 1.540, respectively.
Solid-state 11B NMR analyses of the harvested hydrolysates and solid residues using NaBH4 as an external reference were shown in Fig. 7. Moreover, the 11B chemical shifts were 27 ppm for NH3BH3, 6.6 and 14.7 ppm for H3BO3 and 2.8 ppm for NaBO2 [10]. All samples tested showed the same range of 11B chemical shifts from 0 to 20 ppm, which indicated the existence of the mixtures of boric acid and metaborate on these samples. Contradictive to the XRD analysis, the minor peak with the 11B chemical shift at 27 ppm, corresponding to ammonia borane, was found in the residue of the reacting systems containing both NieCo/r-GO catalysts with H2O/ AB ¼ 2.02. Hence, the 11B NMR analysis presented that not all ammonia borane were completely hydrolyzed (Table 2). Evidently, according to XRD, FT-IR and solid-state 11B NMR analyses the main products of the hydrolysis reaction of the solidstate AB composites studied in this work were the mixture of boric acid and metaborate, coincident with those found in the hydrolysis of ammonia borane over the Co2þ/IR-120 catalyst [10]. Notably, the hydrogen storage capacity could reach as high as 6.5 wt.% in the case with H2O/AB ¼ 2.02, which is relatively superior to those reported in the recent literature (Table S1).
Fig. 7. Solid-state 11B NMR spectra of the hydrolysates and the solid residues harvested after the hydrolysis reaction of the solid-state AB composite containing (a) 5.37 wt.% NieCo/ r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst with different water dosages (H2O/AB ¼ 2.02, 3.97 and 5.93 by mol).
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4. Conclusions In this work, the concept of treating water as a limiting agent in the hydrolysis of ammonia borane for hydrogen production and storage to reduce the mass of the hydrogen storage system has been proven as feasible. The highest hydrogen storage capacity obtained in this work was near 6.5 wt.% from the AB composites with 3 wt.% NieCo/r-GO catalyst reacting with under-stoichiometric amount of water (namely H2O/AB ¼ 2 by mol). However, with insufficient water, the exothermic heat emitted from the hydrolysis reaction could not be properly adsorbed and, thus, the temperature of the reacting system could reach too high to control the release of the hydrogen gas. The hydrolysates harvested after hydrogen released from the solid state AB composites are found likely to be the mixture of boric acid and metaborate. Acknowledgment The authors gratefully acknowledge financial support from the Ministry of Science and Technology of Taiwan through the Grant MOST 102-2221-E-006 -286 -MY2. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.05.091. References [1] T. Umegaki, T. Hosoya, N. Toyama, Q. Xu, Y. Kojima, J. Alloys Compd. 608 (2014) 261e265. rida, Appl. Energy 129 (2014) [2] S. Flick, M. Schwager, E. McCarthy, W. Me 135e146. [3] Q. Yao, W. Shi, G. Feng, Z.-H. Lu, X. Zhang, D. Tao, D. Kong, X. Chen, J. Power
Sources 257 (2014) 293e299. [4] T. Umegaki, Q. Xu, Y. Kojima, J. Power Sources 216 (2012) 363e367. [5] M. Wen, S. Zhou, Q. Wu, J. Zhang, Q. Wu, C. Wang, Y. Sun, J. Power Sources 232 (2013) 86e92. [6] H.P. Veluswamy, R. Kumar, P. Linga, Appl. Energy 122 (2014) 112e132. [7] M. Rakap, J. Power Sources 265 (2014) 50e56. [8] Y. Tong, X. Lu, W. Sun, G. Nie, L. Yang, C. Wang, J. Power Sources 261 (2014) 221e226. [9] C.-H. Liu, Y.-C. Wu, C.-C. Chou, B.-H. Chen, C.-L. Hsueh, J.-R. Ku, F. Tsau, Int. J. Hydrogen Energy 37 (2012) 2950e2959. [10] C.-C. Chou, D.-J. Lee, B.-H. Chen, Int. J. Hydrogen Energy 37 (2012) 15681e15690. € _ Morkan, S. Ozkar, [11] S. Akbayrak, S. Tanyıldızı, I. Int. J. Hydrogen Energy 39 (2014) 9628e9637. _ lu, Int. J. Hydrogen En[12] A. Kantürk Figen, M.B. Pis¸kin, B. Cos¸kuner, V. Imamo g ergy 38 (2013) 16215e16228. [13] M. Chandra, Q. Xu, J. Power Sources 156 (2006) 190e194. [14] M. Diwan, Int. J. Hydrogen Energy 33 (2008) 1135e1141. [15] M. Chandra, Q. Xu, J. Power Sources 168 (2007) 135e142. € [16] F. Durap, M. Zahmakıran, S. Ozkar, Int. J. Hydrogen Energy 34 (2009) 7223e7230. € Metin, E. Kayhan, S. Ozkar, € [17] O. J.J. Schneider, Int. J. Hydrogen Energy 37 (2012) 8161e8169. [18] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Adv. Mater. 22 (2010) 3906e3924. [19] Y.S. Yun, D. Kim, Y. Tak, H.-J. Jin, Synth. Met. 161 (2011) 2460e2465. , J. Ledesma-García, M.F. Sgroi, [20] R. Carrera-Cerritos, V. Baglio, A.S. Arico D. Pullini, A.J. Pruna, D.B. Mataix, R. Fuentes-Ramírez, L.G. Arriaga, Appl. Catal. B Environ. 144 (2014) 554e560. [21] J.-N. Zheng, S.-S. Li, F.-Y. Chen, N. Bao, A.-J. Wang, J.-R. Chen, J.-J. Feng, J. Power Sources 266 (2014) 259e267. [22] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 50 (1958) 1339. [23] M.-K. Chang, C.-H. Chen, B.-H. Chen, Ind. Eng. Chem. Res. 48 (2008) 3021e3029. [24] I. Choi, S.H. Ahn, J.J. Kim, O.J. Kwon, Appl. Catal. B Environ. 102 (2011) 608e613. [25] G.O. Mallory, J.B. Hajdu, Electroless Plating: Fundamentals & Applications, American Electroplating and Surface Finishers Society, Orlando, FL, 1990. [26] C.-L. Hsueh, C.-H. Liu, B.-H. Chen, M.-S. Lee, C.-Y. Chen, Y.-W. Lu, F. Tsau, J.R. Ku, J. Power Sources 196 (2011) 3530e3538. ~ oz, L. Sa nchez-Silva, A. Romero, J.L. Valverde, Chem. Eng. J. 172 [27] S. Gil, L. Mun (2011) 418e429.
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Hydrogen generation from deliquescence of ammonia borane using NieCo/r-GO catalyst Chang-Chen Chou, Bing-Hung Chen* Department of Chemical Engineering, National Cheng Kung University, 1 University Road, Tainan, 70101, Taiwan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Hydrogen produced from the solidstate ammonia borane composite with limited water. The solid-state ammonia borane composite gives 6.5 wt% as the H2 storage capacity. The NieCo/r-GO catalyst was synthesized with the electroless deposition technology. The hydrolysates are mainly metaborate and borate by 11B NMR analysis.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2015 Received in revised form 20 May 2015 Accepted 21 May 2015 Available online
Hydrogen generation from the catalyzed deliquescence/hydrolysis of ammonia borane (AB) using the Ni eCo catalyst supported on the graphene oxide (NieCo/r-GO catalyst) under the conditions of limited water supply was studied with the molar feed ratio of water to ammonia borane (denoted as H2O/AB) at 2.02, 3.97 and 5.93, respectively. The conversion efficiency of ammonia borane to hydrogen was estimated both from the cumulative volume of the hydrogen gas generated and the conversion of boron chemistry in the hydrolysates analyzed by the solid-state 11B NMR. The conversion efficiency of ammonia borane could reach nearly 100% under excess water dosage, that is, H2O/AB ¼ 3.97 and 5.93. Notably, the hydrogen storage capacity could reach as high as 6.5 wt.% in the case with H2O/AB ¼ 2.02. The hydrolysates of ammonia borane in the presence of NieCo/r-GO catalyst were mainly the mixture of boric acid and metaborate according to XRD, FT-IR and solid-state 11B NMR analyses. © 2015 Elsevier B.V. All rights reserved.
Keywords: Hydrogen Ammonia borane Graphene oxide NieCo catalyst Hydrolysis reaction Deliquescence
1. Introduction Energy utilization has become one of the most critical topics in recent decades because it profoundly affects the social development of human beings. Hence, to develop the renewable, clean, and almost pollution-free energy resources has become a goal of a relentless pursuit for many scientists and engineers in the
* Corresponding author. E-mail address: [email protected] (B.-H. Chen). http://dx.doi.org/10.1016/j.jpowsour.2015.05.091 0378-7753/© 2015 Elsevier B.V. All rights reserved.
worldwide [1e3]. Out of these renewable energy sources, hydrogen is one of the promising energy sources since it is not only the lightest and the most abundant element on earth but also carries a high energy density by weight and is environmentally benign [4e6]. Commonly, the utilization of hydrogen energy is often made possible through devices, such as proton exchange membrane fuel cells (PEMFCs), which deliver the electrical energy and emit water as the exhaust [2]. However, the safe and effective hydrogen storage materials, as well as the reliable generation and supplement of hydrogen to PEMFCs are still of grave concern and under extensive
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development to the realization of the hydrogen economy [7,8]. Recently, ammonia borane (NH3BH3, AB), one of chemical hydrides having drawn considerable research interests, is regarded as a promising candidate for the on-board hydrogen storage materials owing to its high hydrogen content (19.6 wt.%), non-toxicity, and high stability in air and water at room temperature [9e12]. Hydrogen gas can be released from ammonia borane compounds through its hydrolysis reaction in the presence of suitable catalysts. In general, the catalyzed hydrolysis reaction of ammonia borane in solution state [9,13,14] can be expressed as follows: catalyst
NH3 BH3 þ 2H2 O ! 3H2 þ NHþ 4 þ BO2
DH
1
¼ 156 kJ mol
(1)
However, the reacting system could be heated up significantly owing to the large exothermic heat of the hydrolysis reaction. Instead, ammonia could be evolved [9]. Therefore, the heat management of the reacting system has to be properly undertaken [10]. Furthermore, Liu et al. have shown that boric acid and ammonia gas are the main product from the hydrolysis reaction of AB in excess water, so the hydrolysis reaction can be also shown as follows [9]: catalyst
NH3 BH3 þ 3H2 O ! 3H2 þ NH3 þ H3 BO3 H2 Oexcess
(2)
In general, precious metal based catalysts, such as Pt, Pd, Ru, Au and Rh, show good catalysis in the hydrolysis of ammonia borane (Eq. (2)). However, their costs are usually too high to jeopardize the prevalence of the ammonia borane in the hydrogen storage applications [13,15e17]. Therefore, the exploration and synthesis of economic but efficacious catalysts, e.g. non-noble metal based catalysts, in catalyzed hydrolysis reaction of ammonia borane for hydrogen production are desired. Notably, to increase the hydrogen storage capacity to achieve the set-goal of the onboard hydrogen storage system by the U.S. Department of Energy (DOE), namely 5.5 wt.% or 40 g H2$L1 in system-level and a lifetime of 1500 cycles, reducing the mass of water involved in the hydrolysis reaction and the use of light-weighted catalyst was attempted and proposed in this work. Previously, the studies on the hydrogen production from ammonia borane fuel system in the presence of precious and nonnoble metal based catalysts have been attempted [9,10]. However, the ion-exchange resin beads, Amberlite IR-120, were used as the catalysts support, on which the specific loadings and dispersion of the active catalysts could still be improved. The good dispersion of the active catalysts in the ammonia borane fuel system is decisive to the complete hydrolysis of ammonia borane with a limited water supply [10]. Consequently, the graphene oxide with a high surface area and a lighter weight was tentatively chosen as the catalysts support in this work. The sparsely and scatteredly distributed charges on graphene oxide provide a way for a better dispersion of the active catalysts, such as Co, to be deposited on the surface of graphene oxide. Graphene oxide (GO), a carbon allotrope with sp2-carbon atoms bonded and honeycomb lattice arrangement, have attracted more attention as a promising support due to its larger specific surface area (~2630 m2 g1), superior electrical conductivity (105e106 S m1), excellent mechanical strength and thermal stability [18e21]. GO can be conveniently synthesized by the Hummers method [22]. Therefore, GO was used as the catalyst support in this work. The electroless deposition process (EDP) was utilized to plate nickel and cobalt metals on the GO support (denoted as NieCo/r-GO catalyst). The EDP is an autocatalytic chemical plating technology that can deposit the metals on the surface of preexisting metal and the front of as-deposited metals in the plating
bath without the use of the external voltage or current [23e25]. In other words, reducing agents such as sodium hypophosphite and hydrazine undergo chemical oxidation and liberate electrons to reduce the metal ions, e.g. Ni2þ and Co2þ, in the EDP [23e25]. As the limited amount of water is used, the effective contact between water molecules and ammonia borane compounds on the catalysts become pivotal in the complete hydrolysis reaction to liberate the hydrogen gas. Hence, ammonia borane powder and NieCo/r-GO catalyst were pulverized as uniformly as possible by high-energy ball milling. The obtained mixture of ammonia borane powder and NieCo/r-GO catalysts is denoted as the solid-state AB composite through the text of this report. Various instruments such as scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) were utilized to analyze the solid-state AB composite. The characterization of the catalysts and hydrolysates as well as the characteristics of the hydrogen production are reported and discussed in this report. 2. Experimental 2.1. Chemicals and reagents Sulfuric acid (SigmaeAldrich), sodium nitrate (SigmaeAldrich), potassium permanganate (J. T. Baker), hydrogen peroxide solution (SigmaeAldrich), stannous chloride, dehydrate (Mallinckrodt Chemicals), palladium (II) chloride (U.R. Chemicals), hydrochloric acid (SigmaeAldrich), sodium citrate, dehydrate (J. T. Baker), ammonium sulfate (J. T. Baker), maleic acid (SigmaeAldrich), cobalt (II) sulfate heptahydrate (Showa), nickel (II) sulfate hexahydrate (J. T. Baker), sodium hypophosphite monohydrate (Alfa Aesar), ammonium hydroxide solution (SigmaeAldrich) and ammonium borane (SigmaeAldrich) were of ACS reagent grade and utilized as received. Graphite was purchased from Bay Carbon Inc. (Bay City, MI). Deionized water from the Millipore Milli-Q ultra-purification system having resistivity greater than 18.2 MU cm was used in the sample preparation. 2.2. Hydrogen generation from the hydrolysis reaction of the solidstate NH3BH3 composite The experimental setup for hydrogen generation were given in details in our previous work [10]. In this work, the enhancement of the hydrogen storage capacity of the chemical hydride fuel was attempted both by increasing as more extents of the hydrolysis reaction as possible and by reducing the overall weights of the reactants. Therefore, limited water near stoichiometric ratio was supplied to the reacting system consisting of the ammonia borane and the catalysts (i.e. the molar ratio of H2O to AB ¼ 2.02, 3.97 and 5.93). Uniform dispersion of the active catalysts in the chemical fuel to increase the water contact with ammonia borane on catalysts also played a pivotal role in achieving this goal. The active catalysts used in this work were synthesized by electrolessly-depositing Ni and Co onto the surface of the r-GO supports, which was prepared in-house from graphite. Prior to the EDP, the proper sensitization and activation procedures were conducted on the nonconductive r-GO nanosheets [23e25]. In two different EDP procedures were adopted to plate Ni and Co nanoparticles on r-GO substrates to give as more uniform dispersion of the active catalysts on the supports as possible (Table 1): Procedure (1): Ni and Co were electrolessly reduced conjointly and simultaneously on activated r-GO substrates to give the catalyst denoted as NieCo/r-GO(j), and Procedure (2): First, Ni was electrolessly deposited on Pd/r-GO substrates and, sequentially, Co on the as-deposited Ni fronts to
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345
Table 1 The reaction condition and the compositions of the electroless plating baths for preparation of the r-GO support and the NieCo/r-GO catalysts. r-GO
NiSO4$6H2O CoSO4$7H2O (NH4)2SO4 Sodium citrate Maleic acid NaH2PO2$H2O
e e 30.0 59.0 8.0 19.0
NieCo/r-GO(j) (electrolessly-deposited simultaneously)
13.1 14.0 30.0 59.0 8.0 19.0
NieCo/r-GO(s) (electrolesslydeposited in succession) Step I
Step II
26.2 e 30.0 59.0 8.0 19.0
e 28.0 30.0 59.0 8.0 19.0
Reaction condition pH Temperature Reaction time
give NieCo/r-GO(s). Additionally, the graphene oxide was undergone with the same EDP in the plating baths without the prior sensitization-activation treatment as well as nickel and cobalt ions present to give the r-GO catalyst support (Table 1). In an EDP, chemical reducing agents, instead of external current, are used to provide electrons for the metal ions to be cathodically reduced to metal on the surface. Common reducing agents for electroless plating of nickel and cobalt include sodium hypophosphite in alkaline baths and dimethylamine borane in an acid bath [23e25]. The composites of the NH3BH3 powder and the NieCo/r-GO catalysts, denoted as the solid-state AB composite, were obtained by pulverizing the predetermined mixtures of NH3BH3 and NieCo/ r-GO catalyst in a high energy ball mill (Pulverisette, Fritsch GmbH, Germany) at 150 rpm for 10 min. Stainless steel balls of 5 mm in diameter were utilized as the milling media. The solid-state AB composite was placed in a three-necked round bottom flask in a fume hood at 30 C. Deionized water was introduced by an prompt injection with a syringe pump into the reactant piles in the flask to start the hydrolysis reaction. A thermocouple was inserted into the reactant pile for monitoring the instantaneous reaction temperature. The evolved gas was directed to pass through a bottle of 0.1 N H2SO4 solution to remove the produced ammonia. Subsequently, the gas went through the other bottle filled with silica gel particles to capture any residual moisture and the elapsed ammonia gas. Finally, the purified hydrogen gas was recorded with a Fujikin mass flowmeter/controller (MFC) to obtain the instantaneous flow rate and the cumulative volume of the produced hydrogen. After termination of the hydrolysis reaction, the hydrolysates and other residues were harvested and dried by a freeze dryer. Subsequently, the solid hydrolysates were analyzed with XRD, Raman, FT-IR and the solid-state 11B NMR spectroscopy. The conversion efficiency of ammonia borane to hydrogen could be calculated with the accumulation volume of the produced hydrogen gas. Additionally, the extent of the catalyzed hydrolysis reaction of the ammonia borane was measured with the quantitative analysis on the hydrolysates by the solid-state 11B NMR. Consequently, the conversion yield of the ammonia borane was obtained, accordingly. 2.3. Instrumental analyses Surface morphology and compositions of the solid-state AB composite after the high energy ball milling was observed by using the Hitachi SU-8000. The XRD patterns of the NieCo/r-GO catalysts were examined with the Rigaku Ultima IV X-ray diffractometer (Tokyo, Japan) over a range of diffraction angle (q) from 2q ¼ 10 to
9.5 90 C 2h
2q ¼ 80 at a 2q-rate of 4 min1 with Cu Ka radiation (40 kV and 20 mA) filtered by a monochrometer. Fourier-transform infrared spectroscopy (FT-IR) was performed on a Nicolet 6700 FT-IR. The DXR Raman microscope equipped with a 10 objective was employed to obtain the Raman spectra excited by 532 nm laser radiation at 5 mW (Thermo Fisher Scientific). An ICP-OES (JY Ultima 2000, Horiba, Kyoto, Japan) was used to quantity metallic loadings on NieCo/r-GO catalysts. The boron chemistry in the hydrolysates were examined with the solid-state 11B NMR spectroscopy using the Bruker Avance 400 NMR spectrometer (Rheinstetten, Germany). Sodium borohydride was used as the external reference in this work. The Micromeritics ASAP 2020 porosimeter using N2 at 77 K as the adsorbate was employed to obtain the BrunauereEmmetteTeller (BET) surface area of the catalysts and supports. 3. Results and discussion According to the ICP-OES measurements, the loadings of cobalt on NieCo/r-GO(j) and NieCo/r-GO(s) catalysts were 14.6 wt.% and 26.1 wt.%, respectively, based on the catalyst weights. The nickel contents were 17.6 wt.% and 39.5 wt.%, correspondingly, on NieCo/ r-GO(j) and NieCo/r-GO(s) catalysts. This indicates that the sequential EDP of Ni and Co would lead to more Co and Ni nanoparticles deposited on r-GO supports. Hence, 5.37 wt.% NieCo/rGO(j) catalyst and 3 wt.% NieCo/r-GO(s) catalyst based on the mass of ammonia borane were used in the preparation of the solid-state AB composites to maintain the same dosage of the active catalysts in the reacting system for the subsequent studies on the hydrogen production from the catalyzed hydrolysis of ammonia borane. The BET surface area of NieCO/r-GO(j) and NieCO/r-GO(s) catalysts are 1576 m2 g1 and 1805 m2 g1, respectively, less than that of the asprepared r-GO support (2285 m2 g1). 3.1. Characterization of the solid-state AB composite Surface morphology of the solid-state AB composite after highenergy ball milling were examined with SEM. Fig. 1 displayed the SEM images of ball-milled particles containing ammonia borane granules and 5.37 wt.% NieCo/r-GO(j) catalyst and 3 wt.% NieCo/rGO(s) catalyst, respectively. The granule size of the solid-state AB composites range from 60 to 95 mm. Moreover, catalysts were uniformly dispersed within the AB composites (Fig. S1). Fig. 2 revealed the FT-IR spectra of the ball-milled AB composites containing 5.37 wt.% NieCo/r-GO(j) catalyst and 3 wt.% NieCo/ r-GO(s) catalyst. In general, IR peaks associated with BeN, BeH and NeH bonds could be noticed on both AB-catalyst composites. For example, IR peaks were observed at 727 cm1 for BeN stretching, at
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797 cm1 for BeN stretching, at 1063 cm1 for BeH torsion, at 1160 cm1 for BeH torsion, at 1605 cm1 for NeH bending and at 2355 cm1 for BeH stretching, all arising from the ammonia borane. The broaden peaks appearing at 3200 to 3400 cm1 might originate from the OeH stretching resulted from the moisture and, possibly, from the NeH stretching of ammonia borane molecules. 3.2. Hydrogen generation from solid-state AB composite
Fig. 1. SEM images of the AB composites with (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst after high energy ball milling at 150 rpm for 100 min.
Fig. 2. FT-IR spectra of the AB composites with (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst.
According to the Le Chatelier's principle, more water present in the reacting system would drive the reaction equilibrium to the favor of hydrogen production and, therefore, accelerate the evolution of the hydrogen gas. However, the increase in water supply also adds up more weight to the system mass and, thus, would decrease the gravimetric capacity of the hydrogen storage system. Consequently, the effect of limited water supply to the hydrolysis reaction of ammonia borane for hydrogen production was studied in this work. A limited amount of water near the stoichiometry was supplied to the reacting system to initiate the hydrolysis reaction for hydrogen generation (Table 2). The molar feed ratios of water to ammonia borane, denoted as H2O/AB, were set at 2.02, 3.97 and 5.93, respectively (i.e., H2O/AB ¼ 2.02, 3.97 and 5.93). Water was injected quickly at once to the reaction flask to moderate the heat emitted from the hydrolysis reaction of ammonia borane. Fig. 3 showed the H2-generation profiles and the temperature profiles of the reaction pile. The temperature of the reaction system dropped very rapidly at first due to the introduction of water droplet to the reaction pile and, then, increased progressively owing to the exothermic heat emitted from the hydrolysis reaction of ammonia borane. With less water injected, the temperature of the reaction system could increase more rapidly and even reached almost 60 C. One possible explanation was that water could adsorb the heat and insufficient water could not effectively suppress the temperature. Afterwards, the generation rate of hydrogen slowed down as AB was consumed up. Notably, it has to mention the danger if the slow injection of water to the reacting mixtures was adopted. It often resulted in the blow-up of the experimental setup, as the reaction temperature could be elevated quickly above the boiling point of water. Consequently, great care and attention had to be paid in the experimental operation. Coincidently, similar trends in the profiles of hydrogen flowrate and the temperature of the reacting system could be observed (Fig. 3). That is, the flowrate of the hydrogen gas produced ascended to the maximum, as the temperature of the reacting system almost reached the utmost. Notably, the hydrogen release profiles from the reacting system containing 3 wt.% NieCo/r-GO(s) catalyst were relatively smoother than those with 5.37 wt.% NieCo/ r-GO(j) catalyst, which could benefit the hydrogen supply to PEMFCs [26]. Another important issue needed to be considered in the chemical hydride fuel system is the hydrogen storage capacity. For example, the US Department of Energy has revised the 2017-target for onboard hydrogen storage system for light-duty vehicles as 5.5 wt.% of the maximum system mass. Consequently, the hydrogen storage capacity of the AB composites were estimated from the accumulation of the hydrogen produced during the hydrolysis reaction of ammonia borane recorded by MFC (Table 2). In brief, the calculation on the hydrogen storage capacity was based on (1) one mole of ammonia borane could produce 3 mol of hydrogen, as shown in Eq. (1) and Eq. (2), and (2) the molar volume of hydrogen was approximated at about 22.892 L at 30 C. For comparison, the solid-state 11B NMR was utilized to analyze the hydrolysates to estimate the conversion efficiency of AB, calculated from the areaunder-curve of the NMR spectra (Table 2).
C.-C. Chou, B.-H. Chen / Journal of Power Sources 293 (2015) 343e350 Table 2 Effect of water dosage to the conversion efficiency of ammonia borane (AB) to hydrogen estimated from MFC measurement and H2O/AB (mol mol
1
)
AB composites with 5.37 wt.% NieCo/r-GO(j) catalyst (a) 2.02 (b) 3.97 (c) 5.93 AB composites with 3 wt.% NieCo/r-GO(s) catalyst (d) 2.02 (e) 3.97 (f) 5.93
347
11
B NMR analysis.
Deionized water (g)
Conversion by MFC/NMR (%)
Hydrogen storage capacity calculated from MFC/NMR (wt.%)
0.236 0.463 0.691
73.2/95.6 100/100 100/100
6.4/8.4 5.8/5.8 4.3/4.3
0.236 0.463 0.691
72.8/96.8 100/100 100/100
6.5/8.6 5.9/5.9 4.4/4.4
In general, the hydrogen storage capacity estimated from NMR analyses was greater than those based on the cumulative volume of the produced hydrogen recorded by MFC. The 11B NMR analysis may overestimate the conversion of ammonia borane. With restricted water supply, the temperature of the reacting system could increase significantly (Fig. 3(a) and (d)). Under the conditions of elevated temperature, the ammonia borane could form
polymeric aminoborane compounds and other borane complexes. That is, not all ammonia borane was converted to metaborate and borate to produce hydrogen. The discrepancy on the results calculated from MFC records and 11 B NMR analysis became obvious when water under stoichiometry, e.g. H2O/AB ¼ 2.02 by mol, was supplied to the reacting system. For example, the conversion of ammonia borane were 73.2% and 72.8
Fig. 3. Effect of water supply on the hydrogen generation profiles and the reaction temperature profiles from the hydrolysis of the solid-state AB composites having 5.37 wt.% NieCo/r-GO(j) catalyst fed with (a) H2O/AB ¼ 2.02, (b) H2O/AB ¼ 3.97, and (c) H2O/AB ¼ 5.93; the solid-state AB composites having 3 wt.% NieCo/r-GO(s) catalyst fed with (d) H2O/ AB ¼ 2.02; (e) H2O/AB ¼ 3.97; (f) H2O/AB ¼ 5.93.
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Fig. 4. XRD patterns of the hydrolysates and the solid residues harvested after the hydrolysis reaction of the solid-state AB composite containing (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst with different water dosages (H2O/AB ¼ 2.02, 3.97 and 5.93 by mol).
with 5.37 wt.% NieCo/r-GO(j) catalyst and 3 wt.% NieCo/r-GO(s) catalyst, respectively, based on the accumulation of the evolved hydrogen gas recorded by MFC. However, the conversion were, instead, 95.6% and 96.8%, if estimated from 11B NMR analysis. The conversion efficiency of AB could reach 100% for both MFC measurement and NMR analysis with the water dosage increased to H2O/AB ¼ 3.97 and 5.93 by mol. This implied that AB could be completely hydrolyzed in the presence of 5.37 wt.% NieCo/r-GO(j) catalyst and 3 wt.% NieCo/r-GO(s) catalyst under excess water. Nevertheless, the most hydrogen storage capacity was only about 5.9 wt.% of the solid-state AB composite under excess water, owing to a higher system mass contributed from the excess water. Instead, the hydrogen storage capacity could reach as high as 6.5 wt.% with under-stoichiometric water supply at H2O/AB ¼ 2.02 by mol, even though not all AB was completely hydrolyzed. 3.3. Analyses of the hydrolysates The residue of the solid-state AB composites after the hydrolysis reaction was harvested and sealed in desiccator to prevent any further hydrolysis reaction caused by the moisture. The chemistry
of the hydrolysate and the solid residue was probed by various instrumental analyses, such as XRD, FT-IR, Raman and the solidstate 11B NMR. Fig. 4 showed the XRD patterns of the harvested hydrolysates and the solid residues. The main XRD peaks at 2q ¼ 15.0 and 30.2 were consistent with those peaks of metaboric acid or boric acid, which illustrated the hydrolysates would be probably the mixture of metaboric acid and boric acid. Interestingly, no peak at 2q ¼ 24 representing the main characteristic of the ammonia borane compound could be observed in Fig. 4, even at the reacting system fed with H2O/AB at 2.02. This would certainly suggest that ammonia borane was nearly converted even with H2O/AB at 2.02. FT-IR analyses on the harvested hydrolysates and the solid residues after the hydrolysis reaction of the solid-state AB composites were performed and shown in Fig. 5. Three main IR peaks were observed: 780 cm1 for BeO bending, 1200 cm1 for BeO stretching, and 1330e1420 cm1 for OeH bending, which were coincient with the FT-IR characteristics of boric acid and metaboric acid. Furthermore, there existed other peaks such as 2300 to 2400 cm1 and 3200 to 3400 cm1, which could be ascribed to CO2 and water in the air, respectively.
Fig. 5. FT-IR spectra of the hydrolysates and the solid residues harvested after the hydrolysis reaction of the solid-state AB composite containing (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst with different water dosages (H2O/AB ¼ 2.02, 3.97 and 5.93 by mol).
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349
Fig. 6. Raman spectra of the hydrolysates and the solid residues harvested after the hydrolysis reaction of the solid-state AB composite containing (a) 5.37 wt.% NieCo/r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst with different water dosages (H2O/AB ¼ 2.02, 3.97 and 5.93 by mol).
Fig. 6 demonstrated the Raman spectra of the harvested hydrolysates and the solid residues. On all Raman spectra of all samples, there existed two broad peaks centered near 1350 and 1580 cm1, commonly known as D-band and G-band of carbon compounds, respectively. The D-band is originated from the lattice defect of disordered carbon structures, while the G-band arises from well-ordered sp2-hybridized carbon structures. Moreover, the graphitic characters could be obtained from the relative intensity ratio of D-band to G-band (ID/IG) [27]. The calculated ID/IG value for hydrolysates from the AB composites with 5.37 wt.% NieCo/r-GO(j) catalyst fed with H2O/AB ¼ 2.02, 3.97 and 5.93 by mol were 2.230, 1.903 and 1.603, respectively. It seems that the ID/IG value would decrease with increasing water dosage to the reacting system. It is possible that more water introduced to the reacting system could buffer the variation of the temperature and, consequently, decreased the formation of the disordered carbon structures on the r-GO supports. Similarly, the ID/IG values found in the hydrolysates from the AB composites containing 3 wt.% NieCo/r-GO(s) catalyst with H2O/AB ¼ 2.02, 3.97 and 5.93 by mol were 2.018, 1.971 and 1.540, respectively.
Solid-state 11B NMR analyses of the harvested hydrolysates and solid residues using NaBH4 as an external reference were shown in Fig. 7. Moreover, the 11B chemical shifts were 27 ppm for NH3BH3, 6.6 and 14.7 ppm for H3BO3 and 2.8 ppm for NaBO2 [10]. All samples tested showed the same range of 11B chemical shifts from 0 to 20 ppm, which indicated the existence of the mixtures of boric acid and metaborate on these samples. Contradictive to the XRD analysis, the minor peak with the 11B chemical shift at 27 ppm, corresponding to ammonia borane, was found in the residue of the reacting systems containing both NieCo/r-GO catalysts with H2O/ AB ¼ 2.02. Hence, the 11B NMR analysis presented that not all ammonia borane were completely hydrolyzed (Table 2). Evidently, according to XRD, FT-IR and solid-state 11B NMR analyses the main products of the hydrolysis reaction of the solidstate AB composites studied in this work were the mixture of boric acid and metaborate, coincident with those found in the hydrolysis of ammonia borane over the Co2þ/IR-120 catalyst [10]. Notably, the hydrogen storage capacity could reach as high as 6.5 wt.% in the case with H2O/AB ¼ 2.02, which is relatively superior to those reported in the recent literature (Table S1).
Fig. 7. Solid-state 11B NMR spectra of the hydrolysates and the solid residues harvested after the hydrolysis reaction of the solid-state AB composite containing (a) 5.37 wt.% NieCo/ r-GO(j) catalyst and (b) 3 wt.% NieCo/r-GO(s) catalyst with different water dosages (H2O/AB ¼ 2.02, 3.97 and 5.93 by mol).
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4. Conclusions In this work, the concept of treating water as a limiting agent in the hydrolysis of ammonia borane for hydrogen production and storage to reduce the mass of the hydrogen storage system has been proven as feasible. The highest hydrogen storage capacity obtained in this work was near 6.5 wt.% from the AB composites with 3 wt.% NieCo/r-GO catalyst reacting with under-stoichiometric amount of water (namely H2O/AB ¼ 2 by mol). However, with insufficient water, the exothermic heat emitted from the hydrolysis reaction could not be properly adsorbed and, thus, the temperature of the reacting system could reach too high to control the release of the hydrogen gas. The hydrolysates harvested after hydrogen released from the solid state AB composites are found likely to be the mixture of boric acid and metaborate. Acknowledgment The authors gratefully acknowledge financial support from the Ministry of Science and Technology of Taiwan through the Grant MOST 102-2221-E-006 -286 -MY2. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.05.091. References [1] T. Umegaki, T. Hosoya, N. Toyama, Q. Xu, Y. Kojima, J. Alloys Compd. 608 (2014) 261e265. rida, Appl. Energy 129 (2014) [2] S. Flick, M. Schwager, E. McCarthy, W. Me 135e146. [3] Q. Yao, W. Shi, G. Feng, Z.-H. Lu, X. Zhang, D. Tao, D. Kong, X. Chen, J. Power
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