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-LiClO4). 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.