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High and rapid hydrogen release from thermolysisof ammonia borane near PEM fuel cell operatingtemperatures: Effect of quartz wool

Hyun Tae Hwang, Ahmad Al-Kukhun, Arvind Varma*

School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA

a r t i c l e i n f o

Article history:

Received 23 November 2011

Received in revised form

18 January 2012

Accepted 23 January 2012

Available online 22 February 2012

Keywords:

Ammonia borane

Thermolysis

Hydrogen storage

Dehydrogenation

PEM fuel cell

* Corresponding author. Tel.: þ1 765 494 407E-mail address: avarma@purdue.edu (A.

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2012.01.098

a b s t r a c t

Ammonia borane (NH3BH3, AB), containing 19.6 wt% hydrogen, is a promising hydrogen

storage material for use in proton exchange membrane fuel cell (PEM FC) powered vehicles.

We recently demonstrated that using quartz wool, the highest H2 yield (2.1e2.3H2 equiv-

alent) values were obtained by neat AB thermolysis near PEM FC operating temperatures,

along with rapid kinetics, without the use of either catalyst or chemical additives. It was

found that quartz wool minimizes sample expansion and facilitates the production of

diamoniate of diborane (DADB), which is a key intermediate for the release of hydrogen

from AB. It was also found that only trace amount of ammonia (<10 ppm) is produced

during dehydrogenation reaction and spent AB products are found to be polyborazylene-

like species, which can be efficiently regenerated using currently demonstrated methods.

The results indicate that our proposed method is the most promising one available in the

literature to-date for hydrogen storage, and could be used in PEM FC based vehicle

applications.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction for which thematerial-based H2 yieldmust exceed the system

Hydrogen powered fuel cell vehicles are expected to play a key

role in future transportation systems since they produce only

electricity and water at point of use. A major obstacle for the

development of such vehicles is the lack of safe, light weight

and energy efficient means for on-board hydrogen storage.

Such hydrogen storage materials should liberate enough

hydrogen so that a vehicle can travel >500 km on a single fill.

The U.S. Department of Energy (DOE) has set a 2015 require-

ment of 5.5 wt% hydrogen for storage systems [1]. Evaluation

of a hydrogen storage system includes all associated compo-

nents such as tank, valves, piping, insulation, reactants, etc.

Currently, no technology can meet the 2015 system target [1],

5; fax: þ1 765 494 0805.Varma).2012, Hydrogen Energy P

target by a typical factor of 2 or more.

Ammonia borane (NH3BH3, AB; powder) has attracted

considerable interest as a promising candidate material

because of its high hydrogen content (19.6 wt%), hydrogen

release under moderate conditions, and stability at room

temperature [2,3]. Hydrogen can be released from AB by either

hydrolysis or thermolysis. Hydrolysis (Eq. (1)) provides low

theoretical H2 yield due to limited AB solubility in water and

requires catalysts [3,4]. In addition, it generates BeO bonds

which are not preferred from the spent fuel regeneration

viewpoint [5], and NH3 which must be removed for use in PEM

FCs. On the other hand, thermolysis Eqs. (2)e(4) requires

either relatively high temperature (>150 �C) to release two

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 en e r g y 3 7 ( 2 0 1 2 ) 6 7 6 4e6 7 7 0 6765

equivalents of hydrogen per AB, or additives (which constitute

weight penalty) for lower temperature operation and shorter

induction period [6e11]. Above 500 �C, AB can be completely

decomposed to form boron nitride (BN). For spent fuel

regeneration, however, BN is not preferred since it has high

chemical and thermal stability which makes AB regeneration

difficult. In addition, under certain conditions, ammonia and

borazine (N3B3H6), a volatile compound and potential poison

of fuel cell catalyst, are generated from AB thermolysis and

need to be minimized or eliminated [12e14].

Hydrolysis : NH3BH3þ3H2O/BðOHÞ3þNH3þ3H2 (1)

Thermolysis : NH3BH3/1xðNH2BH2ÞxþH2; ð90�117

�CÞ (2)

1xðNH2BH2Þx/

1xðNHBHÞxþH2; ð150� 170

�CÞ (3)

1xðNHBHÞx/

1xðNBÞxþH2; ð> 500

�CÞ (4)

It has been reported that, at PEM FC operating temperatures

(w85 �C) in the absence of any additive, H2erelease from solid-

state AB exhibits an induction period of up to 3 h [6,8e10].

After hydrogen release begins, only w1 equivalent of H2 is

obtained even with prolonged duration (>20 h). For this

reason, in prior studies reported in the literature, AB ther-

molysis requires temperature above 150 �C to provide 2

equivalent of hydrogen per AB (i.e. 13.1 wt%H2). However, this

temperature is generally too high to utilize waste heat from

a PEM FC, thus the thermolysis process typically requires

additional heat which constitutes an energy penalty.

In order to decrease reaction onset temperature and

induction period of AB thermolysis, various materials or

additives have been developed. Gutowska et al. found that

a nanocomposite of mesoporous silica and AB releases

hydrogen at 50 �C with a half-reaction time of 85 min, as

compared to half-reaction time of 290min at 80 �C for neat AB

[7]. Bluhm et al. reported that dehydrogenation of AB in ionic

liquids such as 1-butyl-3-methylimidazolium chloride

(bmimCl) releases 0.5, 0.8 and 1.1 equivalent of H2 in 1 h at 85,

90 and 95 �C, respectively [6]. Later, Himmelberger et al. found

that AB reactions in bmimCl containing 5.3 mol % (28 wt %)

bis(dimethylamino) naphthalene (Proton Sponge) released 2

equivalent of H2 in 171 min at 85 �C [10]. Heldebrant et al.

found that addition of diamoniate of diborane ([NH3BH2NH3]

[BH4], DADB), a product of AB isomerization, to neat AB

significantly decreases the induction time and onset temper-

ature at which hydrogen is released [8]. Nanophase boron

nitride (nano-BN) additives to AB play a similar role as DADB

and also serve as a scaffold, both decreasing the onset

temperature of H2 release [11]. Unfortunately, these methods

require either relatively high temperature or involve relatively

large amount of expensive additives, which increase the

overall system weight.

We have recently proposed and demonstrated a novel

noncatalytic AB hydrothermolysis method, where AB-water

mixture is heated to near PEM FC operating temperature

(85 �C) [13,15,16]. Using this approach, the maximum

hydrogen storage capacity was 11.6 and 14.3 wt% at pressure

14.7 and 200 psia, respectively [16]. Unfortunately, AB

hydrothermolysis generates significant NH3 and also BeO

bonds [12,13], which are not preferred from the spent fuel

regeneration viewpoint since they are thermodynamically

more stable than BeN bonds formed by thermolysis [2,5].

As compared to hydrolysis or hydrothermolysis, AB ther-

molysis provides the following advantages: (1) potentially

high hydrogen yield, (2) no BeO bond formation, and (3)

significantly reduced NH3 formation. In this context, the

following challenges remain.

(1) How to decrease thermolysis onset (operating) tempera-

tures to near PEM FC operating temperatures?

(2) How to provide target level or higher H2 capacity at or near

PEM FC operating temperatures?

(3) How to generate H2 at sufficiently high rate on demand?

(4) How tominimize or eliminate NH3 and borazine formation

by AB thermolysis?

It is known that the release of first and second mole of

hydrogen from AB via thermolysis (Eqs. 2 and 3) is exothermic

[2,11,17]. Thus, we expected that with effective heat

management, utilizing the reaction exothermicity during the

first H2 release from AB could trigger release of second H2.

Using inert insulation material, we have recently obtained

high H2 yield (2.1e2.3H2 molar equivalent) by neat AB ther-

molysis near PEM FC operating temperatures along with rapid

kinetics, without the use of either catalyst or chemical addi-

tives [18]. In addition, only small amount of NH3 (<10 ppm)

was detected in the gaseous product. To our knowledge, these

H2 yield values are higher than by any other method using AB

at near PEM FC operating temperatures.

In the present work, the effect of quartz wool on AB

thermolysis was investigated in detail. To better under-

stand the reaction mechanism of AB thermolysis in the

presence of quartz wool, direct observations were made in

an oil bath system and AB sample in the transition state

was characterized by solid-state NMR. Additionally, tran-

sient mass-spectrometry analysis of gaseous products was

carried out to identify generation of hydrogen as well as

ammonia.

2. Experimental

2.1. Direct observations of AB thermolysis

The direct observations of AB thermolysis were conducted in

a silicone oil bath as schematically shown in Fig. 1. The AB

(97% pure, SigmaeAldrich) sample (w0.5 g) was placed in

a test tube (13 mm OD, 100 mm length) and some quartz wool

(4 or 9 mm diameter) was added at the top of the AB sample,

which retains heat of exothermic thermolysis reaction while

permitting product H2 to flow. Before each experiment, the

tube was purged with argon gas for 5 min. The oil bath was

preheated to desired set point and the tube preloadedwith the

sample was submerged in the bath, which was magnetically

stirred to ensure uniform temperature distribution. Apart

from the bath temperature, the sample temperature was also

recorded by inserting another thermocouple inside the

sample. To verify hydrogen generation from the sample, the

Fig. 1 e Schematic diagram of oil-bath reactor system for

direct observation of AB thermolysis.

Fig. 2 e Schematic diagram of the experimental set-up for

transient analysis of gas formation during AB thermolysis.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 6 7 6 4e6 7 7 06766

exit of the tube was connected to a flask with water and gas

bubbles were observed during the gas production.

2.2. Hydrogen release measurements and transientanalysis of gaseous products

The experiments were conducted in a stainless steel reactor

(Parr Instrument Company, Model 4592; volume 70 mL) with

external heating. Starting at room temperature, with 2 �C/min

heating rate, the reaction vessel was maintained for 60-min

hold at the set point value (TSP). Similar to Section 2.1, the

AB sample was placed in a small quartz vial (5 ml) and some

quartz wool (4 or 9 mm diameter) was added at the top of the

AB sample. In addition, the empty space above the quartz

wool layer was filled with glass beads (2 mm diameter) to

provide desired packing densities of quartz wool and AB. The

reactor pressure and temperatures (sample and reactor) are

monitored using an online pressure transducer and thermo-

couples. Additional experimental details are discussed else-

where [13]. For transient mass-spectrometry (MS, Hiden

Analytical HPR-20) analysis of gaseous products, a fixed

volumetric flow of Ar (120 cc/min) was introduced to the

reactor using a mass flow controller and gas generation was

analyzed with time using mass-spectrometry (Fig. 2). For

hydrogen release measurements, the reactor was operated in

a batch mode, while argon gas was continuously injected into

the reactor for the transient analysis.

2.3. Characterization of gaseous and solid products

The product gas composition after reaction and gas formation

for transient analysis were both analyzed by mass-

spectrometry. The hydrogen generation was calculated

using the gas composition, along with pressure increase

during the batch experiment. After cooling the reactor to room

temperature at the end of the experiment, the product gaswas

collected in a sampling bag and then NH3 concentration was

measured using Drager tube.

The solid samples were characterized by solid-state 11B

NMR, where the spectra were recorded using a Varian Inova

300 MHz spectrometer and were referenced to NaBH4

(�42.2 ppm).

3. Results and discussion

Due to slow kinetics of AB thermolysis at PEM FC operating

temperature, as described in the Introduction, neat AB ther-

molysis requires temperature above 150 �C to provide 2

equivalent of hydrogen per AB. For this reason, most prior

studies focused on development of materials or additives to

decrease reaction onset temperature and induction period of

AB thermolysis [6e11]. Unfortunately, these methods involve

relatively large amount of chemical additives or expensive

materials which also increase the overall system weight.

In our method, we take advantage of the thermal charac-

teristics of AB thermolysis. AB starts to release the first

hydrogen mole at w90 �C. It is also known that the release of

first and second mole of hydrogen from AB via thermolysis

(Eqs. 2 and 3) is exothermic, where the latter is more

exothermic than the former [2,11,17]. Thus, we expected that

when the reaction heat is effectively managed, the heat

released during the first decomposition step can drive the

second step. We recently reported that using quartz wool as

inert insulation material at 90 �C, high H2 yield (2.1e2.3H2

equivalent) was achieved and stabilized quickly after sharp

heat/hydrogen evolution [18].

It was also found that most spent AB product after reaction

was confined in the quartz wool layer when thermolysis was

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 en e r g y 3 7 ( 2 0 1 2 ) 6 7 6 4e6 7 7 0 6767

conducted in the presence of quartz wool, while the product

without quartz wool expanded significantly. It is known that

as AB releases H2 during thermolysis, the material expands

(>10 times of original volume) along with foaming [19,20],

which inhibits retention of reaction heat. It is likely that the

quartz wool keeps the sample inside the layer and decreases

expansion, which enablesmore effective retention of reaction

heat, as compared to the case without quartz wool.

3.1. Direct observations of AB thermolysis

In this work, an oil bath system (Fig. 1) was constructed to

observe physical transformations of the AB sample during

dehydrogenation. As described in Section 2.1, the sample was

submerged in the preheated bath and physical change of the

sample was monitored along with sample temperature

change.

The AB sample without quartz wool melts several minutes

after reaching set point temperature (a1 in Fig. 3a). Then, it

starts releasing hydrogen slowly along with significant

volume expansion (a2). Relatively small exothermicity was

also observed during hydrogen release (a2). The AB sample

with quartz wool melts similarly to that without quartz wool

(b1). Unlike the previous case, however, the melted AB is

Fig. 3 e Reaction stages along with temperature profiles for

neat AB thermolysis, (a) TSP [ 90 �C without quartz wool;

(b) TSP [ 90 �C with quartz wool (The detailed

temperatureetime profiles for the rectangular region are

shown in the inset.).

infused into the quartz wool layer (b2). Finally, the sample

temperature increases sharply, with simultaneous gas

evolution (b3).

From the direct observations, it was found that melted AB

must be infused into the quartz wool layer before sharp heat/

hydrogen evolution occurs, resulting in high hydrogen yield.

This indicates that quartz wool facilitates AB

dehydrogenation.

To further examine the role of quartz wool, another

experiment was conducted with a different sample configu-

ration where quartz wool was mixed with AB. In this case, the

quartz wool did not effectively inhibit sample expansion, thus

sharp evolution was not observed, indicating that prevention

of sample expansion is one of the important functions of

quartz wool to enable rapid H2erelease kinetics.

To better understand the reaction mechanism in the

presence of quartz wool, the AB samples before sharp evolu-

tion were characterized by solid-state 11B NMR. While using

the oil bath system, the sample tube was taken out from the

bath when AB was infused into the quartz wool just before

sharp evolution (b2 in Fig. 3). Then, the AB sample at the

transition state was separated into two parts, one below and

the second inside the quartz wool layer, and the two parts

were characterized by solid-state NMR. For comparison, pure

AB was also characterized, as shown in Fig. 4.

As compared to pure AB (Fig. 4a), Fig. 4b and c clearly show

a new pentet peak centered at �37 ppm, corresponding to dia-

moniate of diborane (DADB) [8]. It has been reported that AB

isomerizes to DADB, which appears to be a key intermediate in

AB dehydrogenation since solid products associated with

hydrogen release were observed immediately after its appear-

ance [8,9]. More DADB formation was observed from AB inside

Fig. 4 e 11B solid-state NMR spectrum (a) pure AB, (b) AB

below quartz wool layer, (c) AB in the quartz wool layer at

transition state (b2 in Fig. 3b).

Fig. 6 e Effect of quartz wool amount and diameter on (a) H2

molar equivalent, (b) maximum sample temperature (0.5 g

AB, TSP [ 90 �C, rAB [ 0.32 g/cm3, rQW [ 0.11 g/cm3).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 6 7 6 4e6 7 7 06768

the quartz wool layer than that in the region below it. These

results suggest that quartz wool plays a critical role in forma-

tion of DADB, which reacts rapidly with AB to generate

hydrogen.Gutowskaet al. showed that thekinetics ofhydrogen

release can be significantly improved at lower temperature for

AB infused in high surface area nanoporous silica scaffolds [7].

It is likely that quartz wool also provides sufficient surface area

which facilitates of DADB from the infused AB.

3.2. Kinetic studies of AB thermolysis in the presence ofquartz wool

In the present work, AB thermolysis in the presence of quartz

wool was investigated over a wide range of reactor tempera-

ture, amount of quartz wool and packing density of quartz

wool, while the initial density of AB (rAB) was kept constant at

0.32 g/cm3.

Before investigating kinetics of AB thermolysis, the effect

of glass beads, which were placed on the top of quartz wool

and AB layers to provide their desired packing densities, was

investigated. It was found that H2 yield and thermal charac-

teristics are not influenced by the presence of glass beads.

Further, in the absence of quartzwool, essentially the sameH2

yields (w0.7H2 equivalent) were obtained for both with and

without glass beads.

Fig. 5 shows the effect of reactor temperature on H2 yield.

Starting at room temperature, with heating rate programmed

at 2 �C/min, the reaction vessel was maintained for 60-min

hold at a specific temperature in the range 85e100 �C. In this

study, 90 �C was found to be the minimum reactor tempera-

ture to obtain high H2 yield along with rapid kinetics. For

reactor temperature � 90 �C, consistent high H2 yield

(2.1e2.3H2 equivalent) was obtained while for 85 �C, only

w0.7H2 equivalent was obtained and no sharp evolution was

observed due to the relatively low temperature.

Fig. 6a shows the effect of quartz wool amount on H2 yield

for different types of quartzwool (4 or 9 mmdiameter). In these

experiments, the densities of quartz wool (rQW) and AB (rAB)

were kept constant while the amount of quartz wool was

varied. For both types of quartz wool, H2 yield first increases

with increasing the quartz wool amount, and reaches

Fig. 5 e Effect of reactor temperature on H2 yield (0.5 g AB,

0.2 g quartz wool, rAB [ 0.32 g/cm3, rQW [ 0.11 g/cm3).

a saturation value. It was found that fine quartz wool (4 mm)

retains the AB sample more effectively. Thus, as compared to

coarse type (9 mm), fine quartz wool provides higher or similar

H2 yield for the same amount of quartz wool used. The

maximum H2 equivalent w2.2 was obtained with more than

0.2 and 0.3 g fine and coarse quartz wool, respectively.

Similar to H2 yield (Fig. 6a), as expected from reaction

exothermicity, the maximum sample temperature also

increases with increasing quartz wool amount, as shown in

Fig. 6b. The maximum sample temperature 220e240 �C was

observed when the maximum H2 yield (w2.2H2 equivalent)

was achieved.

As shown in Fig. 7, the effect of quartz wool density was

also investigated for various amounts of quartz wool. It shows

that an optimum density exists for each amount of quartz

wool, which increases with increasing its amount. Optimal

densities were found to be w0.05 and w0.075 g/cm3 for 0.15

and 0.20 g quartz wool, respectively. It is likely that there is

also an optimal density for 0.1 g quartz wool, but it was not

observed in this study due to limited volume of the vial. It is

expected, however, that the benefit from utilizing quartz wool

will disappear if its density is too low since sample expansion

will not be inhibited effectively. Maximum material-based H2

yield (w2.3H2 equivalent) was observed for 0.20 g quartz wool

with w0.075 g/cm3 density, while the maximum overall H2

yield (w12 wt%) accounting for the quartz wool amount was

Fig. 7 e Effect of quartz wool density for different amounts

on (a) H2 molar equivalent, (b) maximum sample

temperature (0.5 g AB, TSP [ 90 �C, rAB [ 0.32 g/cm3).

Fig. 8 e Transient analysis of gas formation during AB

thermolysis (a) hydrogen, (b) ammonia (0.5 gAB, 0.2 gquartz

wool, TSP [ 90 �C, rAB [ 0.32 g/cm3, rQW [ 0.11 g/cm3).

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 en e r g y 3 7 ( 2 0 1 2 ) 6 7 6 4e6 7 7 0 6769

measured for 0.10 g quartzwool withw0.034 g/cm3 density. As

expected, the pattern of maximum sample temperature is

similar to that of H2 yield (Fig. 7b).

It is known that thermolysis of AB also produces some

ammonia [12,13], which poisons the PEM FC catalyst, thus, it is

important to investigate its formation. Fig. 8 shows transient

mass-spectrometry (MS, Hiden Analytical HPR-20) analysis for

neat AB thermolysis at 14.7 psia, reactor set point temperature

(TSP) 90 �C, where a fixed volumetric flow of Ar was introduced

through the reactor and gas generation was measured with

time. Fig. 8a also shows the set-point temperature with time

during the experiment.

For neat AB thermolysis without quartz wool, hydrogen

evolved gradually with time after sample reached w90 �C(Fig. 8a). We have reported that after hydrogen release began,

only 5 wt% H2 yield (w0.8H2 equivalent) was achieved in

90 min [18]. It was also found in the MS analysis that apart

from hydrogen, NH3 evolves simultaneously during the ther-

molysis reaction (Fig. 8b). In contrast, high H2 yield (2.1e2.3H2

equivalent) was obtained from neat AB thermolysis in the

presence of quartz wool. Unlike the neat AB thermolysis

without quartz wool, however, gas formation was completed

quickly after sharp heat evolution. Fig. 8 clearly shows that AB

thermolysis with quartz wool produces more hydrogen along

with less ammonia, as compared to the case without quartz

wool. Some borazine formation was also observed during the

sharp heat/H2 evolution, but was not quantified in this study.

We also measured ammonia formation using Drager tube

after cooling the reactor to room temperature at the end of the

experiment. For neat AB thermolysis without quartz wool,

400e500 ppm of NH3 in the product gas was detected. It is

remarkable that with quartz wool, NH3 concentration in

gaseous product was trace amount (<10 ppm), much less than

that observed in neat AB thermolysis without quartz wool.

4. Conclusions

As noted in the Introduction, for AB dehydrogenation to be

applicable to hydrogen-powered fuel cells, theymustmeet the

following requirements: sufficiently high hydrogen capacity,

fast kinetics at PEM FC operating temperatures (<90 �C) to

utilize waste heat from the fuel cell, little or no ammonia

formation, and easily regenerable spent AB product. In this

study, using quartz wool, we obtained high H2 yield (2.1e2.3H2

equivalent) from AB at 14.7 psia and TSP 90 �C with rapid

kinetics, without the use of either catalyst or chemical addi-

tives. To our knowledge, this value is higher than by any other

method using AB near PEM FC operating temperatures.

In our recent work [18], we observed that the sample

temperature increases sharply along with high hydrogen

release when quartz wool was used, while it exhibited only

modest temperature increase in the absence of quartz wool. It

is likely that the quartz wool keeps AB sample inside the

quartz wool layer and decreases volume expansion, which

enables effective retention of reaction heat. Thus, it is ex-

pected that other materials, which can retain heat of reaction

effectively by preventing the AB sample from expanding,

would also induce high and rapid H2 release fromAB near PEM

FC operating temperatures. In this work, we found that in

addition, quartz wool plays a critical role to facilitate the

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 6 7 6 4e6 7 7 06770

production of diamoniate of diborane (DADB), which is a key

intermediate for the release of hydrogen from AB. It is note-

worthy that with quartz wool, NH3 concentration in gaseous

product was trace amount (<10 ppm), much less than that

observed in neat AB thermolysis without quartz wool

(400e500 ppm). The spent AB solid product was found to be

polyborazylene-like species, which can be efficiently regen-

erated using currently demonstrated methods. This success,

along with the results presented here, suggests that the

proposed method is promising for hydrogen storage, and

could be used in PEM fuel cell based vehicles.

Acknowledgments

This work was supported by the Department of Energy, under

Grant Number DOE-FG36-06GO086050 to the Purdue Univer-

sity Energy Center.We thank Patrick Greenan for his helpwith

some experiments.

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