Environmental Engineering Reference
In-Depth Information
a very low contribution to the worldwide hydrogen production (see Table
2.1
),
because of the high costs of electricity and the high but not complete conversion
efficiency [
72
]. Furthermore, the KOH solution could limit the resistance of used
materials because of corrosion phenomena. In the past years, many studies have
been addressed towards a further improvement of catalysed electrodes to optimize
the efficiency and reliability of the electrolytic process involving alkaline cells, but
the results have not yet satisfactory [
73
-
75
].
New advances of the other two electrolytic cells reported in Fig.
2.4
have been
recently encouraged to exploit the higher potentialities of PEM and solid oxide
technologies, as electrolyser components to be integrated in plants based on wind
and solar renewable sources [
76
,
77
] or nuclear power [
78
], respectively.
As regarding electrolysis with PEM cells (scheme b of Fig.
2.4
), which is just
the reverse of fuel cell operation mode, the interest derives by greater energy
efficiency, ecological cleanness, easy maintenance, smaller mass-volume char-
acteristics and high degree of gases purity [
79
-
81
]. Furthermore, it is expected that
future costs could be progressively reduced due to foreseeable technological
advances of PEM devices working as electric power generation.
The two semi-reactions involved in the process are:
2H
2
O
!
O
2
þ
4H
þ
þ
4e
ð
2
:
24a
Þ
4H
þ
þ
4e
!
2H
2
ð
2
:
24b
Þ
Equations
2.24a
and
2.24b
are the oxidation and reduction steps, occurring at
anode and cathode side, respectively, while the protons represent the ion species
passing through the solid polymer electrolyte. However, the overall electro-
chemical reaction is the same of alkaline electrolysers.
Recent studies have been devoted to the optimization of the already existent
PEM electrolysers, first exploring the possibilities to increase the working pres-
sure. The high pressure electrolysis (HPE) should reduce significantly the energy
costs for the successive fuel compression step. Currently, the on board-storage of
hydrogen in fuel cell cars (see details in
Sect. 2.3
) requires a compression stage
before feeding the vehicle, while the need for an external hydrogen compressor
could be avoided by pressurising the hydrogen in the electrolyser. The energy
required to produce high pressure hydrogen by high pressure water electrolysis is
estimated to be about 5% less than that required for devices working in atmo-
spheric conditions [
82
]. However, high pressure operation could affect the per-
formance of existent Nafion electrolytic membranes [
83
] and yield additional
problems regarding efficiency loss due to cross-permeation phenomena [
81
] that
implies also relevant safety issues. On the other hand, the application of this
technology for electrolytic hydrogen production is mainly related to costs of noble
metals used in membrane electrode assembly (MEA, see
Sect. 3.2
) materials,
requiring the development of new high performance and low cost materials [
84
].
PEM electrolysis process has been proposed especially for wind turbines or
solar photovoltaic (PV) panel utilizations. In this case, the electrolysis represents
