Chemistry Reference
In-Depth Information
recombination center have been found on silicon electrodes in both fluoride and non-
fluoride solutions.
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1.6. OPEN-CIRCUIT POTENTIAL
OCP, or the rest potential of an electrode, is the potential of a freestanding elec-
trode without electrical connection to any other conducting materials. Thus, at OCP
there is no net current flow in or out of the electrode. OCP of an electrode is deter-
mined by the kinetic state of the electrode. It is the most easily measurable electro-
chemical parameter and at the same time is the most convoluted quantity as it is
determined by all the kinetic factors in the system. The electrode at OCP can be at an
equilibrium state or a nonequilibrium state depending on the nature of the particular
electrode/electrolyte system and the reference time scale.
If no net chemical change occurs in the system during the time of measurement,
the electrode at OCP can be regarded as at an equilibrium condition. This occurs, for
example, as shown in Fig. 1.24a, where the Fermi level of the semiconductor has the
same value as the reversible potential of a single redox couple in which the forward
current equals the reverse current In many situations and almost always for silicon,
the electrode/electrolyte interface at OCP is at a nonequilibrium condition and a net
chemical change in the system occurs when two or more redox couples are present in
the electrolyte. OCP in these situations is a mixed potential. This is shown in Fig. 1.24b,
where OCP is established with two redox couples, and with
but In this case, a net chemical change is
involved since and a net amount of reduced species 1 and
oxidized species 2 is produced for a given time peorid.
When one of the redox couples is associated with the dissolution of the electrode,
OCP is also called the corrosion potential and the net dis-
solution rate at OCP is the corrosion current, This is generally the case
with a silicon electrode at OCP in aqueous solutions because the thermodynamic poten-
and
