Fuel Cells for Automotive Applications - Hydrogen Fuel Cells for Road Vehicles - page 72

3.1 Basic Concepts of Electrochemistry

The fundamental process by which a galvanic cell produces DC electricity starting
from chemicals is an oxidation-reduction reaction (redox reaction), defined as a
reaction in which the atoms of the species involved have their oxidation state
changed. The oxidation state is described by the oxidation number of the atom in
the given compound, and expresses the amount and the sign of the charge ideally
assumed by the atom in the specie in question, after attribution of the pairs of
electrons in each bond to the most electronegative atom in the molecule (some
practical rules, compatible with this general principle, are used to calculate the
oxidation number of a specific atom in a compound [ 1 ]). In a redox reaction an
atom is oxidized when its oxidation number increases, whereas it is reduced when
that number decreases. Regarding the reaction mechanism, the variation of the
oxidation state can be realized by a transfer of either electrons or atoms, but in
many cases both of them are observed. If a transfer of electrons is present, an
oxidation corresponds to a loss of electrons, while a reduction implies a gain of
electrons, and the overall redox reaction can be utilized in an electrochemical
system, which can be regarded as an energy converter in which chemical energy is
transformed into electric energy (galvanic cell) or vice versa (electrolytic cell).
In an electrochemical converter an overall redox reaction is divided into two
semi-reactions which take place on physically separate electrodes, anode, and
cathode, for oxidation and reduction semi-reactions, respectively. For a generic
redox reaction, involving the species A and B in equilibrium, the overall reaction
and two semi-reactions can be schematized as follows:

Overall Redox : A red þ B ox A ox þ B red

ð 3 : 1 Þ

Þ : A red A ox þ ne

Oxidation ð anode ð 3 : 2 Þ

Þ : B ox þ ne B red Reduction ð cathode ð 3 : 3 Þ

where n is the number of electrons involved in semi-reactions.
The electrons transferred during the redox reaction move through an external
circuit, exiting from the anode after oxidation, and entering into the cathode for
reduction. The two semi-reactions can occur because the two separate spaces are
inter-connected by a conductive liquid or solid phase (electrolyte) able to transfer
ionic species, thus permitting the closing of the electric circuit. Then the elec-
trolyte has to be ionically conductive, whereas the electrodes have to be electri-
cally conductive and, in the case of gaseous reactants, sufficiently porous to allow
the transfer of reactants and products to and from the reaction sites (see Sect. 3.2 ).
It is possible to build a scale of potentials for different electrochemical pairs in
standard conditions (298 K, 1 atm for gases or unit concentration for solutions)
assuming the pair hydrogen/ion-hydrogen as an arbitrary reference semi-reaction,
and assigning to it a potential of zero volt:

2H þ þ 2e H 2 E ¼ 0V

ð 3 : 4 Þ