Hydrogen is the most
abundant element on the earth's surface, where it is found primarily
in water (H2O) and organic compounds. It is generally
produced from hydrocarbons or water; and when burned as a fuel, or
converted to electricity, it joins with oxygen to formagain water.
Basic chemical knowledge of Hydrogen is relevant
for the understanding of the difference between Borohydride and
Hydrogen as electron supplier.
Names of hydrogen species
|
General |
Term |
Molecular(H2) |
Hydrogen
molecular |
Dihydrogen |
Atom(H0) |
Hydrogen
atom |
Protium |
Cation(H+) |
Hydrogen
ion |
Proton |
Anion(H-) |
Hydride
ion |
Protide |
- Ho (one e-) -Can be
found in metal hydride and H2. The catalyst of
normal
Fuel Cells (PEMFC) separates H2 into two
Ho atoms.
- H+ (no e-) -Can be
found in a H2O molecule.
- H- (two e-) -Can be found in protide compounds such as
BH4-
- H2 (one e-)
-Molecular Hydrogen
THE
4-STATES OF HYDROGEN (Prepared by Prof.
S. Suda)
- Hydrogen (H2: Diatomic
hydrogen): PEMFC (PEFC) and electrolysis
H2
(H-H) Ì 2Ho (internally
reversible) ¨ 2H+ + 2e-
(irreversible) 2H2O Ì
2H2 + O2 (internally reversible)
- Protium (Ho: Monatomic
hydrogen): in metal hydrides for H-storage
H2 Ì2Ho (internally reversible)
- Proton (H+: Hydrogen
ion): in PEMFC (PEFC)
1/2H2 (H-H) Ì
Ho (internally reversible) ¨ H+ + e-
(irreversible)
- Protide (H-: Hydride
ion): ex. in MgH2, LiBH4, and
NaBH4 for hydrogen storage and DBFC
Ho + e- ¨ H-
(irreversible) H2 ¨
2H- + 2e-
(internally reversible)
Protide -- a high energy hydrogen
species
Two electron attached on one
hydrogen atom
In the complex ion of
BH4-, hydrogen exists as a negative ion
(protide: H-). The fact which needs attention is that a
protide can release twice as much electron as a proton (normal
PEMFC).
Protide is highly unstable and has a high energy
level. In order to stabilize the Borohydride, it is resolved into
the aqueous solution of sodium hydroxide or potassium hydride. After
that it is very stable to use und easy to transport.
Through the catalystic reaction on the anode
side of the Fuel Cell, the Borohydride is spited up and 8 electrons
are separated from each Borohydride molecule.
BH-4 +
8OH- ¨ BO-2 + 6H2O +
8e-
Borohydride
(BH4-) 4 Proton (H+) + 8e-
Properties of Borohydride as Hydrogen
Storage Material
By A. Zuttel and L. Sclapbach, Nature
p.353, Vol. 414 (2001)
This figure underlines the fact that sodium
borohydride is a high hydrogen storage material. It is located in
the group of highest hydrogen content, both weight basis and volume
basis.
Major Problems of other hydrogen storage
materials
![](BH4_50.gif) |
Approx. 900atm is required to obtain same
amount of H2 by compression. H2
compression is not suitable for mobile services by
cylinder weight and safety consideration. |
|
![](BH4_50.gif) |
Several hundreds grams is required to
store H2 as metal hydrides (practical hydrogen
content of 2wt%). It is also disadvantage that continuous heat
input is required under hydrogen generation. |
|
![](BH4_50.gif) |
Hydrogen from natural gas or methanol by
reforming requires higher temperature for reaction. It causes
longer start-up duration, delay of response, care for burn
injury, etc. CO, byproduct of hydrogen, should be treated to
avoid poisoning of PEFC electrode and intoxication. It is also
disadvantage that continuous heat input is required under
hydrogen generation. |
|
![](BH4_50.gif) |
In DMFC (Direct Methanol Fuel Cell),
methanol fuel is used as low concentration to avoid crossover
of methanol in fuel cell, leading lower energy density or
recirculation of diluting water. Toxicity of methanol should
be also considered. |
|