Protide compounds in hydrogen storage systems

Article abstract of DOI:10.1016/S0925-8388(02)01241-0

Based on three chemical states of hydrogen: protide (H^-), protium (H⁰) and proton (H⁺), a triangular hydrogen energy system was proposed. The transfers among protide, protium and proton attracted our attention. We experimentally proved that NaBH₄ as a protide carrier can release its energy through a fuel cell (borohydride fuel cell) directly or generate hydrogen gas for polymer electrolyte membrane fuel cell application. The used fuel (meta-borate) can be reverted to NaBH₄ through a reaction with a saline hydride (MgH₂).

Full text of DOI:10.1016/S0925-8388(02)01241-0

Journal of Alloys and Compounds 356–357 (2003) 469–474
L
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Protide compounds in hydrogen storage systems
*
Z.P. Li , B.H. Liu, K. Arai, N. Morigazaki, S. Suda
Department of Environmental and Chemical Engineering, Kogakuin University, Nakano-machi 2665-1, Hachioji-shi, Tokyo 192-0015, Japan
Received 15 July 2002; accepted 25 October 2002
Abstract
Based on three chemical states of hydrogen: protide (H2), protium (H0) and proton (H1), a triangular hydrogen energy system was
proposed. The transfers among protide, protium and proton attracted our attention. We experimentally proved that NaBH4 as a protide
carrier can release its energy through a fuel cell (borohydride fuel cell) directly or generate hydrogen gas for polymer electrolyte
membrane fuel cell application. The used fuel (meta-borate) can be reverted to NaBH4 through a reaction with a saline hydride (MgH2).
2002 Elsevier B.V. All rights reserved.
Keywords: Protide compound; NaBH4; Saline hydride; Hydrogen energy system; Fuel cell; Fuel recovery
1. Introduction
1.3. Acquisition of an electron
It is known that the hydrogen bonding can be classified
into following three types:
The hydrogen atom can acquire an electron, attaining the
1s2 structure of He, to form the hydride ion H2, which is
also named ‘protide’. This ion essentially exists as saline
hydrides like NaH, MgH2 or some complex compounds
like NaBH4, NaAlH4.
1.1. Loss of the valence electron
Borohydrides and saline hydrides are well-known reduc-
ing agents. They often have quite specific uses in organic
and inorganic chemistry, where they may also be the
sources of H2 rather than simple reductants. They also
have attracted attention as a hydrogen storage medium or a
fuel for the fuel cell due to their high hydrogen contents
and high electric potential in electrochemistry. Fig. 1
shows the electrochemical capacity densities of hydrogen
stored by different methods. Borohydrides show higher
gravimetric capacities than metal hydrides and higher
volumetric capacities than compressed and liquefied hydro-
gen. Borohydrides show higher electrode potentials and
reactivities than gaseous hydrogen [1]. They are suitable
for use as a fuel for fuel cells in portable, mobile and
vehicle applications. Borohydrides that are stable in dry
air, are easily handled like common chemicals compared
with other hydrogen storage methods. The hydrogen
generation by the borohydride hydrolysis can operate
under ambient conditions. They are very promising port-
able and mobile hydrogen sources for polymer electrolyte
membrane fuel cells (PEMFCs). In this paper, we propose
a triangular hydrogen energy system based on sodium
The 1s valence electron is lost to give the hydrogen ion
H1, which is merely the proton. The proton usually exists
in liquid, seldom in condensed phases that is invariably
associated with other atoms or molecules. Tetrafluoro-
ethylene–perfluorovinyl ether sulfonic acid copolymer
(Nafion) is one of the few exceptions.
1.2. Formation of an electron-pair bond
The majority of hydrogen compounds contains an
electron-pair bond, such as H2, most of reversible metal
hydrides, and carbon hydrogen compounds. The diatomic
hydrogen of the gaseous hydrogen and mono-atomic
hydrogen in most reversible metal hydrides are referred to
as H0 for simplicity.
*
Corresponding author.
E-mail address: bq96001@ns.kogakuin.ac.jp (Z.P. Li).
0925-8388/02/$ – see front matter
doi:10.1016/S0925-8388(02)01241-0
2002 Elsevier B.V. All rights reserved.

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Z.P. Li et al. / Journal of Alloys and Compounds 356–357 (2003) 469–474
Fig. 1. Capacity comparison when hydrogen was stored in different ways.
2. Characteristics and properties of protide
compounds
Protide usually exists in solid compounds or complex
ions. The protide in saline hydrides behaves like halogens
˚
with a similar effective radius (2.08 A) and crystal
structures. The tetrahedral borohydride ion (BH24 ) or
aluminohydride ion (AlH42) were considered to arise by
the Lewis acid behavior of BH3 or AlH3 toward a protide,
but each H was found to be equivalent [2,3]. They are
usually classified as complex compounds. The hydrogen
contents of protide compounds are usually higher than
those of reversible hydrides as illustrated in Table 1.
Saline hydrides do not easily release their hydrogen by
thermo-decomposition due to their strong ionic bonding.
Unlike in saline hydrides, the stability of the protide in
complex compounds depends on the electron donor. For
example, although borohydride and aluminohydride have a
similar molecule structure, their thermal dissociation prop-
erties are different. Dymova et al. experimentally de-
termined the dissociation reaction of NaAlH4 [4] as
follows:
Fig. 2. Example of the triangular hydrogen energy system.
borohydride consisting of H1, H0 and H2 as shown in Fig.
2, which differs from the linear hydrogen energy system
between H1 and H0.
Table 1
Comparison of protide compounds with metal hydrides on hydrogen content
Metal hydride
Chemical hydride
Protide compound
AB5H6
AB2H4
ABH2
A2BH4
AHx
MIBH4
MIAlH4
MIIBH4
MIIAlH4
Typical
hydride
LaNi5H6
1.4
ZrMn2H3.46
1.7
TiFeH1.9
1.8
Mg2NiH4
3.6
MgH2
LiBH4
NaBH4
NaAlH4
Mg(BH4)2
Mg(AlH4)2
Hydrogen
content (wt.%)
7.6
18.4
10.6
7.4
15.1
10.2

Z.P. Li et al. / Journal of Alloys and Compounds 356–357 (2003) 469–474
471
NaAlH4↔1/3Na3AlH6 1 2/3Al 1 H2↑ ↔ NaH 1 Al 1
1.5H2↑ (1)
3. Triangular hydrogen system
Compared with the linear hydrogen system based on H1
and H0, the triangular hydrogen system based on H1, H0
and H2, can offer a high power output by the fuel cell. As
an example, we studied the possibility of using NaBH4 as
the medium in the triangle hydrogen energy system. Fig. 3
shows the proposed hydrogen system that concludes
following processes:
Their results proved that the protide (H2) in NaAlH4
partially transformed into hydrogen gas (H0). The hy-
drolysis reaction of NaAlH4 is an explosive reaction,
which is not suitable for hydrogen generation. Unlike
NaAlH4, NaBH4 cannot reversibly be associated and
dissociated under moderate conditions, but its hydrolysis is
of interest in hydrogen generation because it is a catalytic
reaction whose rate is controllable. It can generate 10.8
wt.% of hydrogen based on the following hydrolysis
reaction
(1) Electrochemical oxidation of borohydride through a
fuel cell to generate power
NaBH4 1 2O2 5 NaBO2 1 2H2O
(5)
For some applications, borohydride can be reformed into
hydrogen gas by the hydrolysis reaction. The produced
hydrogen supplies to PEMFCs to generate electricity
through the following reactions
NaBH4 1 2H2O 5 NaBO2 1 4H2
(2)
With respect to the high potential of borohydrides, they are
also a promising fuel for fuel cells as several patents have
pointed out [5–8]. Some investigations on the electro-
chemical oxidation mechanisms of borohydride have been
reported [9–11]. For example,
NaBH4 1 2H2O 5 NaBO2 1 4H2
(6)
J
4H2 1 2O2 5 4H2O
The reverse process includes the following reactions.
(2) Electrolysis of water
BH24 1 8OH2 → BO22 1 6H2O 1 8e2 2 ←1.24 V (vs. SHE)
(3)
2H2O 5 O2 1 2H2
(7)
(8)
The protide in borohydrides can also transform into
hydrogen gas (H0) by a hydrolysis reaction and then
transform into H1 by an electrochemical reaction.
Borohydride transforms into meta-borate after release of
its hydrogen. The formed borate can be recovered by a
chemical reduction. For example, sodium borohydride can
be formed by a reaction of NaBO2 with calcium hydride
[12]
(3) Fuel recovery
2H2 1 2Mg 1 NaBO2 5 NaBH4 1 2MgO
or
2H2 1 2Mg 5 2MgH2
2MgH2 1 NaBO2 5 NaBH4 1 2MgO
(9)
J
2CaH2 1 NaBO2 5 NaBH4 1 2CaO
(4)
In this hydrogen energy system, many investigations on
Fig. 3. Comparison of the linear and triangle hydrogen energy system.

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Z.P. Li et al. / Journal of Alloys and Compounds 356–357 (2003) 469–474
PEMFCs, hydrogen production through the electrolysis of
water have been reported. In this presentation, we focused
our attentions on the fuel cell using borohydride as the fuel
directly, hydrogen generation from borohydride and fuel
recovery.
such as a number of chemical hydrides, metal hydrides,
nanotubes and carbon fibers. Protide compounds are
chemical hydrides. Early in the 1950s, Schleshinger et al.
[13] suggested using sodium borohydride to generate
hydrogen under ambient conditions. Their results showed
sodium borohydride solution released 90% of the stoichio-
metric amount of hydrogen. In our investigation, alkaline
borohydride solution in a certain concentration range (,20
wt.%) can release almost 100% of the stoichiometric
hydrogen within 15 min when using Raney Ni as the
catalyst (see Fig. 5). A prototype of the hydrogen generator
has been produced. It produced 90% of the stoichiometric
amount of hydrogen at a rate of 10 l/min at room
temperature as shown in Fig. 6.
3.1. Fuel cell using borohydride as the fuel
Our aim on fuel cell development is to develop low cost
fuel cells using borohydrides that have higher electro-
chemical reactivity than hydrogen gas. For simplicity, this
kind of fuel cell is named borohydride fuel cell (BFC). In
principle, BFC works in an alkaline environment so that
catalytic agents are not limited to noble metals. Low cost
Ni-based catalysts for anode and cathode could be ex-
pected to show good performances like alkaline fuel cells
(AFCs). The Ni-coated steels can be used to make bipolar
plates so that their manufacturing cost can be decreased
greatly by using a cold press forming technique. Because
the fuel is an aqueous solution that can act as a cooling
medium, the cooling plates that are included in the stack of
PEMFCs, are not necessary any more. Based on these
considerations, we constructed fuel cells using fluorinated
AB2 (F-AB2) as the anode catalyst, Pt–C as the cathode
catalyst and Nafion membrane as the electrolyte.
The fuel was an alkaline borohydride solution con-
taining 10 wt.% NaBH4 and 20 wt.% NaOH (containing
2.1 wt.% H2). The bipolar plates were made of stainless
steel. The flow field area of the plates was 70% of the
electrode area (20330 mm). The stack consisting of ten
cells demonstrated a good performance like PEMFCs
(shown in Fig. 4).
3.3. Fuel recovery
In the triangular hydrogen energy system, fuel recovery
is the most difficult issue. In the linear hydrogen system,
only hydrogen reacts with oxygen during energy conver-
sion. For example, hydrogen gas from metal hydrides can
be used to run a PEMFC to produce water. The hydrogen
gas can be produced by the electrolysis of water, and then
reacted with alloys to form metal hydrides. In the triangu-
lar hydrogen system, when protide compounds generate
hydrogen gas to supply PEMFCs by a thermal decomposi-
tion, the process will be the same as that in linear hydrogen
energy system. However, when protide compounds release
their hydrogen by the hydrolysis or are electrochemically
oxidized directly, the protide has not only a relation with
oxygen but also a relation with its donor. The donor will
react with oxygen when the protide is oxidized. Therefore,
the fuel recovery must include processes of not only the
hydrogen cycling but also the donor cycling. For example,
protide in borohydride (BH24 ) runs a fuel cell to produce
meta-borate and water. Protide in saline hydrides can be
formed by reacting alkali or alkali-earth metal with hydro-
3.2. Hydrogen generation
Hydrogen can been stored as a pure substance as a
liquid or compressed gas (say .300 atm.) and in materials
Fig. 4. Fuel cell using borohydride as a fuel and its performance. Anode: 200 mg/cm2 flourinated AB2 alloy, 10 wt.% NaBH4 –20 wt.%NaOH at flow-rate
of 186 ml/min. Cathode: 2 mg/cm2 Pt-black, atmosphere at flow-rate of 200 ml/min.

Z.P. Li et al. / Journal of Alloys and Compounds 356–357 (2003) 469–474
473
Fig. 5. Evolution of hydrogen from sodium borohydride solution at a concentration of 10–100 g/l at room temperature.
gen gas produced by the electrolysis of water. The
borohydride can be obtained by a reaction of saline
hydride with meta-borate through reaction (8) as shown in
Fig. 7. The reaction was conducted with a planetary ball
mill at room temperature. Thermodynamics calculation
proves that this reaction is feasible (reaction DG052
269.7 kJ/mol). Even though the NaBH4 conversion rate
showed a deviation from its theoretical value, almost 100%
of NaBH4 conversion rate was achieved when an excessive
amount of MgH2 (1.25 times more than stoichiometric
Fig. 6. Prototype of a hydrogen generator and its performance.

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Z.P. Li et al. / Journal of Alloys and Compounds 356–357 (2003) 469–474
Fig. 7. NaBH4 formation by ball milling based on the reaction 2MgH2 1NaBO2 5NaBH4 12MgO.
amount) was added. The pure NaBH4 powders can be
obtained by vaporization of ethylenediamine after extrac-
tion and filtration.
of Japan, and the Grant-in-Aid for Scientific Research from
the Ministry of Education, Science and Culture of Japan.
References
4. Conclusions
[1] R. Jasinski, Electrochem. Tech. 3 (1965) 40.
[2] R.A. Ogg Jr., J.D. Ray, Disc. Faraday Soc. 19 (1955) 239.
[3] J.N. Shoolery, Disc. Faraday Soc. 19 (1955) 215.
[4] T.N. Dymova, Yu.M. Dergachev, V.A. Socolov, N.A. Grechanaya,
Dokl. Akad. Nauk SSSR 224 (1975) 591, Eng. 556.
[5] D. Goerrig, DBP 1077644, (1958).
[6] J.Y. Lee, USP 5,599,640, (1997).
[7] S. Amedola, USP 5,804,329, (1998).
[8] S. Suda, USP 6,358,488, (2002).
[9] R.L. Pecsok, J. Am. Chem. Soc. 76 (1953) 2862.
[10] W.H. Stockmayer, D.W. Rice, C.C. Stephenson, J. Am. Chem. Soc.
77 (1955) 1980.
[11] M.E. Indig, R.N. Snyder, J. Electrochem. Soc. 109 (1962) 1104.
[12] D. Goerrig, W. Schabacher, F. Schubert, DBP. 1036222, (1956).
[13] H.I. Schlesinger, H.C. Brown, A.E. Finholt, J.R. Gilbreath, H.R.
Hoekstra, E.K. Hyde, J. Am. Chem. Soc. 75 (1953) 215.
We experimentally proved that a triangular hydrogen
energy system can be established by using NaBH4 as the
medium based on three hydrogen states: protide (H2),
protium (H0) and proton (H1). Borohydride fuel cell
shows a good performance even without using noble
metals in the anode. The used fuel (NaBO2) can be
recovered by a reaction with saline hydride (MgH2).
For now the cost of borohydrides (ca. $58/kg NaBH4)
as a fuel is still unacceptable for large capacity fuel cells,
however it is not a big problem for the small or microfuel
cell. Therefore, borohydrides are potential candidates as a
fuel for small and microfuel cell in portable and mobile
devices.
Acknowledgements
This research is financed by the New Energy and
Industrial Technology Development Organization (NEDO)