Page 43 - Handbook of Battery Materials

1.2 Electrochemical Fundamentals 9

1.2.2
Electrochemical Series of Metals
The question arises, which metal is dissolved and which one is deposited when
they are combined in an electrochemical cell. The electrochemical series indicates
how easily a metal is oxidized or its ions are reduced, that is, converted into positive
charged ions or metal atoms respectively. To compare different metals we use the
standard potential, which is described below.
In Galvanic cells it is only possible to determine the potential difference as a
voltage between two half cells, but not the absolute potential of the single electrode.
For the measurement of the potential difference it has to be ensured that an elec-
trochemical equilibrium exists at the phase boundaries (electrode/electrolyte). At
least it is required that there is no flux of current in the external and internal circuit.
To compare the potentials of half cells a reference had to be defined. For this
reason it was decided arbitrarily that the potential of the hydrogen electrode in a
◦
1 M acidic solution should be equal to 0 V at a temperature of 25 C and a pressure
of 101.3 kPa. These conditions are called standard conditions [6].
The reaction of hydrogen in acidic solution is a half-cell reaction and can therefore
be handled like the system metal/metal salt solution.

+
H 2 + 2H 2 O → 2H 3 O + 2e −
An experimental setup for the hydrogen half cell is illustrated in Figure 1.5.
The potentials of the metals in their 1 M salt solution are all related to the standard
or normal hydrogen electrode (NHE). To measure the potential of such a system,
the hydrogen half cell is combined with another half cell to form a Galvanic cell.
The measured voltage is called the normal potential or standard electrode potential
ε 00 of the metal. If the metals are arranged in the order of their normal potentials,
the resulting order is named the electrochemical series of the metals (Figure 1.6).
Depending on their position in this potential series, they are called base (ε 00 < 0)
or noble (ε 00 > 0) metals.

H 2

Pt

Figure 1.5 Hydrogen electrode with hydrogen-
HCI
saturated platinum electrode in hydrochloric acid.