Environmental Engineering Reference
In-Depth Information
A well-known electro-chemical method to obtain hydrogen using electricity is
the water electrolysis [
70
,
71
], which permits the splitting of water molecule into
H
2
and O
2
according to Eq.
2.22
reported in
Sect. 2.1.1.6
.
The galvanic cells produce electric energy via electro-chemical reactions, while
electrolytic cells, such as those used in water electrolysis, are electrochemical cells
in which a chemical reaction is forced by added electric energy.
The galvanic cells are based on a spontaneous overall reaction characterized by
a negative value of the Gibbs free energy, which corresponds to the theoretical
electric work (see details in
Sect. 3.1
). The electrolytic cells represent exactly the
reverse of the galvanic process, then the overall reaction, characterized by a
positive value of the Gibbs free energy, is not spontaneous, and needs an external
energy resource to force the advance towards the products.
Different electrolysis technologies could be applied, from the commercially
available method based on alkaline cells to the new advanced cells based on proton
exchange membrane (PEM) and solid oxide mixtures as electrolytes. The basic
schemes of these electrolysers are shown in Fig.
2.4
.
The alkaline device utilizes a solution of potassium hydroxide (KOH) as
electrolyte (Fig.
2.4
a). The two semi-reactions of reduction (cathode side) and
oxidation (anode side) that occur in alkaline solution are, respectively
2H
2
O
þ
2e
!
H
2
þ
2OH
ð
2
:
23a
Þ
2OH
!
1
=
2
O
2
þ
H
2
O
þ
2e
ð
2
:
23b
Þ
The sum of the two semi-reactions (
2.23a
) and (
2.23b
) gives the overall
Eq.
2.22
. Hydroxyl-ions represent the chemical species that close the electric
circuit through the electrolyte. The alkaline solution contains about 30 wt%
of potassium hydroxide and operates at about 80C. Today, this technology gives
Electrolyte
Electrolyte
Electrolyte
Cathode
Anode
Cathode
Anode
Cathode
Anode
H
+
OH
-
O
2-
Power
source
Power
source
Power
source
(a)
(b)
(c)
Fig. 2.4 Simplified principle scheme for alkaline (a), PEM (b) and solid oxide (c) cells for water
electrolysis
