ELECTROCHEMICAL METHODS Fundamentals and Applications - Allen.J.Bard

2 • Chapter 1. Introduction and Overview of Electrode Processes
1.1.1 Electrochemical Cells and Reactions
In electrochemical systems, we are concerned with the processes and factors that affect
the transport of charge across the interface between chemical phases, for example, between
an electronic conductor (an electrode) and an ionic conductor (an electrolyte).
Throughout this book, we will be concerned with the electrode/electrolyte interface and
the events that occur there when an electric potential is applied and current passes. Charge
is transported through the electrode by the movement of electrons (and holes). Typical
electrode materials include solid metals (e.g., Pt, Au), liquid metals (Hg, amalgams), carbon
(graphite), and semiconductors (indium-tin oxide, Si). In the electrolyte phase,
charge is carried by the movement of ions. The most frequently used electrolytes are liquid
solutions containing ionic species, such as, H + , Na + , Cl~, in either water or a nonaqueous
solvent. To be useful in an electrochemical cell, the solvent/electrolyte system
must be of sufficiently low resistance (i.e., sufficiently conductive) for the electrochemical
experiment envisioned. Less conventional electrolytes include fused salts (e.g., molten
NaCl-KCl eutectic) and ionically conductive polymers (e.g., Nation, polyethylene
oxide-LiClO 4
). Solid electrolytes also exist (e.g., sodium j8-alumina, where charge is carried
by mobile sodium ions that move between the aluminum oxide sheets).
It is natural to think about events at a single interface, but we will find that one cannot
deal experimentally with such an isolated boundary. Instead, one must study the properties
of collections of interfaces called electrochemical cells. These systems are defined
most generally as two electrodes separated by at least one electrolyte phase.
In general, a difference in electric potential can be measured between the electrodes in
an electrochemical cell. Typically this is done with a high impedance voltmeter. This cell
potential, measured in volts (V), where 1 V = 1 joule/coulomb (J/C), is a measure of the
energy available to drive charge externally between the electrodes. It is a manifestation of
the collected differences in electric potential between all of the various phases in the cell.
We will find in Chapter 2 that the transition in electric potential in crossing from one conducting
phase to another usually occurs almost entirely at the interface. The sharpness of
the transition implies that a very high electric field exists at the interface, and one can expect
it to exert effects on the behavior of charge carriers (electrons or ions) in the interfacial
region. Also, the magnitude of the potential difference at an interface affects the
relative energies of the carriers in the two phases; hence it controls the direction and
the rate of charge transfer. Thus, the measurement and control of cell potential is one of the
most important aspects of experimental electrochemistry.
Before we consider how these operations are carried out, it is useful to set up a shorthand
notation for expressing the structures of cells. For example, the cell pictured in Figure
1.1.1a is written compactly as
Zn/Zn 2+ , СГ/AgCl/Ag
(l.l.l)
In this notation, a slash represents a phase boundary, and a comma separates two components
in the same phase. A double slash, not yet used here, represents a phase boundary
whose potential is regarded as a negligible component of the overall cell potential. When
a gaseous phase is involved, it is written adjacent to its corresponding conducting element.
For example, the cell in Figure 1.1.1ft is written schematically as
Pt/H2/H + , СГ/AgCl/Ag (1.1.2)
The overall chemical reaction taking place in a cell is made up of two independent
half-reactions, which describe the real chemical changes at the two electrodes. Each halfreaction
(and, consequently, the chemical composition of the system near the electrodes)