Production of sodium borohydride by using dynamic behaviors of protide at the extreme surface of magnesium particles

Article abstract of DOI:10.1016/j.jallcom.2005.02.101

An advanced process for the production of sodium borohydride (NaBH ₄) as a hydrogen storage material was developed, which applied the dynamic hydriding and dehydriding behaviors of protide (H^-) in Mg-H system under transitional temperature conditions. An abundant natural resource named borax (Na₂B₄O_7?10H₂O) and the anhydrous sodium metaborate (NaBO₂) recovered from the spent fuel as NaBO_2?4H₂O were used as the starting material in the present process. Powder-state Mg played an important role in the transitional hydriding and dehydriding process where the gaseous hydrogen was converted to protide at the extreme surface of Mg to form NaBH ₄ in exchange with the simultaneous transition of oxygen in NaBO ₂ to form MgO. In the present process, the protide as the most reactive state among the four states of hydrogen is applied for the synthesis of NaBH₄, which can exist in metal-hydrogen complexes, such as NaAlH₄ and NaBH₄. The NaBH₄ yield was reached higher than 90% by a single batch process but was found to be largely dependent on the rate of temperature change and the particle size, i.e., the specific surface area of Mg particles.

Full text of DOI:10.1016/j.jallcom.2005.02.101

Journal of Alloys and Compounds 404–406 (2005) 643–647
Production of sodium borohydride by using dynamic behaviors of
protide at the extreme surface of magnesium particles
S. Sudaa,∗, N. Morigasakib, Y. Iwaseb, Z.P. Lia
a
Chemical Energy Laboratory, Department of Environmental and Chemical Engineering, Kogakuin University (KUCEL) 2665-1,
Nakano-machi, Hachioji-shi, Tokyo 192-0015, Japan
Materials and Energy Research Institute Tokyo, Ltd. (MERIT) 1-1, Sawarabi-daira, Kitayama, Chino-shi, Nagano-ken 0301-0390, Japan
b
Received 7 September 2004; received in revised form 24 January 2005; accepted 1 February 2005
Available online 16 November 2005
Abstract
An advanced process for the production of sodium borohydride (NaBH4) as a hydrogen storage material was developed, which applied the
dynamic hydriding and dehydriding behaviors of protide (H−) in Mg–H system under transitional temperature conditions.
An abundant natural resource named borax (Na2B4O7·10H2O) and the anhydrous sodium metaborate (NaBO2) recovered from the “spent
fuel” as NaBO2·4H2O were used as the starting material in the present process. Powder-state Mg played an important role in the transitional
hydriding and dehydriding process where the gaseous hydrogen was converted to protide at the extreme surface of Mg to form NaBH4 in
exchange with the simultaneous transition of oxygen in NaBO2 to form MgO.
In the present process, the protide as the most reactive state among the four states of hydrogen is applied for the synthesis of NaBH4, which
can exist in metal–hydrogen complexes, such as NaAlH4 and NaBH4.
The NaBH4 yield was reached higher than 90% by a single batch process but was found to be largely dependent on the rate of temperature
change and the particle size, i.e., the specific surface area of Mg particles.
© 2005 Published by Elsevier B.V.
Keywords: Sodium borohydride; Protide; Magnesium; Surface; Hydrogen storage
1. Introduction
drogen” applications, we may use another state of hydrogen
of “protide”, which is the state existing both as solid- and
liquid states in metal–hydrogen complex ions [1,2]. The pro-
tide possesses two electrons and it is the most reactive state
among the four-states of hydrogen, i.e., molecular hydrogen
(H2), proton (H+), protium (Ho: monatomic hydrogen) and
protide (H−: hydride ion).
Hydrogen storage materials have long been searched for
the storage of “gaseous hydrogen” where the application of
hydrogen is very much limited to the source of IC engines
and PEMFC. Conventional metal hydrides could not have
attained H-capacity higher than 3 mass% except Mg-alloys
even after three-decades of effort.
The mass-based H-capacity of materials has been used
as the measure of the storage capacity. However, it should
be evaluated by the weight basis of storage unit as the sys-
tem in any engineering devices. From a practical viewpoint,
the effective H-capacity of most materials under develop-
ment today should be considered not more than 1 wt.% at
maximum. If we leave from the standpoint of “gaseous hy-
In this paper, sodium borohydride (NaBH4, denoted SBH)
as the source of protide will be reported with regard to
its new production process that is based on the natural re-
source, borax, (Na2B4O7·10H2O) and sodium metaborate
(NaBO2·4H2O) recovered as the “spent fuel”.
The hydrogen content in SBH is 10.6 mass% and theo-
retical H-capacity generated by hydrolysis is 10.8 mass%.
However, its H-capacity is greatly reduced by diluting in an
aqueous alkaline solution of NaOH or KOH in order to main-
tain the stability without releasing H2 gas for longer terms.
The H-capacity is a function of SBH concentration and is
∗
Corresponding author. Tel.: +81 426 20 0154; fax: +81 426 20 0150.
E-mail address: bt73093@ns.kogakuin.ac.jp (S. Suda).
0925-8388/$ – see front matter © 2005 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2005.02.101

644
S. Suda et al. / Journal of Alloys and Compounds 404–406 (2005) 643–647
restricted by the solubility limit of SBH in the alkaline solu-
tion, mostly in the 10–20 w−t.% NaOH-solution. The alkaline
solution that contains BH4 is referred to as “borohydride”
in this paper.
Borohydride may be used not only as the source of pro-
tideinelectrochemicalapplicationsforDBFC(directborohy-
dride fuel cell), which is based on the electrochemical charac-
teristics given below in Eqs. (1)–(3) but also as the H2 source
for PEMFC by its catalytic hydrolysis according to Eq. (4).
NaBH4 + 4H2O → 4H2 + NaBO2 · 2H2O
(7.28 mass%) [53–120 ◦C]
ꢀGo (298 K) = −334 kJ/mol-NaBH4
(6)
(7)
NaBH4 + 6H2O → 4H2 + NaBO2 · 4H2O
(5.48 mass%) [below 53 ◦C]
ꢀGo (298 K) = −336 kJ/mol-NaBH4
BH4− + 2xH2O→4H2 + NaBO2 · 2(x−1)H2O (x = 2–3)
2. The protide applications
(8)
2.1. Electrochemical properties and characteristics of
protide [1–16]
3. Production of sodium borohydride as the
hydrogen storage material [23–31]
Borohydride is transformed electrochemically to borohy-
droxide ion by releasing eight electrons at the surface of neg-
ative electrode (anode). The material used as the electrode
canbeselectedfromthefluorinatedintermetalliccompounds.
The electrochemical reaction shown in Eq. (1) occurs only on
the extreme surface of the fluorinated materials. It is unnec-
essary to apply any precious metals and materials so-called
hydrogen absorbing alloys, which involve the hydrogen con-
version to proton via protide.
3.1. The conventional production process
The Rohm & Haas process and the Bayer process are
the two major processes commercially used today for the
production of SBH. As can be seen in Eqs. (9) and (10),
4 moles of NaH or 4 moles of Na is used for produc-
ing 1 mole of NaBH4. The theoretical yield of SBH is
limited to 25% and accordingly, the production cost be-
comes rather expensive. The recovery and separation of
un-reacted NaH and Na and by-products require additional
processes.
Anode: FBH4− + 8OH− → B(OH)4− + 4H2O + 8e−
(Eo = −1.24 V)
(1)
Cathode: F2O2 + 4H2O + 8e− → 8OH− (Eo = 0.40 V)
(a) Rohm & Haas process [23]:
(2)
−
Battery-reaction: FBH4− + 2O2 → B(OH)4
(Eo = 1.64 V)
4NaH + B(OCH3)3 → NaBH4 + 3NaOCH3
(3)
ꢀGo (298 K) = −129.5 kJ/mol-NaBH4
(9)
2.2. Gaseous hydrogen source for PEMFC [17–22]
(b) Bayer process [24]:
4Na + 2H2 + 1/4Na2B4O7 + 7/4SiO2
→ NaBH4 + 7/4Na2SiO3
Gaseous hydrogen, H2, is generated by the catalytic hy-
drolysis of borohydride;
BH4− + 4H2O → 4H2 + B(OH)4
−
ꢀGo (298 K) = −411.3 kJ/mol-NaBH4
(10)
(in aqueous alkaline solution)
(4)
3.2. The advanced processes
The hydrolysis is sometimes misguided by the expression
listed in Eq. (5). It must be noted that SBH exists as borohy-
dride (BH4−) in aqueous solutions but not as NaBH4. Boro-
hydride forms metaborate ions (BO2−) in the diluted aqueous
solutions after releasing hydrogen, and it forms the hydrated
sodium metaborates of several different states as listed in Eqs.
(6) and (7) depending on the temperature levels. Eqs. (5)–(8)
are only valid under the existence of stoichiometric water.
The general formula is given in Eq. (8).
(a) Mechano-chemical process [25,26]
In 1999, a mechano-chemical method that applies the
high-speed ball-milling technique was successfully intro-
duced to synthesize SBH by using MgH2 as the protide
donor and the oxygen acceptor in the reaction shown as
Eq. (11).
However, the energy conversion efficiency was found
highly restricted when mechanical force is converted to
chemical energy although the SBH yield was maintained
higher than 90% and even closer to 100%. In addition, it was
not cost-effective to prepare MgH2, which requires the du-
NaBH4 + 2H2O → 4H2 + NaBO2
(10.8 mass%) [over 270 ◦C]
ꢀGo (298 K) = −321 kJ/mol-NaBH4
(5)

S. Suda et al. / Journal of Alloys and Compounds 404–406 (2005) 643–647
645
plicated hydriding/dehydriding treatments under higher tem-
perature and pressure conditions.
periments under the slower dT/dt rates as can bee seen in
Fig. 1.
NaBO2 + 2MgH2 → NaBH4 + 2MgO
4. Preparation of the anhydrous sodium metaborate
(NaBO2) as the starting material
ꢀGo (298 K) = −270 kJ/mol-NaBH4
(11)
(b) The dynamic hydriding/dehydriding process [27,28,31]
The following simple processes were used to prepare the
anhydrous NaBO2:
The SBH production can be processed by the reaction as
shown in Eq. (12), where the system temperature is changed
rapidly under constant H2 pressure conditions;
(a) From “Borax” as the abundant natural resource
The process that starts from borax (Na2B4O7 · 10H2O)
requires two parts of NaOH to make one part of NaBO2 by
an exothermal steam process as given in Eq. (13).
NaBO2 + 2H2 + 2Mg → NaBH4 + 2MgO
ꢀGo (298 K) = −342 kJ/mol-NaBH4
(12)
1/4Na2B4O7 · 10H2O + 1/2NaOH + 5/4H2O
Typical experimental data are shown in Fig. 1a–d for il-
lustrating the effects of the rate of temperature change, dT/dt
(◦C/min) and Mg particle size (␮m).
→ NaBO2 · 4H2O
(13)
(b) From “Spent fuel” recovered after generating protide and
hydrogen
The higher the dT/dt and the smaller the particle size, the
faster is the reaction rate that was measured as the percent
yield of SBH. The SBH production is initiated when the sys-
tem temperature reached to 500 ◦C and to end up near 600 ◦C.
From this experiment, the SBH synthesis was found to pro-
ceed rapidly at the highest dT/dt rate (◦C/min) and at the
smallest particle size.
The SBH synthesis does not proceed effectively under
isothermal conditions, particularly at the temperature lev-
els between 500 and 600 ◦C that was proven by the ex-
The “Spent fuel” after releasing protide in the electro-
chemical reaction and generating gaseous hydrogen is recov-
ered as the crystalline solution of NaBO2·4H2O, and it is
put in a simple drying process to yield anhydrous sodium
metaborate (NaBO2) as shown in Eq. (14);
NaBO2 · 4H2O → NaBO2 + 4H2O (drying at 270 ◦C)
(14)
Fig. 1. (a) Effect of dT/dt (rate of temperature change) on the NaBH4 conversion rate. (b) Rate of reaction at constant dT/dt as a function of time. (c) Effect
of Mg particle size on the NaBH4 conversion rate. (d) Rate of reaction at constant dT/dt as a function of time.

646
S. Suda et al. / Journal of Alloys and Compounds 404–406 (2005) 643–647
Fig. 2. XRD patterns taken after completing the dynamic H/D reaction.
5. Dynamic states of Mg–H system
detected by XRD were only NaBH4 and MgO. A typical
XRD pattern is illustrated in Fig. 2 that was taken after com-
pleting the reaction. The formation of MgO begins with the
Mg surface toward the center of particles as a function of
reaction time elapsed. This is the same mechanism with the
formation of MgH2 observed under quasi-equilibrium condi-
tions of the Mg–H system during hydriding reactions. In Fig.
3, a sectional SEM view of Mg particle after approximately
30% oxidation being reached in which a clear border between
MgO and Mg phases can be observed.
From this observation, it is known that the Mg particles
should be brought to the smallest size as possible for increas-
ing the specific surface area to complete the oxidation of Mg
to MgO.
The SBH yields depend on the specific surface area of Mg
particles. Accordingly, the amount of Mg required for the
complete reaction as indicated in Eq. (13) is determined by
the particle size of Mg.
5.1. Transitions from the hydriding to dehydriding
regions
The thermal gradient between the initial- and the terminal
points of the reaction provides the dynamic changes of Mg–
H system between hydriding and dehydriding regions. At
the lower temperature region, Mg–H system tends to form
MgH2 hydride at the extreme surface of Mg particles. On
the contrary, it tends to dissociate hydrogen under the protide
state back to the gas phase at the higher temperature region.
However, the Mg–H system is transferred dynamically
from the hydriding to dehydriding regions by the rapid tem-
perature change so as not to form hydride layer at the surface
of Mg but to generate protide at the extreme surface of Mg di-
rectly from the gaseous H2 to form NaBH4. In return, oxygen
is transformed to the surface of Mg particles to form MgO in
exchange with the formation of NaBH4.
5.3. Exchange mechanism of gaseous hydrogen to
protide
The extreme surface of Mg is considered to function as a
catalyst for converting H2 to 2H− during the dynamic temper-
ature changes between hydriding and dehydriding regions.
As MgH2 is not formed under the transitional state of Mg–
H system, the gaseous H2 must be transformed at the extreme
surface of Mg particles to the protide that is transferred in-
stantly to the surface of NaBO2 to form NaBH4. The reaction
mechanism at the contact surfaces between Mg and NaBO2
particles is illustrated schematically in Fig. 4.
5.2. Mechanism of MgO formation
No MgH2 was formed during the dynamic thermal tran-
sition of the NaBO2–H2–Mg system and this was clearly
examined by XRD, EPMA, and SEM where the materials
Fig. 3. SEM and EPMA images of Mg particle (MgO starts from the surface).

S. Suda et al. / Journal of Alloys and Compounds 404–406 (2005) 643–647
[2] M.E. Indig, R.N. Snyder, J. Electrochem. Soc. 109 (1962) 1104.
647
[3] R. Jasinski, Electrochem. Technol. 3 (1965) 40.
[4] M. Jung, H.H. Kroeger, US Patent 3,511,710 (1970).
[5] K. Strasser, J. Power Sources 29 (1990) 149.
[6] J.-Y. Lee, US Patent 5,559,640 (1997).
[7] S.C. Amendola, US Patent 5,804,329 (1998).
[8] S.C. Amendola, P. Onnerud, M. Kelly, P. Petillo, M. Binder, Proceed-
ings of the 194th Meeting of the Electrochemical Society on Selected
Battery Topics 98-15, vol. 47, November 1–6, 1998, Boston, MA,
1999.
[9] S.C. Amendola, P. Onnerud, M. Kelly, P. Petillo, S. Sharp-Goldman,
M. Binder, J. Power Sources 84 (1) (1999) 130.
[10] S.C. Amendola, P. Petillo, M. Kelly, S. Sharp-Goldman, M. Binder,
Proceedings of the 195th Meeting of the Electrochemical Society, May
2–6, 1999, Seattle, WA.
[11] S.C. Amendola et al., J. Power Sources 85 (2000) 186.
[12] S.-M. Lee, J.-H. Kim, H.-H. Lee, P.S. Lee, J.-Y. Lee, J. Electrochem.
Soc. 149 (5) (2002) A603.
[13] Z.P. Li, B.H. Liu, K. Arai, S. Suda, J. Electrochem. Soc. 150 (7) (2003)
A868.
Fig. 4. Schematic illustration of the transitional states of SBH formation.
6. Remarks
A dynamic hydriding and dehydriding process for the pro-
duction of sodium borohydride as the hydrogen storage ma-
terial was developed, which is based on the transitional state
of hydrogen between the gaseous H2 and protide.
The process uses Borax as the starting material and sodium
metaborate as the spent fuel to synthesize the anhydrous
sodium borohydride. The production cost is estimated to be
1/20 times cheaper than the conventional processes available
today.
[14] B.H. Liu, Z.P. Li, S. Suda, J. Electrochem. Soc. 150 (3) (2003)
A398.
[15] B.H. Liu, Z.P. Li, S. Suda, Electrochem. Acta 49 (2004) 3097.
[16] Z.-P. Li, B.-H. Liu, K. Arai, K. Asaba, S. Suda, J. Power Sources 126
(2004) 28–33.
Hydrogen storage materials are the key issues for bring-
ing PEMFC more practical. However, we must recognize that
there has not been developed any hydrogen storage materials
practically available today. Conventional metal hydrides in
general will not be qualified for automobile applications be-
cause of their limited H-capacity and thermal requirements,
and some of the hydrogen-metal complex compounds such
as NaAlH4 will not be employed as “On-board” or “On-site”
materials because of its handling difficulties and safety is-
sues.
The authors convince that NaBH4 as the hydrogen storage
material both for the large-scale storage material and the pro-
tide carrier in a wide variety of fuel cell applications with the
estimated fuel cost of less than US$ 50/kW within a couple
of years.
(b) Hydrogen generation from borohydride
[17] R.L. Pecsok, J. Am. Chem. Soc. 76 (1953) 2862.
[18] S. Suda, Y.-M. Sun, B.-H. Liu, Y. Zhou, S. Morimitsu, K. Arai, N.
Tsukamoto, M. Uchida, Y. Candra, Z.-P. Li, J. Appl. Phys. A72 (2001)
209–212.
[19] S. Suda, Y.-M. Sun, M. Uchida, B.-H. Liu, S. Morimitsu, K. Arai,
Y. Zhou, N. Tsukamoto, Y. Candra, Z.-P. Li, Metals Mater. Int. 7 (1)
(2001) 73–75.
[20] S. Suda, Hydrogen–Metal Systems: Technological and Engineering
Aspects in Encyclopedia of Materials-Science and Technology, Else-
vier Science Ltd., 2002, pp. 3970–3976.
[21] S. Suda, US Patent 6,358,488 (2002).
[22] S. Suda, Handbook of Fuel Cells—Fundamentals, Technology and Ap-
plications, vol. 3, No. 2, John Wiley & Sons, Ltd., 2003, pp. 115–120.
(c) Production of sodium borohydride
Acknowledgement
[23] W.S. Fedor, M. Douglas, D.O. Ingalls, IEC 49 (10) (1957) 1664–
1672.
[24] F. Schubert, K. Lang, W. Shabacher, A. Burger, US Patent, 3,077,356
(1963).
[25] Z.-P. Li, B.-H. Liu, N. Morigasaki, S. Suda, J. Alloys Compd. 349 (1)
(2003) 232–236.
[26] Z.-P. Li, N. Morigasaki, B.-H. Liu, S. Suda, J. Alloys Compd. 354 (1/2)
(2003) 243–247.
[27] Z.-P. Li, B.-H. Liu, K. Arai, N. Morigasaki, S. Suda, J. Alloys Compd.
356–357 (2003) 469–474.
The authors wish to express their thanks for the support
and funding from the following organization; The New En-
ergy and Industrial Technology Development Organization
(NEDO) and the Ministry of Education, Culture, Sports, Sci-
ence and Technology (MEXI).
[28] S. Suda, US Patent Appl. No. 10/721479 (11-25-03) (2003).
[29] Y. Kojima, T. Haga, Int. J. Hydrogen Energy 28 (2003) 989–993.
[30] J.V. Ortega, Y. Wu, S.C. Amendola, M.T. Kelly, US Patent 6,586,563
B1 (2003).
References
(a) Direct borohydride fuel cells (DBFC)
[31] S. Suda, Y. Iwase, N. Morigasaki, Z.-P. Li, in: D. Chandra, R.G.
Bautista, L. Schlapbach (Eds.), Advanced Materials for Energy Storage
II, TMS, 2004, pp. 123–133.
[1] W.H. Stockmayer, D.W. Rice, C.C. Stephenson, J. Am. Chem. Soc. 77
(1955) 1980.